Tatra fuel injection Service Manual

BOSCH

FUEL

INJECTION

SYSTEMS

Forbes Aird

HPBooks

-------- ------

HPBooks

are published by

The Berkley Publishing Group

A division of Penguin Putnam Inc.

375 Hudson Street

New York,New York 10014

First edition: July 2001

ISBN: 1-55788-365-3

@2001 Forbes Aird

10987654321

This book has been catalogued with the Library of Congress

Book design and production by Michael Lutfy

Cover design by Bird Studios

Interior illustrations courtesy of Bosch, Inc. and the author as noted

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the

publisher.

NOTICE: The information in this book is true and complete to the best of our knowledge. All recommendations on parts and procedures are made without any guarantees on the part of the author or the publisher. Author and publisher disclaim all liability incurred in connection with the use of this information. Although many of the illustrations in the pages that follow were supplied by Bosch and used with their permission, this publication is a wholly independent publication of

HPBooks.

I am grateful to Robert Bosch GmbH in the persons of Gerhard Kopany, for permission to reproduce copyright photos and illustrations, and of Wolfgang Boerkel, for answers to some technical questions. Thanks are also due to Wolfgang Hustadt of Bosch North America and to Bill

Roth, for further technical information.

~- -

around just about as long as the automobile

A;thOUghgasoline fuel injection (FI) has been

technology. Until about 1970 it was both rare and expensive, restricted to some aircraft applications, and to a hand-full of exotic, high performance cars and racers. Production cars- even high performance sports cars-

made do with carburetors.

Thirty-plus years later, and as a result of the

"electronics revolution," that situation has been very nearly turned on its head. While cars in many racing classes wear carburetors, virtually every production car in the world has FI! Yet it remains a mystery to most.

'tself, it has always been a mysterious

This book is an attempt to de-mystify fuel injection. While it deals principally with the various electronic fuel injection systems produced by the Robert Bosch company, much of what is said in the following pages also applies to other systems.

No book of this size-indeed, likely no book of any

size-could fully describe the minor variations in FI

installations between one vehicle model and another.

Still, it is hoped that sufficient detail is provided to be of

benefit to mechanics both amateur and professional,

while the general principles described will be useful for those attempting performance tuning, and informative

for readers who simply seek to understand the mystery.

CHAPTER 1 Food For Engines, Food For Thought

CHAPTER 2 Fuel Injection: Then and Now

CHAPTER 3 Bosch Intermittent Electronic FI

CHAPTER 4 Motronic Engine Management

CHAPTER 5 Troubleshooting Bosch Intermittent Electronic FI

CHAPTER 6 Bosch Continuous Injection

CHAPTER 7 Troubleshooting Bosch Continous Injection

CHAPTER 8 Performance Modifications

109

123

f a certain fixed quantity of air-or any other gas- is confined in a closed con-

tainer and then heated, the pressure

inside the container will rise. If one of the walls of the container is moveable, the inter- nal pressure will push that wall outward

with a certain amount of force, according to how much heat was put into the trapped gas.

That, in a nutshell, is the working princi- ple of all internal combustion engines: Each cylinder is a closed container, and each pis- ton represents a moveable wall of that con- tainer; the heat is supplied by the burning of a fuel, usually gasoline, and the trapped gas is whatever mixture of gaseous compounds left over after the burning.

Meanwhile, the other moving parts of an engine are there for one or the other of just two supporting functions. The "bottom end" converts the movement of the pistons into rotary motion and, by returning them to the top of their strokes, restores the closed con- tainers to their original size; the valve gear and everything else atthe "topend" are there simply to provide for the emptying out of the spent gasses and the refilling of the cylinders with a fresh charge of burnable

mixture.

This may all seem very obvious to anyone with even the most basic understanding of how engines work, but lurking within the simple facts outlined above is a wealth of detail. Consider the fuel, for example. Some fuels contain more chemical energy per pound than others, and so can produce more heat when burned. Even limiting the discus- sion to gasoline, the fact is that ordinary pump gasoline is a mixture of hundreds of different flammable compounds, and each of those compounds has a different potential

ability to generate heat when burned. The exact nature of the mixture of these com-

pounds varies from one pump to another and from one season to the next, so a pound of gasoline from one pump on one day might

release somewhat more or less heat when burned than would a pound from another

pump, or from the same pump on some other day.

While each is unique, all the hundreds of

compounds that make up gasoline have one thing in common-they are all hydrocar- bons. That is, they are all made of just two kinds of atoms, hydrogen (H) and carbon

(C). The difference between one of these hydrocarbons and another lies in either the

number of hydrogen and carbon atoms, or in the way in which these two component ele- ments are arranged, or both.

Now, burning is a process of oxidation-a combining with oxygen (O)-so, reduced to its basics, when a hydrocarbon fuel like gasoline burns, individual hydrocarbon mol-

ecules from the gasoline combine with indi- vidual molecules of oxygen from the air. The hydrogen (H) in the hydrocarbon com- bines with some of the oxygen (0) in the air to produce water (H2O), while the carbon

rest of the oxygen to form carbon dioxide

(CO2)' In this process, a large amount of

energy gets released, in the form of heat. This chemical dance amounts basically to a reversal of the processes that went into cre-

ating the hydrocarbons in the first place. See

the box, "Sunlight by the Gallon."

Air, too, is a mixture of substances, although all of them are gasses atroom tem-

perature. About 78 percent of our atmos- phere is nitrogen (N); only about 21 percent

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Burning fuel with insufficient air results in carbon monoxide (CO) being formed, instead of carbon dioxide (CO2)' Excess fuel also leaves unburned hydrocarbons (HC) in the exhaust. Paradoxically, a too-lean mixture causes misfires, which also lets unburned fuel escape. Oxides of nitrogen (NOx) are produced in greatest abundance when the mixture strength is just a bit lean, when combustion is hottest. (Robert Bosch Corporation)

of it is oxygen. The remaining one percent or so is made up of several rare gasses, like neon and argon, plus CO2 and water vapor. The chemical reaction of burning gaso- line-especially inside the cylinders of an operating gasoline engine-is further com- plicated by the presence of these other ele- ments, and particularly the nitrogen.

Nitrogen is a comparatively inert sub- stance-it does not readily react with any- thing much, so in a simplified description of

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combining with some of the oxygen, form- ing various oxides of nitrogen- NO2, NO3

and so on-known collectively as NOx. While for most purposes the minor involve- ment of the nitrogen does not make much difference, these nitrogen oxides are air pol- lutants. Thus, while the idea of "burning" a fuel seems a simple business, here is just one factor that begins to reveal that it is some- what more subtle and complex than it atfIrst

appears. the burning of gasoline in air,the nitrogen is ignored, on the assumption that it passes right through the whole operation unchanged. In fact, that is not quite true. Exposed to the enormous temperatures and pressures in the combustion chamber of an engine, a little of the nitrogen does end up

(that is, weight) of air contains a certain spe-

cificnumberof oxygenmolecules,and any

givenweightof any specificgasolinelike-

wise contains some definite number of

Stoichiometric Mixture

Within narrow limits, a fixed quantity

Vegetables are a lot smarter than you might think. Millions of years ago, they man- aged to harness the energy of sunlight in order to manufacture themselves from the simple raw materials available to them, specifically water (H2O)from the ground and carbon dioxide (CO2) from the atmosphere. A plant-a tree, for instance-grows by using solar energy to split the CO2 into separate atoms of carbon (C) and oxygen (0),

and then to combine the carbon with the water to make molecules of cellulose (C6HlOOS)'In effect, a tree uses water and carbon to make more tree.

From the tree's point of view, the oxygen remaining after the carbon is split from the CO2 is incidental, and so is cast aside, into the air. From our point of view, this aspect

of the process is vital- it is the reason we have a breathable atmosphere. (And you thought those tree-huggers were as dumb as vegetables!)

Usually, once a plant dies, the cellulose breaks down, but the process is not exactly a reversal of the original chemistry. In fact, some of the carbon and hydrogen the tree has spent so much of its sun-energy input combining remain hooked together as mol- ecules having a certain number (say, "X")of carbon atoms and some number (say, "y") of hydrogen atoms, (CxHy), of which there are many hundreds of different ones. If exposed and left to rot, the major product is likely to be a gas-methane (CH4)' other- wise known as "swamp gas," or "fire damp.IIUnder certain conditions, however, entire forests can get buried and, over the course of countless millions of years, subject to enormous pressure from the weight above, the carbon and hydrogen atoms get re-shuf- fled into more tightly clustered hydrocarbon molecules, many of which are liquids. All

of the world's oil (hydrocarbon) deposits are the age-old remains of this process of decay and chemical rearrangement of vegetation.

When we subsequently extract, refine and bum some of this liquid sunshine, we are recombining the hydrogen and carbon with oxygen. When we do so, the swapping of chemical partners that occurs liberates all of the considerable solar energy that, over years, went into the original separation processes. And it is that energy, in the form of heat, that makes the wheels go 'round.

hydrocarbon molecules. Because the burn- ing process amounts to individual atoms combining with each other, it follows that there is only one particular ratio of gasoline- to-air that can ensure that all of the oxygen molecules mate up with all of the hydrocar-

bon molecules. This theoretical ideal is called a stoichiometric mixture.

If there is an excess of oxygen molecules,

some of them will fail to find partners. In terms of the number of oxygen-hydrocarbon pairings, and thus the amount of energy

released, the effect is as if we had started with a smaller quantity of air. At the same time, if there are too many hydrocarbon

molecules in relation to the amount of air, then some of the hydrocarbons will emerge from the combustion process unburned.

Some of the gasoline is simply wasted. Not only that, but a shortage of oxygen means

that there is a likelihood of some of the car- bon atoms in the hydrocarbon fuel to com-

bine with just one oxygen atom, rather than two, yielding carbon monoxide (CO) rather than carbon dioxide (CO2)' While CO2 is one of the" greenhouse gasses" that are part- 1yresponsible for global warming, at least it is only immediately harmful to animal life when its concentration grows so large that it

displaces much of the oxygen we need to breathe. CO, on the other hand, is toxic even in small doses.

It turns out that about 14.7lbs. of air con-

tains the correct number of oxygen mole- cules to pair up with the number of hydro- carbon molecules in 1 lb. of gasoline. The ratio of air to gasoline to achieve a stoichio- metric mixture, in other words, is approxi- mately 14.7:1, by weight. Note we say approximately- there is no single number

that correctly identifies the stoichiometric mixture for all gasolines. To explain, recall that gasoline is a mixture of hydrocarbons.

Each has its own stoichiometric mixture

strength, ranging from less than 13:1 to

more than 15:1, so the stoichiometric ratio for the entire blend depends on the propor-

tions of the differing hydrocarbons that make it up. Apart from incidental variations,

the major oil companies deliberately modify the blend of hydrocarbons in pump gasoline from season to season and from place to place, so the stoichiometric mixture may correspondingly vary slightly, according to where and when you buy the fuel. (See also the sidebar "The Oxygen Battery,"page 34.)

As we have said, gasoline, strictly defined, contains only hydrocarbons, but oil compa- nies have also begun, fairly recently, to include certain additives in gasoline that fur- ther affect the chemically correct mixture. Among the additives commonly found in both pump and racing gasolines are ethyl alcohol (ethanol) and methyl tertiary butyl ether (MTBE). Both these substances are

examples of what are called oxygenates- like the hydrocarbons we have been speak- ing of they contain hydrogen and carbon, but unlike the hydrocarbons they also con- tain oxygen. A fuel carrying its own oxygen adds to the amount inhaled by the engine, so the presence of oxygenates means that a lit-

tle extra fuel is needed in relation to the quantity of air the engine is breathing in, to

take account of the additional oxygen being carried within the fuel- the stoichiometric

ratio becomes a little (numerically) smaller. This is yet another reason why it is not pos- sible to specify one exact stoichiometric mixture strength for any and all gasolines.

Note, too, that the stoichiometric mixture

strength is expressed as a ratio of weights- or more correctly, masses-not volumes. (The mass of something is, in effect, a "count" of the number of molecules in it.) A certain mass of air- that is, a certain number of molecules- will occupy more or less vol- ume, according to its temperature. A cubic foot of hot air contains fewer gas molecules, including oxygen molecules, than a cubic

foot of cold air. Other factors, like baromet- ric pressure and altitude also affect the den- sity of air-the weight of a certain volume, in other words. For that matter, the density of gasoline also varies with temperature, though not nearly as much.

While the ideal of stoichiometry expresses the chemically correct air-to-fuel ratio for any particular gasoline blend, gasoline will, in fact, burn in air over a spread of ratios

from about 6:1 to more than 24:1.Mixtures that contain more fuel than the theoretical

optimum are said to be "rich," while those with an excess of air are termed "lean." For

maximum power production, there is some- thing to be said for mixtures that are some-

what richer than stoichiometric.

To begin to explain, consider a four-cycle

engine turning 6000rpm. At that speed, each

power stroke lasts just 1/400 of a second. To get an idea of just how short a time that is, sight through the shutter of an unloaded

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Maximum power is produced with a mixture just a bit richer than the chemically "correct" ratio. Power tapers off sharply with increas- ing richness beyond that point, and more slowly as the mixture is leaned-out.

camera set to that speed, and push the but- ton. Even though the combustion event involves extreme turbulence that violently

stirs and mixes the different molecules, it is

extremelyunlikelythateachandeveryoxy-

gen molecule will be able to find a hydro- carbon molecule to react with in such a brief

flicker of time. Yet for maximum power we want maximum heat, and the heat comes

from the combining of the hydrocarbon molecules in the fuel with the oxygen mole-

cules from the air.

An engine'scylindersare of a fixedsize, however,so the maximumquantityof air, andsothenumberofoxygenmolecules,that eachcylindercaninhaleis limited.Formax- imumpower,we wantto makesurethatall the oxygenmoleculesavailablein the fixed

amount of air inside the cylinder react with a hydrocarbon, and the way to do that is to provide some extra hydrocarbon molecules. And the way to do that, in turn, is to provide

a mixture that has a little excess fuel- a slightly rich mixture. As noted, however,

that extra gasoline is wasted; it also adds to air pollution. Unburned hydrocarbons, or

"HC," are another of the exhaust pollutants

that environmental laws seek to control.

On the other hand, if we are prepared to

sacrifice a little power, we can make maxi-

mum use of the quantity of fuel burned by providing a slightly lean mixture. In the

same way that a little surplus fuel ensures

that allthe oxygen gets used, a little extra air

helps ensure that every hydrocarbon mole- cule finds an oxygen molecule to mate with.

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A/F ratio

This can reduce, if not eliminate, HC emis- sions. Within limits, it also leads to lower

fuel consumption for a given power output.

Most fuel injection (PI) systems-and

most carburetors, for that matter- take these considerations into account in their design

and operation. During light load operation, such as occurs when cruising at a constant modest speed with a comparatively small throttle opening, the system leans the mix- ture out a bit, to enhance fuel economy and minimize HC pollution. When the driver opens the throttle wide, demanding full power, the system provides a richer mixture, at some cost to fuel economy and HC levels

in the exhaust.

There are other aspects to the rich-mix- tures-equal-maximum-power issue. First, when gasoline evaporates, it soaks up a lot of heat in the process, as you probably know from spilling gas on your hands in cold weather. The internal cooling effect of a slightly rich mixture reduces internal tem- peratures somewhat, especially in critical areas like the piston crowns and the edges of exhaust valves. While modem street engines are boringly reliable, the internal cooling provided by a surplus of fuel can make a

considerable difference to the survival of a

race engine that is running on the ragged edge of thermal self-destruction.

Also, the heat soaked up in the process of boiling that excess liquid gasoline into vapor can reduce the temperature of the air/fuel mixture entering the engine. As we have pointed out, cooler air is more dense than hotter, so a cylinder full of an intake mixture cooled in this way will weigh more (and thus contain more oxygen molecules) than otherwise. This accounts for some slight potential gain in power output.

Detonation

Another consideration relating to the con- nection between mixture strength and power is the issue of the tendency of a gasoline/air mixture to detonate. To explain, the burning

of fuel inside an engine cylinder is often characterized as an explosion, but although the combustion event is extremely rapid, it is not, technically, an explosion. Once initiated by the spark, the burning begins as a small bubble of flame around the plug electrodes. Under normal conditions, the burning process then spreads rapidly but smoothly throughout the rest of the mixture as an expanding ball of fire.

In some circumstances, however, the com-

bustion may start off smoothly enough, but as the flame-front expands through the com- bustion chamber, the rapidly rising tempera- ture and pressure ahead of it causes complex chemical changes in the unburned mixture furthest away, called the end-gas. Squeezed and heated by the approaching fireball, it changes from a predictable, slow burning mix into something far more unstable. As a result, the overheated end-gas ignites spon- taneously almost all at once, the pressure inside the cylinder rises sofast it is more like an explosion than a controlled bum, and the

resulting shock wave rings through the

motor. That is detonation, or "knocking."

The sharp pressure spike that results when this violent secondary event meets up with the original flame front can punch holes in

pistons. Even if it does not, the turbulence created by detonation scours against the sur- faces of the combustion chamber, allowing heat to flow out of the swirling gasses and into the surrounding metal much faster than normal. As a consequence, the gasses lose heat, their pressure accordingly falls, and power drops off immediately. (Although the peak pressure during detonation is much higher than during normal combustion, the average pressure is way down, because of this heat loss.)

Because the changes preceding detonation

are chemical, the ability of a particular blend of gasoline to resist detonation depends on the chemistry of the blend, and thus in turn on the various hydrocarbons that make it up. Overall, the knock-resistance of any sample

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Horsepower

of gasoline is expressed by its octane rating (see the sidebar "Doing Octane Numbers"),

but the number so obtained also depends to

some extent on the mixture strength. Some gasoline components knock worst when run rich; some others increase substantially in

knock-resistance with wholesale enrich- ment. Predictably, these latter are found in

abundance in race gas.

For typical pump gasoline in a typical engine, the mixture ratio for peak power is likely to be in the area of 12:1. Depending on the particular blend of gasoline, anything richer than that may exaggerate detonation problems, and the cooling effect of the sur- plus fuel, if carried to extremes, may sap

some of the heat that we would rather have working to raise the gas pressure. For better

mileage and lowest HC emissions, some- thing closer to 16:1 is wanted. Indeed, at comparatively high engine speeds under very light load, mixtures as lean as 18:1may offer even better fuel economy. Such lean mixtures burn hot, however, and that extra heat, together with all those extra oxygen molecules, makes it more likely that the sup- posedly inert nitrogen will combine with

some oxygen, worsening the NOx emis- SIons.

Optimizing an Engine's Diet

While the generalizations above are

broadlyapplicableto all engines,establish-

ingthecorrectair:fueldietfor anyparticular engineover a fullrangeof speedsandloads canonlybe achievedby a long andtedious

A mixture "fish hook," showing the points of maximum power and of maximum economy. Fuel flow is expressed in terms of the quantity used per hour per horsepower pro-

duced.

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A series of fish hooks established at various engine speeds produce, when linked together, a curved band that represents the practical range of mixture strengths for any given engine. The line forming the upper boundary of this band represents the mixture curve for max- imum power; the lower one for maximum economy.

process that involves dyno testing. The engine is run at some fixed throttle opening,

and the load is adjusted to keep the rpm con-

stant. Starting with a very rich air:fuel ratio, the mixture is adjusted leaner in small steps, and the fuel flow is measured at each setting in, say, pounds per hour. As the mixture is gradually leaned out, power initially

increases until some maximum is reached. Further leaning results in a reduction in

power but, initially at least, the quantity of fuel burned for each horsepower produced actually grows smaller.

This relationship between fuel consump-

tion and power production is described by

the expression brake specific fuel consump- tion, or BSFC. The first word, "brake," sim-

ply refers to the fact that the engine is being run under load on a dynamometer, or

"brake." (The original dynamometer was simply a friction brake; modem hydraulic and electric dynos differ in construction, but the principle of using a retarding device to

produce an artificialload remains the same.)

"Fuel consumption" is pretty obvious; it is simply the number of pounds per hour (lb/hr) of fuel being consumed at some par-

ticular throttle opening and speed.

"Specific" is actually a contraction of "power-specific," meaning that the fuel con-

sumption at any given setting is divided by the horsepower produced at that setting. The results are expressed in units of pounds of

fuel burned per horsepower per hour-

Ib/hp/hr. As a rough rule of thumb, we can

figure that an unsupercharged engine burn-

ing gasoline will have a BSFC of about 0.5

Ibs/hp/hr at peak power. That is, an engine

making 300 hp will need to bum about 150

Ibs. of fuel per hour to do it.

At some mixture strength, the engine will produce a maximum value of BSFC-a maximum amount of power from each pound of fuel, but this will not actually give

the maximum possible power.At somerich- er setting, the power will likely be some- what higher, but disproportionately more

fuel will have to be burned to achieve that slightly higher peak power.As the mixture is

leaned out past the point of peak BSFC, the power falls off markedly, as you might expect. What is particularly interesting is

that the amount of fuel burned, in relation to the power (the BSFC, in other words) actu-

ally increases-although engines can be made to run on such extremely lean mix- tures, it is actually wasteful of fuel to do so. The results of such a test are then plotted on a graph, with BSFC on the vertical axis and power output on the horizontal axis. The resulting curves resemble, and are called,

"fish hooks."

Once one such test has been completed,

the whole thing is repeated allover again at

some other engine speed, until the entire operating speed range has been covered in, say, 500rpm steps. Then the entire set of tests is repeated at different throttle open- ings. As we said, the operation is tedious.

Special Diets

One specialcircumstancethat requiresa

much richerthan stoichiometricmixtureis

cold starting.It maycomeas no surpriseto

learn that the various hydrocarbons that

makeup gasolinehavewidelydifferentboil- ing points and so evaporate at differing

rates. At very low temperatures, some of them may not evaporate at all, so the only way to ensure there is enough of the ones that do vaporize to make a burnable mixture in air is to provide a lot of gasoline overall. Typical cold start fuel-to-air ratios are

between 2-to-l and I-to-l.

Historically, there are two other situations that are (or were) thought to demand a rich mixture-idling and acceleration. Certainly idle enrichment is needed on typical carbu- reted engines, and to a lesser extent on those with throttle body injection (TBI) systems, but this is mainly a matter of the problems that arise from trying to distribute from a

central point all the air:fuel mixture needed

by a multi-cylinder engine. Proof that very

little enrichment at idle is necessary in prin- ciple comes from current emissions-certi- fied production engines, which get by with idle mixtures very close to stoichiometric.

The other situation conventionally thought to demand significant enrichment is acceler- ation. Every successful carburetor ever made had either an accelerator pump that shot in an extra squirt of fuel every time the throttle was opened, or (more rarely) some other means to temporarily richen the mix- ture under sudden throttle opening. It seems that much (although not all) of this "need," too, turns out to be due to secondary fac-

tors- in this case the nature of carbure- tors- rather than a characteristic of the

needs of engines themselves. In view of that, this seems as good a time as any to veer off

from our consideration of the kinds of food that engines prefer and to look at the various

methods for bringing that food to the table.

The Drawbacks of Central

Distribution

For satisfactory engine operation, whatev- er mixes the fuel and air has to closely match, in terms of air:fuel ratios, the various

feeding requirements of the engine under differing conditions, and must be able to move smoothly and continuously between

Ideally, all fuel ingested by the engine would be in the form of vapor. In practice, some droplets remain.

Squirting the fuel through a small orifice under considerable pressure improves atomization, which is one

reason why fuel injection is superior to carburetors. (Robert Bosch Corporation).

them as the situation requires. There is more to it, however, than just keeping the propor- tions right. Large blobs of fuel haphazardly distributed throughout the air just will not do, even if the overall proportions are cor-

rect.

To begin to understand the reasons for

this, imagine setting fire to a tablespoon measure full of gasoline. Yes,it willbum off fairly fast, but consider that an engine mak- ing 225hp goes through about that much gas every second. Allowing for the fact that each power stroke occupies at most half of acom- plete revolution of the crankshaft, and that it

takes two full revolutions for a complete engine cycle, the combustion event in the engine obviously occupies, at most, one quarter of that time. Youcannot bum a table-

spoon full of gasoline in one quarter of a second.

Vaporization-If you were to divide the

same amount of fuel into, say, three tea-

spoon measures and set them all alight simultaneously, then the gas will bum more quickly. If you further divide it into large droplets, it will bum quicker still. The more finely you divide the fuel, the more surface area each particle has in contact with the oxygen in the air, in relation to the volume of fuel within the droplet, so the faster the energy gets released. The ideal would be to divide the fuel into the smallest possible

units-individual molecules. In that case we will not see any liquid fuel at all; it will all

exist as a true vapor. In fact, we cannot usu- ally get quite that close to perfection, so the intake charge will consist of a mixture of air, gasoline vapor and fine droplets. One of the inherent advantages of fuel injection over

carburetors is that the fuel is introduced into

the intake air under comparatively high pressure. In the same way that a shower head produces a fine spray when the taps are cranked all the way open, but gives forth

large drops when the taps are nearly closed, the pressurized mist issuing from a fuel injector helps this process of vaporization.

. Intake PortAirflow-But thereismoreto

it than that. Consider one cylinder of a 320 cubic inch (ci) V-8, turning 6000 rpm. That individual cylinder displaces 40 ci and so inhales that much air every second engine revolution (again, assuming it is a four-cycle engine), for a total of 120,000 ci of air per minute. That air flows through the intake ports in the cylinder head and intake mani- fold which may have a cross-sectional area

of somewhere around 3 square inches (sq in). The average rate of flow through that hole is simply the volume divided by the area of the hole it flows through, so the speed of flow is:

120,000/3 = 40,000 inches per second, or

about37.8mph. .

Now 38 mph doesn't sound like a particu-

larly high speed, but the air on its way to the

cylinder usually has to negotiate some turns, and those turns can be mighty sharp- maybe something like 3" radius. If you both- er to do the arithmetic,you will discover that the airflow negotiating a turn with a radius of 3" experiences an acceleration equivalent to 382 times the force of gravity (382 g).

Now, if all that is flowing through the intake ports is air and fuel in vapor form, those 382 g turns won't bother the gasses at all. But with any arrangement that mixes the fuel and air at a central location, those sharp, high-speed turns really disturb the move- ment of any fuel droplets that are mixed in with those gasses. What will happen, in fact, is that they will get centrifuged to the out-

side of the bend and form puddles of liquid on the inside surfaces of the ports.

At first it may seem that this does not mat-

ter much; the fuel will get carried along by

the air rushing past and will eventually make

its way into the cylinder and the correct fuel- to-air ratio will be maintained, at least on

average. But "on average" is not good enough; the mixture strength in the cylinder

will vary from moment-to-moment, accord- ing to the whims of the puddles. Of course,

if there is only one cylinder, there is less need for bends in the intake plumbing, but things turn really ugly when we are dealing with more than one cylinder eating from the same trough, so to speak.

When multiple cylinders are fed from one

common source, as is the case with TBI and carburetor induction systems, there must inevitably be bends, and probably lots of them. Unavoidably, this fuel-drop-out effect will favor some cylinders and short-change others. In the days before concern about emissions (which means in the days of car- buretors), a variation of four numbers in mixture strength between cylinders in the

same engine was not uncommon-some cylinders might be working on 16:1, others on 12:1.To keep the engine lit at idle, it was necessary to provide a surplus of fuel over- all in order to ensure that the leanest running cylinder got a burnable mixture. With painstaking development of the manifold design, it is possible to reduce this cylinder- to-cylinder variation, and modem manifold designs for engines fitted with carburetors or TBI systems do much better than in the bad old days. Still, the desirability of ensuring as nearly complete vaporization as possible

should be obvious.

At idle speeds, the speed of the flow of gasses through the ports is obviously vastly reduced, so the tendency of droplets to sep- arate out from the gas flow because of cen- trifugal forces will be dramatically decreased. At the same time, the high vacu- um condition existing in the intake manifold of an idling engine encourages fuel droplets to vaporize. In the same way that water boils at a lower temperature (that is, evaporates more readily) on a mountaintop than it does at sea level, gasoline evaporates more readi- ly in thin air than when it is more dense. Ironically, the problem of incomplete atom-

ization remains- at least for carbureted engines-simply because the lower rate of

Single Point Fuel Injection (TBI)

!.Fuel

2.Air

3.ThrollieValve

4.IntakeManifold

5.Injector

6.Engine

===>

{J/0

---

~13

Introducing fuel at a single point-whether from a carburetor or a throttle body injector-"wets" a large area of manifold surface, and gives unevaporated droplets many chances to drop out, forming puddles of

fuel. (Robert Bosch Corporation)

flow meansreducedturbulencewhichmight

otherwise break large droplets up into small- er ones.

Quite apart from the matter of the rate of

flow, there are other factors at work that oblige engines with a "central mixer"-and

especially those fitted with a carburetor-to operate with rich air:fuel ratios at idle. Two of these are charge dilution and reversion,

terms we shall soon define.

Valve Overlap-To explain, in the kind of idealized engine that appears in beginner- level explanations of how piston engines work, the intake valve opens at top dead

center (TDC) of the intake stroke and closes at bottom dead center (BDC). Following the

compression and power strokes, the exhaust valve opens at BDC, and closes again at

TDC. Yet surely anyone reading this book knows that on all real world engines the cam

timing is arranged to open the valves earlier and close them later than this.

The reason for this is the inertia of the

intake and exhaust gasses. Although gasses are very light, they are not weightless-a cubic foot of air, for example, weighs about

0.08 lbs. Thus, as the valves open and close, and the columns of gas passing through the

ports on their way to and from the combus- tion chamber start and stop, the inertia of those gas columns makes them lag behind piston movement. Togive sufficient time for emptying and filling the cylinder, especially at high engine speeds, the intake valve

opens before TDC and closes after BDC, while the exhaust valve opens before BDC

of the power stroke and does not close until after TDC on the exhaust stroke.As a result, there is a period toward the end of the

exhaust stroke when the intake and exhaust

Multi-Point Fuel Injection (MFI)

!.Fuel

2. Air

3.ThrottleValve

4. Intake Manifold

5. Injectors

6. Engine

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Multi-point injection ensures equal mixture strength at each cylinder. (Robert Bosch Corporation)

valves are both open together.

This valve overlap helps an engine to pro-

duce useful power at high speeds, as the

"lead"in the valve timing gets to be in synch with the "lag" in the movement of the gasses. But it also leads to problems at very low speeds, because there is then obviously an opportunity both for some of the fresh intake charge to zip right out the exhaust

and/or for some of the exhaust gasses to make their way backward, upstream, into

the intake manifold.

Charge Dilution-This intermingling of exhaust gasses with the fresh intake mixture that occurs at idle because of valve overlap is termed charge dilution, and for years it was argued that this demanded a rich idle mixture, because the dilution tends to keep the hydrocarbon and oxygen molecules sep-

arate.A hydrocarbon molecule atone side of the combustion chamber, it was argued, might be desperately seeking an oxygen molecule at the other side but they would be unable to meet, because of the crowd of

exhaust gas molecules in between.

While the above argument appears plausi- ble, it is apparently at least partly wrong, as confirmed by the near stoichiometric idle mixtures of modem fuel injected engines.

The difference between these motors and the typical rich-idle engines of a few years ago

seems mainly to be the difference between the degree of variation in mixture strength between one cylinder and another that exist

when the mixture for all an engine's cylin-

ders is delivered at one central point, such as

in a carbureted or TBI engine, compared to

one with multi-point fuel injection, in which

each injector is located near the one individ- ual cylinder that it serves.

TBI engines, at least, are not afflicted by

another complication that affects carbureted engines at idle, that of charge reversion. The violent pulsations that occur in the intake ports and manifold of an engine at idle speed means that a portion of the flow on its way to the cylinders sometimes actually reverses

direction- some of the intake air, already

mixed with fuel, actually pops back outside briefly! This can sometimes be seen as a

pulsating cloud of air:fuel mist hovering

over the intake. Apart from being a potential fire hazard, a consequence of this in-out-in- again motion is that some air passes through

the carburetor three times. Because a carbu- retor adds fuel to gas flowing through it, and

neither knows nor cares which direction the flow is going, fuel gets added on every trip.

On the face of it, it should be possible to adjust the basic idle mixture setting to take account of this triple-dosing. Indeed, a cor- rectly adjusted carburetor will do so, but the reversion phenomenon is unpredictable, so

the mixture has to be set somewhat rich to take account of the instants when it is inef- fective.

Another area where fuel injectiori- at least multi-point Fl, with each injector locat- ed very near the intake valve it serves-has an inherent advantage over carburetors is the ability of Fl systems to cope with sudden increases in engine speed and load. There is certainly a need to shift from the leaner- than-stoichiometric, best-BSFC air:fuel ratio, to the richer-than-stoichiometric best-

power ratio ifthe throttle is suddenly opened while cruising along at light load, but why

should the actual fact of accelerationitself that the engine speed is increasing- intro-

duce any greater need for enrichment at any instant during the acceleration than does any

other situation demanding that same power output at the same rpm?

The answer to this is that, as previously

noted, some of the mixture supplied by any

"central mixer" will wind up as liquid on the walls and floors of the ports and manifold. This fuel will eventually flow to the cylin- ders, but note that the density and viscosity of this liquid fuel is much greater than that of the air, gasoline vapor, and very tiny droplets that make up the remainder of the mixture, so its motion is much slower than

those gaseous and near gaseous compo- nents.

Under steady state conditions, this "pool" of fuel is being consumed and replaced more or less equally all the time, so under those conditions there will always be some more-or-less constant quantity of liquid in the manifold at any moment. That quantity, however, will vary according to just what steady state conditions prevail. Under large throttle opening/high load conditions, the density of the air inthe manifold and ports is higher than otherwise and, as noted above, gasoline evaporates less readily in air that is

dense than when it is thinner.

An engine operating under a large load will thus require a larger quantity of liquid

fuel in the manifold in order to maintain equilibrium between what gets consumed

and what gets supplied. On sudden throttle opening, additional fuel thus has to be sup-

plied to rapidly build this pool up to the larg-

er size of "store" needed to maintain equi-

librium under the new, higher load condi- tions.

It is noteworthy, too, that to encourage

vaporization under these and other condi- tions, carbureted engines for street use are almost always provided with some means to heat the mixture in the manifold, whether by the heat of the exhaust gasses (the familiar

"heat-riser") or of the engine coolant. While

this reduces the problems of having compar- atively large amounts of liquid fuel washing about in the manifold, the heating of the entire intake charge lowers its density. As we have already seen, hot air is less dense

than cooler air, so a cylinder full of such a

warm mixture will contain fewer oxygen

Mixtureformationintermittentinjectionontotheengineintakevalve.

Multi-point injection also provides the opportunity to locate each injector very near the valve it serves. This reduces port "wetting," and so reduces the amount of enrichment needed for acceleration. (Robert Bosch Corporation)

molecules- and thus can make less

power-

cool mixture. The vaporization problem is most severe when the engine is cold, so many engines provided with such a "hot-

spot" have some means to turn down the heat once the engine is fully warmed up. Usually,however, the heat does not getcom- pletely turned off.

Tosome extent, this problem of wetting of

manifold and port walls also affects multi- point fuel injection systems, to a degree that

depends on where the injector is located, rel-

ative to the intake valve. On some engines,

the injector is as close to the back of the

intake valve as possible. Here, clearly, the

length of port that is wetted by fuel is

extremely short, so the additional amount of

fuel needed to fatten up the "store" is mini-

mal. However, on other engines

for reasons of packaging, or access for ser-

thanthesamecylinderfilledwitha

- perhaps

vicing- the manufacturer decides to locate

the injectors a considerable distance from

the valves. The more remote the injectors,

and so the greater the length of the wetted

port and manifold, the more the additional

fuel requirements on acceleration resemble those of a central mixer arrangement, such

as a carburetor.

Carburetors have an accelerator pump to

tide them over this transition, but the amount of additional fuel supplied by the accelerator

pump of a carburetor considerably exceeds the amount that should be needed to main-

tainthe "pool."Toseewhythis is so,andto

better understand some of the other short-

comingsof carburetors,we shouldend this

chapter by taking abrief look at their nature, before moving on to consider the fuel injec- tion systems that have now almost com- pletely displaced them.

The Carburetor

The earliest attempts to feed a gasoline engine hinged on wacky arrangements like a drip-feed of fuel into the air intake pipe, or an arrangement of cotton wicks with their

bottom ends immersed in a pool of fuel and

their upper ends exposed in the intake pipe. These hit-and-miss methods at controlling mixture strength came to an end soon after the invention of a basic carburetor, in 1863,

VENTURI

DISCHARGE

TUBE

A rudimentary carburetor. The essential features are a

"pool" of gasoline main- tained at a constant height by a float-operated valve; a restriction (the main jet) to regulate the flow; a venturi, with a discharge tube con- necting the low pressure

area in the venturi to the fuel supply; and a throttle plate, to control the total amount of mixture flowing

through. There is also a cal- ibrated air bleed, that

meters extra air into the

fuel before the discharge tube, usually through a per- forated "emulsion tube," that helps to atomize the fuel.

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attributed to a Frenchman by the name of Lenoir. By 1894, the German engineer Wilhelm Maybach had advanced the devel- opment of this device by including the now- familiar float-and-needle-valve arrange- ment, to maintain a reliable supply of fuel at a constant height. Arguably, no small part of the success of Karl Benz's "Patentwagen" of

1897 was attributable to its wearing one of

Maybach's float-equipped carburetors.

The adjacent illustration shows the essen- tial workings of a simple carburetor. Fuel is supplied by a pump to the float chamber (or bowl), where a float and needle-valve assembly maintains the contents at a fixed level, very much like the float and valve arrangement in your toilet tank. The float

chamber connects to a smaller reservoir- the main well- so the fuel level in the well

will be at the same height as in the float chamber. A pipe, usually termed the dis-

FLOAT CHAMBER

-----------

MAIN

WELL

charge tube, leads slightly uphill from the well to the carburetor's throat-the main air

passage through it. With the engine stopped,

at least, the level of the fuel will reach part way up the discharge tube, close to- but not quite-spilling out the end. At the point where the discharge tube enters it, the car- buretor throat is fitted with a narrowing piece, called the venturi. Note, too, that both

the well and the float chamber are open to the atmosphere at the top.

As the engine draws air through the carb throat, the restriction created by the venturi obliges the air to speed up. It is no coinci-

dence that the venturi is named after an Italian physicist of that name; it was he,

G.B. Venturi, who established, a couple of

hundred years ago, that when a fluid passing through a pipe or channel is forced to speed up, its pressure drops. As a result of this phe-

nomenon,the pressurewithinthe venturiis

DOING OCTANE NUMBERS I

"Octane" is a measure of a fuel's detonation resistance, compared to two reference fuels, both of them components found in pump gasoline. One, a particular hydrocarbon called iso-octane, strongly resists knocking. It defines one end of the scale-lOa octane. The other reference hydrocarbon, n-heptane, detonates like crazy, so it defines the zero end of the scale. Mix the two together and you get a tendency to knock that varies from zero to 100 according to the proportions of the mix. A fuel under test that behaves like a 90:10 mix of iso-octane and n-heptane, then, would be rated at 90 octane.

The actual measurement is performed using a standardized single cylinder test engine which is specially designed to allow its compression ratio to be varied while the engine is running. During the test, the compression is raised until the enginejust starts to knock. From knowing exactly what blend of iso-octane and n-heptane would also just barely knock at that CR, it is possible to establish the octane number.

Just to keep things baffling, though, there are two octane numbers, "Research" and

"Motor," established under different test conditions. From the chart below it appears that the Motor method provides a more severe test. In fact, the Research method tallies clos- er to real world experience in ordinary cars on the road. Toadd to the confusion, the num- ber printed on a gas pump is the average of the Research and Motor numbers.

TEST CONDITIONS FOR RESEARCH

AND MOTOR METHOD OCTANE RATINGS

Research Method

(RON)

Inlet air temp. 1

Coolant temp.

Engine speed

Spark advance

Air/fuel ratio

25.6 212

600 rpm

13 degrees BTC

Adjusted for maximum knock

lower than that of the surrounding atmos-

phere, which thus pushes the fuel out of the

discharge tube and into the carb throat.

The rush of air past the end of the dis-

charge tube tends to tear the fuel into droplets, thus going some way to achieving atomization. An air bleed at the top of the

main well allows a certain amount of air to leak into the flow of fuel, via a perforated

sleeve called the emulsion tube, before the fuel reaches the discharge tube, thus assist- ing atomization. This is, in fact, almost a

Motor Method

(MON)

300.2 212

900 rpm

19-26 degrees BTC

(varies with CR)

Adjusted for maximum knock

secondary function of the air bleed. Its pri- mary reason for existence is to deal with an odd phenomenon that would otherwise

cause the mixture to become ever richer as the rate of airflow through the venturi

speeds up, as it does with any increase of engine speed.

Because the pressure drop within the ven-

turi is a function of the rate of flow through it, it might seem that the fuel flow would increase in proportion, maintaining a con-

stant air/fuel ratio over a wide range of flow

rates. In fact, the ever-increasing pressure

drop at the venturi as the airflow increases means that the air within it becomes increas-

ingly rarified. Meanwhile, the fuel flow,pro-

pelled by the pressure difference between

the venturi and the atmosphere, increases as

the volume of air increases, but the density

of the air is dropping at the same time. The

result is that the mixture grows ever richer.

A carburetor venturi, itturns out, is a volume measuring device, rather than one that mea- sures mass flow. The air bleed provides a corresponding "leak" of air that compen- sates for that phenomenon (indeed, it is sometimes called a compensating circuit.)

A carburetor as simple as the one just

described would in fact work, and would supply the engine to which it was fitted with a reasonably constant mixture strength over a reasonable range of speeds and loads, but only if those speeds are constant and moder- ately high. Under two sets of conditions, it

would fall flat on its face.

At very low flow rates, such as at idle, the

pressure drop through the venturi is so low that the fuel cannot make it out of the end of

the discharge tube. In these circumstances the carburetor cannot function at all. To deal

with this, most real world carburetors have a separate idle circuit, in fact a miniature

carburetor-within-a-carburetor, that com-

pletely bypasses the main throat and ven- turi. But we needn't bother ourselves with

the details of these.

The other situation that hobbles our simple carburetor is changing from one speed to another. (Actually, slowing down probably would not present much of a problem;

speeding up, however, would.) We have explained that air weighs about 0.08 lbs. per cubic foot. While not weightless, this is vast- ly less dense than gasoline, at about 43 lbs. per cubic foot. Thus, if an engine should suddenly begin to draw much more air through the venturi, the airflow will speed up almost instantly, but the fuel, because of its greater inertia, will lag behind. As a result, the mixture will lean out, to the point where the engine will, in all probability, quit.

To deal with this, most carburetors are equipped with an accelerator pump, that mechanically squirts an extra shot of fuel

into the venturi whenever the throttle is sud- denly opened. Note, again, that this tenden-

cy to run lean on sudden throttle opening is a peculiarity of carburetors. The accelerator pump exists as much to make up for this deficiency as it does to build up the larger quantity of liquid fuel "store" in the mani-

fold, as described above.

espite the fact that there were a thousand and one other problems

of the automobile, auto pioneers were quick to focus attention on the shortcomings of

carburetors. The drawbacks included the

possibility of ice forming within the venturi

in cold, damp air, and a tendency to vapor lock in hot weather when volatile fuels were

used. Conversely, carburetors were unsuit- able for use with fuels that did not readily vaporize (remember, the automobile came first- gasoline followed, almost anything

that would bum was used as a fuel at one time or another in the earliest days). Also,

the venturi restricts airflow, and thus power. Because fuel delivery depends on the level in the float bowl, a carburetor is sensitive to the inclination of the car, such as when climbing hills. In addition, there was some dawning awareness of the problems of feed- ing multiple cylinders from a single "central mixer," as described in the previous chapter. Accordingly, some of the critics set about designing an alternative, specifically what we now call fuel injection.

to be dealt with in the early days

Early Production FI Systems

Although there were even earlier experi- ments, perhaps the first production applica- tion of PI was by the German company, Deutz, who built about 300 fuel-injected sta- tionary engines around the turn of the centu- ry. Hampered by a lack of precision manu- facturing facilities, Deutz abandoned the

scheme soon after. (Deutz is still in the busi- ness of manufacturing industrial and marine diesel engines.) Perhaps aware of Deutz's work, Robert Bosch began experiments with fuel injection in 1912. In 1927, Bosch bought the Acro company, and the patent rights of an Acro employee, Franz Lang,

who had successfully developed the equip- ment needed for high-pressure fuel injec- tion. Shortly afterward, Bosch began manu- facturing diesel injection equipment of a pattern that has changed little in the 70-odd years since (see sidebar "TheReal Jerk").

The adaptation of this system to gasoline- fuelled engines began in the 1930s, initially for use in aircraft. In this application, three advantages of fuel injection over carburetors stand out: freedom from problems of vapor lock, no risk of icing, and a complete indif- ference to the attitude of the vehicle. Vapor lock is a significant problem at high alti- tudes, because the reduced atmospheric pressure there makes it so easy for the fuel to evaporate that it is inclined to boil, form- ing bubbles of vapor in all sorts of awkward places that can effectively prevent any fuel from getting to the engine. Carburetor icing, while merely an annoyance in an automo- bile, can be potentially fatal in an aircraft. And a fuel system that can keep the engine running no matter what the attitude of the

aircraft becomes a matter of first importance during extreme maneuvers, such as inverted flight. Most, if not all, Daimler Benz engines fitted to German aircraft during World War II enjoyed this advantage over their Allied opponents.

Early Racing FI Systems

If we substitute severe cornering for

The straight eight engine of Mercedes Benz's first post-

war FormulaOne race car- this streamlined W196-

injected fuel directly into each cylinder, a system developed from Diesel injec- tion principles. (Mercedes

Benz).

THE REAL JERK

In a diesel, the cylinder at the end of the compression stroke is filled with nothing but air; combustion cannot begin until the fuel is injected. Thus, the moment of introduction of the fuel is equivalent to the ignition timing on a spark-ignition gasoline engine. For that reason, control is needed over not just the quantity of fuel injected, but

alsothe timingof thatinjection. .

Because the fuel has to be kept separate from the air until the correct instant for ignition, the fuel is injected direct- ly into the combustion chamber- the injector nozzle lies inside the cylinder. As a result of that, the injector is exposed to full cylinder pressure during the power stroke. To prevent combustion pressure from blowing the fuel

backward through the fuel lines, the injector is fitted with a very stiff spring-loaded check valve. This check valve

also performs a number of other important functions. It prevents fuel from dribbling out of the injectors between

timed squirts, and ensures that the start and end of each injection is clearly defined. Without this valve, the fuel

spray would taper off gradually toward the end of each injection. The check valve, in conjunction with similar check valves at the pump outlets, also ensures that the fuel lines leading from the injection pump remain filled with

fuel at full pressure, to ensure that the pressure pulse from the pump is transmitted immediately to the injector noz- zle. To overcome the resistance of these check valves, the entire system has to operate at a very high pressure-as much as 3000psi!

The design of the classic diesel injection pump, and the gasoline injection pumps directly derived from that design, follows from the three requirements: to control the quantity of the fuel delivery,and its timing, and to pro- vide a very high pressure output. In construction, it consists of an approximately rectangular body, bored with a number of small cylinders-one for each engine cylinder-each of which is lined with a cylindrical sleeve. The whole thing thus roughly resembles a miniature engine block, a similarity that extends to the "bottom end" of the pump, which has a shaft running lengthwise through it, analogous to an engine's crankshaft. Instead of crank throws, however, this shaft has a number of individual carns, one for each cylinder.Each cylinder in the pump con- tains a piston-likeplunger that is driven up its sleeve by the correspondingcam lobe asthe pump shaft rotates,driven

by the engine at half crank speed, like an ordinary ignition distributor.

The mechanical drive thus provides the timing and the necessary pressure, leaving the problem of control of fuel quantity.To achieve this, each sleeve has a "spill"port drilled through its side,while the upper edge of each plunger has its side cut away to form a sort of spiral ramp. The sleeve fits in its bore with a very slight clearance, and so is free to rotate. Rotation of the sleeve in its bore thus covers or exposes the spill port, according to the angular posi- tion of the sleeve relative to the spiral cutaway on the plunger.With the sleeve rotated so that the spill port is cov- ered even when the plunger is at the bottom of its stroke, the only route out of the cylinder is through a hole at the

top, where the fuel lines to the injectors connect. A single stroke of the piston will thus expel the entire volume of fuel held in the cylinder; this represents the maximum capacity of the pump and thus corresponds to full power.

At anything less than wide-open throttle, a lesser amount of fuel per revolution is obviously needed, and this

reduction is achieved by rotating the sleeve around, relative to the plunger, so that the spill port lies some distance

above the plunger's spirally formed top edge. Upward movement of the plunger thus initially causes fuel to be dis- charged from this port, from where it is fed back to the tank, until the plunger rises far enough to close off the port,

and injection commences.

The rotation of the sleeve(s) is accomplished by a toothed rack that meshes with gear teeth cut onto the outside of each sleeve. The rack is connected directly to the throttle linkage, so moving the pedal rotates the sleeves with-

in the pump, thus controlling the quantity of fuel delivered with each stroke of the plunger.

Although such a mechanical injection pump is, in fact, quite a simple device, and contains few parts-just two

per engine cylinder served, plus the camshaft and rack- the quantity of fuel delivered per stroke is very small, so

all those parts have to be made with extreme precision, involving much hardening, grinding and precision gaug-

ing. Accordingly, mechanical injection pumps-known widely and for fairly obvious reasons as "jerk"pumps- are mighty expensive. When adapted for use with gasoline, rather than the kerosene used in diesels, there is the additional problem that gasoline is a "dry" fuel, lacking the lubricating properties of diesel fuel, thus demanding

the use of extremely hard, wear-resistant alloys, which further adds to the machining difficulty and expense.

The famous Mercedes 300SL "gull-wing" coupe introduced fuel injection to the street. Like the W1.96, a system of direct injection was used. (Mercedes Benz).

inverted flight, most of the advantages listed

are also highly applicable to race cars. Accordingly, by 1937 Mercedes Benz had tested a single cylinder mockup for a racing engine, using high-pressure injection of fuel

directly into the combustion chamber.In the

meantime, a handful of European makers of

small, two-stroke engined passenger cars

had introduced this "direct"fuel injection, in

an attempt to tame the notorious thirst of

two-strokes that results from something like

one quarter of the air/fuel mixture whistling

straight out the exhaust ports during the

intake/exhaust event, when both inlet

("transfer") and exhaust ports are open

together. Because these engines were two- strokes, the injection pump had to run at crank speed, rather than half-speed, as for a four-stroke. This provided Bosch with valu- able lessons in running their injection equip- ment at high pump rpm. Mercedes's first post-war Formula One race engine made direct use of the experience so gained, both

their own and that of Bosch.

In both the M196 Formula One engine

that appeared in 1952 and in the engine for the famous 300SL "gull-wing" sports car, first exhibited in New York in 1954, Mercedes retained the direct (into the com-

bustion chamber) injection scheme used in

.L~

The location of the injector-screwed right into the cylinder head-is apparent in this cross-section of the 300SL engine. (Daimler-

Chrysler Archive)

diesels and in the earlier experiments with gasoline fuel. Combined with suitable injec-

tion timing, this allowed radical valve tim- ing and experiments with "tuned" intake pipes, without introducing the problem of poor fuel economy from portions of the intake charge being lost out the exhaust. Until injection occurred, after the exhaust valve had closed, the engine was inhaling only air.

Diesel Jerk Pump

Giventheexistenceof anestablishedtech-

nologyforinjectingfuel,itishardlysurpris-

ing that the diesel "jerk" pump was adapted in this way for gasoline injection. The adap- tation, however, required hurdling a major difficulty- the problem of regulating the quantity of fuel delivered throughout the full

range of engine operating speeds and loads.

Unlike gasoline engines where power is controlled by closing off ("throttling") the air supply,yet where a very narrow range of

air/fuel ratios must be maintained, on a

diesel there is no "throttle" as such. At any given speed, the engine inhales the same full load of fresh air no matter what the position of the gas pedal; varying the power is sim-

The resemblance to Diesel engine practice is clear in this exterior shot of the 300SL engine. Gasoline's lower lubricity, compared to diesel fuel, compelled the use of expensive, wear-resistant alloys in the injection pump. (Daimler-Chrysler Archive)

ply a mater of injecting more or less fuel.

Power is regulated, in other words, by adjusting the mixture strength. In the sim- pler diesel systems, at least, this is directly controlled by the driver's right foot-the engine runs extremely lean at light throttle, and rich to the point that it visibly smokes when maximum power is demanded. In fact, it is the smoky exhaust that sets the limit for the rated power of a diesel engine; more power would be available simply by inject- ing yet more fuel- if we were prepared to put up with the smoke. (Even at full power, the amount of fuel injected falls short of a

stoichiometric mixture-diesels are always

running "lean," which is one reason they can

never produce as much power as a gasoline engine of the same displacement.)

The inefficient breathing of any piston engine, gas or diesel, at speeds well away from the rpm at which torque peaks means that the mass of air inhaled per revolution, at any given throttle setting, will most definite- ly not be constant across the speed range. Because of the gasoline engine's finicky appetite, the amount of fuel mixed in with

that air also has to be varied on the basis of the quantity of air inhaled, and not just

6 5

7 8

11 12

13 15 16

2 12.Controlrockhead

3. Enrichmentsolenoid

4.Thermostat

1 5.Barometriccell

6. Checkvalve

7. Plungerunit

8.Toothedsegment

9. Controlrack

10.Rollertappet

11.Camshaft

12.Governorcontrollever

13.Contouredcam

14.Centrifugalgovernor

15.Idleadjustingscrew

16.Shut-offsolenoid

d1.Sensoroncontoure cam

The complexity of the mechanical controls for the Bosch system used on the 300SL is mind-boggling. Fuel delivery of the pump (only the camshaft is shown, 1.1.)is controlled by the rack (9). The position of the rack is governed by engine speed, via the centrifugal weights (1.4) acting on a contoured cam (1.3), and by throttle position. Other factors are accounted for by a barometric capsule (5) and a tem-

perature sensitive device (4). (Robert Bosch Corporation)

according to speed and throttle position.

Adapting to Gas Engines-To adapt a dieseljerk pump to gasoline operation, then, means that the rack (see sidebar, page 20) cannot simply be directly hooked up to the loud pedal; the relationship between the two has to be modulated by some other con- troles). The usual way this is done is by adding a set of centrifugal weights, some- what like those in a traditional ignition dis- tributor, to provide a signal proportional to engine speed, plus a diaphragm that "reads"

manifold vacuum, a fairly close approxima- tion of engine load. These additional control devices are connected to the linkage control- ling the rack via a system of cams and links, with the shape of the cams tailored accord- ing to the idiosyncrasies of the engine's breathing.

The monkey motion of these additional

levels of control added even more to the substantial cost of the pump itself, demand-

ed meticulous adjustment, and its complexi- ty mocked the essential simplicity of the

+ 112 hidden pages

Tatra fuel injection Service Manual

CONTENTS

Stoichiometric Mixture

Detonation

Optimizing an Engine's Diet

Special Diets

The Carburetor

Early Production FI Systems

Early Racing FI Systems

THE REAL JERK

Diesel Jerk Pump