The Effect of Higher Compression Ratio in Two-Stroke Engines
Yuh Motoyama and Tohru Gotoh
Yamaha Motor Co, Ltd.
The effect of higher compression ratio on fuel consumption and power output
was investigated for an air-cooled two-stroke motorcycle engine. The results
show that actual fuel consumption can improve by 1-3% for each unit increase
of compression ratio over the compression ratio range of
of improvement is smaller however as compared to theoretical values. The
discrepancies are mainly due to increased mechanical and cooling losses, shortcircuiting at low loads, and increased time losses at heavy loads. Power output
also improves, but the maximum compression ratio is limited due to knock and
the increase in thermal load. In addition, the investigation covered the
implementation of higher compression ratio in practical engines by retarding the
full-load ignition timing.
6.6
to 13.6. The rate
INTRODUCTION
Adopting a higher compression ratio is
one of 'the most important considerations
regarding improved fuel consumption and
power output in gasoline engines. Much
research has been devoted to the effect of
higher compression ratio in four-stroke
engines, but little attention has been given to
two-stroke engines. A two-stroke engine is
different from a four-stroke engine in the gas
exchange process and in the composition,
pressure, and temperature characteristics of
the working gases. Accordingly, the
compression ratio has a different effect on
the thermal efficiency for the respective
engine types. To address the shortfall of
study on two-stroke motorcycle engines, the
effect of higher compression ratio on fuel
consumption and power output was
investigated. This paper specifically
discusses thermal loads as well as knock
problems at high speeds and heavy loads for
higher compression ratios.
FUEL CONSUMPTION IMPROVEMENT
WITH HIGHER COMPRESSION RATIO
The experiment was conducted on an
air-cooled, Schnurle crankcase scavenged,
single-cylinder, two-stroke engine. The
engine specifications are detailed in Table 1.
Compression ratios ranging from
in seven steps were attained by varying the
combustion chamber dome depth. The
compression ratio referred to in this paper
represents the effective compression ratio
calculated by assuming the exhaust portclose timing volume is the volume before
compression.
To ascertain the scavenging
characteristics, delivery ratio and air
6.6
to 13.6
fuel
Table 1 Test Engine Specifications
Engine Type
Scavenging System 1 Crankcase Scavenging
and
*
Stroke
-
Bore
Displacement
Compression Ratio
Exhaust Timing
Scavenaina Timina
Combustion
Chamber
2-Stroke, Air-Cooled,
Sinale-Cvlinder
,
154 x 50
1114.5
16.6
9.5,
91'
122-
(
Two-Staged SemiS~herical
i-
mm
cm'
(STD),
7.8,
11.2, 13.6
A&
BTDC
A
&
B
Tv~e
8.4,
TDC
8.7,
ratio are calculated from the intake air
capacity. Trapping efficiency is calculated
0,
from the exhaust
Unleaded regular gasoline (RON
concentration(1)'.
91)
is used
as the fuel.
In the LA-4 mode, a motorcycle
equipped with an experimental engine is
operated mostly at idle and the load range is
less than
3kW
(350kPa
BMEP)
within 40005000 rpm.(2) Consequently, the test engine
speed was set to 4500rpm to investigate
engine characteristics. Figure
1
shows the
specific fuel consumption results at various
compression ratios. Figure
2
is the
improvement rate for fuel consumption at a
standard. compression ratio of
6.6.
The effect
of increasing compression ratio can be seen
in the results whereby the fuel consumption
improves
by
1-3% for each unit increase in
compression ratio. At any particular
compression ratio and load, however, the
measured values for the improvement rate are
half as much as the theoretical values (air
cycle
y
=
1.40). This is considered to be as
a result of the following:
(i)
Increased short-circuiting
(ii) Reduced specific heat ratio of
the working gases
(iii) Increased mechanical loss
(iv) Increased cooling loss
(v) Increased time loss
The above factors are discussed in more
detail below.
Fig. 1 Fuel Consumption Characteristics
at Practical Engine Speeds
AIR
CYCLE
k,
=
6500
MBT
6
rpm
A,F 15
/
/
a
10
COMPRESSION
/
/
RATIO
/
it
1.41
/
/
'
BMEP
IkPal
IA
Fig. 2 Improvement Rate in Fuel
Consumption at Various Loads
INCREASED SHORT-CIRCUITING
-
If the scavenging characteristics vary with
compression ratio, fuel consumption will
be
affected with the variation of closed-cycle
as
thermal efficiency as well
scavenging characteristics. Figure
the variation of
3
shows
the relationship between the delivery ratio
and the trapping efficiency. The trapping
efficiency is reduced as the compression ratio
rises within the tested delivery ratio range.
More specifically, for the same fuel supply,
increased short-circuiting is induced.
'Numbers in parentheses designate references
at the end of paper
45W
rom
r
.
.
-
-
. - .
,
6
4500rgm*ff15
------
EOUNALENT TRAPPlNG EFF.
TO
-
STANDARD COMP. RATIO
ACWAL
/
./
AIR CYCLE
1
7'1.4
-
BMEP
kP.1
. .
DELIVERY RAT10
Fig. 3 Relationship between the Delivery
Ratio and the Trapping Efficiency
rom
4500
0.85 MET AlF13
>
g
0.80
W
y
W
Fig. 4 Relationship between the Trapping
Figure 4 shows the relationship between
the trapping efficiency and BMEP. The
trapping efficiency improves as the
compression ratio increases beyond a BMEP
of 360kPa. Figure
in fuel consumption for a trapping efficiency
equivalent to the standard compression ratio
(
and assumes the trapping efficiency is
independent of the compression ratio). At
400kPa BMEP, the results show a increase in
the improvement rate with reduced shortcircuiting due to the higher compression
ratios. At 200 and 300kPa BMEP, under
actual operating conditions, however, the
improvement rate in fuel consumption
decreases with the increase in short-
circuiting.
200kPa BMEP, for example, the
improvement rate would be 1.4% higher (an
increase from
increase in short-circuiting.
p,.a..+.-.
0
75
BMEP IkPa)
Efficiency and
At a compression ratio of 12,
9.8%
BMEP
5
shows the improvement
to 11.2%) without an
n
--.-a
----.
--.-0
...--..-
-
'I
C
6.6
7.8
9.5
11.2
r
13.6
0Lb'
6
8
COMPRESSION RATIO
Fig. 5 lmprovement Rate in Fuel Consumption.
with Trapping Efficiency
Equivalent to Standard
Compression Ratio
REDUCED SPECIFIC HEAT
OF
THE
WORKING GASES
in many reports, working gas temperature is
affected by an increase in compression ratio.
The resultant specific heat ratio variation
affects the overall thermal efficiency
Two-stroke engines differ considerably from
four-stroke engines in temperature and
pressure behavior.
conditions, the pressure at the beginning of
compression approaches atmospheric
pressure; and the residual percentage of the
working gases is high especially at low loads.
The thermal efficiency of the fuel-air
COMPRESSION RATIO
Fig. 6 lmprovement
Cycle Thermal Efficiency
10 12 14
RATIO
-
As
observed
(3)(4).
~e~ardiess of load
Rate
of
The Fuel-Air
cycle was then calculated to investigate the
effect of the specific heat ratio of the
working gases. The pressure, the
temperature, and the composition at' the
beginning of compression were set to the
same values as in actual engines. Thermal
dissociation was disregarded.
Figure
6
shows the improvement rate of
the fuel-air cycle thermal efficiency with
A
increasing compressiorl ratio.
ratio of
6.6
was used as the standard for the
compression
calculations. The fuel-air cycle exhibits a
higher improvement rate than the air cycle
value. Variation of the working gas specific
heat ratio does not prevent the effect of
higher compression ratios. To the contrary,
the improvement of fuel consumption is
enhanced.
INCREASED MECHANICAL LOSS
-
Figure 7 shows the variation of mechanical
loss as measured by the motoring method.
Mechanical loss increases by
compression ratio is raised from
27%
6.6
as the
to
13.6.
The pumping work in the crankcase is
excluded from the results.
8
Figure
shows the improvement rate in
the fuel consumption assuming that the
mechanical loss is independent of the
compression ratio. The improvement rate
shows a marked decrease especially at low
loads. At a compression ratio of
200kPa
BMEP
(the same conditions as the
12
and at
earlier example), a 4.5% decrease in
improvement rate is observed, and is higher
as compared to the increase caused
by
short-
circuiting.
Crankcase pumping work varies closely
in proportion to the scavenging volume in
two-stroke engines. Therefore, the ratio of
the pumping work to the indicated work is
almost, but not completely, constant over all
loads. The experimental engine also shows
a small decrease (0.009-0.012) in brake
thermal efficiency due to the pumping work.
The effect of higher compression ratio is
very small for the same load because of the
4500
,Dm
(WID CRANK
CASE
PUMPING
I
lool
COMPRESSION RATIO
Fig.
7
Relationship between the Compression
Ratio and the Mechanical Loss
45W
rum
BMEP 200 kPm
20
.
-
r_
-
:
I
Y
g4
Fig.
8
Improvement Rate in Fuel Consumption
EOUNALENT
l5
'
10 ORIGINAL COMP RATIO
P
OU
5,
6
6
8
COMPRESSION RATIO
B
COMPRESSION RATIO
MECU.'LOSS
ACTUAL
10
12
10
12 la
FlJEL.AIR
,
CYCLE
74
without Increased
Mechanical Loss
reduced delivery ratio.
INCREASED
9
shows the improvement rate in fuel
COOLING
LOSS
-
Figure
consumption assuming the cooling loss is
independent of the compression ratio. The
cooling losses are calculated from cycle
simulations. The heat transfer coefficient at
compression stroke and at expansion stroke
are calculated using the equations proposed
by
G.
Woschini(5). The gas exchange
process has only a slight effect on the
cooling loss and is excluded from the
.
-
..
.
15W
rpm
BMEP
TO
STANOARO
6
COMPRESSION RATIO
lo
1
WE.
COMPRESSION RAT0
100
kP.
COW
8 10 12
ALW
WI
FUELAIR CYCLE
.,'
RATIO
14
FUEL
AR
CYCLE
,
-40
-
20
TOC
CRANK ANGLE I'ATDCI
-A0 - 20 TOC 20 40 60 80
CRANK ANGLE IoATDCI
Fig.
10
Rate of Heat Release
20 40 60 80
Fig. 9 Improvement Rate in Fuel Consumption
wlthout Increased
Cooling Loss
BMEP
kPa
10. Therefore,
and above a compression ratio of
it
seems reasonable to
suppose that the improvement rate in fuel
consumption is smaller with high
calculations. The results show that the
cooling loss considerably reduces the effect
of high compression ratio especially at low
loads, though the decreased rate due to
cooling loss is not so high as compared to
the mechanical loss. At a compression ratio
12
and at 200kPa
of
rate is
3.8%
BMEP,
the improvement
less due. to the increase in
cooling loss.
INCREASED
TIME
LOSS
-
Time loss
variation was investigated using combustion
compression ratio at heavy load.
The discussion above pertains to
improvement in fuel consumption with higher
compression ratios. However, because of
higher mechanical losses and higher cooling
losses, the fuel consumption at very low
loads seems to increase with an increase in
compression ratio.
The
results for idling fuel consumption
are shown in Figure
11.
The fuel
consumption becomes disadvantaged as the
compression ratio increases. It is clear from
pressure analysis. The results of the rate of
the heat release are shown in Figure 10. At
low loads, the greater the compression ratio,
the shorter the combustion time.
.
.
is subsequently reduced. At heavy loads, the
combustion time is longer as compression
Time loss
ratio increases. This is considered to be
caused by the flattened combustion chamber
shape due to the increased compression ratio.
In addition, it is necessary to retard the
ignition timing to avoid knocking above
300
Fig.
COMPRESSION
11
Idling Fuel Consumption
RATIO
Fig.
-
3
-
Z
!
a
$
0
6
a
12
50
40
30
20
10
0
F
C.
Fuel
Consumption
LA.4
MODE
HC
for
0
CO
C
6.6
LA-4
6
5
4
3
2
B
-
E
5
0
"I
Mode
the results that the increase in the mechanical
and the cooling losses are greater than that
for thermal efficiency at such loads.
The fuel consumpfion was also measured
in
an actual motorcycle for the
The compression ratio was set at
LA-4
9.5
mode.
because
severe knocking occurred at a compression
ratio above
Figure
10.
The test results are shown in
12.
The results show a
5.7%
reduction in the fuel consumption and a
4.1%
standard compression ratio of
reduction in HC emissions at a
6.6.
These
values are almost equal to the improvement
4500
rate in fuel consumption at
200kPa BMEP as shown in Figure
rpm and
2.
POWER IMPROVEMENT WITH
HIGHER COMPRESSION RATIO
-
In
typical gasoline engines, power output can be
improved with a higher compression ratio
unless excessive knock problems
predominate. Figure
13
shows the results of
the full load performance and the scavenging
characteristics for compression ratios of 6.6,
7.8, and
9.5.
The performance output is
improved within the tested speed range, but
maximum power occurs at a speed slightly
lower than maximum. This is a result of
lower synchronizing speeds along with longer
relative pressure pulsation due to the lower
exhaust temperature.
evident in the delivery ratio results.
This
tendency is
The
trapping efficiency is slightly decreased. The
engine speeds exhibiting the greatest power
output can be controlled by altering the
exhaust system size.
8
10
110'
and
Scavenging
Fig.
13
Full-Load
2
1
6
ENGINE SPEED
Irpml
Performance
Characteristics
FACTORS LIMITING COMPRESSION
RATIO
Higher compression ratio makes it
possible to improve power output., but causes
serious problems such
as
knock and piston
thermal load increase. To solve the
problems, several reports have proposed low
compression ratio at high-speeds and heavy
loads with variable combustion chamber
volumes.(6)(7). In this paper, retarded
ignition timing is discussed as a remedy for
the problems.
Figure
range for the compression ratio of
14
shows the full-load knocking
9.5.
The
knock intensity is estimated from the sum of
the absolute values of the pressure
differences(8). Under MBT operation, a very
strong knock occurs above
8000
rprn
which
is apt to cause piston top erosion.
COMPRESSION RATIO 9.5
WOT AIF-13
e
lo/
/
~NITl0N TIMING WITH EOLIWALENT
I?
Fig.
The
POWER TO STANOARO COMPRESSION RATIO
14
Full-Load Knocking Range
dashed line in the figure shows the
ignition timing
ENGINE SPEED
with
equivalent power at the
INTENSE KNOCK AREA
SERIOUS KNOCK AREA
LIGHT KNOCK AREA
/
Irpml
x
10'
standard compression ratio. Knocking is
absent even at overspeed. Therefore, above
7000
rpm, using an ignition timing later than
MBT, power output can remain advantaged
and knock problems can be solved.
compression ratio of
9.5
is the maximum
A
limit at which knock can be avoided and
power output ensured.
-
Concerning the thermal load, Figure
15
shows the cylinder pressure and the
instantaneous heat flux calculated from
simulations. The results show that re.tarded
ignition timing is more advantageous for
equivalent power output levels since heat
flux is reduced as well. The engine tests also
indicate that the plug seat temperature is also
lower by this method as
compared to the
results for the standard compression ratio.
The countermeasures taken for problems
associated with high speeds and heavy loads
were discussed above. However, at a
compression ratio of
9.5,
combustion
accompanied by a strong impact was
observed in road tests at very low loads, and
at medium and high speeds. This
combustion under irregular conditions is
called "low-load knock" or "high-speed and
low-delivery ratio knock". These types of
knocking are said to arise from the same
principles as heavy-load knocking(9).
Figure
low-load knock. Figure
16
is an indicator diagram for
17
shows the
Fig.
IMEP 550 kPa
I-
-80-60-40-20 0 20 40
CRANK ANGLE IoATDCI
-80-60-60-20
CRANK ANGLE lDATDCl
15
Calculated Cylinder Pressure and
0
20
40 60 80
60
80
Instantaneous Heat Flux
with
Retarded Ignition
Timing
Fig.
-
0
0
r
P
-
,-
Fig.
16
Low-Load Knock
OCCURRENCE
MORE FREOUENTLY
a
FREOUENTLV
0
LESS FREOUENTLY
4500
COMPRESSION RATIO
9
5
30,
.
',
20
L.
I
I
13 15 17
17
Low-Load Knocking Rage
DELNERY RATIO
AIF
,rn'
knocking range for air-fuel ratio and ignition
timing. The low-load knock can barely be
controlled through ignition timing; so there
are still problems with the avowed similarity
between low-load and heavy-load knocks.
However, removing the deposit shows no
effect implying that this combustion is
clearly not a kind of surface ignition. We
consider that the results
compressed self-ignition. To remedy lowload knock, the use of premium gasoline and
improving cooling are effective measures. If
premium gasoline is not used, the permissible
limit of the compression ratio is
approximately 8.4 without liquid-cooling and
8.7 with liquid-cooling.
CONCLUSIONS
Investigations were conducted on the
effect of higher compression ratio in
cooled two-stroke motorcycle engines. The
results can be summarized as follows:
(1) It is possible to improve fuel
consumption with higher compression ratio,
though, due to various losses, the
improvement rate fails to achieve
theoretically expected values. At very low
loads, such as idling, increases in the losses
surpass the increase in the thermal efficiency
and this can cause unfavorable fuel
consumption in some cases.
(2)
The increases in losses can be
classified into increased short-circuiting and
increased mechanical and cooling losses at
low loads. At heavy loads, the time loss
increases, but the influence of shortcircuiting as well as mechanical and cooling
losses are reduced. Varying the specific heat
ratio of the working gases can be an effective
countermeasure to contain losses.
(3) Higher compression ratios make it
possible to improve power output, though
there is a limit imposed
increased thermal load to the maximum
compression ratio.
indicate
by
knocking and
a
form of
air-
(4)
As
the compression ratio is increased,
it is effective to use an ignition timing later
than
MBT
thermal load avoidance. The maximum
compression ratio needs to be reduced,'
however, because of the occurrence
unmanageable abnormal combustion at
low loads.
REFERENCES
I.
"Characteristics of 2-stroke Motorcycle
Exhaust HC Emission and Effects of AirFuel Ratio and Ignition Timing,"
750908.
2.
K.
Y.
Motoyama, "Improvement of Fuel
Consumption with Variable Exhaust Port
Timing in a Two-Stroke Gasoline Engine,"
SpLE.
850183.
3.
Thermal Efficiency and a Future Concept of
Gasoline Engine and Diesel Engine from the
View Point of
INTERNAL
21, No. 258 (1982), pp. 85-96. (in Japanese)
4.
"Factors Limiting the Improvement in
Thermal Efficiency of
Compression Ratio,"
5.
Applicable Equation for the Instantaneous
Heat Transfer Coefficient in the Internal
Combustion Engine",
6. W.
"Analysis of the combustion Process of
Spark Ignition Engine with a Variable
Compression Ratio,"
7. V. Harne and S.
Compression Ratio Two-Stroke Engine,"
SAE.
891750.
8.
T.
Combustion of Two Stroke Cycle Gasoline
Snowmobile Engine at High Speed and full
Load,"
9.
H.
in regards to knocking and
K.
Tsuchiya and
Nomura, S. Hirano,
S. Matsuoka and
Cqvcle Theory."
COMBUSTION
S. Muranaka,
Y.
S.
SAE,
G.
Woschni, "A Universally
SAE.
H.
Adams, H.
SAE,
R.
Fujikawa and
SAE,
790841.
Nakahara et al,
T.
H.
Tasaka, "The
ENGINE,
Takagi,
I.
Engine at Higher
870548.
670931.
G,
Hinrichs et al,
870610.
Marathe, "Variabie
S.
Abe, "Abnormal
TRANSACTIONS
very
S.
Hirano,
SAE.
Gotoh, and
Vol.
T.
Ishida,
a
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