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Joardder, Mohammad Uzzal Hossain, Uddin, Md. Shazib, & Roy, Mu-
rari Mohaon (2011) Effect of engine backpressure on the performance
and emissions of a CI engine. In International Conference on Mechani-
cal Engineering 2011 (ICME2011), 18-20 December 2011, BUET, Dhaka,
Bangladesh.
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Copyright 2011 ICME
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Proceedings of the
International Conference on Mechanical Engineering 2011
(ICME2011) 18-20 December 2011, Dhaka, Bangladesh
ICME11-TH-013
© ICME2011 1 TH-013
1. INTRODUCTION
Backpressure usually refers to the pressure exerted on
a moving fluid by obstructions against its direction of
flow. The average pressure in the exhaust pipe during
the exhaust stroke is called the mean exhaust pressure
and the atmospheric pressure is called the ambient
pressure. The difference between these two pressures is
defined as backpressure [1]. Diesel engine causes
emissions in the environment; some of them are harmful
for human being. Exhaust systems including catalytic
converter, muffler and resonator in diesel engine reduce
the engine emissions [2, 3]. Increase in exhaust back
pressure decreases nitric oxide, due to the increased
exhaust gas remaining in the cylinder, as has also been
demonstrated by others. Hydrocarbon emissions are also
reduced as exhaust back pressure is increased [4]. Long
term application of the system causes significant
effect on engine performance and emissions.
Particulate matter and other exhaust product adhere
with flow passage of exhaust systems and the passage is
reduced and backpressure is building up on the engine.
The performance and emissions of a diesel engine can
control by backpressure [2]. Excessive backpressure in
the exhaust system create excessive heat, lower engine
power and fuel penalty in the engine cylinder, that may
cause damage of the engine parts and poor performance
[5 ]. The amount of power loss depends on many factors,
but a good rule-of-thumb is that one inch (25.4 mm) of
mercury backpressure causes about 1.0% loss of
maximum engine power [6].
Hence, backpressure can be used up to a certain level
to improve the engine performance and reduce
emissions. In modern diesel engine, diesel oxidation
catalyst (DOC) is inevitably used to control CO and HC
emissions. A new DOC is very effective mean of
controlling CO and HC without any significant penalty
in fuel consumption and power output and efficiency.
When DOC becomes older, its activity is diminished,
and it develops significant backpressure deteriorating
engine performance with fuel penalty. An experimental
investigation was performed at RUET, Bangladesh to
obtain the allowable level of backpressure for which,
there is no significant change in engine performance
and fuel penalty. Its effect on exhaust emissions
including odor was also investigated.
2. EXPERIMENTAL SETUP AND
MEASUREMENT
A four-stroke single cylinder naturally aspirated DI
diesel engine with specification as in Table 1 was used in
this experiment. All experimental data were taken at
engine speeds of 600, 950 and 1200 rpm with low,
medium and high load conditions of 12, 20 and 26N
respectively after engine warm-up. The engine speeds
were selected on the basis of low, medium and high under
EFFECT OF ENGINE BACKPRESSURE ON THE PERFORMANCE
AND EMISSIONS OF A CI ENGINE
Mohammad Uzzal Hossain Joardder, Md. Shazib Uddin and Murari Mohon Roy
Department of Mechanical Engineering, RUET, Bangladesh
ABSTRACT
This study investigated the effect of engine backpressure on the performance and emissions of a CI engine.
Two parameters were measured during the engine operation: one is engine performance and the other is
the exhaust emissions. The result showed that, the brake thermal efficiency and brake specific fuel
consumption (bsfc) are almost unchanged with increasing backpressure up to 40 mm of Hg pressure for all
engine speed and load conditions. The NOx emission became constant or a little decreased with increasing
backpressure. The formation of CO was slightly higher with increase of load and back pressure at low
engine speed condition. However, under high speed conditions, CO reduced significantly with increasing
backpressure for all load conditions. The odor level was similar or a little higher with increasing
backpressure for all engine speed and load conditions. Hence, backpressure up to a certain level is not
detrimental for a CI engine.
Keywords: DI Diesel Engine, Performance and Emissions, Backpressure.
© ICME2011 TH-013 2
available engine operating conditions in practice. The
available loads were divided into three
categories like low, medium and high load.
Loads and speeds were adjusted manually during the
operations. The diesel fuel used in this study is available
in the local market. Loads were measured by electric
dynamometer. Exhaust emissions were measured after
diesel oxidation catalyst and fuel consumption rate were
measured by taking time for consumption of 10cc of fuel.
The calculation of thermal efficiency and bsfc were
measured by following formulae-
The schematic of diesel engine with gas analyzer and
odor measurement arrangement is shown in Figure 1.
Table1: Engine specifications
Engine type 4-stroke DI diesel engine
N
umber of cylinders One
Bore x Stroke 80 x 110 mm
Swept volume 553 cc
Compression ratio 16.5:1
Rated power 4.476Kw@1800 rpm
Fuel injection timing
24
0
BTDC
\
Fig 1. Schematic of diesel engine with gas analyzer and
odor measurement arrangement
Fig 2. Arrangement of mercury manometer to measure
exhaust backpressure.
A flue gas analyzer (IMR 1400) was used to measure
carbon monoxide (CO) and NOx of exhaust gases. A
mercury manometer was used to measure the exhaust
backpressure in the exhaust line. Gate valve in the
exhaust line was used to regulate the backpressure. The
arrangement of backpressure measurement is shown in
Figure 2.
2.1 Human Assessment
Sensual assessment by human nose is one technique
to asses the odor level of exhaust gases. This study used
an odor intensity scale to evaluate the discomfort level
of exhaust gases. The intensity scale and corresponding
explanation of odor rating are shown in Table 2. A
difference in 1 point has been reported as equivalent to
a 10 fold the change in the concentration of odorous
gases [7]. This means that one point improvement in the
odor scale is a significant improvement in exhaust odor.
Deviations in sensual assessment vary from person to
person when the test personnel are inexperienced, while
reliable results can be obtained with experienced
personnel [8]. This study used three experienced
assessors.
Table 2: Odor rating scale
Intensity
rating
Ve rb al
Description
1 Not detectable No odor
2 Slight
Odor but not
uncomfortable
3 Moderate Uncomfortable odor
4 Strong
Limiting odor, long
time exposure not
possible
5 Very strong
Very imitating odor,
exposure even 1 o
2s impossible
3. EXPERIMENTAL RESULTS AND DISCUSSION
3.1 Engine Performance
After the engine reached the stabilized working
condition for each test, fuel consumption, load and
exhaust emissions were measured from which bsfc and
efficiency were computed. The variations of these
parameters with backpressure are presented.
P (kW)
Thermal efficiency=
CV (kJ/kg) × m (kg/s)
m (kg/hr)
bsfc=
BP
(
kW
)
© ICME2011 TH-013 3
Figure 3 shows the variation of brake thermal
efficiency (η
th
) with engine backpressure. Figure 3(a),
(b) and (c) represent the effect of backpressure on
thermal efficiency at 600, 950 and 1200 rpm engine
speed respectively. Brake thermal efficiency was
almost constant for low, medium and high load
condition with backpressure at low engine speed of
600 rpm. At 950 rpm, it slightly decreased for low,
medium and high load condition. At 1200 rpm, it is
decreased with increase in backpressure. Medium and
high load condition could not be examined due to
unacceptable level of black smoke.
Fig 3. Effect of backpressure on brake thermal
efficiency with various engine loads.
Fig 4. Effect of backpressure on bsfc with various
engine loads.
Figure 4 shows the variation of bsfc with
backpressure at various engine load and speed
condition. Figure 4 (a), (b) and (c) represent the effect
of backpressure on bsfc at 600, 950 and 1200 rpm
respectively. From the figure, it is seen that bsfc has
no significance change for low speed condition. of
600 rpm. At 950 rpm bsfc increased with the increase
in backpressure for all load condition.
At 1200 rpm at low load condition bsfc also
increased with backpressure. Hence, up to a certain
limit of speed and backpressure, there is no
significant effect on engine performance. Excessive
backpressure at very high operating condition causes
decrease in the engine performance and fuel penalty.
3.2 Engine Emissions
Figure 5 shows the variation of CO emission with
backpressure at various load condition. Figure 5 (a),
(b) and (c) represents the effect of backpressure on
CO emission at 600, 950 and 1200 rpm respectively.
At 600 rpm CO increases with increasing
backpressure at various engine load condition. At low
load condition, CO is higher than other test load. At
950 rpm high load condition, CO decreases
significantly with backpressure. At low and medium
load condition CO slightly decreases with
backpressure. Also at 1200 rpm CO decreases with
backpressure and at no load condition it is higher than
at low load. Hence, at low engine speed CO is higher
with increased backpressure. However, CO decreased
with increase in backpressure at high engine speed.
Fig 5. Effect of backpressure on CO emission.
Figure 6 illustrates the variation of NOx emission
with backpressure at various engine load condition.
© ICME2011 TH-013 4
Fig 6. Effect of backpressure on NOx emission.
From the figure, it is seen that NOx emission
decreases with backpressure and increases with
increasing engine load at various test load and engine
speed. Increased backpressure causes increased
exhaust gas remaining in the cylinder, which have
higher specific heat and act as a heat sink in the engine
cylinder. Hence, NOx is reduced.
Figure 7 shows the variation of odor with
backpressure for different load conditions.
Fig 7. Effect of backpressure on odor.
From the figure, it is seen that the trend is similar
for all load condition at 600, 950 and 1200 rpm engine
speed, i.e., odor slightly increases with increasing
backpressure.
Figure 8 illustrates the variation of exhaust gas
temperature with backpressure for various load
condition. It is seen that, gas temperature increases
with increasing backpressure for 600, 950 and 1200
rpm engine speed and all load condition. Gas
temperature is low at low load and high at high load.
Hence, backpressure causes increase in the exhaust
gas temperature due to exhaust gas present longer time
in the exhaust line before being exhausted and
increase the temperature of exhaust gas.
Fig 8. Effect of backpressure on exhaust gas temperature.
4. CONCLUSIONS
The following conclusions can be drawn from the
experimental investigation.
1. At low engine speed the backpressure has no
significant effect on engine performance with various
load conditions. At medium and high engine speed the
performance remains constant up-to a certain
backpressure (40 mm of Hg) and then decreased. At
low speed condition, bsfc is almost constant. However,
at medium and high speed, bsfc is little higher for all
load conditions.
2. CO is higher at low engine speed for all load
condition with increased backpressure. However, CO
decreased with increased backpressure for higher
engine speeds and loads.
3. NOx emission always decreased at low, medium
and high engine speed with increased backpressure for
various load conditions due to backpressure increased
the exhaust gases in the cylinder and act as heat sink.
© ICME2011 TH-013 5
4. Backpressure has no significant effect on odor level
up to 40 mm of Hg pressure. However, odor is
increased above the backpressure of 40 mm Hg. The
exhaust gas temperature was always increased with
higher backpressure and engine load.
5. REFERENCES
1. Domkundwar, V.M., 2000, " A Course in
Internal Combustion Engine", Dhanpat Rai
and CO. (P) Ltd.
2. Mohiuddin A.K.M., Mohd Rashidin Ideres and
Shukri Mohd Hashim 2005,"Experimental
Study of Noise and Backpressure for Silencer
Design Characteristics", Journal of Applied
Sciences 5(7): 1292-1298.
3. Huang, L., 2004," Parametric study of a
drum-like silencer", Journal of sound and
vibration, 269; 467-488.
4. Jay A. Bolt, Stephen P. Bergin, Frederick J.
Vesper, 1973," The Influence of the Exhaust
Back Pressure of a Piston Engine on Air
Consumption, Performance, and Emissions",
SAE International, Document Number:
730195.
5. J.E. Bennethum, and R.E. Winsor, 1991,
“Toward improved diesel fuel”, SAE paper
No. 912325.
6. Mondt, J.R., 2000," Cleaner Cars: The History
and technology of emission control since the
1960s, SAE International.
7. Owkita, T. and Shigeta, Y., 1972," Analysis
Method of Low Concentration Gas and Bad
Smell, KOUDANNSYA (in Japanese).
8. Roy, M. M., Tsunemoto, H. and Ishitani, H.,
1999, “Effect of Injection Timing and
Fuel
properties on Exhaust Odor in DI Diesel
Engines”, SAE Paper No.
1999-01-1531.
6. MAILING ADRESS
Md. Shazib Uddin
Department of Mechanical Engineering,
RUET, Bangladesh
shazib0397@gmail.com
