The overall internal combustion engine performance is very much dependent upon
the design and the operation parameters of the inlet and exhaust system. The
inlet and exhaust are used to deliver clean air to the engine and dispose the
exhaust gas quietly with minimum loss of performance. The parameters considered
for properly designing the exhaust pipe system of internal combustion engines
are diameter and length of pipe, material, thickness and insulation of exhaust
pipe system, geometry of pipe connection (junctions) and the position of the
necessary elements of the exhaust system. Numerous studies have been done to
investigate the influence of inlet and exhaust manifold design on the engine
operational characteristics in transient and steady state mode, the engine performance
and engine emission (Kesgin, 2005; Galindo
et al., 2004; Seenikannan et al., 2008a).
Length and diameter are two important parameters in the exhaust system designs
that influence the effectiveness and efficiency of the gas flow. Various modeling
and experimental studies have been also carried out to investigate the fluid
dynamic characteristics in the exhaust pipe and its influence on the diesel
engine performance (Graff and de Weck, 2006; Sekavcnik
et al., 2006; Seenikannan et al., 2008b;
Bell, 1997). The viscous compressible fluid within the
straight circular pipe was excited by an impulse disturbance at the inlet of
the pipe, and the dynamic response was analyzed. The dynamic behaviors of the
fluid are influenced by the variation of the pipe lengths. Other research works
relate the cross-section of the pipe with the power produced and investigate
how the length of the pipe influences of the behaviors of the fluid (Kesgin,
Backpressure exists in the exhaust system and a properly tuned exhaust system
can improve the engine performance by reducing the backpressure (Bell,
1997). The study also showed that a free flowing exhaust system will provide
about 10 hp useful powers due to less amount of pumping loss.
Since most of the studies focuses on the exhaust manifold, the objective of the present work is to experimentally study the diesel engine performance for different exhaust middle pipe configurations. The experiment is conducted using a diesel engine test bed fuelled by pure diesel fuel. The throttle opening is set at two positions i.e., 50% and 100% throttle openings. The parameters to be studied are torque, brake horsepower, Brake Mean Effective Pressure and specific fuel consumption.
MATERIALS AND METHODS
The experiments were conducted at the Universiti Teknologi Petronas Centre of Automotive Research using Diesel Engine Test Bed which is equipped with a cubic capacity of 1753 cc 4 cylinder engine as shown in Fig. 1. The engine is fuelled using pure diesel and the exhaust system consists of a catalytic converter, muffler and 60 mm diameter exhaust pipe. The overall length of the exhaust assembly is 12.8 m. The specification of the engine test bed is tabulated in Table 1.
The experiment was carried out to observe the relationship between exhaust
pipe configurations to the engine performance. The experiment has been conducted
by using three different configurations of the exhaust pipe. The first experiment
was initially done using the original exhaust pipe configuration with total
length of 12.8 m long with three bends. Then, the experiment is repeated using
5.67 m with one bend and 8.24 m long with two bends. Fig. 2a,
b and c below show the schematic diagram of the variation
of the exhausts pipe configuration used for this study. The red circle indicated
the joint of the pipe which can be unfastening to change the configuration of
the exhaust pipe.
Experimental methodology: The experiments were conducted using pure diesel fuel and carried out in two phases based on 50 and 100% throttle opening conditions. In 100% throttle opening, the throttle was set to a maximum level while the 50% throttle opening can be considered as driving in normal condition or daily driving.
|| Specifications of the diesel engine test bed
||(A-c) Variation of the exhausts pipe configuration for the
|| Typical display of the engine test bed panel
The data recorded in the experiment were the Brake Horsepower, torque, Brake Mean Effective Pressure and specific fuel consumption. The readings were taken from the engine test bed panel which is shown in Fig. 3 below.
RESULTS AND DISCUSSION
Torque: Figure 4 and 5 are the variation
of engine torque with respect to the engine speed for three different middle
pipe lengths for the case of 50% and 100% throttle opening as measured in the
experiments. At 50% throttle opening condition, the engine is producing 68 Nm
torque at 2000 rpm engine speed. When the speed increases to 3000 rpm, the torque
value reduces to 46 Nm and decreases further as the speed increases. At 100%
throttle opening, the maximum torque is 88 Nm at 3000 rpm and reduces to 80
Nm at 4000 rpm as shown in Fig. 5.
The pattern of the torque are similar for different length of the exhaust pipe
where the torque increases with the engine speed until reaching the maximum
value and then the torque decreases as the speed increases. These findings agree
with the theory as every engine cycle results in thrust caused by the combustion.
||Graph of Torque vs. engine speed (RPM) at 50% throttle
|| Graph of Torque vs. engine speed at 100% throttle
The faster the engine cycles, the more thrust will be produced until reaching
a point where the inefficiencies of the higher engine speed outweighs the benefits
of the higher speed itself.
Brake horse power: Figure 6 and 7
show the variation of the bhp value with respect to the engine speed for three
different middle pipe lengths for the case of 50% and 100% throttle opening
as measured in the experiment. The graph of bhp vs. engine speed shows that
the brake horsepower (bhp) is proportional to the engine speed. It is observed
that for the 50% throttle opening condition, the bhp value increases with the
increase of the engine speed and reaches the maximum value at 3000 rpm. However,
the bhp value reduces as the engine speed reaches 4000 rpm. The trends of the
bhp values are the same for different length of the pipe. The result for the
exhaust pipe length of 5.67 m and 8.24 m is almost the same but for the 12.8
m exhaust pipe, the bhp value drops by about 40% at 10.2 hp for the 4000 rpm
engine speed as dipict in Fig. 6.
|| Graph of bhp vs. engine speed at 50% throttle
|| Graph of bhp vs. engine speed at 100% throttle
For the 100% throttle opening, the results for all three different exhaust
pipe lengths are almost the same except those at 4000 rpm engine speed. Although
there is no significant difference of bhp values for all three different exhaust
pipe configurations when the engine is running at 2000 rpm, the differences
grow wider when the engine speed exceeds 2000 rpm and 3000 rpm for the 50% and
100% throttle opening conditions respectively.
Specific Fuel Consumption (SFC): Figures 8 and 9
show a variation of SFC with respect to the engine speed for three different
exhaust middle pipe configurations for the case of 50% and 100% throttle opening
as measured in the experiment. The graphs show the fuel consumption by the shorter
exhaust pipe configuration is less than the fuel consumption by the original
exhaust pipe system. The SFC decreases as the engine speed reaching 2000 rpm
after which the SFC increases with the engine speed. The higher SFC might be
due to the warming up session required by the engine where much fuel is required
to put the engine from cold state to its normal operational condition.
||Graph of specific fuel consumption vs. engine speed (50% open
||Graph of specific fuel consumption vs. engine speed (100%
The SFC is higher at 1000 rpm particularly for the longest configuration whereas it should be similar for all configurations because at that engine speed, the engine is at idle state and no work is done yet. The discrepancy in initial SFC because the first experiment was started with the original exhaust system at cold engine condition while the second and third experiments were carried out after the engine is already in suitable temperature to operate. The SFC is lowest at >middle = rpm that is at 2000 rpm because the engine is tuned to develop best cylinder filling at middle revs; the engines breathing is at highest efficiency at these speeds. At higher engine speeds, the frictional loses of the engine rise alarmingly and so the energy of combustion is again being wasted, this time in heating the oil. Low value of SFC is desirable as it use less fuel to produce work. SFC is lower when the engine is in high volumetric efficiency. Nevertheless, the exhaust pipe configuration does play an important role in the engine efficiency as it was related to the power required to push the gases along the pipe as shown by the variation of the SFC for different exhaust pipe configurations.
Brake Mean Effective Pressure (BMEP): Figure 10 and
11 show a variation of BMEP with respect to the engine speed
for three different exhaust pipe configurations for the case of 50% and 100%
throttle opening as measured in the experiment. The result for BMEP is quite
similar with the result for the torque. The graph shows that the highest BMEP
value which is 4.88 at 2000 rpm for 50% throttle opening and 6.29 at 3000 rpm
for 100% throttle opening. The highest BMEP value are achieved by the shortest
exhaust pipe configuration i.e. 5.67 m long exhaust pipe.
For 50% throttle opening, the BMEP value is reduced from 4.88 to 3.29 at 3000
rpm and 2.16 at 4000 rpm. For 100% throttle opening, the BMEP value only decrease
after 3000 rpm. The value increases from 5.06 at 1000 rpm to 6.29 at 3000 rpm
and decrease to 5.7 when engine speeds reached 4000 rpm. It makes the result
look like parabolic curve. Both of the graph (50% and 100% throttle opening)
show that the BMEP decreases with the increasing of pipe length. The longest
exhaust pipe has the lowest value of BMEP. The BMEP is higher at 2000 rpm but
decrease when the engine speed becomes 4000 rpm especially in 50% throttle opening.
BMEP value becomes lower when the engine speed increase because the engine efficiency
is lower due to friction and air viscosity. Longer pipe will have more friction
and thus, the longer pipe has the lowest BMEP value at 4000 rpm. BMEP is simply
an effective comparison tool between different engines.
||Graph of BMEP vs. engine speed at (50% open throttle)
||Graph of BMEP vs. engine speed at (100% open throttle)
It measures the efficiency of the conversion from the indicated mean effective
pressure in the cylinder to the output shaft and the level of pressure attained
in an engine. Higher value of BMEP would indicate higher performance of the
engine. The bend in the pipe also affect the outputs of the engine. In the experimental
result, it shown that the result for 5.67 m and 8.24 m pipe is quite similar.
There is not much difference in term of value of the engine performance. The
graph is almost similar especially in 50% throttle opening. The factor that
differentiates their value is the bend. The bend in 8.24 m pipe causes the gas
to slow down and hence the engine need more energy to push out the gas resulting
in less useful power generated.
Both the length of the exhaust middle pipe and number of bends influence the engine performance characteristic. The number of bends gives greater impact to the engine performance as compared to the length of the exhaust pipe. The more bends installed in the exhaust pipe, the lower the engine output produced. The experimental results also confirm that the length of the pipe influences the performance characteristic of the engine. It can be clearly seen that different lengths do give different output value where the shorter exhaust pipe with less bend produces better diesel engine performance. Even though the variation of the length used in this work is consider small, but the differences in flow pattern and engine performance for different exhaust pipe configurations are noticeable especially at the engine speed greater than 2000 rpm.
The authors would like to express their gratitude to the Universiti Teknologi PETRONAS for the technical assistance provided for the performance of this project as well as during the preparation of this report.