effect of different exhaust configurations on engine performance parameters

7/23/2019 Effect of Different Exhaust Configurations on Engine Performance Parameters

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Effect of different Exhaust configurations on Engine Power

By Shoaib Iqbal

Introduction:-

Exhaust system shape, geometry and design are the factors that control the dynamics of theexhaust gases hence engine performance. Thus the power and torque of an internal combustion

engine is greatly affected by the exhaust system. To investigate how much power can be varied

by changing exhaust configuration and optimize exhaust for maximum power output, we have

carried out tests with various exhaust configurations.

The tests have been carried out on Rotax 912 which is a four stroke; four cylinder; naturally

aspirated spark ignition engine.

The tests have been conducted with the following configurations of exhaust system.

- 4 into 1 collector system without muffler and tail pipe

- 4 into 1 collector system with muffler and tail pipe

- 4 into 2 collector system without muffler and

- Independent exhaust manifolds

Exhaust Theory and Experimental Setup:-

The exhaust configurations have been tested on an engine mounted on a dynamometer with eddy

current absorber. Data acquisition box interfaces all the sensors mounted on the engine with a PC

where the values are displayed on the software interface. Data of the critical sensors has been

extracted from the recorded runs and graphs are plotted to evaluate the data.

There are two primary considerations involved in exhaust testing. The first deals with the

exhaust gas that travels at 200-300 feet per second. Here we are concerned with reducing theback pressure while maintaining flow velocity to get the best cylinder exhaust purging. The

second is exhaust pulses or finite-amplitude pressure waves which travel the speed of sound or at

about 1700 feet per second. These pulses can be tuned to create an effect known as scavenging

(sucking additional exhaust out and intake charge in, with a properly timed negative pressure

pulse).To maximize power an engine exhaust back pressure should be reduced and scavenging

improved.

Basic conditions for the test were

All the primary manifold pipes were of equal length and shape for all experiments Same material and pipe bends/lengths/diameters were used for header and runners All tests were performed with identical load and throttle setting

4-1 Exhaust System without Muffler and Tailpipe

A good 4-into-1 header is considered optimal for the high horsepower. Also, a header does a

pretty good job of additionally "sucking" more exhaust from the combustion chamber, so on the

next intake stroke there's lots more fresh air to burn.


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Figure 1

Test results in figure1 show that engine can make a maximum power of 100.08hp at full throttle.Furthermore, 4-1 systems are known for better scavenging effects as the exhaust pulse from one

cylinder; tend to create vacuum to aid other cylinder evacuate exhaust gases.

4-1 Exhaust System with Muffler

Mufflers indeed reduce the noise but they do so at the cost of power. The test with muffler was

performed to get exact value of power loss with a muffler; the graph in figure1 shows the power

and torque curve plotted against rpm.

Figure 2

Muffler acts as a restriction in the way of exhaust pulses so the engine has to work harder to

expel gases and a part of torque is wasted in pumping these gases out against the restriction. The

restriction also increases back pressure and thus comparatively poor scavenging that leads to

power drop throughout entire rpm range.

0

20

40

60

80

100

4000 4191 5139 5690 5689RPM

Power(hp)

0

20

40

60

80

100

4016 4414 4725 5241 5603 5664 5702 5704 5701 5692

RPM

Power(hp)


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Figure 3

Figure 3 shows the maximum torque values at full throttle for the four exhaust configurations.The drop in torque with muffler system is due to the pumping losses described above.

2-1 Exhaust System without Tailpipe

Figure 4 shows the power curve of 2-1 system.

Figure 4

2-1 configuration shows an evident drop of power at lower rpm range but at full throttle the

power drop is 2-3 hp. The engine produces much less torque as compared to 4-1(with and w/o

muffler) configuration (figure 3). Back pressure is reduced in 2-1 systems but the scavenging is

poor which leads to power drop. At higher speeds, the frequency of exhaust pulses also

increases, which means lesser interval between the exhaust evacuation of the two consecutive

cylinders. The exiting pulse from one cylinder aids in creating vacuum from other cylinders

exhaust gases, hence better scavenging. This phenomenon is less evident at lower engine speeds.

216

217

218

219

220

221

222

223

224

225

4-1 w/o Muffler 4-1 with muffler 2-1 configuration Independent pipies

Torque

0

20

40

60

80

100

4023 4393 4466 4608 4597 4869 5627 5772 5772

Power (Hp)


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Exhaust System with Independent Manifolds

Independent manifolds are known for minimum back pressure as the exhaust gases flow straight

through independent pipes. However, as compared to the above configurations the flow velocity

is also compromised. The faster an exhaust pulse moves, the better it can scavenge out all of the

spent gasses during valve overlap. The general idea is a fast moving pulse creates a low pressure

area behind it. This low pressure area acts as a vacuum and draws along the air behind it. Figure

5 shows how much power is compromised due to the effect of reduced flow velocities on

scavenging.

Figure 5

Conclusions

Figure 6

0

20

40

60

80

100

4000 4446 4952 5489 5632

Power (Hp)

0

50

100

150

200

4-1 w/o Muffler 4-1 with muffler 2-1 configuration Independent pipies

Torque

Power*2

BSFC*1000


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Influence of different exhaust configurations on engine performance is shown in figure 6. 4-1

system without muffler and tail pipe turns out to be best choice for maximum power and torque

output. In addition to that, BSFC value for 4-1 is minimum, indicating that the system is most

fuel efficient.

Exhaust with independent manifolds in least efficient in terms of torque and power, results show

that 4-1 system with muffler can make more power than independent exhaust manifolds.

2-1 system is least efficient in terms of fuel efficiency and produces lowest power and torque

over lower rpm range.

effect of different exhaust configurations on engine performance parameters

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