Intake and Exhaust Tuning

By proper choice of the length of the intake and exhaust piping, the performance of internal combustion engines can be increased. Inlet and exhaust valve opening and closing creates a compressible flow process in which pressure waves flow back and forth through the inlet and exhaust system. The appropriate pipe length can be estimated through solution of the compressible flow equations. Alternatively, a number of heuristic pipe length equations have been developed.

A pressure wave is created when an intake or exhaust valve is opened. The wave propagates through the the fluid in the pipe at the speed of sound c. When this wave encounters a change in cross sectional area, such as the end of the pipe, a wave of opposite sign will be reflected from the end of the pipe. Based on the time it takes for this wave to return back to the valve, the length of the pipe can be determined.

Intake Valve

For example,when the inlet valve opens, a rarefaction wave is sent upstream from the valve. When this wave encounters a change in area such as the intake manifold, a compression wave is generated and sent downstream back to the inlet valve. This compression wave increases the local density of the inlet flow, a process called the "ram effect".

Experimentally, it has been found that a significant gain in volumetric efficiency is attained when the reflected compression wave returns when the piston is at a crank angle of 90o. At this point the piston velocity is maximum. Matching the time it takes for the wave to return with the characteristic piston time, the required length of the pipe can be found.

The velocity of the wave is given by:

velocity = distance / time

The distance is twice the pipe length, 2L. The time is found from the engine speed.

Solving for the pipe length, L, results in:

The term c is the speed of sound. It is dependent on the temperature T (in degrees Kelvin) of the incoming flow, the air ideal gas constant R, and the specific heat ratio k. The speed of sound is given by the following equation:

Exhaust Valve

When the exhaust valve opens, a compression wave is sent downstream and reflects back as a rarefaction wave when an opening in the exhaust system is encountered. Experimentally it has been found that the optimum position of the piston when the wave returns is 120o. At this position the remaining exhaust gas can be scavenged from the combustion chamber. The required length of the exhaust pipe can then be determined.

The intake and exhaust pipe length can be determined from the Intake and Exhaust Pipe Length Applet.