Exhaust Manifold System

The primary purpose of the exhaust manifold is to provide a route for the gases produced by the combustion process to pass to the exhaust system proper. However, the exhaust manifold also assists in another, and much more fundamental, operation: cylinder scavenging. It is possibly the most important device in the process of scavenging spent gases from the combustion area of the engine.

This scavenging is achieved by utilizing the kinetic energy of the outward-going exhaust gases. These are used to set up a compression wave, which is followed by an expan­sion wave, the effect of which is to cause the gas pressure to be reduced to a depression within the region of the exhaust port. This operates in conjunction with the valve overlap period, so that the outward-bound exhaust gases from one cylinder can be utilized to assist the induction of another cylinder. Again, this subject is quite complex because each engine configuration requires separate consideration, and much work using airflow rigs is performed to establish exhaust manifold configuration, long before the engine will see a test bed run.

Instrumentation of the Exhaust System

Instrumentation of the exhaust manifold will be similar to that for the induction manifold. An important point here for consideration is that when individual branched exhaust manifolds require instrumentation, great care must be taken to site the sending devices at the same point (e. g., in relation to distance from the exhaust port) for each branch. The test technician also should be aware that significant changes in measured tempera­tures and pressures will occur as distance from the exhaust port increases. Therefore, it is important to ensure repeatability so that one always positions the thermocouples in the same relative position.

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