Engine Exhaust System:
The engine exhaust system is one of the critical vehicle systems dealing with vehicle emissions. First, the exhaust system collects the exhaust gases from the engine cylinders. Then, it guides them through the central exhaust passage. Finally, it releases them into the atmosphere with minimum power loss, noise, vibrations, and heat transfer to the car’s body.
It consists of an exhaust manifold mounted on the cylinder block or head, catalytic converter, and tailpipe/silencer. The exhaust manifold is a set of pipes that carry exhaust gases from the cylinder head to the silencer. Generally, manufacturers make it of cast iron to endure the heat of the exhaust gases. They employ a two-in-one type manifold for a smoother flow of exhaust gases instead of an integral type. ‘V’ type engines use dual-exhaust systems on each side of the engine. Each unit has an exhaust manifold, muffler, and also pipes.
Some engines have only one exhaust manifold with one muffler and one tailpipe. As a result, the engine exhaust system must create a minimum resistance to exhaust gas flow to prevent power loss. Such resistance creates a back pressure in the exhaust system, which opposes the cylinders’ working pressure.
Furthermore, manufacturers design the exhaust system in such a way that it minimizes the restriction on the flow of exhaust gases. For this purpose, manufacturers provide cast-rib deflectors or dividers inside the exhaust manifold. The dividers smoothly guide the exhaust gases towards the tailpipes/outlet.
Engine Exhaust System Layout:
In practice, manufacturers primarily design the exhaust system and manifold as per the engine–chassis layout. It is because the front suspension and steering system limit the space available for the exhaust manifolds.
Especially, modern-day compact cars have little space in the engine compartment. Hence, manufacturers fit very tiny and compact exhaust systems and manifolds in these cars. In addition, manufacturers design exhaust manifolds to avoid overlapping exhaust strokes as much as possible. Thus, it keeps the back-pressure to a minimum.
Manufacturers often do this by dividing the manifold into two or more branches. Thus, it prevents any two cylinders from releasing exhaust gases into the same branch simultaneously. Increasing the capacity of the exhaust manifold and including large-radius bends into the design eliminates any restrictions to flow. It also reduces back-pressure.
Heat Control Valve:
Earlier generation exhaust systems provided a heat tube to supply the heat to the carburetor’s built-in automatic choke unit. The center portion of the exhaust manifold often connects to the intake manifold thru’ a heat trap and damper mechanism. The exhaust system controls the damper thermostatically to deflect the hot exhaust gases upward and around the intake manifold when the engine’s temperature is below the predetermined value.
The heat control is automatic. When the engine is cold, all the exhaust gases are left. As the engine thoroughly warms up, it bypasses none of the gas around the intake manifold. The heat control valve is offset, being longer on one side than the other. This design permits the exhaust gases’ pressure to force the valve wide open against the spring and counterweight pressure when operating under full-load conditions. In addition, the counterweight on the valve shaft prevents it from fluttering.
The exhaust system & manifold on modern-day engines often have a provision for a thermostatic heat riser valve. This valve directs the exhaust gases through the inlet manifold cross-over passage to pre-heat the incoming air. Exhaust gases externally heat the incoming air-fuel mixture. Exhaust gases flow thru’ specially designed paths inside the intake manifold and heat them.
Furthermore, the exhaust system thermostatically controls and opens the exhaust gas control valve. The gas control valve deflects the gases into the passages during the engine’s warm-up period. The valve is closed when the engine is cold. So, it blocks the exhaust gases from entering the exhaust pipe. Instead, it forces them thru’ the intake manifold’s passages to reach the exhaust manifold and pipe on the opposite bank.
As the engine warms up, more gases pass directly into the exhaust pipe, and less go through the intake manifold passages. In addition, pre-heated air helps better mix with the fuel for complete combustion of the air-fuel mixture.