Engine Exhaust System:
The engine exhaust system is one of the important vehicle systems as it deals with vehicle emission. The exhaust system collects the exhaust gases from the engine cylinders. It guides them through the central exhaust passage and releases them into the atmosphere with minimum power loss, noise, vibrations, and transfer of heat to the car’s body.
It consists of exhaust manifold mounted on the cylinder block or head, catalytic converter, and tailpipe/silencer. Of these, the exhaust manifold is a set of pipes that carry exhaust gases from the cylinder head to the silencer. Generally, it is made of cast iron so that it can endure the heat of the exhaust gases. For a smoother flow of exhaust gases, manufacturers employ two-in-one type manifold as compared to an integral type. ‘V’ type engines employ dual-exhaust systems on each side of the engine. Each unit has an exhaust manifold, muffler, and also the pipes.
Some engines have only one exhaust manifold with one muffler and one tailpipe. To prevent power loss, the engine exhaust system must create a minimum resistance to the flow of exhaust gases. Such resistance creates a back pressure in the exhaust system which opposes the working pressure of the cylinders.
Furthermore, manufacturers design the exhaust system in such a way that it minimizes the restriction to the flow of exhaust gases. For this purpose, manufacturers provide cast-rib deflectors or dividers inside the exhaust manifold. Dividers smoothly guide the exhaust gases towards the tailpipes/outlet.
Engine Exhaust System Layout:
In practice, manufacturers especially design the exhaust system and manifold as per the engine-chassis layout. This is because the front suspension and steering system limit the space available to for in 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 manifold are in these cars. Exhaust manifolds are designed to avoid as much as possible the overlapping of exhaust strokes which keeps the back-pressure to a minimum.
Manufacturers often do this by dividing the manifold into two or more branches. This avoids any two cylinders from releasing exhaust gases into the same branch at the same time. By 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 built-in automatic choke unit of the carburetor. 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 temperature of the engine is below the predetermined value.
The heat control is automatic. When the engine is cold, all the exhaust gases are at the left. As the engine fully warms up, none of the gas is bypassed around the intake manifold. The heat control valve is offset, being longer on one side than the other. This design permits the pressure of the exhaust gases to force the valve wide open against the pressure of the spring and counterweight when the engine is operating under full-load conditions. The counterweight on the valve shaft prevents it from fluttering.
Exhaust system & manifold on modern-day engines often have a provision for 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 passages 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. It forces them thru’ the passages in the intake manifold to reach the exhaust manifold and pipe on the opposite bank.
As the engine warms up, it allows more gases to pass directly into the exhaust pipe and less go through the intake manifold passages. Pre-heated air helps in better mixing with the fuel for complete combustion of the air-fuel mixture.