A computerized emission control system commonly uses engine sensors, a three-way catalytic converter an on-board computer and a fuel injection system.

FIg. 31-31 illustrates the operation of a digital fuel injection system using a three-way catalytic converter and an exhaust gas sensor. Study how the engine sensors and exhaust sensor feed electrical information to the computer. The computer analyses this information and adjusts the throttle body air-fuel mixture for maximum efficiency. Also, note how HC, CO, and NOx are lowest at a stoichiometric (theoretically perfect) fuel mixture

Fig. 31-32 shows diagrams of how a computer (ECU) controls the fuel and emission control systems. Diagram A illustrates how the computer receives data from the engine sensors and the exhaust (oxygen) sensor. It can then control the fuel injection system, ignition timing, EGR valve and charcoal canister purge, diagram B.

Exhaust back pressure

Exhaust back pressure is the amount of pressure developed in the exhaust system when the engine is running. High back pressure reduces engine power. A well designed exhaust system should have LOW back pressure.

The restriction of the exhaust pipes, catalytic converter, and muffler contribute to exhaust backpressure. Larger pipes and a free-flowing muffler, for example would reduce backpressure.

Single and dual exhaust systems

A single exhaust system has one path for exhaust flow through the system. Typically, it has only one engine pipe, main catalytic converter, muffler, and tailpipe. The most common type, it is used from the smallest four-cylinder engines, on up to large V-8 engines.

A dual exhaust system has two separate exhaust paths to reduce backpressure. It is two single exhaust systems combined into one. A dual exhaust system is sometimes used on high performance cars with large V -6 or V -8 engines. It allows the engine to “breathe” better at high rpm.

A crossover pipe normally connects the right and left side engine pipes to equalize back pressure in a dual system. This also increases engine power slightly.

Exhaust manifold heat valve (heat riser)

An exhaust manifold heat valve allows hot exhaust gases to flow into the intake manifold to aid cold weather starting. Look at fig. 22-3.

A butterfly valve may located in the outlet of the exhaust manifold. A heat sensitive spring or a vacuum diaphragm and temperature sensing vacuum switch may operate the valve.

When the engine is cold, the heat valve is closed. This increases EXHAUST BACK PRESSURE. Hot blow into an exhaust passage in the intake manifold, Fig. 22-4. This warms the floor of the intake manifold to hasten fuel vaporization. The heat valve opens as the engine warms up.

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