Exhaust

Note:  This content is directly from our Road Machines Educational CD, available for purchase and download instantly in our store.

Manifold

The exhaust manifold (1) is a collection point for exhaust gases that are exiting the exhaust ports in the cylinder head. It is usually made of cast iron, however, lightweight steel tubing or stainless steel are also used. An oxygen sensor (2), or engine exhaust sensor, is placed near the outlet to measure the oxygen content of the exhaust gas. An exhaust manifold heat valve may also be located at the outlet.

Oxygen Sensor

The oxygen sensor measures the amount of oxygen in the exhaust gas. Low oxygen content indicates a rich air-fuel mixture; a high oxygen content indicates a lean mixture. The sensor's voltage output is proportional to the oxygen content. The electrical signal is processed by the engine's computer with other inputs to control the air-fuel ratio.

Heat Valve

An exhaust manifold heat valve is designed to direct exhaust gas to channels in the intake manifold. The heated gas warms the intake manifold, aiding fuel vaporization when the engine is cold. It is operated by a heat sensitive spring or a temperature sensing vacuum switch and vacuum diaphragm. It is closed when the engine is cold causing increased exhaust back pressure. This forces hot gases into the intake warming channels.