Throttle - Internal Combustion Engines

Internal Combustion Engines

In a gasoline internal combustion engine, the throttle is a valve that directly regulates the amount of air entering the engine, indirectly controlling the charge (fuel + air) burned on each cycle due to the fuel-injector or carburetor maintaining a relatively constant fuel/air ratio. In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle pedal or accelerator.

The throttle is typically a butterfly valve. In a fuel-injected engine, the throttle valve is placed on the entrance of the intake manifold, or housed in the throttle body. In a carbureted engine, it is found in the carburetor.

When a throttle is wide open, the intake manifold is usually at ambient atmospheric pressure. When the throttle is partially closed, a manifold vacuum develops as the intake drops below ambient pressure.

Usually the throttle valve is controlled with a throttle pedal or lever via a direct mechanical linkage. In vehicles with electronic throttle control, the manual throttle control sends a signal to the Engine Control Unit (ECU), which then directly controls the position of the throttle valve. This means that the operator does not have direct control over the throttle valve; the ECU can finely control the valve in order to reduce emissions or maximize performance.

In a reciprocating-engine aircraft, the throttle control is usually a hand-operated lever or knob. It controls the engine power, which may or may not reflect in a change of RPM, depending on the propeller installation (fixed-pitch or constant speed).

The power output of a diesel engine is controlled by regulating the quantity of fuel that is injected into the cylinder. Because the engines do not need to control air volumes, they lack a butterfly valve in the intake tract. An exception to this generalization is newer diesel engines meeting stricter emissions standards, where a throttle is used to generate intake manifold vacuum, thereby allowing the introduction of exhaust gas (see EGR) to lower combustion temperatures and thereby minimize NOx production.

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