Throttle

A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal. What is often termed a throttle (in an aviation context) is also called a thrust lever, particularly for jet engine powered aircraft. For a steam locomotive, the valve which controls the steam is known as the regulator. In an internal combustion engine, the throttle is a means of controlling an engine's power by regulating the amount of fuel or air entering the engine. In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle, accelerator, or gas pedal. For a gasoline engine, the throttle most commonly regulates the amount of air and fuel allowed to enter the engine. Recently, for a GDI engine, the throttle regulates the amount of air allowed to enter the engine. The throttle of a diesel, when present, regulates the air flow into the engine. Historically, the throttle pedal or lever acts via a direct mechanical linkage. The butterfly valve of the throttle is operated by means of an arm piece, loaded by a spring. This arm is usually directly linked to the accelerator cable, and operates in accordance with the driver, who hits it. The further the pedal is pushed, the wider the throttle valve opens. Modern engines of both types (gas and diesel) are commonly drive-by-wire systems where sensors monitor the driver controls and in response a computerized system controls the flow of fuel and air. This means that the operator does not have direct control over the flow of fuel and air; the Engine Control Unit (ECU) can achieve better control in order to reduce emissions, maximize performance and adjust the engine idle to make a cold engine warm up faster or to account for eventual additional engine loads such as running air conditioning compressors in order to avoid engine stalls. The throttle on a gasoline engine 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. The power output of a diesel engine is controlled by regulating the quantity of fuel that is injected into the cylinder. Because diesel engines do not need to control air volumes, they usually lack a butterfly valve in the intake tract. An exception to this generalization is newer diesel engines meeting stricter emissions standards, where such a valve 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. In a reciprocating engine aircraft, the throttle control is usually a hand-operated lever or knob. It controls the engine power output, which may or may not reflect in a change of RPM, depending on the propeller installation (fixed-pitch or constant speed). Some modern internal combustion engines (such as some BMW engines) do not use a traditional throttle, instead relying on their variable intake valve timing system to regulate the airflow into the cylinders, although the end result is the same, albeit with less pumping losses.