Engine braking occurs when the retarding forces within an engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes. The term is often confused with several other types of braking, most notably compression-release braking or "jake braking" which uses a different mechanism. Traffic regulations in many countries require trucks to always drive with an engaged gear, which in turn provides a certain amount of engine braking (viscous losses to the engine oil and air pumped through the engine and friction losses to the cylinder walls and bearings) when no accelerator pedal is applied. Type Gasoline engines The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This causes fuel injection to cease and the throttle valve to close almost completely, greatly restricting forced airflow from, for example, a turbocharger. The restriction causes a strong manifold vacuum which the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect. This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivetrain to engage with the engine. Engine braking is a viable method of controlling the speed at which a vehicle travels downhill. By shifting to a lower gear in a manual transmission, or selecting the "low gear" mode on an automatic transmission, engine braking reduces the need to repeatedly apply the foot brake, lowering the risk of the brakes overheating. While some of the braking force is produced due to friction in the drive train, this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction. On an automatic transmission, engine braking often spontaneously increases the engine RPM, causing a sudden revving to occur even without applying the accelerator pedal. Diesel engines Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold. In heavy vehicles the engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system and to help avoid overheating the brakes. In its simplest form this consists of a butterfly valve that stops the exhaust flow. This is referred to as an exhaust