First we have to understand some fundamental key points.
* A gasoline engine uses a throttle body which throttles the amount of air which can enter the engine. In modern fuel injected engines, the amount of air coming in is read by a Mass Air Flow sensor and the amount of fuel injected into either the throttle body (Throttle Body Injection), intake manifold (multi-port fuel injection), or cylinders (direct injection) is adjusted. In older (carbureted) engines, this was done via the venturi effect (essentially more flow draws more fuel, watch Smarter Everyday’s episode on carbs if you need a visual). Overall process is simple, execution is not (there is a lot of voodoo that goes into how carburetors work).
* On a gas engine when you let off the throttle pedal (which closes the throttle, really returns it to the idle state) you create a vacuum between which the cylinders have to fight on the intake stroke thus slowing the engine down. Many newer vehicles will actually stop delivering fuel when the throttle is closed in a downhill driving situation to conserve fuel.
* Diesel engines don’t have throttles. Engine speed is metered according to the amount of fuel injected into the cylinders (all diesel engines use some form of direct injection). Without the throttle, there is no vacuum to pull so the engine has to be slowed down in a different manner.
* Compression release method, commonly referred to as the Jake brake for the company that initially designed it (Jacobs Vehicle Systems), works by cracking the exhaust valve during the compression stroke which in turn slows down the engine.
* Exhaust brake method, uses what is essentially a throttle body on the exhaust to restrict the flow of exhaust gases (thus slowing the engine down on the exhaust stroke). Most exhaust brake methods are done via a variable geometry turbocharger these days but is still a common upgrade for older diesel engines with fixed geometry turbos which don’t have compression release systems available (such as the Cummins B series, International/Powerstroke light duty engines, and Duramax engines).
* Worth noting that at one point one of the US pickup manufacturers was playing with variable valve timing to simulate the function of compression release braking in conjunction with exhaust braking via the VGT but I don’t know if it ever made into production.
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