Most of us are probably familiar with the term engine braking, but have you ever stopped to think just how it works? Some of you that are a little more mechanically minded probably have a pretty good idea, but there are actually 3 different methods of engine braking that can be used depending on the vehicle. Most commonly, engine braking is used on larger transport trucks, or vehicles that would otherwise be hard to slow down with rotary breaks. However, there is a method of engine braking in gasoline powered engines, as well as two other methods for diesel engines. Essentially, engine braking is using the retarding forces inside an engine (friction, compression, etc.) to slow down the movement of the rotor, and resultingly, the car. You can check out a super in-depth explanation of all of the different kinds of engine braking in the video below.
The first method to discuss is gasoline engine braking, which utilizes the formation of a vacuum to slow down the vehicle. When you let your foot off of the throttle, the throttle body closes, meaning that as the pistons retract into the cylinders, a small vacuum is formed, creating forces that inhibit the continued motion of the car. The first diesel method acts the exact opposite of creating a vacuum, rather it creates excess compression in the cylinder. As the piston moves up to push the exhaust out, the exhaust valve will close creating back pressure in the cylinder, thus slowing the car down.
The last diesel method of engine braking is a little more complicated, and it is dubbed the “jake brake” after the company that created it. The video above explains this method the best, but essentially you are releasing some of the compression built up in the engine so it is not as effective in moving the pistons. You are “wasting energy” in the combustion process by simply combusting gasses, then immediately releasing them, resulting in a negative net work. There are quite a few valves and solenoids used to make each of these systems work properly, and all of them are timed and regulated perfectly to make everything work.
[Image Source: Wikimedia]
Hopefully, now you understand engine braking a little bit better, and you can impress your non-engineer friends by your knowledge of how cars are able to slow down without pressing on their brakes.