eli5: How does engine braking in cars work?


Like, what is the mechanism that slows the car considering the hydraulic brakes aren’t involved at all? How does it vary by transmission type (manual, regular auto, CVT)?

In: 14

An engine is just an energy system. If you put in gasoline and air and spark the mix, it outputs energy into the crankshaft and it spins the wheels.

If you don’t put in any of that (i.e. you let go of the throttle), the system starts requiring energy instead mainly due to the throttle closing and there being no air pumped into the engine anymore. The engine has to spend energy to work against the vacuum that is created, which slows it down.

Just imagine an engine cylinder having to do a full cycle without any air. It’s the same as if you plugged the end of a tube syringe and tried to pull the plunger: it takes effort. That energy is fed back from the wheels, which slows them down.

The engine continues doing the work of intaking air then pushing out exhaust, it just skips the step where it adds fuel into the engine. Pulling air in and pushing it out of the engine cylinders is still a lot of work to do, especially given that its happening 10-50 times a second, thats a lot of air being pushes and that creates a good amount of resistance.

It’s all in the engine. The gear/transmission method doesn’t matter at all, though something with manual control can be good to force lower gears to get a stronger engine braking effect. Most automatics try to shift to a higher gear to minimize engine braking and letting you get the most coasting you can. A full manual, or an automatic with a manual selection mode, helps immensely.

In a typical gas engine, without fuel, there is very little air going into the intake of the cylinder, so as the cylinder moves down on the “intake” stroke there’s a near-vacuum on it. The cylinder goes through the compression+combustion strokes without accomplishing anything since there’s no fuel, then the “exhaust” stroke tries pushing upwards on the cylinder to force out all the burnt fuel. There is none, but this step is done against a full pressure air source on the exhaust pipe. So vacuum on the intake, pressure on the exhaust, is a net negative pressure on the engine making it slow down.

A diesel engine is built differently, so this strategy doesn’t work. Instead there’s what’s called “Jake brakes”, which has a mechanism open the exhaust valve at the start of the “combustion” phase. At this moment the engine has spent energy compressing the air inside the cylinder (with no fuel) which took some decent energy. Normally the “combustion” stroke would have the cylinder move back again, decompressing the air and balancing out the energy loss… but instead, we open the exhaust valve and let the air out immediately in a burst of compressed air so it can’t push the cylinder down again. That’s why trucks doing engine braking are making this loud noise. But it works.

So we just connect the engine to the wheels in spite of the fact we don’t want engine power, and let the engine braking effect slow us down instead.

How this works is subtly different in gasoline engines v.s. diesels.

In gasoline cars, if you want to engine brake, you reduce the gear which, when you let out the clutch causes the engine to rotate faster. This does a couple of things. Because most of the rotating parts in a car engine are lubricated with oil, at high RPMs they experience significant fluid friction. Particularly the transmissions gears which are partially submerged in oil, the crackshaft which also sloshes around in a pool of oil, the pistons are also lubricated by a thing Layer of oil on the cylinder walls which results in hydrodynamic drag it high rpm.

The oil pump and coolant pump also produce significantly more drag on the drivetrain at high RPMs for physics reasons beyond the scope of this post.

Lastly, during engine braking the throttle is closed. This results in an increased throttle vacuum whichnthe engine has to fight against during each intake stroke. Usually the crackcase space under the piston is at slightly higher pressure than atmospheric pressure, which pushes up against the cylinders durning the intake stroke.

Diesel engines don’t have a throttle valve in the intake, because it wouldn’t benefit them and would impair starting. However this means that since they don’t work against a throttle vacuum they produce less parasitic drag in this scenario by comparison. Although fluid friction losses are still in play, like I described earlier.

For this reason many larger diesel powered trucks are equipped with either an Exhaust Brake or a Compression brake. The exhaust brake is a throttle valve, but it’s placed on the exhaust manifold instead. This causes the exhaust to be compressed which creates resistance during the exhaust stroke.