Principles of engine braking
When a vehicle is in gear, the crankshaft of the engine and the wheels are mechanically connected. When turning force is applied to the crankshaft by the engine, the transmission applies that force to the wheels. Conversely, if force is applied to the wheels, the transmission applies that force to the crankshaft and through that to the engine. This is the principle behind 'push starting' a vehicle. Force is applied to the wheels by pushing the vehicle; when the vehicle has some momentum, engaging a gear turns over the engine, enabling it to fire without the assistance of a battery or starter motor.
Engine braking uses this same principle. If you take your foot off the accelerator, so that the engine ceases to apply force to the wheels, the engine begins to act as a brake on the vehicle. As the engine slows down, the momentum of the wheels will keep it turning over. The compression stroke of the engine will soak up some of this driving energy, slowing the vehicle down. Engaging a lower gear will slow the vehicle down more quickly, as the engine will be turned over more rapidly by the movement of the wheels.
Uses of engine braking
Slowing a vehicle down a steep or very long decline just using normal wheel brakes alone can lead to heat build up and potentially dangerous brake fade. It is good practice to slow the vehicle initially with the brakes, then to shift the engine into lower gears and let the inertia of the engine assist in slowing the vehicle, and prevent it from accelerating again. This is safer, because engine braking can be sustained for longer without reducing its effectiveness, and it reduces the wear and tear on the wheel brakes.
'Jake' brakes
In heavy diesel vehicles, engine braking is less effective because more of the compression energy in the cylinders is returned to the crankshaft after the piston reaches top dead center. A 'jake' brake uses an extra lobe on the camshaft to control an auxiliary exhaust valve at the top of each cylinder. When engaged, the jake brake releases the compression stroke pressure before it can be transmitted back to the wheels, thus increasing the braking effectiveness of the engine. The disadvantage with this form of braking is that it is very noisy, producing a chattering, machine-gun sound, usually requiring additional muffling.
