Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression braking is a common legal term for the same mechanism. Large trucks use a device called a jake brake to increase the effectiveness of engine braking.
Most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to air charge during compression to directly ignite the mixture when the fuel is injected.
Regardless of engine type, compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the flywheel. So the engine ends up converting energy that was formerly kinetic energy of the vehicle into heat in the fuel-air mixture. These hot gasses are exhausted from the vehicle and heat is transferred from engine components to the air.
The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gasses are ejected from the vehicle very quickly and the gasses also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.
This engine heat is taken away by the engine's integrated cooling system: usually a liquid circulation system and a radiator. Disc or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they use their mass to retain heat without producing temperatures that would deform and damage the brakes.
Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have less leverage on the engine. This is what allows cars to slow down using their relatively flimsy engine parts. The engine maintains a high rotational speed to dissipate a lot of power without forcing too much strain on the engine.
The jake brake is required in large diesel vehicles because the rate of conversion of mechanical energy into waste thermal energy is low compared to the mechanical returns to kinetic energy from the air-spring effect in the engine.
Engine braking is always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged. This is often called engine drag.
Active use of engine braking (shifting into a lower gear) is only advantageous when it is necessary to control speed while driving down very steep and long slopes. It should be applied before regular disk or drum brakes have been used, leaving the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract the acceleration due to gravity.
Improper engine braking technique can cause the wheels to skid, especially on slippery surfaces such as ice or snow, as a result of too much deceleration. As in a skid caused by over-braking, the car will not regain traction until the wheels are allowed to turn more quickly; the driver must reduce engine braking (shifting back up) to regain traction.
Compression braking, a form of engine braking, produces extreme amounts of noise pollution if there is no muffler on the exhaust system of the engine. Use of a jake brake produces similar effects, due to release of compressed gasses, but the mechanism is distinct from regular car engine braking. Anecdotally, it sounds similar to a jackhammer, however the loudness is between 10-20 times the perceived loudness of a jackhammer. Numerous cities, municipalities, states, and provinces have banned the use of unmuffled compression brakes.
This is often a source of dissatisfaction to professional truck drivers, who may believe that municipalities are taking advantage of them due to their transient nature, and thus disregard the law, believing that they will be gone before a complaint can be lodged.