Engine Braking - Diesel Engines

Diesel Engines

Diesel engines do not have engine braking in the above sense. Unlike petrol engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects.

However, some alternative mechanisms which diesel engines use that replace or simulate real engine braking include:

• A compression release brake, or jake brake, this is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere.

Normally during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel. (This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm.) Having lost the energy stored within the compressed air, the engine is then made to pull the piston down (which sucks new air into the cylinder), and then travel upward again, compressing the new volume of air, which will again be released to the atmosphere after having been compressed. The engine loses energy.

This type of brake is banned or restricted in many locations where people live because it creates a sound loud enough to disturb the peace, including waking people at night. It is very effective however, and creates immense amounts of braking force which significantly extends friction brake life - A 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking force.

• An exhaust brake - This works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one.

A mechanism related to the exhaust brake is back-pressure from a turbocharger. In turbo diesels with variable-vane turbos, the vanes will close when the accelerator is released, which creates a back-pressure braking effect similar to an exhaust brake. Even fixed turbos, especially larger ones, will cause some back-pressure when they are below the turbo threshold (albeit not to the same extent as a variable turbo) and contribute to the braking effect.

Modern diesel engines have engine braking characteristics more akin to petrol engines. This is due to additional devices to allow them comply with emissions regulations. Two in particular cause significant engine braking:

1) Particulate filter

This device filters out soot particles before they exit the exhaust, but it creates a large obstruction in the exhaust path. This causes considerable backpressure, much more than from the turbo charger mentioned above

2) EGR (Exhaust Gas Recirculator)

This device sucks exhaust gas back into the air intake and is usually controlled by a throttle which, depending on the design, can cause a restriction effect similar to that in petrol engines.