With the massive amount of technology going into controlling the deceleration of a vehicle, a form of braking that is often overlooked is engine braking. Engine braking is a way of controlling the air flow through the engine that results in enhanced engine slow-up. In this Engineering Explained video, we look at the different ways that engine braking is achieved.

Gasoline Engine Braking

The concept of engine braking in a gasoline engine is relatively simplistic. This effect occurs when the throttle body is closed and causes a partial vacuum inside the intake manifold restricting the air flow, that limits the effectiveness of the intake stroke, starving the engine of air. This dynamic creates a drag on the engine that results in enhanced engine deceleration. This effect happens on virtually every throttle body engine design.

Diesel Exhaust Braking

The next form of engine braking is the exhaust brake; this is a mechanism that creates a significant restriction in the exhaust system. Creating a substantial amount of exhaust back pressure that effectively chokes the engine exhaust cycle. This gag on the engine causes a retard of the engine rpm, as the piston fights to displace the spent air-fuel mixture, and results in a similar braking effect as a gasoline engine.

This also limits the amount of fresh air to enter the cylinder on the intake cycle, further limiting the power output of the engine. This type of braking system is so efficient that is can successfully decrease the speed of a densely loaded carrier on a steep decline without ever relying on its service brakes. A more sophisticated version of the exhaust brake that is designed to control the back pressure is called an exhaust pressure modulation (EPM) that leads to improved braking performance across a wider range of engine speeds.

Compression Release Engine Brake

Another type of diesel braking system is the engine compression release brake, commonly referred to as a “Jake Brake.” This design utilizes a set of hydraulic pistons, a solenoid, and a check valve to modulate the exhaust valve outside of its normal duty cycle.

Durning vehicle deceleration the engine compression release brake springs into action, being mostly used for steep hill downgrades. As the piston nears top-dead-center on the compression stroke the exhaust valves begin to open, effectively killing the ignition of any remaining air-fuel mixture. Without this ignition cycle, the engine essentially becomes an air compressor resulting in a massive amount of drag on the engine that slows the vehicle. The added advantage of a Jake Brake is that the operator is able to select the number of cylinders that are influenced by its operation, allowing for advanced engine braking management.

Heavily laden vehicles can also be equipped with both the compression release and exhaust brakes for added braking performance. Utilizing these types of braking systems can result in improved brake pad life and a controlled decent when hauling a heavy load.