How does a truck engine brake?

There are two main ways a truck engine can help slow down the vehicle, and they differ based on the engine type:

1. Engine Braking (for gasoline engines):

  • When you lift your foot off the accelerator pedal in a vehicle with a gasoline engine, the throttle body closes, restricting airflow into the engine.
  • This creates a vacuum effect within the intake manifold and cylinders. As the pistons move down during the intake stroke, they’re trying to pull in air, but the restricted airflow creates resistance.
  • This resistance helps to slow down the engine’s rotation, which in turn, slows down the entire drivetrain connected to the wheels. This is a form of engine braking present in most gasoline-powered vehicles, including trucks.

2. Jake Brake (for diesel engines):

  • Many heavy-duty trucks equipped with diesel engines have a device called a Jake Brake, also known as an engine retarder.
  • Unlike gasoline engines, diesel engines don’t have throttle bodies to control airflow. Instead, the Jake Brake utilizes the engine’s exhaust valves to create a braking effect.
  • When activated, the Jake Brake opens the exhaust valves during the compression stroke of the engine cycle. This allows some of the compressed air inside the cylinder to escape prematurely.
  • Releasing the compressed air reduces the pressure inside the cylinder, counteracting the force pushing the piston down during the power stroke. This oppositional force helps to slow down the engine’s rotation and consequently, the entire drivetrain.
  • The Jake Brake creates a distinctive growling or rattling sound when engaged, which is often audible from outside the truck.

Key points to remember:

  • Engine braking in gasoline engines utilizes a vacuum effect to slow down the engine rotation.
  • Jake Brakes in diesel engines use premature exhaust valve opening to create an opposing force that slows down the engine.
  • Both methods ultimately help to slow down the vehicle by reducing the engine’s rotation and transferring that resistance through the drivetrain to the wheels.

It’s important to note that engine braking alone is not sufficient to stop a vehicle completely. It should always be used in conjunction with the regular friction brakes for safe and effective stopping.