What does an engine compression brake do?

• A. Increases acceleration
• B. Reduces engine compression
• C. Engages only when parked
• D. An engine compression brake uses the engine and transmission to impede forward motion by compression of the engine.

Answer: (D)

An engine compression brake slows the vehicle by using the engine itself as the braking mechanism. When you activate it, the system creates extra resistance in the engine during the compression cycle, which builds back pressure through the drivetrain. That resistance converts the vehicle’s kinetic energy into heat, helping to slow the wheels without relying on the regular service brakes. It’s commonly used on heavy diesel trucks to control speed on descents and reduce brake wear. It doesn’t make you go faster, isn’t a parking feature, and it doesn’t reduce compression—the braking effect comes from the engine’s compression creating resistance.