When are exhaust brakes most efficient?

• A. At high engine speed, while in low gears
• B. While travelling at a constant speed on a motorway
• C. During stop/start town work
• D. At low engine speed, while in high gears

Answer: (A)

The exhaust brake works by increasing back pressure in the exhaust system, which slows the engine and converts some of the vehicle’s kinetic energy into heat. How strong this braking is depends on how much exhaust gas is flowing, and that flow is greatest when the engine is turning fast. In a low gear the engine stays at higher revolutions for a given road speed, so the exhaust gas flow is high and the back pressure is substantial. That combination gives the most effective braking from the exhaust brake.

In contrast, traveling on a motorway at a steady pace usually keeps engine speeds moderate, so the exhaust flow isn’t as high and the braking effect is weaker. Stop-and-start town work involves frequent changes in speed and gear, so the engine isn’t consistently at high revs, which reduces effectiveness. And in a high gear with low engine speed, the exhaust flow is limited, producing only a small back pressure and minimal braking.