When is the braking efficiency of any engine brake usually at its highest?

The braking efficiency of an engine brake is typically highest at peak torque RPM. This is because the engine is operating at its most effective point in terms of power output and mechanical efficiency. At peak torque RPM, the engine produces maximum torque, which allows for greater energy absorption by the engine brake system.

During this phase, the engine can utilize the most effective braking force due to the higher combustion pressures and air-fuel mixture being processed, which contributes to improved braking efficiency. The braking capability is optimized since the engine is fully loaded and functioning in a range that maximizes its braking power.

While the high engine rpm can certainly provide strong braking effects, it is not as efficient as when the engine operates at peak torque, where the conditions are ideal for propulsion and retardation simultaneously. In contrast, both idle RPMs and a range within torque rise may not generate significant and effective braking force as compared to peak torque, which firmly establishes that it is the ideal condition for maximum braking efficiency.