When does a diesel engine turbocharger achieve its highest rotational speeds?

A diesel engine turbocharger achieves its highest rotational speeds primarily when rejected heat is greatest. This is because turbochargers operate on the energy of exhaust gases. The turbo uses the high-temperature, high-pressure exhaust gases to spin the turbine, which in turn drives the compressor to increase air intake into the engine.

When the engine is under heavy load or at high power demand, the amount of energy in the exhaust gases increases significantly, leading to greater temperature and pressure. This increased thermal energy allows the turbocharger to reach its optimal performance, resulting in higher rotational speeds.

Other factors such as peak engine RPM, peak torque, and exhaust manifold pressure certainly influence the operation of the turbocharger, but they do not necessarily coincide with the condition of the exhaust gases being at their highest energy levels. Hence, the relationship between heat rejection and turbocharger speed is a critical factor in understanding turbo system performance in diesel engines.