Abstract: A compound traction engine for a heavy vehicle including a high temperature adiabatic four stroke reciprocating diesel engine arranged to drive a boost compressor through a differential gear. The other output of the gear is connected through a torque converter to the output shaft. The exhaust from the engine is fed to a turbine with an adjustable blade nozzle ring and the output from the compressor is connected to the air inlet of the engine and is also connected via a valve to the same turbine. The turbine is connected via a continuously variable transmission to the output shaft. An automatic control unit has inputs from an output torque/speed sensor and a boost pressure sensor and is arranged to control the variable gear and the turbine blade ring.