The problem of highly rated turbocharged diesel engines operating under transient load conditions is now well known, and is due to the inability of the turbocharger to supply sufficient air for good combustion.In Part 1, two methods are discussed for reducing turbocharger lag-air injection onto the compressor rotor and oil injection onto a small pelton wheel mounted on the turbocharger shaft. Results are given showing the benefit of fitting these devices to an engine on a test bed. Engine response is improved in all respects particularly smoke and overall response time.In Part 2, a simulation study of a turbocharged diesel engine installed in a 32 tonne truck is presented to investigate the engine performance during load and speed changes. It is shown that by injecting compressed air on to the turbocharger compressor rotor tip, smoke emissions from the engine during load changes are reduced. The size of air compressor and receiver required for air injection is the same order as for a conventional truck air braking system. Some preliminary engine tests on a computer-controlled test bed confirm the simulation modelling and results.