This is a brief review of Miller cycle use in various engine classes. Results are presented of numerical modelling and stand tests of locomotive diesel engine type 8CHN21/21, whose fuel and environmental performance was improved due to Miller cycle with optimum valve timing. Stand tests confirmed gain in fuel economy by 3,26 % and in noxious waster by over 30 %. This means that emission level in engines powering up-and-coming locomotives may be reduced to limits set forth by legislation in force.