A diesel motorcycle is a motorcycle with a diesel engine. With a traditionally poor power-to-weight ratio, most diesel engines are generally unsuited for use on motorcycles which normally desire low weight, compact size, high RPMs and rapid acceleration. In the 1980s, NATO forces in Europe standardized all their vehicles to operate with diesel fuel. Some forces had fleets of motorcycles and trials were conducted with diesel engines on these. Air-cooled single-cylinder engines built by Lombardini of Italy were used and had some success. This led to some countries re-fitting their bikes with diesel engines.