Diesel engines tend to be the best in large industries, such as ships, boats, trucks and electricity generation. As the diesel engine is down, its benefits are declining.
Compression of diesel fuel and petrol can be compared to the small diesel engine. The gas engine has a compression rate of approximately 8:1 to 10:1. Now only Mazda produces Skyactiv-G serial engines with a compression rate of 14:1, such engine is installed in Mazda 6 and CX-5. But this does not compare with the compression rate of diesel engines, it varies from 18:1 to 20:1 and even more. Such a high degree of compression is required to ignition the fuel mixture. But the components of the engine have to deal with the increased impact loads on the knees, the porch, the caps and the gearbox. This means that the engine and gearbox shall be designed on the basis of heavy loads as compared with petrol engines♪
More metal is needed to create sufficiently strong components of a small diesel engine, which is related and greater weight. Except in motorcycles, weight is a very important aspect.
And there's a question about who the motorcycles are trying to do? Maybe they're people who need a motorcycle as a means of movement and they don't need speed and dynamic qualities. They value in diesel engines the cost-effectiveness and longer service without maintenance.