Asked by: Stephen Lloyd, Cheshire
Actually they have. On the roads of India, plenty of people zoom around on great little diesel-powered Royal Enfield bikes. They’re quite retro but if the classic look is not quite your thing, you can still enjoy diesel-fuelled fun courtesy of sportier-looking bikes produced by Hayes Diversified Technology (HDT). Petrol has always been the predominant fuel in biking, and even though specialist firms are working on diesel engines these days, most of the innovation in non-petrol motorcycle engine technology is in biofuels.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.