The main fuel for engines was petrol, but being volatile, it could catch fire easily. Another German engineer, Rudolf Diesel, perfected the “Diesel” engine which used heavier and less volatile fuel and needed no spark plugs for ignition. Diesel engines are used in heavy transport (trucks, buses, ships etc) as well as modern cars.
Once the engine was established as a safe alternative to horses, it was employed to other applications: powering bicycles (called motorcycles) and boats (now motorboats) amongst others. And helped the invention of aeroplane by providing a lightweight power source.
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