In the early days it was muscle and a crank that prompted an engine to spark into life. This task was then handed on to the electric direct-drive starter, which was invented over 50 years ago … and it has been pretty much doing the job ever since
A combustion engine lacks the ability to fire up on its own; it needs a source of mechanical energy that cranks it up fast enough so the fuel pump feeds atomised diesel into the cylinders. As the pistons move to compress the air-fuel mix inside the combustion chamber, the diesel fuel will eventually self-ignite.
