What happen when petrol burns in car engine?

As it combusts, heat is created. Heat and the expansion of air caused by heat is what drives the pistons. The pistons drive the crankshaft, the crankshaft drives the flywheel, the flywheel drives the input shaft to the transmission, the input shaft drives a series of gears in the transmission, which in turn drives an output shaft to the differential, and from there you get power to your wheels. That's the nickel tour of how it works. The crankshaft will also drive other components, such as your belt pulleys, camshaft, etc.