.During this process, some of the thermal energy is converted into work by exploiting the working substance’s properties. The active substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. Heat engines are often modeled using a standard engineering model such as the Otto cycle. The theoretical model can be refined and augmented with actual data from an operating engine, using indicator diagrams. Since very few existing heat engines match their underlying thermodynamic cycles, one could say that a thermodynamic process is an ideal case of a mechanical machine.
In any case, fully understanding an engine and its efficiency requires gaining a good understanding of the (possibly simplified or idealized) theoretical model, the practical nuances of an actual mechanical engine, and the two’s discrepancies. . . . . . . . .  .Most automobile engines work in a cyclic thermodynamic process that consists of five steps:1:Intake Stroke. The piston is pulled out, drawing the fuel-air mixture into the cylinder at atmospheric pressure.
2: Compression stroke. the piston is pushed back in, compressing the fuel-air mixture and work done on the gas.3: Ignition. A spark ignites the gases, quickly and dramatically raising the temperature and pressure.4: Power stroke. the high pressure that results from ignition pushes the piston out. The gases do work on the piston, and some heat flows out of the cylinder.5: Exhaust stroke. A valve is opened, and the exhaust gases are pushed out of the cylinder. . . . . . . . . . An automobile engine is .a four-stroke engine because each cycle has four strokes consisting of 1, 2,3,4,5 steps during which the piston moves.
Of the energy released by burning gasoline, only about 20 to 25% is turned into mechanical work to move automobiles forward and run other systems (Solanki& Arun, 2008). The rest is discarded. The hot exhaust gases carry energy out of the engine, as does the liquid cooling system.