An automobile such as a car is an automatic self-propelled vehicle. It runs on a gasoline, diesel or electric engine. Petrol or diesel engines used in automobiles are internal combustion engines. In these engines, fuel burns in the cylinder. In an electric engine, there is a motor and a gear box. It is battery-powered and used for small cars on a limited and experimental basis.
Petrol engine is used in most automobiles. However, some automobiles even use diesel engines. Diesel engines are heavier and more expensive than gasoline engines, but they last longer and use less refined fuel.
Both the petrol and diesel engines are four stroke engines. Their construction and working can be understood as follows:
Petrol Engine: It consists of a cylinder containing an air-tight piston. It is connected with the main shaft through a crank by means of a connecting rod. As the piston moves to and fro, its motion is converted into rotational motion of the crank shaft. The cylinder has two valves: one inlet valve and the other, exhaust or outlet valve. Inlet and outlet valves open and close automatically only once in every cycle. Air is mixed with petrol vapour in a carburetor and is made to pass into the cylinder through the inlet valve. The mixture is burnt in the upper portion by means of an electric spark provided by the spark plug. The action of the engine may be explained in four strokes.
When the engine is made to work at the beginning by external force, the inlet valve opens and the mixture 
of petrol vapour and air is allowed into the cylinder. This is known as the charging stroke. Now both inlet and outlet valves close and the fuel mixture is compressed. This is known as compression stroke. The spark plug produces an electric spark and causes the mixture to burn. Due to combustion of the fuel, a large amount of heat is produced. This gives rise to heavy pressure and as a result the piston moves. With the movement of the piston the vehicle moves. This is known as the working or power stroke. Finally the exhaust valve opens, but the inlet valve remains closed. Unused gases, left at the end of the working stroke are thrown out. This is known as the exhaust stroke. In this way, one cycle is over. As the process is repeated, the vehicle goes on moving.
Most automobile engines have four, six or eight cylinders. Most of the engines are in the front and drive the rear wheels.
Diesel Engine: This engine makes use of less expensive diesel as power fuel. It has three valves – one for the admission of air, other for the fuel and the third for the exhaust of gases. These valves open and close automatically.
A diesel engine is also a four stroke engine. In the first stroke, air is drawn into the cylinder and in the second, air is compressed and due to this compression, the temperature of air goes up to 600°C. In the third stroke, fuel admission valve opens and oil in the form of spray is injected into the cylinder. The oil begins to burn and the gases produced exert a heavy pressure on the piston, due to which it is driven outwards. This is the working stroke. In the fourth stroke, the exhaust valve opens and waste gases are expelled. This process is repeated causing the vehicle to move. The efficiency of diesel engine is as high as 40% in comparison to a petrol engine.