PETROL ENGINE(Gasoline Engine)

                     The engine which gives power to to propel the vehicle is a petrol-burning, spark-ignition, four-stroke cycle, internal combustion engine . Petrol is a liquid fuel and is also called by the name gasoline. that is why the petrol engines are also known as gasoline engines. The ability of petrol to furnish power rests on the two basic principles of physics :

• burning or combustion is always accompanied by the production of heat.
• when a gas is heated, it expands. if the volume remains constant, the pressure rises according to Charles's law.

                 The operation of petrol engine in the production of power may be described as, during the downward motion of the piston, the air fuel mixture is sucked from the carburetor into the cylinder. During the upward motion the mixture is compressed by the piston in the cylinder and ignited by an electric spark. When the mixture is compressed by the piston in the cylinder and ignited by an electric spark. When the mixture is burned in the cylinder, the resulting heat causes the gases to expand which exert pressure on the cylinder walls and on the piston. The piston, being movable, is pushed downward by this pressure to the full length of its stroke.
                 The pressure exerted on the piston is transmitted through the connecting rod to the crankshaft that is made to revolve. The crankshaft turns through one half of a resolution as the piston moves downward. A fly wheel attached to the crankshaft stores up energy. The momentum of the flywheel carries the piston through the balance of its motion until it receives another power impulse. The process is repeated over and over again the crankshaft is turning continuously and the engine is running.
                 The events that are repeated make up the cycle of the engine. The number of strokes of the piston required to complete the cycle varies with the type of engine. In modern vehicles, the cycle is extended through four-strokes of the piston or two revolutions of the crankshaft. This is called a four-stroke cycle engine. in two stroke cycle engines, the cycle is completed in two-strokes of the piston or one revolution of the crankshaft.
In four stroke cycle engine, the four strokes are named suction, compression, power and exhaust in accordance with the operation of the cycle which occur during each particular stroke.                                                                               

ENGINES

An engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work. Heat engines are of two types

1. external combustion engine, and 
2. internal combustion engines.

In an external combustion engine the products of combustion of air and fuel transfer it to a second fuel which is the working fluid of the cycle, as in the case of a steam engine or a steam turbine plant where the heat of combustion is employed to generate steam which is used in a piston engine or a turbine. Another example of an external combustion engine is a closed cycle gas turbine plant in which heat of combustion in an external furnaces transfer to gas, usually air, which is used in a gas turbine. Sterling engine is also an external combustion engine. In an internal combustion engine the products of combustion are directly the motive fluid. petrol, gas, and diesel engines, Wankel engines, and open cycle gas turbine are examples of internal combustion engines. Jet engines and rockets are also internal combustion engines. 

The main advantage of internal combustion engines over external combustion engines like steam plants are greater mechanical simplicity, lower ratio of rate and bulk to output due to absence of auxiliary apparatus like boiler and condenser and, hence lower first cost(except in case of very large units), higher overall efficiency, and lesser requirement of water for dissipation of energy through cooling system. 

The advantage of external combustion plants are: use of cheaper fuels including solid fuels, and high starting torque (internal combustion engines are not self-starting). The steam turbine plant, which is the most important external combustion engine, its mainly used for large electric power generation, whereas internal combustion engines are mainly used for transport vehicles - automobiles, locomotives, aircrafts, etc. External combustion engines are less suitable for transport vehicles because of bulk and weight, and difficulty of transporting the working fluid. 

The revolution in the lifestyle of the people caused by transport vehicles, the extensive use of which became possible due to tremendous development of internal combustion engines.