Farm Power & Machinery

Combustion Engines

The heat engine is a device in which heat energy of fuel after burning it properly is used to convert it into mechanical energy (mechanical work). The heat engine is an equipment which generates thermal energy and transforms it into mechanical energy. The heat energy is produced by the combustion of fuel. The heat engines are of two types :

External Combustion Engine

In this type of engine the fuel is burnt outside the engine cylinder, in a device called boiler to produce hot gas or steam, which is used in the engine cylinder to develop the mechanical work. The line diagram of working principle of External combustion engine is shown in Fig 2.1. The best example for external combustion engine is steam engine. The thermal efficiency of external combustion engine is usually in the range of 15 to 20%, which is very low compared to the internal combustion engine.



Fig 2.1. Working principle of External combustion engine.

Internal Combustion Engine

In this type of engine the burning or combustion of the fuel takes place inside the engine cylinder. There are two ways in which combustion takes place in the cylinder:
i) By rapid explosion of air-fuel mixture within the cylinder, when it is ignited by a spark, is called constant volume combustion (C.V.C.).
ii) Combustion takes place by slow burning when the fuel is injected into highly compressed heated air contained in the cylinder. This is called constant pressure combustion (C.P.C.), because when the combustion takes place, the pressure in the cylinder is almost constant.
The power is developed during the power stroke. Most of the engines used in tractors, automobiles, combine harvesters, oil engines and traction vehicles are of this type. The line diagram of working principle of Internal combustion engine is shown in Fig. 2.2. The thermal efficiency of internal combustion engine is about 40%.

Fig. 2.2 Working principle of Internal combustion engine