In Otto cycle the heat addition takes place at constant volume whereas in the Diesel cycle it is at constant pressure.įor this reason, the Diesel cycle is often referred to as the constant-pressure cycle.įrom the above expression, we can see that the efficiency of the Diesel cycle is different from that of the Otto cycle only in the bracketed factor. The difference between Otto and Diesel cycles is in the process of heat addition. This process corresponds to the injection of fuel through fuel injector and combustion due to self-ignition of fuel. Process 2 → 3: heat is supplied reversibly at constant pressure. Process 1 → 2: represents isentropic compression of the air when the piston moves from bottom dead centre to top dead centre and the air is compressed to a higher pressure. The mean effective pressure which is an indication of the work output increases with a pressure ratio at a fixed value of compression ratio and the ratio of specific heats. The mean effective pressure of the cycle is given by: Thus, it can be seen that the work output is directly proportional to the pressure ratio, r p. The use of gases with higher γ values would increase efficiency of Otto cycle. Thermal Efficiency: The thermal efficiency of Otto cycle can be written as:įrom the above expression we can see that thermal efficiency of Otto cycle is a function of compression ratio r and the ratio of specific heats γ.įurther, the efficiency is independent of heat supplied and pressure ratio. Process 4 → 1: represent constant volume heat rejection in which piston is stays at bottom dead centre. Processes 3 → 4: represent isentropic expansion where the product of expands & piston moves from top dead centre to bottom centre. This process corresponds to spark-ignition and combustion in the actual engine starts. Process 2 → 3: heat is supplied reversibly at constant volume. Process 1 → 2: represents isentropic compression of the air when the piston moves from bottom dead centre to top dead centre and charge (mixture of fuel and air) is compressed to a higher pressure.
When the engine is working on full throttle, the processes 0→1 and 1→0 on the p-V diagram represents suction and exhaust processes and their effect is nullified. Dual Combustion or Limited pressure Cycle. The three cycles of great practical importance in the analysis of piston engine performance are Compression Ratio (r): It is the ratio of the total cylinder volume when the piston is at the bottom dead centre, V Total, to the clearance volume, V c. Displacement or Swept Volume (V s): The volume swept by the piston when travelling between both dead centers is called the displacement volume. (b) Bottom Dead Centre (BDC): Dead centre when the piston is nearest to the crankshaft or farthest from the cylinder head is known as Bottom dead centre.
(a) Top Dead Centre (TDC): Dead centre when the piston is farthest from the crankshaft or nearest to cylinder head is known as Top Dead centre. The position of the working piston at the moment when the direction of the piston motion is reversed at either end of the stroke is called the dead centre. The nominal distance through which a working piston moves between two successive reversals of its direction of motion is called the stroke. The cross-sectional area of cylinder is called the piston area and is designated by ‘A’. The nominal inner diameter of the working cylinder is called the cylinder bore and is designated by ‘d’. External Combustion Engines (EC Engines)Įngines whether Internal Combustion or External Combustion can be classified into two types,Īn engine is made up of by combining several components out of which some important components are discussed below. Heat engines can be broadly classified into two categories: Normally, most of the engines convert thermal energy into mechanical work and therefore they are called 'heat engines'. Introduction: An engine is a device that transforms one form of energy into another form.