Air Standard Cycle: Part – 1

To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle.The two most commonly used air standard cycles are Otto cycle and Diesel cycle.

Introduction

The thermodynamic cycles like Carnot cycle, Stirling cycle, Brayton cycle are closed cycles as the working fluid keeps on circulating inside the cycle. In these cycles the addition of heat takes places externally, hence they also called external combustion engines. The most commonly used practical engines used in our day-to-day life are internal combustion engines for our vehicles, motorcycles, construction machineries etc.

The engines in which combustion of fuel takes place inside the engine are called as internal combustion engines or IC engines. The IC engines comprises of the piston and cylinder arrangement with suction and exhaust valves.

In the beginning of the internal combustion cycle, the air-fuel mixture is inducted inside the cylinder. When the fuel is combusted lot of heat is released which makes the engine cylinder very hot. To avoid the deterioration of the engine, it is very important to cool the engine either with water or with external air. After combustion of fuel the pressure of gases becomes very high which pushes the piston producing the work. Thereafter the burnt gases are released to the atmosphere as exhaust gases and the fresh air-fuel mixture is inducted inside the cylinder. Since the burnt air-fuel mixture does not moves into the cycle again and fresh air-fuel mixture is inducted at the beginning of each cycle, the internal combustion engines are said to be working in non-cyclic process.

The Concept of Air Standard Cycles

No matter whether the engine works in cyclic or non-cyclic process, its analysis is very important to find out the amount of fuel consumed and work produced by it. To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle. The addition and rejection of heat is considered to take place with the external reservoir and ideally all the processes are reversible.

The internal combustion engines are considered to be working on the principle of air standard cycles. The two most commonly used air standard cycles are Otto cycle and Diesel cycle. The Otto cycle corresponds to four stroke gasoline or petrol engines also called Spark Ignition (SI) engines. The Diesel cycle corresponds to four stroke Diesel engines also called as Compression Ignition (CI) engines.

Read more: http://www.brighthub.com/engineering/mechanical/articles/4125.aspx#ixzz1WgOns1igRead more: http://www.brighthub.com/engineering/mechanical/articles/4125.aspx#ixzz1WgOgzTt2

DIESEL CYCLE

Diesel Cycle

Processes of Ideal Diesel Cycle

The ideal Otto cycle comprises of two isentropic, one constant volume, and one constant pressure processes. The Diesel cycle is an open cycle or non-cyclic process since the fresh air and fuel is inducted inside the engine during each cycle and the burnt mixture is released to the atmosphere To understand these processes let us consider piston and cylinder engine with air and diesel as the working fluid. Refer the P-V diagram given at the bottom.

1) Air intake process 1-2: During this process the inlet valve of the engine is open, the piston moves towards the bottom position inducting air at constant pressure.

2) Isentropic compression process 2-3: During this process the inlet and exhaust valves of the engine remain close and the air, which has been inducted inside the cylinder, is compressed to the minimum volume. The compression ratio of air is between15 to 20. High compression ratios are very important in the Diesel cycle as we it increases the efficiency of the cycle also allows uniform burning of fuel.

3) Injection of fuel and its combustion at constant pressure 3-4: During this process the fuel valve opens and fuel is injected inside the cylinder consisting of hot air at high pressure. After injection the combustion of fuel takes place instantly. Since the combustion of fuel occurs due to high pressure, the diesel engine is also called Compression Ignition engine. Due to combustion fuel high pressure and temperature is generated.

4) Isentropic expansion process 4-5: Due to extremely high pressure, the piston is pushed again towards the bottom most position of the cylinder. It is during this process that the actual work is produced from the engine.

5) Constant volume heat rejection process 5-6: During this process the exhaust valve opens and all the exhaust gases are ready to be released to the atmosphere. The pressure inside the cylinder falls drastically.

6) Exhaust process 6-1: During this process the exhaust valve is open and the piston moves upwards and removes all the exhaust gases inside the cylinder at constant pressure.

Thereafter the exhaust valve closes, the piston starts moving in downward direction, the inlet valve opens and fresh air-fuel mixture is inducted. The whole cycle is completed in four strokes of engine, hence it is called four-stroke engine.