Thermodynamic Diesel Cycle: Air Standard Cycle: Part – 3, What is Diesel cycle?

Thermodynamic Diesel Cycle: Air Standard Cycle: Part – 3, What is Diesel cycle?
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Apart from Otto cycle, thermodynamic Diesel cycle is one of the most popularly used cycles. The internal combustion diesel engines that are fitted in your cars and others machines run on the principle of Diesel cycle. Like petrol engine running on the principle of Otto cycle, the Diesel engines also comprise of four strokes: suction of fuel, compression of fuel, combustion and expansion of fuel and exhaust of burnt fuel. The four stroke engine has been described in details in article: How Four-Stroke Compression Ignition Engines Work

In diesel engine initially only air is inducted inside the cylinder and it is compresses to very high pressures. Thereafter the diesel fuel is injected inside the cylinder and its combustion takes place to high temperature of the compressed air. Since the combustion of fuel takes place due to compression of air, the diesel engine is also called Compression Ignition (CI) engine.

Four stroke Diesel cycle was invented by Rudolf Diesel in the year 1897. The principle of Diesel cycle is used in the compression ignition (CI) type of internal combustion engines using diesel as the fuel. Originally when Rudolf Diesel discovered this engine, it was meant to run on the coal dust.

Diesel Cycle

Piston and cylinder

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 bottommost 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.

  1. 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.

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Book: Engineering Thermodynamics by P K Nag

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