The four stroke compression ignition (CI) engine is very similar to the four-stroke spark ignition engine as shown in Figure 6.6, but it operates at a much higher compression ratio. The compression ratio of SI engines varies from 6 to 10 whereas in CI engines it ranges from 16 to 20. During the suction stroke, air is sucked alone inside the cylinder and then compressed sufficiently to increase the temperature equal to the self-ignition temperature of the fuel injected at the end of compression at constant pressure. In this engine, a fuel pump and injector are used to inject the fuel at high pressure. The ignition system of CI engine is completely different from SI engine as no spark plug and carburettor are required.
Figure 6.6 All the Thermodynamic Processes in Four Strokes CI Engine
The sequence of operations of CI engine can be explained as follows:
- Suction stroke: In this stroke, piston moves from TDC to BDC and air is sucked alone as vacuum is created inside the cylinder by the piston movement. During suction inlet valve is open and exhaust valve remains closed as shown in Figure 6.6a. On ideal P−V diagram suction is shown by a straight line from 0 to 1 in Figure 6.7.
Figure 6.7 ρ−V Diagram for Diesel Cycle - Compression stroke: Both the valves are closed during the stroke and air is compressed into the clearance volume by the piston movement from BDC to TDC as shown in Figure 6.6b. On P−V diagram it is shown by the process 1−2 in Figure 6.7. At the end of compression at constant pressure fuel is injected as shown in Figure 6.6c. Due to high pressure and temperature, fuel starts to ignite automatically as temperature of air is increased to flash point of the fuel. The heat addition process is shown by the line 2−3 on P−V diagram.
- Expansion or power stroke: Fuel injection starts nearly at the end of the compression stroke. The rate of injection is such that the combustion maintains the pressure constant in spite of the piston movement on its expansion stroke increasing the volume. Heat is assumed to have been added at constant pressure. After the injection of the fuel is completed (after cut-off) the combustion products expand. Both the valves remain closed during the expansion stroke as shown in Figure 6.6d. The expansion process is shown by 3−4 on P−V diagram.
- Exhaust stroke: The exhaust valve is open and inlet valve is closed during the stroke. The movement of piston from BDC to TDC pushes the product of combustion and disposed into the atmosphere through the exhaust valve as shown in Figure 6.6e. The exhaust process is shown by the line 4−5 on P−V diagram.
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