Combustion Process in CI engine
- 1. COMBUSTION PROCESS IN CI ENGINE HARSH PARASHAR 131501013 | PRE-FINAL YEAR DEPARTMENT OF MECHANICAL ENGINEERING
- 2. COMBUSTION Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat. Following conditions are necessary for combustion to take place 1. The presence of combustible mixture 2. Some means to initiate mixture 3. Stabilization and propagation of flame in Combustion Chamber Ignition Limits : Ignition of charge is only possible within certain limits of fuel-air ratio. Ignition limits correspond approximately to those mixture ratios, at lean and rich ends of scale, where heat released by spark is no longer sufficient to initiate combustion in neighbouring unburnt mixture.
- 3. COMBUSTION IN CI ENGINES Unlike the SI engine the air-fuel mixture supplied in CI engine is not homogeneous and fuel remains in liquid particles. The combustion in CI engine takes place at many number of points simultaneously and therefore the number of flame generated are also many as burning of liquid fuel is difficult so first liquid fuel is to be evaporated and to be elevated to ignition temperature and then burn.
- 4. COMBUSTION IN CI ENGINE Pressure variation along with crank angle in CI engine Stages of combustion in CI engine: 1. Ignition delay period 2. Pre-mixed burning 3. Mixing controlled combustion 4. Tail region / After burning Image Source : www.slideshare.net
- 5. COMBUSTION IN CI ENGINE 1. Ignition delay period After the injection of the fuel in combustion chamber, fuel does not ignite immediately it takes a definite period from the time of injection to actual burning this delay is known as the ignition delay period. Ignition delay further can be divided into two part: a) Physical Delay : Since the time of the injection to the beginning of chemical reactions is called physical delay. In this period fuel is atomized, vaporized and mixed with air and raised to self ignition temperature. b) Chemical Delay : In the chemical delay, reactions start slowly and then accelerate until inflammation takes place. It depends various factors like temperature, property of the fuel
- 6. COMBUSTION IN CI ENGINE 2. Pre-mixed burning In this phase the combustion of pre-ignited fuel occurs which was raised to self-ignition temperature after vaporization. The rate of heat release is maximum in this period. The period of pre-mixed combustion depends on the duration of the delay period (the longer the delay higher the pressure rise due to higher heat release) 3. Mixing controlled combustion This phase is called rapid phase because the temperature and the pressure are so high that fuel injected droplets burn as soon as they are injected and any further pressure rise can be controlled by injection rate. It is assumed to be end when maximum temperature is reached inside the combustion chamber.
- 7. COMBUSTION IN CI ENGINE 4. Tail region / After burning The combustion does not stop with the completion of injection process because unburned and partially burnt fuel particles start burning as soon as they come in contact with oxygen. This burning may continue in expansion stroke up to 70 to 80% of crank travel from TDC. Factors affecting delay period in CI engine: 1. Compression ratio 2. Engine speed 3. Desired outputs 4. Injection timing 5. Fuel properties 6. Intake temperature and pressure