In CI (e.g. diesel) engines the only air is inducted and compressed inside the cylinder. Then fuel is injected to be combusted.
CI engine combustion process is extremely complex. It is a 3D, unsteady, heterogeneous process, dependent upon fuel characteristics, fuel injection system design, combustion chamber design and operating conditions.
Combustion an unsteady process occurring simultaneously at many spots in a non-homogeneous mixture at a rate controlled by fuel injection.
The fuel can be injected:
- IDI (Indirect Injection). Fuel injected into a small pre-chamber or swirl chamber attached to the cylinder chamber.
- DI (Direct Injection). Fuel injected directly into the cylinder chamber.
Indirect Injection (IDI)
A pre-chamber or swirl chamber is used to generate a vigorous charge motion during the compression stroke. Air is forced from the main chamber into the auxiliary chamber, through a nozzle or orifice(s) and the charge rotates rapidly within the auxiliary chamber. Fuel is injected into the rotating flow in the auxiliary chamber, combustion starts and the expanding gas is forced into the main chamber entraining and mixing with the main chamber air.
The glow plug shown on the right of the pre-chamber is used as a cold-starting aid. Because when the engine is cold the compressed air during the compression stroke is not hot enough to provide good fuel ignition.
Direct Injection (DI)
Intake generated swirl is used to ensure good fuel-air mixing. These engines have bowl in piston combustion chambers of various shapes and the fuel is injected into one or more locations in the swirling air flow.
Fuel is injected into the cylinder late in the compression stroke by one or more injectors located in each cylinder combustion chamber. The primary purpose of the injector is to distribute and mix the fuel with the surrounding air. In addition to the swirl and turbulence of the air, a high injection velocity (high pressure injection system) is needed to spread the fuel throughout the cylinder and cause it to mix with the air. The injection process consists:
- Fuel is injected into the combustion chamber towards the end of the compression stroke shortly before the desired start of combustion.
- The liquid fuel injected at high velocity through the injector nozzle(s) atomises into small droplets and mixes with the high temperature, high pressure air.
- The fuel vaporises and since the air temperature exceeds the fuels ignition temperature, spontaneous ignition of portions of the mixed air and fuel occur after an “ignition delay period”.
- The combustion increases the pressure and temperature further and reduces the ignition delay period for the fuel subsequently injected.
- Injection continues until the desired amount of fuel (for the load required) has been injected. Typically injection would cease shortly after TDC
- Atomisation, vaporisation, mixing and combustion continue until all the fuel has been burnt. Mixing of the air is, burned and burning gases continue during the expansion stroke
The rate of heat release gives the rate at which sensible energy is imparted to the charge by burning fuel.
The heat release diagram and the pressure-crank angle diagram show that there is a delay between the start of injection and the start of combustion. This time is required for the fuel to atomise, vaporise and mix to give a combustible air/fuel ratio.
At the end of the delay period, a large amount of fuel has been injected into the cylinder (and injection is continuing). A very rapid rate of pressure then occurs as most of this fuel burns spontaneously. This gives rise to the characteristic diesel knock. This also explains why the mixed cycle idealisation assumes part of the fuel is burnt at constant volume.
At the end of the Premixed Combustion Phase, combustion occurs in a controlled manner as the fuel is injected. After injection stops, combustion continues as the mixing process controls the local air/fuel ratio. This process is known as the mixing controlled combustion phase. This is essentially the constant pressure part of the mixed cycle idealisation.
Further, low rate of heat release combustion occurs. During expansion as pockets of rich mixtures disperse into fuel lean mixtures. This is the late combustion phase, which occurs due to imperfect mixing and dissociation.
Some important points to note about the CI combustion process are:
- Since injection occurs just before combustion starts, there is no knock limit as in the SI engine. Hence, a higher compression ratio can be used giving higher thermal efficiency
- It is essential that the ignition period be kept short and reproducible: A long delay period results in increased noise, increased smoke and increased mechanical loading. The cetane number is the measure of the fuel ignition quality. A fuel with a low cetane number (bad for CI) will have a high octane number. Therefore, petrol is a bad duel for a CI engine
- Since the charge is heterogeneous, quality governing can be employed giving improved thermal efficiency, especially at part load. The thermal efficiency is also increased due to the lean mixture operation
- Due to the imperfect mixing in the short space of time available, not all of the air in the cylinder can be utilised. If the air to fuel ratio is too low, excessive amounts of soot or black carbon are produced (the smoke limit is a measure of where the smoke becomes unacceptable. The’ richest air to fuel ratio possible is about 20% lean of stoichiometric. Hence, the maximum mean effective pressure for a naturally aspirated diesel engine is less than that for a SI engine.
- A major limitation on the power output of CI engines is that the combustion process is slow compared to the SI engine due to the mixing process. This severely limits the maximum speed and hence power output
- Due to the last two points, CI engines are often turbocharged to increase their specific power output.