Q3. (a) Explain why a diesel engine cylinder is supplied with excess air. (4)
(b) Explain why fuel droplet size produced during injection has to be within narrow limits in order to enable good cylinder combustion to be achieved. (4)
(c) Explain how the desired fuel droplet size is produced by fuel injectors. (4)
(d) State why fuel injection timing has to be within narrow limits to enable economic engine operation without bearing overload. (4)
Why a diesel engine cylinder is supplied with excess air
>> Complete combustion
- Thermal engines use fuel and oxygen in the form of air to produce energy through combustion. To guarantee the combustion process, certain quantities of fuel and air need to be supplied in the combustion chamber. A complete combustion takes place when all the fuel is burned, in the exhaust gas there will be no quantities of unburned fuel. Air-fuel ratio (AF or AFR) is the ratio between the mass of air ma and mass fuel mf, used by the engine when running:
>> Scavenging
- Air supplied not only aids combustion but also scavenges the exhaust from the previous stroke, recent engines are all equipped with uni-flow scavenging for this reason where, the cold air enters the combustion chamber displaces the hot air (exhaust) out to ensure the combustion chamber clean for the next cycle.
- Scavenging process is highly critical as the time available to get rid of exhaust before the next cycle could begin is practically very low (in microseconds)
Fuel injection timing
- Typically, the fuel injection commences 15 deg before TDC and terminates 20 deg after TDC. The injection should commence and terminate within this short time frame. During this time, the air inside is fully compressed and heated up, all valves are closed.
Early injection
- If the injection is earlier then it leads to longer ignition delay which results in accumulation of large amount of evaporated fuel before the start of combustion. Longer ignition delay is due to lower values of pressure and temperature inside the cylinder during the initial period of fuel injection at advanced injection timings. The longer ignition delay leads to rapid burning rate and the pressure and temperature inside the cylinder rises suddenly. Hence, most of the fuel burns in premixed mode causing higher peak heat release rate and shorter combustion duration.
Late injection
- If injection is delayed, the ignition delay is short causing accumulation of relatively less amount of evaporated fuel due to pressure and temperature inside the cylinder during the initial period of fuel injection being high.
- The shorter ignition delay shortens the mixing time which leads to slow burning rate and slow rise in pressure and temperature. Hence, most of the fuel burns in diffusion mode rather than premixed mode resulting in lower peak heat release rate, longer combustion duration.
Causes of both
- Poor fuel & thermal efficiency
- Overloading of the engine
- High emissions
- Higher Nox values
- Fouling of turbocharger and nozzles.
2020/OCT >>Reasonably well answered but some confusion in details. Structured answers would have avoided repetition.
2020/JUL >> The first three parts of the question are essentially linked and if candidates had read the question completely and looked at the totality they might have been able to answer the question better.
>> Many did not understand what excess air was and some even confused it with supercharging. The first three parts essentially deal with the mixing of air and fuel in a diesel engine to obtain optimum combustion; the excess air is required to ensure that all fuel gets its required oxygen. Unlike premixing of air and fuel in a petrol engine the diesel engine has fuel injected at local point (the injectors).
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