If anybody kindly helps me to clear my doubts, I shall be highly obliged:
We know that as the air-fuel ratio (AFR) is leaned out due to supercharging or turbocharging, the temperature of the burning gases increases, leading to produce more power.
1. It is true for which condition---‘Dissociation’ or ‘no dissociation’?
2. In CI engine, excess air tends to reduce the peak gas temperature (that’s why dissociation effects are not so pronounced in CI engine), then how lean air-fuel mixture rises the temperature due to supercharging or turbocharging? How can it produce more power while supercharging?
3. If maximum temperature and power is obtained at slightly rich air-fuel mixture (in SI engine for dissociation), then what is the purpose of supercharging?
It has to be distinguished between SI and CI engines. SI engines are operated in a air - fuel ration between about 0.8 and 1.2. Leaning the air - fuel ration (means air excess) leads to increased combustion temperature, but not necessarily to higher power output. In general, SI engines are driven at stoechiometric air - fuel ration because of possibility to use 3 way catalyst for efficient exhaust gas after treatment. CI engines are able to run with much higher exceed of air, because of the different combustion process. In this way, CI engines are operated under air - fuel ratio conditions, where a surplus of air reduces peak gas temperature.
Supercharging is able to produce more power because of an increased mass of air in the combustion chamber, which allows the injection of an increased mass of fuel. More fuel and more air means more energy content in the combustion chamber, which may lead to more engine power. In this way, the power increase of supercharged engines is not directly associated with the air - fuel ratio. But of course, the air- fuel ratio is also a parameter, which inflences the power output. At CI engines, the wide varity of possible air - fuel ratio gives the possibility to operate the engine according to the power requirement, but to less air fuel ratio leads to bad exhaust emissions, especially particulate (soot).
Kind regards,
Mario
Thank you very much Dr. Hirz for your kind and valuable reply. The second paragraph is clear to me.
However, for the first paragraph, I want to ask:
Is it like leaning the air - fuel ration (means air excess) leads to increased combustion temperature, but too much leaning (means much air excess) reduces peak gas temperature?
Yes, but it has to be distinguished between SI and CI engines. In SI engines, increasing air - fuel ratio up to a certain value of about 1.1 increases temperature, above a value of about 1.3, homogenous SI combustion does not work. Special case of inhomogeneous SI combustion (achieved by stratified charge in direct injection SI) may push this limit some kind upwards. In CI engines, the air - fuel ratio is higher than in SI engines because of emissions and efficiency reasons. (Remark: In some cases, diesel engines may be operated at low air - fuel ratio, too, but this leads to high soot emissions). The relatively (averaged) lean combustion in CI engines has the effect, that increasing air excess leads to lower temperature due to lower energy content in cylinder. General remark: supercharging at SI and CI may increase temperature because of higher energy content in cylinder and thus higher power output.
Kind regards,
Mario
The temperature increase of lean air / fuel mixture is caused by the chemical reaction behavior. Theoretically, the maximum energy conversion is achieved at a stoechiometric air - fuel ratio of 1.0. In real engines, a certain surplus of oxigen is required to burn all fuel. Depending on different factors, e.g. mixture preparation, combustion chamber geometry, operating point, the maximum temperature in cylinder is reached at an air-fuel ratio of about 1.05 to 1.1. With further increasing air content, the temperature drops down quickly because of lower energy content in cylinder.
Kind regards,
Mario
In starting conditions, the engine is cold. This leads to worse mixture preparation and condensing fuel at cold intake tube and combustion chamber walls. Therefore, a surplus of fuel has to be added in cold start phases.
Kind regards,
Mario
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