As is known, evelating the intake temperature can reduce volumetric efficiency in diesel engines. Likewise, boosting intake pressure can increase volumetric efficiency.
Higher volumetric effcinecy means stronger thermal capacity of in-cylinder mixutre. Hence, it is supposed that lower increase of in-cylinder temperature is obtained with higher volumtric efficiency when same amount of heat is released in the cylinder.
Would anybody show me some references about the relationship between volumetric efficiency and combustion temperature?
Thank you for your help and attention.
It is not true that lower in-cylinder temperature is obtained with high volumetric efficiency. In fact high volumetric efficiency means more fresh charge is intoduced in the cylinder which means more oxygen will be available for the combustion process which will result in high combustion temperatures.
It is important to refer to physics
Temperature (it is air) at inlet is due to preheating and the most often to compression with intercooling. During inlet air is preheated by contact with the cylinder. So we can imagine the most you have air the less it wil be heated.
After you can apply adiabatic compression or polytropic compression to obtain temperature when diesel fuel is injected.
Combustion temperature depends on the ratio between diesel fuel and air and also on the temperature when diesel fuel is injected.
The more the air/fuel ratio is high the less the temperature is high becaus air is in excess but dont forget in diesel combustion the air fuel mixture is not homogenous.
The Brake Mean Effective Pressure (BMEP) of an internal combustion engine can be calculated as the multiplication of the delivery ratio (volumetric efficiency), the net caloric value of the mixture and the overall (effective) efficiency of the engine. This equation assumes stocheometric air / fuel ratio. Considering an increase of energy content in the cylinder of the same engine with increasing volumetric efficiency, the combustion pressure and temperature will increase, too.
Here is are some engine thermodynamics books:
http://www.springer.com/us/book/9783211992760
http://www.springer.com/us/book/9783642029516
http://www.strojar.com/upload/skripta/oborove/Internal_Combustion_Engines_Fundamentals_by_J.B.Heywood.pdf
http://www.rmcet.com/lib/E-Books/Mech-auto/Engineering%20Fundamentals%20of%20IC%20Engines%20%28WW%20Pulkrabek%29.pdf
Best regards,
Mario
Yes Paul at start of injection the mixture is not homogeneous but during the delay period it is expected that fuel will be atomized, vaporized and mixed thoroughly with air and then premixed burning occurs. If the amount accumulated during the delay period is large then a high temperature and pressure rise is expected to take place.
As disscussed above, more oxygen supplied with larger intake pressure is beneficial to higher combustion temperature. However, in some researches, boosting intake pressure is used to extend engine operating range to higher load. These results demonstrated that boosting intake pressure could reduce rate of pressure rise and NOx emissions which is due to lower combustion temperature.
Therefore, the effects of boosting intake pressure on combustion temperature are confused now. Would you please explain the reason for that in detail?
Thank you very much.
As disscussed above, more oxygen supplied with larger intake pressure is beneficial to higher combustion temperature. However, in some researches, boosting intake pressure is used to extend engine operating range to higher load. These results demonstrated that boosting intake pressure could reduce rate of pressure rise and NOx emissions which is due to lower combustion temperature.
Therefore, the effects of boosting intake pressure on combustion temperature are confused now. Would you please explain the reason for that in detail?
Thank you very much.
Dear Yaopeng Li, this effect can be explained by the general equation of gaseous fluid:
p*V=m*R*T
p ... pressure
V ... volume
m ... mass
R ... gas constant
T ... Temperature
When boosting the intake pressure, the temperature in the cylinder also increases. Starting from a higher intakte pressure and temperature, the compression process in the cylinder reaches higher pressure and temperature values. In addition, the effects mentioned above come to play: The higher intake pressure leads to a higher delivery ratio (more oxigen in the cylinder), which allows the injection of a higher amount of fuel. More energy content in the cylinder leads to more BMEP, and thus the pressure and temperature increase, too. Important are different thermodynamics and chemical effects of combustion (you can find detailed explanations of these effects in the above cited literature). But in general an increase of intake pressure increases the temperature in the combustion chamber.
One more thing to be considered: Increasing intake pressure by boosting the intake air, e.g. by use of turbo-chargers or mechanical compressors, also leads to a rising intake air temperature. For this reason, intake air cooling systems might be applied.
Kind regards,
Mario
Boosting inlet pressure has the same effect as a high compression ratio in diesel engines. They both lead to a shorter delay period. It is well known that during the delay period the fuel which is injected during it will mix with the air( called the burning mixture) to within inflammability limits. So if this period is short less amount of burning mixture will be added to the fuel which becomes ready for burning and burns during the next combustion phase( premixed or rapid combustion phase) and so the rate of pressure rise will not be sharp and relatively lower maximum cylinder pressure and combustion temperatures are expected. This is good in itself as it reduces the probability of the occurance of the so called diesel knock phenomena.
Consult : John Heywood " Internal combustion engines fundamentals" McGraw Hill Book Company, pp539 - 548
Higher volumetric efficiency means higher in-cylinder temperature.
volumetric efficiency depends on inlet pressure and temperature. So increase in any one of them will lead to enhance the combustion efficiency or combustion temperature.
Hello,
Increasing in volumetric efficiency is essential in case of a naturally aspirated engines where the engines produces very less sp power. other way to increase it to switch to turbocharged.
The relationship between the combustion temperature and Volumetric eff is proportional.
Higher the Air flow , can inject the more fuel with better lambda ( >1.4 ) values, where the smoke levels are in control. However the Structural stability of cyl head and block to be taken care of when we moved to higher specific power.
Hope you got some information.
Have a nice time.
Regards,
Siva.
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