By certain modifications in diesel engine components, the in-cylinder air swirl is increased and results in better thermal efficiency with an increase in NOx.
NOx formation only increased if your combustion zone temperature increased. At higher temperature reaction rate of NOX formation increased.
I agree with it sir, but in some cases, there is no marginal increase in Combustion chamber temp/pressure but I have got a tremendous increment in NOx.
Brake thermal efficiency is dependent on the difference in peak temperature and outlet temperature of the combustion process. An increased Brake thermal efficiency can be an indication of higher temperatures, and this favours the production of NOx.
Improvement in brake thermal efficiency shows the improvement in combustion. However,
In an engine, NOx formation mainly depends on in-cylinder temperature. As per the zeldovich mechanism, at higher temperature, nitrogen present in the air react with oxygen and forms NOx.
While the broad direction in answers is correct, I see the complete response has needs yet to be provided. Partly, the question itself is very qualitative.
If true answer is required, please determine the increase in fuel flow rate (assuming that it is what is driving the increase in the efficiency) and the air-to-fuel ratio. Please plot it against the adiabatic flame temperature. If you see a significant increase in the flame temperature over this range of increase in brake thermal efficiency (presumably beyond 1400 K because significant thermal NOx gets generated), then surely you must assign this reason.
As per the question, the increase in efficiency and NOx in the diesel engine is brought about by augmented air swirl. Assuming all other factors to remain the same, a likely explanation is as follows:
The primary effect of higher swirl is to enhance fuel-air mixing - this increases the dominance of the premixed phase (uncontrolled phase) of combustion, i.e. there are more sites of auto-ignition and N2 oxidation. This speeds up the combustion process, improving the thermal efficiency and augments NOx formation.
If I ask the question, are you familiar with pre-mixed and diffusion flames, please do not feel insulted, but do you know the difference? NOx formation is temperature dependent (usually it's an exponential relationship) and if the injection droplets vapourise and burn in small packets, local high temperatures are reached before the fuel is distributed throughout the air available. This air is needed to lower the bulk temperature. The swirl helps to locally mix the pre-burned mixture and the burning mixture. If you have high temperatures because of the cylinder design, ammonia in the correct proportion can destroy it giving you the required NOx emissions level. Some companies market suitable liquids for this purpose but it then becomes an engine package problem. I hope that this helps as I have slightly strayed of the question subject. Stephen.
Nox is formed when localized mean temperature is greater than 1500 C combined with localized oxygen avaliability. In case of diesel engine, due to high compression itself, the localized temperature is high enough to create oxygen rich hotspots that favour Nox formation.
While the break thermal efficiency is improved due to it's own high expansion ratio, diesel engine have larger stroke, therefore they extract more work before the exhaust valve open before BDC during expansion storke.
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