MadSci Network: Engineering

Re: does high humidity level in intake air effect combustion engine performance

Date: Thu Apr 6 11:14:50 2000
Posted By: Justin Roux, Engineering and Physiological Scientist.
Area of science: Engineering
ID: 949703489.Eg

Dear Richard,

thank you very much for your question, and please accept my apologies for 
the tardy response. I have no excuse except my for my own poor level of 

The quick answer to your question is yes, a higher level of humidity 
increases the level of engine performance; I have certainly noticed this 
in my car on damp mornings (and we have plenty of them in the UK). I had a 
quick look into the chemistry of combustion because I suspected a 
catalytic effect caused by an increase in free oxygen in the mixture, but 
I'm afraid that I was unable to substantiate my theory when applied to 
that pressurised situation.

In the second case I approached my brother, Paul who works in filtration, 
jet engines, and all things technical. He was also aware of the 
performance boost and pointed me in the direction of some research that 
had been performed noting a marginal boost following water injection into 
a prototype engine. He went on to suggest that the extra 'pull' 
experienced in the car on a damp morning was likely to be from the 
vapourisation of the water adding to the combustion pressure.

I spent some time ruminating upon this answer and decided to speak with 
Alan Walker, a friend of mine who works for a large power turbine company. 
Apparently, some research was performed into the reduction of exhaust 
emissions by the injection of steam or water into the combustion chamber. 
The net result of the research was a 5% increase in the engine 
performance. It was at this point that bells started to ring in my head. 
From my own experience in process plants, I can tell you that it is common 
practice to inject steam (at about 180 degC) into the business end of a 
relief flare. This cuts down the emissions by giving a much fiercer 'lean 
burn' at the top of the stack and proves that combustion is enhanced by 
the water itself, not its vapourisation. However, we must remember that in 
this case it happens at atmospheric pressure.

Summarising these observations, we can say that an increase in humidity 
has a noticeable positive effect upon combustion and that it is the result 
of both vapourisation and chemical influences. Sadly, I cannot furnish you 
with data such as performance curves to illustrate that relationship, but 
it obviously has its limits. Firstly, the air can only accommodate a 
certain amount of water vapour, and secondly, your local fire department 
are also in the habit of adding water to combustion but with a very 
different effect!!!

I hope that I have answered your question - I have certainly learned a lot!
Thank you
Justin Roux.

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