|MadSci Network: Engineering|
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 organisation. 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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