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post #46 of (permalink) Old 10th July 2008
Technical Supremo

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I can understand why most people think that more oxygen is the deal solution for running a petrol engine. The fact is that there is a trade off between too much and too little air.
Petrol concentrations below 1.4%, in air the mixture is too lean to ignite, and for those above 7.6% too rich; at all concentrations between these two limits, a mixture of petrol vapour and air will ignite.
But with more air (think oxygen) the combustion is likely to be complete with some oxygen not being used - this is the most efficient i.e. better economy. On the other hand reducing the air allows more power per stroke to be developed. So in reality allowing more air into the engine may reduce its power output but improve its economy. You just can't have the benefit of both although lean-burn engines such as the IDE and Gdi have attempted to address this problem but as we all know it hasn't been without its difficulties. No doubt engine designers for the public motor market have to strive to get the balance between the two - so it is actual possible that by allowing more oxygen into the fuel mix power output may be reduced. It all about getting the mixture correct for whatever you want an engine to do at a particular time throughout its power range. From my experience I have found petrol engines perform that wee bit better when the fuel mix is slightly rich but tend to be that bit thirstier and I have yet to see a high performance engine easy on fuel.

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