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Old 02-09-2015, 04:57 PM   #21 (permalink)
some_other_dave
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Quote:
Originally Posted by stovie View Post
... if the fuel requires 14.7-1 a/f ratio at atmospheric pressure then it souls only need about 147-1 at a 10-1 compression ratio do to the reduction in the fuel molecules distance apart....
The physical spacing of the molecules has nothing to do with it. What matters is the number of molecules of air, and the number of molecules of fuel. That's why the 14.7:1 ratio is in weights, because weight is a very good proxy for the number of molecules.

Chemical reactions use a certain amount of reactants, and create a certain number of products. For instance, when you burn hydrogen in an oxygen atmosphere, you get 2 * (H2) + 1 * (O2) => 2* (H2O). To make up the two H and one O from water, you need twice as much O2 as H2. Similarly, when you burn gasoline in air, you need very close to 14.7 times as much air as gasoline in order for the reaction to be complete. (That's the "stoichiometric" ratio.) More fuel means you have un-reacted fuel left, more air means you have un-reacted air.

When you have excess air, the combustion reaction changes. It can produce more heat, then as you go even leaner it produces less heat. It also produces more oxides of nitrogen, and can cause mis-fires where the mixture doesn't burn at all. It can even cause incomplete burning if the flame front gets too weak.

Many of those problems can be solved in different ways, but the NOx one is rather more difficult. Since NOx produces stuff like acid rain, it's generally a good thing to try to limit its production. Keeping the mixture to right around 14.7:1 makes the current catalytic converters work well, which reduces all of the emissions from the tailpipe.

I believe that there are catalysts that can work at leaner ratios, but they are more expensive. And the higher MPG is now being produced less expensively (or at least more profitably) with hybrid cars rather than lean-burn cars.

-soD

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