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Name: Jesse
Status: Other
Grade: 12+
Location: CA
Country: United States
Date: March 2009


Question:
I am a student at Palomar Community college and we have started a rather "heated" debate in class. the question is about the air fuel ratio in standard four stroke internal combustion gasoline engine and its relation to power and heat. We know that to have a "perfect" combustion we need a volumetric ratio of 14.7 parts oxygen to 1 part fuel. However we also know that if we richen the mixture slightly, such as 12:1, that we will get more power and the combustion chamber temperature will decrease. Likewise, if we lean the mixture slightly, such as 16:1, we receive less power and the combustion chamber temperature will sky rocket. Of course this prompted the asking of the question why with a focus on the heat.

A question which the teacher could not answer, and thus the debate started. When the mixture is leaned why would the temperature increase even though less fuel is burned over all?



Replies:
Hi Jesse,

You are correct that in order to produce maximum power, a very small enrichment is helpful, especially when accelerating. But it is completely incorrect that a lean mixture burns hotter. The hottest flame temperature occurs at the stoichiometric ratio of 14.7:1. The old wives tale that a leaner mixture burns hotter, came about because years ago as a result of relatively primitive fuel metering devices like carburetors, most engines had to run a little rich (around 13:1 or so was common) all the time to allow reasonable "driveability". So when the mixture was leaned out to a "perfect" 14.7:1, more heat was produced. But mixtures leaner than 14.7:1 burn progressively cooler, not hotter.

The reason for this is pretty obvious when one thinks about it, and you were right to be suspicious. The amount of air an engine inhales at full throttle is the same, no matter what the mixture is. To get a lean mixture, you must inject less fuel. It makes no sense that injecting less fuel, will result in more heat! With that logic, injecting no fuel at all, will result in an infinite amount of heat! Of course, that would be nonsense.

As a mixture is made more and more lean, more and more air that is not used for combustion, and this air only serves to dilute the charge, and cool the combustion process. As stated above, the combustion chamber temperature is at its highest, when a "perfect" 14.7:1 mixture is used, and the temperature falls when the mixture is either made more rich or made more lean.

Regards,

Bob Wilson


The cylinder chamber of an internal combustion engine is approximately adiabatic, that is, the combustion is so fast very little heat is transferred to the walls of the cylinder during the combustion stroke. In addition, under "lean" conditions (excess oxygen), the combustion is so fast that the piston is essentially fixed in same position before and after the ignition of the fuel -- that is the volume is essentially constant. The "art" of the internal combustion is balancing the fuel mix, so that the combustion reaction rate is "in phase" with the expansion of the cylinder chamber after ignition.

A little digression here: One of the problems with using hydrogen gas as a fuel is its extremely fast reaction with oxygen over a wide range of concentration (7 to 74 % v/v). End of digression.

If the fuel ignites prematurely, the piston may be moving in compression mode -- the cylinder volume decreasing while the ignited fuel is trying to push the cylinder to expand the cylinder volume. This causes the classic "pinging" in an engine that is "out of tune".

Now you have to look at what is being heated in the combustion process. If the fuel / oxygen mix is "lean" (excess oxygen), the heat capacity of the gas in the cylinder consists mostly of nitrogen and CO2 -- all the hydrocarbon is already burned. This mixture of gases has a comparatively low heat capacity. As a result the temperature skyrockets (you are heating less gas and those gases have relatively low heat capacity (e.g.) nitrogen.

If you are on the "rich" side, two things happen. First, not all the fuel available in a given piston stroke is burned. You run out of "fuel". Second, the heat capacity of the gases is greater because you have all of this unreacted hydrocarbon available to "soak up" heat. As a result, the final temperature is lower.

It is possible to quantify all of this from thermochemical data, but you can see the trends without doing so.

Engine combustion is much easier to discuss qualitatively. If you really want a quantitative number things get much more involved.

Vince Calder



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