Fuel Ratio and Thermodynamics
Country: United States
Date: March 2009
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?
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,
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.
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.
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Update: June 2012