In addition to the ignition timing,
the other aspect of vehicle tuning that is most commonly
addressed is the fuel delivery. The amount of fuel being
sent into the combustion chambers is commonly measured as
an "air / fuel ratio" which is just like it sounds
- a number representing the ratio of the amount of air to
the amount of fuel being burned in the engine. An internal
combustion engine mixes fuel with oxygen in the air and
then ignites that mixture with a spark plug. From a strictly
scientific point of view, the optimum mixture of air and
common gasoline is around 14.6 parts of air to every one
part of fuel for an air/fuel ratio of 14.6:1. At this ratio
and under the right conditions, all of the gasoline and
all of the oxygen can burn leaving nothing except for the
combustion products. This is called the "stochiometric"
ratio. It is just like the ratio of 2:1 for hydrogen and
oxygen, as when they react (or burn) in that ratio and under
the right conditions, everything is used up and only water
(or H2O) is left. Fortunately for us, the oxygen in the
air is never completely used up when gasoline is burned.
The main reason for this is the fact that air is only 20%
oxygen, and the remaining 80% is comprised of things that
will interfere with a perfect reaction. If one were to mix
gasoline with pure oxygen, the stochiometric ratio would
be approximately 3:1 and the reaction would be entirely
more dramatic with a much greater chance of a "complete
burn."
Higher ratios, or mixtures that contain more air than what
is desired are considered "lean." Lower ratios,
or mixtures that contain more gasoline that what is desired
are considered "rich." The two terms are used
much like "retarded" and "advanced"
when describing ignition timing. Historically, the terms
were used to describe deviations from stochiometric, much
like "retarded" and "advanced" were
used to describe deviations from TDC, but they can also
be used to describe deviations from the desired point and
changes to the current state. "Leaning" the mixture
means adding air (or reducing fuel) and "richening"
the mixture means adding fuel (or reducing air.)
Since there is always some oxygen left in the exhaust gas
stream of a running engine, we have an easy way of measuring
the air/fuel ratio. An "oxygen sensor" can be
used to measure the percentage of oxygen left in the gas
stream, and a computer or other electronic device can be
used to back-calculate the air/fuel ratio that will result
in that particular oxygen percentage. "Narrow-band"
oxygen sensors respond with a voltage output that is sent
to the computer that is between 0 and 1 volt. "Wide-band"
oxygen sensors send a 0 to 5 volt signal which allows for
a much higher resolution and are therefore much better for
tuning. "Lambda" is a commonly used term that
is used in place of the air/fuel ratio number, as many devices
use or report lambda values. A lambda of 1.0 is equal to
the stochiometric ratio (14.6:1 for air/gasoline) and is
adjusted accordingly - a lambda of 0.82 is equal to 12:1
air/fuel ratio.
The "best" air/fuel ratio for a particular vehicle
is a matter of great debate and I will do my best to avoid
that debate in this article. Simply put, there are a number
of factors that one must consider in determining the best
ratio, including power, safety, and fuel economy. Fuel economy
is the easiest to understand, as a lower air/fuel ratio
means more fuel and obviously lower fuel economy. As far
as safety is concerned, richer is considered safer (to a
point) as the extra fuel helps things run cooler. The lower
temperatures help reduce the chance of autoignition and
can literally keep engine components from melting. The safest
air/fuel ratios are continuously being debated, but it is
widely accepted that 13:1 is a good ratio for normally aspirated
engines and 12:1 is good for forced induction engines. Many
choose to go even richer, even 11.5:1. Autoignition (or
"detonation" or "knocking") is considered
a critical concern with rotary engines, and many tuners
choose to go even richer than that. One must also keep in
mind that these "safe" ratios are considered safe
because they have been tried with many thousands of vehicles
over many years by dyno operators that use the same equipment
that most people are likely to encounter. Therefore, a safety
margin that takes into account the accuracy of that equipment
is inherently factored in. If it were common for turbocharged
cars to blow up at 12:1 as measured on commonly used equipment,
then the "safe" air/fuel ratio would have been
lowered.
As far as power is concerned, I'll say only this: Every
vehicle is different. If one wants to find the best air/fuel
ratio for generating power, one should put the vehicle on
a dyno and test it. Many believe that a particular ratio
will result in the most power under any circumstances, and
that belief is just too narrow-minded. There are far too
many factors involved to make such blanket statements.
Regardless of the actual ideal air/fuel ratio number, almost
everyone wishes to see a nice, flat air/fuel graph. This
means that the ratio stays constant throughout the rpm range.
A perfectly flat air/fuel graph is certainly not necessary
for optimum engine performance or safety, but it is a nice
thing to show off when tuning a vehicle. The smoother the
air/fuel curve, the better the drivability will be and the
smoother the power output will be. All good tuners realize
that a little variation with the graph is perfectly acceptable,
especially when one considers the factors involved. One
must consider the accuracy of the oxygen sensor, where it
is placed in the exhaust stream, the velocity of the exhaust
stream at different points in the rpm band, the tools that
the tuner has at his disposal to make changes, etc. Another
important factor is that most air/fuel ratios are measured
via a tailpipe sniffer. This method has proven to be an
excellent way of measuring the ratio, but it is not perfect
at low rpm. At low rpm, an engine may not be producing enough
gas to displace all of the atmospheric air in the tailpipe,
and this will produce a false lean reading because of the
extra oxygen - as one can see in this chart. This phenomenon
is going to be more pronounced in small-bore engines with
large diameter exhaust piping. Two important things must
be considered when one is tuning with a tailpipe sniffer
because of this phenomenon. One, a flat line across the
entire rpm band will mean that the actual air/fuel ratio
is too rich at low rpm. Two, a real-world driver is almost
never at wide-open-throttle at such a low rpm, so the air/fuel
curve at that point is something that the driver will never
see. One can also see from the chart that the catalytic
converter has no significant effect on the air/fuel ratio
in this particular vehicle.
The flatness of the air/fuel graph when one is done tuning
is mainly going to depend not on the competency of the tuner
but on the type of fuel management system being used and
its resolution, and the patience of the customer and/or
his willingness to pay for dyno time. One must also ask
- is a perfectly flat air/fuel curve best? Many assume that
a flat line at 12:1 or 13:1 "across the board"
is best, but why is that? How could it be possible that
the exact same air/fuel ratio be optimum for every rpm and
load? This idea has been largely ignored in automotive enthusiast
circles, as "good" tuners with adequate engine
management equipment produce air/fuel curves that are flat
"across the board" at the desired ratio. Thankfully,
this notion has been challenged recently, and experienced
racers and tuners have begun to realize that air/fuel curves
should not necessarily be flat. Turbos can spool up faster
if the ratio is a little lean during that time, and rich
ratios are more needed in the higher rpm range where more
heat is being produced. Keep in mind that wideband oxygen
sensors have only been in widespread use since the late
'90's, and chassis dyno testing has only become truly popular
in recent years. All of us are still learning. Few people
have been able to perform true scientific experiments, and
therefore few people truly have the knowledge to make blanket
statements concerning what is best for a particular vehicle
or group of vehicles.
When performing dyno testing and tuning, one must ask oneself
"what am I trying to achieve?" If maximum power
is the goal, then just look at the power curve first and
make adjustments accordingly. The fuel curve is only used
as an aid. Many NA race car owners tune in this manner,
and by the time they are done the air/fuel ratio is sometimes
between 14:1 and 15:1. This is usually not considered "safe"
by anyone, but most race car teams accept the fact that
they usually change the engine at least once during a typical
season. Most street car owners are willing to sacrifice
the 3 - 5 hp that they might get by running so lean and
instead opt for an air/fuel ratio that will help their engine
last for many years.