To start we should understand that there is an ideal mix of
gasoline and air that is desirable for the most complete combustion. This ideal
mixture is referred to as the stoichiometric ratio and for typical gasoline
engines it is calculated at 14.67 or rounded to 14.7 parts of air to 1 part of
fuel as measure by weight. This ratio provides the most efficient combustion
with the least pollutants. An air fuel ratio above 14.7 is considered as a lean
mixture while any AFR below that is considered a rich mixture. Now with that
said the stoichiometric mix of air/fuel has changed some over the years partly
due to the addition of ethanol into the fuel and partly due to additives that
are added to the fuel. This puts the ideal AFR closer to 15.2 parts of air to 1
part of fuel. That’s right, the engine can actually use less fuel at idle and
light throttle cruise than it used to, due to these additions in the pump gas
we typically see here in the states. This is, in part, how we are able to
squeeze out increase MPG via ECU tuning. There’s a bit more to it than that but
it’s a considerable contributor to the more efficient combustion we are able to
achieve through tuning. It’s not surprising either when you consider that the
ECU for your car may be operating on calibrations that were intended for
gasoline types that existed 10+ years ago.
What about performance?
The ideal AFR for performance is 13.2 as this mixture ratio creates the fastest
flame front once the air/fuel mixture is ignited in the cylinder. The faster
the flame front travels, the faster the cylinder pressure is built up, which means
we can more precisely time the ignition event to allow the peak cylinder
pressure to act upon the piston top at the exact right moment and with the most
optimal pressure transient. All that geek speak to say, faster flame fronts are
better for engine power development. Now with that said what about engine heat?
Particularly with respect to the pistons… after all they are just aluminum and
with combustion temps reaching over 1600F and aluminum melting as early as 1200F
how do we keep the pistons from melting?
Keeping it cool
Combustion temperature alone is not a direct indication of piston temperature.
The cooling properties of richer air fuel ratios along with piston oil coolers
reduce piston and cylinder wall temperatures substantially. While 13.2 AFR is
the ideal ratio for power it is not uncommon at all to see AFR’s go as rich as
11.4 to add additional fuel that will change state from liquid to gas once it
comes into contact with the cylinders and pistons. That state change absorbs
heat energy as the fuel boils off. This is known in the tuning world as “lambda
protection” and it is done for the sole purpose of keeping the engine
cylinders, combustion chamber, and pistons from overheating. There is no one
ideal AFR for all engine configurations. Each engine can take a differing
amount of heat as compared to the next engine so developing an intimate
knowledge of each engine within a vehicle line is important so as to fully
understand the limits and balance of AFR as it relates to both power and
efficiency. Some engines can take higher temps for longer periods, other
cannot. This balance is what good tuners strive to understand and master.
AFR and the dyno
Chassis dyno testing is a popular way to test the power output of a given
engine and power train, however dyno AFR measurements need some interpreting to
be fully aware of what the data means. One undesirable characteristic of many
chassis dyno testing facilities is the insufficient airflow over the front of
the vehicle to adequately cool the intercooler and radiator. This can cause the
ECU to enrich the fuel mixture to reduce combustion temperatures as the charge
air temperature has risen as a result of this insufficient cooling.
Additionally since the AFR readings are typically taken after the catalytic
convertor, the remaining oxygen content in the exhaust is reduced which can in
turn cause the reading to show richer than the actual ratio. Since 1999 Volvo
models have been equipped with a factory wide band AFR front oxygen sensor so
proper datalogging though the OBDII port is possible and provides the most
As a side note it’s interesting that the factory fueling
maps as early as 1990 would drive AFR targets as rich as 10.7 if necessary to
protect the engine against both high combustion temps and to help ward of
detonation which can be very damaging to the engine and also happen to be
triggered by higher combustion temperatures. For many late model cars that
remains still although it is not as consistent model to model as it used to be.
Each engine configuration has its own particular requirements and
considerations and as engine technology increases, more precise calibrations
make each car a unique case in and of itself.
Robert Lucky Arnold