Application Note: You CAN be too Rich
By Klaus Allmendinger, VP of Engineering, Innovate Motorsports
Many people with turbochargers believe that they need to run
at very rich mixtures. The theory is that the excess fuel cools
the intake charge and therefore reduces the probability of knock.
It does work in reducing knock, but not because of charge cooling.
The following little article shows why.
First let’s look at the science. Specific heat is the
amount of energy required to raise 1 kg of material by one degree
K (Kelvin, same as Celsius but with 0 point at absolute zero).
Different materials have different specific heats. The energy
is measured in kJ or kilojoules:
Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)
Fuel and other liquids also have what's called latent heat.
This is the heat energy required to vaporize 1 kg of the liquid.
The fuel in an internal combustion engine has to be vaporized
and mixed thoroughly with the incoming air to produce power.
Liquid gasoline does not burn. The energy to vaporize the fuel
comes partially from the incoming air, cooling it. The latent
heat energy required is actually much larger than the specific
heat. That the energy comes from the incoming air can be easily
seen on older carbureted cars, where frost can actually form
on the intake manifold from the cooling of the charge.
The latent heat values of different liquids are shown here:
Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Most engines produce maximum power (with optimized ignition
timing) at an air-fuel-ratio between 12 and 13. Let's assume
the optimum is in the middle at 12.5. This means that for every
kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization
of the fuel extracts 28 kJ of energy from the air charge. If
the mixture has an air-fuel-ratio of 11 instead, the vaporization
extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air
has a specific heat of about 1 kJ/kg*deg K, the air charge is
only 3.8 C (or K) degrees cooler for the rich mixture compared
to the optimum power mixture. This small difference has very
little effect on knock or power output.
If instead of the richer mixture about 10% (by mass) of water
would be injected in the intake charge (0.008 kg Water/kg air),
the high latent heat of the water would cool the charge by 18
degrees, about 4 times the cooling effect of the richer mixture.
The added fuel for the rich mixture can't burn because there
is just not enough oxygen available. So it does not matter if
fuel or water is added.
So where does the knock suppression of richer mixtures come
If the mixture gets ignited by the spark, a flame front spreads
out from the spark plug. This burning mixture increases the pressure
and temperature in the cylinder. At some time in the process
the pressures and temperatures peak. The speed of the flame front
is dependent on mixture density and AFR. A richer or leaner AFR
than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results
in a faster burning mixture. The closer the peak pressure is
to TDC, the higher that peak pressure is, resulting in a high
knock probability. Also there is less leverage on the crankshaft
for the pressure to produce torque, and, therefore, less power.
Richening up the mixture results in a slower burn, moving the
pressure peak later where there is more leverage, hence more
torque. Also the pressure peak is lower at a later crank angle
and the knock probability is reduced. The same effect can be
achieved with an optimum power mixture and more ignition retard.
Optimum mix with “later” ignition can produce more
power because more energy is released from the combustion of
gasoline. Here’s why: When hydrocarbons like gasoline combust,
the burn process actually happens in multiple stages. First the
gasoline molecules are broken up into hydrogen and carbon. The
hydrogen combines with oxygen from the air to form H2O (water)
and the carbon molecules form CO. This process happens very fast
at the front edge of the flame front. The second stage converts
CO to CO2. This process is relatively slow and requires water
molecules (from the first stage) for completion. If there is
no more oxygen available (most of it consumed in the first stage),
the second stage can't happen. But about 2/3 of the energy released
from the burning of the carbon is released in the second stage.
Therefore a richer mixture releases less energy, lowering peak
pressures and temperatures, and produces less power. A secondary
side effect is of course also a lowering of knock probability.
It's like closing the throttle a little. A typical engine does
not knock when running on part throttle because less energy and
therefore lower pressures and temperatures are in the cylinder.
This is why running overly-rich mixtures can not only increase
fuel consumption, but also cost power.