Area47

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 from?

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.

shawns94

So basically you are saying that a leaner tune with less timing would create more power?

firestorm

awsome writeup area!
you copy/pasting foo!

i set my afr at a humble 11.2 under wot.
im a little worried about the stock fuel system,
running too terribly lean. im not sure who to trust tho.
id lean towards gm. no pun intended.
so whats the reaL DEAL, AREA?
what afr do you run?

Doc

I run my setup at ~12.3 WOT, it screams. I've been using this setup for over 29k miles without incident.

shawns94

What is your PE table EQ ratio vs. rpm set too?

Red05SS/SC

iTrader: (2)

Some of what he said is true but a good amount is not true, i'm not sure where you got your information from. First off the amount of fuel coming out of your injector has nothing to do with IAT. You can't even measure air temp. at the back of an intake valve where the air carries the fuel into the cylinders. You are correct as far as optimal AFR range and maximum power. Where your wrong though is that richer mixtures actually burn slower and make less overall power. There have been studies done and proven that only changing AFR between around 12 or so to 13.5 or so will do nothing to overall power. When you start going leaner than 13.5 your power will start to diminish slightly up to a point and then anything leaner will result in a major decrease in power. Also you kept stating "denser" mixtures. That doesn't even make sence. You can't compress liquid so saying fuel is less or more dense is false. intake air charge however can be less or more dense, hence blower, turbo, or even just altitude. You have to understand that an engine is nothing more than an air pump. More equals more HP. Less air equals less HP. Running a richer mixture than lets say 12.2 is actually used as an additional cooling method to cool combustion chamber and cylinder walls, not IAT. If you look at a torque curve on absolutely any dyno sheet, if you could measure air consumption of an engine the curve would match exactly to the torque cuve. Now the denser the air the fast it is going to burn thus decreasing timing when increasing boost but adding more fuel will not change the density and burn slower causing less HP like you said.

Area47

i never said it. i was sent that from innovate at work in an e-mail. i thought id share this with you. so take it as you will.


shawn, everyone tunes different. no two tunes will be exactly the same, and every car is different. so this must be taken into consideration.

Red05SS/SC

iTrader: (2)

ur the one that seems to be taking it personally, i'm not trying to personally attack you becasue of the information you provided. thats just how your taking it. i'm just saying that the information is false you can believe whatever you want i'm just stating information. I took an ACP course through the efi university and all of this information is covered. The instructor had published materiels and science showing this so you can take the information as you want i don't personally care i'm just letting everybody know

sunfirejoe

iTrader: (1)

i know im running my buddies cobalt at 12.9 to 13.2 right yah, 13.2

and with 13000 k on it, and running 15 psi, not one hiccup, actually infact it was good enough on slicks to run a 2.775 100 foot drag (we dont have a track its an annual event)

Doc

for ~12.3 - 1.15625 1.15625 1.15625 1.15625 1.15625 1.15625 1.15625 1.15137 1.14551 1.13770 1.12988 1.12500 1.12500 1.12793 1.13574 1.14258 1.14941 1.16016 1.17285 1.17578 1.17969 1.18750 1.18945 1.18945 1.17969 1.16016 1.15039 1.14355 1.14355 1.14355 1.14355 1.14355 1.14355

for ~11.4 - 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.13672 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633 1.23633

shawns94

Thanks for sharing DOC

firestorm

doc, do you have a separate file for race day?
and any objection to pm'ing me you mod list?
and can you confirm your handle on hpt too?
i know i ask a alot.....

Doc

I use two or three tunes. I've been working on a tune for the sucky SoCal gas. Problem is, I don't have the time to run the car.
My mod list is in my vbgarage.
I'm doc.ss on hpt
shoot me your email addy and I'll send you a few tunes I use.

Area47

i never take anything personally on here. i never said was taking this as an attack. i was simply stating where i got the info from. people often read things in a different manor then when people post them in. IE the readers mood.