You CAN be too Rich

[Edub1]

Application Note: You CAN be too RichBy 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/kgWater 2256 kJ/kgEthanol 904 kJ/kgMethanol 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.

[Chezedik]

That is incredibly well put! Good work! I have to say, I learned something.

[Craving4Boost]

I think instead of making AFR's really rich or retarding timing, we should invest in a cheap water injection kit. It wont sacrafice gas milage, and you wont lose power because of the retarded timing. Water is literally free and a small kit is easy to make

[Chezedik]

Yes, but it requires you to refill it. Unfortunately, with water weighing in at almost 8lbs per gallon, it would be hard to make a good setup that will last as long as a tank w/o costing TQ and HP. It has been known for years that water injection beats the crap out of running rich, but the immediate problem is: will you pull over to add more?

[Structure240sx]

my methanol lasts a while. its a half gallon tank and sprays at full once i hit 15psi. depends alot on how you boost

it would be alot better in an water injection kit to run distilled water (not free). you would want to make sure you have a good knock sensor to read.

personally, obviously, i rather run methanol than water. water would help cool but doesnt add the octane that methanol does. methanol also cools

at the local race shop methanol is $2.50/gallon not exactly pricey. i bought 4 gallons a couple months ago and still have about half that

[fiznat]

i rather run methanol than water. water would help cool but doesnt add the octane that methanol does. methanol also cools

I ran a 50/50 filtered water/methanol mix which I thought was a good compromise. The cooling effects of water (filtered in a Brita), combined with the octane boosting effects of methanol.

Water cools MUCH better than methanol alone. It is a function of specific heat, and S.H. of meth is only 2.55 compared to water's 4.184 Kj/kgh. Almost twice the cooling capability.

[Chezedik]

How do you trigger it to activate only at certain pressures? Do you use a throttle microswitch?

[scarboroughdub]

asside from all of that scientific jargin, it is a well known fact that a motor will make way more power leaner than richer. only thing preventing us from running leaner all the time is the engine ability to hadle knock and detonation. so cooling the intake charge is key.

[Craving4Boost]

is 50/50 better than 100 meth? also, do you guys do anything to keep the water for the water injec. kit cool? or it doesnt really matter whether its warm or cool? throw in some ice and let it melt to be super cold lol....

[fiznat]

Not to divert from the topic *too* much, but my meth kit was triggered by vaccum through a switch that ramped the meth spray from 8psi to 20psi (http://snowperformance.net/).

The temperature of the water isnt something that I ever really considered. The main reason water works so well to cool is because it absorbs so much heat (AKA high specific heat)... I suppose it could be even MORE efficient to cool the water down, but its really more than you need imo... with an FMIC AND a meth spray.

I liked doing 50/50 because it gave me both the cooling benefits of water and the octane increasing benefits of methanol. ...on top of that, you run through MUCH less methanol when you dilute it halfway with water .

[Edub1]

asside from all of that scientific jargin, it is a well known fact that a motor will make way more power leaner than richer. only thing preventing us from running leaner all the time is the engine ability to hadle knock and detonation. so cooling the intake charge is key.

You gotta appreciate a guy with no use at all for all that science stuff.

Seriously though, I think the point of the article was that the extra fuel molecules get in the way and cause the burn to proceed at a slower rate.

Because the oxygen is already used up, the gas might as well be water or whatever.

The take home message is that because of the nature of the chemistry, it's better to retard timing a little further than to enrich the fuel. Both meathods will prevent detonation but one will produce more power while doing so.

[4felix20]

Innovative also says that max power is made at an a/f ration 10-15 percent richer than stoich.

[Edub1]

Innovative also says that max power is made at an a/f ration 10-15 percent richer than stoich.

That's who wrote it. I believe him.

Klaus Allmendinger, VP of Engineering, Innovate Motorsports

[huguetpj]

Klaus does now his stuff. I've read all the articles posted in the Innovate site... and that probably is what sold me on buying an LC-1 above any other WB02. Plus, the cutomer support in the forums is great.

Anyway... I'm looking into running a seconday injection... kinda like a water injection kit... but using C16... I don't like the idea of injecting none combustible materials into my cylinders

[Structure240sx]

the guys at SMC told me that with the power i wanted to make with the kit that running striaght meth is the best bet to get the octane i should need

i asked if mixing with water helps more for cooling and they said that it was mainly for jsut economy , saving money on meth. i dont plan on changing it around tho

[huguetpj]

the guys at SMC told me that with the power i wanted to make with the kit that running striaght meth is the best bet to get the octane i should need

Ant, I've been reading this book by A. Graham Bell, a very good read for anybody interested in tuning. That is where I got my idea for the secondary injection. If I remember correctly he said that with methanol it is best to purge out the entire system if it is not to be used for a while, can quite remember how long he said but I don't think it was more than a couple of days, because of the deposits that can build up. There were other reasons I don't remember. He actually recommends Toluol (still trying to figure out where to get some), Avgas (the one with less lead) and Methanol in that order for a secondary injection. That's why I decided not to go with meth... since I don't have the time to be cleaning out an inyection system after each use. The book's in my house, I'll try to look it up for more detail later.

[Edub1]

Klaus does now his stuff. I've read all the articles posted in the Innovate site... and that probably is what sold me on buying an LC-1 above any other WB02. Plus, the cutomer support in the forums is great.

Anyway... I'm looking into running a seconday injection... kinda like a water injection kit... but using C16... I don't like the idea of injecting none combustible materials into my cylinders

Yah well what do you expect, the guy's name is Klaus. I'm wondering though, does that LC-1 hook up to a laptop for tuning? What else would I need?

[huguetpj]

Yes the LC-1 does hook up to the laptop. For precise tuning it would good to at least be able to datalog vrs rpm, for that you'll need the RPM converter (although I'm not quite sure it can be used with the LC-1) or other inputs like the Aux box (this can be used with the LC-1).