Improved FD

pretty good read from the Innovative Motorsports website, dispels some misnomers about rich AFRs, and explains why it does work to mitigate detonation

Rich ("mr. water injection") will be sure to add his 0.02



http://www.innovatemotorsports.com/resources/rich.php

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.

Improved FD

*shrugs* I thought the techies would like this

sdminus

Very nice write up.

At least you got up and looked into the tuning of the thing rather than following the flow

Scott

Kento

Anyone who has some decent knowledge of how gasoline-powered internal combustion engines work knows that overly rich a/f ratios cost power. I've never heard of the misconception that a rich a/f ratio "cools the intake charge", but I guess anything's possible in this internet-fed rumor-milling world we live in now.

For those who still think that water injection substantially cools the intake charge by vaporizing in the intake tract (rather than where it actually does vaporize, in the combustion chamber, dropping temps in that area, where it's actually more effective), note the huge latent heat value of water, especially compared to gasoline.

dgeesaman

iTrader: (7)

Checking my psychrometric calculator, this implies that water injection of .008kgH2O/1kg air has the same effect as 36% change in relative humidity. (100% humid air at 30°C contains .027 kg of water vapor)

That implies that humidity is a much bigger player than the supposed water injection setup. Am I missing something here?

Dave

Kento

Yes. The water in humid air is already vaporized.

dgeesaman

iTrader: (7)

Ok yes, the latent heat of vaporization is the key difference.

Dave

Improved FD

sure

I never thought that, but richening the mixture does have a cooling effect in the combustion chamber, and produces a slower burn...aside from loss of power, the other problem with richening the mixture is excessive carbon deposits

what the article did accomplish, for me at least, is substantiate the science of water injection

dgeesaman

iTrader: (7)

I think the misconception that richer is cooler comes from the idea that leaner is hotter. So relative to running too lean, richer is cooler.

Dave

Improved FD

it's not a misconception....richer combustion chambers ARE cooler, just not as cool as water injection

Kento

I think Dave was referring to cooler intake charge, not combustion chamber.
The fact that there are thousands of racing machines using water injection to quell detonation and working well is substantiation enough. It's the misconceptions on just what water injection can actually do is where people get confused on the "science of water injection", and that sales promo really did nothing to address it.

Improved FD

actually I think that article conveys the science of how it works quite well (although succinctly)

Kento

Uh, yeah, I guess you could say "succinctly"-- the only time he mentions the advantages of water injection are where he lists the latent heat of vaporization values, and where he mentions that a 10% ratio of water would have four times the cooling effect of the richer mixture (but all you need to do is look at the latent heat values listed and do some basic math to figure that out). The rest is a lonnng dissertation on why richer mixtures don't suppress detonation by cooling the intake charge, which is a misconception I've never heard of.

That thermodynamic/chemistry ramble is also a bit misleading (at least in this forum), because there's another reason why rotaries need that extra fuel: the comparatively large surface area of its combustion chamber means that more heat from the combustion is absorbed by the engine, and that extra fuel helps keep internal temps (i.e., the rotor face) in check. This is why tracked FDs need oil cooling upgrades; the rotary doesn't have the contact area of a piston engine (i.e., piston-to-cylinder) to transfer some of that heat, so it depends on engine oil to cool the rotor face, leading to sky-high oil temps under extended hard use.

Improved FD

I guess we all need to ride a new GSXR1000, right kento? or is the ZX-10 top dog this year? I haven't read the Sport Rider comapro yet

the ZX-10 is a monster, I've heard reports of about 168 rwhp bone stock, with a cat!!

t-von

iTrader: (8)

I know with my stock set-up, I get a lot of hesitation and break-up in the upper rpm's (when the A/F ratio goes full rich 10.0) with my home made WI set-up. I also feel a loss of power.

Improved FD

wow, 10.0??? are you running a Pettit or something?