High boost flame propagation and Ign Lead.

[forcefed86]

So I’ve read quite a bit on timing and boosted engines. It is constantly mentioned that the more boost you run, the less lead is required. It is then briefly explained that the added pressure in the CC causes the mixture to burn more quickly. I don’t see an explanation as to why it burns faster.

I’d understand if the A/F ratio was leaned out it would burn faster, but we add fuel to match/maintain the AFR as boost rises. So why would a 10:1 A/F mixture burn any faster at 1psi of manifold pressure than it would at 20psi of manifold pressure? Isn’t it the same ratio of air and fuel being burnt?

 

[pacecarta]

simple explanation is ..when pressurized flame travel distance between the molecules of fuel is reduced

if you burn gas on the ground it burns slow and takes time to travel
higher pressure increases the combustibility of the mix

octane is added to slow the burn rate which is why you dont run a lot of timing with pump gas and why with race fuel you need to increase timing
as pressures increase you need to increase octane to prevent preignition

as for AF is that only means x pounds of fuel vs air , having more or less fuel wont increase the burn rate just the ability to consume all the air ,
if you run more than 14.7:1 AF for gas (rich) then there isnt enough air to completely burn all the fuel (FACT)
having less fuel (lean) it will consume all of the fuel
changing the af wont change how fast it burns

 

[forcefed86]

Ah… density! that’s what I was missing. Makes perfect sense, thank you! Lack of morning coffee does it every time!

Why wouldn't the AFR have a direct effect on “how fast it burns”.

Little blurb from innovates site I just found explaining how AFR does effect the speed of the flame front.

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.