Bengt Sweden

Ok let's beat this one a bit more ;-)

Correct but if the engine could withstand the pressure wave from knocking, then you could increase the compression ratio further and thereby get enourmous gains.
The power in an average (fast) otto engine is proportional to the compression ratio raised to 1.25

Tomas other note regarding rod/stroke relation is also correct but the effect is similar to lowering rpm, you get closer to knock. When the piston moves slowly away from the burning gases higher pressures build which is good for the force you want but bad for knock protection.

My understanding is that this is due to that the extra fuel is used for lowering combustion temperatures allowing more boost. If you just increase fuel from 12.5 without doing anything else you wont get any gains. Or?

Bengt

mark944turbo

"My understanding is that this is due to that the extra fuel is used for lowering combustion temperatures allowing more boost. If you just increase fuel from 12.5 without doing anything else you wont get any gains."

I agree, but obviously there is no way to prove it without a crazy lab setup. There is so much going on in there during the compression and exhaust stroke.

Does all the fuel always get burned during combustion? If not , how does the % vary with different conditions?

Bengt Sweden

No there is not enough air unless you are running closed loop and thereby stoichiometric mix. And still you wont burn all the fuel since not all hydrocarbons will find oxygen in time. And even if there was time, some oxygen would already have married with nitrogen since air consists mainly of nitrogen.
In the catalytic converter you break up the nitrous oxides and use the oxygen to either completely oxidise carbon mono oxides or to burn the hydro carbons that has survived.

Mixing, flame propagation ( and the tooth fairy) is important for a clean burn.

bngt

mark944turbo

Then its safe to say that a richer air fuel ratio will to a point increase the chance of all the oxygen finding carbons. In the exhaust, is the afr derived from the air and fuel that has burned or does the unreacted stuff have an effect as well?

theedge

IIRC that gets the open gap facing the center of the chamber, to allow for a better ignition.

Bengt Sweden

Yes, unfortunately you get other negative effects from too much fuel like multi carbon oxides (soot).

Oxygen sensors measures remaining oxygen. The oxygen content is then related to afr from experience, which means that it probably should be calibrated for every individual engine to be correct?

mark944turbo

"Oxygen sensors measures remaining oxygen. The oxygen content is then related to afr from experience, which means that it probably should be calibrated for every individual engine to be correct?"

I dont think so, im going to look that up.

mark944turbo

When the bulb is exposed to hot exhuast, the difference in oxygen levels across the bulb (between the atmosphere and exhaust gasses) creates a voltage.

So it will know the % oxygen in the exhaust left over, and assumes that the rest of the products are constant to deduce the air fuel ration. In that way, I guess you are correct, it would vary slightly not only between engines but also at different load conditions, intake temps, all that stuff. I think.

TurboTommy

Mark, that's a very good question; i wondered about this myself.

I guess the only way to test it (if you have an installed wideband) is to dial it in pig rich and run the car. Maybe with an adjustable FPR and a fuel pressure gauge (so you have a measured amount of additional fuel). Then we can see if the unburnt fuel has an effect on what the wideband readings should be (calculated from the measured addtional fuel) even if your engine management system is calling for the same amount of fuel (there would have to be enough throttle applied to be in open loop).

FSAEracer03

Well actually part of the reason the forced induction motors create higher numbers at slightly lower A/F ratios is, yes, the extra fuel. But it's not just allowing you to have more of an intake charge being shot in. It's also the fact that the motors run a little hotter, even with a good intercooler on them, and a little extra fuel will cool the combustion process just a tad.

Speaking of flame propogation, does anyone know of any good, extensive tests on the study of turbulent flow in a combustion chamber? I'm curious if any one with a clear-view motor had studied this and has photos to show. I only have a copy of one test performed showing how laminar flame speed robs engines of complete combustion.

By the way, a good book from a thermodynamicists point of view: Internal Combustion Engines by Ferguson and Kirkpatrick, for anyone interested in discussion like these.

mark944turbo

Yes, someone please post any high tech tests they have about combustion studies. Most of the books are over my head at this point in my school life (I havent taken thrermo yet) but I have seen some cool reports with graphs and pictures that are cool to look at.

FSAEracer03

Once you get through Thermo, you might be interested. If you go through Thermo II, you'll be even better off at understanding the material they present. It's up there... and it's not empirical, but wow, do these guys delve deep into the theory!

I'll post the flame propagation study when I get the chance. It's very interesting.

Tomas L

Why would a cooler combustion process give more power, if not the extra margin against detonation that the lower temperature provides, was used to increase boost or compression (as Bengt said)?
Since the fuel is injected close to the intake valve, the extra fuel will hardly cool the intake charge enough before the intake valve closes to give any effect on VE? Then the positive effect will have to be that the extra fuel acts like water injection, taking up heat on evaporation during the compression stroke. But I don't see how any hp gain could be had from this unless you use up the extra detonation margin that this provides by increasing boost or compression.
Am I missing something?

Tomas

BoostGuy951

This is what I have always thought.

Chris White

Most of the cooling from water injection or extra fuel is due to the heat energy absorbed by the vaporization process. The cooling by contact with the water/fuel in the manifold is minimal (unless you have a volatile enough fluid in conjunction with very high air temps).

There were a lot of interesting studies done on flame propagation during WWII for the development of fighter engines. I have (somewhere) a couple of photographic studies of flame propagation done with ‘see through’ engines. Neat stuff, but a little out dated. The test engines ran at very low rpm compared with our engines.

Chris White