Bengt Sweden

It's a general rule that AFR between 12-13 gives maximum HP, but is this due to flame speed, or the fact that you ensure that all oxygen is used?

Bengt

Bengt Sweden

I assume that the faster the burn, the later you can ignite and the less negative work (pre TDC) is done. Hence it would be better to increase boost than ignition if you still have a margin.

Also you would like to stay close to TDC until you have reached dangerous pressures which speaks for longer rods and a crank offset the bore centerline.

Bengt

Tomas L

Since 100% combustion would demand complete atomization of the fuel and complete mixing of the fuel with the air I don't think that 100% combustion will happen, at least in the combustion chamber. I'll guess that if you go from 14.7 to 12.5 you increase the chance of using all availible air for combustion since the air then will have easier to find a fuel molecule to react with.
The power increase could just come from that even if the combustion speed was lower at AFR 12.5.
I just speculating cause I really don't know.

Tomas

OriginalSterm

I used to do a lot of work with an "Optical Engine". Not exactly glass, but you could see everything going on in this one-cylinder engine with various piston geometry/intake port/spark location/timing designs. You wouldn't believe the visual difference from small changes.

TurboTommy

I'm not just assuming that flame speed is fastest at 12.5 AFR because it's a known fact that is the AFR for best power.
I've been interested in this topic for awhile and did some research, for what it's worth.
This is in fact the AFR that supports the highest flame speed. It is actually more accurately said that this AFR is good for best power BECAUSE in part of this highest flame speed mixture.
This best power AFR (for gasoline) is also because the oxygen in the air is more gauranteed a fuel molecule to react with, like Bengt and Tomas said. So, in the limited time available, a little more fuel will get burned. But this is also why the reaction is sped up, so the two dynamics feed each other. The third reason for this AFR being for best power is that slightly better thermo efficiency is achieved. Richer mixtures burn cooler and less heat is wasted into the head and cylinder.

Chris White

A little quote form some tech weenies at TRD -

“gasoline can be ignited in a non-stratified charge type engine between limits of roughly 11:1 and 20:1 A/F rations. Most gasolines will burn fastest at ratios in the 17:1 range. The Stoichiometeric or chemically correct rations is around 14.7:1 which results in the lowest average emissions. Best power is obtained with a rich mixture of around 12:1”

The speed of the flame front does not effect power unless it creates a situation where max cylinder pressure is obtained outside the 12 to 18 degree after TDC range. Faster flame propagation does not equal more power.

Tomas L

Isn't the whole point in this that you want the fastest combustion that will not cause any other ill effects (detonation for instance)?
If combustion is faster we can lower ignition advance and still have max pressure at the right time. And the later we can start combustion the less pressure will we create before TDC, as that pressure will work against the piston.

Tomas

tazman

There is a lot of talk about timing lately and this might not be the correct topic to put this in but since I am trying to understand all of this and it is somewhat related. Chris just mentioned that we are trying to get max cylinder pressure 12 to 18 degree after TDC. My question is how does this relate to values you see in the Tec3 ignition maps. Lets say you set a timing of 17 degree is that 17 degree after TDC and the mixture is ignited that quickly or what??

With the talk about water injection it sounds like you would actually want to retard timing some because the mixture will burn slower.

Up until this point I always thought you wanted to advance timing as much as possible without detonation but from reading this it seems that you could not have detonation and still be firing the ignition at the less than optimal time for best power. If this is true I wonder if a G-Tech would be able to tell the difference in power with the optimal timing.

mark944turbo

17 degrees is 17 degrees of advance, as in the spark plug fires at 17 degrees before tdc, then max cylinder pressure doesnt occur until 12 degrees after due to the time needed for the mixture to burn. The whole point of different ignition advance values is to achieve max cylinder pressure at the correct point, which is later in crank degrees, burning is not instantaneous.

JimV

Not to change the subject but is there a benfiet to filling the small area above the spark plug in the combustion chamber to increase quench?

Bengt Sweden

That maximum cylinder pressure at a certain point is optimal is of course a simplification. What you want is maximum work from a complete cycle.

If you have faster combustion you can have top pressure closer to TDC without too much pressure before TDC. If it wasn't for risk of knock and if combustion could be instant, then you would want to fire it all at TDC in order to make use of the energy for the full stroke.

Chris White

Mark is right, with the Tec3 set at 17 degrees BTDC you will have 29 to 35 degrees of crank rotation to burn the mixture. In timing speak we are referring to Before Top Dead Center and After Top Dead Center. To ‘add’ timing is to advance the ignition timing further forward of TDC and retarding is to take ‘timing out’ and run less advance.

When you think about it, it is kind of odd – to make something happen earlier is to advance it! To make it happen later is retarding it.

The positive numbers in the Tec3 map are all timing before TDC, negative numbers would be after TDC (you won’t see any of those!)

One other note – typically you can get up to 20 degrees of advance on a 951 relatively safely. 17 degrees is a good starting point but you can eek out a little more power with 20 – just be careful about high intake temps….

Chris White

Chris White

Quench is the area where the head and piston are flat and there is very little clearance. When the piston reaches TDC the mixture in the quench area is ‘squirted’ back into the center of the combustion chamber with great force – right at the already burning flame kernel.

This is why you have to be careful about some of the tricks - such as milling the top of 3.0 NA pistons to get a lower compression for a 3.0 turbo – you will ruin the quench and screw up the combustion chamber design.

Chris White

tazman

So going from 17 degree to 20 degree you are actually retarding the timing? That is what it seems you are saying but I thought the total opposite before. Man my lack of knowledge is really showing now

JimV

That's kinda the idea, weld in the area and deck the head. The piston is flat in that area. You would think it would cause more turbulance or mix. Any thoughts?