Engine design and effects on timing..

[David Redszus]

A mixture that is very close to stoich (for that particular fuel) will require the minimum amount of timing (and spark energy).

That is an absolutely true statement. It can be confirmed by any combustion engineering textbook.

Unless something’s changed since Taylor’s MIT research prior to and during WWII the A/F (with gasoline) that has the highest combustion velocity hence requires the least spark advance is very near .87λ (12.8/1).

After a review of Taylors work (as well as Heywood) the texts still indicate that mixtures just slightly rich of stoich are most prone to detonate. The fuel ratio quoted by Taylor and Heywood and several NACA papers are a fuel ratio of .071, which converts to an A/F ratio of 14.08. While that is the ratio most prone to detonation, it is not the ratio for best power.

The A/F that requires the least voltage to ignite is very rich, on the order of 10/1.

Only if the engine is very cold and the fuel won't evaporate. Ignition is a function of energy, not just voltage. Excess fuel increases the energy requirement.

The voltage measurements are reported in 30’s era research so I doubt if there was CD ignition involved.

The spark duration of CD ignitions is very, very short. This can cause misfires if the mixture in the spark plug gap is not suitable. Which is why folks like Bosch have abandoned CD ignition systems as being obsolete.

Just how does anyone know what A/F ratio they are running? An oxygen sensor does not output an A/F ratio; it measures the partial pressue of oxygen and doesn't give a damn about fuel. If we try to use lambda, it must be based on a fuel with a known stoich value. What is that? Does anyone know the stoich value of the fuel they are using?

[Tuner]

You’re right about the F/A ~.071 and self-ignition sensitivity but abnormal combustion has nothing to do with spark ignition and flame velocity for normal combustion. The abundance of oxygen in relatively leaner mixtures (F/A .071) favors self-ignition and an abundance of fuel (F/A >.078) results in more rapid combustion and easier ignition by a spark. One reason F/A near.078 gives best power is because it needs the least ignition advance, the other is the higher mole ratio of exhaust product. Relative to leaner F/A the CO % is higher (twice as many molecules are formed from each O2) and the pressure is high in spite of lower temperatures.

Just how does anyone know what A/F ratio they are running? An oxygen sensor does not output an A/F ratio; it measures the partial pressur of oxygen and doesn't give a damn about fuel. If we try to use lambda, it must be based on a fuel with a known stoich value. What is that? Does anyone know the stoich value of the fuel they are using?

You answered your own question. Because the sensor measures the oxygen side of the combustion equation the stoichiometric value of the fuel is irrelevant.

[RW TECH]

Just how does anyone know what A/F ratio they are running? An oxygen sensor does not output an A/F ratio; it measures the partial pressue of oxygen and doesn't give a damn about fuel. If we try to use lambda, it must be based on a fuel with a known stoich value. What is that? Does anyone know the stoich value of the fuel they are using?

Most folks don't have the means or equipment to properly calibrate a UEGO, so my guess is (at very best) they're swinging it in the air and assuming an oxygen content of 21% or whatever.

I'm pretty sure some of the race fuel producers/suppliers don't know what the specific stoic value is for their fuels, so I have no reason to doubt that consumers don't have this information either.

[Tuner]

If you’re swinging it in the same fresh air the engine is burning then everybody’s on the same page. Production vehicles calibrate the sensor during overrun fuel cutoff to compensate for O2% variation with altitude.

I don’t see why this is so hard to understand?? The stoichiometric value of the fuel, be it cordwood or pure methane is totally irrelevant because the sensor is measuring the oxygen balance relative to the same air the engine is burning.

[David Redszus]

I don’t see why this is so hard to understand?? The stoichiometric value of the fuel, be it cordwood or pure methane is totally irrelevant because the sensor is measuring the oxygen balance relative to the same air the engine is burning.

There are several problems.
The sensor output is millivolts and nothing else. It does not know what air/fuel ratio produced the oxygen partial pressure in the sensor tip.

Sensors outputs are offset by chemical compositions in the exhaust stream, called the rich/lean shift. Also by temperature and by exhaust gas pressure.

Sensors must be calibrated to a specific exhaust gas to be accurate.

But these issues are minor. The problem is converting to an A/F ratio. And now the stoich value of the fuel has eveything to do with it.

[blow-thru]

Since the topic has come up can anyone clarify the use of Lambda values instead of A/F numbers.Is this due to the way fuel is blended and its chemical make up making the conversions eaiser or is it a manufacturers speak which has caught on in the aftermarket world ?????
Or are they basically the same "mixture number" just written differently???

Back on the topic of timing is there a trend to mixtures when varying timing ??? ie I have noticed that the mixtures tend to run slightly leaner when retarding the timing but generally stay the same when the timing is advanced either when cruising {vacuum advance} or full throttle....
Can the heat of the retarded timing be causing a more complete burn hence showing leaner mixtures.
I would also suggest that this would show up on a 5 gas analyser????

One last point, with the trend towards the least timing needed to make best power /torque { drivability aside} what affects does this have on cylinder pressures ie are cylinder pressures irrelivant of the total amount of timing and the timing is used to position the peak presures at the optimum crank angle ??????????

Cheers Carl...

[bobqzzi]

[
problem is converting to an A/F ratio. And now the stoich value of the fuel has eveything to do with it.

How so?

[automotive breath]

...there is more to my second part of my question...

“why such a low total timing figure in my engine?”

Ignition advance influences cylinder pressure development and ultimately
engine power. We initiate ignition before TDC to center the pressure pulse.
As shown in the diagram, increasing the spark advance moves peak
cylinder pressure closer towards TDC with higher cylinder pressure and
temperature. Reducing the ignition advance moves peak cylinder
pressure further from TDC with lower cylinder pressure and temperature.

Increased ignition advance creates more positive work; additional work is
also done against the rising piston (negative work). If we ignite the air
fuel mixture too late, peak pressure is too low with reduced power. The
optimum spark timing becomes a trade off, maximum power occurs when
these two opposite factors cancel. Ideally we want ignition close to TDC
for minimum negative work during compression with peak cylinder
pressure close to TDC for maximum positive work on the power stroke.

So far we haven’t talked about octane. It’s quite possible abnormal
combustion prevents you from reaching optimum ignition advance. I feel
the answer to your question “why such a low total timing figure in
my engine?” has more to do with insufficient octane for the high
compression/early intake valve closing than with it being a super efficient
engine . There is absolutely nothing wrong with your current tune; however
your test would be a good one, add octane and ignition advance in search
for more power. You will likely find more power (at a considerable
cost), not worth the expense because of the application and because:
“I have no problem with the way the engine runs”.

[Duxthe1]

From what I have read about ion sensing technology it was determined that having the peak cylinder pressure timed to occur at ~15ATDC produces the best power. Before 15ATDC the rod has a disadvantageous angle with which to push on the crankpin. After 15ATDC you start to loose peak pressure due to the increasing volume in the cylinder. At ~15ATDC the rod has the best leverage to transfer the enery from the pressure pulse into rotation of the crankshaft, without sacrificing peak pressure and efficiency.

[nitro2]

Increased ignition advance creates more positive work; additional work is
also done against the rising piston (negative work). However if we
ignite the air fuel mixture too late, peak pressure is too low with
reduced power. The optimum spark timing becomes a trade off,
maximum power occurs when these two opposite factors cancel.

True but there can be more to it as well. Some fast burn engines don't have negative work (via combustion) for a wide range of ignition timing settings. Optimum timing is dictated by the burn itself (total energy release and application efficiency). Sometimes changing the timing to apply the energy more efficiently causes a reduction in total energy release, and a net gain of zero. Why? The mixture dynamics change relative to crank angle, quench changes relative to crank angle and heat loss to the combustion chamber changes depending on how things burn. Basically the burn only proceeds properly over a certain range of timing settings and this may not be the range that is needed to generate the highest application efficiency. In this situation combustion chamber modifications are required to get to a win-win situation i.e. more efficient application of the energy without losing the total energy being released.


Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analysis Equipment)
www.tfxengine.com

[nitro2]

From what I have read about ion sensing technology it was determined that having the peak cylinder pressure timed to occur at ~15ATDC produces the best power. Before 15ATDC the rod has a disadvantageous angle with which to push on the crankpin. After 15ATDC you start to loose peak pressure due to the increasing volume in the cylinder. At ~15ATDC the rod has the best leverage to transfer the enery from the pressure pulse into rotation of the crankshaft, without sacrificing peak pressure and efficiency.

15 ATDC is optimum for some engines, but should not be regarded as a rule of thumb. High CR engines want peak pressure sooner than this, low CR engines later than this. Change the fuel, add boost or add nitrous and the picture changes yet again. Engine speed, displacement, chamber design etc. also factor in.

Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analysis Equipment)
www.tfxengine.com

[automotive breath]

... there appears to be a definate bias of mixture motion towards one side
of the cylinder until very late in the cycle,even then its not very uniform
...Is this the development of the "hot spots" or incomplete areas of
combustion due to unhomogonous mixtures ????

When I talk of hot spots, I’m referring to pre-flame reactions or early
stages of auto-ignition. These chemical reactions take place in the
unburned end gas due to excessive pressure and temperature before
the flame arrives.

Poor mixing of the fresh charge and exhaust residual plays a role in
development of “hot spots”, however I don’t completely understand this.
Elevated temperatures of the residual gas are a factor, I think there is
more, related to the chemical composition of the mixture in the area.

In a wedge chamber design, I feel certain the area of concern for
formation of hot spots is the typical arc that normally can be found in the
combustion burn pattern as shown. This area is influenced by several
factors including distance from the spark plug, wet flow and high exhaust
gas residual. In addition unburned air/fuel mixture in this region
undergoes high compression from both the advancing flame front as
well as the upward motion of the piston.

[bobqzzi]

True but there can be more to it as well. Some fast burn engines don't have negative work (via combustion) for a wide range of ignition timing settings. Optimum timing is dictated by the burn itself (total energy release and application efficiency). Sometimes changing the timing to apply the energy more efficiently causes a reduction in total energy release, and a net gain of zero. Why? The mixture dynamics change relative to crank angle, quench changes relative to crank angle and heat loss to the combustion chamber changes depending on how things burn. Basically the burn only proceeds properly over a certain range of timing settings and this may not be the range that is needed to generate the highest application efficiency. In this situation combustion chamber modifications are required to get to a win-win situation i.e. more efficient application of the energy without losing the total energy being released.


Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analysis Equipment)
www.tfxengine.com

I believe we saw exactly this on the engine I ran with TFX equipment. More timing increased pressure and placed it better, but there was only a tiny bit more power and much more heat released. From a tuning perpective it told us the small increase in power was not worth the extra strain on the components.

[MrWOT]

Something occurs to me. Formation of oxides of nitrogen. When we get cylinder pressures and heat way up, we get formation of NOx. Which would reduce free oxygen in the exhaust. But is it enough to skew the results from an O2 sensor significantly?

[blow-thru]

Increased ignition advance creates more positive work; additional work is
also done against the rising piston (negative work). However if we
ignite the air fuel mixture too late, peak pressure is too low with
reduced power. The optimum spark timing becomes a trade off,
maximum power occurs when these two opposite factors cancel.

True but there can be more to it as well. Some fast burn engines don't have negative work (via combustion) for a wide range of ignition timing settings. Optimum timing is dictated by the burn itself (total energy release and application efficiency). Sometimes changing the timing to apply the energy more efficiently causes a reduction in total energy release, and a net gain of zero. Why? The mixture dynamics change relative to crank angle, quench changes relative to crank angle and heat loss to the combustion chamber changes depending on how things burn. Basically the burn only proceeds properly over a certain range of timing settings and this may not be the range that is needed to generate the highest application efficiency. In this situation combustion chamber modifications are required to get to a win-win situation i.e. more efficient application of the energy without losing the total energy being released.


Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analysis Equipment)
www.tfxengine.com

Bingo this is what I was trying to explain in my question and Clint has summed it up perfectly ...
How does advancing total timing effect cylinder pressures ???? I presume it gives the gases further time to expand and consume all available mixture to create more cylinder pressure ????
Thanks AB for the info but I probably didnt explain my question to well, a bit to generalized and I got your definition of "Hot Spots" well worded!!!
I do definatly agree with you on the point of resisdual exhaust gases both being hot { adding to end flame temps} and diluting fresh incoming charge being a major cause of incomplete combustion and detonation.
Mr WOT also made a good point related to NOX's contaminating the charge and this further contributes to the point, it would be very interesting to obtain a chemical breakdown of exhaust gases and how they relate to other chemicals {ie fuel}....

For further info on my combo if people are interested I am running our premium octane fuel over here{OZ} which is apparantly 98 but I'm not sure how it compares to your 93 { I think} over there MON and RON numbers etc ....

Cheers Carl....
ps this is one for AB, I haven't mentioned this so far on purpose but I have grooved the heads as per your pictures{ PM about a year ago} Any comments on this ???
I am planning a manifold swap {dual to single} to test for my own benefit the results on my particular combo and was very seriously considering pulling a head for a look at burn pattens for the sake of a head gasket????
But in the mean time a 20lt of race gas is looking very good any suggestions on octane to use {dont want to use to much and not need it or skew the results}....