Engine design and effects on timing..

[nitro2]

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.

That's what I recall with one of your applications Bob. At a fixed boost, when the cylinder pressure placement was put where it should have been, making it efficient, a bunch of energy release ran away and hid. All in all probably not a bad feature of the combustion chamber for a turbo engine, in the sense that it plays it real safe at the expense of a little power/psi boost. A lesser tuner without the pressure equipment and a dyno may have ended up at the more advanced timing making high peak pressure with little or no more power than at a lower pressure setting.

On a turbo engine changing the combustion chamber to get more power/psi is usually not practical as one can just increase the psi.

In cases where boost is not limited, making a big fat pressure curve with as little pressure as needed (i.e below the max the engine can take but for as many degrees as possible) is the way to go, so long as the burn is completed quickly.

[wfolarry]

I started grooving heads on Harleys last year after reading about it on here. So far all the heads we've done have wanted less fuel & less timing to make the same power.

[automotive breath]

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 ???

Carl,
I have never tried combining a swirl port head with the grooves. My thoughts
are the swirl port would provide high turbulence early in the burning phase
for fast initial flame growth along with enhanced squish action from the
grooves during the final stages of combustion. It sounds like you are pleased
with the results. An iron head SBC running 10.7 comp/measured 212/218 at
.050" is remarkable, can you explain the characteristics of this engine? If
you decide to pull the head, please post pictures.

WFOLarry,
Are you seeing a reduction in detonation and or fuel consumption after
the modification?

[automotive breath]

... 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,
When you talk of losing the total energy being released; what happens to
the energy, is it released too late in the cycle?

When the flame speed is very fast because of an fast efficient engine/chamber
design, does the pressure curve resemble that of an engine with plenty
of ignition advance?

[David Redszus]

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?

Yes it is. There are several things that will skew 02 sensor results including but not limited to: free exhaust hydrogen, hydrocarbon vapors with molecular weights that are greater than diatomic oxygen, unburned mixtures, free radicals that continue to burn in the exhaust, etc.

As was previously well stated, advancing the timing will move peak pressure toward TDC, will raise chamber and piston crown temps and reduce exhaust temps. Retarding the timing will move peak pressure away from TDC, lower chamber and piston crown temps and increase exhaust gas temps.

One important consideration is that although we use the term ignition timing as a surrogate for combustion ignition they are not the same. Just because a spark plug fires does not mean that ignition (and therefore flame propagation) is immediate. Some amount of ignition delay is always present.

Chamber "hot spots" are a topic involving some confusion. Hot spots can be hot surface areas such as valves, spark plug ground straps, carbon build-up. Or they can be hot "volumes" consisting of volumes of residual burned gases.

If the hot spot is the spark plug, the ignition delay period will be reduced and flame propagation will proceed earlier. If the hot spot is the exhaust valve, the combustion flame front will burn toward the heated area first and then continue on to the rest of the chamber volume. If the hot spot is a carbon deposit, flame propagation will increase depending on location and temperature.

While we often hear about pre-ignition, we seldom consider post-ignition processes. It is very possible for the mixture to autoignite at some location in the chamber, after the spark plug has fired. Also, post-ignition can occur whether the spark plug fires or not. This produces an ignition mapping migrane since retarding the spark may have little or no effect on the initiation of combustion.

Inlet swirl velocities can be significant during the induction process but typically die out before TDC contributing little to flame burn rate. Swirl does improve charge mixing which can be very useful. Squish velocity reaches a maximum at about 10deg BTC and then again at about 10deg ATC, the later have higher velocity due to combustion gas expansion. Squish velocity can have a very significant effect on burn rate and therefore on the position of the peak pressure peak.

While we often reference combustion processes by peak pressure location, we are actually concerned about the intergral of combustion pressure rise and its location. This measurement is difficult to obtain without the use of TFX type measuring equipment and is seldom used the peak pressure angle being a usable surrogate.

Last but not least, all air/fuel mixtures are actually local in nature and may not be represented by global mixture values. Consequently, fuel sensitivity values become a critical factor in determining octane requirement and ignition timing curve shape.

[wfolarry]

WFOLarry,
Are you seeing a reduction in detonation and or fuel consumption after
the modification?

Yes. They typically run about 14:1 with 25' total timing. Before the grooves they would be about 13:1 w/35' total. No pinging, cooler running & smoothes out the idle.

[nitro2]

Clint,
When you talk of losing the total energy being released; what happens to
the energy, is it released too late in the cycle?

When the flame speed is very fast because of an fast efficient engine/chamber
design, does the pressure curve resemble that of an engine with plenty
of ignition advance?

Total energy release (measureable) is just that, total. Where it is applied (i.e. early or late) is the application efficiency. The way we work with total energy release and application efficiency is such that in the end IMEP is proportional to (total energy release x application efficiency). Both are equally important and you need both to generate the big IMEP numbers. Sometimes when one goes up the other goes down, which means more changes have to be made.

The pressure curve for a fast burn chamber doesn't normally look like that for a regular chamber with more advance.

Fast burn chambers are generally directed at having better application efficiency. Unfortunately the techniques used to generate it, can reduce the total energy release. Also in many cases the fast burn is only fast for the first half of the combustion process, then not very fast at all for the last half. Being fast in the last half is far more important than being fast in the first half - assuming the same total energy release for both examples.