ignition advance as RPM increases 4

[Mookinator]

i have a question regarding ignition advance.

since the combustion mixture in a gasoline engine always burns at the same rate, as RPM goes up so does the amount you can advance the ignition timing. my question is, what is that rate of advance? i know that every engine has different requirements. but lets say all things being equal and cylinder pressure didn't change with RPM, if an engine runs 10º of advance at 1,000rpm, what would the linear advance be? thank you.

~Mike

 

[MikeHalloran]

The mixture doesn't always burn at the same rate, so you can't just plug a flame front velocity and cylinder diameter into a spreadsheet and work from there. Historical advance curves suggest that that may not even make a good starting point.

For one thing, you've got acoustic effects and turbulence interacting with what would otherwise not be a steady flow anyway.

For another thing, some really odd stuff can happen in a cylinder. More than ten years ago, someone at Yamaha published (probably available as SAE papers) results that started as a study of the burbling noise in two-strokes. Briefly, they found that a plasma existed in some speed ranges, which autoignited the incoming charge without regard to spark timing ... and the engine ran better when the plasma was present, and didn't even need a spark. Then they found they could make the plasma stable over a wider speed range with a sort of throttle valve in the exhaust port. Yamaha went on to mass- produce a two- stroke motorcycle with computer controlled exhaust throttling, and decent emissions and efficiency, for a couple of years.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

The advance rate is not linear to rpm.

The advance does not always increase with rpm, and at very high rpm, some engines respond to some top end retard.

Some factors that change burn rate are, compression ratio, charge temperature, a:f ratio, fuel particle size and degree of vaporisation.

Typically at around 3000 rpm, the turbulence increases burn rate to an extent that no further advance is required.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[franzh]

When we play with ignition timing in the lab, we tend to set a number of baseline factors (when the OEM timing is not known or setting up a new engine):

Maximum timing at torque, where increasing timing no longer has an effect on torque;

Maximum timing before knock;

We then factor in emissions, reducing timing as necessary. The end result is a compromise and an always moving table as my esteemed colleagues have stated.

A number of other factors come into play: ambient air temp, coolant temp, throttle opening, engine loading, fuel quality or octane ratings, EGR activity, catalyst efficiency, and so on.

Franz

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[thundair]

Quote "since the combustion mixture in a gasoline engine always burns at the same rate"

Maybe not the 'same rate' but the same amount of time to burn. As all the pros have said there are a lot of variables to speed that burn rate up or slow it down..
The swirl/tumble and squish is the best way to speed up the burn rate, therefore requiring less ignition advance..
Diesel engines provide a good example of burn with out the aid of ignition advance..Very limited in RPM because of time to burn rate.
Most of the time the advance is created from empirical data and now we have knock sensors that retard the timing if things get out of hand...
My interest is to add another flame front as needed in lieu of auto advance..

Cheers

I don't know anything but the people that do.

 

[70AARCUDA]

...the old "round" reciprocal engines used two coils and two spark plugs per cylinder (especially at takeoff and METO power) which would effectively accomplish the same results...ie: two flame fronts converging upon each other for shorter but rather more consistent combustion duration.

 

[dicer]

Ignition advance in a diesel is accomplished by advancing the fuel injection start point. And all high speed diesel engines have advanced injection. An example of a variable injection advance is the older mechanical Cat 3406.

 

[Fabrico]

Many 2-stroke racing engines respond fairly radically to top end retard. Low to mid range suffer, as does starting, but top end goes wild.

 

[mattsooty]

Mookinator,

In a gasoline engine the speed of combustion does change with engine speed. However, the relationship is in no way linear.

For a given AFR & air mass per stroke (Load) increasing the engine speed will not only change the speed of combustion (ie 0-100% mass fraction burnt) but it will also change the ratios of 0-10 (ignition phase) :10-90 (flame front phase) :90-100% (post flame phase) MFB

Consider this; with a constant AFR & Load, the propensity of the engine to knock is affected thus: -

At low speed, where combustion speed is also low, the flame front will still be progressing through the charge when the end gases reach their Auto Ignition point & detonation will, therefore, occur. In this condition the position of Maximum Advance for Best Torque (MBT) may not be achievable before knocking occurs and the engine is said to be knock limited.

However, at higher engine speeds detonation may never occur because the charge is consumed by the quickly moving flame front before the end gases ever get near their AI point. In this condition it is often quite possible to continue to wind spark in until misfire occurs (because of the reduced residency time of the charge)

The change in speed of combustion is totally to do with the mixture preparation being improved by the increase in gas speed.

As mentioned above, squish & swirl do their part to improve matters at higher speed whereas a more useful mechanism at lower engine speed is tumble. An optimised combustion system makes use of all three in different areas of the engines operating range.


Dicer,

"Ignition advance in a diesel is accomplished by advancing the fuel injection start point"

This isn’t quite true; the point of ignition in terms of crank angle is pretty well constant on a diesel, for a given rpm.

The advance of the start of injection only really controls the rate of heat release; the more advanced the injection the quicker the charge (as a whole) combusts. This is because of the increased residency time of the fuel, allowing more of it to be in a combustible state when the 'ignition point' is reached.

"all high speed diesel engines have advanced injection"

Whilst I agree with that it’s not really the whole story.

The maximum engine speed in diesels is, to a great extent, governed by the speed of combustion of the charge. In terms of increasing this combustion speed both the geometry of the chamber and the atomisation of the fuel are very important (since the charge is inherently heterogeneous).

Whilst it is possible to fire the whole injection amount into the cylinder very advanced of TDC this then causes the most horrendous noise ‘diesel knock’ and also heaps of NOx production because of the greatly increased rate of heat release. Also ignition would occur at the same point if the only a small fraction of the whole injection amount had been injected prior to this point.

Modern diesels combat this by using an advanced pilot injection even before the main charge is injected, thus causing a relatively longer combustion event and a slower rate of heat release.

However, as always, that is not the whole story either. When other factors such as variable fuel pressure are taken into account (which a modern Common Rail Direct Injection engine will have) things become even more complicated, and this is not the time or the place to discuss!

MS

 

[patprimmer]

Thank you matty

I pretty much thought most of what you said, but was not sure enough to actually say so. It is good to see thought confirmed by people with real experience.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[automotivebreath]

Excellent post mattsooty, can you tell us more about altering
the three combustion phase percentages?

Here’s a writing on the effects of turbulence on combustion in
two strokes, instructing.

http://home.earthlink.net/~scloughn/id21.html

"...As the piston displaces fluid during the up stoke it
imparts kinetic energy to the fluid which gets converted at
a certain rate to turbulence. Obviously this increases with
rpm so higher rpm will have a higher turbulent intensity and
thus faster burn rate..."

 

[kilrbee]

Interesting thread. This is such a science (and I agree, with so many variables, there is no linearity) that even the automakers struggle.

Mookinator, if you wanted to consider "all things equal", then you would take your 10 degrees, convert it into time (us) before peak cylinder pressure, and then apply that time to higher rpms. Extreme simplification. IMO, load has more to do with changing up the linearity than anything else.

That is one idea, and excel makes it simple.

This is not a diesel thread, so I will start a new one.

 

[thruthefence]

The old "round" engines (as do the newer "Flat" engines) used two ignition sources for redundancy, not for any control of the flame front. It's a certification requirement.

 

[Lou Scannon]

Nevertheless, the dual system increases power vs a single plug system - recall the magneto test prior to takeoff where each system is switched off temporarily and an rpm drop is observed, which confirms the disabled system was working when switched on.

 

[CrdStang]

It might also be noted that Ford and Nissan produced dual-plug 4cylinder automotive engines. The Ford was a version of their SOHC 2.3L and IIRC the Nissan a 2.4L However, if it made such a huge difference, wouldn't everyone be building them by now?

Some extreme performance engines run dual plugs too.

 

[Junior33]

MikeHalloran, I would be very interested in reading the paper mentioned in your first post, I've searched the SAE database, but I don't seem to have enough criteria to find it. If you could point me in right direction I would be very appreciative.

 

[MikeHalloran]

Look at the references section of this patent:

http://69.64.183.131/6276334.html

Also see the Introduction of this paper:

http://www.llnl.gov/tid/lof/documents/pdf/244343.pdf

Mike Halloran
Pembroke Pines, FL, USA

 

[Junior33]

Thanks Mike that's been a big help I've ordered a couple of papers. I really appreciate your taking the time to point me in the right direction.

 

[thundair]

The multi plug has been beat before "Multi Spark Plugs" thread.

But the reason multiple plugs are seldom used is the packaging problem of fighting for space with the valves and squish.
Porsche has run multiple plugs in their race engines and I remember the info on it, and; advance was retarded 10* for every plug added.

This is related to the original question how?
It is like all the other variables in the combustion chamber that effect timing. There is no hard number nor linear number.
As of now there are too many things going on to calculate even with modern fluid dynamic software.

http://www.fluent.com/solutions/brochures/ic-minibrochure-final.pdf

You are looking for a number to start with, and tune from there until you can write something down in stone...

Cheers

I don't know anything but the people that do.

 

[JWaterstreet]

Thruthefence, not entirely true, as I was just reading about this topic in "the Romance of Engines". (good reading by any techno-geek) The earliest radial engines had two plugs for flame front control, not redundancy.

The redundancy requirement came much later...