Why retard ignition timing for forced induction?

[carwhisperer]

My understanding is that it is beneficial/common practice to retard ignition timing (i.e., fire the spark plug later in the compression stroke, closer to TDC) for a boosted engine vs. a comparable normally aspirated engine, and, to further retard timing in a way that is proportional to boost (not necessarily linearly).

What is the theory behind this? Is it that increased pre-combustion cylinder pressures promote faster flame front propagation, thereby achieving peak cylinder pressure in fewer degrees of crank rotation? It would seem to me that on the other side of the coin would be a need to advance timing to prevent pre-ignition, which increasing cylinder pressure would tend to promote.

Brian

 

[VEBill]

"...need to advance timing to prevent pre-ignition..."

Preempting too-early pre-ignition with even-earlier spark ignition doesn't seem sensible.

I'll let others respond to your question.

 

[Lou Scannon]

Let's come at it from a different direction...
Leaving NOx formation aside for the moment, it is advantageous to have combustion phasing as close to (but not before) TDC as possible, for best power and efficiency.
In a highly developed spark-ignited engine (which every automotive engine is today), combustion knock is usually what limits how close to TDC combustion phasing can be. Of course, one of the main control knobs is spark timing, which has to be set to ensure that the combustion phasing is no more advanced than can be tolerated by the engine.
In a boosted engine, the conditions that promote knock tend to be more severe, these being higher charge temperature, and higher in-cylinder pressures and temperatures throughout the cycle; added to this (at least when speaking specifically of passenger car turbocharged engines) are typically higher proportion of combustion residuals in the gas exchange process, due to the adverse exhaust to intake manifold pressure ratio.
Hence the knob to delay combustion phasing to later in the cycle, i.e. spark timing, needs to be used, resulting in less spark timing advance for a boosted engine versus an otherwise similar normally aspirated engine, and of course the more boost, the less timing advance.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Tmoose]

"to advance timing to prevent pre-ignition"

Retarding ignition timing a few degrees can be pretty effective at reducing/eliminating detonation.

Pre-ignition is a different flavor of aberrant combustion (although in extreme cases can be related).

 

[dicer]

I think the main thing that a boosted engine has most of in the cylinder charge as its being compressed is extra heat.
And for explanation purposes it could be very close to auto ignition temperature, that alone means you would not want to promote it as early as you would on an NA engine. Too early of ignition is a power loser, and detonation promoter.

 

[gruntguru]

What's more - in terms of cylinder pressure at the moment of ignition:
At a given crank angle before TDC, a boosted engine will have a higher cylinder pressure than an equivalent NA engine with the same unfired TDC pressure.

je suis charlie

 

[carwhisperer]

It makes sense to me that advancing timing can combat pre-ignition. But how does retarding ignition combat detonation?

Also, this may have been said another way but does flame propagation tend to complete more quickly, dollar for dollar, in a forced induction engine?

 

[Tmoose]

Hi carwhisperer,

you have said twice something along these lines -
"It makes sense to me that advancing timing can combat pre-ignition."

That effect is unclear to me. Could you explain how that might happen?

regards,

Dan T

 

[BrianPetersen]

I think he's thinking that igniting the mixture well before TDC, so that it has burned via the frame front before it has a chance to ignite on its own, means it doesn't have "preignition".

That might be so, but burning too much of the mixture BEFORE top-dead center is a pretty good recipe for holes in pistons! And blown head gaskets and beaten-up con-rod bearings.

If you start ignition via the spark (obviously before preignition occurs) and you start this process too far before top-dead-center, it raises the pressure and therefore the temperature throughout the combustion chamber including the parts of the mixture that the flame front hasn't reached yet. Starting this process earlier before TDC results in this happening with the piston higher in the cylinder (either closer to TDC on the compression stroke, or sooner in the expansion stroke before the piston has gone down as far) which means the pressure is higher which means the chance of detonation is higher.

Trading off preignition for detonation isn't a good thing to do.

 

[NormPeterson]

Wouldn't earlier before TDC mean lower in the cylinder and further away from TDC?

Over-advancing the ignition sounds like you'd be intentionally making the engine work against itself with the end results being less power at greater fuel consumption and poorer internal parts durability.

Norm

 

[carwhisperer]

Ok that makes sense. Is this a fair way to summarize a couple of these ideas? Advance timing on any engine, as engine speed increases in order to keep peak cylinder pressure in the neighborhood of 15 degrees ATDC, but retard ignition, relatively speaking, With forced induction, because combustion completes more quickly under boost conditions.

 

[BrianPetersen]

Norm, at the moment of ignition if it is well before TDC, advancing the timing might have the piston further down the cylinder, but by the time the combustion gets towards completion (which is when detonation happens), the piston will be higher up!

Peak cylinder pressure as mentioned above is usually somewhere near 15 degrees ATDC plus or minus a few (not many). If you have the ignition (say) 5 degrees too far advanced, and peak pressure happens 10 degrees ATDC, the piston is higher in the cylinder and that peak pressure (and temperature) will be higher ... more prone to detonation.

 

[JayMaechtlen]

Doesn't the mix burn faster when hotter, and also faster when at a higher pressure?
If so, then less advance would be needed to get full burn at the optimum time anyway.
But from what I understand, in lots of boosted engines, if you run higher octane fuel you can run more advance and get better power.


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[Lou Scannon]

But from what I understand, in lots of boosted engines, if you run higher octane fuel you can run more advance and get better power.

Quite so. Any optimized boosted engine today is knock-limited, so the more knock-resistant the fuel, the more timing advance that can be tolerated, with commensurate power and efficiency gain (see my post near the top).

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

Even for an engine that will not detonate under any ignition advance angle and will not detonate even when boosted, there is an optimal advance for maximum torque (MBT). When boosted, the same engine will experience a faster burn speed due to the increased temperature and pressure -so that the advance angle for MBT will be less.
If detonation is a factor, the increase in burn speed will make the allowable advance angle again less when boosted and the effect may be magnified to the point of requiring timing retarded to after TDC.

 

[gruntguru]

Same principle applies for NA engines in throttled operation. Used to be called vacuum advance.

je suis charlie

 

[Panther140]

You added more pressure, more heat, and have higher stakes when the engine knocks.

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Boost = more pressure across the board

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More pressure = Less stable fuel & hotter temps

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More heat = Less stable

Less stable = Faster combustion​

Faster combustion = less crank angle covered during onset of large pressure​

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Too unstable = combustion knock

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Combustion knock = pressure spikes

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Pressure spikes = stressing metals closer to yield strength

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