Why retard ignition timing for forced induction?
Why retard ignition timing for forced induction?
(OP)
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
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





RE: Why retard ignition timing for forced induction?
Preempting too-early pre-ignition with even-earlier spark ignition doesn't seem sensible.
I'll let others respond to your question.
RE: Why retard ignition timing for forced induction?
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
RE: Why retard ignition timing for forced induction?
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).
RE: Why retard ignition timing for forced induction?
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.
RE: Why retard ignition timing for forced induction?
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
RE: Why retard ignition timing for forced induction?
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?
RE: Why retard ignition timing for forced induction?
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
RE: Why retard ignition timing for forced induction?
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.
RE: Why retard ignition timing for forced induction?
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
RE: Why retard ignition timing for forced induction?
RE: Why retard ignition timing for forced induction?
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.
RE: Why retard ignition timing for forced induction?
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
RE: Why retard ignition timing for forced induction?
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
RE: Why retard ignition timing for forced induction?
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.
RE: Why retard ignition timing for forced induction?
je suis charlie
RE: Why retard ignition timing for forced induction?
Boost = more pressure across the board
More pressure = Less stable fuel & hotter temps
More heat = Less stable
Too unstable = combustion knock
Combustion knock = pressure spikes
Pressure spikes = stressing metals closer to yield strength
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