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

 

[dicer]

Mattsooty

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.

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Then you are saying the ignition point in terms of crank angle will not change if the fuel injection equipment is advanced or retarded?
Also are you talking about a certain minimal CR that will cause autoignition?

 

[SomptingGuy]

... also not to be confused with advance devices in older pump-line-nozzle systems, where the length of the pipe had to be considered when deciding when to hit the cam.

 

[dickon17]

Sorry guys, you all seem to be missing one fundamental point about the basic nature of combustion. In spark ignition engines, combustion is governed by chaos theory and is inherently unpredictable and therefore extremely difficult to precisely control.

It is this unpredictability about in-cylinder conditions local to the spark plug at the exact moment of spark generation which leads directly to the chaos. This results in cyclic dispersion or irregularity between power outputs from cylinder to cylinder and cycle to cycle.

Controlling cyclic dispersion is the holy grail of automotive design. Unfortunately, it is the fuel which holds the key and basic fuel technology has remained virtually unchanged for decades. As far as engine design is concerned we are past the point of diminishing returns.

 

[Lou Scannon]

Putting more energy into the ignition event (by orders of magnitude) reduces the variability but so far commercial technology for this does not exist.

 

[automotivebreath]

As opposed to increasing spark energy, consider adding
additional kinetic energy into the air fuel charge at ignition
and during the early stages of combustion. It's proven that
increased turbulence intensity reduces cycle to cycle
variation, reduces burn duration and establishes a more
consistent power output from cylinder to cylinder.

 

[dickon17]

Reducing engine cyclic variability is much easier using fuel technology rather than engine technology. This particular fuel technology was developed in 1992. It is commercially available, but not in widespread use because oil companies are not motivated to improve their fuel technology.