Why does advancing ignition give extra power?
Discussion
You want the greatest gas expansion when the piston is at the top of it's travel, as combustion takes a little time it needs to be initiated just prior to the piston being at top dead centre, hence the ignition advance.
Too much advance and the gas expansion will try to push the piston down while the crank is pushing it up, too little advance or retarding the ignition will push the piston down as the crank is pulling it down which will reduce the force of the gas expansion.
Too much advance and the gas expansion will try to push the piston down while the crank is pushing it up, too little advance or retarding the ignition will push the piston down as the crank is pulling it down which will reduce the force of the gas expansion.
mcford said:
You want the greatest gas expansion when the piston is at the top of it's travel, as combustion takes a little time it needs to be initiated just prior to the piston being at top dead centre, hence the ignition advance.
Too much advance and the gas expansion will try to push the piston down while the crank is pushing it up, too little advance or retarding the ignition will push the piston down as the crank is pulling it down which will reduce the force of the gas expansion.
Thanks - makes perfect sense. So the faster the engine is spinning, the more advance needed I guess?Too much advance and the gas expansion will try to push the piston down while the crank is pushing it up, too little advance or retarding the ignition will push the piston down as the crank is pulling it down which will reduce the force of the gas expansion.
A900ss said:
Thanks - makes perfect sense. So the faster the engine is spinning, the more advance needed I guess?
Yes and it's done automatically even in very old cars, it is an essential feature.Old cars have weights in their distributors that spin and via the increase in centrifugal force, working against springs, and a mechanical link, advance the timing.
Plus vacuum advance for other conditions where advance is required.
It's now all done electronically, pushing the envelope as far as it can go, and automatically scaling back if (say) you put in lower grade fuel, which is prone to premature ignition.
Good explanation here...
http://www.max-boost.co.uk/max-boost/internet_arti...
This is all dependant on engine design though. An engine in a low state of tune is less likely to gain from higher octane/more advance than a highly tuned engine.
On modern stuff with closed loop knock control will adjust advance for you and very little will be gained.
http://www.max-boost.co.uk/max-boost/internet_arti...
said:
Spark-advance control deals with determination of the engine position where the spark plug shall ignite the air-fuel mixture and start the combustion. It is thus used to position the combustion and pressure trace relative to the crank shaft motion. Engine efficiency and emissions are directly affected by the spark advance, due to its influence on the in-cylinder pressure. Work is lost to heat transfer and to the compression if it is placed too early, and expansion work is lost if it is placed too late. The optimal spark advance setting depends on several parameters, e.g. engine speed, engine load, air/fuel ratio, fuel characteristics, air humidity, EGR, air temperature, and coolant temperature.
said:
Development of an engine-fine-tuner for efficiency requires experiments to describe optimal engine output. Such a description is the basis for determining the set-point values to be used in the feed back scheme. In Figure 6, mean values, over 200 cycles, of the PPP (Peak Pressure Position)are plotted together with the mean value of the produced torque at four different operating points covering a large part of the road load operating range for the engine. Two of the operating points have an engine speed of 1500 rpm with different throttle angles, and for the two other operating points the engine speed is doubled to 3000 rpm. The PPP for maximum output torque in the figure lies around 15deg ATDC (after TDC) for all these operating points.
The above is based on the same fuel. If we increase octane and therefore have a slower burn the ignition timing will need to be advanced in order to keep the PPP the same. If a engine has been mapped for use in a wide range of markets, using a minimum octane of 95RON, using good quality fuel can allow more ignition advance.This is all dependant on engine design though. An engine in a low state of tune is less likely to gain from higher octane/more advance than a highly tuned engine.
On modern stuff with closed loop knock control will adjust advance for you and very little will be gained.
Edited by stevesingo on Monday 4th November 09:54
996TT02 said:
A900ss said:
Thanks - makes perfect sense. So the faster the engine is spinning, the more advance needed I guess?
Yes and it's done automatically even in very old cars, it is an essential feature.Old cars have weights in their distributors that spin and via the increase in centrifugal force, working against springs, and a mechanical link, advance the timing.
Plus vacuum advance for other conditions where advance is required.
It's now all done electronically, pushing the envelope as far as it can go, and automatically scaling back if (say) you put in lower grade fuel, which is prone to premature ignition.
I'll go look for a picture.
AW111 said:
996TT02 said:
A900ss said:
Thanks - makes perfect sense. So the faster the engine is spinning, the more advance needed I guess?
Yes and it's done automatically even in very old cars, it is an essential feature.Old cars have weights in their distributors that spin and via the increase in centrifugal force, working against springs, and a mechanical link, advance the timing.
Plus vacuum advance for other conditions where advance is required.
It's now all done electronically, pushing the envelope as far as it can go, and automatically scaling back if (say) you put in lower grade fuel, which is prone to premature ignition.
I'll go look for a picture.
Toltec said:
Going rather off topic, how many of us who do not want electronics interfering with the purity of driving would prefer a car like this?
I don't even want a manual choke back - but that doesn't mean that I want (for example) an automatic gearbox in my sports car or traction control that I can't switch off.My father used to ride a (second hand) Arial with manual advance in his student days.
I remember him talking about retarding the ignition to raise the exhaust temperature so as to de-carbon the exhaust.
My 1976 Ducati had a two-stage advance, where it advanced a fixed amount above about 3000 rpm, as i recall.
I remember him talking about retarding the ignition to raise the exhaust temperature so as to de-carbon the exhaust.
My 1976 Ducati had a two-stage advance, where it advanced a fixed amount above about 3000 rpm, as i recall.
A900ss said:
In simplistic terms, why does ignition advance give more power?
It's worth noting that it only releases more power if the original ignition timing setting was retarded away from a setting that makes the most BMEP. I.E. if the ignition is already advanced to MBT then advancing it some more reduces output!Most production engines are not "spark limited" at peak power rpm (although the latest bread of heavily boosted downsized ones can be) but are at peak torque rpm.
The maximum ignition advance than can be tollerated during spark limited operation depends upon numerous factors, such as mechanical strength and NVH concerns. (Advancing the spark means the charge is burnt at a smaller average cylinder volume, leading to an increase in the rate of rise of cylinder pressure, and that makes the engine sound "harsh" and gives components like head gaskets etc a hard time)
Excessive advance in the spark limited zone eventually results in detonation, and then pre-ignition, and generally results in physical damage to the combustion chamber and pistons.
wormus said:
Google "minimum best timing" or MBT.
FYI MBT stands for Maximum Brake Torque to the best of my knowledge, i.e. it refers to the value of whatever setting you're talking about (timing, fuel quantity, valve event timing and duration etc) that will acheive maximum brake toque at that specific condition (engine speed, altitude, ambient temperature etc)
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