from the Evans Tuning Tech forum.... Combustion, Timing, Cylinder pressure and Knock
posted by imprezor » July 13th, 2005, 8:46 pm
Nice thread. Now here is what I know.
Ignition timing (or spark timing) depends on a bunch of factors. To stay close to the thread subject, I'm gonna talk about a few here.
First of all we're gonna talk a little about PISTON SPEED. As cylinder pressure is a function of piston position (volume) and spark plug timing (and many other things we're not considering right now),
you'll need to combine both to get the maximum cylinder pressure at aprox. 14º ATDC.
Remember that in piston/cylinder engines, the linear movement of the engine is converted to a rotational movement. That's what the rod/crankshaft combination does, so to move the crank you need to torque it, and to torque it you need to apply a force to the crank's arm.
Why 14° ATDC? Why that number?
The torque the crankshaft "feels" depends on the crank's arm length (that's why people stroke engines, stroke = 2*crank's arm length) and the force applied to it, and of course, the angle between the rod and the crank's arm. Cyl. pressure decreases fast as the piston goes down ATDC (so the force applied on the piston decreases too), and the angle betwen rod and crank arm increases. Max torque would be around 90º
(since RxF = F.R.Sin O where O is that angle, and Sin O = 1 when O = 90º)
but at that point cyl. pressure is really low so there's practically no force making work (giving energy to the system). Then, you need the better combination between maximun cylinder pressure (which is forced to be close to TDC) and the widest angle at which you can really get to that pressure.
So 14º aprox. is the point where the relation between cyl. pressure and that O angle is best.
You can get really high pressures at that volume (remember a crank angle means a volume displaced between the piston and the combustion chamber) and a not too good angle for the torque, but is the best you can get!
Now, as the flame front speed is almost always the same (there are small variations because the mixture conditions are fully able to change) you need to adjust spark timing so the time it gets the flame front to increase the cyl. pressure to its maximum value is almost the exact time it takes for the piston to get to 14º ATDC. BUT, that time the piston takes to 14º ATDC depends on the piston speed, which is a wave whose amplitud depends directly on the engine RPM. So as you increase the RPM level, you need to advance timing so you give the flame front the necesary time to increase the cyl. pressure to its max. precisely at 14° ATDC.
Now, after you have the goal of getting max. cyl. pressure to 14º ATDC, (and, on a dyno, you notice you got it right 'cause it's the max. torque output you can take of your engine, I think) you'll go through all the problems talked about below like fuel octane, of compression ratio, camshaft profile and timing, etc.
The reason why higher compression ratios are a lot more difficult to tune is because the graffic Jeff showed below depends directly to it. The safety range Jeff talked about, means that it's you who gets to actually "control" the cyl. pressure increase with more/less boost, plus, that natural increase (the natural pressure increase in the engine without boost) will be a lot smoother with a low comp. ratio, so you have a lot more "room" to move in. What I mean is that, with a low compression ratio, the maximun cylinder pressure will depend more on spark timing and less on RPM, 'cause when you lower the compression ratio, you lower the speed of natural pressure increase the engine has when the piston moves up. And that's why I said up here "it's you who controls the cyl. pressure increase".
Well that's like about it, pardon me if it isn't well written my english is not perfect as I'm from Venezuela.