Dave951M

So how would indexing a spark plug potentially affect the timing? It's an obscure trick that works well on NA engines, but I'm not convinced on forced induction systems.

Chris White

TAZ - nope, thats adding 3 degrees more advance...so its not retarding...
TO make it better to understand I should add BTDC. If the numbers were ATDC then that would be 3 degrees of retarding......

Jim – now we are going form advance & retard to above and below!!! I was thinking you were talking about welding on to the roof of the chamber! You might add more quench area but as to predicting the result…..that’s a tough guess. BTW – did you line up the head and the bowl on the piston to take a look at that interaction?

Chris White

Tomas L

Knock must be the most instant combustion possible to achive? So if we could get the engine to knock at TDC then we would have maxiumum power...
Of course we have to line the combustion chamber walls and the piston top with rubber to dampen the shockwave...

mark944turbo

Nope Tom, you still have it wrong.

20 degrees is 20 degrees of advance, as in the spark will fire 20 degrees before top dead center. "17 degrees" before tdc comes after "20 degrees" in the rotation. That is, unless I have been confused all along.

JimV

Chris,

Yes, I laid the piston on the chamber. The area im' refering to is directly above the flat of the piston, not in the dish.

Chris White

Actually you would have so much instantaneous power that things would (and do) break quite quickly!!
A lot of the knocking you hear takes place after TDC but it is so violent (5000 psi spikes) that it is more like whacking the piston with a big hammer than applying useful downward pressure.

Chris White

mark944turbo

Tomas,you do not want maximum pressure at TDC, that would just cause the connecting rod to explode into the crankshaft head on. I know you were mostly joking, but I wanted to make sure that part was a joke too. And the other thing is that it is pressure under the curve after tdc that matters, not just peak instantaneous power. Similar to how people want max area under the power curve on a dyno chart, not just a spike, like knock.

mark944turbo

OOPS, looks like I was missing this page of posts while writing my replies, that explains why they will seem a little late and random. Oh well.

Chris White

Jim – it might work out – just be careful of any sharp edges – this will cause hot spots and provide a good starting point for auto ignition.

JimV

Thanks!

Bengt Sweden

The force you would get from having all the fuel burnt at TDC ( without detonation) is not extremely high. The difference between 0 and 12 degrees ATDC for top pressure is very small. The problem is to control detonation and to not have adverse pressure before TDC.

As I understand it, a detonation or knocking occurs through ignition by heat induced by a pressure wave. The energy comes still from the normal release of chemically bound energy so the total energy is still the same. The problem seem to be the pressure wave coming from the knock that concentrates the energy release in time and also that there is a higher heat transfer to the piston and combustion chamber. otherwise I agree that controlled knocking would be ideal.

mark944turbo

"so the total energy is still the same" This is interesting, I have not studied chemistry and thermodynamics sufficiently to be an expert on combustion, I am just repeating what a knowledgable person told me. He said "even controlled knock is bad". If there is any real data on this subject I would like to see it.

Chris White

The main difference is combustion is a controlled process resulting in an orderly increase in combustion chamber pressure.
Detonation is a high rate uncontrolled combustion (explosion0 that results in extremely high localized pressures. The collision of multiple flame fronts creates some pretty nasty shock waves (these are the ‘pings’).

Energy over time is still probably the same….until the holes show up in the pistons…

Chris White

Tomas L

Yes my post was a joke but if we look at it seriously, this is what I think.

Knock is produced by the pressure wave from normal combustion reaching out to the outer parts of the combustion chamber before the flame front. Combined with it you also have heat radiation from the combustion. These two things heat and compress (which creates even more heat) the gases that have not yet been burnt. At a certain point these gases will ignite because of the heat. Since this is a volume of gas that ignites more or less simultaneously (contrary from normal combustion that starts at a point and travels from that point) it will produce a sonic (pressure) wave that is very intense but has a short duration, a spike. This spike will kill engine parts.
Energy wise you still burn fuel and you will not get any more energy from knock even if you could take advantage of it.

To get the torque produced at the crank you for a complete engine cycle (720 crank degrees) integrate the force on the piston created by the pressure in the chamber multiplied with the moment arm created by the rod and crank.
This means that all pressure created before TDC gives less torque at the crank and all after TDC gives more torque. In practice due to the rod/crank geometry, the first degrees after TDC will give such a small moment arm that no useful torque will be produced as Bengt already said. The interesting thing here is that a longer rod/stroke relationship will move the point were the useful torque can be produce later in degrees ATDC. The good thing about this is that pressure on the piston BTDC will produce less negative torque, and when combustion is finished and the crank has rotated to a position were the rod/crank geometry is better, it will stay that way for more crank degrees than with short rods.
I hope I expressed myself in a comprehensible manner here...

Tomas

FSAEracer03

Wow.... not sure how I missed this thread. This might be the best I've seen on here yet. This is actually exactly what we have been looking at in my High Performance Piston Engines class for te past 4 weeks or so. I'm going to have to sit down and read through all of this, but in my skimming, I didn't see one closely related item pointed out.

The target A/F for maximum power for an aspirated motor is about 12.5:1, making peak torque through the 12s. But forced induction motors actually have those numbers shifted down a little bit, making peak power around 12:1 and top torque numbers in the high 11s and low 12s.

Lets not let this great thread die away just yet