Engine design and static CR

[kenikh]

I have been reading for ad nauseum about the folks who build their twin plug motors, using a static CR of 10.5:1 as a standard. Invariably these folks are running premium pump gas, but also adjust their ignition timing and mixture extra rich to ensure that detonation risk is minimized. Their goal is to get the extra "snap" and power that a higher CR lends without blowing up their motor from detionation. The question I have is:

What are these folks giving up?

This question arose from some other things I consistently read online and in text books:

1. Stoichiometric mixtures make more power than rich ones
2. The closer the piston to TDC when peak combustion takes place, the more power you make.

Thus, if you are inreasing CR, but compensating for it by making your mixture and timing less than ideal, are you giving back the gains from twin plug compression ratios?

Your expert opinions are welcomed here.

[davidppp]

Hi Kenik.

No expert ...but the ignition lead with TP is much less..

Kind regards
David

[kenikh]

David, I agree, but only at the same CR. BUT, I would guess the ignition lead on an engine 1 point lower is roughly the same (9.5:1 vs 10.5:1, single vs twin plug respectively). Thus the advantage of a twin plug motor would be only attributable to CR; the question is can you make up the difference by running a better mixture at 9.5:1 with the same advance. Thoughts?

[WigglyWorm]

Originally Posted by kenikh

Stoichiometric mixtures make more power than rich ones

Interesting question. I can't wait to see what the experts say. As far as the quoted comment above, I somewhat disagree. I would agree if it were a perfect homogeneous a/f mixture. But with any induction system, and especially with carbs, that is not the case. Some areas in the combustion chamber will be rich, some perfect and some lean. Richening up the mixture allows more areas to be at stoic. Plus a rich mixture will run cooler, which yields a little more power. At least this is my understanding. Again, I can’t wait to see what the experts say.

[304065]

A twin plug setup with NO change to the compression ratio is good for a few more horsepower, but not many. We know this experimentally: when you are doing a runup on a twin plug airplane engine, cutting out one set of plugs typically results in a 125 RPM drop. Admittedly, an O-360 has a 130mm bore dimension (!) so it needs all the flame propogation help it can get, but you get the idea. When one mag is turned off, the timing remains the same, but the combustion event is still going on when the crank is past the point of maximum leverage.

A second plug reduces the distance the flame front has to travel to ensure complete combustion. Since the flame speed doesn't vary much, reducing the distance it has to travel makes complete combustion occur sooner. As a result, you can reduce the amount of spark advance. All you are doing, however, is starting the ignition event later: the peak pressure point in the cylinder is still around the same time, at least for optimum power.

What does an increase in the compression ratio do to the propensity for detonation? Well, we know that the more the air is compressed the higher the charge temperature. When you raise the charge temperature too much and then ignite it, rather than a controlled burn, you get an explosion. Rather like making a good sauce with cognac: if you just pour it in and light it, not much happens. But if you pour it in and let it heat up for a minute and THEN light it, well, I hope you have a good stainless hood and a set of replacement eyebrows. Don't try it at home unless you trained as a Sous-chef with Jacques Pepin.

Now consider the effect of a mixture richer than stoch. A mixture richer or leaner than stoch will burn slower. Richening up the mixture causes the peak pressure to occur later in the cycle. Ideally, you would want the pressure to peak at the point of maximum leverage on the crank, which is around 80 ATDC. So richening up the mixture, by delaying the burn, moves the peak pressure closer to that point. You could do the same thing with a mixture leaner than stochiometric, but any time you go leaner, you are not using as much fuel as you could for a given swept volume of the cylinder. Because you have less fuel, you have less latent heat to be converted into energy to move the piston, hence less power.

This is why best power is found NOT at stoch but at 12.8 AFR. Even though you have more fuel than can chemically combine with the air, the fact that the rich mixture takes longer to burn results in greater mechanical efficiency.

So to review-- Rich-- less efficient chemistry but better mechanical advantage; Stoch-- perfect chemistry but worse mechanical advantage because peak pressure comes too soon, lean, less efficient chemistry, better mechanical advantage, but the piston can't push as hard.

As far as rich mixtures preventing detonation, this is a function of two things, one of which is combustion speed, and the other is the charge cooling effect of extra fuel. We know that detonation can be prevented by water injection. The fact that you use fuel or water doesn't make a difference, it's all about charge cooling. See this good article by Klaus for more info here. http://www.innovatemotorsports.com/resources/rich.php

So to answer your question, if you increase the compression ratio and back off the timing so far that you don't initiate the ignition event until so close to TDC, because you are concerned about detonation, that the peak pressure occurs past about 80 degrees, then you lose power. Likewise, if you richen up the mixture so much that peak pressure comes late, same effect. However, if you run around 26 degrees of advance with a detonation-resistant fuel, using a best power mixture will put the peak pressure point right where you want it, with the resultant increase in power.

Bring on the other experts now.

[kenikh]

Thanks John, that was awesome. This question comes from following guys running MFI so rich it burns the eyes to follow them. Sure they have twin plug motors, but it seems that they HAVE to be giving something up if they are running that rich. 12.8 doesn't seem like an AFR that would leave a "fuel fog" so thick behind a car it could kill cockroaches.

[304065]

I can tell you that I had a problem with my MFI pump and actually got black flagged in a race for "smoking." AFR was 10.8 and it was like Spy Hunter, the old video game.

Most guys with MFI have it adjusted wrong, which is bad for fuel economy, the environment and the car. But if one leaves the system alone, or if the thermostat isn't working properly, about 10.8 (or the warmup mixture) is where you end up. I've spent about five years working on mine and I'm only beginning to understand it. A quantum leap was realized when the LM-1 came out.

But now that you put it in context, those guys could lean the mixture out closer to stoch and retard the timing slightly and probably do better all round. It's the old, if a little is good, more is better phenomenon. . . .

[Eagledriver]

The max power mixture is about 12.5 to1. This is richer than stoch. It is not due to the speed of the burn being slower because if it was you'd just adjust the spark timing and get max power at stoch. It has something to do with getting the most energy out of the available air charge. Maybe due to cooling of the mixture from the extra fuel or by having enough fuel to ensure use of all the oxygen gets used.

The reason your aircraft engine puts out less power on one mag is that the timing is optimized for two mags. I suspect that if you were to adjust the timing (more advanced) you would find no drop in power on one mag. By starting the flame at two places you reach peak pressure sooner so you don't need to ignite the mixture as early.

-Andy

[davidppp]

Hello all.

FWIW the theoretical effeiciency of our heat engines is related to the temperature drop between the hot paart of the cycle and the cold part.

This will be maximal when adiabatic expansion is maximal and this implies there is benefit if the highest temperature occurs at TDC.

The problem with achieving this is detonation..the slower the burn the longer the lead and the more risk of detonation as the piston moves up the cylinder..

And very very fast burn can also be an issue so its not simple!

I do not think there is any real overall effect via leverage geometry : what might be gained at one point in the crank turn is lost elsewhere: it's the total energy converted which matters, not the instantaneous force..

Kind regards
David

[Walko]

Makes sense

[psalt]

John wrote,

Now consider the effect of a mixture richer than stoch. A mixture richer or leaner than stoch will burn slower. Richening up the mixture causes the peak pressure to occur later in the cycle. Ideally, you would want the pressure to peak at the point of maximum leverage on the crank, which is around 80 ATDC.

Hello John,

Peak output occurs when cylinder pressure peaks around 15 degrees ATDC, not 80 ATDC. 80 ATDC is way too retarded and corresponds to an ignition timing point ATDC, not BTDC. With MBT timing, pressure in the cylinder at 80 ATDC is roughly the same as 30 BTDC, in other words, the party is long over. This has more to due with the actual stages of combustion (ignition delay, rapid, controlled and final) than the mechanical leverage point of the crank.

The ideal mixture for best output varys with different engines, but since nothing burns below 14:1, the extra fuel is mostly a coolant to allow more advance. The probability of detonation is highest at the torque peak which is why there is a kink in most timing curves. Generally, you are passing this point briefly at WOT, not a steady state, and a richer than ideal mixture will allow a more aggressive curve. A good programmable EFI ECU allows you to adjust the acceleration enrichment and sustain (time) for this purpose.

Paul

[304065]

Theoretically, perhaps. The trouble I have with that explanation is that we know the engine does not behave as an "Ideal" Otto Cycle engine.

The Otto Cycle presumes constant volume during combustion. Theoretically, combustion occurs instantaneously at TDC.



We know this doesn't happen, however. The combustion process takes time, extending beyond the point where piston speed is zero at TDC.

Now look at the same P-V diagram for a Diesel engine.


Note that volume is increasing while combustion occurs. The piston is moving away from TDC.

Right away, we KNOW a 911 engine acts more like the second illustration than the first, because the engine utilizes ignition advance: combustion begins prior to the "constant volume" period when piston acceleration is zero.

David, question to test your proposition: if the engine power produced is independent of the relationship between peak pressure and crank position, why does advancing or retarding timing appear to affect it? Suppose we take an engine with a wheezy 6.7:1 compression ratio (a '75 930 with no turbo comes to mind) and run the engine on 110 octane leaded racing fuel with a stochiometric mixture. What would you expect to happen to power if the timing were advanced? Retarded? (If I'm incorrectly paraphrasing you I apologize.)

Would we expect to see power increases by advancing the timing or retarding it?

Paul, I agree that 80 degrees is a long way down. I'll go back to the books and see if I can figure out why everyone focuses on max leverage.

[psalt]

John,

If you have Richard Stone's "IC Engines", there is a complete discussion on timing events and output. Also, look at the pressure vs timing diagram in any BOSCH manual. Cylinder pressure at 80 ATDC is almost spent with ideal timing in an actual test engine.

Paul

[davidppp]

Hello, John.

I hope I did not suggest this was so simple!

And as far as I know a lot of the detail is still hotly (sic) controversial.

But the relationship between ignition timing and maximum BMEP is reasonably well accpeted I think...and seems to come with a peak pressure at about 10-15 degrees ATDC.

I would maintain however that were it feasible to achieve without detonation, peak pressure (ie temperature) at TDC would allow greater efficiency in the energy conversion.

None of this Carnot theory has anything to say about crank arm leverage..its simply about the maximum thermal efficiency.

Kind regards
David

[cnavarro]

Peak HP sometimes will vary on the engine too - I have seen some large displacement Porsche 914 engines (with adequate cooling and Nickies) make peak power in the 13.2-13.8:1 range, fuel injected of cource (running SDS with TWM I/R throttle bodies), with lots of dyno time spent developing the map. If memory serves me correct, running about 9.5:1 static compression with about 28 degrees max advance with a very tight deck, single plug. Twin plug running 10.5:1 static, i've seen as little as 22 degrees max advance being the sweet spot. These are all compact wedge chambers, so it's kind of apples and oranges in comparison to a 911.

I have the fuel map and dyno graph for one of these 2563cc type 4's on my web site on the fuel injection page for those curious about it. We dynoed the engine on carbs (44 idfs made the best power on carbs rather than the larger 48 idfs). Then switched to fi, with great improvements. The graphs for carb vs. fi are there too.

As already said, with carbs, it's a compromise- you shoot for what is right and more than likely, will end up with it just right at 12.8:1 at one spot and 10.5:1 on another - this was the case with my 1883 Porsche 356 engine with a large cam with lots of lift and duration.

Someone also mentioned dynamic compression - depending on what octane fuel available, a dynamic compression in the 7:1 to 8:1 range is ok and something to consider when choosing your cam, since it's easily possible to have one cam give you a dynamic compression of 5:1 and another 8:1, with the static being 9.5:1. I run models in Engine Analyzer Pro all the time to find the sweet spot for static compression for a given engine combination.