924Board.org

[TroyDest]

Man, I'm really getting to like Geddy. Every time I'm ready to bail somebody out of a problem or set you guys straight on something, Geddy has beat me to it! Right on, you give valid advice without any BS. Again, I agree with everything Geddy said on this post. -Troy

[Geddy T]

Thanks Troy.
Rick, you wrote:

I like Geddy's answer, but I just thought that my mileage was high because of an exceptionally good balancing job on the engine (i.e., rods, pistons, crank, pully, clutch all turned) and my more gentle break- in driving habits along with a lower compression motor (8:1).

Balancing an engine with gentle break-in will DEFINITELY help your fuel economy. However, a LOWER compression motor will hurt your efficiency as it is the biggest factor influencing volumetric efficiency in the cylinders. Look at Hondas and whatnot (or even motorcycles for a real good example): Relatively high compression ratios out of smaller, yet high horsepower engines, and really high gas mileage. Raising the compression ratio is truly free power. With a high CR, each cylinder is doing more work per crank rotation with the same amount of fuel. The downside is detonation. A well engineered engine is one in which the highest CR possible without detonation is used. Car companies dump huge amounts of money into R&D of more efficient cooling systems and water channels and cooled valves and piston coolers and all of that stuff for the sole purpose of raising the CR and getting more power out of the same amount of fuel.

[Rick MacLaren]

My rebuild UPPED my compression from 7.5:1 to 8:1, so the relationship implied was certainly not that LOWERING compression RAISED mileage. However, Lars asked about a TURBO getting better mileage than a non turbo, as I understand his question. And in this respect my compression was LOWER on the turbo unit. My mileage went UP from previous turbo mileage and UP from my 924 mileage. Is this due to compression alone? No, and I hope my text didn't imply that - instead, I was aiming towards 'optimum' compression. Let's keep in mind no one can write a dissertation on the combustion characteristics of this particular internal combustion engine, just cause it's a 'one off' - you can't determine cause without actually measuring the causal factors you're discussing and showing the data. There just are NOT other cars with 8:1 compression from the factory that have the same electronic brain as this car, unless they are other 'one offs' created by guys like me. All you can say is the compression change 'likely' has made it 'more efficient' in an RPM range where the turbo was not often engaged.

Geddy, great answer, but I get the feeling, by the bike example, that you're implying the relation between compression and gas mileage is positive, nearly linear, and 1:1. Interesting. Still, that relationship you outline can't account for the fact that the city mileage for a 924 is lower than the city mileage for a 924 Turbo, yet the highway mileage for a 924 Turbo is lower than the highway mileage for a 924? This despite higher compression for the 924, and constantly lower compression for the 924 Turbo in the 'under 2500 rpm range'. The relationship isn't a straight line, so other factors MUST impinge on mileage than just compression. Given detonation, isn't the issue of the relation between mileage and compression that of finding an 'optimal' compression?



[ This Message was edited by: Rick MacLaren on 2002-05-29 00:16 ]

[Rick MacLaren]

Hey Vaughan, what's with the dates here?

[Geddy T]

Yes Rick, I totally agree. There is a direct relationship between compression ratio and efficiency, but maybe not 1:1 like you pointed out. I hope I did not imply that. Higher compression helps overall efficiency by increasing volumetric efficiency in the cylinders. Interestingly, an engine can have a volumetric efficiency higher than 100 (only with forced induction). What you said about the NA and the turbo's mileage makes sense to me. A turbo, in a sense, gives you a dynamic CR. When boost is low, so is efficiency. When boost is high, efficiency is high. And like I said before, this is to the limit of detonation. I think from looking at everything that we're both on the same page, though, and just not wording it the same.

[larso]

I'm still not sure if the turbo being really close coupled up to the engine is that much better, like it should be in theort, since there is more heat. Just look at the 951..it has the turbo on the other side of the engine. This may be because once the gases exit the engine and go further along the exhaust system, the gas has more of a chance to burn off unburnt fuel, causing more expansion. I'm not sure how long it takes for the flame coming out of the engine to expand to it's biggest amount of volume, but the placement of the turbo in the area where the gas/flame turns into it's biggest volume at the fastest rate (need a graph of exhaust expansion) would be the best place. When the exhaust first comes out of the engine, it's expanding at a really rapid rate, so you'd think that'd be the best place. Not so with the 951, so what's up? The speed of the gas flow slows down it gets further down the pipe, although with a restrictive turbocharger stuck in the exhaust maybe it does not affect the speed as much, and the flame stays in the exhaust for a much longer period of time. I know that the 951 is a really fast car, so the turbo being coupled up to the engine may not be THAT much of a performance gain as it may seem in theory.


[ This Message was edited by: larso on 2002-05-29 18:40 ]

[Rick MacLaren]

Thanks Geddy! So are the "clean burn" measures done with the local smog check equipment at your local garage 'good' indicators of the degree to which you're achieving good 'volumetric efficiency' or burn? Or how do you measure that efficiency? Or can you infer that efficiency by looking at other measures? Gas mileage isn't always on my list of priorities, but I have an intuition that the cleaner the car burns and the better mileage I get, that it's at least an indirect (and maybe direct) indicator of the operating efficiency of the engine.

Tangential note: I'm hooking up, along the same lines, a gauge to my 02 sensor to indirectly check the fuel/air mixture...the reason being that I'm not confident that my mechanic's appraisal of the richness measured AT IDLE is the same as the richness of the mix across the entire RPM range (esp at boost).


[ This Message was edited by: Rick MacLaren on 2002-05-29 23:45 ]

[Geddy T]

Rick,
Volumetric efficiency is definitely one measure of fuel efficiency. So are emissions levels. Volumetric efficiency is a really simple measure. Quoting from one of my vehicle research textbooks (which this string has gotten me thinking enough to look back through for fun):
Volumetric efficiency is a measure of the effectiveness of the induction and exhaust processes. Even though some engines inhale a mixture of fuel and air it is convenient, but arbitrary, to define volumetric efficiency as (mass of air inhaled per cylinder per cycle)/(mass of air to occupy swept volume per cylinder at 'ambient' pressure and temperature). Assuming air obeys the Gas Laws, this can be rewritten as (volume of 'ambient' density air inhaled per cylinder per cycle)/(cylinder swept volume).

If an engine can "cram in" more air than it can hold at one bar, it has a volumetric efficiency of greater than 100%. Most cars get around with something like 85% due to inefficient breathing or cam phasing. Most cars that achieve 100% or better are forced induction engines. Many really well-engineered engines (motorcycles in particular) have used resonant ram intake and exhaust tuning to achieve higher than 100% volumetric efficiency at peak rpm without supercharging or turbocharging.
Now, as for this relating to your air/fuel ratios and emissions... they're completely unrelated. Volumetric efficiency is related to engine breathing, not fuel. A boost pressure gauge would be more useful than a CO gauge for monitoring it.
The CIS on the 931 has systems built in to help keep up with fuel needs under boost, but I doubt they're perfect. You'll probably notice your CO ratio fluctuate between idle and full boost.
As I said before, volumetric efficiency has no bearing on air/fuel ratio (AFR) and vice versa, but they are related to each other in determining an engine's efficiency. Here's an equation for determining an engine's efficiency:
brake efficiency(Eb-for lack of ability to type true symbol) = [volumetric efficiency(Nv) * brake efficiency(Nb) * ambient air density(Pa) * Calorific Value of fuel(CV)]/AFR
so
Eb = (Nv*Nb*Pa*CV)/AFR

[Geddy T]

One more thing for Larso:
Sometimes turbo placement has more to do than packaging and cooling issues than efficiency in the turbine. That may be the case in the 951, I'm not sure. As for optimal placement of the turbine in reference to the exhaust port, this will take some explaining (I'm one of those people that has to carry a pen with me everywhere I go because I cannot explain anything without drawing a picture).
Okay, let's give this a whirl.
When the piston reaches the bottom of the stroke, the area that the hot gas occupies is very large. Then the piston comes back up and forces it out of the exhaust port, heating up even more both the exhaust gas and (unfortunately) the exhaust port. The exhaust now finds itself confined in a space smaller than the entire cylinder, but infinitely long (seeing how's the pipe goes out to the environment eventually). So the pulse is hot and wants to expand to equalize pressure to ambient and does this. The expansion travels down the exhaust piping, cooling and losing pressure as it does. In your post you state:
"I'm not sure how long it takes for the flame coming out of the engine to expand to it's biggest amount of volume, but the placement of the turbo in the area where the gas/flame turns into it's biggest volume at the fastest rate (need a graph of exhaust expansion) would be the best place."
I would think that you would want the turbine at a place where the gasses are at their smallest volume, not at the biggest volume (when the gasses have expanded to their greatest volume they are no longer able to do work). You want to place the turbine at the point where the gasses are compressed and have the most expansion left in them, which would be right away. By placing the turbine away from the exhaust port, you have already let the gasses expand to a volume equal to that length of pipe (expansion that could have been used to turn the turbine). Now, like I said before, sometimes this just cannot be done. This could be because of inadequate space or ventilation. And, like you said, having a glowing hot turbine and glowing hot header in such close proximity to your intake plumbing would heat your intake charge and cause a negative effect.
There are always exceptions to the rule, but this is just what I've been taught in class.

[larso]