SQ5 and 100 Octane unleaded
04-04-2014, 06:13 PM
#11
AudiWorld Senior Member
04-04-2014, 06:21 PM
#12
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As you can see, plenty of leaded and unleaded choices. I run VP U4.4 and VP MR12 in my race bikes...these are leaded but of course the cat is removed.
04-04-2014, 10:34 PM
#13
AudiWorld Super User
It's the other way around. The octane refers to the number of chemical bonds, and more bonds = more chemical energy. However, they're also harder to break in the first place, which in practical terms means a higher compression ratio is required, or the fuel will only be partially combusted. (The exhaust will smell like farts or popcorn.) Lower octane can be used by retarding ignition until after peak compression has been reached, and it's decreasing. What's counterproductive is running higher octane than the peak compression ratio and combustion speed calls for.
The octane rating is only an average. If you mix 87 and 96 you get 91.5. However, the 87 components will be prone to ignite earlier and the 96 components later. This yields worse performance (slower propagation) than a more pure 91.5. 100 octane race fuel is very narrowly 100 octane, so even if the compression ratio would indicate it's subpar and can't be fully combusted, it can actually work better than the "right" octane.
The octane rating is only an average. If you mix 87 and 96 you get 91.5. However, the 87 components will be prone to ignite earlier and the 96 components later. This yields worse performance (slower propagation) than a more pure 91.5. 100 octane race fuel is very narrowly 100 octane, so even if the compression ratio would indicate it's subpar and can't be fully combusted, it can actually work better than the "right" octane.
04-05-2014, 07:03 AM
#14
AudiWorld Super User
04-05-2014, 09:30 PM
#15
AudiWorld Super User
"You do know higher octane fuel has less power potential, right? 100 octane may actually reduce power in an engine designed for 91. "
Sorry, Xenon, but this is RIGHT. Octane additives are there to make the gasoline burn slower, to prevent explosive detonation caused by high compression.
Which is why high performance, high compression, including turbo and supercharged engines, need high octane gasoline to prevent violent combustion blowing them up under the higher compression ratios.
Now, that's in "ordinary" engines. Modern engines use a knock sensor to detect pre-ignition and change the timing, as best they can, to prevent pre-ignition anyway. So in theory you can use the lowest octane you can find, and the engine will compensate for it.
Fuel maps? Yes, they are a limiting factor. Except the new Q5s are all "flex fuel" vehicles with huge fuel maps in the engine computers. We can run almost pure alcohol, which is VERY high octane even though it has very little fuel density.
Modern engines? The only change you will get with an octane change, is that the higher octane means the turbo can run at a higher compression, so in theory...you can get more turbo boost out of the car. And more raw power.
Except...<G>...that also depends on the fuel map, and whether they are going to allow the turbo to run much (if any) higher boost, for any rate of time. And that's a mystery you'd have to solve by running a boost gauge in the car (or reading boost from the OBD port in realtime) because Audi will simply tell you that is not the gasoline you were ordered to place in the vehicle.
Had the pleasure of knowing & studying under an actual combustion engineer once. He used to laugh and shake his head about "hitest" and say that was only proof that people need to understand the basics of the technology they rely on. And they just don't.
If the 100 octane generates higher boost pressure, then the car is getting something out of it. If the ECU isn't applying higher boost pressure, then the 100 octane is a total waste. Cut back the octane until max boost pressures start to drop, and that's the most effective point for max power.
Shouldn't make any difference for overall economy--just for max power available.
Sorry, Xenon, but this is RIGHT. Octane additives are there to make the gasoline burn slower, to prevent explosive detonation caused by high compression.
Which is why high performance, high compression, including turbo and supercharged engines, need high octane gasoline to prevent violent combustion blowing them up under the higher compression ratios.
Now, that's in "ordinary" engines. Modern engines use a knock sensor to detect pre-ignition and change the timing, as best they can, to prevent pre-ignition anyway. So in theory you can use the lowest octane you can find, and the engine will compensate for it.
Fuel maps? Yes, they are a limiting factor. Except the new Q5s are all "flex fuel" vehicles with huge fuel maps in the engine computers. We can run almost pure alcohol, which is VERY high octane even though it has very little fuel density.
Modern engines? The only change you will get with an octane change, is that the higher octane means the turbo can run at a higher compression, so in theory...you can get more turbo boost out of the car. And more raw power.
Except...<G>...that also depends on the fuel map, and whether they are going to allow the turbo to run much (if any) higher boost, for any rate of time. And that's a mystery you'd have to solve by running a boost gauge in the car (or reading boost from the OBD port in realtime) because Audi will simply tell you that is not the gasoline you were ordered to place in the vehicle.
Had the pleasure of knowing & studying under an actual combustion engineer once. He used to laugh and shake his head about "hitest" and say that was only proof that people need to understand the basics of the technology they rely on. And they just don't.
If the 100 octane generates higher boost pressure, then the car is getting something out of it. If the ECU isn't applying higher boost pressure, then the 100 octane is a total waste. Cut back the octane until max boost pressures start to drop, and that's the most effective point for max power.
Shouldn't make any difference for overall economy--just for max power available.
04-06-2014, 04:38 AM
#16
AudiWorld Senior Member
On a side note has anyone noticed what Mazda is doing with regards to engine compression? Higher compression ratios make for a more effecient engine. They are running 14:1 compression ratios. (due to american crappy fuel, they give us 13:1) which is impressive for a gasoline engine in mainstream production vehicles.
With direct injection and intakes designed to reduce the threat of pre detonation they have been able to achieve this on fuel with an octane rating of 91 or lower.
Mazda has also recently been talking about designing engines that under half load or less, won't use a spark plug to ignite in a gasoline engine but instead ignite the fuel like a diesel. Under full load, it will still use a spark plug of course.
Pretty impresssive stuff.
With direct injection and intakes designed to reduce the threat of pre detonation they have been able to achieve this on fuel with an octane rating of 91 or lower.
Mazda has also recently been talking about designing engines that under half load or less, won't use a spark plug to ignite in a gasoline engine but instead ignite the fuel like a diesel. Under full load, it will still use a spark plug of course.
Pretty impresssive stuff.
04-06-2014, 05:41 AM
#17
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On a side note has anyone noticed what Mazda is doing with regards to engine compression? Higher compression ratios make for a more effecient engine. They are running 14:1 compression ratios. (due to american crappy fuel, they give us 13:1) which is impressive for a gasoline engine in mainstream production vehicles.
Pretty impresssive stuff.
Pretty impresssive stuff.
VW , GM, and MB all have done considerable work on the HCCI engine. Mazda is stating a 2020 release date for Skyactive-2 with HCCI. VW stated a 2015 release in 2007 and that is not happening. Suffice it to say that significant engineering challenges remain.
Last edited by sdiver68; 04-06-2014 at 05:56 AM.
04-06-2014, 11:16 AM
#18
AudiWorld Super User
"Higher compression ratios make for a more efficient engine. "
They do. And the reason that the industry went to LOWER compression engines when emission regulations started coming in, is because lower compression engines are less efficient (goodbye mpg) but they also burn more thoroughly, so there are less combustion products to deal with, and cleaner emissions.
I think after 40 years the auto makers have gotten a better grip on emissions, and they're back to chasing hp from lighter smaller engines, which means boosting compression again. Computers that have more brains onboard than all of NASA owned 40 years ago, has got to help. Radical new stuff like hydraulic variable valve timing...It ain't your grand-dad's Oldsmobile.
Although I might trade it in a flash for a nice 4-4-2 or Vista Cruiser. (VBG)
They do. And the reason that the industry went to LOWER compression engines when emission regulations started coming in, is because lower compression engines are less efficient (goodbye mpg) but they also burn more thoroughly, so there are less combustion products to deal with, and cleaner emissions.
I think after 40 years the auto makers have gotten a better grip on emissions, and they're back to chasing hp from lighter smaller engines, which means boosting compression again. Computers that have more brains onboard than all of NASA owned 40 years ago, has got to help. Radical new stuff like hydraulic variable valve timing...It ain't your grand-dad's Oldsmobile.
Although I might trade it in a flash for a nice 4-4-2 or Vista Cruiser. (VBG)
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