Fuel Octane differences?

[GScholz]

They all do eventually. The shelf life of gasoline is typically around three months. It will eventually turn into a vaseline like jelly. However the problem with 100/150 avgas was separation of the TEL octane booster, causing lead fouling of the engine.

[Oldman731]

They all do eventually. The shelf life of gasoline is typically around three months. It will eventually turn into a vaseline like jelly. However the problem with 100/150 avgas was separation of the TEL octane booster, causing lead fouling of the engine.

Yikes!  GScholz is back!  Dude!

- oldman

[GScholz]

Hey Oldman 

I pop in now and then to see if there's anything interesting going on. Not much lately though.

[colmbo]

I just wanted to clarify that the power gains are not from the fuel but the engine tuning.

What engine tuning?  What specifically are you saying is done?  Is it timing? Mixture?  What?

I have no experience with the super high-octane fuels but do have time in the B-17 and B-24.  Present day they use 100LL. We used the performance numbers for the 96 octane fuel since that is the closest spec in the operating specs for each airplane.  Using the 96 specs meant a slightly lower maximum manifold pressure but there were no changes to the engine "tune".  Just wondering what you're talking about.

[Devil 505]

What engine tuning?  What specifically are you saying is done?  Is it timing? Mixture?  What?

I have no experience with the super high-octane fuels but do have time in the B-17 and B-24.  Present day they use 100LL. We used the performance numbers for the 96 octane fuel since that is the closest spec in the operating specs for each airplane.  Using the 96 specs meant a slightly lower maximum manifold pressure but there were no changes to the engine "tune".  Just wondering what you're talking about.

My knowledge in engine mechanics comes from experience as an auto mechanic, but the principles of internal combustion engines remains the same. 

The manifold pressure is a measurement of the air/fuel mixture volume before entering the cylinder. It relates to the throttle setting. Where Octane comes into play is in ensuring that the fuel can handle the pressure and heat inside the cylinder during compression. As the mixture is compressed, it heats up, and is ignited by the spark plug as the piston reaches top-dead-center to propel it downwards. If the cylinder is too hot or the mixture is compressed too much, the fuel will ignite early and resist the piston's upward momentum towards top-dead-center - causing reduced power and possible engine damage. Camshaft and spark timing must be adjusted to ensure that the fuel ignites at the exact moment to provide maximum force on the piston. FYI: Diesel engines operate by being designed to compress the mixture until combustion.

While higher throttle settings will increase mixture volumes to a small degree, the large gains are made by making mechanical changes to increase either the volume of the cylinder or the volume of fuel/air mixture. Increasing the cylinder size would happen at a manufacturer level in the case of a military engine. So let's concentrate on increasing the volume of fuel/air inside the normal sized cylinder. An adjustment in camshaft timing can give some performance gains, but it is better to change the cam to one with a lobe shape to open the intake valves earlier and close them later. Another method would be to change the the gearing of the supercharger to force more air into the manifold and open up the fuel jets/injectors to account for more air. Same idea for turbochargers as well. The final method would be to add a cooling additive (such as nitrous oxide) into the manifold to super-cool the air into a denser charge, allowing for higher volume in the cylinder - also requires additional fuel delivery.

Hope that helps.

[GScholz]

[awrabbit]

good stuff.


I agree there would be some tuning involved and even with that,
 the real advantage would be with  the compression upped a few notches and some porting on the intake side of the heads to handle increased volume. larger valves intake and exhaust,  would keep the exhaust side on the outer port a little tighter tapered or, even maybe stock to increase the bottom end grunt of an engine. also the camshafts would need to be ground to optimize  and handle the the higher fuel air volume with more lift and duration/overlap. then ya get into valve springs, turbo charger gearing, better rings...... yada yada yada.  with that said engine life would suffer and would create more service work for the ground crews.

could you imagine running up on a 51 with those kind of mods and the fuel to go with it?   yikes.... then some NOS just for fun 

however the bottom line would be with a hotter plugs, hotter mags, and some induction tuning there would create some advantage with the higher octane fuels.



 

[GScholz]

It's not just a matter of compression ratios as these engines all used forced induction. With forced induction the fuel-air mixture is already compressed by the blower/turbo before being pushed into the cylinder where it is compressed again. With just some minimal tuning you could increase the boost from the blower and gain significant power without modifying the engine itself. The same with ADI systems like water injection, or adding an intercooler between the blower and carb. Just by adding water injection in the blower on the DB 605 in the 109G-6 (turning it into a 109G-14) they could increase the boost and get 300 more hp out of it.

It is essentially the same thing you see with the insane power outputs the "ricers" get out of their tiny Japanese engines running on racing fuel and huge turbo/intercooler setups.

[Devil 505]

With just some minimal tuning you could increase the boost from the blower and gain significant power without modifying the engine itself.

Right, but that still involves mechanical changes to the gearing and impeller of the supercharger. That is not exactly "minimal work." You are still looking at a manufacturer level change in design followed by the production of new parts.

[GScholz]

That depends on the blower design. On most designs you would at least have to change the belt/chain sprockets or change internal gearing. On the DB 600 series however the blower was not directly driven off the motor by a belt or chain, but by a hydraulic coupling. It was a large single-stage blower with variable speed independent of engine RPM. That's why the 109s have such smooth power curves. The speed of the blower was automatically controlled and also depended on air pressure/altitude. So on that engine increasing boost pressure was just a matter of tuning this blower control system.

[icepac]

Concerning testing of captured enemy planes on high octane US fuel...................

Yes the fuel is higher octane and better quality but it will not provide "extra horsepower" over the fuel the airplane was designed to run..............unless the "testers and technicians" who prepared the planes for flight deviated from the factory settings and advanced the ignition timing and raised boost pressure.

The only advantage likely seen by the testing would have been a safety margin concerning detonation.

The engine management systems are not like the ones in modern cars that can take advantage of higher octane fuel because the ECU will be able to use more advance.


I seriously doubt the guys prepping the captured planes for testing deviated from the factory settings as defined in the manuals and likely left timing and boost pressure unchanged from when they acquired the aircraft.

[GScholz]

It's not only octane rating that's important, but also the aromaticity and chemical makeup of the fuel. German synthetic fuel was too different from Allied petroleum based fuel that the RAF were unable to properly test the 109E before they had captured some German fuel in Africa. The DB would run rough and foul its spark plugs on Allied avgas.

[awrabbit]

We used to put 100LL in our cars in the old days for street racing and it helped out. was much cheaper than race fuel at the track. even with the additives for aircraft.   we would run a richer mixture in the secondary side and also jet up a little on the primary's and pull up the timing. I know for a fact Octane does increase power. for naturally aspirated  or blowers/superchargers.
you can see it in time slips at the drag strip.  pump gas 93/94 octane will produce a couple 10th's lower ET ( even better with cold dense air) and MPH. hotter fuel+ more air = more torque and Horsepower.

you can tell the difference between 8 lbs of boost and 10 for sure.

then I went to nitrous uhhg.... everything comes down.... timing,shift points.  the problem of stages and also the damage it does to motors when not tuned right. Valves/plugs/ lifting heads.   the weather factors in big time. unless you very conservative.

also you are correct increasing the boost and raising over all compression would be redundant.( what was I thinking )    with the super charging.  increasing the compression with the blower drive would give increase the power.  dropping the compression and using variable supercharger gearing. the 51 has a 2 stage super charger so there are options with gearing.

as for turbos.... I am just starting to get involved in the turbos and there is is a lot of tun-ability in the modern turbos.. not so sure about the technology of what was available back then. I know I am planning on twins on my Challenger this year.    going for 1100 hp on 94 pump gas.


well.... the USA fuels were better quality overall and if we are going on the test numbers from US testing then I think the over all performance is askew a little. just my thoughts.
perhaps not a big difference but, maybe enough to count.  I am sure that if we tested these aircraft the mechanics made sure they were well tuned. 

[GScholz]

In other words you had already made up your mind before even starting this thread, and it is essentially just a rant. Check.

I know for a fact Octane does increase power. for naturally aspirated  or blowers/superchargers.
you can see it in time slips at the drag strip.  pump gas 93/94 octane will produce a couple 10th's lower ET ( even better with cold dense air) and MPH. hotter fuel+ more air = more torque and Horsepower.

you can tell the difference between 8 lbs of boost and 10 for sure.

Octane has nothing to do with the fuel being "hotter" or not. There is no more energy in a gallon of 100PN gas than in a gallon 87PN. If you drive on 87 PN and 8 lbs boost you will make just as much power as on 100 PN and 8 lbs of boost. It doesn't matter what fuel they put in the captured Zeke as long as the engine liked it and the pilot stayed within the RPM and boost restrictions in the pilot's handbook.

[awrabbit]

In other words you had already made up your mind before even starting this thread, and it is essentially just a rant. Check.

Octane has nothing to do with the fuel being "hotter" or not. There is no more energy in a gallon of 100PN gas than in a gallon 87PN. If you drive on 87 PN and 8 lbs boost you will make just as much power as on 100 PN and 8 lbs of boost. It doesn't matter what fuel they put in the captured Zeke as long as the engine liked it and the pilot stayed within the RPM and boost restrictions in the pilot's handbook.

No was curious as to if the higher octane fuel was used for testing captured aircraft or if the performance numbers were from the country of origin of the aircraft.

I knew the allies used hotter fuel but, was unclear as to what was used or how the performance numbers were gathered.

 


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