Compression Ratio and Octane

[[TWB]Sauerkraut-]

I'm a bit confused when it comes to the octane levels used during WW2.

 

For example, the Merlin engine had a compression ratio of something like 6:1. It required 150 octane fuel to boost to 25 lbs.

The DB 605 had 7.5:1, and on 100 octane couldn't even reach 14 lbs.

 

Modern performance engines with more than double the compression ratio can boost to 25 lbs on 100 octane fuel w/o knocking. Why is this? The fuel should be the limiting factor when it comes to knocking, so where does the difference come from?

[Nibbio]

Maybe electronic ignition control?

[40plus]

Intercooling allows for a much higher boost level.

 

Modern engine management systems can also detect and react to knock and dial it back a bit. That allows for teh engine to be run much closer to teh knock threshold without risk of catastrophic failure. Closed loop controls systems can detect air fuel mixture through the o2 sensor and react accordingly. 

 

In short, we're just better at it now.

Edited September 18, 2018 by pfrances

[Talon_]

Tighter tolerances, better machining, less oil where it shouldn't be and more where it should be, faster valve returns, better injection systems.

 

I guess the main thing to remember is that it's crazy we were able to get to 2,000hp from a V-12 at all only 40 years after the dawn of powered flight. We've had twice that long again to improve and the results are clear!

Edited September 18, 2018 by Talon_

[Kurfurst]

1 hour ago, itsthatguy said:

I'm a bit confused when it comes to the octane levels used during WW2.

 

For example, the Merlin engine had a compression ratio of something like 6:1. It required 150 octane fuel to boost to 25 lbs.

The DB 605 had 7.5:1, and on 100 octane couldn't even reach 14 lbs.

 

Modern performance engines with more than double the compression ratio can boost to 25 lbs on 100 octane fuel w/o knocking. Why is this? The fuel should be the limiting factor when it comes to knocking, so where does the difference come from?

 

You have already answered it yourself - there is a large difference in compression ratio. The high-octane DBs usually run at 8.3-8.5 : 1. That's almost 50% more load, so its a correspondingly lower manifold pressure. And these are very highly supercharged engines, developing insane amount of power. German fuel (C-3) itself was not an issue, since it had very good knocking properties at rich mixtures, comparable to 150 grade fuel.

 

There are also other factors, mechanical ones for example. Even if your fuel can take a given pressure without detonation, the power that the engine develop may still crack your engine block, wear out the bearings quickly or break the connecting rods. It took a lot of time and trial to come up with components that could stand up to the stress for a reasonable time. Even so, aircraft engines were of very low lifespan - a hundred or two hours of operation at best under practical circumstances. Prior the war, racing engines could pump out insane boost and powers - but their lifespan was measured in minutes, rather than hours. 

 

You also have to provide for sufficient cooling and lubrication for the engine. All that power generates heat. Then there is supercharging - in order to produce such high manifold pressures to decent altitudes, the supercharger also needs to be improved. 

[40plus]

Something else to add. While I have limited knowledge on war era engines, I've built a few race engines for both sportbikes and cars. Nothing incredible, but properly fun engines doing more than they really ought to be doing. For race engines, reliability is not a huge concern. They are built to run on the raggedy edge of what they can do. Chances are you will rebuild it at the end of a race whether it fails or not.

 

In a war plane reliability is hugely important. I would imagine engineers would not risk running the engines at such a level since a failed engine isn't just a lost race. It's a lost plane, pilot and if common enough, war.

 

Other things to take into consideration are dyanamic compression ratio vs absolute, fuel mixture efficiency, combustion chamber design, cooling efficiency yada yada yada

[Talon_]

26 minutes ago, VO101Kurfurst said:

German fuel (C-3) itself was not an issue, since it had very good knocking properties at rich mixtures, comparable to 150 grade fuel.

 

In fact C-3's octane content was closer to Allied 100 octane, which is why it required additives such as MW-50 and GM-1 to achieve similar horsepower levels.

[Kurfurst]

[Talon_]

Well known typo. This is known as "arguing in bad faith".

 

We know the Luftwaffe had to run captured Allied aircraft on C3.

Edited September 18, 2018 by Talon_

[ZachariasX]

3 hours ago, itsthatguy said:

For example, the Merlin engine had a compression ratio of something like 6:1. It required 150 octane fuel to boost to 25 lbs.

Magic of the intercooler.

 

3 hours ago, itsthatguy said:

The DB 605 had 7.5:1, and on 100 octane couldn't even reach 14 lbs.

No intercooler. 

 

3 hours ago, itsthatguy said:

Modern performance engines with more than double the compression ratio can boost to 25 lbs on 100 octane fuel w/o knocking. Why is this? 

Now we have good fuels and we do have intercoolers (sometimes triple intercoolers) on those high compression engines.

 

3 hours ago, itsthatguy said:

The fuel should be the limiting factor when it comes to knocking, so where does the difference come from?

You got that right. But it is one factor among several. Today we have good fuels. Allied fuels were just plain bad compared to what you get on the pump today and the German turd is literally the bottom of the barrel.

 

 

I see most have absolutely no idea of what constitutes a „good fuel“ for high performance gasoline piston engines. Hence also the numbers fetish that is much less relevant than you might think.

 

Let me just say this, octane rating, especially past 100, where it stops making sense technically, is not a measure of the fuel quality. It just is a measure of how fast it will ignite for practical purposes.

 

Diesel and methanol have a very high octane rating in that sense, however they do not have an octane rating as both compounds are not octane molecules.

 

The better fuel basically allows the very same engine to be run at a leaner mixture. It gives you mileage. And it will not foul your sparks. And keeps your engine happy in general. That is about it. If the Allies had 100LL as is common today, they would have been happy as clams. For most purposes it wouldbe far superior to what they had.

 

If you are really really interested in why that is happening, what is a good fuel and what good fuels enable you to do as engine designers (or why some design choices were made when having liquid turd at had) I can tell you. But I won‘t bother typing all that if the info above answering your questions will do.

[40plus]

17 minutes ago, ZachariasX said:

 

The better fuel basically allows the very same engine to be run at a leaner mixture. It gives you mileage. And it will not foul your sparks. And keeps your engine happy in general. That is about it.

 

Thermodynamic efficiency of the diesel and otto cycles scales with compression ratio. Mileage benefit is due being able to run a high compression ratio not the lean-ness of your AFR (though that does have an effect too, just a lot less so). Forced induction artificially boosts CR and thus thermodynamic efficiency. "Better" fuel allows us to run engines at these high CRs and not die while enjoying the benefit of better mileage.

[Talon_]

29 minutes ago, ZachariasX said:

If the Allies had 100LL as is common today, they would have been happy as clams. For most purposes it wouldbe far superior to what they had.

 

For what it's worth, we still manufacture 145-octane avgas especially for running these old warbirds at high boost because 100LL is not enough. Mostly for the Reno air races.

[ZachariasX]

1 hour ago, Talon_ said:

 

For what it's worth, we still manufacture 145-octane avgas especially for running these old warbirds at high boost because 100LL is not enough. Mostly for the Reno air races.

Actually, we do not do that and even racing fuel for obscene manifold pressures at Reno etc. are 100 octane, but not just any 100 octane, it is almost pure isooctane (stoichiometric ratio H:C = 2:1), the best you can have for gasoline engines. Just 10 bucks a gallon. But hey, you can afford a Mustang. ARCO is not for us, right? Plus it uses ADI, of course, water mostly.
 

Just so you get an idea of what fuel quality means, here:

 

Fact is that both German and British/America engines had about the same power output. They differed in design, but the design reflects the fuel at hand as much as the desired use. If Daimler Benz had Esso as fuel supplier, it would have made different engines. Same for Rolls Royce, had the only IG Farben as supplier. In both cases, they would hacve drawn about the same power from whatever they would have come up with as specific design.

 

On a somber note, 100LL AVGAS, that ist basically your Allied 100/130 octane (just WAY BETTER) is in peril, as leaded fuels are expected to be banned sometime soon in general, basically leaving no replacement for warbirds requiring this kind of fuel.

 

1 hour ago, pfrances said:

Thermodynamic efficiency of the diesel and otto cycles scales with compression ratio.

That is not your main concern if you are looking just for power. But if you are looking for milage, read again what I've wrote.

 

1 hour ago, pfrances said:

"Better" fuel allows us to run engines at these high CRs

If you put Diesel in your gasoline engine, you'd have even higher CR's but it would really run rough. But you are right, you can do it. But it's really not what you want.

 

That said, the aromatic compounds that are used (up to 50%!) to lace gasoline for these high CR are much farther from Gasoline is than Diesel. The engine just suffers a bit less with that turd.

 

(If you accidentally put in diesel in your gasoline engine, you can actually drive somewhat with it. Well, if you put in too much. Silly driving 400 miles on the wrong fuel. But you can do it, although your engine will hate it and it will run rough. The other way around, gasoline in diesel, that is fatal, you need to evacuate the tank. The diesel injectors are lubricated by the diesel, something that doesn't work with gasoline flowing through and you will kill your engine quickly.)

 

 

Since it is not really obvious what makes a good fuel, I might type up some facts on that that illustrate both what you have as fuels, and what it means in terms of designing an engine. Hopefully this will give people following these discussions a bit more insight and being able to judge the documents tossed around. I can always try and hope dies last.

 

 

[Kurfurst]

I think they made fuel for existing engines, rather than the other way around.

 

Much easier the change the composition of fuel than to design a brand new engine for a specific fuel.

 

Better milage is of importance if its significant - 10% better milage can allow you to save 10% of the fuels weight you need to carry to obtain any given range.

Edited September 18, 2018 by VO101Kurfurst

[ZachariasX]

Just now, VO101Kurfurst said:

I think they made fuel for existing engines, rather than the other way around.

 

Much easier the change the composition of fuel than to design a brand new engine for a specific fuel.

 

They should like to do so, but fuel is something you need to have for cheap in obscene quantities. You're stuck with what you have to start out with. Now, we can do that, and what we are producing is almost 98% isooctane (that will be your V-Power or comparable fuel). Today, we don't even have the same crude oil als 70 years ago. At that time the British (and Americans) could drill a hole in the sand and *splash* out came liquid black "gold", that through relatively simple destillation gave you almost 80 octane grade fuel. That was rather good stuff to invent the gasoline engine around it.

 

Today, we all drained this stuff from the ground and it doesn't exist anymore in that quality. Hence we do it almost like the Germans, we have to deal with the large rest, that in most cases is not even liquid. But by blowing hot water along with poisenous chemicals in the ground, we can wash out the residual dirt (at the cost of pollution the other ressource that we need much more, subterranean water).

 

What we get is something we can now, through elaborate sythesis (comapred to what they did in the 40's) make isooctane. Or your shirt.

 

The Germans could very well make isoparaffins as well as isooctane. But it was tedious and you would never ever have made enough for whole armies.

[Ehret]

10 minutes ago, ZachariasX said:

What we get is something we can now, through elaborate sythesis (comapred to what they did in the 40's) make isooctane. Or your shirt.

 

So much trouble, now - how good would be the methanol+ethanol, then? Per unit of weight alcohols have less energy but burn cooler and have high octane rating. Maybe, high compression engine at relatively fast RPM could be feasible and good enough?

[Talon_]

41 minutes ago, ZachariasX said:

Actually, we do not do that

 

http://www.warteraviation.com/fuels/avgas115-145/

[Kurfurst]

The thing is, the development of aero engines you see in the planes in WW2 started out in the early 30s or even earlier. They were all designed for the fuel that was available back then (basically 87 octane or even lower), but since it took about 10 years to design a completely new block from scratch, and at least 3-4 years to come up with something semi-new, fuel had to be adopted, since it was much quicker - within reasonable limits of course. Spark plugs of the time were not great either and there is all kind of problems you will run into, especially with highly leaded or highly aromatic fuel. 

 

Armies did not needed high quality fuel though - octane ratings for motor gasolines were horrendously low, something like 68 octane IIRC. Trucks, tanks cobbled around on that stuff. They could do with the most basic stuff, the aviation engines had to get something better - hence all the German avgas was synthethic, since they could produce iso-octanes as well, which was added to the base stock.

 

 C-3 itself contained something like 20% iso octanes and 40% aromatics and some .12% TEL to boot - late B-4 even pumped up TEL to .16%, w-o added iso octanes) . Its quite a nasty goo and was probably horribly expensive to make,  but as I understand the process was such that optimal production was to produce about 1 part B-4 and 2 parts of C-3 to get the maximum total volume.

 

Training fuels like A3 (and early biplane fuels IIRC) were a bit of a cheat, containing copious amounts ethanol - less energy content, but cleaner burning and with less knocking.

 

We have something similiar today for cars - stuff like E85 is basically Ersatz fuel.

Edited September 18, 2018 by VO101Kurfurst

[ZachariasX]

13 minutes ago, Talon_ said:

 

http://www.warteraviation.com/fuels/avgas115-145/

Oh, haven‘t found that one. But it is slmost the same than the other, twice the lead. It‘s also good fuel if you don‘t mind the lead.

Edit: only 5% aromatic compunds added, compare that to 50% of the 150 octane from „the good old days“.

 

13 minutes ago, VO101Kurfurst said:

They were all designed for the fuel that was available back then (basically 87 octane or even lower),

Biritish and German engeneers started out with different assumptions on fuel quality available. The result you can see in their design choices.

Edited September 18, 2018 by ZachariasX

[Kurfurst]

They did not, actually. The fuels to come simply did not existed when development of said engines started at around the start of the 30s. The design choices simply stemmed from earlier experience, and the desire was usually to make a bigger engine than the previous one.

 

Some (especially radial engine producers) even went to simplest way - they took an existing engine, and bolted it together.

[MiloMorai]

4 hours ago, Talon_ said:

 

In fact C-3's octane content was closer to Allied 100 octane, which is why it required additives such as MW-50 and GM-1 to achieve similar horsepower levels.

 

Don't fall of your chair and hurt yourself Kurfy. ?

 

The Germans had C3 late in the Bob and was comparable to the Brit 100PN fuel.

The PN later increased to 125 and near the later stages of the is said to have ~140PN.

 

On the net there are British tests of German avgas. (I lost the link when Microshaft messed with my computer)

[Cpt_Siddy]

6 hours ago, Talon_ said:

Tighter tolerances, better machining, less oil where it shouldn't be and more where it should be, faster valve returns, better injection systems.

 

I guess the main thing to remember is that it's crazy we were able to get to 2,000hp from a V-12 at all only 40 years after the dawn of powered flight. We've had twice that long again to improve and the results are clear!

 

 

We got to the moon 25 odd years after first turbo fed rocket engine so there is that....

[NZTyphoon]

2 hours ago, Cpt_Siddy said:

 

 

We got to the moon 25 odd years after first turbo fed rocket engine so there is that....

Little do most people know that the moon landings were achieved using P&W R-2800-77s fed on 779000/9500 grade zap fuel,  with superhydrogenated MW-50000 injection and 1,000,000,000 in Hg boost: all that stuff about using big rockets is bunkum.

[Cpt_Siddy]

17 hours ago, NZTyphoon said:

Little do most people know that the moon landings were achieved using P&W R-2800-77s fed on 779000/9500 grade zap fuel,  with superhydrogenated MW-50000 injection and 1,000,000,000 in Hg boost: all that stuff about using big rockets is bunkum.

 

 

I heard that this rocket was later cleared for the mythical 1.98 ata, but the fire in local NASA shed destroyed the secrit documents....

[ZachariasX]

On 9/18/2018 at 10:56 PM, VO101Kurfurst said:

They did not, actually. The fuels to come simply did not existed when development of said engines started at around the start of the 30s.

They did. And fuels to come were made the same way as the fuel in 1933 when IG Farben got the contract to produce most of Germanys fuel.

 

Ok. But taking it slow here. In the 30's fuels used in the USA or Great Britain were as different from German "gasoline" as apple juicce is from orange juice. When you say they "had the same octane number", then it is like you saying applejuice is the same as orange juice, because they are equally sweet.

 

Octane rating, at least the way you are usin it is an arbitary correlation of a functional behaviour of the juice. It by no way describes what is in there. Also yoi seem pretty resistant to the idea that different organic compounds may very well but at different efficiencies.

 

Both facts

1) German aircraft fuel was chemically different from British/American fuel

2) Octane number in the sense used in this contect (ratings past 100) does not correlate to chemical composition of the liquid.

had effects on the design choices of the engines built in the 1930's. It is not about "fuel that wasn't invented yet".

 

If you understand (and most here in this forum are seemingly oblivious of that) how fuel was sythesized and why it was done so, you get also some further insights. For instance why B4 (the stuff everyone in this forum hates) fuel was retained for very good reason until the end of the war. And it was not "because they had that and they didn't have enough C3" that they kept B4. They could have made C3 exclusively if they wanted. If.

 

Long story short, unless you are running an engine at divine ata ratings, C3 fuel is turd. B4 is better fuel at lower ratings. Just let that sink in for a while.

 

The Britsh had a tremendous head start in fuel technology, as they had excellent crude oil that with very little work on it gave you a relatively good fuel with a clean burn that even had an octane rating of around 80. It was cheap, easy made and great for gasoline engines. If they threw in TEL to a point just shy of engine failure, they got 100 octane ratings. Awesome.

 

The Germans were in a much different predicament since the 1930's. Regular crude oil was only good for about 30% of it's economy. At that point, one is not even thinking about the war, but industry and transportation. What did they do at that point? In 1933, they made the "Benzinvertrag", subsedizing hydration technology to make fuels from coal. Now this was not just "some" fuel. Although in principle, you could make isooctane in both the "IG-Verfahren" (high pressure process) and through Fischer-Tropsch synthesis, IG Farben pushed hard for the high pressure synthesis because they were not just interested in making fuel, they were interested in making anything synthetic. "just making fuel" was not economically viable, that everyone involved at that time agreed. that was also stated during the Nürnberger Trials. Be it methanol or explosives, platics, whatever had to be made from the production intermediates. As a result, it did not very readily produce isoctane but annilin rich internediaries, that juice made up for a relatively high octane rating, higher than what you get from making isoparaffins through Fischer-Tropsch sythesis. Thus, frim the very beginning, they had to make a blend of compounds that worked "reasonably well for their purposes". They had everything. And fuel.

 

Now, what does that mean, "resonably well for most purposes"? It had to have a high octane rating, but it still had to burn somewhat. Now the power you can get from an engine is limited by the air you pump through it. Fuel, you can always throw in as much as required. In consequence, if a fuel doesn't burn that efficiently, you have to throw in more to get the same power.

 

What did we learn from that? Bad fuel makes you losing mileage, not neccesarily power. Going overly rich mixture settings ("C3 injection") allows you some leeway there. At the cost of getting anywhere. The German engines had to be built such that they could either run on low octane, higher efficient fuel, or high octane lower efficiant fuel. To do so, they were forced more to increase displacement than the British. You can absolutely design a new block every year or so. It doesn't have to be financially viable. As long as it keeps you alive, all well.

 

The high aromatic content of those highest octane fuels was throughly problamatic. One point is also storage. Those compounds form raisin. Just becuse they like to do that and if that raisin enters your injector, you engine will run badly. Or quit. There's a limit of how much from that turd you can have in your fuel. Allied 150 octane was phased out after the war was over. It's just trouble. But they used it when required. Same as the Germans used their ugly C3 Schnapps that was pushed to borderline ratings, sacrifying any other quality a fuel should have.

 

 

 

[HagarTheHorrible]

Thankyou, I've enjoyed reading this thread.  Very informative.

[ZachariasX]

Maybe one more thing, just to illustrate how fuel quality and engine devoelopment is interlinked.

 

Back in the prehistoric times, when short, chunky pilots as these were defining flight

 

they also preferentally flew aircraft that consisted of engines with some controls and some wings attached to it. Planes looked like this:

Spoiler

 

or, if for some weird reason flotation was an issue, like this:

Spoiler

 

Or if they felt like having a good time, it looked like this:

Spoiler

 

Making combat aircraft from that gave in worst case this:

Spoiler

A plane "as deadly as it looks".

 

Best case, we get the Me-109 or the Spitfire. Still "engines made flyable", and somebody even brought a gun to put on them. Now some poeple were wondering why Douhet come up with the doctrine of "the bomber always getting through", but I guiess these planes are part of the answer.

 

What these aircraft have in common, is that they are very small (as small as possible) and they are very light. Being overpowered, you used two main modes, either full bore (racing, intercepting, impressing your neighbors) OR you used them at rather low throttle settings that still would make them fly comfortably well.

 

Now, what is the connection to fuel quality and engine design?

 

Remember, above, I said that "bad fuel" costs you mileage, as you have to put in more to suppress knocking and cooling the burn, cooling your cylinders in consequence. BUT, if I let the very same engine run at lower power outputs, I can lean the mixture, making the burn more efficient, hence increasing your mileage. Now we come to bottom of the issue. The higher I push an engine on given fuel, the lower its efficiency.

 

There is a thing called cruise speed. Cruise speed is a speed at which you cruise somewhere, making your mileage a metric of paramout importance. And this is why at cruise speed you lean your mixture. After what I've written above, the point where you had to enrich the mixture for going faster is a limit to efficient cruise. Now, this delta, from max. cruise speed to max. speed (max cruise power to max. power) is largely dependent on your fuel quality, as "bad fuel" will lower that point, even though you might get the same cruise speed.

 

Both the Spit and the 109 started out with rather smallisch V12 engines of about 27 litres, meaning that the limit of efficient power output is very low compared to possible max power outputs, and it will also be very hard to up that efficient power output unless you increase fuel quality in terms of burn efficiency.

 

The result is (besides abyssmal range) the low cruise speeds of both the Spitfire and the 109. If you fly faster, your engine will be progressively less efficient.

 

In the Spitfire, at least there they found some empty space for more gasoline to offset higher boost ratings. The 109 on the other hand was greatly helped by the fact that work came to him, instead of him having to fly all the way to work.

 

Starting out with 27 litre engine, at roughly 1000 hp, what do you do when you want more power. K said correctly that people hate to make new engine blocks, as that cost money and tends to delay production. So what did Rolls Royce do? They were working on blowers to pump more air through the same block. This works well IF you have fuel good enough that doesn't force a rich-rich mixture early on, negating any hope for range in combat. Because they had a fuel that had to be set on a rich mixture later, they could do so. The octane ratoing only set them a cap about the maximum boost rating. And this was always in a region, where the engine stared to come apart and the progresively had to reinforce the engines to follow possible boost ratings.

 

The Germans were not so lucky. At DB, they knew that with their synthetic fuel, they soon had an engine that got them nowhere, literally. So they had to come up with an alternative. They actually had to "split" the use of their engines between running like mad and getting somewhere.

 

That required two fuels, one that got you somewhere (B4) and one that didn't get you shot down right away (C3). In consequence, they were compelled to build engines that accepted both low and high octane ratings and make the most of it. And for the B4, that compelled them to up displacement right away. And they did so. The DB60X series progressively went from 26 to 44 litres in just a couple of years. And you understand now why even the most fancy "big block" of those must accept B4 fuel. As well as C3, of course.

 

Rolls Royce on the other hand the pumped up that little engine like there's no tomorrow, up to +25 boost. Amazing. BUT with what I've said above, while the engine was about to produce 2000 hp instead of 1000 hp, the point of where it lost efficiency remained the same. Even the Spit IXe on steroids had an abyssmal cruse speed. Tempest pilots were mocking them as themselves having a higher landing speed than a Spitfire had a cruise speed. To make matters worse, the slower you fly at give fuel consumption, the shorter your legs are. It's not just that the Spit didn't have much fuel abouard, it also could not make much from it as it was slow. It took some work, like removing weapons and lightening the airframe to give it faster an longer legs in the PR versions.

 

The Germans on the other could make C3 or B4 fuel, as they liked. But making one came eat the cost of making the other. It's not that they had one and were working on the other and "kept the bad one as extra". And where was B4 mainly used? Bombers, recon, transport, all of which actually needed to go somewhere. It's not that they wouldn't have fancied to get 100 extra hp on the He111, not at all. It's just had they done that, they had less range of the very same aircraft plus all the other issues you have with that aromatic turd. Hence, you have large displacement engines.

 

The British used "small blocks" even in their bombers. But only up to a point. The AVRO Shackleton had to have Griffon engines, as the Merlins would have to be used at a boost rating, where efficient burn is no longer possible, hence they installed the Griffon that shifted this point to a workable power region.

 

It can be said in all fairness, the Spitfire should have been discontinued in 1942 (as should have the 109) and be replaced with the Griffon powered MB5. But both the Spit and the 109 were still great aircraft. And MB had a habit of not completing their job. The Tempest on the oher hand clearly shows what you can do with larger engine. Cruising efficietly at almost 400 mph is truly a whole new sport. Sure, you can say the 190 Dora and the late 109 could still  catch them at that speed, but this is saying a sprinter is fater than a Marathon runner.

 

 

And for yourself, your car is most likely specifed for 98 octane gasoline to perform as specified (it doesn't, don't put it on a bench, you'll be disapointed). This means, V-Power with 100 octane is absolutely unneeded, as your engine will not reach compression settings requiring that. BUT getting such a fuel is deeply desirable to keep your engine happy for the simple reason that it burns very clean. Also. if you dilute your gasoline with alcohol, it gets lighter and you lose milage per gallon of gas. If your fuel is turd (did I mention ARCO above?) and you put in more turd to get octane to at least 91, well, your engine will not live up to its specs and it will accumulate dirt.

 

 

 

 

 

 

 

[NZTyphoon]

Here is a modern analysis of the Allison V-1710, R-R Merlin and DB series engines, from the perspective of Dwight Thorn, who rebuilds engines to power aircraft participating in the likes of the Reno air races:

 

Quote

Thorn’s specialty is replacing Rolls-Royce rods with beefy, never-run Allison connecting rods and adapting them to fit Merlin crankshafts and pistons. This allows the engine to operate at 135 inches of supercharger pressure but at lower rpm because of the rods’ greater mass. Before Thorn’s imaginative fix, racing Merlins with their lighter connecting rods turned as much as 3,800 rpm, the propeller spinning so fast the blade tips were supersonic, which meant they weren’t creating thrust. Now racers can back the revs down to 3,300 or 3,400, allowing the prop to get a better bite but sending cylinder pressures into the stratosphere.
Read more at https://www.airspacemag.com/flight-today/masters-of-the-v-12-3039083/#W8JRW76cJEFHKyM5.99

 

Thorn’s best engines are built with what the cognoscenti call “transport banks.” Between 1948 and ’50, Rolls-Royce turned out the strongest and most durable Merlins ever for Canadair-built Douglas DC-4s known as Northstars. These 1,760-horsepower engines could pound away for hours without missing a beat, and they made use of every trick Rolls had learned about building durable V-12s. They are the gold standard, and if you want a racer, they are what you need.
Read more at https://www.airspacemag.com/flight-today/masters-of-the-v-12-3039083/#KmZsU9Whr3fiAZ2K.99

https://www.airspacemag.com/flight-today/masters-of-the-v-12-3039083/

Part of the reason the superchargers of these race prepared engine are getting stratospheric supercharger settings, compression ratios and power ratings compared with their wartime counterparts is that the racers are burning mostly synthetic fuel blends and alcohols that are far, far superior to anything produced during the 1940s: these fuels burn quicker and cleaner, and have a higher calorific value, thus generating more energy.

ftp://ftp.energia.bme.hu/pub/AviationEngineerSpecialist/PowerPlant/AvEng_Fuelproperties.pdf

http://www.whitfieldoil.com/171.287/racing-gasoline

 

Rating a fuel on octane levels alone is a relatively crude way of determining how efficient a fuel is. Most fuel testing equipment cannot measure above 100 Octane, thus, as mentioned by Zacharias X earlier, ratings above 100 make no technical sense. Other factors in determining a fuel's efficiency include:

Burning speed - how quickly does the fuel release its potential energy?

Energy value - the potential energy that can be released by the fuel.

Cooling effect - (quoting) The cooling effect of fuel is related to the heat of vaporization. The higher a fuel’s heat of vaporization, the better its ability to cool the intake mixture.

 

https://vpracingfuels.com/about-us/faq/

 

 

[ZachariasX]

@NZTyphoon

Good links, thank you. maybe I shold also add this one, from Shell some facts and the about the future of AVGAS.

 

One minor correction, high performance fuels tend to burn slower than antique fuels. The slow burn makes combustion more controllable, greatly aiding in achieving high power ratings as well as efficient engines. Nowadays you can also inject in only part of the chamber, using a fraction of fuel, mixing obnly with a fraction of the air in a specific location, while giving an efficient burn. You couldn't do that if you don't have tight control over the burn.

 

It is also of note that slow burn, as we have it for the "good fuels", is not always desirable. Ancient engine concepts like the Monosoupape rotary engine in fact relied on a fast burn, as they ejected the combustion gases in the same stroke when the burn is taking place. This means, for "efficient" operation of these rotaries, combustion has to be completed way before the piston travelled all the way downwards. In the lower quarter of the stroke, the valve opens and lets the pressure out and if you have residual burn then, ll you get for that is a little fire next to the engine instead of power cranking the prop. This way, you actually have a design that relies on the low octane, fast burning turd to operate efficiently.

 

Also, think of high performance engines as EFFICIENT engines. I remember (lacking any source though) the chief technitian of VW saying that, after being asked why VW would develop something like a W16 engine for a Bugatti, he said that only engines that have the economy can be good racing engines. Fuel quality is key to that.

 

Think of the engine and the fuel as a combination of technology. Fuel is made. Engines are made. They should match to give you that extra power.

 

[unreasonable]

Very interesting stuff, thank you. A question - given the spectrum of the UK and German engine/fuel design philosophies outlined above, where do the Soviet engines stand?   

 

 

[ZachariasX]

35 minutes ago, unreasonable said:

where do the Soviet engines stand? 

I'm mean here and I say, they stand with the French, as they made a living on the Hispano Suiza that was turned into a Klimov. But these are also larger displacement engines, the Klimov 105 is still a Hisso in some way.

 

Both are relatively large ~36L displacement and similar compression something of 7:1. The Klimov has a slightly better power to weight ratio (1.42 to 1.32 kW/kg) probably due to a better supercharger mainly. It is actually close to the DB601 with similar power output at 34L displacement. The cylinder compression of 6.9:1 is marginally lower than the Klimovs 7.1:1, but essentially it is the same. The Daimler however has a 1.47 kW/kg power to weight, showing what you can do with a neat supercharger, even though it is single stage.

 

Lets have a look at the Merlin. It has a very low 6:1 cylinder compression. It is the same as in the Peregrine or the Kestrel, the baby V12's. But if you are about to add a big blower to the engine, you don't have to go all too high with compression in the cylinder. At 2 ata, you'll have 12 ata in the cylinder insetad of 6.

 

It can be said that all Klimov, Hisso and DB tried to get more out of the piston engine itself, whether RR readily relied on the supercharger. That the super(turbo)charger is the way to go is especially obvious as you can see it in todays cars. There are hardly any naturally aspirated engines anymore. Those that are have compression (since the 1960's) ratios up to around 11:1, basically mimiking what the Merlin is capable of doing at +16 boost or so.

 

Both Klimov and DB are simply not made for the same turbo installations and hence can't be compared in absolute boost values. The DB needs less boost for the same power. But it comes at the price of being essentially a heavy engine for a light aircraft (in case of fighter AC).

 

American engines running at ~6.5:1 compression were a bit of an intermediate. After all, they were fond of turbosuperchargers and were thinking in that direction as well. But as that was mainly envisaged for altitude performance, you could give the engine a tad more compression than RR boffins would do.

 

In this sense, if you have low compression in the piston of a 27 litre engine and you know you can add twice the air in the cylinder before presure gets critical, you have built essentially a 54 litre engine. Th Merlin is a tiny big engine and for good reason is a great choice for racers.

 

If you are not convinced that you can fetch that much air at low temperatures, you better increase piston pressure such that with the added air it will reach no more than 13:1 or so. This shows how borderline both +25 boost and 1.8 ata in fact are for those engines.

 

 

TL;DR

It's just Cyril Lowesey's superchargers on one side, and then the whole rest of the world on the other side.

[unreasonable]

Thanks, I think I understand that. 

 

Cyril Lowesey got just a CBE, by the way. Terry Wogan got a KBE.  We (I mean us Brits) have had many great engineers but have never really given them due recognition. 

[Sgt_Joch]

The Russians did not push their engines as much as the Germans or British. The Yak-1b had a top boost of 1.36 ata (+ 5 lbs) and the La-5fn 1.6 ata (+ 9 lbs). This on top of a fairly conservative initial 7:1 or so compression ratio.

 

The Russians had several grades of domestic fuel. Top of the line fighters were supposed to use the top grade which had a 94-95 octane rating, however I have seen various references to quality issues with Russian fuel. The Russians also received large quantity of 100/150 octane AVGAS through lend lease, but this seems to have been reserved for lend-lease ACs.

Edited September 21, 2018 by Sgt_Joch

[HagarTheHorrible]

Still enjoying this thread 

[MiloMorai]

Slower burning fuels are also easier on the engine, mechanically wise.

 

I don't see BMEP mentioned.

[ZachariasX]

2 hours ago, MiloMorai said:

I don't see BMEP mentioned.

That‘s for you to type up why this is an interesting metric for determining what the engine can do.

[EAF19_Marsh]

I thank you sincerely for taking my basic understanding of the subject and improving it in a coherent and logical fashion.

 

Please continue to add your thoughts on other engines and aspects, this is fascinating for an amateur enthusiast.

[NZTyphoon]

6 hours ago, unreasonable said:

Thanks, I think I understand that. 

 

Cyril Lowesey got just a CBE, by the way. Terry Wogan got a KBE.  We (I mean us Brits) have had many great engineers but have never really given them due recognition. 

And who better to help explain the development of the Merlin than...(drum roll)...Cyril Lovesy

 

Edited September 21, 2018 by NZTyphoon

[69th_chuter]

All I know is today's Sunday flying Mustangs are limited to 55 inches on blue gas (100LL) and the racers are running 150 inches using 180 octane manganese laced gas along with WM injection*.  (Yeah, we can't actually have 180% octane, but its an extrapolated performance reference engineers came up with to make life easier.) 

 

~And XX boost in one engine isn't necessarily comparable to the same boost numbers in another engine.  Raw boost numbers can be dramatically altered by cam timing among other things.

 

*Referencing Dago Red's 2009 462mpg Gold Race Unlimited win.

[NZTyphoon]

Here's a video on Stanley Hooker of Rolls-Royce