Why was Luftwaffe fuel Octane so low?

[gruad]

We are told Allied engines could rely on up to 150 Octane which allowed greater supercharging whereas Nazi planes topped out around 85 to 95 and they relied on higher capacity and water/methanol injection.

Now as I understand it Octane rating depends on 1. the distillation of fuel and 2. additives like TEL

1. The Germans had access to proper oil early on and then had to switch to creating avgas from coal. But if their tech was sufficient to do this surely they could come up with a high octane fuel.

2. Tetra ethyl lead was used to improve the Octane. You can see the white streaks on the wings of the Lancaster. Surely the Germans could reverse engineer this from shot down planes, or is there some special chemical secret to its formation?

So I am a bit perplexed, or was it that the Germans believed less in super/turbo charging.

I look forward to hear what the forum knows on this matter...

 

[ThomasP]

Part of the problem was the supply of the necessary chemicals/materials. Processing chemicals like those of the Benzene family in order to make common octane boosting chemicals like toluene and xylene is expensive, and (I think) more time consuming - at least in comparison to the 'non-synthetic' forms of fuel refining/cracking production. IIRC the production of TEL had similar issues.

While the Germans were capable (technology wise) of making the chemical compounds/mixtures necessary, they had to balance their use in Avgas against the need for the same chemicals in other areas.

Early-war, there was considerable concern among the Allies as to whether there would be a stable and large enough supply of the various octane boosters needed for the quantities of Avgas that were projected - and there were times when the supply of the octane boosters slowed production. There was a significant effort to obtain alternative compounds that would provide the same performance numbers, particularly for the 100/130 grade fuel.

 

[tomo pauk]

We are told Allied engines could rely on up to 150 Octane which allowed greater supercharging whereas Nazi planes topped out around 85 to 95 and they relied on higher capacity and water/methanol injection.

Luftwaffe used on BMW 801D, on DB 601N and some late-war DB 605s the so-called C3 fuel. It was about hi-octane as the Allied 100/130 grade fuel. BMW 801 engines typically didn't used the water-methanol injection, the late-war DB 605s did; Jumo 213 was also using the MW 50 (German nomenclature for water methanol 50%:50% mixture)
There was a host of US (and Japanese) engines that used water-methanol injection from winter of 1943/44 on, like the R-2800 on fighters, R-1820 on FM-2, or the 2-stage supercharged V-1710 on P-63. All of them used the 100/130 grade fuel, the P-47D was tested with 150 grade fuel + water-alc injection for 2800 HP.
(engine on P-47m and N was making 2800 HP on 130 grade fuel + water+alc injection)
Yes, having a big engine helped, so did having the water-alc. What also helped on German fighters was that they were smaller than Allied fighters, especially when compared with P-47 - size and weight mattered.

Now as I understand it Octane rating depends on 1. the distillation of fuel and 2. additives like TEL

1. The Germans had access to proper oil early on and then had to switch to creating avgas from coal. But if their tech was sufficient to do this surely they could come up with a high octane fuel.

2. Tetra ethyl lead was used to improve the Octane. You can see the white streaks on the wings of the Lancaster. Surely the Germans could reverse engineer this from shot down planes, or is there some special chemical secret to its formation?

Germans knew what it takes, and they made a lot of hi-oct fuel from coal. Their problem was that total quantity of fuel available was pitiful vs. what Allies had, and, to make matters worse, production of hi-oct fuel required another set of production facilities vs. what the low-oct fuel needed.

So I am a bit perplexed, or was it that the Germans believed less in super/turbo charging.

I look forward to hear what the forum knows on this matter...

They believed in supercharging, whether with turbo or not. Turbos required a lot to make work, especially in choice of materials (again the German weak spot), and in tweaking the system so it works reliably. We can recall that only Americans put the turboed engines in mass use in ww2 - it helps when one has a lot of rare materials, time to iron out the bugs, plus money and factories to spare. Neither Soviets nor British made turbos for military aircraft apart from some experiments, and agin just some companies did that.
Granted, Germans were lagging with introduction of 2-stage superchargers, again with a notion that not all companies made working 2-stage engines on the Allied side.

 

[msxyz]

The problem of 'low octane' fuel was foreseen even before the war. That's why German firms developed direct fuel injection. In addition to many other advantages (lower consumption, not affected by high G manoeuvres, better combustion at all altitudes/pressures) direct injection allow higher compression ratios, or higher boost ratios without the risk of premature detonation. This advantage is unique to direct injection because fuel is vaporized during the compression, and it acts like a heat sink preventing 'engine knock'. Manifold injection (i.e. what is employed by most cars) or single point injection (a simpler form of injection that happens near the butterfly valve as in a carburettor) don't have this advantage.

Then, of course, there was the issue of procuring crude oil. Natural distilled gasoline has an octane rating between 50-70 because it's a mixture of different hydrocarbons. Distillation is a simple process that separates various hydrocarbons only according to their boiling temperature. The octane rating varies a lot because different oil fields yield a crude of different quality and that contains different hydrocarbon chains. For example, the gasoline distilled from oil pumped in Romania or in Borneo was known to have higher octane rating. Even when TEL became widespread, higher quality gasolines were much sought after because they allowed to use less TEL for the same effect or, conversely, to reach higher octane ratings with the same amount of TEL.

I've seen aircraft engine manuals of the period which recommends different amount of TEL blended into the gas, depending on the gasoline origin.

Even with TEL there's a limit on the octane rating you can reach before the effect diminishes or the gasoline contains too much lead that ends up on spark plugs, shorting them.
With high quality gasoline, by blending TEL alone, it should be possible to reach a rich rating 96-98 octanes. To obtain a fuel with better anti knock properties you've to start adding other chemicals to the mixture.

Sports car of the inter war period typically used a blend of gasoline, methanol (an alcohol) and benzene, a compound which was retrieved mostly as a by product of coal gas production. Alcohols however don't mix well with gasoline (that's a problem which isn't even completely solved today on ethanol blended fuels) and it tends to separate at low temperatures, such as those encountered at altitude. Alcohols are also less energetic than gasoline, therefore they're a poor choice for a plane, since weight has a severe adverse effect on performance and range, certainly more than in a car.

You're left with benzene, a compound which is also naturally occurring in gasoline, and other similar compounds which are called 'aromatics'. Benzene, along with two other molecules called Xylene and Toluene, were encountered as by product of coke and coal gas production. These aromatics have very high octane ratings if they're used as fuel in a combustion engine. Remember those Formula 1 cars of the 80s with over 1000Hp from tiny engines? They ran on straight toluene, which has an octane rating of around 150!

Aromatics are also present in natural gasoline but in a small fraction. To increase their percentage (and therefore the octane rating of the whole fuel) the methods used back then were very inefficient and energy consuming, so, at the time, only the US had the necessary oil output to allow wartime production of gasoline with higher aromatics content.

Germany was however following a different route. Having recognized that with Carbon, Hydrogen and Oxygen you can possibly build every kind of organic molecule right there, German chemistry concentrated on producing all kind of chemical feedstock from "simple' sources like carbon and natural gas; considering that Germany had a lot of coal at disposal, it seemed a safer route to industrial progress rather than relying on oil imports.

Most of the synthetic gasoline produced in Germany used the so called 'Bergius' process that used coal as feedstock. The process was sound and efficient (by memory, I think over 90% of the coal by weight gets converted into gasoline and the rest is spent, as heat, to sustain the process). The resulting gasoline is of higher quality (less pollutants like sulphur) and has a base octane rating of around 70-80, which was high enough for cars of the period. A modest quantity of TEL gave it a lean/rich rating of 87/91 making it suitable for aircraft use.

That was the fuel for which German aircraft engines of the period were designed. It was domestically produced with a process which was very efficient and relied on resources (coal) which could be mined locally. It duplicated, more or less, the characteristics of gasoline that could be obtained by distillation. That meant that Germany could either use gasoline from oil (if it could secure oil fields by conquering new lands) or synthetic fuel in case it was cut off from oil.

What Germany could not afford was to invest in technologies that were resources intensive. High Octane gasoline was one of them. The Bergius process was affordable and simple. Whatever high octane fuel Germany could produce was obtained by mixing its base synthetic fuel with around 20% of straight run BTX (Benzene, Toluene, Xylene) obtained mostly as by-product from coke and coal gas production.

That's also what made jet engines so attractive to the Germans: jet engines can burn any fuel than can be vaporized by pressure in the burner and that includes naphta/diesel oil of all sorts.

 

[dedalos]

We also should note that high octane fuel was serious problem , but not the most serious. By using high quality fuel and increased pressures you need the engine to have the structural strength to withstand the resulting much higher internal forces. That needs improved alloys and at the end specific raw materials. Simply not available.

 

[Timppa]

This advantage is unique to direct injection because fuel is vaporized during the compression, and it acts like a heat sink preventing 'engine knock'. Manifold injection (i.e. what is employed by most cars) or single point injection (a simpler form of injection that happens near the butterfly valve as in a carburettor) don't have this advantage.

I believe the opposite is true. You get charge cooling effect when the fuel is injected (carburettor or or manifold injection) to the heated air after the the supercharger, not when the fuel is injected directly to cylinders.

 

[msxyz]

I believe the opposite is true. You get charge cooling effect when the fuel is injected (carburettor or or manifold injection) to the heated air after the the supercharger, not when the fuel is injected directly to cylinders.

Fuel (or water, or methanol) injected in the manifold absorbs some of the heat caused by compressing the air. Fuel sprayed in the cylinders works on the same principle, but it is vaporized closely to the hottest part of an engine, that is around the spark plug and the exhaust valve, where metal is constantly at 300 deg or more. The cooling happens right during the compression stage, when the chances of spontaneous ignition due to pressure and heat build up are highest.

DIrect ingition is going through a revival of sort also in modern gasoline cars. The Skyactive engine by Mazda reaches a 14:1 compression ratio using common 95 octane 'green' gas and employs high pressure direct injection.

 

[BiffF15]

Fuel (or water, or methanol) injected in the manifold absorbs some of the heat caused by compressing the air. Fuel sprayed in the cylinders works on the same principle, but it is vaporized closely to the hottest part of an engine, that is around the spark plug and the exhaust valve, where metal is constantly at 300 deg or more. The cooling happens right during the compression stage, when the chances of spontaneous ignition due to pressure and heat build up are highest.

DIrect ingition is going through a revival of sort also in modern gasoline cars. The Skyactive engine by Mazda reaches a 14:1 compression ratio using common 95 octane 'green' gas and employs high pressure direct injection.

Isn't that same with diesel engines (automotive)? We had one of the diesel gate VW Jettas, with a TDI engine. Really liked it, great fuel mileage (38mpg at 78mph w/AC on), but VW paid too much money not to give it back.

 

[Milosh]

Didn't the Germans use a different method calculating octane? Also they used the low, not the high, PN number.

Late war C3 fuel is said to be approaching 150PN.

There was a web site that had test results by the Brits on German fuel.

 

[Shortround6]

For some reason the Germans didn't take advantage of the "mixture" results.

Now for the Germans you can't use the "rich" mixture results if you don't test for rich mixture and don't label the resulting fuel accordingly.

According to some sources (which could be wrong) some of the German fuel could be as much (but often less) than 40% aromatics.
Allied fuel was held to no more than 20% aromatics by practical matters, they were allowed to use somewhat more.
But if you used 20% benzine you will get a a different result than if you use 20% toluene. Or any other mixture.

You will also get different lean mixture and even if you blend it to get the same lean mixture you still won't have the same rich mixture.
Likewise if you different amounts of TEL the rich mixture changes more than the lean mixture.
and so on.

If the Germans supplied a hypothetical 96/130 fuel in one train load of fuel but only suppled 96/125 in the next train load of fuel you could be blowing up a bunch of engines.
Allied fuel suppliers were suppled with standardized test engines and the government inspectors/purchasing agents were also supplied the same standardized engines so everybody knew what going on. The Allied fuel could be one of dozens (if not hundreds) of actual blends but the the fuel had to meet certain standards.

Another wrinkle (there were a lot of them) was that some of the aromatics were limited as to how much you could use. Benzine, for example, might be very useful fuel as a race car/speed boat/ aircraft speed setting fuel but compare to regular gasoline it didn't work very well at low temperatures.
Italy did use a high Benzine blend in the 1930s (or later?) but then Italian temperatures were lower than northern European temperatures. Not saying they didn't have issues.
Toluene was in competition in the raw materials market for explosive (the 3rd "T" in TNT)

For Allied aviation fuel in WW II there was a standard that ALL the aviation fuel had to meet of 18,700 Btus per pound of fuel/ min.
unfortunately most (all?) of the aromatics are less than 18,700 Btus per pound for the allies that meant they couldn't just add another 5-10 % of aromatics to boost the rich mixture.

Perhaps the German fuel was a bit lower in Btu per pound?

But for the Germans the problem may have been figuring out the limit/s of 96/"130" fuel (and it changed from each engine, just because you could use MAP XX in engine YY doesn't mean you could use it in Engine ZZ) and establishing a uniform standard.

The Germans, especially on the DB 6 engines, used higher compression ratios, which gave better fuel economy but also limited the amount of boost you could use with a given grade of fuel.

I have my doubts about the German fuel injection system offering better cooling than Allied carbs, throttle body fuel injection.

Were the German fuel injectors opening up the injectors with the pistons near the bottom of the stroke? Were they opening up the injectors once the valves had closed but the pistons were a bit below top dead center? Were they opening up just a bit before the spark plugs fired?

In any case most Allied supercharged engines used quite a bit of excess fuel as coolant, quite a bit over what was considered even a rich mixture as far as combustion went.

 

[msxyz]

I have my doubts about the German fuel injection system offering better cooling than Allied carbs, throttle body fuel injection.

Were the German fuel injectors opening up the injectors with the pistons near the bottom of the stroke? Were they opening up the injectors once the valves had closed but the pistons were a bit below top dead center? Were they opening up just a bit before the spark plugs fired?

In any case most Allied supercharged engines used quite a bit of excess fuel as coolant, quite a bit over what was considered even a rich mixture as far as combustion went.

Direct injection is just a bit better at preventing premature ignition.

The direct injection systems employed was a purely mechanical one derived from diesel pumps. But, while in a diesel the injection starts a few degrees before top dead centre and continues while the piston is going down, in gasoline engine, the direct injection usually starts when aspiration is well underway or even during compression stage (therefore when the piston is around bottom dead centre)

Nowadays you have fancy systems to further improve the concept (i.e multiple injections, stratified charges, etc.) but the main 'selling point' of gasoline direct injection is being able to squeeze fuel directly into the cylinder when all the ports are closed, so A) no fuel is wasted exiting from the exhaust valve while it is still open B) better metering of fuel C) less prone to knock.

As for German gasoline octane, yes pretty much every country had it's own way of measuring it. Standard B4 fuel specs were OZ(Oktanzahl) 87; density 0,71 - 0,76 kg/l, TEL up to 0,12 Vol%. temperature of ebullition between 40° to 170° C (90% vol evaporated @ 160°C), Cold-resistant to -60°C. In Italian use this fuel was called '91 ottani'. Why is that? If we had to rate this fuel according to modern standards, it would be an 87/91 fuel (lean/rich rating).

As for C3 fuel, aside from the claims contained in some post war report, that late war C3 matched 100/130 allied gasoline, I believe the situation was more complex: probably not all the batches had the same properties, and the formula also evolved during the war, not least because of shortages.

 

[Deleted member 68059]

For some reason the Germans...

We are told Allied engines could rely on up to 150 Octane which allowed greater supercharging whereas Nazi planes topped out around 85 to 95 and they relied on higher capacity and water/methanol injection.

Now as I understand it Octane rating depends on 1. the distillation of fuel and 2. additives like TEL

1. The Germans had access to proper oil early on and then had to switch to creating avgas from coal. But if their tech was sufficient to do this surely they could come up with a high octane fuel.

2. Tetra ethyl lead was used to improve the Octane. You can see the white streaks on the wings of the Lancaster. Surely the Germans could reverse engineer this from shot down planes, or is there some special chemical secret to its formation?

So I am a bit perplexed, or was it that the Germans believed less in super/turbo charging.

I look forward to hear what the forum knows on this matter...

Th full answer to that required a very lengthy book. Tomo (who has a copy) has given some of the salient points but it's worth adding that metallurgy played a fundamental driving role to the maximum permissible boost Germans could use. This was without question the main reason for the German engines not using anything like the full boost potential of the C 3 fuel, the British ran a bmw 801 cylinder on 100/130 and it actually lowered the maximum detonation free boost limit relative to C 3

The reasons for the very late arrival of decent high altitude superchargers in Germany was mostly due to management stupidity and absolutely not lack of knowledge. They had two stage supercharged DB 600s running in 1936 or so, but it appears to have been deprioritised

 

[tomo pauk]

The reasons for the very late arrival of decent high altitude superchargers in Germany was mostly due to management stupidity and absolutely not lack of knowledge. They had two stage supercharged DB 600s running in 1936 or so, but it appears to have been deprioritised

I'm sure that you know this far better than I do; some people might find it interesting:
Junkers flew a 2-stage supercharged version of their L88 engine in 1932; Bristol flew a 2-stage S/Ced Pegasus in 1936. Alas, apart from a tiny number of the Jumo 213E and even smaller number of 213F, and for different reasons, neither company series produced a 2-stage S/Ced military engine for ww2.

 

[Shortround6]

Mercedes was using two stage superchargers on Gran Prix cars but they were roots superchargers.

Engineering handbooks mentioned multi stage superchargers (or air compressors) before WW I, water pumps in the fire service go back well before WW I for two stage centrifugal pumps (and 4 stages piston pumps).

But engineering text books many times only tell the student that something was done, and give some basic formulas.

One of Hookers claims to fame was that he realizing that one or more formulas in some of the engineering text books on superchargers at the time had one or more errors in the formula/s.

Getting the basic formula's to actually produce results may be a bit trickier.

The auxiliary supercharger was not even engaged until the plane was over 20,000ft (or 30?).

Getting the right balance between the stages and the right amount of intercooling also took some doing.

 

[tomo pauk]

Getting the basic formula's to actually produce results may be a bit trickier.

Intercooler wwill indeed need some relocation; British/Bristol has about 3 years to do it for ww2

This is what is reported for what Farman was doing back in 1935:

 

[Shortround6]

The Farman diagram makes no mention of an intercooler. Of course in 1932-35 most engines weren't using a lot of boost to begin with.

The 1938 "Janes" has a description of a Farman engine (and a photo) for an inverted liquid cooled V-12 of 110 X 120 cylinders of 11.4 liters.
The altitude capabilities are almost identical to the 1935 story. However it seems a little too good to be true.
ground level of power of 450hp (Boost and even engine rpm are not given) with ground level power being given as available at 4,000 meters (13,120ft) and 8,000 meters (26,240ft. )

All for a a weight of 616lbs.

Please note that the air cooled Renault V-12 was 19 liters, offered 450hp at 3,600 meters (11,800ft) and was said to be the equivalent* of 690hp at sea level and weighed 848kbs without accessories. The idea was that IF the engine could be opened up at sea level without blowing the engine up (material failure or detonation) that would be the theoretical limit of the engine power.

The sea level "equivalent" was some times by used by authors or companies to differentiate between unsupercharged engines or different models of superchanged engines.
A supercharged Kestrel XVI was supposed to be good for 690hp at 11,000ft (normal power) with a "potential ground power" of 1000hp for a weight of 970lbs.

Now at 8,000 meters (26,240ft ) the Kestrel should have lost around 260hp giving the Kestrel about 430hp left(?) . But the Farman can give that power at less than 2/3rds the weight?
No intercooler?

Something smells and it is not the French cheese (this time)

 

[tomo pauk]

Now at 8,000 meters (26,240ft ) the Kestrel should have lost around 260hp giving the Kestrel about 430hp left(?) . But the Farman can give that power at less than 2/3rds the weight?
No intercooler?

Something smells and it is not the French cheese (this time)

Like many times, it is much more about the idea (a 2-stage S/C) rather than about the particular engine.

 

[Shortround6]

Again, note Allison made a prefect hash out of their first attempts at 2 stage superchargers by trying to use two impellers the same diameter.
And this was not only after Farman but after P & W was building engines using two stage superchargers for the 1938/39 Pursuit trials.

I am not saying it couldn't be done. Obviously it was done.
But it took a lot more time than some people expected.
Jumo used intercoolers on single stage 2 speed superchargers. It seemed to work fairly well on bomber engines. Especially ones that were using 87 octane fuel.
I don't think anybody else used intercoolers on single stage superchargers. Most designers may have thought the increase in drag was not worth the extra power. (if you had 100/130 fuel it really may not have been worth the drag).
The P-63 didn't use intercoolers (in part because the intercooler manufacturer couldn't deliver.) and while superior to the single stage supercharger the two stage supercharger was thousands for feet below what the intercooled two stage engines could do.
Granted the Germans weren't using the amount that boost that the Allisons did.

RR never used the Merlin 46 supercharger with a 2 speed single stage drive.
They did fool around with a Merlin 46 with an intercooler and a higher single speed drive ratio and got 1100hp at 26,000ft with 9lbs of boost.
Not as good as a 2 stage but they got about 4.5 times the pressure out of the supercharger.

 

[Engineman]

Were the German fuel injectors opening up the injectors with the pistons near the bottom of the stroke? Were they opening up the injectors once the valves had closed but the pistons were a bit below top dead center? Were they opening up just a bit before the spark plugs fired?

Hi Shortround6,
The DB605 was timed for the commencement of the injection at 48 degrees (+/- 2) ATDC on the intake stroke. This was with the inlet valves open and exhaust valves just closed after the valve overlap. I estimate that the maximum injection period would be about 152 degrees of crank angle and so injection would cease before 20 degrees ABDC on compression stroke at the latest. The maximum ignition advance was 45 degrees BTDC so, there was at least 115 degrees of crank rotation on intake/compression after fuel injection finished before ignition.

Cheers

Eng

 

[Shortround6]

Thank you.

It still looks doubtful as to the Fuel injectors ability to cool the fuel and cylinder in that period of time compared to a fuel/mixture coming in 25 Degrees C cooler. Granted the Germans, with their less boost weren't heating up the intake as charge as hot.
I have seen fuel figures for DB engines but they often do not list the specific fuel consumption at full throttle.
The Allied engines are using around 50% more fuel per hp than they are are best cruise. And a lot of that is being used for cooling.