Amateurs study aircraft design. Professionals study oil production.

kool kitty89 said:

Let alone dealing with Russian winters ...


Oh, but a neat document relevant on the alternate fuel blends topic:
http://www.google.com/url?sa=t&rct=...=w36EQGfs91GL3iFCwcrfOg&bvm=bv.89947451,d.cGU

It includes use ethanol, n-butanol, acetone, various grades of gasoline, naphthas, and diethyl ether (poor octane but good vaporization characteristics, so tested to see what blends could still achieve acceptable octane ratings).

The naphtha fuel studies would be somewhat in line with Koopernic's comments on using kerosene in gasoline type engines. (but more in line with my own suggestions in focusing on normal gasoline types and blending other fuel types to improve vaporization and combustion characteristics)

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That link is quite helpful.

German A3 grade aviation gasoline had a Research Octane Number or RON of 80 and consisted of 30% alcohol. It was used in training and light aircraft. Interpolating the charts you have linked suggest that using a straight run gasoline an upgrading of 55 RON to 80 RON was possible with 30% addition of alcohol without TEL. (Ethanol/methanol behave almost exactly the same way). This is interesting because the Fischer-Tropsch archives (the US Navy researched ones) suggest that there was a fraction of 55 octane in German WW2 Fischer-Tropsch.


(note this is late C3 fuel, early C3 was less)
German Fuel Rating system - Axis History Forum
BIOS-119


A duel fuel gasoline aviation engine able to run from 80 octane A3 and then switch to higher octane B4 or C3 only when required in combat, takeoff or climb should have been quite helpful though it would come at the complexity of two fuel tanks. I do know that Junkers Ju 352 had a system that allowed a switch from B4 to C3 fuel for takeoff and that post war R-3350 DC7 could use 115/145 for takeoff.

The Finnish experiment with the Saab B engine (in Talbots and Saab 99 Petro models) offering duel fuel petrol in one tank and able to run on Kerosene or Turpentine from the other larger one seems to have been based around a fuel which was called TVO (Tractor Vaporising Oil) in the UK and Australia which is a blend of Kerosene and Gasoline. They were used in Ferguson Tractors. The engines that normally used TVO fuel were not fuel injected near Top Dead Cntre, as in a Hesselman engine, but relied on using exhaust manifold and engine heat to help vaporise the kerosene into a carburettor. Since the mixture was carburized and drawn in during induction there was a possibility of auto ignition causing pre ignition during compression and even detonation so an octane rating was required. The gasoline component increased ease of vaporisation and increased RON to an acceptable level. Such fuel was relatively cheap in parts of the world.

"Pure" Kerosene like Diesel has a RON of about 25 not 67. It also has a Cetane rating of around 25, this being a measure of the slowness of the fuel to burn. The fuel injected Hesselman engines could burn both diesel and kerosene fuel quite well once warm and it was easy to achieve a compression ratio of 7:1 or greater. An octane rating as high as 67 was not required. The choice of a 67 octane "kerosene" reflects I think that there was a substantial infrastructure in place to distribute TVO for agricultural machinery in Finland. The octane rating was more incidental than necessary though I suspect it made it possible to start directly in summer rather than having to use gasoline.

The point of the Hesselmen engine, compared to the diesel, was to produce an engine in which the problems of very high compression ratios (mechanical stress) and the problems of precise high speed injection were very relaxed. Hence if your diesel technology is good enough you don't need Hesselman. Better alloys lightened diesels. Modern aviation diesels actually run of aviation jet turbine fuel.
The Hesselman engine is constained by air fuel ratios that can be ignited by spark.

The curious question is then why the very high compression ratios of diesels given the low octane number of their fuels should allow easy ignition. It appears to be from the need to achieve ignition over a wide range of air-fuel ratios since diesels adjust power via modulation of fuel only not air. (hence their efficiency due to low suction losses)

It appears incidentally that natural turpentine has a very high RON in excess of 100. I recall reading that Japanese collected pine cones to make fuel for their fighters.
https://data.epo.org/publication-se.../20110302/patents/EP2290037NWA1/document.html

From my point of view the most efficient way for the Germans to have provided fuel was to run 100% methanol fuel injected engines (fuel injection for easy starting) with the otherwise difficult
fuel. The process involves coal gasification, purification and adjustment of the syn gas followed by production of methanol over a catalyst. Most of the gas is reacted in one pass and there are few unwanted by-products. The complexity of heat exchangers, refrigeration and distillation columns downstream can thus be avoided. Furthermore the engines that would run of this fuel would be very small and efficient.

Multi fuel diesels or multi fuel Hesselman engines would also be useful as in they would have consumed fuels from both fischer-tropsch as well as Hydrogenation that have poor octane rating but non the less are produced in quantity.

I suspect that the manufacture of so many fuel injection systems would be problematic. It would also be hard to ramp up in the few short years there was to produce the synthetic fuel plants.
The Germans seem to have been betting on gas turbines to power not only aircraft but tanks and smaller ships such as e-boats and torpedo boats as they are inherently multifuel and such fuels are easy to synthesise from coal in many ways.

Nevertheless attempting to mimic the gasoline obtained from crude oil via synthesis from coal creates the problem of low yields and low efficiency that is caused by having to make repeated adjustments of the fuel.

To improve the efficiency there would be choices:

1 Go to pure alcohol fuels, seemingly the most efficient.
2 Introduce a multiplicity of engines more able to operate on fractions other than high octane gasoline such as kersonese ie naphta,or diesel, low grade gasoline in multi fuel diesel and hesselman engines, (gas turbines eventually) and perhaps steam.
3 Use blends.

Incidently the Finnish Government killed the fischer-tropsch experiment through tax greed. Because the fuel was so cheap to produce and also because it had lower taxes as a primary industry input they increased the taxes to give parity to normal gasoline, thereby destroying the project.

Koopernic said:

A duel fuel gasoline aviation engine able to run from 80 octane A3 and then switch to higher octane B4 or C3 only when required in combat, takeoff or climb should have been quite helpful though it would come at the complexity of two fuel tanks. I do know that Junkers Ju 352 had a system that allowed a switch from B4 to C3 fuel for takeoff and that post war R-3350 DC7 could use 115/145 for takeoff.

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Even for gasoline engines alone, using the same engine with higher or lower boost limits depending on fuel type used would make sense. The issue of many C3 specific German engine models using higher compression ratios (rather than just different boost limits) was just brought up here:
http://www.ww2aircraft.net/forum/aviation/luftwaffe-technological-gambles-42998-6.html#post1196883

You lose some fuel efficiency at lower compression ratios, but with mixed fuel supplies, that's not much of a disadvantage. (plus, lower compression ratio + higher octane can still mean SOME gains in cruise efficiency due to higher lean octane rating and ability to run at higher boost+lower RPM compared to lower octane fuels in the same engine)

The point of the Hesselmen engine, compared to the diesel, was to produce an engine in which the problems of very high compression ratios (mechanical stress) and the problems of precise high speed injection were very relaxed. Hence if your diesel technology is good enough you don't need Hesselman. Better alloys lightened diesels. Modern aviation diesels actually run of aviation jet turbine fuel.
The Hesselman engine is constained by air fuel ratios that can be ignited by spark.

Click to expand...

Hesselman (or similar type) engines might have been easier to develop for use in place of some of the light aircraft engines too, where diesel technology would have been impractical. There's still some requirement for octane rating in regards to detonation/knocking for any spark-ignited engine, but widening the allowance for vaporization qualities and burn rates would certainly be useful. (the engines would be more flexible fuel-wise than typical diesels as well since poor cetane ratings wouldn't matter)

Plus, fuel injection would avoid G-force related carburetor problems on aircraft. (not that that was a major concern on trainers and utility aircraft, but still useful -perhaps more so for trainers and the few combat/recon aircraft using those engines)

And yes, kerosene type jet fuel tends to work rather well as diesel fuel. (naphtha type fuels don't work well as diesel)

From my point of view the most efficient way for the Germans to have provided fuel was to run 100% methanol fuel injected engines (fuel injection for easy starting) with the otherwise difficult
fuel. The process involves coal gasification, purification and adjustment of the syn gas followed by production of methanol over a catalyst. Most of the gas is reacted in one pass and there are few unwanted by-products. The complexity of heat exchangers, refrigeration and distillation columns downstream can thus be avoided. Furthermore the engines that would run of this fuel would be very small and efficient.

Click to expand...

Methanol vaporizes easily enough to probably work well enough in conventional carbureted gasoline engines (or something close), but hesselman engines would certainly improve the flexibility/versitility on the whole along with being methanol-compatible. (so long as corrosion issues were addressed -be it in engine/fuel system design, fuel blending components, or both)

Whatever blends of fuel were used would likely need to be of a fairly consistent energy content per gallon or at least similar in terms of flow rate for air/fuel mixes. (different dedicated fuel/vehicle types could vary more there -like aviation fuel vs ground fuel- but just having consistent grades of fuel for different purposes ... which I suppose is implied given the requirements for the fuel industry in general)

I suspect that the manufacture of so many fuel injection systems would be problematic. It would also be hard to ramp up in the few short years there was to produce the synthetic fuel plants.
The Germans seem to have been betting on gas turbines to power not only aircraft but tanks and smaller ships such as e-boats and torpedo boats as they are inherently multifuel and such fuels are easy to synthesise from coal in many ways.

Click to expand...

A total switch to gas turbines seems rather impractical ... in fact the emphasis on turboshaft/turboprop designs that were being pursued were a bit of a waste (albeit so were a fair number of the more advanced -or just plain complex- turbojet designs -with the possible exception of the HeS-30 given its mix of simple and advanced design features, early progress ... and cancellation in favor of MORE complex/risky designs). But gas turbine engine development is best left for another thread (like the axial vs centrifugal thread or the above linked technical gambles thread)

In any case, Hesselman (or similar) engine designs would probably be a lot more practical than gas turbines in the majority of ground applications almost indefinitely (let alone during the 1930s/40s) and even without hindsight they were a reasonably proven technology that's relatively similar to the existing automotive and aero engine industries, unlike gas turbines.

Nevertheless attempting to mimic the gasoline obtained from crude oil via synthesis from coal creates the problem of low yields and low efficiency that is caused by having to make repeated adjustments of the fuel.

Click to expand...

If the added complexity of hesselman (let alone diesel) engines was seen as excessive, there should have been compromises to allow for more efficient synthetic fuel blends compatible with (relatively) conventional carburetor type engines without being totally interchangeable gasoline. (I still think nearly pure methanol should have worked well enough in this sense -the corrosion/storage issue seems bigger than the vaporization one)

Use of fuel preheaters/vaporizers as in those tractor engine designs could be another compromise compared to hesselman style fuel injected engines.

Though using hesselman-like engines for light aircraft engines might be more attractive. (cases where the properties of methanol are still unattractive due to weight/bulk and possibly chemical properties, but where the most efficient denser, high octane fuels would be poorer in terms of vaporization properties -using methanol or methanol/acetone/ether blends as starter fluid might still be significant)

1 Go to pure alcohol fuels, seemingly the most efficient.
2 Introduce a multiplicity of engines more able to operate on fractions other than high octane gasoline such as kersonese ie naphta,or diesel, low grade gasoline in multi fuel diesel and hesselman engines, (gas turbines eventually) and perhaps steam.
3 Use blends.

Click to expand...

Using an array of standardized fuel grades that could be made up of varying blends of alcohols, hydrocarbons, ketones, and possibly ethers derived from synthesis as well as direct liquid fractions from coal distillation (or materials processed/extracted from coal tars) and potentially pine-tar/resin derived fuels as well. (ethanol from fermentation might be useful too, but I'd think most of the useful material for that would be better used for human consumption while agricultural waste -and other biomass waste- along with wood/bark/etc could be used as feedstock for gassification as well -and some smaller amounts of direct distillates from wood/biomass in terms of turpentine, tar, methanol, etc)

Hell, municipal waste could be used too, especially if citizens were encouraged (or compelled) to sort their garbage ... something already going on to some extent in regards to war-time recycling efforts. (I'm not sure if pyrolysis/gassification of sewage would have been economically viable at the time, but it's at least an interesting consideration as well)



Additionally, you'd still need heavy lubrication oils, wax, etc, so (for whatever mix of fuel synthesis/processing/refining was used) you'd end up with at least some supply of heavier oils as well, and likely less desirable byproducts. (kerosene and diesel grade fuel oils -or heavy fuel oils for shipboard use- would be plenty useful, but some of the remaining byproducts might still be useful for blending into other fuels -especially with more flexible base fuel standards for engines)