1930-1945: alternative engines for military purposes

[tomo pauk]

Includes the engines for tanks, aircraft and other big-ticket vehicles a military of the day might be interesting in. Also includes the different applications of the known engines. Fuel can be normal gasoline, diesel, a fuel with big % of alcohol in it (mostly ethanol or methanol, to keep the things simple), any type of natural gas (compressed methane, propane and/or butane). A bi-fuel engine type also, where it makes sense. Engine conversions are also fine. Any layout of the engine one finds to be better (best?) suitable for the intended vehicle, be that from technical, tactical or logistical standpoint. Extra points for the engines that make a lot of sense in economical terms, and that can be sold abroad. Same if the new and better, or more suitable engine types can be made from the existing types.

Military ships will need to sit this one out, I'm afraid.

To strt this one - the V12 air-cooled engines for the tanks. As it was, Japanese favored the air-cooled inlines and the like, while the Praga air-cooled diesel was slated for the German purposes, but I'm not sure that ever happened bar the prototypes. Probably a better spot for the V12 Gypsy engine than on an aircraft, and similar for the Argus V12 engines.

 

[Shortround6]

To strt this one - the V12 air-cooled engines for the tanks. As it was, Japanese favored the air-cooled inlines and the like, while the Praga air-cooled diesel was slated for the German purposes, but I'm not sure that ever happened bar the prototypes.

I believe they put around 400 of them in armored cars? The big 8 wheelers late in the war.
Information in an old book is a 14.8 liter V-12 of 220hp at 2250rpm.
There is some interesting information on America's biggest automaker using air cooled engines in this Website.

Client Challenge

the company failed in 1934 and that pretty much ended American interest in air cooled cars and trucks.
The air-cooled engines of the time required better materials and turned out not to be simpler than the liquid cooled engines. At least not in the higher powered models.
Materials made huge advances during and just after WW II so what was possible in the late 40s or early 50s was not possible in the mid 30s.

 

[tomo pauk]

I believe they put around 400 of them in armored cars? The big 8 wheelers late in the war.
Information in an old book is a 14.8 liter V-12 of 220hp at 2250rpm.

Thank you

The air-cooled engines of the time required better materials and turned out not to be simpler than the liquid cooled engines. At least not in the higher powered models.

Militaries might be interested in the things that civilians might not be. On a tank, a protection of the liquid cooling system was an important thing, and failing to do a good job there made the tank susceptible to the battle damage (as in the Char B1), or made the cooling system bad in it's job (Crusaders, mainly). Liquid cooling system is yet another thing that can malfunction, it requires antifreeze in winter, and it is no fun having it pierced in Africa and the similar dry enviroement. Tank can be shipped without the liquid in the system, a thing that the tanks powered by the 'normal' engines will demend that the inspection and filling of these tanks is done before the engine is started in the place of reception.
On the aircraft, the higher-powered engines require that the cooling system is robust during the normal operation (a thing where British and Americans were much better than the Germans), it mattered if a lot of the material used is copper or it's alloy (better cooling properties than alluminium alloys - again the Germans score a minus here, since their sully of copper was bad). A place for the liquid is also requied, and on a small aircraft the suitable volume might be at premium.

This should not be read as 'liquid cooled engines bad', but I'm trying to show that there were merits for the idea of air-cooled V and inline engines.

On the aircraft tasked with the low-altitude bombing and other close-support work, liquid cooled engine might not be the best choice.

 

[Shortround6]

Differences in fuel seem to come down to about 3 levels.
Best fuel economy = diesel
2nd best = gasoline
3rd best = everything else

This sort of lumps in fuel storage/transport problems.
Alcohol works very well as a fuel most of the time but the BTUs per pound means you need a lot more of it to move a truck 400km cutting a bit into the payload.
A lot of the alternative fuels are worse.
Use of natural gas requires heavy pressure cylinders.
Granted just about anything beats horse and wagon (even with rubber tires).
If an army has to resort to such alternatives before the war starts perhaps they should not be starting the war?

tank cooling systems seem to a subject of their own. I think the American M-26 had some real problems, manly with fan belts.
The Post war M-41 with air cooled engine had a reputation for setting itself on fire. Maybe using a supercharged flat 6 of 14.7 liters and 500hp had something to do with it. Fires got a lot fewer when fuel injection replaced carburetors? Hot engines with carbs inside a tank in hot weather is not a good thing?
Getting enough air through the air-cooled systems was often a problem.
Crusader suffered from have a poor, hasty designed fan drive. But you need some sort of fan drive system for an air-cooled engine.

Allison converted a few Liberty engines to air cooling in the 1920s. Allison also converted a lot of Liberties to better bearings. The Air cooled Liberties used a smaller bore reducing displacement to 1410 cu/in. power was about the same or a small bit higher. A supercharger with just enough boost to overcome losses in the intake system was used. Engines used the original crankshaft and con rods (?)
Air cooled engines used, in general, small bores for better cooling. This meant smaller valves which hurt breathing unless something else was done (more boost?). As a general rule an air-cooled engine got rid of 75% of the heat through the cylinder head. This helps account for the wider bore spacing on the inline engines, which means longer/heavier crankcases and crankshafts. This means more torsional vibration which can be solved by more weight. maybe only a little.
Yes, the Military can buy more expensive engines than commercial operators. For airplanes this doesn't matter much. Using aluminum cylinder heads for truck engines may raise eyebrows though.

 

[tomo pauk]

Differences in fuel seem to come down to about 3 levels.
Best fuel economy = diesel
2nd best = gasoline
3rd best = everything else

Agreed.
A wholesale dieselization will likely never happen, but having the 25+ ton tanks being powered by diesel engines has a lot going for it. A very-long range transport aircraft should've also benefitted if powered by the good diesels.

Alcohol works very well as a fuel most of the time but the BTUs per pound means you need a lot more of it to move a truck 400km cutting a bit into the payload.

If the engine is tailored towards alcohol fuels, it can be of a higher compression ratio due to the good anti-knock properties of that fuel. In the time when really high-octane fuel was not graned to be available, ethanol fuel made a lot of sense, be it mostly-ethanol of mixed with gasoline in the desired ratios. Higher CR engines have better mileage than the low CR engines. Ethanol has a much better BTU/pound values than methanol.
Another application of the alcohol fuels might've been in the engines outfitted with compressors, be that for the complete duration of the voyage/flight, or for tanking-off. Similar like the 100 oct fuel was used for take off on the RAF's Blenheims of the day, and the 87 oct was used for the rest of the mission; same with the Fw 200Cs powered by the BMW 132H engines.

Use of natural gas requires heavy pressure cylinders.

Yes for methane, that makes only sense if it is compressed.
No for butane and/or propane.

 

[GTX]

A good example of non standard engines is that of the Australian AC1 Sentinel tank. The tank needed a 400 hp engine. The first candidate was a Guiberson radial diesel engine, but it was quickly rejected as there were issues with delivering them. The second option was a pair of General Motors 6-71 (GM 6046) engines. Finally, a third option cropped up at the end of February 1941. This was the 5.7 L Cadillac engine used on the luxury 75 series of cars. This engine could develop up to 150 hp. The most important consideration was that the engine was readily available. However, it was clear that one engine was not enough, and so several had to be used - three in fact:

 

[Howard Gibson]

Includes the engines for tanks, aircraft and other big-ticket vehicles a military of the day might be interesting in. Also includes the different applications of the known engines. Fuel can be normal gasoline, diesel, a fuel with big % of alcohol in it (mostly ethanol or methanol, to keep the things simple), any type of natural gas (compressed methane, propane and/or butane). A bi-fuel engine type also, where it makes sense. Engine conversions are also fine. Any layout of the engine one finds to be better (best?) suitable for the intended vehicle, be that from technical, tactical or logistical standpoint. Extra points for the engines that make a lot of sense in economical terms, and that can be sold abroad. Same if the new and better, or more suitable engine types can be made from the existing types.

Military ships will need to sit this one out, I'm afraid.

To strt this one - the V12 air-cooled engines for the tanks. As it was, Japanese favored the air-cooled inlines and the like, while the Praga air-cooled diesel was slated for the German purposes, but I'm not sure that ever happened bar the prototypes. Probably a better spot for the V12 Gypsy engine than on an aircraft, and similar for the Argus V12 engines.

If you have an air cooled engine, you need a source of fast moving air. In an aircraft, this generally can be found somewhere. I know that some successful American tanks used radial engines. Either you build an engine from the ground up to be cooled by still air or by a fan, or you run an aircraft engine at substantially lower power levels, not necessarily a problem on a tank. If I convert an aircraft liquid cooled engine for tank use, all I have to do is install larger radiators.

Consider that on a fighter aircraft, I will go into a frenzy to get another 5mph of top speed by whatever means. On a tank, reliability and easy of maintenance are way more important. Out in the field, I would stick with gasoline or diesel fuel. Keep it simple.

Alfa Romeo 1500cc 450hp Roots Supercharged. How did they do this?

This is from Greg's Airplanes and Automobiles. Using methanol fuel, Alpha Romeo was able to use a two-stage supercharger. Its gas mileage (kilometerage?) was awful. Compared to gasoline, Methanol has a high ignition temperature, and much lower energy per kilogram.

 

[GTX]

A good example of non standard engines is that of the Australian AC1 Sentinel tank. The tank needed a 400 hp engine. The first candidate was a Guiberson radial diesel engine, but it was quickly rejected as there were issues with delivering them. The second option was a pair of General Motors 6-71 (GM 6046) engines. Finally, a third option cropped up at the end of February 1941. This was the 5.7 L Cadillac engine used on the luxury 75 series of cars. This engine could develop up to 150 hp. The most important consideration was that the engine was readily available. However, it was clear that one engine was not enough, and so several had to be used - three in fact:

View attachment 902913View attachment 902914

Speaking of the Sentinel, another different engine option was the Quad-Gypsy engine:

This comprised of four air-cooled, four-cylinder de Havilland Gypsy Major engines (same engine used in Tiger Moth as can example) mounted in an H configuration on a common crankcase. Starting in 1941, Gypsy Major engines were produced under license at General Motors' Holden plant. With its 4.65 in (118 mm) bore and 5.51 in (140 mm) stroke, the Quad-Gypsy engine would have displaced 1,495 cu in (24.5 L) and produced 510 hp (380 kW) at 2,500 rpm. The 16-cylinder engine weighed around 1,500 lb (680 kg). It was planned for use by the Sentinel IV variant.

 

[Shortround6]

Yes for methane, that makes only sense if it is compressed.
No for butane and/or propane

A 20lb propane tank (popular for barbques) weighs about 17lbs empty, some older ones are heavier. 20lb tank holds about 4.8 US gallons volume.
A 100lb tank weighs about 70lbs empty. propane has about 21,500btus per pound.

A wholesale dieselization will likely never happen, but having the 25+ ton tanks being powered by diesel engines has a lot going for it. A very-long range transport aircraft should've also benefitted if powered by the good diesels.

Which aircraft diesel can be considered good in WW II?

If the engine is tailored towards alcohol fuels, it can be of a higher compression ratio due to the good anti-knock properties of that fuel. In the time when really high-octane fuel was not graned to be available, ethanol fuel made a lot of sense, be it mostly-ethanol of mixed with gasoline in the desired ratios. Higher CR engines have better mileage than the low CR engines. Ethanol has a much better BTU/pound values than methanol.

Depending on a lot from high compression
Ethanol does have a much higher BTU per pound. 11,500 compared to 8,600. Gasoline has 19,000 or just under. Alcohol has problems as a motor fuel in some applications. It has a real affinity for water. Ethanol also has problems with evaporation/vapor lock. So does methanol but it falls more in the middle of the range for gasoline. Methanol was favored as a racing fuel partially because it offered much better cooling in the intake tract. Ethanol was about 2.64 times better than gasoline per pound and methanol was 28% better than ethanol. Both alcohols in practice were much better than gasoline because they used more fuel per lb of air and cooling effect was much greater. You can run trucks/cars on Ethanol but racing vehicles really love Methanol because you can burn more pounds of it to one pound of air. Not a lot but it does come out ahead. Not so good as truck fuel, You need more of it for the same range.

Another application of the alcohol fuels might've been in the engines outfitted with compressors, be that for the complete duration of the voyage/flight, or for tanking-off. Similar like the 100 oct fuel was used for take off on the RAF's Blenheims of the day, and the 87 oct was used for the rest of the mission;

British may have done it but it seems somewhat over sold.
A few American transport aircraft had separate fuel tanks for 87 octane and 80 octane.
One was 5 seat transport powered by a 450hp P&W R-985 engine. The 87 Octane tank was only 10 gallons, the rest of the fuel for 1000mile range airplane was the 80 octane tank/s.
Blenhiem only had two tanks per engine. The smaller tank was good for around 400 miles of range at cruising speeds. The Blenheim used the 100 octane "in combat" or in dangerous areas in addition to take-off. Also made a nice reserve on long flights.
Italians were using using a blended fuel mix in the CR 32s if not later. But the more fuel blends a country uses the more difficult the logistics get in trying to get the right fuel to the right units/vehicles.

 

[tomo pauk]

A 20lb propane tank (popular for barbques) weighs about 17lbs empty, some older ones are heavier. 20lb tank holds about 4.8 US gallons volume.
A 100lb tank weighs about 70lbs empty. propane has about 21,500btus per pound.

Thank you.
A sheet metal tank for gasoline will weight a good deal, even if not in the category of what the propane tank weights (I've carried these many times, that were used for gas stoves, before we switched to induction cooking ).
A tank for CNG is the one that is heavy due to it's robust construction?

Which aircraft diesel can be considered good in WW II?

Jumo diesels were good, and were the rare German engines that have no problems when turbocharged.
I'm sure that other people might've done a more advanced job than Guiberson or BMW did with their radials.

Depending on a lot from high compression

One of few real-world examples is the BMW 801. The C version used between 2% ( on lowest power) and 15% (on higher power) more fuel than the D version, with greater consumprion of the C being at high engine power settings. That is despite the better altitude performance of the D, that also used more HP to turn the impeller.
CR was 6.5 for the C, 7.2 for the D.
Spec consumption of the DB 605A was even better than on the 801D, let alone the total fuel flow (assumes all engines are fully-rated).

Ethanol does have a much higher BTU per pound. 11,500 compared to 8,600. Gasoline has 19,000 or just under.

On one un-supercharged engine, the spec consumption was .47 to 0.50 lb/hph of pump gas at hig RPM and high power, for some 560-580 HP. The spec consumption of the E85 was 0.60 to 0.62 at high RPM and high power, for some 580-590 HP. Unlike what cab be expected - the ~50% worse spec consumption if we look just at the BTU values - it was about 25% worse.
A similar engine, but with the higher CR that gives 12-15% better spec consumption at high power, might've been a ticket, with no problems with knock and with a bit more power. Or, keep the CR same, add a supercharger and power can be doubled without knock problems.

 

[z42]

A sheet metal tank for gasoline will weight a good deal, even if not in the category of what the propane tank weights (I've carried these many times, that were used for gas stoves, before we switched to induction cooking ).
A tank for CNG is the one that is heavy due to it's robust construction?

LPG liquefies somewhere between 200 and 2000 kPa, depending on the composition, so once you manage to liquefy it there's no reason to increase pressure further. CNG however, doesn't liquefy at room temperature, so there you need lots of pressure to pack the gas tighter. For comparison, volumetric energy density is around 26 MJ/L for LPG (but note tanks can be filled only to about 85% as there needs to be a bit of gas there to maintain a steady pressure), CNG compressed to 25 MPa about 9 MJ/L, and common liquid hydrocarbon fuels like petrol or diesel around 34-39 MJ/L.

In general, not sure there was much NG production and infrastructure around in the WW2 time period?

 

[z42]

I believe they put around 400 of them in armored cars? The big 8 wheelers late in the war.
Information in an old book is a 14.8 liter V-12 of 220hp at 2250rpm.

I believe it was the Tatra air cooled V-12 that was indeed used in some armored cars. Also used in the Tatra 111 heavy truck that remained in production all the way to 1962 (and successor models to this day AFAIU).

Tatra 111 - Wikipedia

en.wikipedia.org

A bit too late to have a big impact on Germany's fortunes in the war, but evidently a pretty good design.

 

[tomo pauk]

Some possible tanks' engines:
- A much greater proliferation of V6 engines. Would've looked just fine on many tanks that used either inline 6 engines, or the small V12s. Like the Char B1 (shortens the required engine compartment by a good deal), Pz-III and -IV (shortens the engine compartment requirement, and it should be easier and cheaper to make than the V12s), as well as many British tanks. A V6 can have genes in an aircraft engine, but also be with ever less light alloy components and more iron/steel as the engine progresses. Either gasoline or diesel.
- Seven cyl radials, that historically only the Americans really took advantage of. G&R were maning such engines for aircraft, British can try with the Aquila-sized type, or the even the Cheetah spin-off. Germans can make sorta half of the BMW 139/801. Crankshaft will probably be from steel for cheaper manufacturing. Advantages - high power at small weight and on a short engine, no liquid cooling system. Disadvantages - the diameter of the engine needs to be taken into consideration when designing a tank; lower cylinders will not be easy to access for maintenance.
- Shorter stroke 9 cyl engines coming from the 'mainstream' types, sorta like Bristol Mercury in dimensions.

 

[z42]

One engine concept that IMHO would have deserved wider usage was the loop scavenged two-stroke diesel. Yes, like other two stroke diesels it requires a blower, but you want that anyway for higher power. But crucially, no valves are needed. Thus no maintenance intensive valve gear. And the engine is lower in profile as there's no valve gear on the top of the cylinder heads. Could have been a nice tank/truck/etc/etc engine in various sizes.

 

[tomo pauk]

LPG liquefies somewhere between 200 and 2000 kPa, depending on the composition, so one you manage to liquefy it there's no reason to increase pressure further. CNG however, doesn't liquefy at room temperature, so there you need lots of pressure to pack the gas tighter. For comparison, volumetric energy density is around 26 MJ/L for LPG (but note tanks can be filled only to about 85% as there needs to be a bit of gas there to maintain a steady pressure), CNG compressed to 25 MPa about 9 MJ/L, and common liquid hydrocarbon fuels like petrol or diesel around 34-39 MJ/L.

Thank you.

In general, not sure there was much NG production and infrastructure around in the WW2 time period?

Data is very sketchy, most of the graphs and tables I've read are for the post-war era.
I don't know how high the NG production was on the priority list of any of the beligerents, how much of the NG was simply burned/wented off, and how much was used, either on the site or on other locations.

 

[Shortround6]

Some possible tanks' engines:
- A much greater proliferation of V6 engines. Would've looked just fine on many tanks that used either inline 6 engines, or the small V12s. Like the Char B1 (shortens the required engine compartment by a good deal), Pz-III and -IV (shortens the engine compartment requirement, and it should be easier and cheaper to make than the V12s), as well as many British tanks. A V6 can have genes in an aircraft engine, but also be with ever less light alloy components and more iron/steel as the engine progresses. Either gasoline or diesel.
- Seven cyl radials, that historically only the Americans really took advantage of. G&R were maning such engines for aircraft, British can try with the Aquila-sized type, or the even the Cheetah spin-off. Germans can make sorta half of the BMW 139/801. Crankshaft will probably be from steel for cheaper manufacturing. Advantages - high power at small weight and on a short engine, no liquid cooling system. Disadvantages - the diameter of the engine needs to be taken into consideration when designing a tank; lower cylinders will not be easy to access for maintenance.
- Shorter stroke 9 cyl engines coming from the 'mainstream' types, sorta like Bristol Mercury in dimensions.

A lot of these things are sort of country by country.
What makes sense for one country may not make sense for another.
US had the largest selection of truck and industrial engines but very few were suitable for tanks. In part due to size, not weight.

Tanks are sort of a special case. Weight of the engine is almost immaterial.
A difference of 100lbs in a 5-8 ton tank? Even 400lbs in 25-30 ton tank?
Size of the engine installation is much more important. Radiators, oil coolers, fans, ducts, air cleaners all are inside the armor envelope.
An inline 6 is long but it allows somethings to be placed alongside instead of ahead or behind. Assumes the engineers do a good job with laying out everything, which they often didn't.
In peacetime tank production was not big enough to pay for bespoke engines. In war time often did not allow for new engines.

Radial aircraft engines, even in tanks, needed aircraft construction. Trying to substitute steel for aluminum in crankcases means new form of construction. Changing cylinder barrels was not going to work. Changing cylinder heads was going require a lot of work, major change in heat transfer so major changes in size and shape of fins.
Changing a liquid cooled aluminum engine to iron/steel is easier, but there were very few aluminum liquid cooled aircraft engines that were not large V-12s.
Updating the WW I H-S V-8s means investing in an old engine. It is 20 years old in 1935. 2-3 years older than the Liberty and building an iron 11.7 liter 180-200hp engine of dubius durability (50-100 hours before overhaul?).
British went with the Liberty because it was cheap, at least from a design cost and possibly tooling cost. Certainly not the best from a design point of view or best 1935-36 practices or best power/volume density.
Knowing when to settle for existing commercial engines and when to go for more specialized engines is hard and again, depends on countries. US often went for size of vehicles and could use large commercial engines and/or the US was satisfied with less than optimal but available and cheap but serviceable. Twin V-8 Cadillacs in light tanks, bulky but serviceable.
Now find or build a 200-240hp 10-14 liter single V-8 engine? Or a big 6 ?

French tried using large 4 cylinder engines. Standard for large commercial trucks in Europe in the 20s and 30s but of limited potential. Also large 4s have serious vibration problems. maybe the engine survives, but crew, radios and other gear suffers. Sixes are supposed to be smooth, but some are smoother than others. Some V-6s are not so smooth.
French had very few large truck engines and/or low production of such engines. Which means small production capability of such engines of any design.

US used Continental R-670 7 cylinder radials in tanks but they needed Continental to make trainer aircraft engines rather than tank engines. US had plenty of manufacturing capability to make tank and tractor engines. Continental themselves made over 5000 large water-cooled inline 6 cylinder engines to power the M-5 artillery tractors. Continental had 22 different commercial industrial, bus, car, truck engines in their catalog in 1940.
Other nations were not so fortunate.
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