A question about aero diesel engine in WWII

[donkeyking]

I'm quite wondering why aero diesel engine wasn't popular like avgas engine in WWII.


I know a main advantage the diesel engine has much higher compression ratio than avgas engine.

What disadvantages were aero diesel engine in WWII?
Thanks

 

[Shortround6]

They were heavy. Because of the high cylinder pressures a diesel operates at they require heavier construction than a gas engine. The better fuel economy doesn't equal the combined engine+fuel weight of the gas engine until long flights are required. There was a lot of interest in diesels during the 20s and 30s but gasoline kept getting better allowing higher compression to be used in gas engines for better economy so the goal post kept moving. The diesels that were built needed careful maintenance and didn't like the frequent throttle changes that combat required.

 

[evangilder]

Another thing that could be problematic is the cold temperatures at altitude. Diesel fuel gels in cold weather and unless an anti-gel agent is added to the fuel, you could have a rough time when you need power the most. Some diesels are "winterized" by mixing with a more refined diesel, but these are local mixes that are dependent on how cold it will get, etc.

 

[PWR4360-59B]

For worries about gelling, using jet fuel would be the way to go for an aircraft diesel engine.

 

[Snautzer01]

N.A.C.A. Technical memo nr.565

Development of the Junkers-Diesel aircraft engine

.
View attachment Development of the Junkers-Diesel aircraft engine.pdf

 

[johnbr]

good post.

 

[Trilisser]

How reliable are the sources on the issue of the built diesels not liking rapid throttle movements? E.g. if US fleets submarines it was found that the Fairbanks-Morse tolerated rapid starting and stopping and throttling much better than other designs and the F-M engine was based on the Junkers principle.

I also believe that had some maker built a "normal" air-cooled radial using 4-stroke diesel principle with pure turbocharging the power/weight might have been quite good.

 

[Shortround6]

Fast throttle response in a submarine and fast throttle response in an airplane might be two different things. Full power in a F-M Submarine diesel being 720 rpm and Junkers aircraft diesel being 2500-3000rpm.

Pre- WW II radial Diesels include Packard and Guiberson in the US. a few experimentals in England and Germany, All 9 cylinders I believe (?). There was a French experimental Clerget 14F of 940hp at 1477lbs. 14 cylinder two row radial, 4 stroke supercharged.
The Clerget company had also built a V-16 engine, perhaps for airships or flying boats?, it was turbo charged and while it gave 2000hp it also weighed 3750lbs and was of 4970cu in (81.4liters) displacement.

There were a few other experimental or airship engines in the 30s, every time the diesel guys got close the gasoline engine moved the goal post with better fuel that allowed higher compression and fuel economy in the gas engine and/or a better power to weight ratio that meant that the combines weight of the engine and fuel for given length of flight stayed in the gasoline camp for all but the absolute longest of flights, dening the diesel any market. The Junkers was the only commercial use of the aircraft diesel. The Russians tried a few in long range bombers.

 

[davebender]

Do-18
Do-26
Bv-138
Bv-139
Bv-222

Blohm Voss and Dornier built quite a few seaplanes powered by Jumo 205 diesel engines.

1930s Dornier was probably the most experienced seaplane builder in the world. They must have had good reason to power the state of the art Do-26 with 4 Jumo 205 diesel engines rather then 2 Jumo 211 V12s.

 

[Shortround6]

1930s Dornier was probably the most experienced seaplane builder in the world. They must have had good reason to power the state of the art Do-26 with 4 Jumo 205 diesel engines rather then 2 Jumo 211 V12s.

They had two reasons. For the length of flight they were planning the diesels and their fuel weighed less than the gas engines and their fuel. These were very long ranged aircraft.
Two Jumo 211s were never going to replace four Jumo 205s. In the early planes the four diesels offered 2400hp vs the 2000hp of a pair of Jumo 211s and in the later planes the four diesels offered 3520 hp vs the 2400-2800hp of a pair of later Jumo 211s.
with 44-49,000 planes a few hundred horsepower would make a difference even with a catapult launch.

 

[GrauGeist]

A version of the Ju86 (Ju86C) used the Jumo205 in the 1930's as a commercial carrier as well as a couple military versions, which also used the 207 208 types.

 

[donkeyking]

Thanks your reply.

Another question: why does diesel engine not react fast for throttle changes?

 

[davebender]

I've wondered that myself. I have driven diesel powered cars and trucks that responded to throttle changes just fine.

 

[Shortround6]

Descriptions of engines of the time don't say they didn't respond to throttle changes, they say the engines didn't "like" them or didn't "respond" well to them in terms of service life or maintenance. The Jumo 205s seem to have done well enough in airliner service given careful maintenance but didn't seem to perform well in combat service in Spain.

 

[davebender]

Those would have been early model engines. I've got to assume there were improvements by the early 1940s.

 

[Shortround6]

The only service Jumo diesels in the early 40s were used in long range flying boats and high altitude reconnaissance/bomber aircraft whose flight profiles would entail long periods of constant throttle settings. They were used for specific reasons. The flying boats used them for the extra range their better fuel consumption gave them and the high altitude planes used them both because the cooler, lower temperature exhaust of a diesel made the manufacture of a working turbo charger easier and it reduced the fuel load needed to be lifted to the high altitudes.
Numerous throttle changes per flight were still not being used often and the planes essentially flew single flights and not formations of planes.

 

[vikingBerserker]

I thought I remember something about trying to or actually attaching superchargers to them to help performance.

 

[Shortround6]

The Jumo diesles were 2 strokes and needed superchargers to run. The had a piston at each end of the cylinder and ports in the cylinder walls that were uncovered by the pistons. when the pistons were 'down' (bottom of stroke) one set off ports let in the pressurized air and the ports at the other end let the exhaust out. as the piston 'rose' the ports were cut off and when the piston nearly meet in the middle the injector/s sprayed the fuel in, it ignited and the pistons went down to uncover the ports to repeat.
without the pressurized air from a supercharger helping to blow the exhaust out power would have been very restricted. The high altitude engines had turbo superchargers which allowed higher take off power (1000hp vs 880hp ?) but allowed them to carry the 1000hp to 29,000ft.

 

[Trilisser]

But what was the actual technical reason for not liking rapid throttle changes? Frankly I find those usual remarks in general literature dubious until someone produces a credible technical reason for such problems. I wonder if the experience with the Leyland engine (of 2-stroke opposed piston design) in the Chieftain tank would be of use...

 

[tyrodtom]

Even modern diesel engines don't rev as fast as a gasoline engine of equal displacement.

Diesel engines have very heavy pistons , rods, and crank, to withstand the very high compression ratios, we're talking compression ratios of 20 to 1 and up. And i'm just talking static compression ratio, who knows what the dynamic ratio is when you add a supercharger.

A very heavy rotating assembly is slow to speed up, losses rpm slowly to.