A question about aero diesel engine in WWII

[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.

[Shortround6]

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...

My impression is that it had more to do with maintenance issues than actual response "times" in combat. Perhaps ring problems or oil consumption? The engines may have given adequate service in airline use but had shorter overhaul times when used for combat?

[davebender]

Why would a diesel powered heavy bomber have shorter overhaul times? Normally they cruise to and from the target at a slow and steady speed. Not much different then a Do-18 hauling mail to South America.

[Trilisser]

In general all engines in military servive have shorter overhaul lives than their civilian counterparts. Even in the case of bomber the handling of the throttles will be more violent than in a commercial plane and the time limits for particular power settings may be exceeded which rarely happens in commercial service.

[Shortround6]

Why would a diesel powered heavy bomber have shorter overhaul times? Normally they cruise to and from the target at a slow and steady speed. Not much different then a Do-18 hauling mail to South America.

As noted by Trilisser, bomber engines had a shorter overhaul life the same engine used in transports. Fighters were even shorter. There is no reason to suppose that diesel engines would follow a different pattern.

Only the Germans and Russians tried to use diesels in combat aircraft. While they did complete combat missions neither country found them satisfactory for combat use.

[davebender]

Pratt & Whitney JT3D turbofan engine.

Pratt & Whitney JT3D - Wikipedia, the free encyclopedia
Major applications.
B-52H heavy bomber.
Boeing 707 passenger liner. Douglas DC-8 passenger liner.
C-141 cargo aircraft.
KC-135 tanker aircraft.

The same engine. Four different aircraft types. 8,600 engines have been built and they have been in service since 1959. So there must be all sorts of real world data for engine life.

What is the average engine life for each aircraft type?

[tyrodtom]

World war 2 bombers flew usually in close formations, for mutual support. That required a lot of throttle changes, that's not good for any engine.

Even when used in Vietnam, the B-52's formations were not in anyway what anyone would call close. The only time a B-52 would require any sudden throttle changes would be during aerial refueling.

Comparing modern jet overhaul times military versus civilian isn't valid when comparing the same for WW2 bombers verses civilian.

[davebender]

This engine was produced in massive numbers and used to power all sorts of aircraft. What was the average engine life for each aircraft type?

B-17 and B-24 heavy bombers.
F4F and P-36 fighter aircraft.
TBD Devastator torpedo bomber.
DC3 passenger liner.
C-47 cargo aircraft.
PBY Catalina naval amphibious aircraft.

[Shortround6]

The B-17 didn't use R-1830s.

The R-1830 was built in a variety of models over a number of years with improvements in materials and manufacturing techniques as time went on.

This also applies to the jet engines. It is also worth noting that the JT3D is a development of the earlier JT3C with many common parts. Service life increased drastically over the years in all aircraft types.

There are a few figures for the Merlin available from "The Merlin in Perspective-the combat years", Alec Harvey-Bailey, Rolls-Royce Heritage Trust. Make of them what you will.

In 1939 they are saying the life of a fighter engine was 240 hours and a bomber engine was 300 hours. For 1944/45 they are saying that the life of a fighter engine was 300/360 hours, bomber engine was 360/420 hours and transport engines 480/500 hours.

Please note the increased life rating (not always achieved) even with much increased max power ratings.

[NVSMITH]

-This link: http://www.oldengine.org/members/die...rd/avrange.htm will take you to some pages with technical information on early diesel aero engines.
-Both supercharging and turbocharging were tested but one has to remember the state of super/turbo charging before and during the war. The turbocharged Allison V-1710 only went to the P-38. That is one of the reasons the supercharged Merlin was such a popular replacement for the Allison.
-Naturally I can't find the information now, but there was at least one annual publication on diesel aircraft engines before WW2.
-This is off WW2 topic but there have been a number of recent experiments in producing small aircraft diesel engines. With modern technology the engine weights have come down considerably and the specific fuel consumption figures are still good.

[GrauGeist]

Simple mistake regarding the P&W R-1830...it was used in the B-24 while the Wright R-1820 was used in the B-17