A question about aero diesel engine in WWII

[razor1uk]

Depending on the few methods of operation and controls used in a 'constant speed prop', these could perhaps induce slight rpm and fuelling changes automatically more often than just manual controls - dependant upon the mission, range and airframes; this could maybe cause more problems with diesel engines without more modern electronically programed and mapped settings for ignition, fuel pump loadings, injection requirements and of small throttling increments that we can enjoy nowadays.

As a note to the modern high performance diesels, the small cylinder high power diesel format, AFAIK, this is more likely to work towards and into the is region of engine speeds when the/each cylinder volume is under 500cc - typically thought to be around 330cc; and was partly (re-)discovered by the team or group that was tasked in developing the USMC Diesel (engine, for their) off road motorbike which was based upon the Kawasaki KLE/KLR 650 bike and (petrol) motor which was effectively copied and redesigned internally where needed to be 'Dieselized'.

 

[Trilisser]

Another factor may be the limited RPM range of a diesel. Not familiar with aero diesels, but a 3406 Cat, for instance, has a power band from 1500-2100 RPM. A similar power gas engine will have a band much wider, like up to 3300 RPM for a Griffon-like engine. That's one reason why gas engines remained common in tanks, the wider power band required less gear shifting. A diesel aero engine would require a much more sensitive CSU (governor) to adapt to throttle settings.

Not quite so. The need for gear shifting is dependent on torque rise. There is a most excellent graph on this in Walter Spielberger's Panther book. IIRC the same driveability was obtained with a 9-speed gearbox + an engine with zero percent torque rise vs 4-speed gearbox + an engine with 50 % torque rise.

 

[Dogwalker]

Given that a diesel engine produce less heat than a gasoline engine for the same power, and given the inherent strenght of radial engines, is rather surprising that so few tried to do a radial diesel. First than WWII, in it's 14F engine, Clerget managed to obtain first 640hp from 508kg dry weight, and then, with a better supercharger, 940hp from 670kg. A reasonable power/weight ratio for a radial engine of the time. With 1270mm diameter the engine was even rather small.

 

[Shortround6]

Not sure what you mean by "inherent strength of radial engines"?

While the short crankcase was an advantage the separate cylinders are not. A disadvantage of the Diesel is the High cylinder pressure, while the average cylinder pressure may be lower the PEAK pressure is higher and the engine has to withstand the peak pressure or else things like cylinders can depart for places unknown or pistons/con rods can wind up in a heap in the crankcase.

 

[Dogwalker]

WWII Radials are generally not known for cylinders departing for unknown locations, and, as you stated, the short crankcase - short chrankshaft is an advantage.

 

[Trilisser]

One thing to mention: diesels would have been much more suitable for pure turbocharging (no mechanical engine stage at all) than WW2 SI engines due to their significantly lower exhaust gas temperature plus that they had no carb to deal with. With turbocharging the power/weight ratio could have been significantly improved.

 

[Trilisser]

BTW, someone mentioned common rail injection as a modern invention. Not so. E.g. many WW2 British subs had engines with common rail injection...

 

[Shortround6]

It depends on how how much you pushed them. Bristol launched a cylinder through the test house roof and into the parking lot before WW II.
As some engines progressed (like the Wright Cyclone) from 5-600 hp engines in the late 20s to 1400-1500hp engines post WW II the number and size of the cylinder hold down bolts changed. Cyclone went for 12 on the early engine to 16 for the late 1930s/early WW II engine to 20 on th elate war post war engines. Wright also changed the Crankcase several times.

Limits were established in the test houses/ test stands and as long as those limits were observed little trouble was had, exceeding those limits could lead to disaster. Under detonation conditions some radials did loose cylinders, somebody posted a picture on this site of a cylinder sticking through a cowling.

 

[Dogwalker]

It depends on how how much you pushed them.

As anything.

Bristol launched...

I wrote "generally" precisely with the purpose of avoiding anecdotal stories. Not hard to believe that an engine break down every now and then. This shows that radial engines have a tendency to throw cylinders around as champagne corks?

As some engines progressed (like the Wright Cyclone) from 5-600 hp engines in the late 20s to 1400-1500hp engines post WW II the number and size of the cylinder hold down bolts changed.

This is not exactly surprising.

Limits were established in the test houses/ test stands and as long as those limits were observed little trouble was had, exceeding those limits...

Had someone proposed to exceed some limit?

 

[Dogwalker]

One thing to mention: diesels would have been much more suitable for pure turbocharging (no mechanical engine stage at all) than WW2 SI engines due to their significantly lower exhaust gas temperature plus that they had no carb to deal with. With turbocharging the power/weight ratio could have been significantly improved.

Supercharging (even better turbocharging) is the perfect addition to a diesel engine, while it causes some more problems do deal with in the gasoline engine (detonation). The disadvantage is the need to have a very precise timing system for the direct injection. Obtaining it mechanically (the real innovation of the common rail of the '90s is the use of ECU commanded and electrically activated valves) isn't easy.

 

[Trilisser]

Considering that vast number of diesels have been built using just mechanical injection pumps I'd say that the issue was solved very much prior to WW2. It should not be forgotten that the the primary reason behind the introduction of electronic fuel controls have been political, i.e. emission legislation. If general aviation engines had to meet equal emission standards as automobile engines, it would be an instant goodbye to Lycomings and Continentals. Not that I agree with all emission directives, especially those dealing with NOx and particles as meeting these two seriously compromise engine efficiency.

 

[Dogwalker]

I did not say "impossible", however those diesels that have been built using just mechanical injection pumps first than WWII were not, in general, high-speed, high-energy-density engines. As the RPM and the supercharger pressure increases, the pressure of the injected fuel have to be increased and the timing have to be more precise as well, and that has to be done trying to not increase much the weight of the engine. These were problems that the designers of the time had to deal with. The practical effect of the electronic common rail tecnology was not only to make diesel engines cleaner, but even lighter, as the more precise timing (with the possibility to even have several different fuel sprays in the same cycle) reduced the mechanical stress the engine was subjected.

We already saw, in the Isotta Fraschini engines case, that a WWII engine, "dieselized" using modern tecnology, could reach a power output almost double of what it had using gasoline at that time. But first some problems had to be solved, and the field was little known in the '30s.

 

[swampyankee]

There was also relatively little incentive to solve them, although numerous companies (Guiberson, Packard, Rolls Royce, Napier [I know the Culverin was a license built version of a Junkers engine; the Coatelen was not], Daimler, Beardmore, Maybach, Charomskiy, Clerget, and others) did get to at least prototype stage with diesel aircraft engines. The reasons that none of them except the German engines had any kind of commercial success are probably complex, but mostly due to the weight sensitivity of aircraft and the lack of an aviation-based distribution network for diesel fuel. Junkers is really the only one that had any success as an airplane engine, although the Guiberson diesel was produced in quantity, but primarily as an AFV engine.

There is a lot more incentive to solve the problems of aircraft diesels today, mostly as avgas is getting increasingly difficult to get, especially outside North America and Western Europe (avgas is a small percentage of refiners' output). Modern technology also makes it much easier to get high power/weight ratios with modern diesels: systems such as Kiva, Ansys, Nastran, and Fluent mean engine analysis is much better than people like Ricardo or Fedden could even dream of in the 1930s and '40s.

 

[Trilisser]

I disagree with Dogwalker. E.g. the primary reason for those multiple sprays is noise reduction, not stress reduction. Speaking of injection pressures, a common automobile diesel of the 1980s had an injection pressure of less than 200 bars. The injection pressure of the Jumo 205 was some 550 bars.

I'd say that whoever claims that electronically controlled common rail injection has improved power/weight or power/displacement in any significant way is exaggerating quite a bit (=talking like Ananias). Compared to e.g. variable geometry turbochargers it's peanuts. After all, today's automobile diesels do not have any practical increase in piston speed compared to those of the 1980s, for example. E.g. the legendary 200D Mercedes from the early 1980s gave 60 hp at 4400 rpm (piston speed 12.3 m/s). A current VAG group 2-litre diesel develops about 150 hp at 4000 rpm (piston speed 12.7 m/s). For comparison: Jumo 205D piston speed at TO power is 14.9 m/s.

 

[Dogwalker]

However, with the benefit of hindsight, for Germans and Italians at least, there was some incentive. Apart from fuel efficiency and inflammability, diesels does not need 100/115 100/130 octane fuel to fully benefit from supercharging and, as Trilisser said, to make a turbocharger for a diesel is much simpler and less demanding in therms of exotic alloys than for a gasoline engine (infact there were production marine turbocharged diesels already in the '20s). For example, since these two engines had mainly overheating problems, and, as said, the diesel generates less heat, it would be interesting to see what could be done with a dieselized and turbocharged Isotta Fraschini Delta, or Zeta, if the technicians of Isotta Fraschini had been put to study the problem in the '30s rather than in the '50s.

 

[Dogwalker]

I disagree with Dogwalker. E.g. the primary reason for those multiple sprays is noise reduction,

What noise? A turbodiesel had a low exhaust noise, since the exhaust gasses are silenced by the turbo itself. The combustion / mechanical noise, that is high in diesels, is another face of the mechanical stress, an yes, it was lowered by electronic controlled direct injection.

not stress reduction. Speaking of injection pressures, a common automobile diesel of the 1980s had an injection pressure of less than 200 bars. The injection pressure of the Jumo 205 was some 550 bars.

Low pressure '80s diesels were prechamber / indirect-injection ones. More weight, less fuel efficiency, less thermal efficiency, and less benefits / more danger in being turbocharged than a direct injection one.

I'd say that whoever claims that electronically controlled common rail injection has improved power/weight or power/displacement in any significant way is exaggerating quite a bit

May be that the raising of performances, and lowering of weight, specifically since the direct injection has become common in diesels, came from the sky.

 

[Trilisser]

No offense, but at least in automobile manufacturers' own advertizing material it is clearly stated that multi-phasing of injection is primarily intended to make the engines less noisy plus to reduce emissions. Not in a single text has it been stated that it is intended to prolong engine life and/or reliability.

Not all 1980s diesels were prechamber. And even then pre-chamber diesels' main attraction was softer noise. Plus in construction machinery, tractors etc. direct injection has been significant for quite a while. Even WW2 diesels like the Soviet V-2 tank diesel had direct injection. And that engine had excellent power/weight ratio.

As for the improvements in performance, the main reason for that has been with at least 99 % share of the impact has been turbocharging (especially variable geometry turbochargers) coupled with intercooling. In the 1980s intercooling was quite rare.

 

[Dogwalker]

No offense, but at least in automobile manufacturers' own advertizing material it is clearly stated that multi-phasing of injection is primarily intended to make the engines less noisy plus to reduce emissions. Not in a single text has it been stated that it is intended to prolong engine life and/or reliability.

Quite strange, since I happened to read it several times.

Not all 1980s diesels were prechamber.

And those that were not prechamber had 200 bar pressure of the injector?

And even then pre-chamber diesels' main attraction was softer noise.

The prechamber diesels main attraction is to have less pressure / longer time injection, so to need less precise injection time. I already explained the drawbacks.

Plus in construction machinery, tractors etc. direct injection has been significant for quite a while.

Precisely were RPM are lower and so injection timing could be less precise. Who knows why...

Even WW2 diesels like the Soviet V-2 tank diesel had direct injection. And that engine had excellent power/weight ratio.

Have I EVER said "impossible", or that "none has been done"? I Already posted another that had.

As for the improvements in performance, the main reason for that has been with at least 99 % share of the impact has been turbocharging (especially variable geometry turbochargers) coupled with intercooling. In the 1980s intercooling was quite rare.

Rare? Prechamber turbodiesels, injecting fuel during compression, to have little more than mediocre performances needed MASSIVE intercoolers to avoid preignition and a hole in the piston. It was part of the inefficiency of the formula, and of the causes that have made that it was erased as soon as a reliable form of direct injection has been available. You are reasoning as direct injection and turbocharging are independent variables. They are not.

 

[Trilisser]

Yes, turbocharging and direct injection are independent variables. Plus, why speak of direct injection when your original claim was electronically controlled common rail injection and its blessings on power increases? Regarding pre-chamber engines, the "automobile technical handbook" by Pentti O. Savolainen states that the advantages of pre-chamber diesel are
-lesser possibility of injection nozzle clogging
-lesser quality fuel may be used
-lower noise

None of your reasonings is listed.

 

[Dogwalker]

Yes, turbocharging and direct injection are independent variables.

The fact that you have decided that way does not means that's true.

Plus, why speak of direct injection when your original claim was electronically controlled common rail injection and its blessings on power increases?

It's not so difficult to read the posts and see how the discussion has gone that way.
I stated that "the real innovation of the common rail of the '90s is the use of ECU commanded and electrically activated valves", and that obtaining mechanically a precise timing in a high speed direct injection diesel isn't easy.
You stated that the problem was solved prior than WWII and that the reason under the adoption of electronic controls is political.
Ecc... ecc...
Or are you asking what '90s common rail has to do with direct injection?

Regarding pre-chamber engines, the "automobile technical handbook" by Pentti O. Savolainen...

I'm very sorry for Mr. Savolainen, who probably has the only fault of his work being misquoted.
"in respect to the direct injection chamber, the pre-chamber has the advantage to allow lower injection pressure" Treccani Encyclopedia (more or less the Italian equivalent of the Britannica)
You are certainly able to search the sources which state that a indirect injection diesel has a efficiency of about 15% less than one direct injection one, and to imagine how the head of a prechamber diesel had to be heavier than one of a direct injection one.
As a methodological note, I must say that the fact that something is not written in a certain book, as authoritative the author is, does not mean it does not exist. No book it says "everything". Usually it says what is needed in that moment.