You are fairly close to the mark on your comments. The Maybach's were good engines, they do not appear to be the cause of many of the reliability problems that plagued German tanks. Which should be expected, considering that they were developed as specialized tank engines over the course of more than 10 years. However, your criticism of them is also on the mark. They were developed for high horsepower and low torque - not a problem neccessarily in and of itself, especially in the early, lighter German tanks - but coupled with the increased weight of later German tanks and the poor material quality in their later transmissions, it was disastrous. The transmission in the Panther appears to be the cause of 80+ percent of the failures in the late-model Panther, a problem associated with the use of poor quality steel in the gearing (there was barely enough hi-quality steel available for the better-made Tiger transmissions) and the overly-complicated, multiple gear design.

But as to the US tank engines, that was a product of parsimony and short-sightedness, as well as some good luck and clever engineering. The original US "tank" engine was nothing of the sort. It was actually a de-rated radial aircraft engine. Designing tanks around them was a matter of economy, decided on in the 30's. The engines were light weight, with excellent power output and torque, but were also bulky and were fuel hogs. Worse, the outbreak of war resulted in huge increases in aircraft production, and a huge increase in the demand for radial aircraft engines, leaving insufficient capacity (at least initially) to produce enough for tanks.

The result was a frantic scramble for engines of any description, that were then hurriedly bodged into the existing tank designs. This meant that the M4 Sherman eventually had a multitude of powerplants, with different logistic requirements (some were gas, some were diesel). And they varied in quality, the Chrysler multibank was probably the worst - multiple engines working a common driveshaft? Aside from being almost impossible to maintain - to access the sparkplugs in the lower bank you had to yank the entire engine - it tended to stall easily, not a good thing in combat. The GM twin-diesel was at least reliable, if a little large for its output (the Sherman hull in the M4A2 had to be enlarged slightly to accomodate it, another manufacturing compication), but it was a diesel, when all other US Army vehicles were designed to run off of gasoline. That was solved by making them a primary Lend-Lease item, and by giving them to the US Marine Corps (that many of the Navy's landing craft used diesels evidently was part of that decision). But for the British it later created another problem - the M4A2 could not be modified into a Firefly configuration, the engine and transmission layout interferred so badly with the crew compartment that ammunition handling and stowage for the larger rounds proved insoluable.

The "best" of the US tank engines was the Ford GAA, a modified truck engine which became the preferred standard for the US Army. But it only had a maximum power output of about 500 horsepower, which was insufficient when the decision was made to uparmor the US second generation tank, the T25, creating the T26 Pershing. The Pershing in some ways had even more problems than the German tanks, since its powerplant simple could not produce sufficent power or torque to move its bulk. Engine failures and engine related transmission failures plagued it until 1948, when the first real purpose designed US tank engine was produced (note the similar timelines - the Germans began tank engine development and had the Maybach line pretty much at its peak of development by 1942-1943, about seven years of development - the US development began seriously in 1941 and bore fruit in 1948, after about seven years).

So, a long winded answer to a very good question, and one that many haven't asked or thought about.

But the Chrysler multibank may have been in theory a good engine. In practice I have yet to find anyone who liked it. Mostly for the maintenance requirements.

And the M4A2 was actually pretty good, it just didn't meet the US Army requirement for a gasoline-based engine. And did cause some problems with space allocation.

And yes, the Maybach design was pretty much of a dead end. In a sense they appeared to have trapped themselves in a similar way as did the US, allocating diesel development and production to U-Boot programs. All in all they had a greater headstart, but more limited production and design capacity - good for the world in the long run of course.

And I must agree that the US Army's reluctance to use diesels was very peculiar. It sounds like they simply turned their back on any diesel development. IIRC the first purpose-built tank diesel engine wasn't developed until well into the 50's.

Another factor may have been availability of diesel fuel. I have no data on this. I don't know what the German domestic crude oil and imports yielded in terms of a gasoline/diesel ratio, neither do I know if the German synthetic fuel plants could produce diesel fuel (they certainly did produce gasoline type fuel).

Forgive me for mainly speculating, I have little knowledge on the tank engines. I too believe that overall the arguments for a diesel engine are strong, just wanted to throw some fuel (petrol or diesel) on the discussion.

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Originally posted by Rich:
And yes, the Maybach design was pretty much of a dead end. In a sense they appeared to have trapped themselves in a similar way as did the US, allocating diesel development and production to U-Boot programs. All in all they had a greater headstart.

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The MAN and Daimler-Benz diesels for the German Navy's coastal forces may have been a factor, because their development efforts must have diverted potential resources away from panzer engines.

As far as I can tell, the main driving force behind the Schnellboote diesel programs was avoiding the fire hazard of using gasoline in a wooden boat. Fuel economy and range must have been another factor. D-B apparently had fewer development problems and their MB 500 series diesels were the most common propulsion units for S-boats.

The Raumboote and the Mehrzweckboote also used diesels but they were different designs. Then there was the Marineartilleriefahrprahm, converted ferry barges loaded up with deck guns and armor, but I'm not sure of the engine types.

The panther and tiger engines did use aluminum. But represnted a much smaller percentage of ger tank production then the rus T34/KV?IS. The german AF needed all the aluminum for its planes and engines. The rus got a huge chunk of al from LL allowing them to make more al tank engines than the gers. Perhaps the allied ship of al to rus combined with the huge AF requirment meant little was avilable for the army.

Diseal eng all have higher ratios which requires more stell to withstand the higher pressure. Diseal engines could easily be twice the weight and outer size of the equivalant gas engines. Al offsets some of the wieight considerations but increases size or bulk. Remeber a larger engine means more arm to cover it making the tank heavier because of this.

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Originally posted by Darrin:

All tanks even shermans during the war needed at least 1 rep or maintainace each day.

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This can of course negate the advantage with diesels having to refuel less often.

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Originally posted by Jukka Juutinen:
To Niklas, flat torque curve is rarely searched for since it kills the flexibility of the engine and requires more complicated transmission (I assume you already knew this...).

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Motorcycles typically have much flatter torque curves that car engines. Despite this I have found them to be more flexible than the car engines. But this of course may be due to a generally high torque value (relative to the engine size).
As to transmission complexity, a flat curve may cause the user to want an extra gear or two, but that need not mean a more complex transmission, there are other factors to consider too.

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Originally posted by Jukka Juutinen:
And see the book for volume requirements also. It is far too simple to think that "since the diesel has 39 litres and Maybach 23 litres their external volume must be proportional to that". For example, auxiliaries and their location play an important part in this.

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Originally posted by Jukka Juutinen:
Niklas, AFAIK Germany would have had substantial supply of diesel fuel since naval stocks were quite large, according to some sources. Of course, fuel used for steam propelled ships and ships with very large diesels could not be used but fuel used for U-boats and other smaller vessels having high speed diesels could have been used.

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Originally posted by Jukka Juutinen:
To Niklas, flat torque curve is rarely searched for since it kills the flexibility of the engine and requires more complicated transmission

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A flat torque curve makes an engine much more driveable because acceleration is the same at any point on the curve. Transmission can be less complicated, not more. Engines with peaky torque curves require a lot of gears as acceleration is maximum only in a narrow rpm range.It is just difficult to get with max hp output.

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It is far too simple to think that "since the diesel has 39 litres and Maybach 23 litres their external volume must be proportional to that". For example, auxiliaries and their location play an important part in this.

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Generally it does work out fairly proportional for engines with the same cylinder layout. Diesels (especially big industrial diesels) tend to be far larger than equivalent gas engines because of the stresses involved. Starter, fuel filter system, radiator, batteries, oil capacity are also far larger and heavier.

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Darrin, if a tank weighs 50 tons or so and you add say 2 tons, would anyone even notice it

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That is two tons less armour on the turret, front glacis, etc. The Soviets compensated for the inherant heavy construction of a diesel engine by casting the block and heads out of aluminum, which the Germans could not afford.

The Maybach had a lot of advantages for the Germans, such as the high power rating, the very short block (which allowed the superior mid mounted turret design), light weight, easier starting, quicker warmup, less smoke in cold weather. Plus it was basically a scaled up version of the Maybach that powered their smaller tanks.

The V-2 had a lot of advantages for the Soviets, such low-end torque which compensated for their horrible to shift transmissions. It is also much more successful to crudely build a low rpm, high torque engine than a high rpm, high horsepower one.

I would much rather drive a tank with a big diesel than a big gas, based on my driving experience with heavy equipment, but the Maybach had a lot of advantages for the time.

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Originally posted by Jukka Juutinen:

Niklas, your point re diesels stocks taken since I do not have adequate evidence to disagree. It would be interesting to know how large the stocks indeed were.

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Neither have I any data, and it would definitely be interesting to know.

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Originally posted by Jukka Juutinen:
Have you tried putting that high revving motorcycle engine in a tank?

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Of course not, but what I am speaking of is engine characteristcis and output relative to engine displacement. If a 39 litre engine in a tank gave as much torque and power as a motorcycle engine does (relative to engine size), and given suitable gear ratios, the automotive performance would be very much greater. I don't however think that such an engine would last very long in a tank, so its not what I recommend. It would most likely also consume fuel faster than an alcoholic drinks whisky.
What I really wanted to say is that what matters is not primarily the shape of the torque curve, but the level of it and a t what revs it operates. Remember, if a tank is driven on a gear that allows it a top speed of, say 40 km/h, and it currently runs at, say 3000 rpm and develops (at that rpm) 1000 Nm of torque, this will be equal to a lower revving engine, developing 2000 Nm at 1500 rpm.

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Originally posted by Jukka Juutinen:
Paul, please check out data and ads for modern large truck and tractor diesels (30-40% is now mandatory). The truth is that rising torque curve (rising curve with dropping rpm) is what is needed for heavy vehicles.

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Maybe we are talking about two different things here. A flat torque curve can have high torque rise, it just holds it over a larger rpm range. The longer the torque curve holds a high level, the less shifting is needed. I know that sometimes when people say 'flat torque curve' they mean 'torque curve with low amounts of torque', but that is not a correct assumption. Both a low level of torque and a peaky torque curve means more gears needed.

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Paul, you did not get my point of that two ton example.

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Oh I did. Tanks are designed to a certain weight. What goes in as engine has to be taken off as armour. The Panther is a good example of a design where the weight was increased beyond the designers specs, and had plenty of trouble as a result. MORE weight would not have been a good idea.

Or look at another way, what if you were driving a T-34/85 into combat and you could have a 80mm glacis plate in front of you and a Maybach instead of a 40mm one and a V-2? That thin glacis was a major reason why the T-34 is the most destroyed tank of all time. I'm sure they would have loved to uparmour the hull, but the upgunned chassis was overloaded as it was.

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Originally posted by Jukka Juutinen:
Niklas, consider that 40 km/h tank encountering a smallest of obstacles with engine of zero torque rise (torque=ability to do work, power=rate of doing that work).

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Those definitions of torque and power does not correspond to what I remember from my studies of math, physics and engineering science at university. Torque is not ability to do work, since it depends on gear ratios and since the tank actually shall not rotate, but rather move on a flat (at least compared to a rotating shaft) surface.

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Originally posted by Jukka Juutinen:
If that 40 km/h was at full throttle and rpm, any increase in inclination or other form of resistance will require immediate shifting, or the vehicle stops! The more torque rises with lowering rpm, the greater the ability to negotiate increasing resistance without shifting (and with mech. transmissions there is allways a momentary cut in torque available to tracks).

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Yes, but the comparison, as I see it, is not between two engines running at full throttle at the same speed, see below.

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Originally posted by Jukka Juutinen:
BTW, why does e.g. MTU advertising material emphasize excellent torque rise characteristics if it was a meaningless feature...

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It is, if the hp output is similar. However, this need not be the case. Lets take the Maybach 23 litre engine and comparing it to the V-2 diesel. All figures I mention here are taken from memory, so if someone finds better figures, just put them into the calculation.
The Maybach engine produce 700 hp at 3000 rpm, corresponding to a torque of 1640 Nm. At 2500 rpm the engine gives 600 hp, corresponding to a torque of 1687 Nm.
The V-2 diesel gives 500 hp at 1800 rpm, corresponding to 1950 Nm.
Now if we assume a drive sprocket diameter of 80 cm and a gear allowing the tank to make 36 km at top rpm (assumed here to be equal to peak power rpm, this is hardly a daring assumption, especially the diesels usually fall off quickly after peak power). This means the sprocket has to rotate four revoultions per second, or 240 per minute. That will make it necessary for the Maybach to have a 12.5 : 1 ratio, giving 20500 Nm on the drive sprocket at peak power rpm. the V-2 will need a 7.5 : 1 ratio, giving 14625 Nm on the drive sprocket at peak power rpm.
If we assume that the Maybach has a completely flat torque curve, that will also be its best torque at the drive sprocket. If we assume a 30 % torque rise for the V-2, it will at most give 19000 Nm on the drive sprocket.
Thus, even at its best, the V-2 will produce less force on the tracks to move the tank forward. If we assume a 39 % torque rise for the V-2, it will at its peak equal the Maybach, but on all other rpms be inferior.
As I see it, the V-2 gets its torque rise characteristics not by being stronger at low rpm than the Maybach, but by being weaker at higher rpm. This is also why MTU talks of its torque rise characteristcs (I assume you speak of the MTU diesel in the Leo 2 tank), because at 1500 hp it is at least as powerful as any other tank engine. In that situation, torque rise is really a rise, not a loss at higher rpm.
I surely agree that torque rise is a good feature, but not if it is gained on a diagram by lowering the output at higher rpm.
To go back to your example above (two tanks drivning at 40 km/h). I think that is not correct, because one of the engines simply would not go at full throttle (if we compare the Maybach and the V-2). In one of the tanks the driver could have countered the incline by giving more throttle, and not losing speed at all.

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Originally posted by Jukka Juutinen:
Niklas, my comparison was not of a 700 hp Maybach vs. 500 hp V-2. It was for engines of equal power, of which one has zero torque rise, the other has rising torque curve.

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Well, if both have the same maximum power output, it is obviously better with torque rise than flat torque. However, as we began discussing diesels vs gas engines, this assumption was not clear to me. After all, diesels rarely give as many hp as gas engines of similar size.

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Originally posted by Jukka Juutinen:
Besides, with supercharging, the V-2 could easily have been boosted up to 700 hp. Then it would have been a case of "Hasta la vista, Maybach!"

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Perhaps, but there is alsway possibilties for improvemed output of engines. Supercharging could also have been used on the Maybach. It usually comes down to if it will have penalties in the form of fuel consumption, reliability, overheating etc.

Jukka