What was the problem with the allison engine?

It's a lot more complicated than it appears. Those lousy British British carburetors (and the American ones) helped cool the intake charge by about 25 degrees C over what the fuel injected German engines could due to fuel evaporation in the supercharger. They were also much cheaper and simpler to make, requiring about 1/4 the parts and much, much less precision machining which meant that the machines and operators could be making something else.

If you are going to use a "snapshot in time" like the BoB of under 2 months you had better make sure your facts are right.

The Merlin III could make 6-6.25lbs (roughly 42in or 1.4AtA) of boost in the intake manifold with no ram at 16,250ft. which is better than any other supercharger in squadron use in Aug of 1940 could do. It made 1030hp at that altitude.
The Allison -33 in the long nosed P-40s made 1040hp at 14,300ft using 41in of MAP or 5.5 lbs? British Spitfire or Hurricane would have lost about 2,000 ft of ceiling (service,operational and combat ceilings). That assumes of course that that the British got -33 engines after
the first 288 or so. The early engines broke and had to be sent back for a "re-work" that included such minor items as a modified crankshaft. Allison fixed the engines at company expense and continued to improve the engine in may ways resulting in an ever improving engine not only in power but in durability and reliability. However d o not confuse mid or late war engines with engines being built in 1940 (or with the electric Curtiss propellers that were slow to change pitch and allowed the engines to overspeed)
The mid 1940 DB 601 engines didn't come close in supercharger performance. The supercharger on the DB601A-1 model was good for about 39in (4.5lbs) at 4000 meters (about 13,200ft). An improved supercharger was brought into service during 1940 that boosted the 1.3 Ata Limit to 4500 meters or 14,850ft.
Of course in Aug/Sept Spitfire IIs began to trickle into service with Merlin XII engines with better altitude perfroamcne than the Merlin III and Sept of 1940 saw a few Hurricane IIs start to show up with the Merlin XX engines which not only had two speeds but the supercharger with Hooker improvements. This supercharger could carry 6lbs boost (42in or 1.4ata) to over 20,000ft. In other words the Fall of 1940 (after the BoB?) Merlin supercharger could match the performance of the DB605A of 1942, or perhaps we should say that it took Daimler Benz until 1942 to match the performance of the Merlin Superchargers of the fall of 1940.

pbehn said:

I believe that once the USA agreed to supply it (at a cost) they all did.

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Went looking and came across this, Use of 100 Octane Fuel in the RAF during BOB

Stocks of 100 Octane
30th September 1939 153,000 tons(b)
27th February 1940 220,000 tons(b)
31st May 1940 294,000 tons(a)
11th July 1940 343,000 tons(b)
31st August 1940 404,000 tons(a)
10th October 1940 424,000 tons(c)
30th November 1940 440,000 tons(a)

There is further data in the quite heated at times discussion mainly due to a well known anti Brit person.

We have a number of threads covering these topics, including some on enduring myths of WW II

The US Army didn't get what they asked for in 1939-40-41 for superchargers, they got what was technically possible.
1. Allison was a small company with only about 25 men in the engineering dept and not all of them were engineers, that number includes the guy/s who ran the blueprint machine. They didn't have the staff to work on multiple supercharger set-ups.
2. Allison had one of the best superchargers in the world in 1939-40 in terms of the Pressure it would deliver vrs the pressure of the air entering it. Pressure ratio. NO American company had a single stage supercharger that was any better.
3. One of Hooker's claims to fame was that he very quickly realized that some of the formulas people had been using to design superchargers for over 20 years were flawed. Doesn't matter how many engineers you have, if you are using flawed formulas/design tools you are going to get flawed results.
4. Fuel capable of using high manifold pressure was relatively new. Many people didn't realize how bad their superchargers were because 2-4lbs boost (34-38in MAP) was all the fuel would support. 87 octane would support 6lbs boost in some engines (not all).
This was the big reason for the Armies push for turbochargers. By using two stages separated by an intercooler they could use more boost with the same fuel than a single stage supercharger would allow.

Contrary to legend Britain was NOT saved by the timely arrival of a tanker full of US 100 octane gas. The British had been stockpiling 100 octane fuel since the middle of 1939.
One also has to be careful in reading as while quite a bit of British fuel came from the "Americas" that doesn't necessarily mean it came from America (the United States). There was a large refinery in Trinidad for instance since before WW I that had been progressively enlarged. At some point in 1940 it reached the capacity of 285,000 barrels per DAY. It had reached 9000 barrels a day back in 1919.

We are also back to the American 100 octane vs British 100 octane thing. American refineries could certainly produce fuel to British specifications but it was far different than USAAC 100 octane fuel. Using USAAC 100 octane with 12lbs boost would have seen blown up Merlin's all over southern England in 1940. This also goes back to the Allison. There was only so much you could do with 100 octane fuel and that is what the American fuel was, 100 octane or PN number lean, not rich and in fact more than one batch actually measure under 100 PN under rich conditions. British 100 octane at the time of the BOB was actually between 115 to 120 PN number when measured later.

Milosh said:

There is further data in the quite heated at times discussion mainly due to a well known anti Brit person.

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I dont know anything about the "anti Brit person" I just seem to remember that to perform at best the RAF needed 100 Octane fuel but didnt have the refining capacity to satisfy demand. Supply of 100 octane fuel became a political issue because of neutrality. Eventually there was an agreement based on "a dollar per barrel" (as I remember it).

Read the thread. The Brits had enough capacity and even increased stocks without American help.

I believe there was a refinery (at least one?) in western England that was also producing 100 octane fuel from imported feed stocks.

Now the British may very well have been working to assure even more supply (or as back up in case of bomb damage?) which is only a prudent thing to do, especially with the U-boat war (what percentage of fuel gets sunk/lost en-route?) But that is a far cry from "American" gas saved England which is where some of these arguments go.

Shortround6 said:

I believe there was a refinery (at least one?) in western England that was also producing 100 octane fuel from imported feed stocks..

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The UK could produce 100 octane, it could also import it. No one knew how much effect German bombing would have on refineries, they seem vulnerable but are not as vulnerable as they seem. I live in Teesside which had a large petrochemical industry, the LW did not make any significant/successful raids on it in the whole war.

GregP said:

The issue with the altitude capability of the Allison was the direct result of Air Corps requirements. They got what they ordered. They also declined to fund the development of a higher-altitude version. Allison's use of the auxiliary, 2nd-stage supercharger was a low-cost development that was not optimum, but could be considered for a bomber's larger nacelles. Had the Air Corps / USAAF decided to make a 2-stage integral engine, Allison could have done it. Not sure about the timeframe, but it wasn't magic ... only unfunded.

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The Merlin of 1940 only had a single stage supercharger but it gave higher altitude performance than the V-1710.

Perhaps because the V-1710 was originally intended to be used with a turbocharger it ended up with a smaller than optimum supercharger, which impacted altitude performance somewhat.

Possibly true Wuzak, can't say for sure as I have never seen a real reason why the supercharger wasn't developed when it went from airship to aircraft.

If I had been around at the time and in the position to do anything about it, I'd have at LEAST paid to have the Merlin 2-stage S/C combo fitted to an Allison to see how much difference it made, as I expect it would have. But, they didn't and I really wonder why. Doesn't matter anymore, but would be nice to know. Or they COULD have tried the hydraulic coupling the Bf 109 used. Didn't do THAT, either.

I love the Merlin, for sure. Have to say the same for the Allison. Both are good engines today, but the Merlin would have been the better choice at 17,000 feet and higher during the war by far, unless the Allison were a turbo (P-38) or a 2-stage (P-63). Merlin might have been slightly ahead anyway since Hooker flat KNEW his business. Seems like a child would have seen that when he switched from the airship to the aircraft, mid-teens in altitude would have been eclipsed quickly.

Nothing we can do about it now, though. Things were what they were, and I'm really not too much on "what-ifs."

Allison did test a Merlin 2 stage supercharger on the V-1710 (but not mounted to the engine) and found, unsurprisingly, they got almost exactly the same power numbers as the Merlin.

Two items

One of the strengths of the Allison (designed long before the Merlin) was that it was (probably) the first modular engine (like all modern military and transport engines).



You could change a C model Allison to an E or F model or an x hand rotation engine to y hand rotation engine with the minimum of effort and you could almost always use late model parts in the early model engine within very simple rules.

Changing from C to E to F model (and with C and F engines changing reduction gear ratio) was merely a gearbox change.

Reduced to the most basic level, converting rotation involved removing the reduction and accessory boxes, splitting the crankcase, rotating the crankshaft end for end, adding/removing one or two gears in the accessory box, rewiring the ignition harness and reassembling.

Fitting a set of high compression type three of four pistons and type two or three piston pins to an earlier low compression engine was a standard option (though fitting type one or two low compression pistons or early pins to a high compression engine was naturally not permitted).

The Merlin did not have any of these abilities.

Stupidly, in my opinion, the USAAC never approved Allison designing a one piece two stage supercharger version of the accessory drive housing for the V-1710 because, almost certainly, this could have been introduced without disrupting the earlier engine production (unlike the Merlin where such a significant change meant a whole new production line, new crankcase etc). The nearest Allison came to a two stage was the ASB engines where the accessory gearbox was changed and the Axillary Stage Blower externally mounted and driven from the new gearbox.

Another outcome of this Allison modular design strength was that the company (and every USAAC/F heavy maintenance shop) could convert engines during overhaul to later, higher powered versions of the same basic engine (or salvage many early engine parts for use in late model engines).

The Brits used a similar process to convert Spitfire Vs to Spitfire IXs but there was no equivalent process for Merlin's because they were not a modular design.

Secondly

In the mid sixties I spent some time in Reno at the Harrah Casinos engine shop where they were using late model Allison E type engines (from P-63's) in racing hyroplanes. These engines were reliably turning out over 5,000bhp on the dyno though some of that power came from the ASB being driven, not by the Allison itself, but by a full race Chrysler hemi developing some 800bhp. The Allison's could make and take the power because they were originally designed to be water cooled so had much more coolant capacity than the Merlin which was designed for Glycol/Prestone from day one.

wuzak said:

The Merlin of 1940 only had a single stage supercharger but it gave higher altitude performance than the V-1710.

Perhaps because the V-1710 was originally intended to be used with a turbocharger it ended up with a smaller than optimum supercharger, which impacted altitude performance somewhat.

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I am constantly amazed that people criticize the Allison for not having 1942/43 altitude performance in 1939/40. Nobody else did.
The Allison had better altitude performance than the R-1830 with a singe stage supercharger, Better than the R-1820 with single stage supercharger and better than the R-2600 A with single stage supercharger, it was better than the 1940 DB 601 or Jumo 211. Or ANY Italian engine and most if not all Japanese 1940 engines. Or any service Russian engine in 1940 (the AM-35 was not in service for most of 1940). No French engine came close.
Granted it did hit it's limits fairly soon but the same basic supercharger ( a few modifications to inlet and diffuser and new gears) was able to make a bit over 7lbs of boost at 15,500ft while flowing enough air to get 1125hp to the prop.
P & W was making two stage superchargers for a reason, their single stage wasn't very good and the designs provided by GE to american manufacturers, to be polite, sucked.

Shortround6 said:

It's a lot more complicated than it appears. Those lousy British British carburettors (and the American ones) helped cool the intake charge by about 25 degrees C over what the fuel injected German engines could due to fuel evaporation in the supercharger.

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"It's a lot more complicated than it appears."

Too true!

The American carburettors such as the Stromberg PD series were not carburettors in the normal sense of the word but injection carburettors and exempt from the gravity considerations that plagued the Merlin and other British (and pre-ww2 American) engines.

A normal carb depends on the reduction in air pressure through the throat (Bernoulli's theorem) to suck the fuel from the float chamber into the airflow.

The Stromberg PD uses that pressure differential to meter fuel which is injected into the eye of the supercharger where it is mixed not only by the spray pattern of the injection nozzle itself but also by the inherent turbulence and temperature rise of the supercharger. A win win on both the British and German systems.

Unlike Rolls Royce, Allison and other American companies spent considerable time ensuring that the fuel mixture at each cylinder was as nearly exactly the same as possible, relatively easy on single row radials but more difficult on twin rows and much more difficult on the R-4360 and V-12 engines where induction lengths were not exactly identical (and now you know why the Allison has such a weird induction manifold and Merlin such a simple manifold)

As a result the Allison had far superior fuel mixture consistency than the Merlin, though not as good as the injected German engines, but that produces a weight penalty that the Merlin did not suffer.



Allison




Merlin


Which was the better engine? That very much depends on what criteria you use to define better.


Mi

Th standard Allison carb isn't bad at all. Neither is the Merlin carb. If they WERE, the engines would not be good to great ones, and both ABSOLUTELY WERE. They weren't fuel injection, but they weren't bad.

Just look at who won the war, and you'll see it plainly.

After the war, piston engines didn't matter much, though Argentina DID make some very good piston aircraft (Nancu comes to mind, used Merlins; 460 mph) in addition to the Soviet Union (Lavochkins and Yaks). Mostly today, the Soviet fighters (Yak-3s) use Allisons.Says something about the relative availability of the two, but they perform quite well relative to the Merlin crowd in peacetime fun flying. Ask anyone who flies one. I have. They don't much fly at 25,000 feet and above ... though they CAN. I'd surely fly one!

GregP said:

Th standard Allison carb isn't bad at all. Neither is the Merlin carb.

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I believe the Allison used an injection type carb from quite early on. It was, in effect, a single point injection system.

The early Merlins and Griffons used an actual carburetor, which had a float chamber, etc.

The Merlins would, therefore, cut out under negative G. The V-1710s, as I understand it, wouldn't.

wuzak said:

It was, in effect, a single point injection system.

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Commonly known stateside as a "pressure carburetor", the poor man's fuel injection. Good old Bendix.
Cheers,
Wes

MiTasol said:

The Stromberg PD uses that pressure differential to meter fuel which is injected into the eye of the supercharger where it is mixed not only by the spray pattern of the injection nozzle itself but also by the inherent turbulence and temperature rise of the supercharger. A win win on both the British and German systems.
Mi

Click to expand...

AN excellent post aside from the bolded part. I believe the British system also got a good mixture due to "inherent turbulence and temperature rise of the supercharger." as the carburetor was before the supercharger.
Unlike the French, the Russian Klimov series and most (all?) Italian inline/V engines where the supercharger blew threw carburetors spaced down the sides of the cylinder banks.


Granted this did free the engine from the threat of carburetor icing.

Now which is harder to make, one BIG carb or six little ones. Or harder to keep "in tune".

pbehn said:

I dont know anything about the "anti Brit person" I just seem to remember that to perform at best the RAF needed 100 Octane fuel but didnt have the refining capacity to satisfy demand. Supply of 100 octane fuel became a political issue because of neutrality. Eventually there was an agreement based on "a dollar per barrel" (as I remember it).

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The UK didn't need to prouce their own 100 octane fuel as supply was considerably in excess of demand during the BOB. One refinery did produce 100 octane for a short time as a back up to ensure that we could if we wanted to but that switched back to normal production after the trial.
Interestngly, the only time I was able to find evidence of a shortage of 100 octane fuel was in May 1944. With the vast amount of fuel being used in the preparation for D Day, the problem was distributing it fast enough to keep up with operational requirements. This did cause some problems.

MiTasol said:

"It's a lot more complicated than it appears."

Too true!

The American carburettors such as the Stromberg PD series were not carburettors in the normal sense of the word but injection carburettors and exempt from the gravity considerations that plagued the Merlin and other British (and pre-ww2 American) engines.

A normal carb depends on the reduction in air pressure through the throat (Bernoulli's theorem) to suck the fuel from the float chamber into the airflow.

The Stromberg PD uses that pressure differential to meter fuel which is injected into the eye of the supercharger where it is mixed not only by the spray pattern of the injection nozzle itself but also by the inherent turbulence and temperature rise of the supercharger. A win win on both the British and German systems.

Click to expand...

The float-type carburetors, as used on Merlins and the like before 1943/44 were robbing the power at altitude, being too restrictive and necessitating the ice guard. The switch to 'fuel pumps' (pressure carburetors, as noted by Wes) gained rise in the rated altitude and ceilings for the Spitfires: the 8-10 mph speed gain was recorder on the Spitfire Vs. (link)

Unlike Rolls Royce, Allison and other American companies spent considerable time ensuring that the fuel mixture at each cylinder was as nearly exactly the same as possible, relatively easy on single row radials but more difficult on twin rows and much more difficult on the R-4360 and V-12 engines where induction lengths were not exactly identical (and now you know why the Allison has such a weird induction manifold and Merlin such a simple manifold)

As a result the Allison had far superior fuel mixture consistency than the Merlin, though not as good as the injected German engines, but that produces a weight penalty that the Merlin did not suffer.

Click to expand...

The last paragraph is bashing the Merlin. Allison tried 3 types of intake manifolds before they got it right. 1-stage Merlin with pressure carb also consumed less fuel on usual war-time powers - not a sign of a lousy fuel distribution.

Which was the better engine? That very much depends on what criteria you use to define better.


Mi

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Merlin was a better engine.
It's propulsive power at altitude was always better (equled or barely suprpased by turbo V-1710 that comes wih own set of issues), they introduced 2-stage variation almost 2 years earlier, V-1710 never got the 2-speed variant, the 2-stage V-1710 never received intercooler. A most powerful Merlin was an easier retrofit on an existing aircraft than a most powerful V-1710.

Modularity in factory line does not help a pilot in a combat. BTW - the change from a small , 9.5 in supercharger to the big 10.25 in (as suggested when Americans saw the Merlin) was judged as setting back the V-1710 programe by 2 years in 1938.

Merlin vs Allison, Ford vs Chevy, Rolls vs Bentley, conservative vs liberal; ENOUGH ALREADY!! THERE IS NO DEFINITIVE ANSWER that will vanquish the opposition to acknowledge defeat! That's the point, isn't it?
Cheers,
Wes