What was the problem with the allison engine?

[tomo pauk]

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.

American carburetors (at least the ones used on main military engines) were initially better than British types, ie. pressure carbs were better than float-type carbs. The much glossed-over things with using one big carb on a V12 engine of ww2 vintage, vs. other types of fuel distribution was that it necessitated backfire screens (a.k.a. 'flame traps') that were 'stealing' rated height even for non-ram conditons (= static engine). Venturi in the carb, along as the small pipes/tubes act as restrictior.
Thus, even if the fuel spray is indeed cooling the mixture, two areas where rated altitude is decreased can be hardly cancelled by one area where rated altitude is increased.

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.

Unfortunately, the C series used weaker crankcase and crankshaft than the mid-war F and E series, so the gearbox change is of no help. The impeller gearing was also different and weaker on the C engines than on those F and E series of engines. Earlier pistons were high compression (for 6.65:1) vs. late pistons (that never made it to the war, 6:1 CR). Now that we're at it, let's chage the intake manifold, the early ones were problematic.
In the Merlin, the chagnge from x to y rotating engine was a merely gearbox change.

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.

As noted above, installing the parts from early V-1710s is a show stopper. Conversion from one engine to another is not equivalent to the conversion from one aircraft to another.

 

[Zipper730]

I think this is the main problem. The U.S. Army Air Corps bet on the wrong horse.

I wouldn't say that, but I think the problem is that they almost exclusively focused on turbochargers over any twin-stage supercahger system

 

[Shortround6]

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)

It may be that the British simply did not build larger carburetors for the later engines? You don't normally use the same carburetor/s on a 1000hp engine that you would use on a 1500hp engine any more than you use the same carburetor on a 40hp motorcycle and a 60hp motorcycle.
The US used different sized carburetors on single stage and two stage Merlins. There were carb changes on the early Allisons.

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.

The Allison may very well be the better engine. Rolls-Royce may well have supplied a better powerplant.
Semantics perhaps. but there is little doubt that the Allison block/heads, crankshaft, con-rods etc were better. However only by use in post war racing applications, few wartime Merlins failed due to lack of strength. The Merlin superchargers were always better, but here everybody keeps trying to make it a two horse race. The Allison supercharger wasn't as good but was good for 2nd or 3rd place for most of the war in comparison to all the other horses in the race. German, Italian, Japanese, Russian. It took until 1943/44 for P &W and Wright to beat it using single stage superchargers.

I would also note that for both the US and British the engine makers did NOT make their own carburetors, they bought them from outside suppliers for most of the war. So the engine makers cannot take credit for better fuel systems and can only shoulder the portion of blame for bad ones in regards to not switching to another available system. I would also note that many times the engine maker and/or airframe maker was also at the mercy of outside suppliers when it came to radiators, oil coolers and intercoolers. The P-63 was supposed to have an intercooler. the outside supplier failed miserably.

 

[tomo pauk]

...
The Allison may very well be the better engine. Rolls-Royce may well have supplied a better powerplant.

Semantics perhaps. but there is little doubt that the Allison block/heads, crankshaft, con-rods etc were better. However only by use in post war racing applications, few wartime Merlins failed due to lack of strength.

The items you're refering to are the part of 'power section' of the engine. As a whole engine, a ww2 piston engine for military application, Merlin was better.

The Merlin superchargers were always better, but here everybody keeps trying to make it a two horse race. The Allison supercharger wasn't as good but was good for 2nd or 3rd place for most of the war in comparison to all the other horses in the race. German, Italian, Japanese, Russian. It took until 1943/44 for P &W and Wright to beat it using single stage superchargers.

Main problem with the supercharger of the V-1710 was it's small size, for a desired airflow and pressure ratio required above 15000 ft, not helped by initial 'slow' gearing. The resricted size of the intake (pre-impeller) also contibuted to the insufficient performance above at high altitudes. That, coupled with small cubic capcity of the V-1710, made te engine lacking vs. main German and British types, until a 2-stage version appeared (by what time the bar was upped considerably by Merlin). Let's recall that 1st useful 2-stage V-1710 produced barely more power above 20000 ft than the Merlin 46 (1-stage supercharger) and a DB 605A.

 

[Shortround6]

I wouldn't say that, but I think the problem is that they almost exclusively focused on turbochargers over any twin-stage supercahger system

A certain points in time the turbo simply offered much more. Like in 1939 the turbo was offering sea level power to somewhere between 20 and 25,000ft.
The P & W two stage R-1830 engine used at the 1939 fighter trials was good for 1200hp at take-off, but only 1050hp at 11,000ft in low gear and 1050hp at 17,500ft in high gear. This was "improved" to 1000hp at 19,000ft in the first F4F wildcats.

This was the only existing system in actual development with a flyable engine in 1939 and 1940 in the US, P & W didn't get the first two stage running until 1938.

The turbo also offered a number of theoretical advantages, including the fact that the auxiliary stage was almost totally independent of the engine in regards to rpm/boost. In other words, volume of exhaust gas permitting, you could adjust to turbo unit to give anything from no boost, to all the boost the engine would take, delivered to the mouth of the carburetor and be infinitely variable, no fixed gear ratios and clutches, hydraulic couplings (which had upper and lower limits).
The turbo didn't work as quick and easy as promised but then some of the other supercharger systems had a few problems too.
Also note that both systems benefited from the improved fuels.

 

[Shortround6]

The first engines used in the P-38 were developed to run on 92 octane fuel. The advantage of the turbo can be seen in that engines with 6.44 gears to the small supercharger offered a take-off rating of 1150hp using 39.4in ( 4.75lbs?) MAP and would still give 1150hp at 25,000ft using the same 39.4in MAP (this was the goal I don't know how it worked in practice)

Now compared to the C15 engine (running US 100 octane) and making 1040hp the F2 engine as above was supposed to be running about 112 degrees F in the intake manifolds with a 70 degree intake while the C15 would be running a mixture temperature of 196 degrees F.
The F2 engine used 240hp in friction and to drive the supercharger with the small gears, The C15 engine used 290hp for friction and to drive the supercharger with the 8.80 gears. Basically 50 more hp driving the supercharger. The C15 was making 1330 indicated HP vs the F2s 1390 but the difference in cylinder pressure was 205 IMEP vs 214.5 IMEP.
A two stage engine with a mechanical blower is going to need higher cylinder pressures to drive the blower even if the intercooler works as good as the turbo set up. The 2nd stage Allison came up with took over 200 hp to drive to get 10,000lbs of air per hour, this is the amount of air you need to make 1672hp in the cylinders. So subtract say 210hp to drive the 2nd stage and the 290 hp for the friction and the supercharger drive for the C15 engine and you are down to 1172hp, only trouble is what is the IMEP? and what kind of fuel do you need to run at those pressures without suffering from detonation?

The early unsupercharged airship engines ran on either 80, 83, or 87 octane fuel, there are 23 model designations of Allisons running on 91-93 octane fuel, I don't know if all were built or how far they were tested, a number were rebuilt into later models, most never flew.
Point is that the size/type supercharger you design for 87-91 octane fuel is smaller and has less pressure than the one you design for 100/130 fuel and in 1938-39 no one knew that 100/130 existed or could be made.
Please see loss of power at low altitudes for the DB605 with the supercharger from the DB603 engine. Better high altitude perfroamce put less power for take-off and low altitudes

Now please note that Allison had built 30 engines total from 1930 to the end of 1938 and only built 46 more in 1939, in 1940 they built 1178 engines.

 

[wuzak]

Now please note that Allison had built 30 engines total from 1930 to the end of 1938 and only built 46 more in 1939, in 1940 they built 1178 engines.

Which is why the following statement is simply not true.

There is no doubt that had RAF fighters been powered by Allison during BoB ,the result would have been the same

 

[wuzak]

In the Merlin, the chagnge from x to y rotating engine was a merely gearbox change.

I'm not 100% certain on that.

If the engine was a left hand rotation engine then, yes, it was just a matter of removing the reduction gears and idler and replacing them with the reduction gears from a right hand rotation engine.

But the right hand rotation engine did not have the position for the idler gear shaft.

http://users.skynet.be/BAMRS/dh103/pics/crankcase.jpg

 

[Shortround6]

Even with a magic wand creating over 1000 engines in 1939 the British fighters would have lost several thousand feet of altitude.
Spitfire may have survived, much more doubt about the Hurricane.
And that is assuming perfectly sorted out C15 engines.

People should NOT confuse the engines/supercharger and the aircraft. Germans got altitude performance from a 3rd tier engine (altitude wise) by using the smallest, lightest fighter of the BoB fighters, not because of a better supercharger.

 

[GregP]

You cannot change a left into a right-hand turn Allison with very little effort. I worked at an Allison shop and we built them.

True, there are very few parts changes, but ... if you have a left turn Allison and need a right turn, it's pretty much a major disassembly.

There is zero to is very little difference in effort to assemble either left or right-hand turn from parts, but to change one to the other is NOT a simple thing. The camshafts must be swapped end for end, there is an idler gear to add in the nosecase AND in the cam train (meaning the accessory housing also comes off ... that's the whole engine!), and you will need a starter that turns the other way. Last, but not least, you need the ignition harness for the other-handed turn. Maybe one or two more, but that's it. Still, to change from one to the other means disassembly of the intake track, and engine down to the crankscase, removal of the nosecase, adding a gear in the nose case, etc.

But assembly is basically the same for either-handed turning if you are building up from parts. I'd decline to do it unless I completed disassembly, stripped, and repainted the engine case, cylinder heads, and valve covers. Might as well LOOK like a fresh engine when you're done ... if you're going to all the trouble anyway!

 

[TheMadPenguin]

Kinda like this picture from Flight Global of a turbocharged Merlin.

http://www.flightglobal.com/airspac...olls-royce-merlin-xx-supercharger-cutaway.jpg

AirSpace community site is gone? Has anybody a new link to a re-incarnated site?

 

[GrauGeist]

AirSpace community site is gone? Has anybody a new link to a re-incarnated site?

They have bent to the pressures of Social Media, unfortunately.
Their most active community can be found at facebook here: FlightGlobal