Superchargers: Early V-1710 vs. Merlin

[Shortround6]

From my understanding, some of the limits and/or differences between supercharger presure and compression are;

1. The engines power is usually limited by detonation (or engine strength). Detonation is delayed by using higher octane ( or if over 100, performance number) fuels. Detonation limits vary widely from engine to engine, by this I mean from air cooled to liquied cooled and even from one type of aircooled or liquid cooled engine to another.

2. Three general factors the affect detonation (aside from individual engine characteristics) are the pressure of the intake mixture or air, the temperature of the intake charge and the compression ratio of the engine. Lowering the compresion ratio will allow a higher boost pressure to be used at the same temperature which means more power because a higher pressure means more air/fuel is being burned.

3. A higher compresion ratio means a higher thermal efficency which means more economic operation or better fuel milage. While it does increase power to the propellor it usually limits the amount of boost to an extent that a lower compression, high boost engine will deliver more peak power.

4. lowering the intake charge temperature with inter/after coolers will allow either higher boost or higher compression to be used but does not change the the point about a lower compresion higher boost engine making more power than a high compression lower boost engine.

 

[siznaudin]

Most (all...?) of the fuel injected German engines missed out on the charge temperature reduction afforded the allied aero engines as the latter had cooling through latent heat of vaporisation of the fuel helping them. And if there was an inter(after)cooler in the system, even better.

 

[Shortround6]

Going back to the original question there really wasn't much to chose between either the superchargers or the engines. They were just set up differently.
The Early Merlin was set up to give a critical altitude at 16,250ft. Using 87 octane fuel this ment that the engine could NOT use full throttle below that height without the risk of detonation problems. The lower the altitude the more the throttle had to closed until the engine was rated at 880 HP for take off.
There is no way to limit the amount of boost. The supercharger is geared to the engine and if the engine is turning a certain rpm, like 3,000 the super charger is turning a certain rpm. In the case of the Merlin III that would be 25,740rpm. It doesn't matter if it is at sea level or at 20,000 that is how fast the impellor will turn. Gearing a single speed supercharger to give good performance at 15-20,000ft means that it will deliver way too much boost at sea level and will require high power to drive it in the thick sea level air. Engines have to be built to withstand GROSS power. Net or shaft HP plus friction, supercharger losses and any auxilary requirements like pumps. It will also heat the intake charge reducing it's density while increasing the risk of detonation.
Using 100 octane fuel moved the detonation threshould allowing the throttle to be opened more at low altitudes.
The Allison's were set up with a lower critical altitude or full throttle height. THe early P-40 engine for instance was 13,200ft. THis ment that the supercharger was taking less power at low altitudes, and heating the intake charge less. THE C15 (-33) Allison made 1,090 HP at 13,200ft. 60HP more than the merlin at 3,050 lower. The differences would have been more if both engines were measured at the same altitude. The Melin would be loosing power at it went lower while the Allison would have lost power as it went higher. The Allison was rated at 1,040 for take off. the extra 160hp fover the Merlin III for take off and intial climb may have been useful in some cases.
At the start of the war most peaple's superchargers were capable of delivering a pressure ratio of about 2.3 at an effiency of between 60-70%. since you need a pressure ratio of 2 to 1 just to deliever sea level pressure at 20,000 ft that doesn't leave much extra boost for higher performance. Especially considering that many engines were already running a pressure ratio of 1.4 at sea level. 1.4 X 2 meaning the supercharger would need a pressure ratio of 2.8 to get sea level power at 20,000ft. After Hooker got through they were getting a bit higher pressure at 70-75% effecinecy. the higher the effecency the less power is needed for a given amount of boost leading to more NET HP. It also means less charge heating for given level of boost. Later they got pressure ratios of over 3 to 1 from single stage superchargers at good effiency. But with improved fuels the level of boost went up at sea level. 12lb of boost means a pressure ratio of 1.8 at sea level and a need for 3.6 pressure ratio to maintain power at 20,000ft.
A two speed supercharger gets 2 critical or full throttle heights but still runs into the pressure ratio problem. This is why 3 speed superchargers were never very popular. The extra speed would give more consistant power while climbing (an extra peak in the sawtooth chart) put it couldn't do anything about the pressure ratio limit. It aslo couldn't do anyting about the charge heating at altitude. Using large pressure ratios ment lots of charge heating.
Two stage superchargers sovled the pressure ratio problem and offered even more performance because a two stage supercharger will casuse less charge heating for a given pressure ratio than a single stage supercharger.

 

[gjs238]

Shortround6:

This last post is very enlightening and gets to the core of the initial question.

So it seems the early V-1710's were not so bad after all, at least in reference to the early Merlins.
It's just that the Merlins took leaps forward in evolution when Hooker became involved.

It also explains the American preference for turbocharging, which seems to "solve" the supercharger limitations noted...
...And the German "solution" with a variable speed supercharger, not unlike a torque converter in an automobile automatic transmission.

Speaking of German superchargers, which wasn't included in the scope of the original question, can you provide any insight as to how they kept pace with Hooker's developments of the Merlin?

 

[siznaudin]

Shortround6:

This last post is very enlightening and gets to the core of the initial question.

Agreed!

Shortround6:

Speaking of German superchargers, which wasn't included in the scope of the original question, can you provide any insight as to how they kept pace with Hooker's developments of the Merlin?

Nitrous? ... aka "chemical supercharging"

 

[MiTasol]

Interesting brief history of the Rolls Royce R engine and its relationship to the Buzzard is in this NACA pub from 1931

 

[GregP]

The shortcomings of the Allison supercharger were:
a/ it was too small. Its diameter was only 9.5 ins compared to the 10.25 of the Merlin. When Hooker analysed the R-R supercharger, he quickly found that it was strangling the engine. If this was the case with the R-R product, then the Allison must have been really suffering as evidenced by the huge amount of trouble the P-40 engines had with detonation.
b/ it was mounted too close to the back of the engine. Drawings of the V-1710 s/c clearly show the very tight angle between the outlet of the s/c and the start of the inlet manifold.
c/ the V-1710 induction manifold was extremely convoluted. This posed problems for the mixture flow as evidenced by the necessity to provide special pickups for raw fuel that gathered at the base of the main inlet branch.
d/ development of the V-1710 s/c was, for a long time, not given much attention because it was felt that the turbocharger would provide the main source of supercharging for all US aero-engines.

b, c, and d are incorrect.

The flow from the supercharger into the intake is pretty smooth and the intake was fixed with a turbulator. After that, there were no more issues with detonation, which was caused early-on by a combination of incorrect fuel formulation for the way the Allison was set at the factory and the fact that the air-fuel mixture tended to separate in the manifold at cold temperatures. Both of those were cured within 8 months or so.

The turbocharger was not the main source of supercharging. It was supposed to provide the high-altitude portion of the boost and it did that in the P-38, which, in the J and L models, had service ceilings of m40,000 a,d 44,000 feet respectively.

Could the Allison have been made better? Sure.

All it would have taken was approval and money. They never got either.

 

[thunderbird]

It seems to me, that the biggest difference between Rolls Royce and Allison, is in how quickly one adapted to the needs of war, and how one didn't. Was it because Alison was a General Motors company, or was it because the American Army technical staff were mind boggling incompetent?

 

[MiTasol]

because the American Army technical staff were mind boggling incompetent?

 

[Bretoal2]

Or because US WWII war began more than two years AFTER British WW II..... And because Munich crisis was a non-event in USA.... and so on...

 

[SaparotRob]

The U.S. did seem able to build a proper radial engine, though. Could it be that "doctrinally" the U.S.A.A.C was concentrating on strategic bombers (radial engines) while the fighter/interceptor designs (inline) were a bit of a side show? The U.S. did have a few radial engine fighters.

 

[Jugman]

It seems to me, that the biggest difference between Rolls Royce and Allison, is in how quickly one adapted to the needs of war, and how one didn't. Was it because Alison was a General Motors company, or was it because the American Army technical staff were mind boggling incompetent?

Neither. The Merlin was in production for about three years before the Allison. There was far more time to sort out technical, production, training, and operational issues.

 

[MiTasol]

and followed a long line of successful, and less successful, v12s

 

[MIflyer]

They did try an experimental 10.25 inch supercharger impeller on the Allison F-11R but it appears that the difference between it and the 9.5 inch version was not that great.

I recall reading somewhere that an NACA engineer was given the task of improving the V-1710 and his attitude was that it was a piece of junk and thus a waste of time. Adding another speed to the supercharger should have been pretty simple, so it is a shame that feathermerchant did not do his job.