Merlin 70s operting at +21 and/or +25 lbs/sq in: any such luck?

[tomo pauk]

Hello,

I wonder whether the Merlin 70 series (in Spitfire and/or Mosquito) were ever operating at boosts above +18 lbs/sq in; if yes, what were the capabilities? Any good data is appreciated

So far only one report, about Merlin 72 in Mossie. +21 lbs/sq in.

 

[Mike Williams]

Hello,

I wonder whether the Merlin 70 series (in Spitfire and/or Mosquito) were ever operating at boosts above +18 lbs/sq in; if yes, what were the capabilities? Any good data is appreciated

Hi tomo:

The Merlin 70 was cleared to operate at +25 lbs boost.

 

[Juha]

Very interesting
Thanks, Mike!

Juha

 

[tomo pauk]

Indeed, many thanks

 

[Mike Williams]

You're welcome gents.

Regarding capabilities of Merlin 70 series equipped Mosquitos and Spitfires, please see the following:

 

[Edgar Brooks]

I haven't found anything on the Merlin 70, but some Merlin 66s were cleared for +25psi, in the Mk.IX, but only for a short lifespan. The Spitfires had to use 130 octane fuel, and were fitted with a special throttle box, with a physical stop fitted, so that, if using 100 octane, the pilot couldn't go above 18psi. The engines were known as "Basta," and I don't know their purpose (possibly for anti-V1 patrols, or V2 hunting, but that's just a guess.)

 

[fastmongrel]

Was +25psi the maximum any Merlin was cleared for. I find it amazing how engineers and chemists managed to take an engine from +3 (iirc) to +25 in such a short time

 

[Siegfried]

Was +25psi the maximum any Merlin was cleared for. I find it amazing how engineers and chemists managed to take an engine from +3 (iirc) to +25 in such a short time

The merlin didn't have a particularly high compression ratio, it was only 6:1. 25psi relative is 39 psi absolute. The Germans would have referred to this as 2.785 ata ie atmospheres or BAR the US as 83.57 inches of mercury.

As fuels and spark plugs improved there was considerable scope especially with the use of inter cooling.

The low compression ratio has a considerable cost in power and efficiency though it does allow higher supercharging pressure with some recovery in the form of jet thrust from the high pressure. The inter cooler also allows higher boost though again it is throwing away energy.

By contrast the DB605DC had a compression ratio of 8.5:1. It reached boosts of 1.98 ata (about 14psi) using C3 fuel plus MW50. C3 fuel was equal to about 100/130 and I assume c3+mw50 was almost equal to 100/150

If you multiply 1.980 x 8.5 you get 17 for Daimler Benz engine
If you multiply 2.785 x 6.0 you get 16.71. For merlin engine.

The construction of the DB without head bolts allowed a larger volume without much weight increase though its RPM was less.

The Packard Merlin in the P-51H did use 100/150 plus ADI water injection though the British didn't seem too interested in ADI on the merlin.

US P-51D and H were flown at 90" Hg which is 3 Ata or 28 psig by using water injection Don't think they saw service.

The DB engines never saw service with inter coolers so and even the planed two stage versions of these engines added little in weight unlike the Merlin which added about 150kg.

 

[tomo pauk]

Yay, seem like the we're going to have a zillionth RR vs. DB debate.

(*grabs popcorns*)

 

[cimmex]

There is nothing wrong in siegfried's calculation example
cimmex

 

[Edgar Brooks]

There is nothing wrong in siegfried's calculation example

Except that it has absolutely nothing to do with the originator's enquiry, and doesn't take him forward, in his quest, by a single iota,

 

[Milosh]

How many German a/c flew with 1.98ata? I would say not to many as it was only cleared in late March 1945, iirc. The a/c also had to carry around the empty MW50 tank.

 

[Shortround6]

Nothing really wrong but a few things that are not right.

Power is NOT dependent on "1.980 x 8.5 you get 17".

It IS dependent on the amount of fuel and air you can get through the engine in a given period of time. An engine running at 2 ATA or 60 in or 15lb boost ( numbers rounded off) is putting 33% more fuel and air through the engine as the SAME engine using 1.5 ATA, 45in or 7.5lbs of boost. Start with a basic; displacement times rpm. Then factor in the extra air flow due to the supercharger. That is your potential energy. You can't make more than you put in. The compression ratio helps you get more (harvest?) of the potential energy but does not create more.

There are few problems with trying to use compression to GAIN power in a supercharged engine. Unsupercharged engines do not have all of these problems. an old book show a study from 1937 that gives the increase in MEP (mean effective pressure), maximum pressure and fuel economy for increasing compression ratios starting with a base of 5 in percent.

compression ratio 5 is 100%.
Compression ratio 6 shows a MEP of 108%, max pressure of 120% and a fuel consumption of 92%.
Compression ratio 7 shows an MEP of 113-114% Max pressure of 143% and a fuel consumption of 88%.
Compression ratio 8 shows an MEP of 120% Max pressure of 170% and a fuel consumption of 83%
Compression ratio 9 shows an MEP of 125% Max pressure of 190% and a fuel consumption of 80%

The MEP (which is the indicator of power) is a curve that shows diminishing returns as the CR goes up (14 to 1 compression is just over 70% fuel consumption and just under 140% MEP) as is the fuel consumption curve. Max cylinder pressure however is a straight line, which at 14:1 is over 300%. Since the engine has to be built to handle the max cylinder pressure, higher compression engines (everything else being equal) have to built heavier than low compression engines. one reason diesels are heavy for power. Raising the compression ratio 33% (from 6:1 to 8:1) gets you 20% more power (at best) and saves 17% in fuel?
Raising the manifold pressure 33% will give you 33% more power NOT COUNTING the extra power to drive the supercharger and any losses due to the higher intake temperature. If these losses are less than 13% the higher manifold pressure will give you more power than raising the compression. It does not help with fuel economy however.

 

[Mike Williams]

Fwiw, Lumsden noted in his British Piston Aero-Engines and their Aircraft the following tidbit regarding the Merlin 70: "The maximum power on +25 lb boost at 3,000 rpm was about 1,950 hp at 5,000 ft. in MS gear and 1,700 hp at 18,000 ft. in FS gear."

With respect to the Merlin 72, with which some Mosquitos were equipped, Lumsden noted: "At +21 lb. boost, this engine would give 1,800 hp at 6,000 ft. and about 1,580 hp at 18,000 ft."

 

[Mike Williams]

Here's more data on the Merlin 72 operating at +21 lbs./sq.in. boost:

+21 lbs./sq.in. authorization for the Merlin 72:

 

[Kryten]

what interests me in that excerpt is the sentence-

"authorised in service for periods of not longer than five minutes at a time"

That suggests to me that it is restricted by temperature, in other words the engine would be able to use the 21lb on several occasions provided the individual periods were not longer than 5 mins?

 

[Tante Ju]

The question seems rather theoretical as the only units actually using 150 grade fuel were those Merlin 66/266 powered Mark IX/XVI Squadrons of the 2nd Tactical Air Force in NW Europe for a couple of months in 1945 before problems with the fuel made them to switch back to 100 octane fuel. Apart from that, two Sqns of Merlin 66 Spit Niners were using this fuel in operational trials during 1944. Wheter these saw combat action at all is open to question.

There were only about 2 squadrons of Merlin 70 in RAF service during the war. I am pretty sure they were not with 2nd TAF and did not have access for 150 grade fuel, so for all practical purposes, they continued flying on +18 boost.

So cleared or not cleared, it appears it was never used in service for the Merlin 70.

 

[Siegfried]

@Shortround, my calculation was not meant to show anything but the fact that overall compression of the DB605DC and 2 stage merlin was roughly the same but by different routes. I realise all these things are very non linear. Thanks for the data, very good.

@Milosh, Some Me 109 were probably operating at 1.98 ata in Jan 45 or dec 44. Problems forced a partial retreat to 1.9 for old units and 1.8 ata for new but the 1.98 ata rating was restored around March.

The thing to remember is that the DB605 at 1.8 ata produced 1850 metric hp which pretty much matched the Merlin at 25psig.

Direct comparisons are difficult because of the Merlins use of different gear ratios to optimise at different altitudes and the resultant performance peaks. Nevertheless although the Daimler Benz engine wasn't as dependent on high supercharger pressures due to its higher CR and swept volume it still gave up at high altitude slightly earlier especially when compared to the higher ratio Merlins. Hence we see the appearance, at least in prototype form of the two stage DB605L for the Me 109K-14.

The DB 605L could produce 1160ps at 9600m/32000ft which is the same as the DB601A1a could do at sea level as installed in the Me 109E4 in 1939 as the engine was not inter cooled little weight was added.

The 605 and Merlin solved many of the same problems by different routes which makes them interesting to compare. The jumo 213 looks much the same as a Merlin or Grifon. The 605 emphasising fuel efficiency at military power to suit the me 109 size and use of lower grade fuels to suit Germany's limited capacity in that area.

I imagine one effect of the V1 campaign was to divert much of the 100/150 program to defense of Britain?

 

[Milosh]

Siegfried, a staffel of II./JG11 was testing 1.98ata in Dec/Jan. Then after late March when 1.98ata was cleared for use, there was the problem of supplying C3 fuel to those units. The Fw190A had priority of C3 fuel.

Tante Ju,
http://www.spitfireperformance.com/2taf150_112044.gif

British 100/150 Grade Fuel Production

That is a lot of fuel.

The Americans used 150 fuel in their Merlin powered P-51s.

How many DB605L engines were operational?

 

[Mike Williams]

As usual the use of 150 grade and increased boost by the 8th Air Force is ignored:

2nd TAF began converting to 150 grade in December 1944 and were still using it in May 1945 when the Nazis were ultimately defeated.

126 Wing, Canadian reverted back to 130 grade in May 1945 in time to perform stylish formation shows over vanquished Germany.

Some units of ADGB began using 150 grade in the summer of 1944 and were still using it in 1945.

Fwiw, Lumsden's British Piston Aero-Engines and their Aircraft shows 1,000 Merlin 70's were built at Crewe and equipped Spitfire HF VIII, IX and PR XI.
1,000 Merlin 72's were built, 500 at Crewe and 500 at Derby, which were fitted to Mosquito B.IX, BXVI, B.30 and PR.IX