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moo
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RR Merlin Engine - Exhaust Developments

Mon Sep 10, 2012 6:28 pm

Throughout the WW2 period, the RR Merlin was used on a wide variety of aircraft, and it enjoyed a huge development period that saw its power range double by the end of the war.

One of the things that is most obvious on later aircraft as compared to earlier aircraft is the exhausts - the Spitfire for example goes from three on each side to six. Up until now, having no real in depth knowledge of the engine beyond the famous stuff, I always had assumed that the reason for the increase in exhaust horns was due to increase in cylinders on the engine - which I now know to be completely wrong as the Merlin was 12 cylinder right through the war.

So why the doubling of exhaust horns then? Anyone know?
 
Stealthz
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RE: RR Merlin Engine - Exhaust Developments

Tue Sep 11, 2012 12:56 am

Not an expert on this but from my motor racing experience.
Exhaust design is part science, part art. More science to those expert at it, more dark art to those that are not.

As IC engine exhaust gasses are not a smooth stream design of exhaust systems is quite complex to maintain the correct back pressures etc, joining the exhausts of adjacent cylinders is likely not the best choice. However the short length of the exhasts in most Merlin installations makes this a pretty moot point.

As the power goes up and it did increase substantially over it's lifetime. All else being equal ie number of cylinders etc the only way you will get more power is to increase the fuel and air being burned. That increases the gasflow in the exhaust system so pairing the exhausts may have become impractical.

The design of the "Ejector" exhausts on the Spitfire actually contributed measurable thrust to the aircraft, reportedly the equivelent of 70 hp when tested on a prrototype Spitfire
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Ozair
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RE: RR Merlin Engine - Exhaust Developments

Tue Sep 11, 2012 1:02 am

I believe it is related to when they moved to a two speed, two stage supercharged Merlin. This is the best explanation I have seen on it.

http://www.pprune.org/aviation-histo...-merlin-siamese-exhaust-stubs.html
 
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moo
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RE: RR Merlin Engine - Exhaust Developments

Wed Sep 12, 2012 8:32 am

Thanks guys, much appreciated  
 
L-188
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RE: RR Merlin Engine - Exhaust Developments

Thu Sep 13, 2012 4:52 pm

Any thoughs to the reason being differences between Rolls built engines and the re-engineered Parkard built motors.

Many things where changed in the latter to suit US production practices
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moo
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RE: RR Merlin Engine - Exhaust Developments

Thu Sep 13, 2012 6:00 pm

Quoting L-188 (Reply 4):

The only Spitfire to be Packard Merlin powered was the MkXVI, which was identical to the standard MkIX in every way except for the engine, so no it had nothing to do with the US produced engines.

While RR redesigned parts for the US production line, it produced the same basic package although you couldn't interchange parts with a RR Merlin.
 
ThePointblank
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RE: RR Merlin Engine - Exhaust Developments

Fri Sep 14, 2012 12:26 am

Another thing was that the Octane being used increased during the war for the Allies. The British, through the Americans, introduced and standardized on 100-octane fuel in late 1940 from the pre-war 87-octane fuel. Later on, the Allies had access to 130 and 150-octane fuel, which allowed them to increase boost pressure on their engines for more power.
 
dlednicer
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RE: RR Merlin Engine - Exhaust Developments

Fri Sep 14, 2012 8:33 pm

During WWII, there was a lot of research done on exhaust systems. The Spitfire, in particular, was the subject of a lot of this interest. Here is one report NACA WR L-680 Flight Tests of NACA Jet-Propulsion Exhaust Stacks on the Supermarine Spitfire Airplane (NTRS is down right now, but you can also find this report here: NACA WR L-680 Flight Tests of NACA Jet-Propulsion Exhaust Stacks on the Supermarine Spitfire Airplane)

If you look through "Spitfire; The History" by Eric B. Morgan and Edward Shacklady, you will see all kinds of exhaust configurations that were tested on Spitfires. Many of these were attempts at flame damping.

In Joesph Smith's comprehensive paper "The Development of the Spitfire and Seafire" published by the Royal Aeronautical Society in April 1947, he credits multi-ejector exhausts with increasing the Spitfire's top speed 4 mph.

At the Reno Air Races, most of the Mustangs have used pretty much stock exhaust configurations, but there have been some attempts at improving on them:

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[Edited 2012-09-14 13:50:37]

[Edited 2012-09-14 14:03:17]

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