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RR Merlin Engine - Exhaust Developments  
User currently offlinemoo From Falkland Islands, joined May 2007, 5205 posts, RR: 4
Posted (3 years 8 months 3 weeks 15 hours ago) and read 11409 times:

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?

7 replies: All unread, jump to last
User currently offlinestealthz From Australia, joined Feb 2005, 5896 posts, RR: 38
Reply 1, posted (3 years 8 months 3 weeks 9 hours ago) and read 11367 times:
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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

If your camera sends text messages, that could explain why your photos are rubbish!
User currently offlineOzair From Australia, joined Jan 2005, 1380 posts, RR: 2
Reply 2, posted (3 years 8 months 3 weeks 9 hours ago) and read 11366 times:

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.


User currently offlinemoo From Falkland Islands, joined May 2007, 5205 posts, RR: 4
Reply 3, posted (3 years 8 months 2 weeks 6 days 1 hour ago) and read 11104 times:

Thanks guys, much appreciated  

User currently offlineL-188 From United States of America, joined Jul 1999, 30408 posts, RR: 57
Reply 4, posted (3 years 8 months 2 weeks 4 days 17 hours ago) and read 10923 times:

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

User currently offlinemoo From Falkland Islands, joined May 2007, 5205 posts, RR: 4
Reply 5, posted (3 years 8 months 2 weeks 4 days 16 hours ago) and read 10901 times:

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.

User currently offlineThePointblank From Canada, joined Jan 2009, 2458 posts, RR: 0
Reply 6, posted (3 years 8 months 2 weeks 4 days 9 hours ago) and read 10839 times:

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.

User currently offlinedlednicer From United States of America, joined May 2005, 560 posts, RR: 6
Reply 7, posted (3 years 8 months 2 weeks 3 days 13 hours ago) and read 10731 times:

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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Photo © David Lednicer

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

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

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