The Case Against The Float Carburettor

[Deleted member 68059]

I`ve got so fed up with people saying "But I bet you`ve never read Stanely Hookers autobiography!" to me when I discuss the various forms of fuel injection that the British totally failed to develop before WW2, that I`ve started writing up an article for my website (not yet finished).

While I`m writing, you can find much of interest here by Chris Starr (ex. RAF Tornado pilot and restorer of the engine of Bf-109 "black-six".

Guests Corner: Chris Starr - Calum Douglas

This section of the website is dedicated to distributing the writings of Chris Starr, former RAF Tornado pilot, RAF Tanker pilot and responsible for the restoration and maintenance of the Daimler-Benz DB605 engine of the RAF owned and (previously operated) Messerschmitt Bf109G, otherwise known...

www.calum-douglas.com

For interest, here is a very interesting conclusion written by the Royal Aircraft Establishment, Farnborough, England in late 1942, comparing Merlin-46 supercharger
performance with the standard AVT carburettor, vs a jury-rigged two-nozzle fuel injection system the stuck on a test-bed.

Please note that the author of this somewhat confusingly refers to the float carburettor jets as "fuel injection tubes" in paragraph 2, and
that the "removal of the carburettor choke tubes" requires the use of a fuel injection system.

Very briefly, my conclusions will be that Hookers writings in his autobiography almost certainly only really apply to the two-stage Merlin
(where very high temperatures are experienced through the supercharger), and that the benefits of evaporating fuel through the
supercharger, did not work to anything like the same degree with the most numerous versions of the Spitfire produced (the MkV),
because the fuel could not evaporate through the blower at high altitude, due to the very low external air temperatures, and only
moderately high temperature increase through the single-stage, single-speed blower used.

This is thoroughly supported by compressor maps generated on the high-altitude supercharger test rig at Farnborough, in
fact not just suported, but I am essentially just reproducing what they found at the time, and adding some modern commentary.

 

[pbehn]

Very briefly, my conclusions will be that Hookers writings in his autobiography almost certainly only really apply to the two-stage Merlin
(where very high temperatures are experienced through the supercharger), and that the benefits of evaporating fuel through the
supercharger, did not work to anything like the same degree with the most numerous versions of the Spitfire produced (the MkV),
because the fuel could not evaporate through the blower at high altitude, due to the very low external air temperatures, and only
moderately high temperature increase through the single-stage, single-speed blower used.

That would be understandable from Hooker's point of view, wouldnt it? The request for a high altitude engine for the Wellington came in March 1940 so I presume that was what he was mainly involved in? The Spitfire MkV may have been the most numerous of the marques produced, but if that was the end of its development it would have been an ignominious end. It was the two stage Merlin that cemented the name "Merlin" as a legend in many aircraft.

 

[Deleted member 68059]

That would be understandable from Hooker's point of view, wouldnt it? The request for a high altitude engine for the Wellington came in March 1940 so I presume that was what he was mainly involved in? The Spitfire MkV may have been the most numerous of the marques produced, but if that was the end of its development it would have been an ignominious end. It was the two stage Merlin that cemented the name "Merlin" as a legend in many aircraft.

I think the point is that everyone reads that book and thinks that means "all" merlins, when in fact the reality is a lot more complicated, indeed, the float carburettor two-stage Merlin-60 series itself was a very short lived beast. As soon as the 66 arrived it was given the American Bendix pressure carburettor, which is basically a simple single point injection system, and significantly better than the British SU`s.

 

[Reluctant Poster]

That would be understandable from Hooker's point of view, wouldnt it? The request for a high altitude engine for the Wellington came in March 1940 so I presume that was what he was mainly involved in? The Spitfire MkV may have been the most numerous of the marques produced, but if that was the end of its development it would have been an ignominious end. It was the two stage Merlin that cemented the name "Merlin" as a legend in many aircraft.

I always have to take exception to the statement that the the Mark V was the most numerous Spitfire Mark. The IX and XVI were identical airframes built on the same production line with basically the same engine and the same performance and also the reality is that the Mark VIII shared the same engine and the same performance and the differences between it and the IX are mainly the wing. Mark numbers are not necessary logically assigned. The wing was substantially revised between the VB and the VC, more than just the armament. I could just as well been given a sperate mark number.

 

[Deleted member 68059]

I always have to take exception to the statement that the the Mark V was the most numerous Spitfire Mark. The IX and XVI were identical airframes built on the same production line with basically the same

Well thats a perennial issue with many things, for example. most people would say that the Mk21 was not even the same aircraft, let alone a new Mark.

For reference, the MkV had slightly LESS design hours spent on it than the Mk-III, which was never even produced, and half the hours the Mk21 absorbed.

The marks which absorbed markedly high design hours were the Mk1 (obviously), III, V, VII and 21 (which was 170,000 man hours)

You could infer therefore that these were all major revisions, and every other mark was a relatively small refinement.

On Jigging and Tooling, the picture is different still, with the Mk VII and VIII each absorbing about double the effort of the V (about 200,000 man hours each)

 

[pbehn]

I always have to take exception to the statement that the the Mark V was the most numerous Spitfire Mark. The IX and XVI were identical airframes built on the same production line with basically the same engine and the same performance and also the reality is that the Mark VIII shared the same engine and the same performance and the differences between it and the IX are mainly the wing. Mark numbers are not necessary logically assigned. The wing was substantially revised between the VB and the VC, more than just the armament. I could just as well been given a sperate mark number.

Certainly true that the performance of the Mk IX and XVI were identical but the engines were different not only physically but in their finance. That doesnt change my point that without the two stage Merlin, the Spitfire and the Merlin itself would have a much smaller place in history.

 

[Reluctant Poster]

Certainly true that the performance of the Mk IX and XVI were identical but the engines were different not only physically but in their finance. That doesnt change my point that without the two stage Merlin, the Spitfire and the Merlin itself would have a much smaller place in history.

I am actually agreeing with you. The 2 stage Merlin was a league above others. The most numerous Spitfire engine was the Merlin 66. The RAF called the Packard version the 266 because it was essentially the same. You can swap a 266 for a 66 and vice versa. The cowling is a bit higher for the 266 because of the taller US intercooler. Many parts are interchange between Packard and RR Merlins. An example in the use of post war RR engine blocs in racing Merlins. Other than the supercharger drive (not the supercharger itself) the difference lies in the accessories which are US made but built to RR patterns.

 

[Deleted member 68059]

I am actually agreeing with you. The 2 stage Merlin was a league above others. The most numerous Spitfire engine was the Merlin 66. The RAF called the Packard version the 266 because it was essentially the same. You can swap a 266 for a 66 and vice versa. The cowling is a bit higher for the 266 because of the taller US intercooler. Many parts are interchange between Packard and RR Merlins. An example in the use of post war RR engine blocs in racing Merlins. Other than the supercharger drive (not the supercharger itself) the difference lies in the accessories which are US made but built to RR patterns.

Data please >

You are picking a single engine, whereas the V series used many variants which are all identical for the purposes of the discussion of this thread, like 45,46,50, 55 (single stage, single speed with float carburettors).

A great many IX`s didnt have 66`s either.

Spitfire production by mark>

 

[Reluctant Poster]

Data please >

You are picking a single engine, whereas the V series used many variants which are all identical for the purposes of the discussion of this thread, like 45,46,50, 55 (single stage, single speed with float carburettors).

A great many IX`s didnt have 66`s either.

Spitfire production by mark>

View attachment 673771

Based on Robertson's "Spitfire Story of A Famous Fighter" I count 6289 LF VIIs, IXs and XVIs with Merlin 66s or 266s. V Production was divided between Melin 45s and 46s which had different superchargers with different performance bands with respect to altitude. The VIIIs , IXs and XVIs are also identical for the purposes of this discussion. I will say that 1522 of the earlier production VIIs and IX s were equipped with earlier Merlin marks that did have float carburetors.
If you want to compare Spitfires with SU vs Spitfires with Bendix (including Seafires and Griffons), the SU wins by approx. 12900 to 9900. I am not sure of how many Mark XIs were powered by Merlin 70 series so I put them in the SU category

 

[Deleted member 68059]

Based on Robertson's "Spitfire Story of A Famous Fighter" I count 6289 LF VIIs, IXs and XVIs with Merlin 66s or 266s. V Production was divided between Melin 45s and 46s which had different superchargers with different performance bands with respect to altitude. The VIIIs , IXs and XVIs are also identical for the purposes of this discussion. I will say that 1522 of the earlier production VIIs and IX s were equipped with earlier Merlin marks that did have float carburetors.
If you want to compare Spitfires with SU vs Spitfires with Bendix (including Seafires and Griffons), the SU wins by approx. 12900 to 9900. I am not sure of how many Mark XIs were powered by Merlin 70 series so I put them in the SU category

I`m not certain of the IX split between Bendix and non bendix either , sadly my copy of "Spitfire The History" by Shacklady which has every single registration recorded is in my family home a few miles away.