The Secret Horsepower Race - WW2 Aero Engines in detail

[samoht]

I've always been interested in the piston engines used in WW2 aircraft. They're close enough to our car engines to be relatable and relevant, and yet many of them used technologies such as direct injection, forced induction, four-valve heads and water/methanol/nitrous injection, that only arrived in road cars much later.

The other fascination is that during the war additional horsepower was worth more than bragging rights or quarter-mile times, it was literally a matter of life or death. If the other guy can do 400mph and you can do 410, then you've little to fear from his guns; vice versa and you could be shot down. This pushed the combatants into intense competition, each needing to respond to the others' advances. The closest peacetime analogy I can think of is F1.

I've read a bit about the Merlin and how it was developed during the war, Stanley Hooker's autobiography of how he realised that the existing supercharger design was mathematically incorrect, and redesigned it giving the Spitfire a crucial mid-war boost, then followed up with the two-stage supercharger for more power at high altitudes. However it's been harder to find real detail about the German efforts.

Anyway, I just finished reading The Secret Horsepower Race, written by a former Toyota Motorsport engineer who's spent years crawling through archives, and it more than fills this hole. It's a real treasure trove of archive photos, performance charts and cutaways of US, British and especially German engines, telling how the far-sighted Helmuth Sachse laid down the template for German engines as being inverted-vee, single stage supercharged and fuel injected, before he was 'retired' from the air ministry to BMW for being insufficiently enthusiastic a Nazi. Then how the Germans had to battle against shortages of crucial materials for bearings and valves, and for high-octane fuel, while bringing improved engines to production, before being crushed by allied bombing.

The book isn't perfect, but issues are mainly ones of omission, based on lack of available original documents to back up areas such as the ongoing workarounds the British implemented to make carburettors work under negative g conditions. But it provides a great overview and fantastic original detail of the areas it does cover, especially although not exclusively the German side.

Did you know that post-war, Garrett turbochargers employed as head of research the wonderfully named Werner von der Null, a German aristocrat with duelling scars who had been a key figure in aircraft super and turbocharger development during the war?

Basically, if you're interested in aero engines, or arguably any piston engines, I'd highly recommend seeking out a copy - I found it absolutely fascinating.

[samoht]

jet_noise said:

Does it mention specialist fuels such as synthetic or 150(!) octane?

Probably the best answer I can give to that is "yes", see index sections below:





I'd had no idea the extent to which the Germans synthesised avgas from coal with hydrogenation and de-hydrogenation, the fact that German high-octane fuel was different from Allied high-octane due to high aromatic content, the fact that fuels have a different octane rating at 'lean' and 'rich' mixtures, etc. The German high-octane fuel programme in particular, blending various different exotic hydrocarbons, reminds one of nothing more than the turbo cars of 1980s F1. It seems that they were able to produce high octane ratings, but not always able to do so in the high volumes that the Allies could.

[samoht]

Simpo Two said:

There may be some hertiage from the Schneider Trophy there, where fuel was equally important for wringing the last bit of power from an engine. After the contest was won, the Macchi-Castoldi MC72, with its remarkable 'double' Fiat engine and potion from British fuel expert Rod Banks, achieved 440mph in 1934 - a record for a piston engine seaplane that still stands.

Yeah, it's interesting how the Schneider Trophy race turned into effectively a biannual weapons test for the competing nations. I have fond memories of seeing one of the Supermarine winners in the Science Museum. There's a chapter on the Schneider Trophy in the book also, as you say it was a significant prelude to the aircraft of WW2. I hadn't realised that surface cooling was used to avoid radiator drag, nor that on average one pilot was killed at each event, so hairy were the racing planes, nor that the Germans were banned under Versailles restrictions (although it makes sense).

[samoht]

Piginapoke said:

It's an interesting subject. My understanding is that the Spitfire Mk 1 and Messerschmitt 109 were close in enough in performance that the pilot and tactical situation were the defining factors, rather than another 10mph top speed. Is that right?

Yeah, that's my understanding also - when both pilots wanted to fight each other, the aircraft were pretty equal and pilot skill and initial conditions were most significant.

However, there were also cases where e.g. a German pilot would run for home, or a German bomber would be too high/distant for a British fighter to reach before they hit their target. In these cases the relative top speeds of each aircraft were crucial.

Another example were the photo-reconnaissance Mosquitos and Spitfires which could snap away over Germany with near-impunity, whereas the Germans had much less ability to do so, contributing to their failure to see the Normandy landings coming.

[samoht]

Yertis said:

It's easy to forget that the Germans were feeling their way as much as everyone else with regard bomber tactics in the early part of the war. I don't think they ever expected the 109 to be flying escort missions. The BoB was the first time they'd come up against and serious opposition, and their dedicated 'escort fighter' the Me110 was completely outclassed by the Hurricane and Spitfire.

Yeah, pre-war German doctrine seems to have focused quite heavily on treating the Luftwaffe as an assistant to the Wehrmacht, in support of fighting on the ground as an integrated part of Blitzkrieg, rather than as an independent strategic force. I also think that Hitler was half-hoping not to have to fight Great Britain at all, hoping we'd be content with our Empire and let him have control of the Continent.

[samoht]

Simpo Two said:

samoht said:

Another example were the photo-reconnaissance Mosquitos and Spitfires which could snap away over Germany with near-impunity, whereas the Germans had much less ability to do so, contributing to their failure to see the Normandy landings coming.

I believe they had special Ju88s which were pretty damn fast.

According to wikipedia "Ju 86P-2 photo-reconnaissance aircraft. Those operated successfully for some years over Britain, the Soviet Union and North Africa. In August 1942, a modified Supermarine Spitfire V shot one down over Egypt at an altitude of some 14,500 m (49,000 ft); when two more were lost, Ju 86Ps were withdrawn from service in 1943. "

I recall reading elsewhere that there was work on higher-altitude Mosquitos specifically to intercept these German PR planes, but they stopped after the Germans gave up coming over. So sounds like they were able to for a period, but then lost that capability.

[samoht]

2xChevrons said:

What doesn't seem to get as much covering was the Bf109's (well, really the DB601 engine's) supercharger drive. At the time the Merlin in the Spitfire and Hurricane used a simple single-speed, single-stage supercharger with automatic boost limit but no other form of control.

The DB601 drove its single-stage supercharger through a hydraulic coupling which allowed infinite gradation of drive ratios between the minimum and maximum available.

... (lots more great info & explanation) ...

Interestingly, Geoffrey Wilde joined Rolls-Royce in 1939 and proposed a continuously variable hydraulic coupling drive for the Merlin supercharger (having previously worked at the British arm of Daimler). However, while Hooker was very supportive, after a discussion with suppliers they decided that it would be too much of a change too late in the day, so had to persist with the two fixed speeds. Wilde's proposal used a torque converter akin to an automatic gearbox, whereas the DB production solution was like a viscous LSD with alternating plates in a fluid bath, but with drive put through it and then the slip controlled by changing the oil level - a through-flow of oil also removing the heat generated from the slip. (Info from the aforementioned book, which also has a couple of neat diagrams of the two mechanisms).