Reluctant Poster
Tech Sergeant
- 1,737
- Dec 6, 2006
Airless injection was actually first used by Vickers in 1910 for submarine engines.
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Airless injection was actually first used by Vickers in 1910 for submarine engines.In regards to German direct injection. There are three basic types of fuel injection
1 Throttle body injection where a single spray injector puts fuel where a venturi type carburettor would normally be. This allows more precise control, provides vaporisation from the high pressure of the spray and has no negative G issues. The Allied engines tended to move from carburettors to this type of injection though they were often misnamed carburettors.
2 Multipoint injection into the valve intake plenum, the injector sprays on to the back of the intake valve rather than direct into the cylinder. This is the kind most often seen in cars up to about 2010 or so.
3 Direct Injection. This is the technical high end, it involves direct injection into the cylinder. This is what the Germans used. As shortround6 says this was mechanical but mechanical systems are no less precise than electrical. Modern engines can use very advanced techniques to put in multiple squirts of fuel to stir up the mixture and create stratified charge (rich near the spark plug to get good ignition and lean elsewhere to get good economy). The high pressure creates excellent vaporisation.
Now as to why. Anthony Kay in "Junkers aircraft and their engines" says that American and British companies had so tied up carburettor patents the Germans had no choice anyway.
Robert Bosch had developed direct injection to essentially invent the high speed diesel. Up until then Rudolf Diesels invention relied on compressed air injection. It worked well but was rather bulky. The expertise developed by German companies such as Robert Bosh, Prosper L'Orange and Junkers was merely applied to petrol engines.
Now as to the advantages and disadvantages.
1 Good Cold starting. During the Stalingrad airlift the most effective transport was the He 111 because its engines would start reliably. The Ju 52 needed elaborate heated ducts to each engine and often couldnt start.
2 No danger of fuel fractionating in the ducts (as happened to the P38) and no issues with getting vaprisation of fuels that dont meet spec.
3 The biggest advantage was that direct injection allowed large valve overlaps so that intake and exhaust was open simultaneously. This allowed the end gases to be effectively scavenged and swept away all of the dead exhaust that didn't produce any power and could cause preignition. The DB601 also used a tuned resonance effect like a trumpet, to resonance scavenge the exhaust. On the DB601E onwards this was so large that the engine idled poorly and variable length intake ducts were introduced. Variable intake ducts were not feasible on a radial so BMW were planning variable valve timing on the BMW 802. Do this on a throttle bodied injection engine like a merlin you would loose a bit of fuel.
Now the disadvantage.
The British and latter American engines used aromatic fuel like 100/130 as a charge precooling mixture sprayed in before the supercharger. This dramatically contracted the air and increased charge density and reduced supercharger work.
In about 1942 the Germans improved their green dyed C3 fuel from about 94/115 to 96/125 then 100/130. Towards the end of 1942/early 1943 they introduced 'rich mixture injection' on the BMW 801D2 of Fw 190F ground attack aircraft. Essentially they must have turned down the direct injection and injected the majority of fuel into the eye of the supercharger as on the Merlin. Fw 190A fighters received only 'increased boost' which just increased boost settings to take advantage of the better fuel. Latter the two systems were combined.
The BMW 801 didn't really have large valve overlaps as it was a radial. However the DB605A did so it would loose a lot of fuel if rich mixture injection was used, I think this is why water injection was preferred, its not such a tragedy to waste water as as to waste fuel.
The BMW 801 didn't really have large valve overlaps as it was a radial. However the DB605A did so it would loose a lot of fuel if rich mixture injection was used, I think this is why water injection was preferred, its not such a tragedy to waste water as as to waste fuel.
Was there any benefit of direct injection over carbs in WW2 land vehicles?[/QUOTE
The Continental AV1790 used in the M48 tank and other vehicles used a fuel injection system based on the SU/RR system used on late model Merlins. There is an article in the following link.
Armor
See page 23
I am sitting here looking at a chart of 801D valve overlaps superimposed over 605A.... and would be most interested to hear where you read the 801 had low valve overlap from - as its over 90 degrees ! (thats about triple that of a DB601A, which DID have pretty standard overlap).
If you have extreme overlap is about if you have DI or not, and not about the overall engine configuration (radial/inline/air cooled/water cooled etc etc) - as otherwise loads of fuel gets chucked down the exhaust pipe.
Can you upload your diagrams please.
From SR6, at the beginning of this thread:One would think those reasons would be obvious Tomo.
From the ME-109E onwards, the automatic mixture control reduced the pilot's workload.Was there any benefit of direct injection over carbs in WW2 land vehicles?
Yes, they were; that meant more HP. And eventually they adopted the US Bendix Pressure Carb, which sprayed the fuel into the "eye" of the supercharger and eliminated that pesky "lean cut/rich cut" that occurred under negative G. And which US manufactured engines had from before the war.The evaporation of the fuel in the supercharger lowered the intake mixture temperature by 25 degrees C on the early Merlins and RR was very hesitant to give that up.
You are on a limb, a weak one!From SR6, at the beginning of this thread:
"During the war (and very early in it) RR had looked at Fuel injection of the German type (direct injection) and rejected for a number of reasons. Some may have been good, some may have been cover your ass reasons. The British carburetors and American single point/pressure injection carbs were much easier to make requiring hundreds fewer parts. The evaporation of the fuel in the supercharger lowered the intake mixture temperature by 25 degrees C on the early Merlins and RR was very hesitant to give that up. Granted the mixture distribution was not all it could be with simpler systems."
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I'd go on a limb to say RR maybe, just maybe, has the expertise.
Yes, they were; that meant more HP. And eventually they adopted the US Bendix Pressure Carb, which sprayed the fuel into the "eye" of the supercharger and eliminated that pesky "lean cut/rich cut" that occurred under negative G. And which US manufactured engines had from before the war.
Please post a link to Callum's website.Members may well be aware of the dire limitations and carburation faults that British-built fighters were faced with until at least 1942 due to the float-type carburettors used. In particular, during the Battle of Britain, British Spitfires and Hurricanes suffered with engine cut-outs in manoeuvres. The Germans took full advantage of the British weakness by using negative-G in combat to cause the cut-outs in the British fighter engines, whereas the German engines with Direct fuel injection ran happily in negative-G because the fuel injection continued to operate correctly.
This awful situation owed its cause to several failings in the slow British development of carburation technology in approximately 1930 to 1942. The British aircraft and engine industry was privately owned, with a Government Air Ministry that presided over regulation and government contracts. The Air Ministry also ran the Royal Aircraft Establishment (RAE) which
was a technical research and development department-heavily biased towards official policy and procurement.
Under the Direction of the Air Ministry, the RAE slowly developed the idea of the Injection carburettor in the mid 1930's onwards. This development was offered to Rolls-Royce as a development partner by the Air Ministry, but Rolls-Royce declined to be involved and just continued with their procurement of float carburettors from Claudel-Hobson (CH) and later, Skinners-Union (SU). Air Ministry policy was to encourage the individual carburettor companies (and other ancillary producers), rather than have engine companies monopolise the whole industry. Nonetheless, with each engine development, Rolls-Royce was very much in control of their own carburettor details, working directly with the carburettor companies on the carb features and detail design.
During development of Rolls-Royce Merlin engine, competitive float carbs from CH and SU were considered and the SU type was chosen by the time that the Merlin started production. Note that, the Air Ministry accepted this and so did the RAF.
By 1938 the Merlin was in production with the flawed float carb, the RAE was slowly developing the injection carb (the slow rate due to low finance and low priority) but in the USA, Bendix had a production injection carb for the V-1710 and Germany had Fuel injection in production! So, this is how the boat was missed! Rolls-Royce, the Air Ministry, the UK carb companies and the RAF all missed the need for something better than a float carburettor that was inoperative under negative-G.
For a detailed essay about the subsequent Crisis development of Anti-G carbs and Restrictors, see Calum Douglas' home Website and the article on it.
Eng
Please post a link to Callum's website.