Rolls Royce Fuel Injection

[Reluctant Poster]

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.

Airless injection was actually first used by Vickers in 1910 for submarine engines.

 

[Deleted member 68059]

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.

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.

 

[Reluctant Poster]

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

 

[Koopernic]

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. The DB601E introduced the large overlaps and this made it onto the DB605A. They handled the idling issue by having variable length intake ducts.

When C3 fuel with a rating of around 97/125 or 100/130 or so became available in 1942/43 the BMW 801D2 received the following emergency boost systems:
1 Increased boost used on fighters allow increased manifold pressure and increased power up the limits of the supercharger.
2 Rich mixture injection used on fighter bombers which sprayed fuel into intake, ahead of the supercharger, which precooled the mixture and allowed more efficient supercharging, more air and slightly increased boost. It had a lot of restrictions, I think limited to use below about 1000m or so.
3 Eventually the systems were combined with altitude restriction removed.

So I'm thinking that a large overlap will help scavenge the exhaust but direct injection is needed to avoid fuel loss. If you go the "Rich Mixture Injection" you no longer have the direct injection system operating during emergency power and losses will be incurred. Maybe they didn't care because it wasn't combusted anyway.

I know that the BMW 802 (an 18 cylinder double row radial with the same bore but longer stroke as the BMW 801) were putting in provision for variable valve timing on the exhaust side. They wanted more overlap.

 

[Deleted member 68059]

Can you upload your diagrams please.

I`ve taken the trouble to put them all in a single source for everyone with some explanatory text.