Rolls Royce Fuel Injection

[yulzari]

Just to avoid confusion. Direct injection squirts neat fuel directly into the combustion chamber. Indirect injection squirts it into the incoming air charge and the joint mixture goes into the combustion chamber down the inlet port. Either can be adjusted mechanically or electronically depending upon the technology of the day.

Basic examples are diesel engines which draw in a pure air charge and then squirt diesel fuel into the chamber or a petrol carburettor system where the sir is mixed with the petrol in the carburettor and the mixture is drawn into the inlet port and thus into the chamber.

 

[fastmongrel]

Apart from the negative G problem which was solved fairly quickly what adavantage did the DB engine have over the RR engine or vice versa.

Having taken direct and indirect Injection systems and various types of carburettors apart in the past I know which I would prefer to tinker with.

When I raced classic Motorbikes there were many a time I took a Dellorto, AMAL or Mikuni carb apart cleared a blockage or put in a different needle or jet during practice or between races. That was often in a muddy field laughingly called the Paddock by the race organisers with basic tools every racer carried in his beat up ex Post Office van that was the usual race transport.

Having to do the same with an injection system would have made me run away and hide till racing was over.

 

[Shortround6]

Britian, once the shooting started, was hard pressed to make everything they needed. Many things had to prioritized and choices made on the amount of machine time or manufacturing time needed. The British made no attempt to copy the German MG 151 cannon for example. Captured specimens were examined but it was judged too costly in machine time to duplicate.
If you could equip several engines with carburetors for the same cost (man hours of machining) as one engine with fuel injection the fuel injection had better offer some real advantages.

 

[wuzak]

Rolls-Royce were testing stratified charge direct injection on the Crecy.

 

[XBe02Drvr]

Just to avoid confusion. Direct injection squirts neat fuel directly into the combustion chamber.

In diesels, yes. In recip gasoline engines, (I was taught in A&P school) it means injected into the intake manifold just upstream of the intake valve of each cylinder. A "pressure carb" or "pressure injection carb" injects fuel metered the same way as a direct FI system, but into the throat of the throttle body venturi, where it is then distributed as a fuel/air mixture to the cylinder intake valves through a manifold similar to those used for float carb systems. This does away with the need for a fuel rail and high pressure fuel manifold, as well as precision machined injector-valves at each intake port.
This renders retrofitting of a pressure system much easier on an existing float carb equipped aircraft. There's still additional airframe plumbing to be done, as the fuel pickups and delivery lines are going to operate under higher pressures, and a fuel return line back to the tank system added, if not already installed. If the header tank is not already equipped with a negative G fuel pickup, that will have to be added, too.
Cheers,
Wes
PS: Negative G engine oil scavenging needed also, if not already equipped.

 

[Reluctant Poster]

The Aircraft Engine Historical Society has some very good articles on the Rolls Royce Speed Density system. It is not a carburetor as it does not use a venturi to measure airflow. It is what is often called single point or throttle body fuel injection. If you are not a member of AEHS you should join forthwith. It is the best 12 dollars you will ever spend.
https://www.enginehistory.org/Accessories/HxFuelSys/FuelSysHx09.shtml
http://www.enginehistory.org/members/aehsdc/AEHSDCscans/2700.php

 

[Reluctant Poster]

Rolls-Royce were testing stratified charge direct injection on the Crecy.

I believe it used captured Bosch injectors

 

[Reluctant Poster]

Sometime ago I posted a NACA paper on fuel injection vs carburetors. It concluded that multi point fuel injection was only advantageous only if the engine has fuel distribution problems, hence the interest in multipoint injection for the R-3350

 

[wuzak]

I believe it used captured Bosch injectors

Certainly they used a captured fuel pump.

 

[Admiral Beez]

Was there any benefit of direct injection over carbs in WW2 land vehicles?

 

[XBe02Drvr]

Was there any benefit of direct injection over carbs in WW2 land vehicles?

Other than adding complexity and parts count (more profit on a cost plus government contract) and increased probability of breakdown in combat? I doubt it. The KISS Principle is a good guideline for combat vehicle design.
Cheers,
Wes

 

[Shortround6]

Fuel was often dirtier in WW II than now. Clogged or fouled fuel injectors are harder to clean than a carburetor. Even the problems of running out of fuel are more difficult for a fuel injected engine. Once you have fuel back in the tank you have to get the air out of each fuel line. On a direct injected engine (or one with the injectors just upstream of the intake valve) each injector line needs to be purged/bleed.

 

[Admiral Beez]

Fuel was often dirtier in WW II than now. Clogged or fouled fuel injectors are harder to clean than a carburetor. Even the problems of running out of fuel are more difficult for a fuel injected engine. Once you have fuel back in the tank you have to get the air out of each fuel line. On a direct injected engine (or one with the injectors just upstream of the intake valve) each injector line needs to be purged/bleed.

I do like the ease of maintenance of my Triumph's Amal and vintage Suzuki's Mikunis.

 

[tommayer]

Other than adding complexity and parts count (more profit on a cost plus government contract) and increased probability of breakdown in combat? I doubt it. The KISS Principle is a good guideline for combat vehicle design.
Cheers,
Wes

Questions that seem relevant to several posts. This seems as good a place any to ask. 1) Anybody have any hard evidence about percentage of efficiency gain of direct injection v pressure carb? How much better was the German direct injection system than the Bendix and Stromberg units? Forget EFI. It did not exist, therefore comparisons irrelevant when discussing WWII recips. My guess is the pressure carbs were as good as mechanical injectors in real life. My guess based on 1800 hours experience, divided roughly into thirds, between float Marvel updraft carb, Bendix pressure carb, and Bendix mechanical point injection system in the same Lyc 320 in the same airplane. In my real world, fuel burns, throttle response, inverted functionality, & max power were indistinguishable between pressure carb and point injection. The injection system for that Lyc was simpler. It did not require a return line. The float carb was awful compared to either. Only good thing about it was the engine would run on gravity, so no need for a fuel pump, assuming you had a tank high enough to provide some 2-3 lb gravity feed. I've also spent thousands of hours behind float carbs and injectors in radials. For clarity for non geriatric pilots, we used to call pressure carbs injectors instead of pressure carbs when used in 985s, 1340s, 1300s, 1820s, 1830s etc. I kept track for a few years on a couple of 985s. Injection made cold starts far easier, throttle response better, allowed the engines to make about 1 inch more manifold pressure, and reduced fuel burn by 10-12%. Huge $$ savings on fuel over a run to TBO. Downsides were you needed a return line on those units, and a header tank. W/O header tank usable fuel in any tank was decreased by 5-7% if you were in any kind of chop. My guess--repeat guess--is that WWII US-Brit pressure carbs were 6, DB and Junkers mechanical injection a half dozen. If you read the relevant P&W tech stuff, I think you'll find at SL with an injector a 985 can achieve a BSFC of under .4, which is competitive with early Honda EFI on tin cans.

 

[tommayer]

Questions that seem relevant to several posts. This seems as good a place any to ask. 1) Anybody have any hard evidence about percentage of efficiency gain of direct injection v pressure carb? How much better was the German direct injection system than the Bendix and Stromberg units? Forget EFI. It did not exist, therefore comparisons irrelevant when discussing WWII recips. My guess is the pressure carbs were as good as mechanical injectors in real life. My guess based on 1800 hours experience, divided roughly into thirds, between float Marvel updraft carb, Bendix pressure carb, and Bendix mechanical point injection system in the same Lyc 320 in the same airplane. In my real world, fuel burns, throttle response, inverted functionality, & max power were indistinguishable between pressure carb and point injection. The injection system for that Lyc was simpler. It did not require a return line. The float carb was awful compared to either. Only good thing about it was the engine would run on gravity, so no need for a fuel pump, assuming you had a tank high enough to provide some 2-3 lb gravity feed. I've also spent thousands of hours behind float carbs and injectors in radials. For clarity for non geriatric pilots, we used to call pressure carbs injectors instead of pressure carbs when used in 985s, 1340s, 1300s, 1820s, 1830s etc. I kept track for a few years on a couple of 985s. Injection made cold starts far easier, throttle response better, allowed the engines to make about 1 inch more manifold pressure, and reduced fuel burn by 10-12%. Huge $$ savings on fuel over a run to TBO. Downsides were you needed a return line on those units, and a header tank. W/O header tank usable fuel in any tank was decreased by 5-7% if you were in any kind of chop. My guess--repeat guess--is that WWII US-Brit pressure carbs were 6, DB and Junkers mechanical injection a half dozen. If you read the relevant P&W tech stuff, I think you'll find at SL with an injector a 985 can achieve a BSFC of under .4, which is competitive with early Honda EFI on tin cans.

 

[XBe02Drvr]

1) Anybody have any hard evidence about percentage of efficiency gain of direct injection v pressure carb?

I've never seen any, but operationally, the German ability to tweak individual injectors to compensate for intake manifold anomalies or uneven cylinder temps looks like a valuable feature.

My guess is the pressure carbs were as good as mechanical injectors in real life.

Concur

Injection made cold starts far easier, throttle response better, allowed the engines to make about 1 inch more manifold pressure, and reduced fuel burn by 10-12%.

Our flight school had a Sundowner and a Sierra with nearly identical O360s, except the Sierra had multi point injection and a C/S prop and the Sundowner F/P and float carb. At the same charted % power they had near identical fuel burns, though the Sierra engine was rated 20 HP higher and made nearly 20 Kts more speed. The FI engine was always an easy starter, except maybe on the third or fourth flight on a hot day, whereas you had to establish a relationship with the Sundowner's engine and maybe even exercise your scatological vocabulary to get her going. BTW, what sort of "inverted functionality", gravity feed, O320 powered machine did you have? Sounds interesting.
Cheers,
Wes

 

[b0ned0me]

For direct or port fuel injection there might be an advantage in avoiding an intake manifold full of fuel/air mix, with the distribution/pooling/backfire problems that could lead to. Not sure how significant that would be but these issues seem to have absorbed a bit of engineering time on some engines.

 

[XBe02Drvr]

For direct or port fuel injection there might be an advantage in avoiding an intake manifold full of fuel/air mix, with the distribution/pooling/backfire problems that could lead to. Not sure how significant that would be but these issues seem to have absorbed a bit of engineering time on some engines.

I think that until you get up to more than three cylinders/bank those problems would be pretty minimal. V12s, OTOH, might offer all kinds of possibilities. Radials, of course are another story; they're a fire looking for an opportunity.

 

[Koopernic]

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.

 

[XBe02Drvr]

1 Throttle body injection where a single spray injector puts fuel where a venturi type carburettor would normally be.

Our flying club had a T34 with a Bendix "Pressure Carburetor" (manufacturer's terminology), which I think was a PS5C. I saw it once off the aircraft, and it had what looked for all the world like a venturi with the injector nozzle in the throat of it. Our mechanic, whose day job was the Air Station's R1820 and O540 powered aircraft, never much liked the float carbs on our Cessnas and Pipers,* but was a fan of the T34. He didn't fly much, but when he did she was his chariot of choice.
*Believe it or not, carb ice happens in South Florida when you least expect it. We averaged at least one a year of surprised northcountry pilots coming a cropper to it.
Cheers,
Wes