Could you have designed a better P-39?

[Shortround6]

A cam timing change doesn't make the pistons stronger,or the con rods, or the crankshaft or the crank case. At 2000hp the DB 605 was already making 35% more power than it started with. Now you are claiming it can do 57%???

Allison never sanctioned 30" more boost on it's early 1150hp engines ( the ones after the "C" series.)

It may have been a field expedient. The Allisons that were factory rated for 75-76in of manifold pressure had a number of internal modifications. Over 2000 of the later ones had a new 12 counter weight crankshaft that was 27lb heavier than the older crankshaft. While it might have been no stronger the improved balancing significantly reduced the loads on the main bearings. A new design of piston ring cut down on blow by and the loss of oil through breathers at high power levels in addition to improved cylinder lubrication.
Boosting power is easy. Getting the engine to last at the higher power levels is the hard part. It took until the 1960s for most racing car engines to reach the power to weight ratio that was common for aircraft engines at the start of WW II. That is Formula 1 or sprots racing cars like at Le mans, not 10 second drag engines.

 

[Siegfried]

Add how was the German use of MW-50 any more outside the box than the allied use of water injection? Tested by the USAAC in the mid/late 30s on a P&W wasp engine.

How much would a DB 605DAM that used N2O to get more than 2000hp have weighed? these aren't drag car engines, the pilots expect not only to be able to use the power but actually get back to base after doing so. The US would not approve a WER power level until a single engine had racked up 7 1/2 hours at that rating (5 min at time) I imagine that most other countries did something the same.

Neither MW50 or the cryogenic N2O system added much weight. Both substances were added at about the same rate as the fuel flow rate (about 1/14th of the air mass flow rate).
A 110L insulated tank (about 25 gallons) whose fluid weight would add about 100kg (220lbs) could contain either cryogenic N2O (SG 1.2), or MW-50 or fuel depending on mission. Roughly MW-50 was used below the full pressure altitude ( 1 atmosphere manifold pressure) of the engine, N2O was used above that altitude. Due to plumbing supply issues the tanks usually were not '3 purpose' as described. If one imagines an DB605 engine with a sfc of 0.42lbs/hp then 1600hp would consume about 666lbs/hour i.e. that tank would have enough fluid for 20 minutes at full flow, perhaps a little less at WEP but note the DB engines had good fuel economy even at Military power since it didn't use rich mixtures. The Ta 152H1 had prodigious fuel volume in 6 wing tanks and one was used for MW50 while the N2O was in the fuselage. (from memmory). From the top of my head there were 3 settings for N2O flow rate of 130 grams, 80 grams or 50 grams per 100 grams of fuel. Ruediger Kosin (Ar 234 designer) in his book "The German fighter" gives a flow requirement of 360kg/hour of N2O in addition to 300kg/hour of fuel to restore a DB605 from 1000hp back to 1340hp at 9000m. I think the book "war prizes" mentions that on the Ta 152 the boost system could be used in two 20 minute bursts; they were quite long. Obviously the system was not for sustained high speed cruising but it should be considered a system good for only 3 or 5 minutes like some allied WEP systems.

The Germans engines used direct in cylinder fuel injection so there was no charge cooling effect from injecting high aromatic fuel into the supercharger. The injection of MW-50 makes a lot more sense from this perspective. The Jumo 213A on the FW 190D9 did have a rich mixture injection system added just after it entered service, from my understanding of what I've read the system worked by bleeding the air line to the fuel injection systems air flow sensor to trick it into into lowering the injection of fuel simultaneously the the "lost" fuel and some extra was then added into the eye of the supercharger to cool the charge, this boosted power from 1769 to 1900hp. MW-50 was latter added as well with two systems in use: a field modification known as the "Oldenburg" system which used supercharger pressure to blow MW50 into the inlet and a more capable Junkers factory system which pumped in the MW50 at higher flows and atomization and required factory technicians to visit the squadron due to the control system modification required. I think this further boosted power to 2000 and 2100 respectively, probably as much as 2240 with C3 fuel.

Fuel injection means fuel doesn't displace air and it also allowed the German engines to run radical valve overlaps to thoroughly scavenge end gases without loosing fuel into the exhaust however it meant no charge cooling from rich mixture injection into the supercharger unless special mods were provided as described above.

The MW50 system became standard on Me 109G6AM and Me 109G6ASM around March 1944, standardised on the Me 109G14A and Me 109G14AS around May/June.

FW 190's rarely used MW50 as it cracked the cylinder heads, the BMW 801D2 produced about 1729hp standard and 1950hp with rich mixture injection of C3 fuel. The MW-50 "Ribbentrop" system was added to the FW 190A9 in 1944 and I suspect brought the engine to 2200hp. I believe from readings on a newsgroup a small number of FW 190A5 "Jabos" used in hit and run raids had an MW-50 in use in 1943 but I think it was rare.

Amazingly Rudiger Kosin reports the minutes of a November 4th 1942 meeting which had been set up to study high altitude fighter development. The meeting reported that a two stage BMW 801 could be flying today but as it was expected to eliminate this engine from the fighter production program by the turn of the next year work was suspended in view of BMW's workload. Obviously they were expecting to move on to other fighter types and engines that never came about!

The allies also used Water Injection on the P-47, the system was unpopular in winter as the ethanol anti-freeze was inadequate (this is the reason the Germans used methanol) in a European winter and could blow up the engine. Some nitrous oxide was added to P-51's assigned to attack Me 262, to give them a better chance. Mostly the availability of 100/130 fuel is how the allies boosted their engine. The first number is the octane rating (octane to heptane ratio running stoichiometric ie 14:1 air fuel) while the second figure is the PN or performance number the 130 meaning 30% extra power is available if the mixture is run about 12:1 and over boosting the engine by adding 30% more pressure (the test engine however adjusted compression ratio but its a good rule of thumb. Much of the German fighter and bomber force had to make use of B4 which was rated at only 87 octane lean.

 

[Siegfried]

Germans didn't think 'outside' the box when it came to power development. they chose a different approach to begin with and not all that different from their early V-12 engine ( the big BMW) or the Hispano V-12. Large displacement, low rpm, low boost, low weight for displacement engines. Once on that path a lot of the development becomes rather predictable and just like the Allison and Merlin, not easy to change in midstream.

The DB600 series was highly innovative in some areas.

The DB605 ran a compression ratio of: 7.5/7.3 (B4 87 octane optimized engines) or 8.5/8.3 (C3 96 octane optimized engines). This compression ratio is much higher than the
6.2 approx used on the Merlin, Griffin and Alison even without considering the 100/130 fuel these engines used. For the same air mass flow the power losses from suction are about the same but the proportionately longer stroke allows recovery of more of the energy of the burnt gases. This means a much better power to weight ratio and much better fuel efficiency. This high compression ratio explains why the DB605 could get away with a modest single stage supercharger and low boost levels. It's supercharger was used primarily to compensate for altitude whereas the allies used their superchargers to squeeze in more air to obtain higher power levels. The highest boost level the DB605 used was 1.98 ata which is equal to 14psi in British terms (probably used by 1942) and 60 inches of mercury in US terminology. By the end of the war Merlins were operating at 28 psi/ 90 inches which is 3 ata in German terminology. It also explains why the DB603L and DB605L (with two stage superchargers) didn't need inter-coolers.

Secondly the DB600 series engines from 1941 onwards (starting with the DB601E) used a radical 105 degree valve overlap (both inlet and exhaust valves open simultaneously) which allowed tuned resonance scavenging of the end-gasses. This allowed more air into the cylinder and got rid of the end-gas species that tend to cause pre-detonation. In order to ensure good idling and low RPM opperation the inlet manifold was made variable length to re-tune the inlet like a trumpet. This is only possible because of the 3rd innovation which is direct in cylinder fuel injection which ensured the fuel was not lost during scavenging since the fuel was only injected on the in the upstroke. The 4th innovation was the over head cams, engines like the Griffin, Merlin and Allison used push rods. The rocker covers probably needed to be bigger but this does not matter as the engines were only installed inverted.

When the DB600 series had problems it was with the bearing lubrication system design, which had to run at high pressure which lead to poorly understood frothing at high altitude, this was dealt with by with a de-aerator but took a long time to discover. German spark plug development also lagged for a while, though they placed less demands on their spark plugs.

The DB series didn't use 4 bolts to tie down the cylinder head, they used (I believe) a single large nut that used the threaded cylinder sleeve as a bolt. This ensured even stresses and allowed a thinner cylinder wall and the engine to have a high volume in compact light dimensions. The Hispano-Suiza 12Y I believe bolted the head to the block and had an integral head.

The use of direct in cylinder fuel injection was partially motivated by the need to avoid patent issues with British and American companies, however it should be noted there are many technical advantages and two problems with the Allison and CW R-3350 would have gone away had the US used direct fuel injection. The P-38's fuel in European conditions fractionated in the inlet manifold leading to low octane fuel getting to some cylinders which then pre-detonated while the R-3350 had uneven fuel distribution which lead to burned cylinder (the top 5 in the second row).

 

[fastmongrel]

The Merlin had overhead camshafts unless by pushrods you mean valve rockers

 

[wuzak]

The Allison also had an OHC with 4 valves per cylinder and pent roof combustion chamber.

All Rolls-Royce poppet valve piston engines from the Kestrel, save the early Merlins, had the same style combustion chamber shape and 4v ohc. Kestrel, Buzzrd (and the R derivative), Goshawk, Peregrine, Merlin, Griffon and Vulture had the style of head shown in the picture fastmongrel posted.

Most had single piece head and blocks, with wet liners - something that the German engines did not do, IIRC (wet liner has the piston and combustion chamber on the inside, and is directly exposed to the coolant on the outside. Dry liner sleeves fit into the block and do not have direct contact with the coolant.) . The Merlin started out with the ramp head, which required 2 piece construction, but when they proved to be disappointing they went back to the single piece Kestrel style head/block with the valve layout as above. Then there were problems with coolant leakage, and the Merlin was redesigned with two piece heads and blocks. I believe that the Grifon was designed from the outset with 2 piece head and blocks.

 

[Shortround6]

The German engines used a different approach to getting power than the Allison or Merlin. Airplane designers do not care what the HP per cu in or cu liter is. They do care what the horsepower per pound of engine weight is. You can build a large displacement slow turning engine for the same weight as a small displacement high rpm engine. Picking either approach is not "thinking out side the box" as this choice had been faced by engine designers for may years. The availability of materials such as good bearings at a given time can influence the designer one way or another. As could previous experience (in the late 20s the internal combustion engine had been around for just 30 years) or a designers own ideas/ prejudices, as in air cooled or liquid cooled. designers or even companies often picked one or the other and stuck with it.
Once a designer/team starts down a certain path certain developments follow. Some of these developments over improvements in one area or aspect (or more) of performance but have limits or drawbacks in others. The Germans were very interested in fuel efficiency, as well they should be given their chronic shortage of fuel. High compression helps here, but high compression limits the amount of boost that can be used with a given grade of fuel. The Merlin used a 6:1 compression ratio while all but a few Allison's used a 6.68:1 if memory serves. It was estimated that the Allison got 5-10% better fuel economy than the Merlin but couldn't use as a high a boost pressure. At the end of the war (and post war--the P-82 twin Mustangs) some Allisons were fitted with 6:1 pistons to allow for higher boost and power levels.

Airplane engines are very lightly built. The stresses on the Crankshaft and bearings go up with the square of the engine speed (rpm) increasing the max RPM of an aircraft engine (and keeping anywhere near the same engine life between overhauls) by even a few hundred rpm usually called for a new crankshaft and a strengthened crankcase. Cylinder size also plays a part in rpm limits. The gasoline mixture in the cylinder burns at a pretty constant rate, that is if ignited on one side of the cylinder the flame front travels across the cylinder at pretty much the same speed in all engines. Too wide a cylinder at high rpm means that the burn isn't completed when the piston reaches bottom dead center or the exhaust valve opens, which ever is first. Big heavy pistons also don't reverse direction very well. Too long a stroke for a given bore size goes the other way, combustion is finished (for the most part) before the piston gets near bottom. While the gases are still expanding some what peak pressures are long gone. It might be good for fuel economy but not for high power output. These are generalizations and don't really apply to the the engines we are discussing because their designers knew about this to begin with. It does help to explain why the Russian AM 35-42 series was the largest V-12 used in WW II though and it used the same bore and stroke as the BMW V-12s used in many of the Luftwaffe's early airplanes. See:

BMW VI - Wikipedia, the free encyclopedia

Notice the weight. Large, light,slow turning engines were not new, they were not "outside the box". In fact they actually represented the normal in aircraft engine design when you consider the Hispano engines and the Italian V-12s and W-18s.

In the 1930s with 80 and then 87 octane fuel every bodies superchargers were pretty much the same. The fuel just wasn't going to support much more than 6:1 to 7:1 compression ratios and 4-6lbs of boost. The German move to fuel injectors may have been an attempt to push that boundary at bit. It also offered less chances of icing in the intake, freedom from backfires and a few other advantages. When better fuel came along and superchargers could offer higher boost (Early Merlins and Allisons got a good part of their power from RPM not boost) the British and Americans were able to use the evaporation of the fuel in the supercharger as a low level charge coolant. I would guess that this was more by chance than a long term plan. Once noticed however it was a strike against going to the German style injectors if other problems could be solved.

Since it can take 4-6 years (or longer) from start of pencil on paper to squadron service of an aircraft engine decisions made in the mid 30s affected engine choices and design till the end of the war if not beyond.

I am not trying to say one side was better than the other or smarter.

Choices were made early on that lead to other choices later or that limited options in one way or another later on. There was little that was "NEW" in engines as far as ideas went. Turbo chargers had been used (experimented with) since WW I in a several countries. Diesel aircraft engines had been worked on by the Americans, British, French and Russians as well as the Germans (maybe the Italians too?) Fuel injection systems had been worked on by a number of countries/companies. The USAAC wanted Allison to work on fuel injection in the mid 30's, Allison turned down the request because they didn't have engineers to take on the project. Bristol in England had several fuel injected prototypes in the early 30s. Water injection had been tested in the Mid 30s. exotic fuel blends had been a staple of both auto racing and speed record aircraft flights for years.
The Germans were not "thinking outside the box" any more than anybody else was to end up where they did. They may have devoted more resources to a particular area than other people did while other countries/companies devoted their resources to a different aspect/area of engine development.

As an aside the P-38 troubles with fuel distribution were known, predicted and a solution was in the works even as it happened. The Specifications for 100/130 fuel was changed allowing for a higher amount of heavier compounds which would separate out at low temperatures. This could potentially affect ALL Allison engines, not just P-38s although due to the altitudes at which they flew they were the most affected. A new intake manifold was designed to solve the problem and was being fitted to engines at the factory in Nov/Dec of 1943. Hundreds were shipped overseas for refitting to exiting engines. Most of the problems occurred in those few months while the change over was taking place. Work had started on the Manifold back in the summer of 1943. This is part of the problem the US and to some extent the British faced. It could take weeks if not 2-3 months to get a factory modification to units in the field.

 

[Ratsel]

A cam timing change doesn't make the pistons stronger,or the con rods, or the crankshaft or the crank case. At 2000hp the DB 605 was already making 35% more power than it started with. Now you are claiming it can do 57%???

No, it dosn't make it stronger but cam timing will change the Dymamic compression ratio (not the static c/r).

Siegfried explained everything else very well.

 

[Shortround6]

Siegfried explained everything else very well.

Siegfried explained a lot of things but there is more than one error in his explanations, but then nobody is perfect and it is only through such explanations that we can arrive at a valid conclusion and stop the "German stuff was better because it was German" or "British stuff was better because it was British" or " >insert country of choice< was better........) nonsense.

 

[Ratsel]

, with wet liners - something that the German engines did not do, IIRC (wet liner has the piston and combustion chamber on the inside, and is directly exposed to the coolant on the outside. Dry liner sleeves fit into the block and do not have direct contact with the coolant.) ..

This DB 605 seems to have bolt-on heads (kinda sorta), wet sleeves, 4-valve/cyl, OH camshafts. If it leaked tons of oil.. its running right

 

[Shortround6]

It was not uncommon for the cylinder block ( which is different from the engine block) to be detachable from the crankcase. In the world of cars crankcase, engine block, and cylinder block are often used interchangeably. In many cases it makes no difference because they are all referring to the same casting in many cases.
In the case of the DB engines and many other aircraft engines (and few car engines, mostly race cars) making a one piece casting that big was too hard. SO they split it up. the crankcase holds the crankshaft. The cylinders are cast in rows of six for a v-12 and TWO cylinder blocks are fastened to the crankcase. Want to do a valve job? pull the cylinder block off the pistons and have at it.

The picture actually shows the the dry liners. The serrated rings at the lock nuts that hold the cylinder block in place, the shiny tube they are threaded onto is the cylinder liner which fits into the cylinder block casting. the green colored serface is where the coolant hits the cylinder block. A tight fit is needed on a dry liner to transfer the heat from the cylinder liner (wall) to the cylinder block and then to the coolant. an air gap would act as an insulator and and cause hot spots or cooling failure. In a wet liner engine parts of the cylinder block would be cut away to expose the liner to the coolant.

 

[fastmongrel]

Interesting stuff guys keep it coming.

 

[Ratsel]

The picture actually shows the the dry liners..

actually it shows the top half of the cylinders being the conventional 'wet' design.. as you know, thats where 90% of the heat exchange takes place, scratch that, the top 10% of the cylinder does 90% of the cooling. the lower parts, only bearing 10% or so of the heat, the oil does more then an adequite job in cooling, as its designed to too. dunno why you keep refering to car engines, but on some of our car V8 engines we use to fill the lower 1/2 of the cooling passages around the sleeves with readyset, effectively reducing the cooling capacity by 1/2.. with never a cooling issue, and a stiffer bottom end to boot good for about 15lbs of boost, a 300hp shot. street driven.


edit for the post below,

same difference, just a different way of implementing it.

 

[wuzak]

I think you'll find that the green bit you refer to as the sleeve is in fact the aluminium casting into which the iron or steel liner is pressed. The advantage doing it that way is, providing that the casting is good, is little chance of coolant leaks. Which plagued the Merlin with its one piece block and heads.

 

[Ratsel]

 

[Shortround6]

Ok, what does the second picture show?

we know that the cylinder blocks are made in piece with the heads. We know the crankcase was a separate piece. We know that teh crankcase was full depth and not split horizontally like an Allison. Bith Allison and the Early Merlins used one piece heads and cylinder blocks with separate crankcases.

 

[wuzak]

I thought that the Allison was always a two piece block/head design.

 

[Ratsel]

Ok, what does the second picture show?

its show the condition after being pulled from underground after 65 years, + the word "Achtung" and a different firing order. They changed it to squeeze more ps out of the DB 605D.

 

[Siegfried]

The Allison also had an OHC with 4 valves per cylinder and pent roof combustion chamber.
SNIP.

Thanks everybody, I was clearly wrong about the Allison Merlin not also having an OH camshaft.

There is a brief description of the DB600 cylinder block setup. I assume the engine has an integral head.
Daimler-Benz DB 601 - Germany

"Daimler-Benz DB 600 series engines was one of those designs, that turned out to be right from the very start. It was an inverted vee, and the original engine displaced 2069 cu in (33.9L). It had three main features; it had roller bearing connector rods, it used dry cylinder liners and had a unique system of attaching the cylinders to the crankcase."

"CONSTRUCTION DB 605: Cylinder barrels of steel are screwed and shrunk into the cast Silium-Gamma-alloy cylinder blocks. These dry liners project beyond block providing attachment by means of threaded rings which pull the liners against the finished face of the crankcase. This feature helped to save the weight of the studs and avoided the possibility of distortion."

 

[fastmongrel]

This feature helped to save the weight of the studs and avoided the possibility of distortion."

However it meant you needed extra weight for lockrings and thicker walled liners to take the threads. To restate Shortround6s point you save weight in one place and gain it somwhere else.

 

[Siegfried]

However it meant you needed extra weight for lockrings and thicker walled liners to take the threads. To restate Shortround6s point you save weight in one place and gain it somwhere else.

I'm not so sure I'd agree; 16 small bolts are better than 4 large ones from a stress distribution point of view and the ultimate is then just a huge sleeve. The key 'accomplishment' of the DB600 series was that it produced competitively high levels of power from B4 (87 octane fuel). The idea of the Db600 series seems to have been to produce a large swept volume in a small space through use of think walls between cyclinders suitable for use high power to wieght ratio outputs using lower grade fuels. The Merlin jumped from about 1030 to 1280 or 1310hp when 100/130 became available and allowed a jump in boost pressure from 6psi to about 12psi, an increase of almost 30%. Without that fuel I suspect the usefullness of the Merlin was limited or Rolls-Royce would have needed to start using water injection.

The production of C3 fuel required so much additional input an additional 30% of ordinary B4 could have been made. (though I believe the introduction of the process of alkylation and hydroforming improved this from sometime in 1943 onwards).

The Reich had available about 20 billion tons of fuel per year while the USA had at least 10 times as much: 200 billion. There is not way that the Germans could have afforded to throw away that much energy and so most of the bomber and transport fleet as well as the half of the fighter fleet had to run of B4.

For Instance the DB605ASB and DB605DBM (both with ovesized superchargers) managed 1850hp using B4 (87 octane) and MW50, the DB605DCM managed this power level of C3 (96/130) without water injection.
The Luftwaffe page , Daimler-Benz DB 605
Note allied 100/130 was really 104/130. The Merlin 66 using 100/130 managed 1720hp on 100/130 at 18 psi boost. Greater power levels used 100/150.

Both engines weight was 745kg and 744kg respectively.

The two stage Merlin 66 running 100/130 was a superior engine to the DB605A1a running B4, at altitude, from its introduction to the appearance 1 year and 8 months latter of the oversized supercharger variants of the DB605; at this point the Daimler Benz engine caught up. The delay seems to have been related to lack of production of the supercharger rather than the engine itself and a failure to anticipate the need for high altitude engines.

I am aware that the Merlin 100 series (Packard V-1650-9) as developed for the P-51H was able to produce around 2100-2200 using 100/150 and water injection combined but it looks like the DB605 with suitable fuel could have also run at higher pressures. Kurfurst claimed it was benching 2.4 ata which means 2400hp.