Merlin vs. DB601 (1 Viewer)

Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules

It doesnt quite work that way on a supercharged engine, its not about fuel, compression ratio or capacity though bigger is often better. Its all about how much Air you can move into and out of the engine and thats down to the air compressor and the intercooler.

Take a look at BMWs M12 Formula 1 engine of the 80s, it was a 1500cc 4 cylinder engine with an Iron block from a saloon car that ended up producing 1,400 hp in practice trim and 1,000 hp in race trim with the boost wound back to save fuel. The Turbo just got bigger and bigger and the engine became almost a gas generator bolted to a Turbine. If BMW had kept on strengthening the engine (it was Iron because Aluminium technology couldnt cope) and making the Turbo bigger and the radiators bigger its mind boggling to imagine what power they could have achieved if the FIA hadnt changed the regulations to try and slow the cars down.

Scroll down to the 9/03/15 entry;

McCabism

I think we're saying pretty much the same thing. Cooling aside, with a stoichiometric mixture fuel mixture, air or fuel are a function of each other. I'm assuming that both DB and RR designed so there was an optimum of both engine strength and weight. The FIA BMW obviously has a great overhead in strength at the cost of avoidable weight.
 
It doesnt quite work that way on a supercharged engine, its not about fuel, compression ratio or capacity though bigger is often better. Its all about how much Air you can move into and out of the engine and thats down to the air compressor and the intercooler.
Superchargers (mechanically driven or otherwise) add compression and increase mass flow, but are only ONE of the factors contributing to that. The bare (normally aspirated) engine will induce air into the manifold through the vacuum created by the cylinders themselves, and larger displacement and higher RPM mean larger volumes of air being drawn in (and of course, superchargers of similar mass flow will provide lower pressure to a large displacement engine than a small one).

Engine displacement, RPM, throttle body/plate design, and overall intake manifold design are all factors that come into play for mass flow through an engine. (manifold design was the key difference between the Merlin X and XX series, more aerodynamically clean and un-kinked supercharger+intake manifold improving mass flow and compression while reducing charge heating -reduced drag/friction- and thus improving performance without increasing supercharger size or engine power consumed to drive it)

The V-1710 had a known manifold bottleneck as well around the carburetor/throttle body, but it was never corrected due to some combination of war-time expediency and lack of funding. (as well as relative irrelevance for turbocharged installations using low gear ratios for the integral supercharger -for the 8.8 and especially 9.6 geared engines, it was a major bottleneck both for mass flow and charge heating, and is the main factor that prevented the 9.6:1 engines from matching or exceeding the performance of the Merlin 45 in both power and altitude performance) Similar overhead on the production line is likely why 2-speed gearing was never used on the V-1710, and external auxiliary supercharger stages were preferred due to not disrupting existing manufacturing tooling. (the integral supercharger and manifold arrangement remained unchanged)

Junkers adopted an annular throttle plate (swirl throttle) that avoided aerodynamic losses common to conventional butterfly style throttle plates and greatly improved power curves for both single and multi-speed supercharger drive when employing this mechanism. (gains were most significant at sea level and critical altitude for higher gear speeds where the throttle plate does the most constricting of the intake, little or nothing is gained when the throttle is allowed to be wide open, but the swirl arrangement greatly improves airflow characteristics at partial throttle to the point power curves are often best at low level for any given supercharger speed, and dropping slightly in power as critical altitude is reached rather than loosing massive amounts of power to intake drag at lower altitudes and failing to take advantage of the denser atmosphere -which in theory, should allow the supercharger to work somewhat less hard, and is demonstrated in practice with a well designed swirl throttle)


There's also the issue of cylinder compression ratio, but this doesn't impact mass flow, but instead improves power and fuel efficiency though higher peak compression in the cylinder, and smaller volume of the charge when ignited. (ie the cylinder extends slightly further at full stroke in higher compression ratio configurations)

German engine designers typically opted for higher compression ratio over higher boost pressures for engines targeting higher octane fuels. This had the advantage of avoiding larger and more power-sapping superchargers and the intercooling necessary to use such (or charge heating suffered without such cooling) while offering an increase in power at all altitudes without increasing fuel consumption. (the V-1710 is a bit of an interesting case there given it uses a slightly higher displacement and higher compression ratio than the V-1650, but not nearly to the extent of any of the German V-12s of that power class ... of course the V-1710 was bottlenecked by politics and funding issues)

Ever increasing engine RPM was the primary way german engines developed higher and higher capacity throughout the war, and one of the reasons they needed larger (or faster running) superchargers as well, in spite of little (or sometimes no) increase in boost pressure. (mass flow increased due to the increased operating RPM) Aside from the case of the DB-605 increasing the 601's bore and volume. (prior to that the 601 had increased its operating speed continually, and had it not been for the roller bearing shortage, would have been worth continuing development to that end -along with introducing high altitude versions using the DB-603 supercharger a la DB-605AS) The Jumo 211 and Jumo 213 progression to higher and higher RPM without increase in volume is an exceptional example of this progression, particualrly with the impressive operating speed of the 213. (The V-1710 might have been rated too conservatively in this respect too given that, unlike the Merlin/V-1650, there had been fairly dramatic improvements in crankshaft design that should have allowed reliable operation well above 3000 RPM and well above the limits of the early-war C-series engines present on the P-40B/C/Tomohawk, though like overboosting there may have been quite a lot of unofficially sanctioned operation of the V-1710 beyond its official specifications -unlike the Merlin which didn't function well enough under such conditions to tolerate some of that sort of abuse and was also noted to run too rough to allow low-RPM cruise like the V-1710 offered, the crankshaft not being as well balanced and not nearly as tolerant to high or low RPM beyond its general sweet-spot for operation; I suspect part of the V-1710's conservative rating was lack of funds for more extensive testing but also due to more conservative USAAF standards in general and particularly due to some of the failures experienced with the C series V-1710 when pilots/crews operationally pushed them out of spec -notably the Flying Tigers- and suffered failures with those much less tolerant early series engines -the crankshaft design in particular was both less suited to overreving and running at higher torque levels, so overboosting to excessive power levels at lower RPM would also overstress it)


Rolls Royce (and several American engien designers) also seemed to have more capable centrifugal compressor engineering than German designs, or at least a significant number of cases, so relying less on supercharger performance also sidestepped this serious bottleneck. Junkers Jumo's pre-war superchargers were particularly poor with the 210 and (pre-F series) 211 using a somewhat ridiculous looking 'spouted' impeller with a series of square outlets formed into the outer edge of a shrouded impeller and thus producing a substantial amount of drag when spinning. This was replaced with a far better designed and somewhat novel fully-shrouded impeller in the 211F, though probably still not as aerodynamically efficient as most Allied engines impellers it seems to have been a good design and one Junkers stuck with and built on for their later war designs (and multi-stage superchargers) and did have the advantage of simplifying the supercharger casing itself. (an unshrouded impeller requires very tight tolerances in the casing to maximize compression and avoid leakage and drag around the edge of the impeller blades/vanes, the shroud effectively has a built-in casing that contains and guides the airflow into the diffuser independent of the outer casing)

http://www.enginehistory.org/Sarah Clark/Finding Aid/Jumo211A SC.jpg (Jumo 211A supercharger -note the 'spouted' impeller, yes that's one solid piece that rotates at high speed inside the casing ... years ago when I first saw it, I assumed it was 2 parts with those spouts being part of the diffuser guide vanes)

Access forbidden! (nice allied report on the 211F including details on its supercharger)

To be fair, American supercharger design had been pretty awful when General Electric had been the only supplier (super, not turbochargers) and engine manufacturers finally decided to go it alone and develop their own, superior centrifugal compressor designs (and consequently forcing GE to both step up their own game in compressor design and to focus more on turbocharger development).

With that in mind, it's surprising Hans Von Ohain and Whittle both did as well as they did with their impeller designs. (Ohain making some compromises to allow sheet metal blades rather than a machined aluminum disc, but still rather well designed compared to plenty of late-30s superchargers, particularly Jumo's ... perhaps another reason axial compressors were pursued more heavily by some was due to the underdeveloped nature of centrifugal compressor designs? -often claimed to be a mature technology of the time, but really not, Whittle in particular targeting unheard of 4:1 and higher compression ratios on a single stage -Ohain's 2.8:1 in the HeS 3 was pretty much cutting edge for 1939, while the Halford H.1 remained within the range of Ohain's early work until later in development when pushed over 3:1 when approaching its full RPM and 2,700 lbf design thrust and 3.3:1 in the post-war Goblin II with improved diffuser casing and combustion chamber design)
 
The only real "apples-toapples) comparison in engines is using Mean Effective Pressure, and thsi shows them all to be VERY close to one another, as I expected before I started. For reference, I used the model that came up in Wiki for the numbers, but chose the Merlin XX since I had access to all. The merlin starts to look better up high with the 2-stage units, but not at takeoff, which is where these come from.


But Greg, according Wikipedia - Mean effective pressure - Wikipedia:
W=Pme x Vd;
work or power directly depends on displacement (Vd), power is product of Pme (MEP) and Vd (displacement) so if you talk power and want "apple to apples comparison" then displacement should be included, isn't it?
There is an old saying - no replacement for displacement
 
But, not because of fuel injection, but because of notably higher compression ratio?
I can say for DB60x series only - without comparison to Merlin.
A higher compression at DB601 was (is) possible due to fuel injection... There is no fuel injection as a factor in basic power equation for internal combustion engine per cycle (W=Pme x Vd).

By the rule, if we have two identical engines, one without and with fuel injection then fuel injected version is more efficient. There is an example of Db600 vs. Db601.
1629463659882.png


Of course, there is a big technical issue of building controlling unit for injection - but assuming it solved, injection of fuel (direct or indirect) must make the engine more efficient - mostly due to cooling effect of injection and atomization - even if there are other positive effects of fuel injection. Controlling issue is far more simple to be solved for stationary regime engines (let say aircraft engines) than than ever changing (cars). Fuel injection is nothing new or modern - is as old as internal engine itself (Fuel injection - Wikipedia) - by the way, it wasn't mentioned there, but even Nikolaus Otto himself, filed a for a patent on direct fuel injection in 1877.

Regards
 

Users who are viewing this thread

Back