Merlin vs. DB601

[wiking85]

Which engine was better the Merlin or DB601? From what I can tell the Merlin had a lower displacement, but the same weight (roughly) as the DB; why was that? What did the Merlin have that made it capable of the same or better HP at lower displacement than the DB?

 

[davebender]

What did the Merlin have that made it capable of the same or better HP at lower displacement

RR Merlin employed higher compression and/or supercharger boost.

Higher RPM is the other primary method for getting more power out of small displacement engines.

 

[GregP]

The Germans were operating on lower octane (or performance number if greater than 100) fuels and so could not use the same compression ratio and rpm. They had to limit cylinder pressure to prevent detonation.

In my view, the DB 6 series were excellent engines with very few negatives. That the Germans used manual starting was their own choice. Today an DB can be fitted with an electric starter. One main issue in a wartime pistion was reliability and the DB seems to me to have been VERY reliable.

We know the Merlin was good one.

Which was best? Can't say but, in terms of engineering, neither took a back seat. We KNOW the DB 601 / 605's propelled Bf 109's up to 35,000+ feet with hydraulically-driven superchargers quite effectively.

I have not seen any average TBO's for the DB, but Merlins were in the neighborhood of 250 hours or less. Well. let me temper that, the military prescribed overhaul was around 250 - 400 hours. The Allison was anywhere from 300 - 1,000 hours depending on the expected engine stress in the installation. Typical was around 450 hours.

I've seen a 1942 review of the DB 601 in "Flight" magazine that estimated overhaul every 100 hours, but would love to see real overhaul numbers for the DB series as well as the Jumos. The DB 601 manual I saw never mentioned overhaul, but did require a major inspection at 150 hours. Canada has interpreted this as 150 hour TBO for the Russel Bf 109E.

So ... I can't pick which is "better" and would gladly fly behind eitther the Merlin or a DB. I HAVE flow behind a Merlin (and Allison) on a couple of occasions and would love to have the chance to ride in or fly a DB-powered aircraft. The only major wartime engines I might decline to ride behind are the Japanese Aichi Atsuta (unless they drilled the front gearcase oil hole that was left out on the original Japanese version) and an early Nakajima Homare (had a lousy reliability reputation). I might think really hard about riding behind a Russian WWII piston engine, but that is due more to the state of the current part supply than the design of the engines.

I can tell you the Nakajima Sakae 21 in our A6M5 Model 52 Zero has been flawlesssly reliable.

Nothing to do with the Merlin or the DB, but is just for info.

 

[davebender]

Water injection became common on aircraft engines during 1944 and it made quite a HP difference. Not sure why it wasn't employed earlier.

 

[tomo pauk]

I reckon it you are talking about single stage Merlin?
The things were very much 'fluid', power-wise, with newer engines quickly supplanting older ones.

At any rate, Merlin turned notably bigger rpm than DB-601A (3000 rpm vs 2400). It's supercharger intake section were probably designed for greater flow of air (1030 HP at ~16000 ft vs. 1006 HP at 14500 ft). Merlin was using one of lowest compression ratios (6:1), that meant it was able to use much greater boost levels than other engines. RR took advantage of that, and Merlin III was able to produce 1300 HP at 9000 ft, boost pressure being there +12 lbs per in^2 (vs. +6 lbs per in^2 on 87 oct fuel). Please note that benefits of better fuel were confined at lower altitudes - Merlin III was producing same power above 16000 ft be it with 87 oct fuel or with 100 oct. The DB-601 was later (later in 1940) allowed for 2600 rpm, here, though the increased power is not noted?
The DB-601 have had advantage of having the variable supercharger drive - instead of only one supercharger drive ratio in Merlin III, the DB-601 was able to vary the speed of it's supercharger, so it will take just a small power to drive it under 2 km of altitude (= more power left for the propeller). The net benefit is that take off power was 1100 PS (~1085 HP), vs. 880 HP of the Merlin III.
RR was aware of this Merlin's shortcomings, so they introduced Merlin X (2-speed supercharger) and Merlin XII (improved Merlin III, with reinforced internals so it was able to withstand +12 lbs boost even with engine in static position). The Merlin XII (Spitfire II) was able to give 1175 HP for take off, and, due to changes in supercharger system, a bit more power higher up. Merlin X never went in the fighters, the power was comparable with Merlin III above 16000 ft, take off power was 1065 HP.

Germans also moved on, with DB-601N. It was using C3 fuel (roughly 100 oct?), vs. B4 fuel (~87 oct). The RPM were increased to 2600, so power at altitude also went up considerable - at ~15700 ft, the 1-minute power was ~1160 HP, the 30-minute power was 1035 HP at about same altitude. Take off power also increased (+75 PS), and 1-minute rating was available at all altitudes, vs. only SL to ~2km for the 601A. At 3 km (~9700 ft), the 601N was giving ~1220 PS, vs. 1040 for the DB-601A. Benefits of better fuel? Though, it seems like benefits of better fuel were not used maximally (boost pressures were about the same as at 601A).The rev limit was upped to 2800 rpm quickly (again, in late 1940), but I don't know how much the power was increased.

British introduced, in about the same time as the Germans introduced the DB-601N (~2nd half of 1940), the Merlin XX. Stanley Hooker redesigned the supercharger system, so now the power high up was about 1150 HP at 18000 ft, take off power went to 1280 HP. Engine was used on Hurricane II with those power levels. Combat power (usually limited to 5 minutes) was equal or better than 1400 HP, under 14000 ft. The Merlin 45 (for Spitfire V) was basically the XX, but single speed supercharger. High gear was used only, so the power above ~10000 ft was about as what the XX was capable for, the power between SL and 10000 ft was slightly lower (1185 HP for take off).

Shortly after that, DB introduced the DB-601E. It reverted back to B4 fuel, introduced changes in supercharger, valve timing other stuff (see here for more), allowed initially for 2500 rpm and 1,30 at boost. The power curve was comparable with Merlin 45. Some time in early 1942 (January, probably), the DB-601E was allowed for 2700 rpm and 1,42 ata, and power was notably increased on all altitudes (vs. Merlin 45 and XX: slightly more power above 15500 ft, slightly less under ~12000 ft; better TO power vs. Merlin 45, almost 150 HP).

Basically: power-wise, both engines were competitive enough, advantage see-sawing through time and desired altitude. The DB-601 series was a better choice if a country have had problems acquiring high octane fuel. The DB engines were also to accommodate a cannon firing through the prop shaft.

 

[GregP]

Pretty good summation, tomo.

Maybe we should undertake to do one for a few more engines, including some radials.

Perhaps a discussion of Merlin - Griffon - Sabre contrasted with the DB - Junkers Jumo (probably at least the 213 used in the Fw 190D) contrasted with the Allison contrasted with the Klimov and Mikulin. Maybe in a new post as not to hijack this one.

I left out the Aichi Atsuta since the Japanese never quite got it right.

 

[Milosh]

Greg, afaik the DB601 engines used 6.9:1 CR. This compares to 6:1 for the Merlins.

The DB601N engine used C3 fuel which late war had a PN of 140+ and had a CR of 8.2:1.

The DB engines were more fuel efficient due to fuel injection.

 

[tomo pauk]

Could you post some numbers comparing the consumption, Milosh?

 

[GregP]

Hi Milosh,

Right on about the CR. Merlin was 6.0 and the British could thus use higher boost before detonation set in. It's kind of that way at Reno these days. The winning engines are running a CR of about 5.0 and insane boost numbers. I've heard 145 inches (56 psi or so) being bandied about by some in the unlimited gold pits ... unofficially, that is. For comparative purpose, 1.42 ata equates to 41 inches of MAP or 5.5 psi of boost, so racing these days is stressful on the engines, to say the least.

According to the German fuels report I read and recorded, six samples of C-3 fuels averaged 95.7 Octane lean (95 to 97.5) and a rich mixture rating of 123.75 (118.5 to 125). Another report said they found two different 50-gallon samples that averaged 95.5 lean with one sample being 150 rich and the other 145 rich. These were German reports.

So we have eight samples from varying times, taken from German reports, all near 95 lean and varying from 118.5 to 150 rich. Sounds like some wartime variation was happening for sure, and you could NOT be sure what rich rating of fuel you would get. So, a good pilot who knew his engine might do some leaning and get some benefit, but a prudent commander would simply order them to use what the average could be expected to be unless in a fight for life ... and then you might as well melt the engine or die since it WAS a fight for life.

I would assume that you could NOT expect most samples to be 140+ since 6 of 8 samples from the German reports were below that number.

Note: All C-3 samples were about 40% aromatics, 37% parafins, and 21% Napthenes with specific gravities of 0.7705 - 0.7706.

 

[Aozora]

Could you post some numbers comparing the consumption, Milosh?

Merlin fuel consumption could vary according to the fuel used and boost pressure: the BASIC specifications were:

Merlin II (and III) from an R-R handbook issued in May 1938:

Supplements from 1939 1941 (Merlin X specs)

Merlin 45-50 series:

Merlin 60, 70, 85 series (A.P. 1590P,S U Volume 1):

To ask whether one engine was better than the other is a little simplistic; both were fine aero engines, designed to different formulas, albeit for a similar purpose and power requirement.

 

[GregP]

If I compare the Allison V-1710 (F-30R) against a Merlin XX against a DB 601 against a DB 605, I get data as follows:

1) Allison F-30R:
Displ (cu in) = 1710.6; rpm = 3,000; power (HP) = ,1500. MEP = 15.97 Bar (sea level, takeoff).

2) Merlin XX:
Displ. (cu in) = 1649; rpm = 3,000, power (HP) = 1,480. MEP = 16.34 Bar (sea level, takeoff).

3) DB 601:
Displ. (cu in) = 2073.9; rpm = 2,500; power (HP) = 1,159. MEP = 12.21 )sea level, takeoff).

4) DB 605:
Dsipl (cu in) = 2,180.5; rpm = 2,800; power (HP) = 1,775; MEP = 15.88 Bar (sea level, takeoff).

So the Merlin XX is 33.8% better than the DB 601. But if we drop the DB 601 and only compare the DB 605, the difference between the best and the worst is only 2.9% in MEP! Pretty damned close, if you ask me, and I know you didn't. I dropped the DB 601 because it was supplanted in production early-on.

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.

I think if we compared and earlier Allison, an earlier Merlin, and the DB 601. they'd all be VERY close in MEP.

 

[Aozora]

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.

Again, this depends on which version of the 2-stage Merlin is being discussed; for example, the Merlin 66 running on 150 grade at +25lbs boost is a very different animal from the Merlin 61 running on 100/130 grade at +18 lbs

It can be so hard to make comparisons, particularly when comparing variations of the same basic item:

 

[GregP]

I like the one without all those distracting clothes, myself.

However, in this case, the cup sizes are the same, whereas in the case of the engines above, the DB 605 is the busty one with the prop shaft poking out of the cup centerline. I still say they're very close ... but beauty is in the eye of the beholder, after all, and each person must pick their favorite. Somebody MUST want Phyllis Diller! Huh?

I'll stick with someone like Barbara Eden in her prime. I'd post a drawing, but it probably violates forum guidelines.

 

[wuzak]

If I compare the Allison V-1710 (F-30R) against a Merlin XX against a DB 601 against a DB 605, I get data as follows:

1) Allison F-30R:
Displ (cu in) = 1710.6; rpm = 3,000; power (HP) = ,1500. MEP = 15.97 Bar (sea level, takeoff).

2) Merlin XX:
Displ. (cu in) = 1649; rpm = 3,000, power (HP) = 1,480. MEP = 16.34 Bar (sea level, takeoff).

3) DB 601:
Displ. (cu in) = 2073.9; rpm = 2,500; power (HP) = 1,159. MEP = 12.21 )sea level, takeoff).

4) DB 605:
Dsipl (cu in) = 2,180.5; rpm = 2,800; power (HP) = 1,775; MEP = 15.88 Bar (sea level, takeoff).

So the Merlin XX is 33.8% better than the DB 601. But if we drop the DB 601 and only compare the DB 605, the difference between the best and the worst is only 2.9% in MEP! Pretty damned close, if you ask me, and I know you didn't. I dropped the DB 601 because it was supplanted in production early-on.

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.

I think if we compared and earlier Allison, an earlier Merlin, and the DB 601. they'd all be VERY close in MEP.

Which DB 605 made 1775hp and when?

 

[Aozora]

Which DB 605 made 1775hp and when?

That would be the DB 605AS/ASM series, from early 1944, or the DB 605DB from late 1944

 

[R Pope]

The Germans fitted a captured Spitfire with a DB engine (not sure which one) and there was very little performance difference noted. The Spaniards put Merlins on 109's with little diff, too. Small advantages in some flight conditions, losses in others, I believe.

 

[cimmex]

Well, power wise both contemporary engines were rather close. IMO the DBs offered some advantages.
Inverted V-design offered better view over the nose,
fuel injection ensured supply at all flight situations,
hydraulic supercharger drive was more efficient,
side mounted supercharger allowed an engine cannon.
cimmex

 

[wuzak]

That would be the DB 605AS/ASM series, from early 1944, or the DB 605DB from late 1944

Thanks for the info Aozora.

Regarding Greg's analysis, an equivalent period single stage Merlin would make more power than the 1480hp of the 1941/42 Merlin XX.

The Merlin 24 had a take-off rating of 1610hp, low blower of 1640hp @ 2000ft and high blower of 1500hp @ 9500ft. These are at +18psi boost.

The two stage Merlins were similar in weight to the DB 605 - the XX and 24 were lighter.

Of course, the major performance emphasis on the Merlin was the 2 stage engines. And their BMEPs were higher, though probably not at sea level.

A Merlin 76/77 was rated at 1710hp in MS gear, +18psi boost and 3000rpm, giving a BMEP of 18.9 bar, or about 18% more than the DB 605.

 

[Milosh]

Greg, please look at the dates those fuel samples were obtained. I think you will find that most of them were early to mid war.

 

[tomo pauk]

As for fuel consumption - the DB-601E was using 445 L/h, for 1350 PS, take off. Or, circa 118 US gals per hour. The V-1650-1 (basically, the Merlin XX), 133 US gals for 1300 HP, take off power. Maximum continuous powers: 300 L/h for DB-601E (about 79 us gals/hr) for 1040 PS; the V-1650-1 was using 93 us gals for 1010 HP. The DB-601A was using 300 L/hr when producing 960 (hi-alt) or 990 PS (SL).
So, yes - DB-601 was more fuel efficient than Merlin. But, not because of fuel injection, but because of notably higher compression ratio?