Superchargers: Early V-1710 vs. Merlin

[gjs238]

When the British first began acquiring P-39's, P-40's, Mustang Mk.I's, how did the supercharger in the Allison V-1710 in those planes compare to that of the Rolls Royce Merlin used in the Spitfire Hurricane at that time? Were they both single-stage single-speed? If so, did the Merlin s-s/s-s supercharger outperform the Allison s-s/s-s supercharger?

I read over and over how the Allison powered planes were at such a disadvantage due to the single-stage single-speed supercharger, but I'm wondering what the Merlin was using at that time.

If they were both using single-stage single-speed superchargers, then why did the Merlin powered engines perform so much better at high altitude?

 

[red admiral]

Its difficult to get information on the superchargers themselves to make a proper comparison (power, boost, efficiency etc.) I don't think there was a great deal of difference between the early V-1710s and the Merlin II. Hooker redesigned the supercharger intake and supercharger itself (better matching of characteristics) which got incorporated into the Merlin XX and I don't think the V-1710 progressed much further.

 

[jerryw]

The shortcomings of the Allison supercharger were:
a/ it was too small. Its diameter was only 9.5 ins compared to the 10.25 of the Merlin. When Hooker analysed the R-R supercharger, he quickly found that it was strangling the engine. If this was the case with the R-R product, then the Allison must have been really suffering as evidenced by the huge amount of trouble the P-40 engines had with detonation.
b/ it was mounted too close to the back of the engine. Drawings of the V-1710 s/c clearly show the very tight angle between the outlet of the s/c and the start of the inlet manifold.
c/ the V-1710 induction manifold was extremely convoluted. This posed problems for the mixture flow as evidenced by the necessity to provide special pickups for raw fuel that gathered at the base of the main inlet branch.
d/ development of the V-1710 s/c was, for a long time, not given much attention because it was felt that the turbocharger would provide the main source of supercharging for all US aero-engines.

 

[Thorlifter]

Good technical information Jerry. I learned something new today!!! ha ha

 

[davebender]

What was the supercharger diameter of the DB601 engine?

 

[jerryw]

The diameter of the DB 601 supercharger was 10.24 ins, ie same as the Merlin.
In the later 605 engines, the diam. was increased to 10.47 ins.

 

[Colin1]

...If this was the case with the R-R product, then the Allison must have been really suffering as evidenced by the huge amount of trouble the P-40 engines had with detonation...

Before I ask
what is meant by 'detonation' in this context?

 

[pasoleati]

Detonation=sudden, uncontrolled ignition of the fuel within the combustion chamber leading to excessive pressure and temperature within the same with probable engine failure coming up quite quickly.

 

[Colin1]

The normal thing then
So how long was the service life-span of a V-1710, bearing this problem in mind?

 

[hrandy]

The British Army Cooperation Units approved a war emergency rating on the 1710-39 engine used in the Mustang 1 of 56" Hg, significantly above the 44" Hg that was the standard USAAF rating. They also ran the -39 at 72" Hg for up to twenty minutes without engine damage. This makes me wonder if the P-40 problems could be fuel or spark plug related. (see the full report at E-GEH-16 )

 

[davebender]

The diameter of the DB 601 supercharger was 10.24 ins, ie same as the Merlin.

That cannot be a coincidence. RR and DB engine designers must have based their supercharger specifications from the same calculations.

 

[red admiral]

That cannot be a coincidence. RR and DB engine designers must have based their supercharger specifications from the same calculations.

When you have similar capacity engines rated at similar altitudes its hardly a surprise. The supercharger has to supply a similar amount of air at a similar pressure. Not coincidental that the results were similar.

 

[mad_max]

I understand pretty well the sizing and operational characteristics of Roots type superchargers and turbos, but have always wondered why the German, British, and USA went about designing their engines and boosting them in opposite ways.

The British seemed to have used lower compression with higher boost, USA not quite as low boost as the British and the 601 had higher compression with less boost...the 605 had even higher compression ratio.

With single stage superchargers my thoughts are higher compression would lead to better performace under 5-6 K in alt, would provide good performance at mid alts, but power would drop off at higher alts.

Lower compression would need to be boosted higher (more fuel consumption) at lower alts, should have good power to 20k, but run out of boost over that alt.

Any aviation engine folks care to take a stab at it?

 

[jerryw]

.

The British seemed to have used lower compression with higher boost, USA not quite as low boost as the British and the 601 had higher compression with less boost...the 605 had even higher compression ratio.

?

Do you mean "USA not quite as HIGH boost as the British"??
Higher CR on the 605 compared to the DB 601 was due to higher octane fuel.

 

[davebender]

I think the DB605 got it right. Moderate boost + nitros oxide injection for WEP + variable speed hydraulic supercharger coupling + automatic control of fuel mixture and propellor pitch. The entire package was relatively compact, lightweight and inexpensive.

 

[vanir]

Ouch. Sorry I do apolegise coming in all arrogant as a transient visitor among the company of experts within their specialised fields but I just need to correct a few misgivings.

The DB-601 and Merlin aren't anywhere near the same capacity.
The Merlin weighs in at just under 27 litres swept capacity for the early series and just over 27 litres coinciding with the supercharger redesign (ie. from the Merlin X, XII and XX onwards).
The Daimler is a shade under 34 litres.
I think what Red Admiral meant to say, in engineering terms power output is most directly related to fuel intake rather than engine capacity and the Merlin II and DB601A have similar power outputs with similar full throttle heights so it is natural their supercharger requirements may be very similar.

The shift you get between them is torque x rpm = hp so the Daimler being a much larger engine produces the same power output at much lower rpm. This is handy because due to much larger throw weight inside the engine it isn't going to like revving its ring off much. There are a lot of pros and cons you could weigh up between the two approaches, historically they were pretty evenly matched it seems but my ears would start ringing after a while in a Spit I think.


wrt low-comp/high-boost and high-comp/low-boost firstly air cooled radials aside, larger capacity inline engines tend to march their rated altitudes upwards from smaller capacity ones. I guess this has to do with flow dynamics through large bores and no small amount of complex diversity I'm sure.
Smaller capacity engines have less torque particularly as altitudes rise and so may require complex supercharging systems sooner than the larger engine and are thus more limited overall in terms of extended high performance application. A good comparison for this is the 605AS large bore motor compared to the complex Merlin 60-series for roughly similar rated altitudes.

As far as compression ratio goes a higher comp with lesser boost tends to bring the full throttle height back down for good performance at medium combat altitude, whilst maintaining good cruise and climbing conditions at much higher rated altitudes. Best of both worlds without complex supercharging and intercoolers. For maximum high altitude performance however either multiple supercharger stages or an intake oxidiser would still have to be used (nitrogen oxide).
Of course for outright medium altitude high performance applications a charge coolant like MW50 can bring the full throttle height down further to allow significant overboost in the thicker air (ie. it is like using the high supercharger gear in a two-speed engine at low altitude, with the knock index improved by charge coolant), with outputs similar to maximum emergency power at sea level or better. Problem here is glow effect on the plugs which limits its use and necessitates frequent cooldown periods.

Larger bores are also useful in dealing with lower knock rated fuel as you can more effectively shape the piston crown to help delay predetonation. The higher compression ratio in the 605A to the 601E at the same pressures using the same B4 fuel is most likely due to larger bores and better piston crown shaping. Quite a bit of research was performed in Germany on this, and it was decided that was a better result than the increased valve corrosion due to the use of higher octane fuels (unless absolutely necessary).

Finally on fuel consumption at cruise, again comparing the Spit and 109 what you have is two aircraft that'll cruise around at about 2000rpm. Both will have fairly neutral manifold pressure. Ah the Daimler has larger bores you say, it is using more fuel at the same engine speed.
Ah but no I say, it has much more torque, has a rated altitude for cruise a kilometre or two higher than the Merlin unless it is using positive boost, and the increase in fuel consumption is offset by much less engine loading for higher average speeds. These offsets provide similar aircraft range between early Spit and Me109 with similar fuel loads.

Aircraft range as engine capacity increased actually went up with the same fuel load for the later Messerschmitt, probably helped also by improvements in drag, whilst fuel loads had to be increased as the Spit developed to keep similar internal fuel ranges, whilst maintaining ~fairly competitive performance (adjusted by variant moreso than year of issue but I think tending to favour the Spit in the later years bar a particularly nice example, like a clean G-14 fitted with an ASB and Mk103, be pretty nasty for anything short of a XIV).

Here's a nice little paper outlining some of the research into fuel qualities that was going on in Germany.
http://www.fischer-tropsch.org/primary_documents/gvt_reports/BIOS/bios_1612.htm

 

[jerryw]

.
The Merlin weighs in at just under 27 litres swept capacity for the early series and just over 27 litres coinciding with the supercharger redesign (ie. from the Merlin X, XII and XX onwards).

tropsch.org/primary_documents/gvt_reports/BIOS/bios_1612.htm[/url]

Some new and interesting info from Vanir here. Could you post the BORE and STROKE of each of the two versions of the Merlin you mention ("early series" and post-supercharger redesign) so that some comparison of the two types can be made?
Thanks.

 

[vanir]

What I've got is an Aviation listing page of basic specifications on all aero engines in major manufacture giving 26.89 litres for the Merlin II, III and IV and 27.02 litres for the Merlin X onwards.

I'm currently scouring engineering sources to find precisely why. I figure if the bore x stroke is the same, it may be a cylinder head chamber change when the new superchargers were fitted (I spoke a little hastily and misread Merlin X for XX, it was before the new supercharger redesign but coincided with the introduction of the two-speed supercharger).

Both the very minor increase in capacity and the two-speed supercharger definitely suggest, with the assumption a professional civil aviation source is good, that the capacity increase is derived from a further revised cylinder head chamber design. This would not necessarily affect compression ratio as generally chamber design is matched with a new piston crown. The net result in mechanical terms is a better knock rating for basically the same engine using the same fuel.

Still looking up some definitive sources.
Indeed I shouldn't have spoken so hastily in that specific regard. One gets used to their own library and personal satisfaction with conclusions, and forgets sometimes that at the public forum a little more humility is required.

My apolegies if it all turns out to be a wild goose chase. But I shouldn't think so.

 

[hrandy]

The displacement or swept volume of a piston engine is traditionally determined by the bore and stroke of the cylinders and the number of cylinders and does not include combustion chamber volume. Numerous Rolls-Royce Heritage Trust books state the bore and stroke of all versions of new Merlins as 5.4 inches by 6.0 inches and give the displacement as 1650 cubic inches or 27 litres. Obviously Rolls-Royce rounded off the displacement to the nearest whole number since if one uses the classic formula of bore radius X 3.1416 x stroke x number of cylinders the result is 1648.96 cubic inches or 27.0216 litres.

Since the according to the Heritage Trust books the bore, stroke and number of cylinders of newly manufactured Merlins never changed from 5.4 x 6.0 x 12 your Aviation listing page of basic specifications giving 26.89 litres for the Merlin II, III and IV is wrong. Seeing that they got the displacement of the Merlin X and onwards right I would guess that this is a case of conversion error and poor editing.

According to the Heritage Trust books Rolls-Royce was very reluctant to change the basic 5.4 x 6.0 formula. They preferred to grind worn cylinder liners overbore and then chrome plate them back to 5.4 inches rather than allow oversized pistons. Only in one circumstance, with third rebuild Merlin Xs on long service coastal command aircraft, did they allow .10 inch oversize pistons. This would have resulted in 1655.07 cubic inches or 27.1217 liters dsiplacement.

 

[siznaudin]

For anyone interested in the subject matter of this thread, try to get hold of "Not Much of an Engineer" by (Sir) Stanley Hooker. Utterly absorbing and enlightening material.