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.