Radial engine vs In-line engine, fuel consumption

[Todd Secrest]

So in WW2, both radial and in-line engines are used, if both engines had the same cubic inch volume, which would be more fuel efficient?
(If there was a difference)

 

[swampyankee]

Overall, liquid-cooled engines could do slightly better, at least in theory, but since the main market for liquid-cooled engines was military fighters, which were less concerned with fuel efficiency than military multi-engine and commercial users, who tended to use radials, it's likely radials were a bit better at cruise.

 

[Shortround6]

So in WW2, both radial and in-line engines are used, if both engines had the same cubic inch volume, which would be more fuel efficient?
(If there was a difference)

A simple question but the answer is not simple.

The Merlin (1650 cu in) is sometimes criticized for poor fuel consumption. It did use one of the lowest compression ratios of any high performance WW II aircraft engine (6:1) but that also allowed more boost to be used before detonation set in. The Allison (1710 cu in) used 6.7:1 compression and is generally credited with a bit better fuel consumption but a bit lower limit on boost (both engines went through a variety of models with upgraded parts so beginning of war and late war engines should not be compared to each other).
P & W never upgraded the R-1690 Hornet so we are left with the R-1820 Cyclone and the R-1830 Twin Wasp as our air cooled examples ( I want to try to stick to the US engines as we can assume the same fuel and the same or at least close technology in piston rings/bearings and carburetors/single point fuel injection). and both use compression ratios of 6.7:1 or 6.65:1 depending on exact model (or source).

At high power the advantage may lie with liquid cooled engines. The air cooled ones start running really rich and use the extra fuel as an internal coolant. The Liquid cooled ones use the same trick but generally at even higher powers. When the engines are running lean they are pretty close to each other. If the engine cooling system is set up to the cool the engine when it is making 1200hp (give or take) then cooling it at cruise (around 750hp?) is no trick at all and any benefit of liquid cooling over air cooling is marginal.
In the early years the two radials often came with 2 speed superchargers and at low altitude when running in low gear didn't use up as much power driving the supercharger.

However a big wrench in the comparison is the actual installation of the engines, Until the FW 190 and later the air cooled engine was simply a higher drag installation and required more power to go the same speed as the liquid cooled powered fighter (a P-36A could fly 285mph/15.000ft using 750hp from it's P&W R-1830 while an early P-40 could fly at 286mph/15,000ft using 600hp from it's V-1710 Allison.

A later installation of the R-1830 in a P-40 test mule got the drag difference down to about 8%.

When cruising the American engines could get into the high 0.40s (lbs of fuel per HP hour) for either the radials or the Allison. The Merlin was usually rated around 0.50 or just above. At Military power (or combat power if allowed) fuel consumption could rise to around 0.70lb per hp hour for any of the engines.

 

[fliger747]

The only radial that I have a lot of experience driving around is the R985. My comment on operation of that engine (which had only manual mixture control) was that in common with most radials, had cooling issues. The typical radial solution for a hot cylinder (#9 in a 985) is to run with a rich mixture. On takeoff Radials are run in full rich, which is usually mixture richer than optimum for power, for cooling. The function of water injection was to allow better cooling from greater mass flow through the engine and also a better mixture closer to best power, allowing higher MP's without detonation.

In cruise, a plane such as a DC6 the flight engineer can carefully monitor and tune the engines for optimal performance. A fighter pilot had his hands full enough without doing much engine fiddling.

All that said, the radials seem to have won the longevity battle. A lot of R2800 CB16's still in commercial service hauling freight and fuel.