Article Type: Military History
Article Date: July 27, 2001
Is there anything more unsolvable than an argument about which engine is best? When I was a kid all we cared about was the speed of planes. As a teenager the debate changed somewhat to which auto engine was superior. After many years of "grown-up" life I've learned a little more and there's no sense arguing. The mechanics were the true heroes of the WWII air war and engines were the alloy hearts of their big iron machines.
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Allison 1710 Series  |
In the early days of aero engines the air-cooled radial offered lighter weight and required no elaborate cooling system versus greater power but the greater weight and cooling system plumbing of the "V" engine.
The main manufacturers of radials were BMW and Pratt & Whitney while Daimler-Benz and Rolls Royce, as well as Junkers to lesser extent and Allison, built V-types.
The V-12 layout offered a sharper nose profile all around due to their inherently slim cross section size which made of better streamlining while all country's naval aircraft used the fatter air-cooled radials due to their more reliable nature over vast stretches of water where a damaged cooling system meant certain engine failure.
Fifty hours was usually a maximum average life span before rebuild.
RADIALS-BMW
BMW's basic workhorse was the 801A. The layout was twin rows of seven cylinders each with a displacement total of 48.1 liters or 2,550 cubic inches from a weight of 1,213 kg or 2,670 lbs. It produced 1,600 hp at 2,700 rpm. Four gear-driven valves per cylinder fed the air/fuel mixture and extracted exhaust gasses while two spark plugs atop each cylinder ignited it. A separate cam for each cylinder orchestrated this. Its light weight made it quite useful in relative pounds per horsepower of a given aircraft.
As with all engines, modifications arose owning to different sub-model number classifications. The "D" offered 1,700 hp and with the MW 50 boost method of injecting a methanol-water mixture for short periods 2,100 hp could be attained.
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| 801 In FW 190 |
BMWs were used most in the Focke-Wulfs principally the FW-190 derivatives. The engines were supercharged and fuel injected plus they had a clever device called the Komandogeraet (command device) that controlled ignition timing of the two Bosch magnetos supercharger boost settings and the fuel flow with use of the throttle lever only. Later the 801TS added an exhaust-driven turbocharger that complimented the supercharger's compression of intake air by using the normally wasted exhaust flow to produce power. By 1945 over 61,000 were delivered.
A very late war development of BMW was the 803 which was a 28-cylinder arrangement mounting the cylinders back to back in pairs siamesed under a single cylinder head sharing a camshaft. It weighed 2,950 kg (6,500 lbs.) but produced 4,000 hp.
P & W
Pratt & Whitney developed its 2800 series Double Wasp of two rows of nine cylinders each. The type was used in every US Naval fighter of the era and the USAAF P-47 Thunderbolt. The B-26 and B-24 used P & Ws where the B-17 and B-29 used equally excellent Wright Cyclones.
Displacement was 45.9 liters or 2,800 cu in. It had the normal four valves per cylinder with two plugs for each cylinder sparked from twin magnetos. Weighing in at a mere 1,068 kg (2,350 lbs.) it pumped out 2,000 hp at 2,400 rpm in its initial incarnation. By later in the war the R-2800-57(C) was producing 2,800 hp. All used "war emergency power" (WEP) methanol-water injection systems. The boost was usually restricted to ten minutes for safety sake. By the end of the war the P & W 4360 had 28 cylinders and over 3,000 hp. Well over 100,000 of all types were manufactured during WWII.
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| P & W 2800 Series |
In most applications the turbo-supercharger was mounted directly behind the radial engine but on the P-47 the whole affair was mounted in the aft fuselage behind the cockpit though the effect was the same.
Nakajima
The excellent 14-cylinder twin-row air-cooled radial of the Nakajima aided Japan's entry into combat aviation. Though Mitsubishi power plants were generally more powerful, the infamous Zero used the Sakae (Prosperity) and it was built in large numbers, about 11,000. Its modest output was quite sufficient for the light Zero.
Initially the A6M2 used the 925 hp Sakae 12. It allowed extreme fuel economy making Zeros able to roam nearly 2,000 miles in range. The 1,130 hp Sakae 21 was used in the A6M3 through A6M6 models and development culminated in the 1,560 hp Sakae 31. Range was decreased to about 1,200 miles in the heavier A6M8c.
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| A Sakae 21 being rebuilt- Chino CA USA |
Unfortunately, information on Japanese engines is sketchy on many specifications and production quantities.
Hercules
In England the standout radial was the Bristol Hercules XVI twin-row 14-cylinder engine. It was the normal four-valve, two-plug, 14-cylinder twin row configuration with a displacement of 38.7 liters (2,360 cu in) driving a supercharger behind it. Early ones produced 900-1,300 hp and in later ones, such as the XVII installed in the Beaufighter, 1,770 hp was seen. A few thousand were rated at 2,000 hp. Besides the Beaufighter, the Wellington, Stirling, Halifax, Hastings, Hermes and the Viking used the Hercules. 57,400 engines were built.
The Centaurus V and VI development came late in the war as various complexities were overcome and it was attached to the Hawker Tempest II. Its turbo-supercharged 18-cylinder twin-row layout displaced 53.6 liters (3,270 cu in) and had the usual four-valve two-plug, dual magneto arrangement for a total weight of 1,580 kg (3,490 lbs). In the Tempest II it was giving 2,520 hp and soon was developed to produce 3,220 hp with the addition of methanol-water injection. Only about 400 were produced during the war.
The BMW and the Centaurus both used engine-driven cooling fans in the cowl inlet where the Hercules and Pratt & Whitney tailored air flow baffling and cylinder fin design to direct air from the roots of the fins following the contour of the head. Much more aluminum was used in radials than in V types such as the forged or cast cylinder heads. Of course the alloy pistons rode in steel sleeves inserted in the cylinder bores. Octane of 100-115 was the general rule for wartime fuel.
Airliners in the postwar era benefited from the war technology by usually having four 28-cylinder radials of generally 3,500 hp each to propel them comfortably.
V-Types
Though radial design ultimately became the horsepower winner by adding rows of cylinders like so many Lego blocks, the 12-cylinder V was also easy to plug onto the firewall of most aircraft giving superior performance owing to the fact that its overall cross section dimensions were more compact than the bulky radial though it was longer.
Depending on the application they could be installed upright or inverted. Look at frontal views of fighters and you can see how narrow the fuselage was even though height might be close to radial-engine fighters. In the profile the nose tapered down to a very aerodynamic form. Its ballistics produced much less drag as it pierced the air.
DAIMLER-BENZ
Arguably the finest piece of engine design comes in the Daimler-Benz 600 series of engines beginning with the 600A. It is not so much that it was powerful, as Merlins ultimately produced a bit more muscle relative to displacement, but that its design was clean and uncluttered taking into account all the "bolt-ons" that complete the package. As we know the real heroes of the fighters were the mechanics who had to service them. A quick turn around due to simplified maintenance and accessibility of components allowed the 600s to shine. No matter how hot a pilot was the airplane he flew needed to be in excellent condition.
Weighing nearly the same as a Merlin at 756 kg (1,663 lbs.) the last evolution, the 605 ASCM/DCM found on the Bf 109K displaced 35.7 liters (2,179 cu in) and belched out 2,000 hp with MW-50. The 605 A-1 used on the 109G rated in at 1,475 hp. The 605 D and E with MW 50 pulled 1,800 hp. The DB 603A used in the Me 410 was rated at 1,750 hp. The DB 603LA with MW 50 had 2,300 hp and was found in the Ta 152C.
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| DB 601-605 Series |
The 605 was a direct evolution of the 601 that was used on the Bf 109E and F. The RPM limit was increased to 2,800 rpm due to block and crankshaft redesign along with modified valve timing with the standard four per cylinder. The alloy block now was bored as large as it could be for maximum piston volume within the steel liners. Each alloy piston had five hard steel rings and the one-piece forged crank rode on bronze bearings.
One unique feature was the hollow airscrew shaft connected to a tunnel longitudinally through the crankshaft for the cannon. Obviously this designed-in feature was integrated into the engine on the drawing board. The two-stage supercharger attached to the port side at the rear of the engine and drives from gears off of the crankshaft at a ninety-degree angle. The first stage cut in at about 5,000 feet and the second at about 12,000 feet in altitude.
The DBs were always installed and operated in the inverted position by design. The engines were all dry sumps with oil residing in a separate tank (size depending on application-10 gallons for a Bf 109/110 & 44 gal. on a Do 217!) and circulated through the engine via pumps located in each camshaft case. Dual Bosch magneto units that fed two spark plugs per cylinder provided ignition. Cooling was done with a 50/50 mixture of glycol and water.
Twin pumps on the lower rear of the engine delivered 87-100 octane fuel to a precision fuel injector system. An inlet for each cylinder ingested fuel directly into the ports for an unheard of equalization of mixture. This ended icing conditions and, more importantly, allowed inverted and negative-g maneuvers that no Merlin could match. The total number of 109 variants totaled around 33,000 but no figures were found on DB engine totals.
Merlin
This power plant seems to have more magic than the character it was named after. Much of it is deserved. Swapping a Merlin for the Allision gave the P-51 Mustang its true personality and the rest is, pretty much, history. Packard built the Merlins in the US. The Merlin ultimately grew into the Griffon in England.
1934 saw the first Merlin B. As it evolved from this ancestor into WWII the refinements were many. The Merlin 66 was one of the most widely applied models along with the V-1650-7 Packard.
Its displacement was only 27 liters (1,649 cu in) weighing 768 kg (1,690 lbs.). It could rev at 3,000 rpm and produced 1,720 hp (1,695 hp Packard). Spark came from a dual magneto system firing one plug per cylinder. One was mounted toward the rear on each side of the engine. The two-stage supercharger was mounted directly behind the engine and below it in the same housing were two SU carburetors later replaced by Bendix-Stromburg units in 1943 featuring a diaphragm to assist inverted maneuvers. 100-115 octane fuel injected directly into the supercharger at 5psi. The carburetors we speak of here were more on the order of automobile-type throttle body fuel injection systems rather than carbs as we knew them on autos.
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| The Magical Merlin |
The Merlins performance advantage over the Daimler with 530 more cubic inches is due to Stanley Hooker's redesign of the supercharger, impellers and inlet flow. It was found that duel impellers added power substantially though an engine aftercooler (not to be confused with the radiator aftercooler) was needed to prevent premature fuel detonation during compression in the cylinders. The aftercooler increased the fuel/air charge density proportionately producing extra power.
Certainly cooling was a critical factor and Spitfires ultimately required larger then dual radiators. The Mustang profile is dominated by its huge single air scoop for the big radiator. Its cooling system of 16.5 gallons at 30 lbs. pressure used a 70/30 mixture of glycol/water. The radiator aftercooler had 5 gallons under 20 lbs. pressure. The coolant and oil pumps resided on the rear bottom-side of the engine along with the fuel pumps. Oil capacity was 12.5 gallons. When overheated the Merlin did not immediately freeze up. The cylinder heads would develop tiny cracks allowing coolant to seep into the crankcase forming a tell tale white exhaust stream warning of impending danger.
The final variant of the Merlin, the 130/131 developed 2,030 hp with one that was tested at full power for fifteen minutes at 2,640 hp. 168,040 were produced including Packard's output.
But as hot-rodders say, "there is no substitute for cubic inches," so entered the Griffon at 36.69 liters (2,239 cu in) displacement. Overall dimensions were close to the Merlin so interchangeability to the same aircraft was accomplished. Spitfires, such as the XIV, using the Griffon had 2,050 hp and the Spitefire with a Griffon 69 using a three-stage supercharger could hit 494 mph. With a single stage blower the Mk 58 developed 2,455 hp at low level and was used on the Shackelton bombers.
Allison
America's only V-12 was the turbo-supercharged Allison V-1710 series. The V-1710-89/91 used in the P-38 H through M models was rated at 1,425 hp at 3,000 rpm. The 28-liter (1,710 cu in) engine weighed a scant 1,345 lbs. The earlier models had a single stage supercharger, which limited the performance at altitude in the aircraft in that it was installed like the P-39/P-40, mounted on the rear. The P-38 installation placed the blower units atop the engines. 10.8 gallons of oil lubricated it and dual magnetos fired one plug for each cylinder. The Allison never saw the extensive developments of the Merlins since the P-38 was ample with the 89/91 and Packard Merlins and P & Ws were powering other supreme US fighters of the day.
Sources
- Genreal Motors War Products
A Power Primer
General Motors Corp., Detroit, MI, 1944
- Green, William
Fighters Vols. 1,2,3 & 4
Doubleday & Co., 1960-61-62 & 60
- Green, William & Swanborough, Gordon
The Complete Book of Fighters
Salamander Books Ltd., London, 1994
- Gunston, Bill
Classic WWII Aircraft Cutaways
Osprey Publishing, London, 1995
- Headquarters AAF Training Command
Pilot Training Manual for the Mustang (AAF Manual 51-127-5)
Office of Flying Safety, Winston-Salem, NC, 1945
