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Multi-crankshafts opposed piston engines (2)

by François Dovat


The most extraordinary diesel built by Junkers – and perhaps the most extraordinary diesel ever built – was the Jumo 223. This engine grouped four banks of 6 cylinders laid out at 90° on the faces of an equilateral rhombus whose apices were formed by 4 crankshafts. Being the logical development of the 2 crankshaft engines, it comprised 24 cylinders and 48 pistons. The number of cylinders was quadrupled while multiplying by only two the number of crankshafts. In 1939, in attempting to win the war, one didn't retreat in the face of tremendous technical difficulties and exorbitant development costs.

Figure 9 : Jumo 223

The one and only specimen ever built finally reached the power of 2200 hp at 4400 rpm on the test bench by the end of the war. It is not confidently known whether it had been fitted as the central engine on a three-engined Ju 52 and had achieved one or more trial flights. Its cylinders were smaller than those of the Jumo 205 and 207, with 80 mm bore and 2·120 mm stroke. It was envisaged to outfit it with a turbocharger for some 2500 hp at the same rpm – which implied a mean piston speed of 17.6 m/sec. That seems incredibly high. However, it should be remembered that in a 2-stroke engine the separating force on the piston-con-rod assembly at TDC is counterbalanced on each stroke by the compression and firing pressures; not just on one stroke out of two as in a 4-stroke engine.

Detailed technical information concerning this engine remains untraceable and the single specimen is assumed to have been either destroyed during an allied bombing raid on the Junkers factory at Dessau, or taken to the USSR. It is said that engineer Manfred Gerlach remained on site after the arrival of the Soviets, and continued the development of the larger Jumo 224 version under their occupation. This project had been launched in 1943 and progressed at a low level, as for the Jumo 223, since the prime development resources were reserved for the inverted V 12 Jumo 213 with direct gasoline injection, as well as for the Jumo 004 turbojets propelling the Messerschmitt 262 and Arado 234. The Jumo 224 project made use of cylinder dimensions identical to those of the Jumo 207 D, that is to say 110·(2·160) for a displacement of 73 liters, and a power of some 4500 hp at 3000 rpm. Its construction and development would have continued at Dessau, then later in Moscow.

According to the three poor photographs of the Jumo 223 which represent about all documentation available about it, the intake crankshafts were on the sides and the exhaust ducts appear grouped around the upper and lower crankshafts. Two cylinders were in the same phase of the cycle and thus two of them fired simultaneously. With the crank pins of each crankshaft spaced at 60° and two piston-connecting-rod assemblies at 90° to each other on each crank pin, even 30° intervals between each double firing were obtained. The 4 crankshafts spun all in the same direction and each one of them was synchronized with its diagonally opposed pair, any phasing discrepancy inducing unequal timing between the four banks of cylinders. The cylinders of each bank were grouped by three in two blocks, bolted together at half engine length. The four crankshafts engaged a median power transmitting gear which was geared directly onto the propeller shaft via a large central gear installed before the front and rear cylinder blocks were assembled together. The four camshafts actuating the injection pumps were driven by the side crankshafts power transmitting gears. The centrifugal scavenge compressor located transversely at the rear had four outlets, one for each cylinders bank.

It is known furthermore that the ratio of the reduction gear was 3.9:1, that the weight of the engine was 1400 kg, and that its dimensions were 2.37 m length, 1.37 m height, and 1.24 m width. The compression ratio is said to have been 17:1 and a bsfc of 238 g/kW/h was obtained. If the power of 2200 hp was actually achieved, it resolves to 76 hp/l, this in spite of a modest mean effective pressure of some 9 bars.



After the war, the development of the Junkers diesels also continued in Great Britain where the Napier firm had, in the Thirties, built under license some Jumo 205 prototypes, renamed "Culverin". Under the technical direction an engineer of genius, Ernest Chatterton, Napier undertook the development of a diesel in inverted equilateral triangle based on the technology of the Jumo diesels, the "Deltic". Initially intended for marine propulsion, the Deltic had 3 banks of 6 cylinders at 60° and a crankshaft at each apex of the triangle according to a concept which is said to be due to an obscure engineer named Norbert Penwarden. Its 18 cylinders and its 36 pistons provided a swept volume of 88.2 liters thanks to the dimensions of 130.17 x (2x184.15), hence a stroke/bore ratio of 1.42, very near that of the Jumo 207 D and 224.

Figure 10 : Napier Deltic

By phasing its three crankshafts 20° apart, one obtained at the same time the necessary retard of the scavenge pistons relative to the exhaust pistons and a spacing of 20° (360°: 18 = 20°) between each cylinder of the same transverse plane, since 3·20° = 60°, precisely the angle between two cylinders banks! So, the firings were evenly spaced. Contrary to the case of engines such as the Jumo 223 and 224 which were laid out in a square (or rhombus), the triangular construction of the Deltic meant it was necessary to mount an intake piston/ con-rod assembly and an exhaust one on the same crank pin. In this case, the two upper crankshafts rotated clockwise, and the lower one anti-clockwise. The exhaust lead was thus intrinsically related to this design and could not be modified. An exhaust lead of 20° was probably a little more than the optimal value, but the consequences were not excessively serious since bsfc was between 218 and 224 g/kW/h at the rated speed. Besides, the power was also evenly distributed between the three crankshafts and the engines functioned with a remarkable absence of vibration.

Figure 11 : Napier Deltic

The three cylinder blocks were identical light alloy castings in which wet liners machined from steel forgings with chromium plated bores were introduced. The nitrided crankshafts had hollow crankpins. Both upper ones were identical while the counter rotating lower one had opposite handed throws. The exhaust pistons were attached to a forked connecting-rod reinforced at the big end by a nitrided steel sleeve on which the inner diameter was the bearing for the crankpin, while its outer diameter provided a journal for the plain rod of the intake piston.

Figure 12: Napier Deltic

The Deltics were successful, but complex beasts due in part to their gear trains transmitting the power of the three crankshafts to a center output shaft. The exhaust ports covered the external face of the circumference of the liners only because it was out of the question to arrange exhaust manifolds inside the triangle formed by the cylinders banks. This triangle was closed at one end by the phasing and power transmitting gears, and at the other end by the turbo-machinery and its drive. This was quite intricate, especially in the later versions which included a compounded exhaust turbine, mechanically connected by a step-up gear to the output shaft, as well as three intercoolers. These versions (CT18-52B) intended for fast patrol boats, weighed 6200 kg complete with their transmission and were able to develop up to 3100 hp at 2100 rpm.

The Deltic was also employed for main-line rail traction, being installed in 22 locomotives of the same name (or class 55) of the British Railways. They finished their career in 1982 after 20 years of front-line service. These 99 ton CC diesel-electric locomotives were equipped with two Napier Deltic 18-25B engines with a continuous power rating of 1650 hp each at 1500 rpm. They showed outstanding performances and could exceed 160 km/h. Six of them have been preserved in an operational state and still pull charter trains occasionally.

Outstanding tech information about the Deltic is available at



The British Chieftain tanks were powered by two opposed piston engines. A small 1 liter 3 cylinder Coventry-Climax H-30 APU (Auxilliary Power Unit) to drive the generator and a hydraulic pump for starting the main engine. This was a 6 cylinder multi-fuel Leyland L-60 of 19 liters, rather similar to the Junkers Jumo 205, but with cylinder dimensions of 117.6 x (2·146) mm giving a stroke/bore ratio of 1,24. Scavenging was ensured by a Roots blower, quite visible in figure 13. The development of this engine was problematic; thus the specified power of 750 hp at 2100 rpm was reached only on the later versions during the Seventies, and it seems that its reliability was poor.

The Rolls-Royce K-60 is a similar engine but of 6.6 liters for 240 hp at 3750 rpm, and was widely used for smaller military vehicles in the 1960's an 70's.

Figure 13: Leyland L-60

Another 6 cylinder opposed piston engine is produced in the Ukraine by Kharkov Engine Building Design Office (KhkBD) for battle tank propulsion. The displacement of 16.3 liters is obtained by the square dimensions of 120·(2·120) mm. It weighs 1180 kg, measure 1602·955·581 mm and is deliverable in two versions:

6TD-1: 1000 hp at 2800 rpm, 215 g/kW/h, 2500 Nm at 2050 rpm

6TD-2: 1200 hp at 2600 rpm, 218 g/kW/h

Figure 14 : KhkBD 6 TD-1

Various other opposed piston engines existed or are currently in development, among them some with a single crankshaft which were used for automotive propulsion. (File to come soon).

The author expresses special thanks to Philip J. Dingle for the several corrections and suggestions which greatly improved this translation.

Main sources:

- Gastertaedt J., "Junkers Heavy Oil Engines" in The Engineer, Feb. 5th, 1937

- Von Gersdorff & Grassman, "Flugmotoren und Strahltriebwerke"

(© François Dovat)

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Idée & conception © 1999-2011 van Damme Stéphane.

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