Wednesday 17 June 2015

Opposed-piston engines: here we go again

The arrival of Cummins Inc. on the scene has once more thrown the limelight onto opposed-piston two-stroke diesel engines for military fighting vehicles.
Whether these new ‘headless wonders’ use compacted graphite iron (CGI) for strength, lightness and low noise remains to be seen. But the opposed-piston engine is not new even though today they are described as ‘new’ and ‘revolutionary’.

And they are being proposed for passenger cars and light commercial vehicles, as well as heavy commercials.

But in the latest twist, fighting vehicles have entered the fray with a $14 million contract from the US Army implying their use in armoured vehicles, such as battle tanks.

There appears to be two main contenders, although there may be others yet to emerge. These are Achates Power Inc. of San Diego, California and EcoMotors Inc. of Allen Park, Michigan. Which will have an engine in production first as yet remains to be seen.
                               British and German development

A look at past history is perhaps not out of place, especially as various opposed-piston engines have been developed in the UK over the 70 years, most notably by Rolls-Royce (K60) and Leyland Vehicles (L60) for fighting vehicle application in the 1960s. The K60 appeared in the British Chieftain battle tank and the British Army’s FV432 military vehicle, as well as the Swedish S tank.

There was also the TS3, known as the ‘Commer Rocker’, for use in commercial vehicles. The Commer TS3 was a three-cylinder, two-stroke diesel engine fitted in Commer trucks built by Rootes Group in the 1950s and 1960s. The engine used a Roots blower for supercharging. Largely the product of Tilling-Stevens, the engine was developed by Rootes Group when it bought out Tilling-Stevens. The engine was the first diesel engine used by Rootes Group and was of unorthodox design.

Rootes' intention for the engine was to produce a new range of Commer trucks with the modern "cab forward" design, which required an engine low enough to mount under the driver's cab rather than in front of it as previously.

Eric W Coy, Rootes' chief engineer, was responsible for the development of the engine by a core team of seven people at the Humber plant at Stoke HYPERLINK ""Aldermoor. "TS" in the engine's name derives from its Tilling-Stevens, acquired by Rootes in 1950. From 1954 Rootes diesel production was moved to the Tilling-Stevens plant in Maidstone, Kent.

In addition to the three-cylinder two-stroke engine, a four-cylinder version (the TS4) was designed and a few prototypes built. This engine did not enter production however. Engineers familiar with the TS3 engine recall fuel consumption as being poor and emissions disappointing; also the engine was far from quiet.

The TS3 developed 125bhp (gross) at 2,400rev/min. The torque amounted to 431Nm (318lbft) at 1,250rev/min. The swept volume of the engine was 3.52 litres or 215 cubic inches. The declared dry weight of the engine was 475kg or 1,048lb.

Another British company which built two-stroke engines (but not opposed-piston designs) was Foden of Sandbach, Cheshire. Detroit Diesel in the US also manufactured two-stroke engine for use in commercial vehicles.

                                      Opposed-piston engines

Opposed-piston engines are not new. The most famous British opposed-piston engine is the Deltic diesel engine designed and developed by D. Napier & Son of Acton, London.

The Deltic story began in 1943 when the British Admiralty set up a committee to develop a high-power, lightweight diesel engine for Motor Torpedo Boats. Hitherto in the Royal Navy, such boats had been driven by gasolineHYPERLINK "" engines, but this fuel being highly flammable, made them vulnerable to fire, and at a disadvantage compared with the German diesel-powered E-boats.

Until this time, diesel engines had poor power-to-weight ratio and low speed. Before the Second World War, Napier had been working on an aviation diesel design known as the Culverin after licensing versions of the Junkers Jumo 204.

The Culverin was an opposed-piston two-stroke design. Instead of each cylinder having a single piston and being closed at one end with a cylinder head, the Jumo-based design used an elongated cylinder containing two pistons moving in opposite directions towards the centre. This negated the need for a heavy cylinder head, as the opposing piston filled this role, hence the term ‘headless’,

On the downside, the layout required separate crankshafts on each end of the engine, and some form of gearing to take off power and combine it into a single shaft. The primary advantage of the design was that it led to a rather "flat" engine, intended to be buried in the wings of large aircraft.

The British Admiralty required a much more powerful engine, and knew of Junkers' designs for multi-crankshaft engines of straight six and diamond-form. The Admiralty felt that these would be a reasonable starting point for the larger design which it required.

The result was a triangle, the cylinder banks forming the sides, and tipped by three crankshafts, one at each apex. The crankshafts were connected with phasing gears to drive one output shaft. In this arrangement, there were six banks of pistons driving three crankshafts, the same as three separate V-engines of the same overall size.

Various models of Deltic engine could be produced with varying numbers of cylinders, though nine and eighteen cylinders were the most common, having three and six cylinders per bank respectively. In 1946, the Admiralty placed a contract with the English Electric Company, parent of Napier, to develop this engine.

One feature of the engine was the way that crankshaft-phasing was arranged to allow for exhaust port lead and inlet port lag. These engines were described as uniflow designs, because the flow of gas into and out of the cylinder is one way, assisted by mild supercharging to improve cylinder exhaust scavenging.

Although the engine was cylinder-ported and required no poppet valves, each bank had a camshaft, driven at crankshaft speed. This was used solely to drive the fuel injection pumps, each cylinder having its own injector and pump, driven by its own cam lobe.

The Deltic saw service in locomotives and military vessels. While the Deltic engine was reputedly successful and powerful for its size and weight, it was a highly-strung unit, requiring careful maintenance. This led to a policy of unit replacement rather than repair in situ.

Deltic engines were easily removed after breakdown, generally being sent back to the manufacturer for repair. However, after initial contracts had expired both the Royal Navy and British Railways set up their own workshops for overhauls.

                                   Turbo-compound Deltic

The E.185 or "Compound Deltic" turbo-compound variant was planned and a prototype built in 1956 and tested in 1957. This capitalised on Napier's experience with both the Nomad aero engine – itself a compound engine of gas turbine and piston engine – and its increasing involvement with gas turbines.

It used the Deltic as the gas generator inside a gas turbine, with both a twelve-stage axial compressor and a three stage gas turbine. Unlike the Nomad, this turbine was not mechanically coupled to the crankshaft, but merely drove the compressor. It was hoped that it would produce 6,000 bhp, with fuel economy and power-to-weight ratio "second to none". 

Predictions by engineers closely connected with it were that connecting rod failure would be the limit on this power, failing at around 5,300 bhp. On test it actually produced 5,600 bhp before throwing a connecting rod through the crankcase just as predicted. 

Naval interest had waned by 1958 in favour of the pure gas turbine, despite its heavier fuel consumption, and no further development was carried out.

                                  Junkers Jumo engines

Perhaps the most famous opposed-piston engine is the German Junkers Jumo design. The Junker Jumo 205 aircraft enginwas the most famous of a series of dHYPERLINK ""iesel engines that were the first, and for more than half a century the only, successful aircraft dHYPERLINK ""iesel engines.

The Jumo 204 first entered service in 1932. Later engines in the series were styled Jumo 206, Jumo 207 and Jumo 208, and differed in stroke and bore and supercharging arrangements. In all more than 900 of these engines were produced.

These engines all used a two-stroke cycle with 12 pistons sharing six cylinders, piston-head to piston-head in an opposedHYPERLINK ""-HYPERLINK ""piston configuration. The configuration required two crankshafts, one at the bottom of the cylinder block and the other at the top, geared together. The pistons moved towards each other during the operating cycle.

Intake and exhaust manifolds were duplicated on each side of the block. There were two cam-operated injection pumps per cylinder, each feeding two nozzles, for four nozzles per cylinder in all.

A main feature of both the Deltic and the Junkers designs were their extreme complexity.

                                  Modern engines

Protagonists of modern opposed-piston two-stroke diesel engines claim they are simple, economical and ideal in meeting future emissions requirements.

On 4 September 2013, Zhongding Power and EcoMotors announced in a statement  that a “commercial agreement to produce the opoc engine, a breakthrough technology that has the potential to be the world’s cleanest, most efficient, lightweight, and lowest cost internal combustion engine.  One of the largest automotive component conglomerates in China, Zhongding will finance and construct the first opoc plant in the Anhui Province.  The plant represents an investment by Zhongding of over US $200 million and will have a capacity of about 150,000 engines per year – over US $1 billion in revenue potential.  High-volume production is expected to begin in 2014.”

The plant is intended to make the EcoMotors opposed piston (opoc) engine which, according to EcoMotors, has the potential to be the world’s cleanest, most efficient, lightweight, and lowest cost internal combustion engine, with the potential for 20 to 50 per cent better fuel economy compared to a state-of-the-art diesel engine. 

But now Achates Power Inc. has teamed with Cummins in the latest turn of the screw. It now remains to be seen what happens next.

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