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.
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 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 gasoline
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 engine was the most famous of a
series of diesel
engines that were the first, and for more than half a century the only,
successful aircraft diesel
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 opposed-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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