Wednesday, October 04, 2006

Stranger than this it does not get

With a couple of hours of free time in Coventry I had two possibilities. Visit Coventry Cathedral, and have my spirit uplifted by one of the masterpieces of post-war British architecture, or go to the Museum of British Road Transport.

The choice was really no choice. I knew the museum had an example of the Commer TS3 two-stroke diesel engine which has been an object of fascination ever since I gained a rudimentary understanding of engine design.

This remarkable engine was inherited by Rootes Group through their 1950 takeover of Tilling-Stevens, a small lorry and bus manufacturer based in Maidstone, Kent. Although a comparatively small operation, Tilling-Stevens' history could be traced back to last years of the 19th century, and the company had a lifelong reputation for bold and innovative design.

The two-stroke TS3 upheld this tradition with distinction - there cannot be many production engines which so wholeheartedly defy convention in almost every aspect of their design.

First commercially produced in 1954, the Roots-supercharged TS3's opposed piston design entailed each cylinder having two pistons, four connecting rods and a pair of rocker arms finally transmitting motion to a conventional crankshaft situated beneath the cylinder block. The engine's distinguished antecedents were the Junkers 'Jumo 205' aircraft engine of the 1930s and the 1950 British Napier Deltic engine. The latter, an 88 litre, 18 cylinder unit, was developed to power minesweepers, but achieved iconic status in the eponymous railway locomotives.

Despite its complexity, the Commer engine was a reliable and versatile power unit used in trucks, buses and even a late 1950s bespoke racing car transporter built for Ecurie Ecosse by Alexanders of Falkirk. The engine's unique selling proposition was that, by providing power comparable to an eight litre conventional diesel in a much lighter and smaller package, operators could carry more freight within the prescribed GVW limits. The TS3's specific power output, astounding for the time, anticipated today's high-performance diesels by nearly half a century. In its final production form in the early 1970s, the 3.3 litre TS3 developed around 130bhp. The contemporary petrol Rover V8, 200cc larger in capacity, produced 145bhp in its most powerful version.

The opposed-piston design offered the possibility of developing a modular range and although all production versions had three cylinders, a batch of four-cylinder prototypes was developed in the 1960s but never proceeded to series manufacture. Development of the Commer engine ceased following Chrysler's takeover of Rootes' car and commercial vehicle interests. Those who have heard one say it is one of the best sounding engines ever. It is a pity that the baton has never been taken up by another manufacturer - perhaps they are wary of the two-stroke's problems with emissions compliance.

Good ideas abandoned in the past often re-emerge when the time and supporting technology are right - in 1906 Tilling-Stevens introduced their first petrol-electric vehicles, and continued to produce them for the next two decades.
The observant reader may have noticed that such 'hybrid' powertrains have become something of a cause celebre of late...

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