The Commer Double Knocker

Monday, August 18, 2014

Tags :  con-rods

This article is unusual in that it looks at a concept that will almost certainly never be applied to a race engine but is technically interesting enough to merit examination. In the 1950s a British truck maker made a production engine the likes of which we will never see again. Commer, a name that disappeared many years ago, came along with an engine called the TS3, but widely known as the Double Knocker.

It was a three-cylinder two-stroke diesel engine with a single crankshaft, six pistons and 12 con rods, plus six rockers that formed part of the cranktrain. Two-stroke diesels are an interesting concept; we don’t see them widely used but they are among the most efficient internal combustion engines. Even the latest energy recovery systems used in Formula One – which have been nothing short of a revolution in fuel efficiency in motorsport – cannot easily compete with a well developed two-stroke diesel. Brake thermal efficiencies in excess of 45% are possible for power units developed for commercial transport, and huge stationary engines can achieve considerably higher figures than this.

In the TS3 engine, each pair of pistons moved towards each other in a cylinder and, compared to the similar concept of opposed pistons used on the Napier Deltic (which used three crankshafts to do the same thing in a much larger package size) the Commer was a very tidy and compact design in terms of packaging. The link at the end of the article shows the engine in section, and if you refer to this or another sectioned drawing of the engine, it will make the process of understanding this unique engine much easier.

The inlet charge for each cylinder was compressed by a pair of pistons. Each piston had a short con rod attached to a very substantial rocker. In turn, the rocker was connected to the crankshaft by a further con rod. The rocker appears not to have had a 1:1 ratio – that is, the piston stroke was larger than the crankshaft stroke. The rocker also had some offset, which was necessary as the pistons in each cylinder clearly needed to be coaxial, but the motion was imparted by separate throws on the crankshaft spaced 180° apart.

So, in the cranktrain, we have a much more complex arrangement than in a normal engine, but this is offset in an opposed piston engine design by the fact that the engine has no cylinder head.

Does this have any relevance to race engine designers? Some years ago, when the possibility of hybrid systems was first being considered in Formula One, and when I was working on Formula One engine development, I took part in a discussion of what we might do if given a set amount of energy to start a race with, and no other restrictions. A turbocharged two-stroke diesel engine was mentioned at the time. If fitted with systems for energy recovery and running at near-constant speed, powering a generator, it might prove to be a  good solution. In terms of packaging, an opposed piston design such as the Commer Double Knocker might well be a good solution.

At the moment though it seems unlikely that this sort of power unit would be used in any kind of motorsport, but for passenger and freight applications a similar concept could prove to be a better solution to the ‘range extender’ engines now being developed for electric vehicles.

For sectioned views of the engine go to:

Written by Wayne Ward

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I used to work on this engine in the early 70's.  They would wear out the pins at the end of the rockers and sound like two skeletons turning over in a tin bath, especially on cold start.  The front-mounted Rootes type supercharger was driven by a long shaft that passed through the bock to a rear mounted gear drive.  Occasionally the supercharger would overheat and seize, snapping the shaft.  I sometimes ponder if modern lubricants and/or surface engineering would have overcome it's problems.