Vintage Thing No.4 - The supercharged two stroke Trojan engine

Just a glance at this drawing had me intrigued

My web host Eric Cowan lent me a textbook entitled The Motor Vehicle by Newton and Steeds over Christmas. And there among its pages was a cut away diagram of a supercharged two-stroke four cylinder Trojan engine.

This raises more questions. I've looked at the engine of a typical Trojan and this is nothing like what lurks beneath their floorboards. They're mid-engined and the cylinders lie down flat.

There's no date of production but the book is the 1953 edition, originally published in 1929. There are two split cylinders and within these are two pistons and two conrods that share a common combustion chamber, which was a feature of other, more normal Trojans where more normal Trojans is an entirely relative term.

With a bore and stroke of 65.5 mm by 88 mm, the capacity of the power cylinders is a nominal 1186cc, since the offset cylinder axes vary the strokes slightly. The two compressor cylinders measure 92.6 mm again with an 88 mm stroke, giving a swept volume of 1293cc and 9% displacement margin over the cylinders they feed. All the conrods are the same length and interchangeable and the two pumping cylinders are clearly air-cooled, while the - for want of a better description - combustion cylinders are water-cooled.Also, the pumping pistons are cast iron while the combustion cylinders are aluminium alloy.

There's a single transfer pipe and twinned inlet ports for each set of pumping and burning cylinders. Number 1 charging cylinder feeds the number 2 pair of power cylinders and automatic cage valves, which operate a bit like reed valves on yer two-stroke Yamahas and Suzukis in the seventies, feed the compressor cylinders.

For a better view, click on the images.

Trojan automatic valves and cage

The offset geometry determines the port timing and reduces side thrust on the delivery stroke. According to the text of Newton and Steeds, the stroke timing is advanced 8º relative to the 90º angle between the vee. The effect of the cylinder offsets, combined with the positions of the leading edges of the ports, gives a lead to the exhaust period of "about" 22º and an overlap of about 83º. This slight vagueness to the timing is part of the charm of this engine together with the nominal cubic capacity. The  duration of the exhaust period is 106º compared to 104º for the inlet.

The happy smiley line at the bottom of the graph is the fuel consumption. Much lower and it would be below the axis of the graph, which would mean the powerplant would be a net producer of fuel. Of course, this is unlikely but from my understanding of quantum physics, entirely possible.

There is a power curve and I include it out of interest as it's not a complete contradiction in terms. It revved up 2,500 rpm and pumped out a maximum of 24 bhp. Much was made of the flat torque curve so it's understandable why they were so good at climbing hills, albeit slowly.

According to G N Georgano, later Trojans were rear engined. A six-cylinder 2.2-litre prototype was unveiled at the 1935 Motor Show but this never went into production. I can find no mention anywhere of Trojan using superchargers!

As for the constructional details of normal two-stroke Trojan, Colyn Thomas and I have a pretty good idea of how the plot works but are still searching for a cut away view like this one to enlighten us.