Vintage Thing No.12 - The Trojan engine

Cut away engines seem wrong to me but are pretty well essential in understanding the workings of the normally aspirated Trojan split-single two-stroke. This the view from above.

A mate of mine sent me this illustration of a cut away Trojan engine the other day. This was in response to my earlier post (VT 4) about a supercharged Trojan engine. Well, I still know nothing about the blown sort but here's a pic of the common or garden variety.

Although it looks like a twin, the Trojan was a split-single. Yeah, I know, it's a contradiction in terms. Think of it as a saimese twin then. And they weren't split-singles, either. They were four cylinder split twins!

Still with me?

Some people call split-singles twingles so what would the quad-twin Trojan be - a quin?

But why bother with split singles, or twingles, in the first place?

The big problem with two strokes is spent exhaust gases mingling with a fresh incoming charge of fuel into the cylinder. Two strokes don't have separate mechanical valves, they have ports in the cylinder that the piston uncovers and rely on pressure waves and black magic to go really well. And much of this witchcraft centres around scavenging - the art of getting all the spent gases out of the cylinder without losing it down the exhaust pipe.

Leslie Hounsfield designed the Trojan as utilitarian transport but still needed good scavenging. He obtained it by using two pistons running side by side. The incoming mixture comes in through a common inlet port under the pistons and they squeeze it down into the crankcase by the pistons. As the pistons reach the bottom of their stroke, a transfer port into the cylinder wall is uncovered and the fuel/air mixture rushes in to fill the partial vacuum in the upper cylinder. The transfer ports are closed as the pistons come back up and the incoming charge is ignited in the combustion chamber, which is shared by both pistons. As the pistons go down the cylinders under the force of the explosion, the exhaust port is opened.

This the view from the side. There are four pistons running in two pairs of split cylinders(!)

Thorough scavenging is the result since the incoming charge has to cross both cylinders before it reaches the exhaust port. And by then it's given fulfilled its destiny in the combustion process and isn't wanted in the cylinder anyway.

The vee-shaped conrod that connects the pistons results in asymmetrical port timing since one piston will always be ahead of the other, even at top and bottom dead centre. They might be level but only for an instant as one is going down when the others coming up.

Another consequence of this layout is that both the transfer port and the exhaust port can stay open for the same duration, which again leads to a well scavenged and well filled cylinder. The transfer period can be timed to begin say 10 degrees of crankshaft rotation after the exhaust port has opened and then close a similar interval after the exhaust port has closed.

The duplication of parts in a split-single means they are more expensive to produce than a conventional engine and more moving parts mean more friction. Mind you, there were only Also, the exhaust piston runs a lot hotter than the inlet piston so Hounsfield chose water cooling for the Trojan. It had a capacity of 1488cc and produced 10 bhp but had a flat torque curve, prodigious hill climbing abilities and was cheap to run.

The Trojan slogan at the time was "Can you afford to walk?" They were cheap to run and appealed to frugally minded (read tight) motorists during the years of the Depression.

I like two strokes because they have a power stroke twice as often as a four stroke. Theoretically they should be twice as powerful as a four stroke. Other people like two strokes because - theoretically - they should use half the fuel for the same power output. It's two sides of the same coin but we all agree that they are lighter and simpler but, generally speaking, smokier and noisier.

The Trojan engine lived under the floorboards of the Trojan car and that under te floor boards power output seems like a challenge to a Horsepower Whisperer like Nick Hob.

Are the MCC Trojans that compete today in long distance trials hopped up in anyway? I suspect not. They are reliability events and the long stroke of the Trojan engine would encourage either low revs or the tangling of those gangly conrods.

I am assured that for the Trojan layout to work properly the conrods have to flex slightly, which doesn't sound right - conventional practice demands they should be as stiff as possible

I still don't know any more about the blown variety. It's obvious Hounsfield got his idea for the supercharged Trojan from his multi-conrod layout single but what did he do with this supercharged engine? And could anyone do better than 24 bhp?