Ever hear of Norman Hossack? Of course you have. He designed the Hossack suspension, often confused with the far more basic girder front suspension and adapted by BMW under the name Duolever, but the Hossack engine? What's that? Well, it's just your average square piston two stroke, that's all.
When I received an email from Norman, I read through his description of the engine and couldn't ever recall hearing about this. He included a number of images and videos and it looked pretty interesting and I thought all of you engine buffs might like to hear more.
It seems Norman Hossack started thinking about this idea, way back in 1969 while still living in Rhodesia. He moved to the UK and spent several years working for McLaren as a mechanic and was part of their 1974 Indy 500 team, but kept thinking about his engine, so he worked out the design and found some assistance getting the patterns made and the engine cast. After putting it all together, the engine ran, but no one was showing much interest, so he shelved the project and moved on, he had some ideas about motorcycle suspension he wanted to develop. Last year, he dusted off the old engine and carefully rebuilt it, installing a new carburetor while paying special attention to the seals, but keeping everything else as it was. The engine runs, quite well, actually, and is currently installed in a pocket bike.
You have to watch the video, but the piston has no wrist pin, it rocks side to side as the rod travels around the crankshaft, giving this engine its unique character. Very interesting.
It's probably better if I just let Norman explain the details himself:
The HOSSACK engine is an internal combustion engine that employs a novel set of internal components and works on the 2-stroke principal. The 2-stroke type of engine has fallen out of popularity in recent years for several reasons. Prime amongst these is low specific output due to the fact that these designs waste up to 30 percent of their fuel charge unburned through the exhaust. The HOSSACK engine design seeks to correct these losses and gain from some of the advantages that the 2-stroke design offers, namely low weight, low part count and more power strokes per cycle.
The HOSSACK engine crank and the lower end of the connecting rod are the same as any conventional 2-stroke engine. However from there on up to the combustion chamber things are very different. The working chamber that would normally be considered the cylinder is now rectangular in cross-section and the connecting rod and lobe (piston) becomes one fixed item. There is no wrist pin. The crown of the lobe rocks within the working chamber. This means that to maintain a shape that fills the chamber, it has 2 radii opposing each other and these radii provide the running faces. The center point of these 2 radii provide a focal point for the strip seal that seal against the chamber wall opposite the loaded running face.
There are several gains to be had from this format.
Prime among these is the ability to make the port timing asymmetric. Conventionally piston ported 2-stroke engines open the exhaust port first followed by the transfer ports. This is an acceptable sequence but as the cycle continues the transfer ports close before the exhaust and because of this up to 30 percent, by some studies of the fresh unburned charge is lost down the exhaust port. The HOSSACK design seeks to avoid this loss by varying this sequence. This is achieved by positioning the ports on different sides of the lobe. In this way the transfer ports can open later and close later and the exhaust can open sooner and close sooner. By this it is claimed the losses normally associated with 2-strokes can be limited.
Secondly the HOSSACK engine having dispensed with the wrist pin and its associated structures can be built much lighter. The normal wrist pin requires a strong support structure at the proximal end of the connecting rod as well as a beefy structure in the piston. Add to this the retaining circlips and a bearing and the weight of the rod itself. The HOSSACK engine provides a lighter structure allowing for higher engine RPM or lighter construction.
Thirdly in a conventional engine all these just mentioned masses stop and start 2 times every cycle. At TDC and at BDC these masses decelerate to and accelerate from a complete stop. The HOSSACK design changes this by virtue of its shape. The lobe never stops. It rocks. There is no instant in which the lobe is stationary.
It takes up to 30 degrees of crank rotation for the total mass of the lobe to change direction over TDC and up to 40 degrees of crank rotation over BDC. By this change in the reciprocating pattern the end loads generated are reduced which could lead again to a lighter component or higher RPM.
Norman thinks the engine has a lot of potential and would like to see some academic involvement to analyze its operation. He believes the first engine has run well enough to justify further research. His thought is the engine would be ideal if developed for small engine applications like drone aircraft or lightweight military power packs.
I think it's very interesting and innovative. Square pistons! Don't forget, this was all pre-CAD and pre-CNC, just drawings and machining the old school way. I like it.