The Hossack Engine – a Square Piston Two Stroke

Hossack engine - square piston 2 stroke

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.

Hossack engine - square piston 2 stroke

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.

Videos below:

Link: Hossack Design
Link: Hossack youtube channel

More from Norman Hossack on The Kneeslider:
Hossack Trellis front suspension for Ducati 800
Front drive system for Hossack front suspension


  1. Joe Bar says

    Drone use is out, at least for military. All military engines must run on diesel/JP5.

    • GuitarSlinger says

      Seeing as how its a two stroke , it wouldn’t take much at all to convert the engine to diesel

      Hmmn ….

      • Hooligan says

        The Science Museum in London has some examples of two stroke diesels that the Germans tried to develop for aircraft use in Bombers.
        This is the third time I have mentioned two stroke diesels recently. What is wrong with me?.

      • Joe Bar says

        Yes, I have run several model airplane diesels. All rely on volatile fuel components like ether, and a contra-piston to run.

        I am not sure how practicle conversion of the Hossack engine would be. US military drone power is moving towards small turbine power.

    • Travis says

      considering the military’s useage of battery operated drones, not all drones run on JP8.

    • jagoo says

      Isn’t this just the same approach though, by saying that a certain technology or method must be employed, that Norman Hossack’s design innovations seek to dispel by taking an open minded look at the way in which we operate?

      Just as the Hossack motorcycle suspension designs were rejected through a rather short sighted view of the proposals and ultimately, a more carefully considered approach to the proposals has allowed BMW to advance technology and expand within that industry.

      Let’s not get stuck in a rut simply because “this is the way we do things”!


  2. BenK says

    Interesting idea from Norman, as always. Up there with Tony Foale as an underrated British shed genius.

    There are a few areas that I’m curious about, mainly to do with sealing. Providing a gas-tight seal against a flat face is quite tricky, and tends to entail significant increases in friction. This is particularly problematic, as the ring/bore interface is responsible for for the largest part of the engine’s total friction. This facet has been the downfall of numerous rotary, vane and flap engines (both steam and IC), and was the main source of difficulty for many years with the Wankel. As Mazda’s experience shows, these can be addressed, but would probably need to be a significant area of development if the engine was to be progessed.

    Interestingly Honda holds (held? seem to remember they were due to expire around now) a series of patents on just this area relating to the development of the NR500 and 750. The flat faces of the ‘spamcan’ pistons presented a lot of similar issues.

    From the illustrations it’s not clear how the other faces are sealed. The motion of the piston imparts a twist on this axis at TDC and BDC which one would think would impart a lot of stress on the seals.

    None of this is intended as criticism; I certainly haven’t got the skills or knowledge to build my own engine, and my understanding of engineering is essentially that of the curious layman. Just points that didn’t seem immediately obvious.

    • GenWaylaid says

      Those face seals make me dismayed. Mr. Hossack has demonstrated that he can make an engine that combines the balance problems of a reciprocating design with the sealing problems of a rotary design. I would ask him to try applying his talents in the opposite direction!

  3. EGeek says


    I’d like more info on the port timing and how it stops the flow of raw fuel air out.

    After that I’d like to see it compared to the Husqvarna “X-Torq” and it’s unique porting, charging port and piston in regards to emissions and BSFC specifics…

    • O'malley says

      The X-torq and similar designs are a very good addition to two strokes, however they are exactly that, an addition.
      Two strokes can be made more efficient but the trade off is they become more complicated, which somewhat defeats the purpose of a two stroke in the first place.
      Has anyone here ever thought of making two strokes (kind of) simpler? It is a very different approach to all other designs
      Not to say a comparison wouldn’t be cool to see though XD

  4. john says

    The side loads on the edge of the piston are going to be problematic, I think. Just a hunch. It is basically operating with a controlled exaggerated piston slap. In a cylindrical bore, this would be horrible because it will wear a cylinder out of round. But in this motor doesn’t have round bores. Maybe some wear on the cylinder sides would not have as much impact on ring seal.

  5. BoxerFanatic says


    But with edge seals and apex seals… I think I’d rather go with a wankel rotary, and just lose the reciprocation almost altogether.

    I am kind of wishing small rotary engines would make a comeback to motorcycling.

    With the idea of an electric traction motor, and on-board electrical generation, a rotary paired as a generator, or even co-built AS a generator, makes some sense.

    I really want a BMW K1200R-Sport, with Hossack suspension… but this engine seems like one should just go further to a wankel rotary.

  6. Sick Cylinder says

    I picture this motor with ceramic or special plastic parts to overcome the friction / seal problems. I remember reading about IC engines which would not need cooling systems because of the use of special materials – I gues we are not there with those materials yet.

    I also imagined this engine running on compressed air or carbon dioxide – I don’t know if torpedoes still use that technology, but they used to use small piston engines powered by gas. Perhaps this engine made of teflon and running on compressed air powering a torpedo.

    Regardless of itspossible use, it is a very interesting design and hope a University takes time to investigate this. What about contacting the University of Belfast – I beleive they were experts in two stroke technology and used to tune the 250cc GP bike ridden by Jeremy McWilliams.

    • Mean Monkey says

      @Sick Cylinder
      The late, great Indy car builder, Smokey Yunick had a fuel efficient ceramic engine running about 25 yrs ago, but couldn’t get serious support from anybody. The auto manufacturers wanted to buy up his design and prototypes, but he refused to sell. Smokey knew that they would just sit on the project to make it die.

      Maybe we need hoardes of invading Hossacks (on Hossback, of course) to shake the auto industry.

  7. Scotduke says

    It’s an interesting idea and I’ve no doubt it could be made to work. The questions are how much it would cost to make it work and how efficient would it be?

  8. B50 Jim says

    Mean Monkey–

    Smokey Yunick was a brilliant, self-taught engineer, but his biggest talent was pulling the wool over observers’ eyes. Part innovative engineer, part showman, part huckster and all competitor, Smokey could convince reporters that his creations would generate 800 horsepower, deliver 80 mpg and run using no oil or cooling system. Smokey knew the best way to tell a lie was to tell part of the truth. His ceramic “adiabatic” engine might have delivered some of the performance he claimed, but it probably had enough serious drawbacks to make it unsuitable for production. Remember he spent most of his career as a race-car builder and owner looking for ways to circumvent the rules and gain an advantage on the others. Savvy observers knew that ol’ Smokey always was trying to put one over on them. Sure, he held several patents and was Chevrolet’s unofficial racing shop; his ideas and designs often were technologically brilliant, and he could demonstrate that a theory was workable, but he always followed P.T. Barnum’s adage that a sucker was born every minute, figuring that enough of his schemes would work well enough that someone would pick them up and cough up the R&D money to try and take them to production. I take nothing from Smokey. He was one of racing’s most colorful characters and I loved reading of his exploits, but his brilliance was more in generating ideas that had great potential but ultimately were unworkable in volume production. Some of his ideas did find their way into manufacturer’s products, but once those ideas made it to production they had to be diluted sufficiently to make them workable, rendering them much less effective than the original concept. What works on the track usually doesn’t translate to everyday driving. That’s why there are so many innovative, even brilliant, concept engines and other components out there that never make it to the market.

    • Wave says

      If you don’t have crankcase compression on a two-stroke engine, then you need some other means of compressing the intake air. There are several ways of doing this. Some two strokes, like a lot of diesels, use a roots-type supercharger to do this. In the past, some engines like the Ricardo “Dolphin” engine have used a second cylinder to compress the air for the firing cylinder,

      Anyone interested in unique engines should check out a book called “Some Unusual Engines”, by L.J.K Setright. It was published in 1975, so might be hard to find, but I borrowed it from my university library recently and it’s fascinating!

    • Tin Man 2 says

      I remember reading the original article in Hot Rod back in the day, Snake Oil comes to mind. The Detonation was hidden by a rattley exhaust system and the test was controlled by Smokey on his terms. The Laws of Physics apply to Smokey just like everyone else.

  9. says

    Looks like a design that would work well with short stroke big bore, 39.4mm stroke 50mm bore, less rocking of the piston, less stress on sealing system, less stress at connection of conrod and piston. A 90mm rod so a larger clearence between crank and piston for full crank reed induction. not sure how the piston is going to handle high revs, but hay it looks like an interesting design.

  10. Nortley says

    Those inside corners look like good places to gather residue and start a hot spot to upset ignition timing. But, for an engine intended to be cheap and short lived, that shouldn’t be a problem.

  11. Thomaskwscott says

    Just visualising in my head (the most inaccurate modelling tool by far) but i don’t think it needs to be square. It should be possible to construct a barrel that would allow for an oval piston though it may need to allow for movement of the piston perpendicular to the conrod (not pivoting). Am i nuts?

    p.s. i love 2 strokes and this is amazing, where can i get mine?

  12. says

    “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.”

    But would you still like it if he started using CAD and CAM in further developement?

  13. Steve says

    Those little seals look a whole lot like apex seals and we all know how that works out. I do think its a cool design though. I’d like to see a 2 stroke that utilizes something where a re-ring / rebuild wouldn’t be needed so often without sacrificing power, but that seems like a dream.

  14. Brad says

    Why not a round (spherical) pistol in a round cylinder like this? Sealing would still be a problem but it would solve the corner problems in the bore?