Startup: Commer Knocker!

It’s not normally made of plexiglas…

Ever since I did an article last year about the Ten Craziest Old Engines over on Hooniverse, this particular one has stuck in my head. It’s called the “Commer Knocker”. It’s a three-cylinder, two-stroke, six piston, horizontally opposed, direct-injected, high-revolution diesel engine. Yeah, I know.

It’s a bloody hard one to understand, in a way, but it utilizes opposed pistons, whose movement also causes the valves to open and close. Kind of. The pistons are horizontally opposed, but connected to a common crankshaft by way of rocker arms, in much the same way that the valvetrain is connected to the camshaft on a conventional pushrod engine. Kind of.

It’s one of those moments of engineering that make you sit back and think, “And you did this… why?” After all, there were boxer and flat-four or flat-six engines in production at the time; since the motivation was to try and create an engine that would be nice and flat to accommodate the new “cab-forward” (or what we North Americans would call “cabover” or “COE”) trucks they were developing, one wonders why it wasn’t possible to develop a boxer diesel.

You know, besides the horrible experiences that Subaru has had with theirs in the last few years.

Sounds pretty cool though:


  • dwegmull

    I wonder if this arrangement causes the intake side to be always a bit cooler than the exhaust like a uniflow steam engine… This alone could improve efficiency over a conventional design although I'm not sure it would warrant the added complexity.

  • Number_Six

    I'm curious: did it work with any sort of reliability or did the Commer Knocker become a mechanic's punchline? Given British dogged pursuit of things that don't work very well, it would be interesting to know what these things were like to live with.

    • Well, it was fitted to Commers and Karriers through the 50's and 60's, It can't have been too bad, unless Commer insisted in persistently forcing underdeveloped products down our throats like, er, every other sector of the UK motor industry used to…

      • Number_Six

        My dear departed Grandad had a fleet of them but when he rambled on about the old days I always wanted to know about his Landies, DAFs, Austin 7s, and the airfield he ran during WWdeux, not the daily workings of his hire plant. I'll probably see some of his old crew in a couple of weeks and see if I can get them talking.

        • Get the brown beers flowing. I'll bet they have a story or five to tell you.

    • Deartháir

      Apparently they worked quite well!

      Although, now that I think of it, I believe they worked quite well in British terms. Which could, admittedly, put reliability anywhere from "broken" to "usually broken".

      • Oh, don't try to cover for them. You know full well the actual range is from "perhaps inert" to "aggressively attempting to maim and/or kill its owner."

        • Deartháir

          I think we need to develop an actual scale for British reliability; applies to vehicles, appliances and non-comedic television shows and movies. I suspect most of the latter would be towards the lower end of your scale.

          • Am I guessing that only the lower portion of the scale need be calibrated decimally, you know, for accuracy? In fact after three or so you may as well round it straight to ten.

          • Mr_Biggles

            "Warm Beer" pretty much sums it up, no?

      • Hey, at least they're consistent! Steely British resolve to the last!

  • I believe this was one of my favorite engines of the Ten Craziest Old Engines, for the sheer fact that it is so goddamned crazy. I love it.

  • Wait…. what?

  • Pingback: The UK and the Napier Nomad vs. the US : Atomic Toasters()

  • Opposed piston engines are huge on the funk factor. Here's one I saw last fall, it was fabricated by the owner from some old blueprints that he ran across. Of course, the hit-and-miss engines are pretty cool, too. Maybe there should be a separate article on them here on Atomic Toasters. [youtube li_RcEolXHI youtube]

  • discontinuuity

    Ecomotors is building a similar engine, and it looks like it will have performance advantages over a traditional 4-stroke.

  • Manousos Pattakos

    Here is PatOP, another Opposed Piston engine.

    [youtube j53v75mJj_4 youtube]

  • texlenin

    That sounds like a perfect replacement engine for a Tatra 77/87/603. I was going to mention Ecotech as well. What about an article on
    the MITY engine from a few years back? Or toroidal cylinders in general.

  • franly devaere

    here is a toroidal opposed engine :
    [youtube AM335uWoYBQ youtube]

    • texlenin

      Does your website have an English version? So yours has 8 combustion chambers, holding 16 opposed cylinders? Do I have that right? I also looked deeply into Reisser's CHB engine; some interesting ideas there as well. Stackable, for one. This may not be the best forum for in-depth discussion; that will be up to the moderators.
      That being said, I'd be happy to volunteer my old Datsun 280Z as a long-term test-bed vehicle. 😉

  • Ploughguy

    Now why don't you all leave it alone, poor thing. I did a 600 mile trip in a Commer-knocker powered semi when I was a kid and I have been in love with the sound ever since. And the design has benefits – no head, crankshaft or valves, and the piston linkage load is half what it would otherwise be. And it is a two stroke so you get twice the power for the revs, more or less. And they sound great. The nearest thing I have heard to it, satisfaction-wise, is the 5 cylinder diesel in my Landrover.

    It would be interesting to get someone with actual knowledge to compare the cost of running the head and two wasted strokes (or the extra half an engine) on a conventional donk to the cost of running the rockers and the scavenging blower on a knocker. It feels like it should be way more efficient. Certainly the Deltics seem to have been a great success.

    I am impressed that an engine made by the Rootes Group has a Roots blower which is spelled differently, comes from a completely different root (yok!) and even a different country.

    As for why they built it? Admit it – if someone showed you the idea and offered you a chance to have a go at it, would you say no?

  • Paul Carson

    Interesting article and I have known about Commer Knockers (saw them often in Australia about 40/50 years ago. They were common. Didn't know the engine worked this way though. It's ingenious! 90 BHP from 3 liters that otherwise required 8 liters. So why doesn't someone build these now if they are so powerful? The Hillman Car Club explanation makes it all easy to understand, but the animation they provide could be slower! And I was looking for an explanation as to why they knock.

  • John

    Sounds similar to GM 2 stroke diesels and EMD's. Opposed piston 2 stroke design was used in Doxford and Burmeister & Waine marine diesels.