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Thread: Smallest model true diesel?

  1. #1
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    Default Smallest model true diesel?

    I have been looking around for the smallest true diesel models, and what I find seems to be about 30mm cylinder diameter as the smallest I see.Possibly that is due to the heat losses and the problem of the injector.

    I mean true diesel operating on compression ignited kerosene (or light diesel fuel) with an injector, not the carbureted model airplane "diesel" engines.

    There is also the Hvid type, which is in between the semi-diesel and true diesel, with maybe slightly less injector trouble, since it does not use a high pressure injector. I do not know of any Hvid types modeled, although hot bulb engines have been made, especially by Find Hansen in Denmark Even those are fairly large.

    Spark engines have been built as small as 4 cylinders in a block maybe 2 inches long, but they do not have the same heat loss issues.

    Anyone know of smaller examples of true diesel, significantly below a 30mm or so bore?
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    Hashim Khan

  2. #2
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    You gonna build one?

    I'd think it mostly all depends on how small you can make an injector. Does a nozzle and pump count as an injector? Does it have to mechanically or electronically inject fuel? Direct injection a necessity too? If not a carb isn't very far off a "fuel injector", it just needs to have pressure behind the fuel instead of using vacuum.

    There is quite a lot of variance in diesel engines over the years and styles.
    Andy

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    We had a diesel piledrivef building a bridge here a while back..
    And there has been gas pogo sticks.. maybe you could do one with diesel ?

  4. #4
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    Most successful models seem to use a "pintle" injector, pressure activated, as opposed to a common rail valve type, with direct injection. A carb almost requires both an adjustable counter-piston, and a low energy ignition fuel additive, such as ether.

    I have a russian diesel aircraft engine, but those are fundamentally different to a normal or "real" diesel.

    Hot bulb I would call "semi-diesel", not sure what to call a Hvid type. Real diesels vary, with prechamber types, and ones with no such devices. I would suppose that simple is better, and prechamber type model diesels would appear to have a heat loss problem. Hot bulb type certainly do in small sizes, per some of the builders.

    Injectors seem to be small already, 2mm or so od for the whole thing
    1601

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    Hashim Khan

  5. #5
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    What is the smallest true diesel engine single cylinder displacement? I'm wondering if there are some specialty multi cylinder engines where each single cylinder is not smaller than 30mm.

    I also wonder why you don't consider the model airplane diesels as not being "true" diesel engines. OK, sure.... They use ether and often an ignition improver additive to run. But that simply means that they are a different sort of fuel. If you were to switch heads in place of a fixed slightly higher than optimum compression head and arrange an injector for the same ether/kero blend the engine would certainly ignite the fuel and run. The trick would be in lubricating the lower end and cylinder walls. But if those small size engines were in fact four strokes and the lower end was other wise lubricated they'd run. And is the kero with ether really any different than the "soup" that changes annually for big trucks to suit winter and summer condition changes?

    This post also reminded me of some smaller size true (injected) diesel engines I've seen on YT. 20mm bore by 40mm stroke. So smaller bore but perhaps not a smaller displacement.

    https://www.youtube.com/watch?v=zXCGyItsgQs

  6. #6
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    Well, I DO mean models, not commercial units, although I suppose there is no reason to exclude a small commercial unit if it existed.

    I exclude the carbureted engines because I want to exclude them.... That is not the type of diesel I am interested in, I am talking about diesels running on pure kerosene or a blend not including an ignition fluid.

    The 3/4" bore is the smallest I have seen so far. Find Hansen's engines are always extremely nice. And they work very well. With 21:1 compression, the 40mm comes down to a space of about 2mm at TDC, so it is not outlandishly long stroked.
    Last edited by J Tiers; 12-20-2018 at 01:57 PM.
    1601

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    Hashim Khan

  7. #7
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    There has to be clearance between the piston and the head at tdc which is wasted space lowering the compression. Unfortunately, as cylinder capacity goes down, the clearance takes up an increasing proportion of the volume. One way around this problem is the design which incorporates a larger secondary pumping cylinder delivering ait to the diesel cylinder at above atmospheric pressure.
    I don't know how you would make an injector with the required diameter hole in it.

  8. #8
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    Quote Originally Posted by 754 View Post
    And there has been gas pogo sticks.. maybe you could do one with diesel ?
    Ahh yes, the Hoprod.


  9. #9
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    What you don't want to end up with is an engine with a combustion chamber shaped like a coin.

  10. #10
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    Quote Originally Posted by old mart View Post
    What you don't want to end up with is an engine with a combustion chamber shaped like a coin.
    Nope...that would be bad, and it would be difficult to get a good spray pattern in it, I'd hate to try to make a fan-pattern injector at that size. The ideal chamber would likely be a sphere at TDC. Smallest surface area, highest compression temperature, etc. Quite a few full size diesels have a recessed piston, although they may not be fully spherical.

    I do not know what shape the ones Find Hansen makes are, I would assume they must be recessed, if not semi-spherical, since his injector comes down vertically in-line with the piston stroke. Otherwise the spray pattern would probably hit the piston, unless he has perfected a radial pattern (which, given his meticulous workmanship, he very well might have).
    1601

    Keep eye on ball.
    Hashim Khan

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