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Model Diesel engines - questions about scaling upwards

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RipSlider:
Hello folks.


I am interested in model boats - specifically making fast model boats - and one of the area's I would like to investigate is diesel engines - a diesel should, in theory at least, drive a much coarser prop, and so give more preferable performance for the designs I'm looking at - which will come in later posts when I need to bother bits of metal to make them.

There are plenty of diesel engine designs available up to about the 5cc mark. There is the Taplin Twin at 8cc and ( I think ) 10.5cc and a couple of 8cc singles - all running on the two-stroke cycle. There are also some plans around for flat twins at about 6cc - 3cc per cylinder.

What I really want is a flat twin, or flat four - at the 15cc mark and the 30 cc mark.

So, naturally, the idea of scaling up one of the existing plans popped into my head. This wouldn't be too awful in theory, becuase diesels are chunky, solid things and so will ( I think ) suit a ham fisted newbie like myself down to the ground.

The problem that I have is that any discussion of a 15cc 2-stroke diesel automatically generates comments along the lines of "oh dear me no - you certainly wouldn't want one of those...". Further sleuth like probing as to WHY it would be a bad idea in most cases leads to a response of "I don't know - but it WAS a good idea it would have been done already".

The only two sensible responses I have got are:
1) It might be a bit fierce - noise wise and vibration wise - I'm OK with both of these as the engine can go on rubber mounts and will be out at sea so I don't care how loud it is.

2) that the pressure in the chamber is an issue, and the head is liable to tear itself off. Discussion then went into the fact that this problem could well be copable with - tougher material, bigger threads etc.


So, dear modders of madness, I would like to pose two questions to you based on all of this:

1) What do *you* expect a 15 or 30 cc Diesel 2-stroke to behave like? Any other possible issues/thoughts/experiences that would make you shudder? Is the concept complete stupidity?

2) If such an engine is possible viable - and Sensible answer No.2 - from a very clever chap indeed - suggests it might be - would I be able to take an existing plan - say the 6cc twin - and scale everything up by a common factor and expect to find myself with a working engine? I.e cylinder increases, but so does wall thicknesses, bolt sizes and threads etc etc etc? Or would you expect that I would run into less than obvious issues that would scupper this idea and mean that it would have to be entire scratch designed?

As an example of 2 - I'm not entirely sure that if I double the displacement, and keep the same fuel/air mixture and the same compression ratio I might run into issues. I know that use of inter-coolers in diesels is directly related to the size of an engine - although this is in the 4-stroke world.

Many thanks in advance for any thoughts/idea's/well phrased insults you might care to share.

Steve

Darren:
I have a 1 lt singe cylinder diesel and it's head is still there after 60yrs or so...... :)

Ok it's not the same thing I know, but why should a 30cc present any problems?


Diesels generally run slower than petrols, don't they?

HS93:
Hi as  Boat fan have you any pictures of your boats , we like pictures on this site, and I am very interested in steam

     :ddb: Peter  :ddb:



:worthless:

bogstandard:
In the late 60's I knew a chap who had exactly the same idea as yourself. He made a 20cc horizontally opposed twin diesel.

It ran like a bag of nails, and nothing he could do would sort it out.

The problem was the reciprocating weight. The reason model diesels aren't made much larger than 6.5cc is because of the speed they have to run at to achieve the power, 4 to 5,000 RPM was about the normal operating range of an engine of that large size. He could never get the revs high enough to get efficient compression combustion, purely because he had to make the parts out of heavier steel rather than aluminium used in the much smaller engines, to withstand the high stresses involved.

I used to run 5cc diesels in my early model boats, and any sign of a cool day and they just wouldn't run, so eventually I changed over to glowplug and never looked back.

Diesels in model boats are OK, very economical with plenty of low down torque, fine for a slow running scale boat, but absolutely no contest when it came to challenge the speed and power of a glo engine, and they are filthy runners, oil everywhere, because you never got complete combustion.

Bogs

RipSlider:
Bogs,

Is the issue the TOTAL reciprocating mass, or the mass of each piston in isolation?

I.e, the chap had trouble with 2x10cc cylinder. Would he have had the same issues with 4x5cc cylinder.

Asking the same question the other way around: is the issue related to total volume, or volume per cylinder?



I'm interested for two reasons. The first is that I know that British army moved a couple of years ago to convert their motorbikes for their DR's into 2-stroke diesels of approx 250cc and having had a chat with a guy who's ridden them, they seem to be pretty popular with the troops and so obviously functional. They are being used in Helmand etc in 50 degrees C tempuratures and would seem to have a reputation of being solid as a rock.

The second reason for interest is that a lot has changed in 40 years. For example, there are now very efficient designs for model sized inter-coolers and turbo chargers for these sized engines courtesy of the R/C Car racing brigade. Materials have improved and are better understood etc.

Thanks for all the answers so far.

Steve

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