Author Topic: Model Diesel engines - questions about scaling upwards  (Read 3993 times)

Offline RipSlider

  • Jr. Member
  • **
  • Posts: 25
Model Diesel engines - questions about scaling upwards
« on: June 10, 2009, 07:48:05 PM »
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.


Offline Darren

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 3795
  • N/Wales
Re: Model Diesel engines - questions about scaling upwards
« Reply #1 on: June 10, 2009, 08:02:47 PM »
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?
You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)

Offline HS93

  • In Memoriam
  • Hero Member
  • *****
  • Posts: 788
  • Country: gb
Re: Model Diesel engines - questions about scaling upwards
« Reply #2 on: June 10, 2009, 10:37:40 PM »
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:

I am usless at metalwork, Oh and cannot spell either . failure


  • Guest
Re: Model Diesel engines - questions about scaling upwards
« Reply #3 on: June 10, 2009, 11:23:21 PM »
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.


Offline RipSlider

  • Jr. Member
  • **
  • Posts: 25
Re: Model Diesel engines - questions about scaling upwards
« Reply #4 on: June 11, 2009, 01:54:06 PM »

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.



  • Guest
Re: Model Diesel engines - questions about scaling upwards
« Reply #5 on: June 11, 2009, 02:36:02 PM »
You are quite correct in saying that materials and designs have come a long way, you only have to look at what Darren has to say about his engine, designed and built like a brick s**thouse and will go on for lifetimes, but only at a certain rev range and power output. Modern designs are another concept, being more like a four stroke engine rather than the heavy cast iron jobs from times gone by.

I can't comment on the modern stuff as I have had very little to do with them, but in my day, the large model diesel was the holy grail for modelling enthusiasts.

There is a massive difference between the reciprocating weights and forces of four 5cc engines stuck together, or two large 10cc units trying to work in unison. The bigger you went, the more strength had to be built in, and more problems trying to raise the compression ratio to get a good detonation and burn. If I was going to try it, I would go down the route of the Taplin twin, small cylinders, but more of 'em. Because they are basically a two stroke, it is very easy to put multiple cylinders along a common crank. To me, an 8 cylinder 20cc engine will be easier to achieve than a twin cylinder 20cc.

I am sure that using modern designs and materials, then your ideas might come to fruition, but I am sure most of it will be frustration and burning the midnight oil to achieve your final goal.

You have to remember, they might be available in the outside world, but the manufacturers have money to throw at projects such as that, because they have a final market and profit. You will be working by yourself, with the only prize of a massive grin when you eventually get it going, and most probably a write up in a model mag. The big boys were most probably at your stage 20 years ago, so it might be to your advantage to see if any of those can give you any pointers along the way. There must be a method of getting over the large cylinder/bad combustion phase. On the engines that I worked with, they were very low tech, maybe you should be looking for high tech solutions, to get you thru the barrier.


Offline SPiN Racing

  • Madmodder Committee
  • Hero Member
  • *****
  • Posts: 505
Re: Model Diesel engines - questions about scaling upwards
« Reply #6 on: June 20, 2009, 12:20:38 AM »
A few thoughts as I have done the model airplane thing in the past.. for around 15 years or so. Involved in 2 stroke pylon racing, pattern, and unlimited acrobatics.

The diesels I saw back then (20 years plus ago) were mostly 2 stroke engines with a revised head. This being a diesel head. The issues run into by these engines were VERY hard starting from the added compression, and a load of wear on the cases from the added power of the diesel hit, and added stress from the compression increases. A number of racers I knew were running shaved heads on teh glow engines, and bumping compression and running significantly more expensive higher content nitro fuel. THey also had to run very odd glow plugs, so they wouldnt burn em out.

Since then I have been extensively involved in the racing side of Rotaries(wankels), as well as the piston world, but mostly in the tuner side of things rotary racing wise.

The rotary engine itself is very very good with turbocharging up to around 18 pounds of boost. From there as the combustion pressures increase, there are a world of issues you run into failure wise. Almost all of them cause fairly significant failures. From melting the face of the rotor, to snapping eccentrics(crank) to breaking rotor gears, and mostly warping, and or melting housings. These failures are a result of problems with the engine materials not being able to handle the added stresses of the engine being put under that added pressure created under those levels of boost.

Now... the failures on well tuned, suitably prepared engines are all basically the same. The housings deform from the pressure, and wipe out other parts of the engine. This is not in a overly high horsepower situation in most cases.
A rotary engine with average porting, and 18 pounds of boost is in the 450-500 horsepower range. If you were to bump the boost to say.. 25 pounds of boost, the engine will be bumping 600 horsepower.. and very very quickly fail. Not because of the horsepower developed.. but because of the added chamber pressure of the high boost.

There are Normally aspirated rotaries making 450+hp in the higher end world of rotaries. 3 and 4 rotor engines are making upwards of 800-900 horsepower in full race trim without turbos, or added boost. There are only a very small few shops that can actually build a 3 or 4 rotor N/A engine properly.. but they DO make that power all day.

There are a few shops that make rotaries WITH boost over 20 PSI. And they make 1000-1500 HP on a 3 rotor engine. A VERY FEW.. (like 2 I know of in the world... and IIRC both are in AU)

The point is... (sorry to ramble) The engines making high power with high boost have had a world of VERY VERY extensive work done to get them to live for any length of time at those higher pressure levels.

To make a small engine as you are interested in will take a goodly amount of homework.. and likely will require some innovative work, with what you will need to allow it to live, and on the other hand make some power.
After some of the wild things I have seen here and other places.. I have no doubt it is very possible. I am certain it can be done.. but it will take a good amount of thought and planning. Dont be afraid to try something new with the design. Dont be boxed in with what the thoughts were 20 or 30 years ago. DO LOOK at them. Learn from what they discovered.. and try something different.

An example...
Rotary engines have been made by Mazda since the mid 1960s. A smart Auzzie came up with a new idea.. a few years back.  Under high boost, and or high RPM situations, the rotary crank (eccentric) would get a harmonic, and begin to wobble. As its riding on two bushings.. one in the front and one in the rear. When this harmonic happened.. one of two things resulted. 1. the crank sheared. 2. The rotors hit the housing, and then engine failure ensued.
The auzzie came up with a novel idea... To take the crank/eccentric.. and make it a 2 piece unit. With a BEARING in the middle. so when high rpms were achieved (16K RPM) the eccentric didnt whip around and fail.

Dont be afraid to think out of the box. And keep us posted on the progress!
SPiN Racing