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Model Diesel engines - questions about scaling upwards

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bogstandard:
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.

Bogs

SPiN Racing:
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!

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