Engines Designed for Beginners

Created: Nov, 2005
Last update: Feb, 2007
Click on photographs to view in more detail

This series of pages began as some thoughts in the October 2005 Issue of Model Engine News on what would constitute a good first choice engine design for a budding model engineer new to both engine building and model engineering. This page is an expansion of that material. We will examine the things known to cause difficulty and design features that present fewer problems. With these defined, we'll look at a selection of designs intended for beginners and see how they compare to our criteria.

   Things that cause problems
   What's Good and Why?
   Design Reviews:
Schroeder Simple Single
       Schroeder Victor
       Owen Mate
       Dean 2.5cc Diesel
       Roy L Clough Jr's Little Dragon
       The Gordon Alpha
       Topsy (not really for beginners)
       MAN SIMPLEX 25


Many years ago, Roger Schroeder authored a paper called
How to Build a Model Airplane Engine that Runs. Roger has kept this material up to date and it should be read to understand the basics of what is required. Many amateur and not-so-amateur model engine designers have contributed their thoughts to the problem of a simple model engine for home construction. In some cases, the designer/constructor was their own guinea-pig! Examination of the literature and correspondence with builders and designers suggests to me that there is no "ideal". The variability in skills, experience, equipment, availability of time and materials, and a host of other factors make a definitive proclamation impossible.

However, we can examine techniques and details of construction that are known to present difficulty to some first-time builders, while paradoxically, not to others—who encounter totally different hurdles! We can also provide more information about the choices available to those trying to decide on what to tackle first. Ultimately, there is no substitute for YOU making an informed choice, followed by swarf and scrap—although hopefully not too much of the latter!

Things that cause problems

Before we look at "good" features, I'll list tasks that have been known to cause problems to first-time model engine builders and why. In no particular order, these are:
  1. Thread cutting
  2. Workholding
  3. Unintentional tapers
  4. Soldering or brazing

1. Thread cutting

Cutting a pair of mating threads that fit together correctly is not hard, provided your lathe has the capability! Even if it does, the things that can go wrong are many and varied, ranging from an aluminium to aluminium fit that cold welds itself together while being trial fitted, to improper thread engagement due to the way the male and female cutting tools are ground. Not infrequently, the thread cutting will be one of the last operations on a complex part. A serious mistake will cause the whole part to be scrapped which will be rather discouraging. Avoid designs requiring thread cutting unless you have both the capability and have practiced cutting good threads.

2. Workholding

Castings in particular can be difficult to hold for machining operations. They will almost certainly require a lathe face-plate and accurate angle-plate in one of the operations. Again, these are items that not every beginner is going to have initially, so unless you do, look for designs that permit complex things like the crankcase to me held in a three or four jaw chuck.

3. Unintentional tapers

Many "side-port" engines feature a cylinder liner that appears simple to make: a tube with a flange and some holes drilled in it below the flange for the ports. This drops into a bore in the crankcase where more openings and channels match up with the ports. Designs like this, typified by the
Deezil and ML Midge, depend on the closeness of the fit between bore and liner to prevent gas leakage between the three sets of ports. If the case bore is a bit large or tapered, the engine will not run because gas will lead between the ports that are supposed to be isolated. The larger the engine displacement, the more chance of a taper. The smaller the size, the more difficult it will be to get a good cylinder to piston fit. A loose fit or taper in the bore for the crankshaft can also result in loss of primary compression making the engine hard to start. A design that allows for some latitude here is better than one requiring absolute precision.

4. Soldering or Brazing

The venerable "side-port" is generally a very forgiving engine in relation to timing, but many require that bypass (aka transfer) covers and inlet tubes be soldered onto the cylinder. For example, Sparey diesels, the Weaver-Ransom diesel, and many of the Classic Model Engines ignition kits. Diesels run relatively cool and "soft" soldering can be used provided that the mechanical loading of the joint is not too high. Vibration and the weight of a tank/venturi assembly can cause the joint to fracture. This can lead to a hair-line crack that leaks air causing erratic running, right up the the whole thing falling off! On the bypass, butt-joining a tiny shell with edges about 0.010" thick to achieve a gas-tight seal requires skill. Glow and spark engine run much hotter and joints will need to be silver-brazed. That makes this another aspect to avoid unless you are correctly equipped and confident.

Remember, not everyone is going to be troubled by all of these, but forewarned is forearmed. All of them can be overcome with the right equipment and sufficient practice exercises. Take a close look at an intended design and plan in your mind how you will tackle each part. If you are a bit uncertain, it's probably best to choose another design, or make a trial on some scrap before committing.

What's Good and Why?

This is a question I'm frequently asked and it's a difficult one to answer. I lean towards a compression ignition (CI) type, purely from an ideological predisposition (and because I love the smell of ether fumes in the morning ). This is probably an actual negative to modern builders who have no experience with operating this type of engine, compounded with the increasing difficulty of obtaining the correct fuel, or even the ingredients to mix your own! But all that aside, let's look at characteristics that are important:

  1. A proven, developed design with enough prototypes made to iron out bugs due to manufacturing tolerances, etc.
  2. A displacement of somewhere in the 0.09 to 0.15 cuin range (1.5 to 2.5 cc). Much smaller than this and the parts require watchmaker skills; larger and mistakes become expensive. Additionally, the bore of engines in this range is "sweet"; beginners will have no great difficulty achieving the required finish and uniformity. Above and below though, extra care and effort are required.
  3. No screw-cutting required. While I believe the ability to cut accurate threads is ultimately mandatory for all model engineers, reports from some first-time builders suggests that removing it from the equation for the first attempt is a distinct positive.
  4. Just simple turning, drilling, and tapping operations required—no special tooling, milling, or slitting required. These things too should be part of a model engineer's skills inventory, but they can come later.
  5. No bush for the crankshaft; simply a reamed hole in the crankcase makes for one less thing to worry about. Pressing in a bush presents some 'gotchas', as does using a Locktite fit. Easier to skip it. Engine life and performance will be reduced, but we can live with that—many commercial engines have!
  6. Builders' choice for ignition type: glow, CI (diesel), maybe even spark. Some early English engines like the ED Mk III offered both CI and glow heads out of the box. This would let the builder use the system they are most familiar with, then try the other after confirming that the engine will run.

The question of using a casting for the crankcase verses one hogged from a lump of aluminium is contentious. A casting simplifies things. Machining it is generally the first task as other parts will be made to fit it. This provides the beginner a grand opportunity to have an expensive mistake when skills and confidence are at their lowest. On the other hand, a "hogged" case requires even more skills, but permits a few stuff-ups at an acceptable cost in material and time (to obtain a replacement that is, not in the overall effort invested). But this comes at the expense of either a dependency on milling operations, or a lot of sawing and filing—both of which are skill requirements we wanted to avoid.

If pressed, right now I'd recommend the ML Midge, or the Schroeder Deezil. The Midge is smaller than I'd prefer, and requires hogging the case. The Deezil is the right size, but requires screwcutting. The Owen Mate would be another contender, but as the crankcase extrusion is no longer available. A builder is faced with both hogging and screwcutting! All these engines are proven designs and have been successfully tackled by many first time builders with positive results.



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