Even though ABC engines are more “lean” tolerant, ringed engines have the same, maybe longer, projected service life. This is true only if the ringed engine is never run too lean or improperly maintained.

Because two-strokes are simple, reliable, less expensive, easy to operate, and powerful, most new pilots start with them. To date, all the RTF trainers on the market use two-strokes (except for one electric-powered version). RTF trainers usually feature an installed engine, a fuel system, a radio, and a completely prebuilt airframe that requires no previous modeling experience and less than two hours to complete.

But many new pilots prefer the realistic sound, fuel efficiency, and more available torque of a four-stroke engine, so they forego the convenience of an RTF airframe and purchase an ARF trainer. ARFs require a little modeling “know-how” (i.e., gluing wing halves together) and approximately 20 hours to complete. The new pilot must also buy and install the radio and engine. This is currently the only way to have a four-stroke on your trainer.

Model four-strokes are still fuel- and air-cooled, glow-catalyzed machines. Most important, unlike larger four-strokes such as automotive, most lawn mower or generator engines, model four-strokes remain fuel lubricated. This is vital to remember.

Except for needing fuel lubrication, model four-strokes work much like those blue-and-red engine diagrams we learned about in school. (Of course, those diagrams never showed the black, messy grease we had to wade through to get to those “simple” parts.)

Instead of drawing the fuel/air mix through a venturi-fed, hollow crankshaft and then through transfer ports into the combustion chamber, model four-strokes feed their combustible gas mixture into the upper cylinder area through an intake valve. As can be seen from a diagram and photos, this change makes for a different engine that operates using an entirely different process.

As a four-stroke starts, the air/fuel mix is created inside a carburetor that is almost identical to that of a two-stroke. But instead of then being drawn into a hollow crankshaft, the combustion area’s partial vacuum caused by the downward-moving piston draws the fuel/air mix from the carburetor through an intake pipe (intake manifold), then through the open intake valve and into the cylinder’s combustion area. Of course, the intake valve has to be “open,” pushed away from its base, for the gases to flow into the cylinder.

Since a steel valve is unintelligent and can’t figure out when to open on its own, it is forced open by a device known as a “pushrod” that is powered by the “camshaft,” which is usually located near the engine’s crankshaft.

The camshaft is connected to the crankshaft by a timing gear. When the camshaft opens the intake and exhaust valves, and for how long, determines the timing of a four-stroke. Like the two-stroke's, the four-stroke's timing is advanced. Fresh fuel/air mix is compressed by the upward-moving piston and ignited before the piston reaches TDC.

You’ll find a lot of interesting information just by studying Diagram 3 of the O.S. 70S. I will cover four-stroke model engines in an upcoming installment.

Click on photo to view large image with caption

The brief description shows that four-strokes operate different from two-strokes. While a two-stroke has a fuel/air explosion every time the piston reaches the top of its travel, a four-stroke has such a power-producing explosion every other time the piston reaches the top. Since a four-stroke has half the number of explosions per series of revolutions, it should, in theory, produce only half the horsepower of a two-stroke engine. However, as you have seen, two-strokes have several inefficiencies that cost horsepower.

Rather than twice the power, four-strokes originally had roughly 60% of a two-stroke’s power. But manufacturers have learned that four-strokes are more tolerant of timing advances, can have larger carburetor openings and intake valves, and are easier to “supercharge” than most two-strokes. A modern four-stroke can produce approximately 75%-80% of the horsepower produced by an equivalent-displacement two-stroke. If supercharged, a four-stroke can equal a nonsupercharged two-stroke of the same size in horsepower.

But four-strokes do have one advantage over two-strokes. For various technical reasons, such as combustion duration, timing, and the nature of valve intake combustion, four-strokes produce their maximum twisting force, or torque, at rpm that most sport pilots can prop their engines to reach. Many two-strokes need to reach speeds faster than 13,000 rpm to reach maximum torque, and that speed is hard to reach with a sport engine running on 10%-nitromethane (a power enhancer)-content fuel.

The more available torque means that a four-stroke can, in practice, safely turn larger-diameter propellers, with equivalent pitches, than a comparable two-stroke. Four-strokes are also quieter, and their exhaust note is slightly lower pitched because of lower rpm.

As for two-stroke horsepower ratings, they are of little practical use when selecting an engine. Such ratings are usually computed at high rpm using small propellers. It is possible to use the same small propeller on your trainer, but the average trainer airframe has so much drag that the fast-turning small propeller has little thrust effect and your trainer will barely move through the sky.

What’s thrust? How do I find the right propeller for my engine/airframe combination? What good is a spinner? As this series progresses, I’ll cover these subjects and many more of the technical details, similarities and differences between engine types, and their care and feeding. I’ll also discuss field accessories, tools, fuel types, and many other subjects in great detail.

You may not become a model-engine expert, but you will learn everything you need to know to select an engine, keep it running reliably, make it last for years, and get the best flying performance possible.