CHAPTER3 INTRODUCTION TO MODEL ENGINES

The basic physical differences between four-stroke engines and two-stroke engines are quite apparent. On the left is a Saito .40 four-stroke engine and on the right is a K&B .40 two-stroke engine. The rocker arm covers on top of the Saito's cylinder head can be removed to adjust the valve clearance. The two-stroke engine is much more powerful, but it requires a bulky muffler to reduce its noise output. By contrast, the Saito exhausts through an open pipe.

GLOW ENGINES

Glow model airplane engines are the smallest and most powerful internal combustion engines ever developed. Luckily for hobbyists they are also among the easiest engines to operate, if you take the time to learn a few basics. For those who have little or no experience with model engines, the following background information can provide a basic understanding of how these small, but very powerful motors work.

2-STROKE GLOW ENGINES: The vast majority of the glow model engines being used by R/C'ers run on the "two-stroke cycle" principle of operation. Every time the piston travels up (one stroke) and back down (second stroke), it has completed one power cycle. In other words, a 2-stroke engine fires every time the piston reaches the top of its travel, providing a power stroke on each revolution of the crankshaft. This super efficient system enables a 2-stroke engine to achieve very high running speeds and power in a relatively simple design that has a minimum of working parts and a minimum of weight. In fact, 2-stroke engines achieve higher speeds and a better power-to-weight ratio than any other type of engine!

A 2-stroke model airplane engine typically runs very fast, normally developing its maximum power between 10,000 - 20,000 RPM (revolutions per minute). Exactly where within that broad RPM range the top speed will be depends upon whether the engine was designed for sport or racing airplanes. Until you become an expert flier, you will only be dealing with sport engines that run between 10,000 - 12,000 RPM on the top end. These sport engines are easy to run, very reliable, and with minimal care they will last for many years. Lubrication is provided by oil mixed with the fuel (more on that later). Due to their simple air-cooled design, 2-stroke engines are also relatively cheap and easy to manufacture. Overall, when you consider the high power, low weight, simplicity, and reliability of 2-stroke engines, you can see why they have become the number one powerplant for flying model airplanes.

4-STROKE GLOW ENGINES: These engines operate on the "four-stroke cycle" principle of operation, meaning that the piston travels up and down twice (thus four strokes) during the power cycle. Like an automobile engine, a 4-stroke model engine has valves, rocker arms, pushrods, and cams, all of which add to greater size, weight, and complexity compared to a 2-stroke engine. Inevitably, because of the physical limitations of all the moving parts and since it fires only once for every two revolutions of the crankshaft, a 4-stroke engine cannot turn as fast or develop as much power as the same size 2-stroke. A 4-stroke model airplane engine typically produces 10 - 20% less power than a 2-stroke engine of equal displacement.

Four-stroke engines are ideally suited to scale models. This beautiful Sig Citabria is a typical example of a civilian-type scale model powered by a four-stroke engine. The low-pitched sound of the engine adds to the realism of the aircraft in flight.

Even though 4-strokes are not nearly as efficient in power-to-weight ratio as a 2-stroke, they do have several favorable characteristics. First off, 4-strokes develop their maximum torque at a lower rpm, usually in the 6,000 - 9,000 rpm range, which enables them to turn a larger propeller than a 2-stroke engine of the same size. This low rpm/big prop combination is especially good for models that you want to fly relatively slow in level flight but still retain excellent climb characteristics, like an R/C scale or sport aerobatte model. Four-strokes are also much more economical on fuel than 2-strokes. The world's record R/C endurance flight is held by a model with a 4-stroke engine! However, perhaps the main reason for the growing popularity of 4-strokes among R/C modelers is their quieter, more pleasing, and more realistic sound. It's a sharp contrast to the typically unrealistic, high-pitched whine of a 2-stroke model engine.

ENGINE SIZE: Engine size is measured in terms of displacement, which is the internal volume swept by the piston in making its stroke up and down in the cylinder. In the United States, displacement is normally stated in cubic inches (cu. in.). In most other countries it is stated in cubic centimeters (cc). The following chart converts one form to the other: