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R/C model aircraft can be powered by several different types of 'engines' including electric motors, petrol engines and most commonly by 2 stroke and 4 stroke glow engines. Each have there own advantages and disadvantages but all are widely used world-wide. Your choice will depend on the type and size of your aircraft, and most importantly, the noise limitations at your flying site, and of course how much you want to spend.
In a 2 stroke engine the fuel/air mixture metred by the carburettor is forced into the combustion chamber during the down stroke of the piston. During the upstroke the mixture is compressed and when the piston reaches the top of its stroke, the glow plug ignites the compressed gases, forcing the piston down. On the way down exhaust gases escape through the exhaust port while the fuel mixture enters the chamber. The entire power cycle takes place in 2 strokes of the piston.
In a 4 stroke engine the fuel/air mixture metred by the carburettor is delivered into the combustion chamber during the downstroke of the piston through a valve operated by a pushrod via the crankshaft. On the upstroke the valve closes and the mixture is compressed. When the piston reaches the top of its stroke, the glow plug ignites forcing the piston down. On the next upstroke of the piston a second valve opens and allows the exhaust gases to escape. The fuel mixture then again enters on the down stroke. The entire power cycle takes place in 4 strokes of the piston.
The glow plug is common to both 2 stroke and 4 stroke engines and is made up of a very small coil of platinum wire. To start the engine, an electric current, provided by a 1.5 volt battery, must run through the coil to heat it. The engine is then turned over to make the engine start running. Once the combustion cycle has started, the coil in the glow plug can retain heat between firing and the electric current is no longer required.
The mixture for both types of glow engines is usually adjusted by two needle valves on the carburettor. One needle valve adjusts mixture for idle and low speed operation while the other is for high speed mixture adjustment. Engines for control line models do not usually have carburettors and operate only at full throttle. A needle valve is mounted at the air intake and provides mixture adjustment for high speed only.
Throttle control for R/C engines is usually accomplished via a rotating barrel in the carburettor. This barrel controls the amount of fuel/air mixture going to the combustion chamber and is activated by a small arm mounted on the side of the carburettor and controlled from one channel & servo.
Two stroke engines are the most common model engine found on our aircraft. They are simple, light, easy to operate and easy to maintain, and are generally inexpensive. They operate at a high RPM but can be quite loud therefore needing a good silencer. Four stroke engines are increasingly popular. They produce a lower, more scale- like sound. They produce their power at lower RPM than two stroke engines. Because of their high precision engineering, and more parts they are usually more expensive than two stroke engines. They also need more maintenance and adjustment than their two-cycle counterparts but they are not difficult to operate and maintain.
Electric power is usually provided by a series of NiCad cells made up into packs. Six and seven cell packs are most common while larger motors may require up to 28 or more cells. These are light and can be recharged quickly in about 15 to 30 minutes. The run time is usually in the 4 to 10 minute range.
Bushes or Ball Bearings?
More recently a method of construction has been developed called ABC (Aluminium, Brass, Chrome) where an aluminium piston runs in a chrome plated brass sleeved cylinder. The piston and cylinder are matched at the factory to give a perfect fit and provide optimum compression. An ABC engine starts easily by hand, and produce more power than their ringed cousin. They also have a good lifespan, and are less prone to damage when run lean. They are a little more expensive to buy but very expensive to restore compression if required, as the entire piston/cylinder assembly must be replaced. One great advantage is that they require little or no running in period.