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John's R/C Flying Pages


Engines

To propel your model through the sky you'll need an engine. We can safely ignore gas turbines for the moment - if only on grounds of their cost. Electric powered flight is becoming increasingly popular, but for the moment the internal combustion engine is still king. Most beginners will start with a 2-stroke glowplug engine, but other types are available.

2-stroke engine The 2-stroke engine (pictured left) fires every time the piston reaches the top of the cylinder and is mechanically simpler than the 4-stroke which fires every second revolution.

4-stroke engine The 4-stroke (pictured right) needs valves in the cylinder head to let the fuel in and the exhaust out plus cams, pushrods and rockers to operate the valves. Generally for a given size a 2-stroke engine will be more powerful and noisier than a 4-stroke. It will also cost about half as much to buy.

Most engines used by modellers are glowplug engines. These operate on methanol-based fuel and have a glowplug in the head which ignites the fuel in the cylinder. The glowplug is initially heated by connecting a 2V battery to it. Once the engine is running the battery is removed and the plug continues glowing without it.

Diesel (or compression-ignition) engines are used to a limited extent. These engines are very much more popular in free-flight and control-line flying. Operating on a parafin (kerosene) based fuel they rely on the heat generated in compressing the fuel/air mixture as the piston rises to ignite the mixture. In place of the glowplug you'll find a control to adjust the compression ratio for optimum performance.

Spark ignition - as used in car engines - is popular for engines above about 20cc capacity. This size is unlikely to be of relevance to most beginners!

The capacity of an engine is usually quoted in cubic inches - or rather hundredths of a cubic inch. The most common engine size is probably a '40' or 0.40 cubic inches, about 6.5cc.

Engine design

The most significant differences in engine design have already been briefly mentioned - 2-stroke or 4-stroke and glow or diesel (or spark ignition). There are other design differences that are significant.

Bearings

Cheaper engines have the crankshaft rotating in a bronze 'bush'. This is cheaper to manufacture than the alternative, but generates more friction - hence the engine generates less power. In the UK these are described as 'plain bearing' engines. In the USA they are called 'bushing' engines.

More expensive engines (and probably all 4-strokes) have the crankshaft held by a pair of ballraces. One small ballrace is located just behind the propeller driver and the bulge in the crankcase is plainly visible on most ballraced engines. A bigger ballrace supports the crankshaft at the front of the cylinder. Both engines pictured above are ballraced.

ABC or Ringed

Before 'ABC' engines were developed, small engines had plain pistons and required a lot of careful running-in. This was to allow the piston and liner to wear down any 'high-spots' so that the engine could run successfully at full power. The hazard was that these highspots generate heat through friction. If the engine got too hot the piston would expand and eventually jam or seize in the cylinder. Larger engines partly got around this problem by allowing a sloppier fit between piston and liner and using a piston ring to improve the seal between the two.

ABC engines have Aluminium pistons and Brass cylinder liners that are Chromium plated. This combination has much more satisfactory expansion characteristics. As the engine heats up, the cylinder liner actually expands more than the piston so increasing the clearance between the two and reducing the friction that is partly responsible for the engine heating up! An ABC engine is therefore less inclined to seize and requires less running-in. ABC engines often feel very tight when cold, especially when the piston is at the top of it's stroke. This is normal.

Fuel

The 'classic' fuel is a mixture consisting of 80% (by volume) methanol and 20% castor oil. The methanol burns and produces the power, the oil is required to lubricate the moving parts. Ordinary mineral oils do not mix with methanol, so castor oil has been used for many years. Often now the castor oil is replaced totally or partially by synthetic oils. Follow the guidance supplied with your motor if they specify using - or not using - synthetic oils.

Nitromethane (usually just called 'nitro') is generally added to fuel replacing some of the methanol. It boosts power output from the engine and allows a little more flexibility in tuning the engine. It is however not cheap and, in the UK at least, use of fuel containing 5% of nitro can be regarded as normal. 15% would be thought of as extravagant by most people for general use, although in USA this is more common. Competition flyers often use higher percentages of nitro. Small engines - up to about 1.5cc often require higher nitro fuel to run satisfactorily.