There are two main propulsion systems used by R/C models today: The internal combustion systems (glow engines) and the electric motors. Combustion engines' energy source has so far a higher energy/weight ratio than the batteries used to power the electrics. However, the combustion engines are usually more noisy and more prone to oil spillage than the electric motors.
There are two types of glow engines: The four-stroke and the two-stroke.
Two-stroke engines are the most used, mainly because they are simple made, light, easy to operate, easy to maintain, and are usually inexpensive. Two-stroke engines operate at a high RPM and therefore can be quite noisy without a good silencer.
Nevertheless, the four-stroke engines also enjoy some popularity, mainly because they produce a lower, more scale-like sound and consume less fuel. They have lower power/weight ratio and lower RPM, but provide more torque (use larger propellers) than theirs two- stroke counter-parts.
However, since the four-stroke engines require high precision engineering and more parts to manufacture, they are usually more expensive. They also need more maintenance and adjustment than the two-stroke, yet they are not too difficult to operate and maintain.
Ball Bearings vs. Bushings
Some nitro RC engines are available with ball bearings supporting the crankshaft. Others have bushings that support the crankshaft. The picture to the right is what the bearings look like. There are two sets of bearings. One set in the front and a larger set in the back.
A bushing is nothing more than a bronze or brass disc the with a hole in the center for the shaft.
It is easy to tell if a nitro RC engine has bearings or not just by looking at it. Take a look at this O.S. 46AX engine to the left. The area highlighted red is where the bearings sit in the housing.
Ringed vs. ABC Nitro RC Engines
There are two different ways that nitro RC engine manufactures seal the piston with the sleeve. It is critical that the piston and sleeve form an airtight seal in order for the engine to produce compression. Some RC engines simply have a ring that goes around the top of the piston that seals with the iron sleeve. This is called a “ringed” engine. The picture to the right is cross sectional view of the piston/sleeve assembly of a ringed engine. It is greatly exaggerated in order to illustrate how a ringed piston seals with the sleeve. The portion in red is the ring.
Some nitro RC engines have an aluminum piston that moves in a tapered chrome plated brass sleeve. This type of engine is called an ABC engine for Aluminum, Brass, and Chrome. The picture to the left is exaggerated to illustrate what the piston/sleeve assembly looks like at ambient temperature. At ambient temperature the inside diameter of the sleeve is slightly smaller at the top of the stroke. The fit between the piston and sleeve gets tighter as the piston rises. When the engine gets to operating temperature the top of the sleeve expands and the sleeve is no longer tapered. The reason the top of the piston expands is because it sees all of the heat from the combustion.The bottom of the sleeve is much cooler and does not expand nearly as much.
When the top of the sleeve expands the piston will fit perfectly with the sleeve at both the top and bottom of the sleeve. When the engine is cold the sleeve only seals with the piston when the piston is near the top of the stroke. This can make ABC engines a little more difficult to start, especially in cold whether. ABC engines are more tolerant if you accidentally run the engine slightly too lean.
The repair cost for a ringed nitro RC engine is much less that that of an ABC engine. The ring itself is usually the only thing that fails in a ringed engine, provided that you run the engine with the correct fuel mixture. The rings are very easy to replace and are inexpensive. The cost for replacing the ring is typically under $20. To rebuild an ABC cylinder you will need to buy a new sleeve and new piston. As with anything, the sum of the individual parts of an engine cost a lot more than the whole. It is usually not economical to rebuild an ABC engine.
The benefit of an ABC engine is that the break-in time is a lot shorter. An ABC engine will tolerate the occasional lean run, as where a ringed engine will be damaged immediately with a lean run. The benefit of a ringed engine is that it can be rebuilt relatively cheap. A ringed engine will tend to hold its maximum power longer than an ABC engine which slowly decreases in power as the sleeve and piston wear. If you’re new to the hobby I recommend getting an ABC engine. It is much more forgiving and easier to break in. It’s worth noting that just about all four-stroke engines are ringed engines.
Operation & Maintenance of Nitro RC Engines
A nitro RC engine will last you a lifetime if properly operated and maintained. On the same toke, the engine could last a very short time if not cared for properly.
A glow engine consists basically of:
The glow motor's Carburetor consists basically of:
All glow engines require a special fuel, called "glow fuel." It consists of methanol as base, with some amount of nitromethane to increase the energy and pre-mixed oil into the fuel, which lubricates and protects the engine parts.
Two-stroke engines operate by igniting the fuel in its combustion chamber once every turn of its crankshaft.
The fuel is mixed with air at the carburettor and forced into the cylinder during the down movement of the piston (1st stroke).
While the piston moves up, the mixture is compressed and when the piston reaches the top, the glow plug ignites the compressed gases, forcing the piston down (2nd stroke).
On the way down exhaust gases escape through the exhaust port while the fuel mixture enters the cylinder again.
In a four-stroke engine the fuel/air mixture enters the combustion chamber during the down movement of the piston through a valve operated by the camshaft (1st stroke). When the piston moves up, the valve closes and the mixture is compressed (2nd stroke). When the piston reaches the top, the glow plug ignites forcing the piston down (3rd stroke). On the next up movement of the piston, a second valve opens and allows the exhaust gases to escape (4th stroke). The piston moves down and the fuel mixture enters the combustion chamber again, repeating the 1st stroke.
The glow engines usually have a simple ignition system based on a glow plug made up of a little coil of platinum wire rather than a spark plug. A 1.5V battery is used to heat the glow plug only during the starting procedure and is removed when the motor reaches a certain rpm. This is possible because the glow plug keeps glowing by the heat produced during the compression and combustion without needing the battery.
There are two lengths of glow plugs available. The short ones are normally used on engines smaller than 2.5cc (.15cu in). Some have a metal bar across the bottom of the plug called for Idle Bar, which prevents raw fuel from dousing the heat from the element during idle.
There are also the so-called "hot" and "cold" glow plugs, which refer to their effective coil operating temperature. The glow plug's temperature depends on several factors, such as the coil's alloy, thickness and length, the size of the hole in which the coil is located as well as which material the glow plug's body is made of.
Usually smaller engines and those that run on less nitro prefer hotter plugs. In case of doubt just follow the engine manufacturer's recommendation.
Turbo glow plugs have a chamfered end that matches the threaded hole on the engine's head. It is claimed to give less compression leakage around the glow plug and less disruption of the combustion chamber. Also the hole in the cylinder head, which exposes the glow plug to the air/fuel mixture in the cylinder is much smaller, resulting in fewer rough edges that could create unwanted hot spots. The turbo plug is shown on the left of the picture below.
Glow engines may have plain bushed supported crankshaft or ball bearings. Ball bearing engines usually have a better performance, run smoother, and last longer but are more expensive than those with bushings.
The model engines' piston and cylinders construction are usually based in two methods: Ringed engines or ABC.
A more recent method is the ABC, which stands for Aluminium, Brass, Chrome where an aluminium piston runs in a chrome plated brass sleeve. The piston and cylinder are matched at the factory to give a perfect fit and good compression. ABC engines start easily by hand, give more power than the ringed engines, have a good life-span and are less prone to damage with a lean run.
Schnuerle ported engines have several fuel inlet ports on three sides of the cylinder allowing more fuel to flow to the combustion chamber. This gives somewhat more power than with standard porting, which has only one fuel inlet port on the side of the cylinder opposite the exhaust outlet. A Schnuerle ported engine is usually slightly more expensive due to higher manufacturing costs involved.
The fuel tank size and location affects the engine operation during the flight. A typical tank placement is shown on the picture below:
When the engine is in the upright position, the fuel tank's centreline should be at the same level as the needle valve or no lower than 1cm, (3/8in) to insure proper fuel flow. A too large fuel tank may cause the motor to run "lean" during a steep climb and "rich" during a steep dive. Normal tank size for engines between 3.5cc (.21) and 6.5cc (.40) is 150 - 250cc.
In order to emulate the full-size aircraft jet-power systems, it is often used the so-called Ducted Fan, there a glow-engine drives a fan fitted inside the model.
There are also glow engines specially designed for Ducted Fans, which have a special shaped head, also having the exhaust port facing towards the rear of the model.
These engines are often equipped with a tuned pipe exhaust in order to improve their efficiency at high rpm.
Since a special method to start is often required due to the reduced access to the engine, this arrangement is not recommended for beginners.