Engine Answer Pages

For modeling purposes we can break the different types of engines into 3 categories of internal combustion engines -- 2-cycle, 4-cycle and ???. The first we'll cover is the two cycle glow plug engine.

The two cycles are by far and away the most popular class of model engine. They have been around in one form or another since the early 1900's. The terms two cycle or two stroke are used because the engine produces a power stroke on every up and down movement of the connecting rod or on every revolution of the crankshaft.

The first stroke draws the intake fuel charge into the cylinder and the second fires the mixture and produces the power that we use to drive a propeller or rotor on a helicopter. These designs are fundamentally simple, produce tremendous power relative to their weight and are very easy to repair. It's no wonder they are the powerplant of choice for use in model airplanes and helicopters.

While the principles of two stroke operation are the same for all engine types there are very important design differences that various manufacturers employ to produce the final products they offer for sale. The most fundamental difference between manufacturers is the type of piston to liner fit they employ. There are basically four different types of designs on the market today.

ABC or ABN

I'll count this as two different designs but really they are the same idea. Most manufactures today have adopted this type of piston liner fit for engines up to the .90 cid size. The letters stand for an Aluminum piston in a Brass liner that is Chrome plated. In the case of the ABN the last letter stands for Nickel rather than chrome. The liner fit relys on very tight tolerances between the piston and liner to provide the seal for the combustion process to take place.

I often have new ABC or ABN engines returned by customers who are convinced there is something seriously wrong with the motor they just purchased because these engines seem so tight when you try and turn them over. This tightness over top dead center is perfectly normal and the squeaky metal-on-metal feel disappears after a few minutes running. It is this tight tolerance that gives the superior compression ABC and ABN engines enjoy.

In my opinion the chrome plated liner is vastly superior to the nickel plated one since chrome is a much harder surface than nickel and proves to be far more durable in use. OS is a manufacturer that has used nickel plated liners from day one and while having an excellent reputation for product reliability, their engine's piston-liner assemblies do not last as well as some of the other manufactures. The nickel is a less expensive way to manufacture engines but in my opinion not a better way. Given a choice, take the ABC engine every time!

The ABC or ABN engines are the best way to extract maximum performance from a model engine since it is the most efficient way to effectively seal the combustion chamber and increase performance. They rely on the brass liner expanding just enough so that the aluminum piston doesn't gall or seize inside the liner when the engine is running. This type of design is quite tolerant of a lean fuel mixture setting without causing damage to the engine -- as long as you catch it early before the engine get's too hot.

ABC's by their very nature will allow the piston liner to expand at a faster rate than the piston, causing the engine to lose compression and either slow or quit in extreme cases -- quite the opposite of an engine that uses a piston ring (covered below). This is a great feature if you are aware of the idiosyncrasies of this design.

What should be avoided at all cost is breaking the engine in or running it at any time with a slobbering rich needle setting since this causes the engine to run too cool and the brass liner won't expand enough to clear the tight tolerance of the aluminum piston (galling-or seizing occurs). All these designs should be run on the lean side of rich -- never 4 cycle an ABC or ABN engine with an overly rich needle valve setting!

Piston Ring - Steel Liner

Up until 5 years ago most engines used this type of piston liner seal on everything except the highest performance engines. They are typically easier to break in than an ABC engine, and in the hands of the sport flyer, a better choice since they are so easy to handle. They were abandoned for a very simple reason -- manufacturing cost.

These engines have more parts and machining steps involved than a comparable ABC engine. In the competitive world of engine manufacturing two or three dollars saved when manufacturing a hundred thousand engines can make the difference between making a profit or not!

These designs rely on a tempered steel piston ring that fits in a groove machined into the piston for the combustion chamber seal. These engines are much cheaper to repair than an ABC engine if you damage this assembly since most of the time compression can be restored by replacing just the piston ring ($25.00) versus a piston liner assembly on the ABC type ($80 - 160).

These engines are not as efficient at sealing the combustion chamber as the ABC designs. This performance shortcoming is more than compensated for in ease of break-in, cost of repair and trouble free operation. Most larger engines (4 cycle and helicopter engines) still use this design for the reasons I've mentioned.

Steel Piston - Steel Liner

These designs are not in wide use any longer since the ABN and ABC piston liner fit is superior with much less break-in, albeit at greater cost. Since two similar metals are used for the fit, it requires a great deal of careful running and break-in before the piston and liner are burnished to provide the seal required for optimum performance. FOX and COX are the only two manufactures I can think of that still use this type of fit. In their defense, if broken in properly, they provide years of service and are very cheap to manufacture.

Highly recommended reading -- All About Engines by Harry Higley