Break-in: The secret to lonegvity and reliability

Model Airplane News, Nov 2001 by Gierke, Dave

The scene is all too familiar: an engine is fired up for the first time at the flying field; that white-- knuckle first flight is almost at hand. The anxious modeler surveys his shiny new model and reflects on the time and money required to get to this moment.

The engine, if it's lucky, has had a few tanks of fuel run through it before the pilot advances the throttle for takeoff. As the model breaks ground and climbs for altitude, he may have his hands full with an out-of-trim machine when the worst happens: the engine quits!-a nasty thing to occur so close to the ground with a nose-high attitude and low airspeed. If down-- elevator isn't applied immediately, the model will likely experience a stall or stall spin; the consequences are usually catastrophic.

If fortunate, the model lands safely. Although the engine is still smoking and too hot to touch, opinions on its troubled performance are offered by several modelers: "This brand of fuel isn't any good." "Did you use the right glow plug?" "The prop's too big." From the background, a timid voice is heard to say, "The engine sounded a bit lean to me. Does it have much running time on it?"

All of these problems could be responsible for the engine's sudden stoppage, but in my opinion, the number one cause is the lack of proper break-in. Some say they always achieve satisfactory break-in while flying the model. This is possible; I've done it myself when pressed for time. However, modelers should become familiar with the running characteristics of their engines while attaining reasonable reliability before installing them in models. Lean, hot-- running engines are not only unreliable but also often damaging to internal components, affecting both performance and longevity.

The following discussion is designed to convince those of you who have no opinion about the necessity and benefits of test stand break-in. For the others who routinely do it in the air: good luck!

WHAT IS BREAK-IN?

The late Peter G.E Chinn, longtime engine review columnist for Model Airplane News, defined engine break-in as: "... the process involved in aiding an engine's transition from a newly assembled conglomeration of assorted metal parts to an efficient working whole." He elaborated, "It means running the engine under carefully controlled conditions at the beginning of its life [my emphasis] in order to avoid the risk of immediate damage ... and to help working surfaces to become properly smoothed and aligned for maximum mechanical efficiency and performance." This statement, made almost 40 years ago, still holds true.

CONSEQUENCES

If you don't take the time, or if you don't perform the break-in correctly, your expensive new engine could be damaged almost immediately. Here's a partial list of deficiencies that will limit the engine's effectiveness:

* Difficulty in setting the high-speed needle valve (narrow range);

* Reduced peak power (at what rpm the engine will turn a given propeller);

* Difficulty in setting the idle needle valve (narrow range);

* Difficulty maintaining an acceptable idle (unreliable);

* Poor throttle-up characteristics (poor mixture control through the mid-range); and

* Hot running, cranky operation, similar to a varnished engine with a piston and cylinder that need to be cleaned.

These problems are directly related to the condition of the piston and cylinder fit. Excessive piston to cylinder clearance produces:

* Combustion gas blowby (wasted power, high piston temperatures due to poor heat transfer to the cylinder, poor cylinder-wall lubrication retention, piston skirt and/or ring damage); and

* Poor crankcase compression (2-stroke idle and throttle transition problems).

INSIDE THE RUNNING ENGINE

When the engine is running, we can't see what's happening inside, but here is what researchers tell us probably occurs:

* Cool air and fuel mixture enter the cylinder on one or more sides of the piston while hot exhaust gases exit the other; unequal piston expansion and distortion is probable in a 2-stroke engine.

* Cylinder temperatures are greater above the ports where combustion occurs; therefore, the cylinder expands more at the top than at the bottom, affecting the wear pattern of the piston in a 2-stroke engine.

* In 2- and 4-stroke engines, as the piston is being pushed away from the cylinder head by expanding, high-pressure gases, the connecting rod forms an angle with the axial centerline of the cylinder. This produces a side thrust (vector force) that generates a distorting load for the piston against the cylinder. This load isn't nearly as great on the opposite wall for the return stroke because the force is minimal.

* In 2- and 4-stroke engines, if air cooling to the cylinder is uneven, the aluminum-- alloy jacket (crankcase) may distort the cylinder which adversely affects the running fit of the piston.

The extent to which each of these factors affects an engine's performance is the subject of debate among experts. One thing is certain: you can't force these changes to occur unless the engine is actually run, brought up to temperature and then allowed to cool, time after time.