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BEFORE PROCEEDING to the third article in the "From the Ground Up" engine series, I am going to review the first installment, published in the April issue. MA has received numerous comments on the information presented in that article. Many were favorable, but several pointed out that the theoretical information was incomplete, poorly explained, or just plain wrong. And in some instances they have valid points.

So I'll review some of "Engines 101"'s basics, keeping in mind the comments that were offered. In all cases they were offered in good faith and in the hopes of improving the series and modelers' understanding of engine basics.

 Probably the most incomplete section was the beginning of the article. I did not take the space, which is precious in any article, to properly explain the intent of the engine theory I was writing about or the manner in which it was to be presented. I'll do it now as it should have been done in "Engines 101."

The engine theory I am presenting is intended solely to provide beginning RC pilots with enough knowledge of a model-engine's workings to understand why choices about proper mixture settings, fuel, propellers, and other items to be discussed later will be made.

There is no intent to fully detail an engine's intricate machinery. The new RC pilot is not going to be designing or disassembling (I hope) his or her first few engines, but this person will be setting high- and low-speed mixture settings.

All the theory I present will be from a strictly operations viewpoint. Proper engine operation is the only goal in this discussion. Where true technical names for parts may be confusing, I will use descriptive terms instead. Since most new modelers have no knowledge of two-stroke engines, but do have at least a passing familiarity with their cars' engines, I'll try to reference parts with confusing names in more recognizable terms.

As with all things mechanical, a model engine's true operation is complicated. Operations that are explained separately and appear to be independent actually overlap, and sometimes interfere with, other operations. To simplify the theoretical presentation, I will explain each action as if it were the only one happening at that time.

A great deal of confusion would have been avoided if I had started "Engines 101" with the preceding. Because of the decision to simplify and avoid confusion, the rotary disk induction valve became the crankshaft intake slot.

The true engineer's name is correct but leads one to look for a moving valve such as those found in a car or a rotating valve. There isn't one, and the "valve" is a slot. The name also sounds as if some sort of "pumping" action is happening when it is not. The same rationale was used when discussing the "intake" and "boost intake" ports, actually known as transfer (also called bypass) and boost transfer (bypass) ports.

Why "transfer"? Because these ports allow the fuel/air mix in the crankcase to transfer from the crankcase to the combustion chamber, but their function is that of fuel/air intake ports. When traveling, you do not need to know a street's name—just where it goes. In this case it goes into the combustion chamber.

I also simplified the partial vacuums found in our engines' operations—actually low-pressure areas since there is nothing even approaching a physicist's definition of a partial hard vacuum in our engines (there is just too much gas density everywhere inside)—as just "vacuums," as most automotive books do.

I completely ignored the function of an engine's timing advance, all references to Top Dead Center (TDC) operations, and especially all references to interference and benefits one operation may have compared with another.

None of this theory would help newer RC pilots operate their engines better. Explaining these operationally irrelevant, but theoretically important, functions would have taken almost a full article themselves.

Similarly, I called the methanol in our fuel a "heat exchanger" and pointed out that methanol helps cool the engine; that is why a richer mixture is important. I felt that "heat exchanger" was a simple, generally understood term that would not require definition but get the point across.

However, in technical terms, methanol cools our engines because it has a high heat of evaporation. During carburetor air intake, methanol in the fuel is transformed into a gas requiring a great deal of heat. The process—called refrigeration—therefore removes heat from the surrounding lower engine sections to have the energy to transform the methanol.

The same process cools the food in your household refrigerator, but without the combustion part. Your refrigerator substitutes an electrically driven pump and evaporator for crankcase pumping and venturi action at the carburetor. But this process is not technically one of heat exchange. A heat exchanger does not transform into another state of matter, but absorbs heat from one source and transfers it to another.

There is a true heat exchanger in our fuel that removes heat from the engine and transfers it to the earth's atmosphere via the exhaust. We call it the "fuel's oil." As everyone who has cleaned his or her model aircraft knows, some oil is not burned during combustion and escapes through the exhaust. While escaping, it also transfers some of the engine's heat.

The main point remains, however. Whether through heat exchange or refrigeration, the fuel lubricates and cools our engines. RC engines are not cooled solely through contact with the air; therefore, the proper high-speed fuel mixture settings are critical and must not be ignored.

In retrospect, I probably should have used the technical terms and part names for the preceding and for other items in "Engines 101." It is usually better to explain the technical, rather than "street," names at the start, even if doing so requires much additional explanation. In this way, regular RC pilots and engine technicians will eventually have the same reference names.

So sit down, tighten your engine cap on your head, and let's explore the true, fully detailed, roller-coaster operation of the thermodynamically controlled contraption we call the two-stroke, internal-combustion engine. Along the way I will point out areas where "Engines 101" was unclear, poorly explained, or technically incorrect.

"Engines 101" started with the engine before the first combustion and then followed the operation cycle. It was assumed that there was a fresh air/fuel charge in the crankcase. That caused confusion about fuel transfer (intake) timing versus exhaust timing.

Since no combustion had yet occurred, I ignored the fact that the exhaust port opens slightly sooner than the transfer ports do. The main point was that the exhaust port became fully open at the same time the transfer ports were opened completely.

But that might lead one to wonder why the fuel/air mixture just doesn't go right across the cylinder and out the exhaust. Rather than explain that yes, it does do that (somewhat), I ignored it.

Actually, some fresh fuel/air mixture does exit the exhaust, especially during start-up. This is one of the two-stroke engine's inefficiencies that engine designers strive to minimize. This is also one reason why you may notice some fuel condensing in the muffler during operation—especially during "rich" operation. Only the fact that the engine's parts are moving quickly helps reduce this unwanted fuel/air loss.

This time I'll start with the engine's piston at Bottom Dead Center (BDC), meaning that it is as far down in its movement (called stroke) as it can get. The engine has not started, and there is no fuel anywhere inside the engine. There is no fuel anywhere except in the fuel tank. (Operationally, it is important to have fuel in the tank before trying to run the engine.)

Starting an engine from this position is difficult until fuel flows from the tank, through the fuel lines, and into the carburetor. Therefore, we need to draw the fuel from the tank, into the carburetor. We will use the "suction" effect that permits the engine to run in performing this task. Where does the suction come from? While at BDC, the rotary disk induction valve—the intake slot in the crankshaft—is fully closed.

As the engine is hand-rotated counterclockwise, the piston begins to move upward. It first closes all the transfer (intake) ports. At this point the rotary valve (for short) begins to open, but the exhaust is also still slightly open. However, there is no connection between the exhaust port and the engine's lower crankcase at this point, so that is irrelevant now.

As the piston continues to move upward, the crankcase volume (not area, as was written in "Engines 101"; that was a misstatement) begins to increase. As this volume increases with continued upward movement of the piston (not cylinder—another misstatement in the original article), a low-pressure area is created in the crankcase.

This happens because the now-sealed crankcase volume is bigger than it was, but it still contains only the original amount of air. The air expands to fill the increased volume and therefore has a lower pressure.

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