Engines 101

BOB ABERLE CONCEIVED the “From the Ground Up” article series to enable all new pilots to get the most from their RC modeling experience. As successful as his concept was, Bob was also so successful in the execution of the first two parts–radios and electric power–that it is a challenge to even try to continue the series. But try I will, and I hope to continue the spirit and detail of Bob’s series.

As Bob covered so well in his radio series, “From The Ground Up” will continue detailed coverage of all aspects of becoming a successful, experienced model pilot. Selecting and building that first airplane, learning to fly it, maintaining it, progressing beyond the solo, and those first steps after the trainer will be discussed.

However, gaining experience as a pilot first requires flying! For the pilot to get flying practice, the airplane has to leave the ground, and the most common way to make that happen is to drag it up there behind an engine. That makes engines the next step in this series.

Having the correct engine for the airplane, making it easy to start, and running it reliably and without excessive wear are some of the most important aspects of learning this hobby/sport. Learning to fly is challenging enough without having to worry about flights that end too quickly and far too quietly.

Engines come in bewildering series of variations; these are .60s. L-R: K&B .65 is ABC, non-Schnuerle; SuperTigre .61 is Schnuerle ported, ABC; blacke-headed Rossi .61 is high rpm, high compression, rear exhaust; O.S. rear-exhaust, long-stroke .61 produces enormous torque at lower rpm and has fuel pump.

If you are flying with an instructor (always recommended), engine failure too far from the runway is almost the only way your trainer will be damaged during your flight instruction (if you follow Bob’s great radio advice). Having to repair your airplane may improve your building skills (covered later in the series), but it does little to speed your flight training.

So the goal here is to present engines, installation, mufflers, break-in and tuning techniques, fuel, fuel tanks and supply lines, propellers, support equipment, spinners, maintenance, minor repair, and, above all, safety practices in as much detail as possible to help you forget about your engine while you are learning to fly. Your engine is important but should not be the center of your attention during the early stages of becoming a model pilot.

The engine theory presented is intended solely to provide beginning RC pilots with enough knowledge of its workings to understand why choices regarding mixture settings, fuel selections, propellers, and other topics will be made.

There is no intent to fully detail an engine’s intricate workings. The new RC pilots are not going to be designing or disassembling (hopefully) their first few engines, but they will be setting high- and low-speed mixture settings.

All the theory presented will be strictly from an operations viewpoint. Proper engine operation is my only goal in this discussion. As with most things mechanical, a model-engine’s true operation is complicated.

Operations that are separately explained and appear to be independent actually overlap, and sometimes interfere with, other operations. But I will explain each operation as if it were the only action happening at that time, to simplify the theoretical presentation.

What is this tool called a model “engine”? It is an air- and fuel-cooled, fuel-lubricated, venturi-fed, catalytically enhanced (the glow plug) combustion-ignition machine constructed from aluminum, with some steel in high-stress areas. It is designed to convert a fuel’s chemical energy into something that will turn a propeller.

Considering each aspect of that boring description helps you understand and avoid some of the most common model-engine problems. Having a machine convert fuel into mechanical energy releases heat. This heat has to be removed, or the machine will literally begin to melt and fuse its moving parts.

Our engines remove this heat by directing the propeller’s airflow over most of the engine; they are air-cooled. But airflow is a poor means of engine cooling. Unlike water or glycol (antifreeze), air is not the best “heat exchanger” and does not reach all parts of the engine equally. The parts in the propeller’s slipstream receive more airflow than parts that are not. Plus the air does not remain in contact with the engine for long and therefore does not have time to absorb much heat.

Unlike water-cooling, the cooling air cannot reach deep into the engine to cool the moving parts directly. Our engines use “fins” to increase the surface area contacted by the cooling air, but air-cooling remains a surface-contact process and is thus inherently inefficient.

To help remove heat the air can’t, our engines use fuel cooling as well. The lubricating oil in the fuel acts as a heat exchanger while the fuel’s methanol cools the lower internal engine parts by refrigeration.

Refrigeration, you ask? Methanol cools our engines’ lower areas 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 refrigeration process removes heat from the surrounding lower engine sections to have the energy to transform the methanol. But I don’t suggest you try using this “refrigerator” to keep your iced tea cold.

The fuel also helps cool the engine’s combustion chamber. Some of the fuel’s oil content is not burned during combustion but does absorb heat. As the heated oil is exhausted, it removes that absorbed heat. The important point to remember is that the fuel cools the engine as it powers it.

Equally important, our model fuel is the engine’s sole lubrication source. The fuel contains oil that keeps the moving parts separated from each other, reducing friction and lowering the engine’s temperature.

Unlike most car engines which have an independent oil source, the amount of oil applied to a model engine’s moving parts depends entirely on the engine’s rate of fuel supply, or “mixture setting.” The mixture setting adjusts the amount of fuel that is mixed with engine’s incoming air supply.

An engine’s maximum air supply is fixed by the diameter of the carburetor opening and adjusted by the area opened by the throttle barrel. But the pilot adjusts the amount of fuel mixed with that incoming air supply using high- and low-speed fuel-metering devices known as “needle valves” and/or “air bleed” adjustment screws.

By properly adjusting these fuel-metering devices, the pilot is responsible for the engine’s operating temperature and therefore its reliability and durability. This is true no matter what type of engine is used–a two-stroke or a four-stroke. There are several other types of model engines, such as gas ignition or true diesel, but the two- and four-stroke alcohol-fueled types comprise the majority of the engines that new pilots use.

A quick look at the exploded engine views and photos reveals the differences between the two major engine types. The first (Diagrams 1 and 2), and simplest, is called a “two-stroke” engine. These are almost as operationally simple as engines get. Their design may be complicated, but provide them with the proper fuel/air mixture and a “lit” glow plug (the catalytic enhancer), and they will start and run every time. But how do they work?

To best illustrate how it runs, we start with the engine not running and totally without fuel. The engine’s piston is at Bottom Dead Center (BDC). This means that it is as far down in its movement (called a stroke) as it can get. The engine has not started and there is no fuel anywhere inside it. In fact, there is no fuel anywhere except in the fuel tank. (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 and 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 slot cut into the hollow crankshaft just under the carburetor–is fully closed. The induction valve (for short) connects the carburetor to the engine’s lower crankcase when it is open, allowing fuel and air to flow into this lower area.

As the engine is hand-rotated counterclockwise, the piston begins to move upward and closes all the transfer (intake) ports. These ports are cut into the cylinder wall opposite the exhaust port and connect the lower crankcase section to the sections above the piston and transfer fresh fuel and air into these upper sections.

The intake slot in the crankshaft is clearly visible. This SuperTigre engine’s crankshaft is ball-bearing supported front and rear. Check out the “bulges” that house the bearings.

At this point, the induction valve begins to open. As the piston continues to move upward, the lower crankcase volume begins to increase. As this volume increases with continued upward movement of the piston, 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.

But remember the induction valve that was just opening as the transfer ports were closing? It opens more as the piston travels upward. The valve is fully open at this point, and that means that the crankcase section is no longer sealed. If the carburetor throttle barrel is open, air rushes through the carburetor, through the rotary valve (crankshaft), and into the crankcase. Remember this process; it will be repeated shortly once fuel is added to the mix.

The connecting rod connects the piston to the pin on the crankshaft, converting the piston’s up-and-down motion into rotational motion.

Now there is lots of air rushing into and through the engine as we mechanically hand-rotate the propeller. What happens if we put an obstruction such as a thumb over the carburetor’s air inlet?

Low pressure returns to the lower crankcase since it is again sealed, even when the rotary valve is open. But the piston is still moving and re-creating the low-pressure condition with each revolution. You can actually feel the suction with your thumb. This suction effect draws fuel and air into the carburetor. It is the engine’s only fuel-draw mechanism except for gravity. Since this suction is never as strong as a fuel pump, fuel-tank placement is critical.

The low-pressure condition seeks relief from wherever it can, and since the only possible pressure relief is the small brass fuel inlet inside the carburetor itself—the fuel jet—fuel is drawn from the fuel tank and into the fuel jet.

Larger brass “tube” on right is fuel jet. Smaller brass fitting on left is low-speed needle valve that regulates amount of fuel flowing through fuel jet at reduced throttle levels.

Remove the obstruction. As the induction valve opens, the crankcase’s lower pressure draws fuel through the fuel jet and air from the atmosphere into the induction valve. As the air is pulled through the carburetor, it speeds up to go through the narrow intake passage. The added velocity means that the intake air gains kinetic energy, and to maintain balance, the potential energy (temperature and pressure) drops.

When the engine is running or hand-cranked, this lowered pressure is seen at the fuel jet, and the difference between this low-pressure area and the outside air pressure (seen at the fuel-tank vent) “sucks” fuel into the carburetor as if your thumb were still there!

When the piston reaches as far upward as it can—Top Dead Center (TDC)—the fully open rotary valve begins to close but is still drawing fresh air and fuel into the crankcase for another 70-90 degrees of crankshaft rotation. The valve closes completely before the exhaust port begins to open. The crankcase and combustion chamber are again sealed. But the piston still has a ways to go before reaching BDC. It continues downward, compressing the fuel/air mixture inside the engine’s crankcase.

This results in the lower crankcase becoming a high-pressure area. The piston continues downward, compressing the crankcase mixture and increasing the pressure. But before reaching BDC, the piston uncovers the transfer and boost transfer ports (extra ports cut into the engine to increase transfer efficiency). The high crankcase pressure now has an exit.

The cylinder liner has been removed from the engine. It is positioned to show the large exhaust port.

The fuel/air mixture under pressure rushes up through the transfer ports and fills the entire volume above the piston. Since the exhaust port is also fully open at BDC, some of this precious mixture is lost out the exhaust port. But some remains above the piston.

(One advantage of the Schnuerle boost transfer port system is that less incoming fuel/air mix flowing from these side-mounted ports is lost out the exhaust. The Schnuerle ports are not aimed straight out the exhaust port as the main transfer port is.)

The liner has been positioned to illustrate the Schnuerle port (center) and one of the two standard intake ports (lower right).

As BDC is passed, the piston travels upward, pushing more of the fuel/air mixture upward and into the already filled combustion chamber. Yet some still goes out the exhaust port—another inefficiency. Once the exhaust port closes, the piston begins to compress the fuel/air mix as it continues upward. If the glow plug is lit and the fuel/air mixture is in the proper proportions, a prolonged, controlled explosion called “combustion” occurs.

How far upward the piston travels determines the engine’s compression ratio. This is the ratio of the entire cylinder volume above the piston when at BDC to the remaining cylinder volume with the piston fully raised at TDC.

For example, if the volume above the piston is 10 times larger when the piston is at BDC than the cylinder’s combustion area at TDC, the compression ratio is 10:1. The higher the compression ratio, the more power is produced by the final fuel/air combustion. But compression ratios can be too high, causing preignition, hot running, burnt glow plugs, and piston damage. Most sport engines do not have high compression.

The engine is now running at full speed. The piston is at BDC with most of the exhaust gases gone, receiving a fresh charge of fuel/air from the crankcase into the now-vacant volume above the piston, right? Well, not really.

The exhaust port opens only slightly before the transfer ports, called the exhaust lead or blowdown. The exhaust gases have not fully exited the cylinder when the transfer ports begin to open. The relationship between these openings is part of the engine’s timing.

Diagram 4 summarizes many sport engines’ timing in this regard. In practice, this timing means that fresh fuel/air mixture is flowing into the cylinder even as exhaust gases are exiting. Why would an engine designer do this?

The hot, still-expanding exhaust gases are exiting at a high velocity. This forms a low-pressure area just above the piston, “behind” the exiting exhaust gases. The fresh air/fuel mixture is “pulled” through the transfer ports into the low pressure in the cylinder at the same time as the descending piston is compressing the mixture in the crankcase and pushing it into the bypasses. We say the exhaust gases “scavenge” the fuel/air mixture into this section. The scavenging effect increases the velocity, hence the amount, of the fresh fuel/air mixture that is drawn into the engine.

As the scavenge action is completing (the momentum of the exhaust gases is exhausted) and the pulling of intake from the crankcase through the transfer ports is ending, the induction valve opens. This helps start the flow of fresh fuel/air mixture into the crankcase for the next power stroke.

At extremely low speeds, such as like idle, the scavenging action goes to completion and you are back to having pressure in the crankcase at the moment of the induction valve’s closing because of the descending piston. You can sometimes tell this is happening as the engine spits fuel from the carburetor at slow speeds.

The dark line around the top of this Webra .91 engine’s piston is the ring. Also notice the idle mixture-adjustment screw in the center of the throttle arm.

ABC SuperTigre .61 has no ring. The thin line on the piston is actually the oil retention groove that helps to keep the engine’s top end lubricated.

Therefore, the scavenge effect is the major force our engines use to put fuel and air into the combustion chamber. Yet crankcase pressure does play an important part. Together, these alternating, thermodynamically produced high- and low-pressure conditions allow our engines to run.

Several exhaust systems are available that will increase the scavenging effect. They are for a later discussion, but now you understand how and why they could increase an engine’s power by increasing the scavenging effect.

During the charge cycle, some fresh fuel/air mixture is drawn out the exhaust along with the escaping gases. This is lost power and poor fuel economy that engine designers strive to recover as much as possible.

An additional complication is that the combustion occurs before the piston reaches TDC. It continues even when the engine reaches TDC and ends at or after TDC. The amount of advance is in Diagram 4.

It may seem strange to put combustion pressure against the piston’s upward movement, but combustion takes time and our fuel doesn’t burn all at once. Therefore, the prolonged explosion used to burn as much of the fuel/air charge as possible is made possible by the “advanced timing.” The relationship between the piston’s movements and ignition is a delicate balance. Too much advance and the piston may be damaged. Too little means insufficient combustion occurs.

However, running an engine too “lean”—not enough fuel and too much air in the mix—produces extra heat that can change this delicate balance. Hot engines can experience timing that becomes so advanced that detonation occurs. This means that the fuel/air mixture ignites before it should.

This condition may sometimes be identified by a loud “frying egg” sound (crackling) as the engine is run at full speed. When you hear this sound, your engine may be in for problems from overheating and detonation. Land. Readjust the high-speed mixture.

The piston is rapidly moving up and down. But the propeller needs to rotate. Therefore, the piston is connected to the crankshaft by a device called, for some strange reason, a “connecting rod.” The piston’s up-and down-movement is converted to a rotating crankshaft that turns the propeller.

At least one side (sometimes both sides) of the connecting rod uses a bronze bushing that must be well lubricated. The connecting rod is the part that breaks if a too-large or too-small propeller is used. When the rod breaks, the rest of the engine is usually destroyed.

As I mentioned, the combustion process creates a lot of heat, which is used to keep the glow plug glowing even when the battery is disconnected. In effect, the glow plug is working to catalytically create each combustion cycle just as a diesel’s combustion, created by a high compression ratio, creates its combustion. Neither requires outside ignition energy to continue the process.

Simple and elegant, with an explosion occurring every time the piston moves up and down, two-stroke engines are the definition of power and reliability.

Two-strokes come in several types and designs. Each has its own strong and weak areas. The new pilot should be concerned with a few of these varied designs because they are important for proper engine selection. Look at the photo of the two .60 engines. The engines are called “.60s” because the piston displaces .60 cubic inch as it travels.

The total volume of the space—stroke length multiplied by the cylinder’s cross-sectional area—occupied by the piston in its travels is called the engine’s “displacement.” Most sport two-strokes used on trainers have a .25-.61 cu. in. displacement.

The larger the displacement, the more powerful the engine and the larger the propeller it can use. But larger-displacement engines use more fuel per minute and cost more. Larger engines also require larger airframes that could cost more.

Look carefully at the two .60-size engines’ sides in one of the photos. One engine has a straight cylinder and one has a raised area (under the “S”). That raised area is the space for the boost transfer ports designed and patented by Dr. Schnuerle in the late 1930s but not first used until after World War II. “Schnuerle porting” is nothing more than extra transfer ports that permit extra air/fuel mixture to flow into the combustion chamber.

Look carefully at the sides of these engines. The SuperTigre on the left has a bulge (under the “S”); that is the Schneurle boost port.

Some engines have several extra boost ports. Almost all sport engines made today have at least one such Schnuerle boost port. Schnuerle engines use more fuel and are slightly harder to adjust at idle but are more powerful than non-Schnuerle engines.

The opposite side port cut into the cylinder sleeve is the intake boost port. The front intake port is the light area toward the right. A rear intake port is visible if you have great eyes.

Another point to consider is the crankshaft’s support system. If you look at Diagram 2 of the O.S. .25 FX, you’ll see a front and rear ball bearing supporting both crankshaft ends. Diagram 1 of the .40 LA does not have ball bearings. Instead, there is a bronze bushing in the crankcase housing. The crankshaft slides into, and is supported by, this bushing.

Bronze bushings provide more crankshaft support area but also more crankshaft friction that robs engine power. Ball bearings provide better support because they reduce friction. But ball bearings are also more susceptible to corrosion if not properly maintained. Bronze bushings tend to wear sooner than ball bearings and are more difficult to replace. But bushings do not rust or corrode. Electric starting is less problematic with ball bearings; bushed engines require a lighter “touch” when pressing the starter against the propeller.

The K&B engine in the foreground uses a bronze bushing for support. The front housing does not have the bearing-support bulges as does the ball-bearing-supported SuperTigre in the rear.

There are two main two-stroke design types: ABC (AAC) and ringed piston. Ringed-piston engines have one or more compression rings on the piston, just like an auto engine. The rings provide drag and do not have as tight a seal against the cylinder wall as ABC engines do. The ABC type is unique to model engines. The piston is made from Aluminum and the cylinder is made from Brass plated withChrome. The piston actually has the same diameter as the top of the cylinder does. This can be felt when turning the propeller. The propeller rotation tends to “stick” as the piston reaches the top of its stroke and enters the section of the cylinder with the same diameter.

How can this work? The varied construction materials expand at different rates as the engine heats when running. The cylinder expands more than the piston does. The final diameter of both allows some clearance at TDC for the piston to move when the engine runs. This makes for a tight compression seal at the top end without ring friction, which could absorb some power. But many non-long-stroke ABC engines have slightly lower torque than ringed engines since compression drops slightly as the piston progresses downward into the larger cylinder area. This expansion difference also provides protection if the engine overheats.

When overheating, the cylinder continues to expand faster than the piston, providing extra clearance that protects these vital parts. But the engine still sags, reducing power, while the destruction process begins in other parts of the engine.

ABC engines work well in the smaller sizes, up to roughly .61 cu. in. Most engines that size sold today are ABC or AAC (Aluminum piston, Aluminum cylinder, Chrome plated), but several fine ringed designs are also manufactured. Most engines larger than .61 cu. in. are ringed.

Even though ABC engines are more “lean” tolerant, ringed engines have the same, maybe longer, projected service life. This is true only if the ringed engine is never run too lean or improperly maintained.

Because two-strokes are simple, reliable, less expensive, easy to operate, and powerful, most new pilots start with them. To date, all the RTF trainers on the market use two-strokes (except for one electric-powered version). RTF trainers usually feature an installed engine, a fuel system, a radio, and a completely prebuilt airframe that requires no previous modeling experience and less than two hours to complete.

Shown are four- and two-stroke engines’ “heads” (tops of combustion chambers). Tubes are four-stroke’s intake and exhaust “manifolds.” Both heads are hemispherical (recessed half spheres) in shape for more power, made popular by Chrysler’s “Hemis” of the 1960s.

But many new pilots prefer the realistic sound, fuel efficiency, and more available torque of a four-stroke engine, so they forego the convenience of an RTF airframe and purchase an ARF trainer. ARFs require a little modeling “know-how” (i.e., gluing wing halves together) and approximately 20 hours to complete. The new pilot must also buy and install the radio and engine. This is currently the only way to have a four-stroke on your trainer.

Model four-strokes are still fuel- and air-cooled, glow-catalyzed machines. Most important, unlike larger four-strokes such as automotive, most lawn mower or generator engines, model four-strokes remain fuel lubricated. This is vital to remember.

Except for needing fuel lubrication, model four-strokes work much like those blue-and-red engine diagrams we learned about in school. (Of course, those diagrams never showed the black, messy grease we had to wade through to get to those “simple” parts.)

Instead of drawing the fuel/air mix through a venturi-fed, hollow crankshaft and then through transfer ports into the combustion chamber, model four-strokes feed their combustible gas mixture into the upper cylinder area through an intake valve. As can be seen from a diagram and photos, this change makes for a different engine that operates using an entirely different process.

As a four-stroke starts, the air/fuel mix is created inside a carburetor that is almost identical to that of a two-stroke. But instead of then being drawn into a hollow crankshaft, the combustion area’s partial vacuum caused by the downward-moving piston draws the fuel/air mix from the carburetor through an intake pipe (intake manifold), then through the open intake valve and into the cylinder’s combustion area. Of course, the intake valve has to be “open,” pushed away from its base, for the gases to flow into the cylinder.

Since a steel valve is unintelligent and can’t figure out when to open on its own, it is forced open by a device known as a “pushrod” that is powered by the “camshaft,” which is usually located near the engine’s crankshaft.

The camshaft is connected to the crankshaft by a timing gear. When the camshaft opens the intake and exhaust valves, and for how long, determines the timing of a four-stroke. Like the two-stroke’s, the four-stroke’s timing is advanced. Fresh fuel/air mix is compressed by the upward-moving piston and ignited before the piston reaches TDC.

You’ll find a lot of interesting information just by studying Diagram 3 of the O.S. 70S. I will cover four-stroke model engines in an upcoming installment.

The brief description shows that four-strokes operate different from two-strokes. While a two-stroke has a fuel/air explosion every time the piston reaches the top of its travel, a four-stroke has such a power-producing explosion every other time the piston reaches the top. Since a four-stroke has half the number of explosions per series of revolutions, it should, in theory, produce only half the horsepower of a two-stroke engine. However, as you have seen, two-strokes have several inefficiencies that cost horsepower.

Rather than twice the power, four-strokes originally had roughly 60% of a two-stroke’s power. But manufacturers have learned that four-strokes are more tolerant of timing advances, can have larger carburetor openings and intake valves, and are easier to “supercharge” than most two-strokes. A modern four-stroke can produce approximately 75%-80% of the horsepower produced by an equivalent-displacement two-stroke. If supercharged, a four-stroke can equal a nonsupercharged two-stroke of the same size in horsepower.

But four-strokes do have one advantage over two-strokes. For various technical reasons, such as combustion duration, timing, and the nature of valve intake combustion, four-strokes produce their maximum twisting force, or torque, at rpm that most sport pilots can prop their engines to reach. Many two-strokes need to reach speeds faster than 13,000 rpm to reach maximum torque, and that speed is hard to reach with a sport engine running on 10%-nitromethane (a power enhancer)-content fuel.

The more available torque means that a four-stroke can, in practice, safely turn larger-diameter propellers, with equivalent pitches, than a comparable two-stroke. Four-strokes are also quieter, and their exhaust note is slightly lower pitched because of lower rpm.

As for two-stroke horsepower ratings, they are of little practical use when selecting an engine. Such ratings are usually computed at high rpm using small propellers. It is possible to use the same small propeller on your trainer, but the average trainer airframe has so much drag that the fast-turning small propeller has little thrust effect and your trainer will barely move through the sky.

What’s thrust? How do I find the right propeller for my engine/airframe combination? What good is a spinner? As this series progresses, I’ll cover these subjects and many more of the technical details, similarities and differences between engine types, and their care and feeding. I’ll also discuss field accessories, tools, fuel types, and many other subjects in great detail.

You may not become a model-engine expert, but you will learn everything you need to know to select an engine, keep it running reliably, make it last for years, and get the best flying performance possible.