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Powertrains 102: Advanced Guide To Fuel Powered RC Engines

Article by Kenny McCormick

Before I cover the finer points of 2-cycle and 4-cycle engines, first I want to go over how engines work in the first place. Engines have one goal in life: To turn heat into rotation. Engines get this heat from a fuel of some sort. That fuel can be anything, and it doesn't even have to be burned inside the engine! Steam engines don't really care how they get their steam, you can make it electrically if you want. All they need is steam under high pressure. That steam is then used to opreate the engine. In RC, we use internal combustion, and our fuel of choice is either methanol or gasoline.

The dominant form of ICE powerplant in use is the piston engine. This holds true for RC as well. In a piston engine, you have, of course, a piston. It's a simple round slug, usually aluminum but it can be made out of any metal sufficiently durable, that rides inside a cylinder. In RC, pistons can be either ringed or non-ringed, depending on the cylinder they are intended to run in. This cylinder, too, can be made of any metal, but it is typically made of four constructions: Steel, cast iron, brass with nickel-plated bore, and brass with a chrome plated bore.

Cylinders and Bores

With the brass cylinders, the cylinder is machined to be tapered when cold, narrower at the top than the bottom, and the piston is matched to the bore of a given sleeve. It does not use a ring, the taper of the bore combined with the oil in the fuel provides the necessary seal. Also, the taper does more or less go away as the engine heats up and the top of the bore will expand more than the bottom. Ringed engines used hard steel liners with a somewhat softer iron or steel ring around the piston. These bores are cut straight. Iron bores tend to be matched with iron pistons and are usually machined straight but can be cut with a taper, and do not use a ring. The piston is lapped to a perfect fit. This is not used in modern engines, but you will find a lot of 1/2A stuff constructed in this manner. Cox engines are a very notable example.

rc engine

A note on ABC and ABN engines: You will not find a mass produced four stroke using one of these two cylinder constructions. The reason for this is that the pinch tends to quite literally grab the piston. High performance engines have a tendency of completely seizing up when cold because of this, with the only remedy being heating the cylinder to operating temperature to free the piston. Trying to force such an engine to turn over anyway risks breaking the connecting rod or crankshaft, as the parts are not really designed to pull the piston out of this pinch beyond the few strokes it takes to get the engine started. When a two stroke engine is running, there is always a constant downward force on the piston and rod, so this isn't really much of a concern. Should you attempt to build a four stroke with a tapered bore, however, you will need a heavily overbuilt connecting rod, which drives rotating mass up and can cause balance issues in an engine design already known for having balance issues. Not good. Hence, four strokes don't use tapered bores.

Pistons

Pistons, as mentioned, can be made out of anything. Typically, for steel, ABC and ABN bores, the piston is made of lightweight aluminum. Iron bore engines tend to use a matching iron piston. Most RC engines have most of their skirt left, although increasingly cutouts are appearing to lighten and provide clearance. The top of the piston is, 99% of the time, flat, although some older engines may have some special features designed to aid combustion in some manner.

The piston does not rotate, instead moving up and down, and these movements are called strokes. The connecting rod attaches the piston to the crankshaft, which allows the engine to cycle and allows us to retrieve useful rotation out of it. The cylinder head caps the cylinder off and holds glow/spark plugs, and in four strokes the valves, and the crankcase holds the whole thing together. Two strokes stop at this point, they need no further parts. Four strokes, in addition to those basic parts, have a camshaft, lifters, pushrods, rocker arms and valves, these parts all work in unison to control how gases move into and out of the engine. I'll cover them more in Powertrains 104. Other technologies exist, but they are not commonly used in RC. These include the gas turbine, used in aircraft but not anywhere else due mainly to fuel consumption and noise, and the Wankel, which is most famously found in the RX7 and RX8 sports cars and seldom used in RC. OS makes one for aircraft, but it's more a novelty engine than anything else.

Four Important Things

All internal combustion engines need to do four things in order to work: Suck, squeeze, bang, and blow... and in that order. Remove one and the engine stops running. We usually remove the "bang" part when shutting an engine down. Suck is simply drawing in the fuel and air. It is during this phase that the fuel is mixed with the air in our engines, even diesels(more on them later). Squeeze refers to compression. The more you compress the fuel and air mix the more energy you extract as motion when it burns. This is why racing engines have astronomically high compression ratios compared to lawnmowers. Compression also has effects on ignition, which glow engines take full advantage of. Bang is the fuel actually burning. The fuel doesn't explode...well it isn't supposed to anyway. It burns quickly and smoothly, and in doing so, increases the pressure inside the cylinder immensely. Heat is also produced, and as we all know hot gases expand. This drives the pressure up even more. This pressure is what drives the piston back down, producing useful power which we turn wheels and props with. The fourth part, blow, refers to getting the burnt gases out of the engine. These gases are no longer useful, and leaving them in the engine would impede the next cycle, so out they go.

The Suck Cycle

Now, let's focus a bit on the suck cycle. Specifically, the fuel and air. In our engines, air is drawn in through the carb. The carb then meters out a specific quantity of fuel and atomizes it. This mixture is then drawn further into the engine, either into the crankcase or directly into the cylinder depending on what kind of engine it is, and is eventually burned to produce power. Atmospheric pressure handles forcing the air in, as the piston draws a vacuum. Air rushes in, the carb mixes fuel with it, and this fills the engine.

The fuel is now in the combustion chamber and has been compressed. So how's it begin burning? There are three methods used in RC to accomplish this. One is spark ignition, one is glow ignition, and one is compression ignition. Ignition almost always occurs before the piston finishes compressing the mixture, this is because it takes time for it to burn and by the time the pressure builds the piston is just starting to move down. This gives the best power and efficiency. Glow ignition is the simplest system in use, so I'll go over it first.

Glow Ignition

Glow ignition relies on three things to function. It needs compression, a chemical reaction between the methanol fuel and the platinum in the element, and a fair bit of latent heat. When starting the engine we only have the first two, the heat is supplied by running an electric current through the element. When the engine is running, methanol reacting with the platinum in the element combined with heat from combustion keeps the plug lit, which allows us to remove the battery. Ignition timing is automatic, the fuel/air mix ignites when it wants to. Compression is the main means we have to control ignition timing. The more compression we have the earlier the base timing. Less compression means later base timing. The ignition must occur earlier in the cycle as RPM goes up, as well as when the throttle opens, if we want our engine making the best power it can with the least fuel needed. Heat is responsible for this. When you open the throttle the fuel mix burns much hotter inside the chamber. This puts more heat into the glow plug element, which means it lights the next charge off just a few degrees earlier. High RPMs have a similar effect, only instead of each charge burning hotter, you have more combustion cycles in a given timeframe. The plug has less time to cool off before it ignites the next charge.

Spark Ignition

Simple, wasn't it? Spark ignition is next. In a spark ignition engine compression and combustion heat play absolutely no role in ignition...well, at least, they aren't supposed to. Instead, the position of the piston and a blast of high voltage electricity handles lighting the fire each time. On top of this they run a different fuel, gasoline. In most modern gasoline RC engines there is a sensor on the crankshaft. This sensor determines where the crank is, and by proxy, where the piston is. This information is fed back to an ignition box mounted on the firewall. This box uses this data to determine A: where the piston is, and B: how fast the engine is running. A and B are computed, giving you some form of ignition advance, and when the box is happy, it sends a burst of voltage, 50,000 volts is about average for spark ignition engines, to the spark plug. This spark then jumps a small gap within the cylinder, igniting the gasoline within. Fun fact: Your daily driver uses an even more advanced version of this. It monitors more parameters and has to handle more cylinders, of course, but it is still similar.

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