Please note: this article is incomplete and in development.
Turning up to 50,000 RPM in a modified version and cranking out almost 2 horsepower, nitro engines are the ultimate R/C powerplant for the scale model gearhead. Although the advanced technology of nitro engine expertise is beyond the knowledge of this author, there are a few important points that everyone should know about these little screamin' demons.
Two-Stroke Nitro Engines
The heart of any nitro-powered vehicle is the engine. Most nitro-powered vehicle run a .12 or .15 cubic inch engine, and many 1/8 scales run .26-.28 c.i. engines - but this is by no means a standard. At the time of this article, the current largest ground vehicle engine is a .46, standard with the CEN Genesis monster truck.
When selecting an engine size, look closely at the unit of measure used. One manufacturer produces a 2.5 engine. Being the standard is in cubic inches, this indicates something far larger than what we're used to - but the reference is 2.5 cubic centimeters, which is pretty close to .15 cubic inches. More marketing at work!
The simple but ingenious configurations of most nitro engines consists of a simple carburetor with an adjustable high speed needle (main fuel flow,) a low speed needle (spray bar to assist atomization and reduce fuel flow from the main jet for low speeds,) an idle speed adjustment screw, and may contain a third, factory preset "mid" speed needle. The vaccum created by the piston draws fuel/lubricant mix into the carburetor where most of it is atomized into a mist, down into the crankcase, up the sides of the piston sleeve through precision-positioned intake ports, and into the combustion chamber where the gases are compressed and ignited. Precision positioning of the intake and exhaust ports allows exhaust gasses to exit the chamber and draw the next cycle of fuel in with only the movement of the piston to manage the transport of the gasses.
The engine internals are also deceptively simple. A bearing-mounted stainless steel crankshaft, aluminum or steel connecting rod, aluminum or steel piston, and an aluminum-brass chrome (ABC) or aluminum-brass-nickel (ABN) removable piston sleeve are the only major internal parts, encased by a cast aluminum crankcase and topped off by a finned aluminum cooling head.
The top of the piston sleeve is slightly tapered, known as a sleeve's "pinch," so that as the piston comes to top dead center, it seals more tightly insuring maximum compression during combustion. As the engine wears and the pinch decreases, the piston and sleeve will need to be replaced, or a process called sleeve pinching or squeezing can regain the pinch lost due to wear.
Perhaps it is the simplicity of the engine that makes them so difficult to tune for many beginners. Precise high-speed and low speed mixture settings are required for the engine to perform correctly throughout the power band without overheating or failure. Either side of the tuning "sweet spot" and the engine won't start, or stalls at inopportune times. Good engine tuning comes only with experience, but can be learned in a very short time with the help of an experienced nitro hobbyist. To avoid the nitro newcomer blues, if you'e planning on buying a nitro, buddy up with someone who already owns one. You'll also gain a new racing partner in the process.
New nitro engines, like their full-scale counterparts, require a break-in period. While break-ins are well described by the engine manufacturer manual, most break-in procedures consist of an overly rich fuel mixture to insure ample lubrication while driving around at low to mid speeds. Continue this procedure for at least five tanks, allowing the engine to completely cool between tanks to obtain a full heating and cooling cycle. As you approach the later tanks, you may begin leaning out the engine for performance and increasing the throttle range. Breaking in the engine by allowing it to idle for several tanks is not recommended. You do not reach the correct temperatures to obtain a good heating and cooling cycle.
Most nitro engines will run well with a 10% - 30% nitro percentage, but the cost of fuel increases pretty drastically with percentage increase. Most owners run 20% nitro, but racers who apply various engine modifications to get the most out of their engine will run 30% or 40% nitro. Whatever you decide to use, it's important to stay with the percentage of fuel you break in with. You may change nitro percentage at a later time but will need to re-run a small break-in of the engine to allow it to adjust to the new percentage.
An oil-soaked air filter, usually with a one or two-stage foam element, is found over the air inlet of every nitro engine; the oil helps trap any dust that might otherwise find its way past the filter element. Never, ever ever run your nitro engine without an air filter! The slightest bit of dust will kill any nitro engine.
The Glow Plug
Nitro engines don't use an electrically charged spark plug but a glow plug that has a fine platinum element inside its hollow core. The coil of the glow plug is initially charged to red hot with a glow starter, a special tool that clamps onto the glow plug while it's installed in the engine head, powered by a single rechargeable sub-C cell. Once the engine begins running, a catalytic action between the alcohol fuel and the platinum of the glow plug's coil keeps the coil glowing bright and ready for each rotation of the engine.
This is also a reason why correct engine tuning is often sensitive and hard to find. If the fuel mixture is too lean (not enough fuel for the oxygen entering the engine,) the glow plug will starve and go out. Too rich (too much fuel for the oxygen entering the engine,) and the lubricants in the fuel will douse it out.
Glow plugs come in various temperature ranges - thicker elements in the glow plug are "cool" plugs, and thinner ones are "hot plugs" - and generally are paired with the nitro percentage in the fuel. Hot plugs work well with lower nitro percentages (0-10%,) medium temps with average nitro (10%-25%,) and cool plugs should be used with higher nitro content (25%-40%.) There are variations on this rule of thumb, but they are based on advanced modifications and more importantly, what works best for a particular engine and fuel.
Starting a nitro engine involves:
- Fill the fuel tank,
- "Prime" the engine - plug the exhaust slightly and crank it over, forcing fuel to travel up to the carb (don't over -prime or it will flood!)
- Turn on transmitter and electronics,
- Check again to make sure you turned on transmitter and electronics,
- Performing pre-startup checks as described in Radio Systems below,
- Place the glow starter on the glow plug (make sure it's charged!)
- Rotate the engine quickly to begin the starting sequence.
There are various methods of rotating the engine for a startup. Many engines have a pull-start, a spring-loaded mechanism similar to the pull-start on any lawn and garden equipment. When using a pull-start, it is IMPORTANT not to pull the start on a new engine unless you heavily lubricate it or are preparing for a start. Never pull the pull-start cord out more than 3/4 of it's length when trying to start. This can break the cord or the delicate mechanisms holding the spring in place inside the pull-start.
They're often a bugger to fix if you break the pull-start internals.
A second popular method, used on racing engines and engines without a pull-start mechanism, is to use a starter box, a box containing one or two electrical motors inside, powered by a 12 volt gel cell. A rubber wheel driven by the electric motor(s) protrudes through the top of the starter box. The box has pins that can be positioned so that when you place your vehicle on the box, the flywheel access hole in the chassis lines up PERFECTLY with the starter box wheel without the wheel touching any other part of the chassis. The top of the box is spring-loaded with a pressure-actuated switch in one end of the box; when you press down on the box, the starter box wheel cranks over, turning the flywheel, and starting the engine.
When using a starter box, if the flywheel presses immediately against the starter box wheel, it is likely to stall the starter box, which can eventually burn up the motors inside it. The idea is to press the box so that the motor(s) spin up without touching the flywheel, then an additional push "bumps" the flywheel against the starter box wheel in bursts. This is also why the starter box has aquired the name "bump starter."
The gel cell, also called a Sealed Lead Acid battery (SLA,) that is used in a starter box can be bought at any battery shop and fits neatly inside the box itself. It should be charged with a "float charger," used in recreational vehicle applications. This allows you to leave the charger attached to the cell at all times when it's not in use without danger of overcharging.
Gel cells are not meant to be charged at a higher rate than 1700 mah. A car battery charger's "slow charge" rate is 2 amps, or 2000 mah. While some use these to charge their gel cells, this is not recommended as it is likely to burn up the battery, shorten it's life, or worse, cause it to explode. Use the float charger peviously mentioned; it is safe and designed for charging SLA's.
In the absence of a gel cell, two ordinary 6-cell RC packs connected in series or a regular car battery can be used to power a starter box, but exercise caution in connecting the leads as car batteries can explode if shorted out.
Another method that only works with pullstart engines is to remove the pullstart unit completely and insert a power drill-driven attachment, called a Yank Eliminator or a Roto-Start, to the pull-start interface in the back plate. When the engine starts, simply pull the attachment away from the engine.
And finally, some of the newer models are emerging with onboard starting devices built onto the engine. More difficult to manage when you have to remove or work on the engine, onboard starting systems are pretty slick, but they are not for racing, and neither are pull-starts. In racing, all unneccessary equipment is removed to lighten the vehicle load.
Stopping
There are four ways to stop a nitro engine:
- Run it until the fuel tank runs dry.
- Using your shoed toe or a rag (it is hot!) plug the exhaust pipe. Many do not like this method claiming it floods the engine but is the fastest and most accessable method with the body on the vehicle.
- Using a toe or soft item, such as a screwdriver handle, apply pressure to the engine flywheel from the bottom of the vehicle via the flywheel access hole. A small touch is all that is required; this stalls the engine without flooding it.
- Pinch the fuel line at the carburetor to starve the engine.
. . . to be continued . . .
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