Ignition engines becoming more popular
Eric Henderson | [email protected]
THIS MONTH we are going to look at the growth and development of ignition engines.
There has been a large trend in recent years to use electric motors to power model aircraft instead of glow engines. The growth of electrics has somewhat overshadowed the significant increase in popularity of ignition engines.
You might note that I wrote “ignition” rather than “gas.” This is because existing ignition engines use either methanol- or gasoline-based fuels. (I am unaware of any model engines that use both types because of the effect of different-based fuels on components such as diaphragms, fuel lines, and seals.)
The new popularity of ignition engines can be attributed to many factors, including the increased availability of purpose-built designs for model aviation.
There was a time when a gas engine for an RC model was really just a converted weed whacker or chainsaw power plant. Those who did these conversions tended to be highly skilled. Leave it to the creativity of modelers to take a garden tool and make it fly an airplane. They were mechanically oriented people and knew how to not only make their creations work, but get the best they could out of designs that were not that great for flying.
These “wee beasties” were often hard to start with badly timed ignition systems and were not that great in the rpm and associated power department. Starting a converted gas engine was often dangerous and occasionally downright nasty. Hand-flipping a propeller on one of these creations often caused a backfire or premature ignition. The piston was not far enough up the barrel and would go back the wrong way.
Two things caused this. Sometimes the propeller, shaft, and piston were not moving fast enough. The other cause was the ignition timing was fixed and too soon (advanced) for hand starting. At normal running speeds this ignition timing was fine, but during starting it was not very friendly.
Also included in this column:
- Two-cycle engines
- DX8 telemetry use
This all changed when transistorized (aka electronic ignition, EI) systems came along. Most of these new EI systems don’t allow the spark plug to fire unless the crankshaft is spinning at more than 300 rpm. This is equivalent to a hard flip of the propeller. If the propeller is turned over slowly during an action such as priming the carburetor, the spark plug will not flash.
Generally the current breed of ignition engines is built and designed with safety and ease of operation in mind. There are water-cooled versions for model boats. You will see ignition engines in helicopters and remote-control cars. The airplane variants are now lighter and more powerful than their predecessors.
Because of their origins, ignition engines were once quite large and heavy. In the last 10 years the weight has decreased. You can find gas engines as small as 17 cc and as large as 350 cc. Some airplanes can now be pulled vertically with ease.
Smaller, 40- to 60-size RC aircraft, powered by glow engines, can now reap the benefits of ignition engines. One not-so-obvious benefit of a gas engine is that the exhaust burns much hotter than glow-fuel counterparts. This makes fitting smoke systems a breeze. Squirt some kerosene into a gas engine exhaust system and you will have white smoke billowing everywhere.
One sidebar-type question often asked is, "Why are model gas engines predominantly two-cycle?" It is most likely an issue of lubrication.
There are plenty of four-cycle, gas-fueled engines in the world. Most of us drive one. The lubrication for model two- and four-cycle engines is typically included in the fuel. Full-size four-cycle engines usually use an oil-feed system with some kind of sump and pump—not the most practical or lightest system for a model engine.
The advantages of ignition over glow or electric power can be argued, depending upon personal taste. For the sake of observation rather than argument, here are a few of the positive points that have contributed to the growth of the gas engine movement. I did write "gas" this time because the majority of ignition engines use a two-cycle gasoline/oil mixture. (I can cite the YS-170 ignition engine as a current exception.)
The most obvious advantage of a gas-powered airplane can be seen at the flying field. If you look at what a gas jockey takes compared with a glow engine user, you will only see a small gas can and most likely a heavy leather glove. That's it!
Most gas engines only need a few flicks of the propeller to prime the carburetor and they fire. You just open the choke, flip again, and they purr to life. The typical glow engine flight box—if there is such a thing—has a glow igniter, an electric starter motor, and a 12-volt battery.
The one winning fact is the cost of the gas/ignition fuel. A gallon of glow fuel can easily be five times the cost of a gallon of premium gasoline and the associated two-cycle oil. Additionally, gas engines are legendary when it comes to their miserly fuel consumption. This leads to smaller fuel tanks and/or much longer flight times.
Less power comes from burning gasoline (petrol to the rest of the world) than methanol. In essence, methanol is alcohol, which is poisonous to humans. Drag racing cars use alcohol for a reason: it gives a better power burn. However, there is still plenty of power that can be obtained from burning gasoline.
Gas engines require an ignition system to provide a spark to ignite the gas/air fuel mix when it is compressed by the piston. The EI systems need a power source and employ a small battery pack that stays with the airplane for the entire flight. Magneto-based systems that do not need a battery still exist, but they need a mechanical way to advance and retard the ignition for starting.
Ignition engines run at lower rpm, but compensate with better lugging power (torque). They willingly turn larger or higher-pitch propellers than their glow counterparts. They also will idle at slow and reliable speeds. They don't tend to cool off, load up, and die like their glow plug counterparts.
Another reason for their increase in popularity is that the knowledge base of operation has increased. The basic setup for an ignition engine is not the same as a glow engine. It can take up to five hours to break in a two-cycle gas engine. During this break-in period, the accumulated experience of a glow plug veteran often gets in the way.
Glow-plug gurus just can't leave the fuel mixture needles alone. They hear a gurgle or a misfire and immediately think "adjust the mixture!" Generally it is not the fuel-to-air ratio that needs to be changed. They should be referring to the manual to check the oil-to-gasoline mix charts. The manufacturer already knows what the right fuel/oil mix should be once the engine is broken in.
Getting a gas ignition engine to run smoothly could be as simple as using less oil in the gasoline. This is pretty counterintuitive for an experienced glow guy. For example, reducing a 30:1 break-in fuel/oil mix to 40:1 will often let a coughing gas engine run smoothly. If the engine is fully broken in and the fuel mix is dialed in correctly you may never have to change your carburetor's original high- or low-needle settings. If you do, it will usually be a very small change.
Reducing the oil content produces more power. Oil does not combust as gasoline does. In the same space more oil equals less combustion and provides less power. Less oil means there is more gasoline and air in the combustion chamber, so you will get a bigger and better bang. Of course you still need some oil to lubricate the moving parts.
More on DX8 telemetry use
In the review of the O.S. FS-95V ringed four-stroke engine, I made a slight departure from the more traditional test mount and fitted a Spektrum radio system. The new Spektrum DX8 telemetry module was employed with rpm and temperature sensors fitted to the engine.
This same system has some super benefits when fitted to a model airplane. You can tell the radio to set off an alarm when the engine is not running. This is great when you are flying a busy field or at a large meet such as a Giant Scale event.
The DX8 will beep if you set the low-rpm alarm level to zero. You can turn the alarm off by touching the "clear" button to avoid being distracted during the mandatory landing that will follow.
A temperature reading that is too high can also be used in a similar way. I can think of four occasions where I could have saved an engine if I had known that it had reached a critical head temperature. A setting of 200°F would be a good place to start. Once again, you can turn off the alarm while you try to land. You can also throttle back and/or put the airplane into a shallow dive to see if the engine cools down and the alarm turns itself off.
One really cool thing that you can do is fit a voltage tap to show the state of your ignition battery. This channel is normally used to monitor a Li-Poly electric motor flight pack, but that would not be required on a gas airplane.
The condition of the ignition pack is normally an unknown to the pilot. With the DX8 telemetry you can now see the condition while the engine is at rest, running, and in the air. It is good data for safety and airplane survival. This instrumentation can, will, and should change the longevity of your airplanes and maybe the use of your hobby credit cards—just ask your spouse!
Sources:
- O.S. Engines
(217) 398-8970 www.osengines.com
- Spektrum
(217) 352-1913 www.spektrumrc.com
Transcribed from original scans by AI. Minor OCR errors may remain.
