In our hobby we rely heavily on the performance of our trusty two cycle glow engines, however for the most part we never give a single thought about them until they fail to perform in some way. Least of our thoughts are about how their ignition is timed. Well, ignition timing is a very important factor for reliable operation, and is critical to engine longevity. However before I discuss problems caused by improper ignition timing, I will digress to discuss some of the basics of two cycle engine theory. Note: These theories may also be applicable to four cycle engines, however I will not be discussing those applications here. Also, I will operate under the assumption that the mixture is set for peak power, not run too rich or lean. Further, I will only equate these processes to glow engines, and not diesels.
So, to begin, let's look at what goes on inside your typical two cycle engine as it completes one cycle of operation. With the piston at bottom dead center (BDC), the exhaust port is completely open and the crank port (or reed valve) is completely closed, as the piston moves up the cylinder, the volume in the cylinder above the piston is reduced, and the volume inside the crank case is increased. At some point, the crank shaft port (or reed valve) opens to allow fuel air mixture to flow into the crank case in an attempt to fill the partial vacuum created there. The fuel air mixture in the cylinder above the piston is compressed, increasing the temperature, as the volume is reduced (increase in pressure is always associated with an increase of temperature). At some point during the pistons upward travel, near top dead center (TDC), the temperature/pressure are correct for the metal in the cherry-red hot glow plug to catalyze the fuel air mixture, and the chemical reaction known as ignition occurs. This reaction is exothermic (gives off heat), so the pressure in the cylinder is now greatly increased, and the piston is driven downward to uncover the exhaust port, and some time later the intake port. Once the piston has traveled far enough downward, the crank case will have sufficient pressure to allow fuel air mixture to pass into the cylinder above the piston, and start the process over. This seems complex but when observed is actually very simple, and is the fundamental that allows all two cycle engines (in our hobby) to run.
Now, let's look only at that single step in the entire process, the catalyzing of the fuel air mixture I am calling "ignition." The timing of this is critical to allow the engine to start easily, burn the fuel completely, thereby liberating maximum possible energy, and to maximize the longevity of the engine. If ignition occurs too long after TDC (post-ignition, or retarded), it will not have time to burn all the fuel in the cylinder and excess unburned fuel will be lost out the exhaust. If the ignition takes place too much before TDC (pre-ignition, or advanced), the engine will be prone to start backwards, and will labor when running forewords. This pre-ignition process is often called detonation, and can have catastrophic effects on the moving parts of the engine. For example I have witnessed broken connecting rods, crank shafts, wrist pins and pistons, all caused by pre-ignition. Pre-ignition is commonly manifested by the top of the piston appearing to be sand blasted, or pitted uniformly across the entire upper surface. The sound of an engine suffering pre-ignition is labored, or may have a metallic or pinging sound.
Now let's look at the causes of both pre-ignition, and post-ignition: Fuel nitro-methane content; engine temperature, specifically cylinder temperature/pressure at TDC; and glow plug temperature.
As is over-reported in our hobby, an engines power output may be increased by using a fuel of greater nitro-methane content. I myself have done so, many timesI sport fly on 10% nitro fuel (for all engines .039 to .61 c.i.d.), and compete with 25%. So, why is this true, and why is it in many cases a very bad Idea? Well the function of nitro-methane in our fuel is as an igniter or ignition enhancer if you prefer. Its kindling temperature is lower than alcohol, and it liberates more energy per molecule than methanol, thanks to the rather high energy nitrogen bond present. So an engine which is running with the ignition occurring sometime after TDC, and appears to be running to your satisfaction, for your day to day needs, on 10% nitro-methane; may have the ignition timing advanced with the addition of a fuel with higher concentration of nitro-methane. Or to put it more concisely, up the nitro, advance the ignition timing. Suppose now that the ignition timing of an engine is occurring before TDC already, and the nitro-methane content of the fuel is increased. What do you suppose will happen? Only an empirical test will give conclusive results, but more than likely pre-ignition will occur. This excessive amount of ignition timing advance will by characterized by a propensity for the engine to start backward, overheat, and labor when running forewords.
Engine temperature has a direct effect on the volume of the intake charge in the cylinder. The hotter the engine, the greater the expansion of the atmospheric gas as it enters the crank case, and thus the more advanced the ignition will be. An engine running either too cold or hot will have trouble flying the model it is on. Let's look first at an engine that is too cold. The common method for this is to have the needle valve set to rich, or too large a propeller to be used. The engine starts fine, runs on the ground and gets hot, while under the static load of the propeller. Upon release the engine unloads, gains in revolutions per minute (RPM), cools off and goes rich. In severe cases this gets progressively worse until the engine is too cold to run and it dies. So the cure for this is to turn the needle valve in, and testfly the aircraft. Or as stated above, you can put on a smaller propeller, or one of lower pitch. This allows for the engine to be unloaded on the ground and therefore to be cooler while running static; and is actually closer to the temperature it will be when flying. Supposing your engine set-up is exactly as you like and your engine is still suffering from either too much advance or retard. You can add or remove gaskets from under the head, or head button. This is the best solution for running engines with glow heads, or factory head buttons of only one heat range. So the only solution is to raise or lower the nitro, or increase or decrease the actual cylinder pressure, with head gaskets (shims).
Last on our list of causes of improper ignition timing, is the actual cause of ignition, the heat range of the glow plug being used. Many manufactures make more than one heat range of glow plug, such as those made and sold under the McCoy name. They came in (decreasing in heat) a four-stroke, number 5.9, number 8, and number 9. Many a time, I switch from the number 8, to a number 9 at a contest, to prevent pre-ignition. So, if your engine is going rich in the air, and the needle is set to your satisfaction, and the prop is correct for your application, then try a hotter plug. On the converse, if your engine is prone to starting backward, labors on the ground and never seems to unload in the air, try a colder heat range plug. From time to time you will hear a person, say: "Put an OS A7 in that engine and forget about it!" Well, I assume they have some experience with plug, engine and application in question; otherwise, I would avoid any advice that uses only one solution to a problem that has three possible causes.
In closing, I believe that ignition timing causes perhaps 75% of the engine problems I see at the flying field, however, the most common solution is to crank on the needle valve, and when this only worsens the problems, the flyers simply throw their arms up and say: "That engine is a rotten so and so!" Don't let that happen to you, use my three pronged approach to solving ignition timing problems, and have fun flying.
This page was upated May 5,