Reid engines are known to be unsteady runners. The main cause for this, in my opinion, is the irregularly shaped and very voluminous combustion chamber. A single-cylinder Reid, producing a mere 15 HP, has as much displacement as a 6-cylinder semi-truck engine making 500 HP or more. An engine making that little power with that much volume is going to have a very difficult time dispersing a tiny amount of fuel evenly throughout its chamber, resulting in lean and rich areas in the combustion chamber. When one of these less-than-ideal areas happens to be at the point of ignition when combustion is supposed to occur, a miss will result. The remedy to prevent this is to avoid single-point ignition and cause ignition to occur throughout the mixture. This can be done by causing ignition to occur by dieseling, in other words, from the heat of compression.
The lowest temperature at which a fuel will ignite is called the ignition temperature. This should not be confused with the flash point. The flash point of a fuel is the lowest temperature at which a fuel will evaporate to form a flammable vapor. These temperatures are different for every fuel. Diesel fuel has a higher flash point than gasoline (it must be pretty warm to evaporate), yet the ignition temperature of its vapor is actually lower than gasoline. This is why diesel fuel is used in engines that rely on their compression temperature for ignition. Kerosene also has a comparatively low ignition temperature.
When a carbureted engine is under heavy load, it draws in more fuel and air than when it is under light load. Because there is more air being compressed, there is more heat from compression. Likewise, a lighter load results in less air drawn in and less heat from compression. (This is particularly true of a single-speed engine like a Reid; a car’s engine works at a higher revolution and idles at a lower revolution, allowing the intake per cylinder to be more constant over the varied load range.) With a true diesel engine, the amount of air drawn into the engine is constant, resulting in a consistent compression temperature, and the power output is controlled by the amount of fuel that is injected. This fuel is injected during compression at the optimum time for ignition. Therefore, the injection controls the timing. In an engine that does not inject the fuel at the time of ignition, but which still is expected to ignite by the temperature of compression, it is necessary to control the timing in some other way. One way to do this is to formulate the fuel to have the correct ignition temperature so that it will ignite at the correct time during the engine’s cycle.
Compression ignition can be achieved in a Reid engine if the fuel used has the right ignition temperature for the load at hand. The ignition temperature of pure gasoline is too high to ignite reliably with a Reid’s low compression, and gasoline with 10 percent ethanol is even more difficult to ignite. But by adding diesel fuel or kerosene to the gasoline, the ignition temperature may be reduced to the point where the heat from compression is sufficient to cause ignition to occur regularly and spontaneously throughout the chamber. Again, this means the timing of the engine is determined by the mixture of the fuel – the more gasoline, the later the ignition will occur; the more kerosene, the earlier ignition will occur. If the fuel ignites too easily, the engine will pre-ignite and knock; if it is too difficult to ignite, it will miss or flood. By adjusting the fuel’s composition, the time of ignition can be adjusted for excellent performance.
Knowing that kerosene ignites at a lower temperature than gasoline (due to its lower ignition temperature), engines designed to run on kerosene typically have a lower compression ratio in order to prevent pre-ignition from the compression’s temperature. (Some engines also inject water into the cylinder to cool the fuel and prevent this pre-ignition while burning kerosene.) However, since kerosene must be warmer to create a volatile vapor (it has a higher flash point), kerosene-burning engines typically warm up on gasoline and then switch to kerosene after they are hot. For the same reason, I start my Reid on propane and then switch to liquid fuel when it is warm.
While running my line shaft, my 15 HP Reid engine always has a draw of at least 5 HP, because that is what is required to turn the 90 feet of line shaft in my shop. This load will double or triple depending on what machines are being used at any given time. At the time of this writing, I have experimented with 15 different fuel mixtures to find what runs best under different loads and what mixture produces the best economy: I have mixed gasoline with diesel fuel, with oil, and with kerosene, and I have experimented with pure gasoline and with gasoline containing ethanol. The mixture I am currently burning, which produces even ignition and good timing under varying loads, is 60 percent gasoline containing 10 percent ethanol, and 40 percent kerosene. With this mixture, and with proper carburation, my Reid simply doesn’t miss, and it does this with neither a spark plug nor a hot tube.