Some Thoughts on Diesel Power
and Comparison With Glow Engines
APPEARANCE. The most obvious difference between diesel and glow engines is the absence of a glow plug and presence of a threaded "T" bar at the top of the engine which is known as the Compression Vernier. Diesels are invariably taller than glow engines and generally slightly larger overall and heavier for the same displacement. Slightly smaller carburetors or venturis provide adequate and balanced intake. Smaller and more restrictive mufflers may be used as diesels are less sensitive to exhaust back pressure.
THEORY OF OPERATION. These diesels breathe by the same general principles of intake, transfer, combustion and exhaust applicable to the conventional two-stroke glow engines; although port timing, areas, and volumes are all specially matched to diesel operation. The definitive difference for diesel operation is the method of ignition. Glow engines have an electrically and chemically heated glow filament for starting that carries residual heat from stroke to stroke. Once running, the filament stays hot without electrical assistance. Glow fuel (consisting primarily of alcohol, plus some portions of nitro and oil) will ignite from the heat of the glow plug at a relatively low compression pressure. Diesels obtain ignition directly from compression pressures without the need for a glowing element or electrical assistance. Diesel fuel (consisting primarily of ether, kerosene, and oil) will ignite directly from compression pressure. To enhance the ease of starting and to allow fine balancing of fuels to speed and temperature, all modern diesels have variable compression. This is conventionally achieved by the use of a moveable contra piston located at the top of the cylinder. The contra piston is adjusted downward and closer to the piston to increase the compression by screwing in the compression vernier or "T" bar on top of the cylinder. When the compression vernier is unscrewed to lower the compression, the compression/combustion pressures push the contra piston upward to follow the compression vernier. The contra piston is a very tight fit in the cylinder and usually it will not follow the compression screw upward under compression alone. It usually requires combustion. Sometimes a hot contra piston will stick in the cylinder, and the compression will not come down when the adjustment screw is hacked out while the engine is running. In this circumstance it is necessary to cool the engine down a bit at low throttle or rich settings in order to reduce compression. (More about operation and tuning later.) Adjustable compression not only allows for perfect ignition timing at one given engine load speed; it also allows for retuning for optimum ignition timing at very different engine load speeds. The important practical advantage is that diesels are much more flexible than glow engines in adapting to a wide range of conditions, including temperature, altitude, fuel blend, propeller, etc. Adjustable compression indirectly provides adjustable ignition timing. This allows a diesel to turn relatively small propellers at a very high speed at advanced timing or to turn very large and efficient propellers with high torque at a retarded timing setting.
APPLICATION. Current recommendations are to use a sport diesel of about the same size or one size smaller than a specific size two stroke glow engine. For example, you should generally be able to replace a .40 glow engine with a .35 or .40 diesel. When the design of a particular plane calls for a two stroke glow engine of a certain range, you should expect that a diesel of the smaller or mid-size suggested to work well. For example, if the plans call for a .29 to .40 two stroke glow engine, you should try a .29 or .35 diesel engine. These suggestions are for planes of a medium wing loading, flown at medium airspeeds. For large, light planes, which are expected to fly relatively slowly, you should select a diesel even smaller than suggested above; because diesels are especially effective in pulling large, light airplanes at relatively low airspeed, because they are capable of turning such large propellers. These are the same applications where four stroke glow engines are often selected. A .60 diesel can usually replace an .80 to .90 four stroke glow engine, for example.
PERFORMANCE. Diesels are typically more powerful than an equivalent sport glow engine of the same displacement. Moreover diesels develop their power at a relatively lower rpm which allows for the use of a larger propeller and these are more efficient at converting power into thrust. The diesel's combination of more power and more efficient propeller results in much more final thrust; and it is thrust which pulls the airplane not power alone. The gear reduction units developed for large scale aircraft are an example of the importance of large efficient props even though some horsepower is necessarily lost in the unit and extra weight is added, the more efficient propeller more than compensates for the losses. Diesels develop the high torque to turn large propellers directly. Diesels fitted with large low pitch propellers can readily develop the tremendous static thrust necessary to pull a seaplane up on the step. Large propellers are not only more efficient for a given horsepower, but are much more effective (and realistic) when large cowls are necessary for a scale craft. In tightly cowled installations, diesels are particularly welcome as they are much cooler running and less susceptible to overheating.
FUEL. CARLSON ENGINE IMPORTS stock a variety of commercially blended fuels for all model diesels. There are no additional shipping charges for shipping fuels beyond the standard shipping charge per order. The per order shipping charge applies to any size order and any combination of products. Fuels can only be sent by surface to the first 48 states. The best hobby shops now stock commercially blended model diesel fuels. If your favorite local hobby dealer does not yet stock model diesel fuels, please have him contact us for sources of commercially blended model diesel fuels.
Diesel fuels are composed primarily of Ether, Kerosene, and Oil. For the smallest diesel or for operation at very high rpm, 2-4% of a cetane booster such as amyl nitrate, amyl nitrite or iso-propyl nitrate, is added to smooth ignition, without excessive compression. There are a variety of Cetane Boosters available, but most modelers who home brew their fuel find various combinations of the three basic components to be quite adequate. Most manufacturers suggest specific blends for various applications. Typical recipes follow:
Mix fuels outdoors in small quantities, keep in tightly capped, properly labeled metal cans and store in a cool location. Properly stored, blended diesel fuels can survive many years on the shelf, but a can of fuel, left open, can he ruined in a matter of minutes. The ether component, which is essential for easy starting evaporates away quite quickly. The remaining fuel, if it starts at all, will require excessive compression, run hot and possibly damage the engine. IT IS THEREFORE IMPERATIVE THAT YOU USE ONLY FRESH FUEL WITH AN ADEQUATE ETHER COMPONENT TO ENSURE EASY STARTING AND AVOID DAMAGE TO THE ENGINE. We discourage the use of automotive starting fluids for the ether component of model diesel fuel, as these are generally not very pure sources of ether and can be hazardous to handle. All manufacturers specify castor oil as the preferred oil or at least as an essential part of the oil to be used in model diesels.
Newcomers to diesels are encouraged to use fresh commercially blended fuels, at least until they are fully familiar with the starting and running characteristics of their new engine.
Diesel engines are much more fuel efficient than glow engines. Expect about 50% longer flight times from any given fuel tank or install a fuel tank of about 2/3 the size you would select for a glow engine of the same size.
"Hot Fuel Proof" paints are not required with diesel fuel. Fuel tanks, lines and pumps should all be diesel fuel compatible; as silicone and other glow-fuel-only products will deteriorate over time when used with diesel fuels.
Glow engines run at so much higher temperatures than diesel engines that they even burn the lubricating oil into smoke or bake it into caramel on the engine surfaces. Diesel engines run so cool that they simply spit their oil through the exhaust, which can create a rather messy airplane if no steps are taken to plumb the exhaust oil overboard. This is usually no problem, as diesels are rather insensitive to exhaust back-pressure. CARLSON ENGINE IMPORTS stocks the "INDUSTROCLEAN" concentrated heavy duty cleaner which makes for easy cleaning of this diesel exhaust oil if the exhaust is run open.
Diesel exhaust smells quite different than glow fuel exhaust. Some like it; some don't. Diesel exhaust will sometimes appear inky black. Not to worry-this is something like the black soot which kerosene lamps emit when the wick is up too high. Some folks claim that the blackness of the exhaust will guide them in fine tuning a diesel, but I never found this to be helpful personally. Diesel engines have a softer, quieter sound than two stroke glow engines; they sound more similar to the four stroke glow engines.
Diesels have a longer life expectancy than glow engines because diesels are built heavy duty and are cooler running.
Diesel fuels, like Glow fuels, are very flammable and very toxic. Both should be handled with care, kept tightly closed in metal containers and stored in a cool location.
Diesels do not generally require pressure feed fuel systems, because their smaller carburetors produce better fuel draw than glow engines. Exhaust back pressure is also less of a concern to diesels as they simply do not need to breathe as much of the fuel air mixture to produce their full power.
Diesels may be operated inverted if care is taken not to flood the engine while inverted. Some modelers turn their airplane over for starting, then back right side up once running if they have an inverted installation. Three bladed props my be fitted for scale appearance more readily with a diesel because of the large torque available. One may also wish to consider a three bladed prop as a means to use the full torque of their diesel when the ground clearance of their design will not allow for a long enough two bladed prop.
PREPARATION: As delivered from the factory, most diesels have the compression adjustment set for a good-running position; and this setting should not be lost upon initial examination. (If the compression adjustment screw is completely free, turn it in lightly until it just touches the contra-piston and note this position). Most engines will benefit from a careful cleaning prior to operation. I generally suggest that a new engine he immersed in a safe solvent and the crank be gently rotated back and forth to flood the engine case; then use compressed air to blow the solvent out while rocking the crank gently back and forth. Do this repeatedly until all traces of contamination are removed from the engine and all hearings and working surfaces are free of any grit, etc. Promptly lubricate the cleaned engine with a high quality after-run oil. I like Marvel Mystery Oil. (It is especially important to use an after-run oil after running the diesel on any fuel which contains any cetane boosters).
OPERATION: As noted above, the theory of diesel engine operation is similar to glow engine operation, except that no glowing element is required to obtain ignition and that ignition timing can be adjusted by varying compression.
Diesels supplied by CARLSON ENGINE IMPORTS usually include brief instructions for operation of their specific diesels, but usually assume that the purchaser is generally familiar with model diesels. CARLSON ENGINE IMPORTS has developed these rather detailed instructions with a view to the American modeler who may have never even seen a diesel operated before. While these instructions have enabled many modelers across many years to learn to handle and love their diesels; the newcomer to diesels is still encouraged to seek the assistance of an experienced diesel flyer where possible. Much of the learning to handle diesels well involves getting the "feel for it, rather than just understanding the theory. It is suggested to use relatively large and heavy props for initial testing, as these provide a nice flywheel effect for easy starting.
Diesels operate at slightly higher compression than glow engines, so they should feel a bit "bigger" than a glow engine of the same displacement. A properly adjusted .15 diesel will feel more like a .19 or .25 glow engine when the prop is flipped. If it starts to feel more like a .29 or .35, it is probably overcompressed. AVOID OVERCOMPRESSION! If you have lost the initial setting or have any doubts about the setting; reduce compression and be sure to begin from an undercompressed setting. If your engine has a readily detachable silencer, we suggest that it be left off for initial bench testing so that you can readily prime the engine through an open exhaust port. The fuel tank should be located such that the fuel level in the tank will be level with the spray bar when starting. Use a transparent fuel line for bench testing and ensure that the fuel when drawn to the carburetor by choking will neither continue to drain into the engine, nor run back down the fuel line to the tank. For normal starting, fuel should be waiting in the line the moment the engine first fires off. A conservative practice recommended for beginners is to practice starting the engine on a few drops of prime alone without even connecting up the fuel line. In this way a beginner can get a feel for the proper amount of prime and compression required to get a short burst on prime alone. Experiment to find the minimum setting of compression which wilI fire off the prime. Once this position is established, you may connect up the fuel tank and experiment for a needle setting which will continue to run after starting on the prime as before. (Cold start needle settings will generally be richer than best run needle settings and cold start compression settings will generally be higher compression than best run settings for compression). If the engine runs off the prime, in similar sounding short bursts, but does not continue running after the tank is connected up, and will not fire again until it is reprimed; the needle is probably set too lean. Unscrew the needle a bit and try again. If the engine runs momentarily in softer sounding bursts and will fire again sometimes without repriming; the needle is probably set too rich. Lean the needle out a hit and try again. You may also want to try a slightly higher compression setting if needle experimentation as no ted will not yield continuous running. Some engines require a slight increase in compression to continue running immediately after they first fire off. If you do this, use just enough compression to keep the engine running. Once the engine is running continuously reduce compression to where it is miss firing from undercompression, then adjust the needle (in whichever direction is needed) for slightly faster running. Then reduce compression further to keep the engine miss firing from under compression. Continue to make alternate small adjustments improving the speed with the needle, then reducing the compression to somewhat below top speed; until no further adjustment of the needle, in either direction, will improve the engine speed. From this position, and only after the engine is fully warmed up, increase the compression just until the engine smoothes out into a steady two-stroke running. (If you
always approach the final compression setting from the undercompressed side, you will never encounter difficulty in reducing compression from a stuck contra piston in a hot engine). I describe this setting as the "Minimum 2-stroke compression with optimum needle setting. From this position, speed will fall off from any adjustment of the needle in either direction and miss firing will result from any further reduction of compression. I use this setting, or one SLIGHTLY richer, for initial break-in of a new engine. Do not set a diesel slobbering rich for break-in, because you may windup using over compression to compensate for the excessively rich needle setting, and this can be quite hard on the engine, If you wish to slow the engine down further, reduce the throttle setting if an R/C engine or reduce the compression setting if not R/C. It is quite acceptable to run a diesel far under compressed for test flying free flights, etc. You should use short engine runs, followed by periods of time sufficient to allow the engine to fully cool down between short runs for break in. Use progressively longer and faster runs in the break-in process, until you reach the point where the engine will run at optimum needle settings with optimum/peak speed compression settings without fading down at full temperature. NEVER INCREASE COMPRESSION PAST THE TOP SPEED SETTING! Over compression causes overheating which has the effect of more over compression, etc. This results in the speed fading and a binding feeling/sound even though it is still running with a smooth 2- stroke sound. Experience will help you recognize and avoid this tight and labored condition. Beginners encounter this experience sometimes when they are impatient to make final adjustments of the engine prior to their first flight of the day, etc. The engine gets set to the optimum needle and the compression is increased to peak power before the engine is fully warmed up; then the temperature continues to rise resulting in over compression. Starting or running with over compression can over strain the internal parts of the engine or create premature wear, especially on the connecting rod. Such abuse typically results in a bent or broken connecting rod or crank pin. Similar damage can result from forcing a flooded engine through compression when badly flooded and/or over compressed. For this reason electric starters are discouraged, especially for the smaller displacement diesels. Like glow engines, finger guards or "chicken sticks" are recommended for safety purposes.
As with glow engines, diesels should not be operated with excessively lean mixtures, because of the lack of lubrication and excessive heat. Because they run so much cooler, diesels are much less susceptible to damage from lean runs than glow engines, but they should be avoided in any event, especially when new. Unlike the glow engine which will usually sound tight and labored when running lean or just die out; the diesel will continue to run when lean and can make a quite distinctive sound. Because diesels are less susceptible to dying out when slightly out of adjustment, they are popular in multi-engined aircraft. The diesel may miss fire, but it will keep running, even when substantially out of tune.
Idle adjustments are similar for diesel engines as with glow engines. Final compression and main needle settings should be reached at full temperature and at full throttle. Then idle mixture (if adjustable on the particular R/C carburetor) should be set for highest idle speed and the minimum throttle travel speed adjustment screw should be set for the lowest dependable idle speed which still yields adequate throttle response. Both diesel and slow engines tend to cool off and "load" up with unburnt fuel and oil when left idling for prolonged periods of time. Glow engines commonly die at such a low idle whereas diesels may die just when the throttle is reopened. These characteristics can be somewhat minimized by SLIGHT increases in compression and slightly leaner needle settings, but be sure to avoid substantial over compression at full throttle settings. Large R/C glow engines seem to idle a bit better than large R/C diesels, but small displacement diesels seem to idle better than small displacement R/C glow engines.
We suggest that you become familiar with all of these adjustments on the test bench before mounting your engine in the aircraft. After mounting your engine, you should go over all of these adjustments before test flying your airplane and practice the starting and tuning drill again. The adjustments described above should be adequate for initial test flying, but you should expect to need to refine these adjustments slightly for the optimum flying adjustments. In particular, you should expect to need just a bit more compression for final flying adjustments, because once the prop unloads in the air the higher engine speed will require a slightly advanced ignition timing from a slightly higher compression setting. I like to adjust the compression just high enough to where it will 2-stroke in level flight, but miss fire in a prolonged dive. If this setting turns out to he much higher than peak speed on the ground; avoid prolonged engine runs at full throttle on the ground.
Once your new diesel is fully broke in and you become fully familiar with her starting, tuning and flying characteristics; we believe that you will find her to he a very satisfying type of model engine to fly. You will also find that your familiarity with the characteristics of undercompression and overcompression will aid you in correcting for these conditions when encountered in a fixed compression glow engine.
We welcome your comments and suggestions for the improvement of this summary as well as your further questions in regards to diesels or any of the other products offered.
Copyright 1985 (Revised 2000) Edward R. Carlson