Diesel Fundamentals

A model diesel engine is a "compression ignition" engine, physically the same as a glow two-stroke engine apart from the design of the cylinder head, and we'll get to that part later.  

One of the facts of physics is that if a gas is compressed quickly, its temperature rises. If the compression ratio is high enough and the compression rapid enough that very little heat is lost from the gas, the temperature achieved is high enough to ignite an air-ether mixture. 

This ether (di-ethyl ether, once used as an anesthetic) is a critical component of model diesel fuel because of its low flash point. Typically it makes up 32% or more of the mixture.  The main power ingredient is kerosene (or jet fuel, which is more highly refined). It has more energy per unit volume than ether. The ether is required to ignite it. The rest of the fuel is lubricant, usually castor oil. It is much better if there is also 1.5 - 2% of an ignition improver like the amyl/hexyl/octyl/iso-propyl nitrate. This makes starting easier, makes the engine run more smoothly, and reduces loads on internal parts by reducing the compression ratio required to run the engine. 

Those are the fundamentals, but they don't begin to explain the advantages of a diesel engine. I consider the biggest advantage of a diesel to be its great flexibility, its ability to turn a large variety of propellers.

Why this is important leads first to a discussion of propellers. A given engine can drive a propeller of large diameter and low pitch or one of small diameter and high pitch. A large-diameter, low-pitch prop moves a large volume of air at moderate speed and provides high thrust. It is like low gear in a car: lots of pulling power, but not able to move the vehicle very fast. And a small-diameter prop turning at high speed provides a high-speed blast of a relatively small volume of air. It is like high gear: it doesn't have the thrust to get the car moving from a standstill, but can drive it to high speed once the car is moving. 

On a racing plane, a large, low-pitch prop would scarcely get the model flying. On a slow-flying model, a small prop turning at very high speed would generate very little thrust and would have difficulty getting the model moving.  For a slow-flying model, the optimum large prop will be turning at a relatively leisurely pace, say 6,000 - 11,000 rpm, while the smaller prop of a fast model will be going a zillion rpm. Converted from metric, a zillion might be 11,000 up to more than 30,000 for racing. In any case, the pitch must be high enough to move the plane at the intended speed when it is turning at the speed that the engine is able to turn it.  

Glow engines are usually designed to develop their power at quite high rpm, say 11,000 to 15,000 rpm or more. They are happiest with relatively low loads, in the form of props with relatively small diameters. If loaded down with a large prop, they may overheat due to pre-ignition. Here's a brief description of why.  In a glow engine, the fuel will begin to burn at a fixed point on the upstroke (with some caveats). With a relatively small propeller, this point will be such that the engine fires at just the right time so that the maximum pressure is reached just at TDC. But if you put a large prop on that engine, it will still fire at exactly the same point on the upstroke.  However, the higher drag of the prop is causing the crankshaft to turn more slowly and prevents the piston from moving as quickly. The result is that maximum pressure will be reached <before the piston reaches TDC. This puts a strain on all moving parts of the engine: the piston crown, piston pin, con rod, and crankpin. This preignition can be heard as a pinging, and the effect is that the engine will overheat. It can be seriously damaged. (Note that the fuel in a properly operating engine <burns>. Very rapidly, but it is still burning. It does not "explode" or "detonate". Those words describe undesirable pre-ignition.)  

Flexibility of Diesel Engines

A diesel, on the other hand, can turn the ordinary props used on glow engines at very respectable speeds, but it can also swing larger props because the ignition timing can be varied so that the fuel begins burning at just the right point on the upstroke, just as the spark is timed precisely in an automotive engine. How is this done?  A model diesel has a screw of some kind in the head. (There are a few exceptions, but they don't count.) It pushes against a contrapiston, a movable "plug" inside the top of the cylinder that forms the top of the combustion chamber. Pushing the contrapiston down with the screw decreases the volume and thus increases the CR, so the point on the piston upstroke where the required temperature is produced is lowered. This advances the timing. Conversely, turning the screw out allows the contrapiston to be pushed farther up the cylinder when the piston rises and the trapped gas presses against the bottom of the contrapiston. (Sometimes they stick, but that's one of the things that make diesels so interesting!) Raising the contrapiston lowers the CR and retards the timing so that a larger prop can be driven with no risk to the engine.  To show the effectiveness of this control, the people at PAW once put a huge 18-6 prop on a PAW 35 engine and started it. It could turn only 3500 rpm, but it ran for <five> hours straight with no damage to the engine! The test was stopped just because the people wanted to go home. 

		The English Mills diesels in 1.3cc and 0.75cc are remarkably efficient in turning really large propellers. A number of replicas have been made of these still popular sport flying engines.

To vary the ignition point of a glow engine it is necessary to try different glow plugs, install or remove head shims to reduce or raise the compression ratio (in effect, a glow-engine is a glow-assisted diesel!), and play around with the percentage of nitromethane in the fuel. Compare this with merely turning the compression-adjusting screw of the diesel engine.   

Other advantages

 In addition to the greater flexibility of a diesel engine vis a vis glow, a diesel offers the following other advantages: -lower noise level -more pleasing, more "masculine" sound quality -longer running time on the same volume of fuel -no need to buy glow plugs or batteries -a certain aroma that is guaranteed to identify the modeller as a real "diesel man” 
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STARTING AND ADJUSTING  FUEL  

The fuel consists of kerosene for power, ether to ignite the kerosene, lubricant, and an ignition improver. Here are some notes about the ingredients.   

Lubricant  

There have been many fuel formulas, with oil content ranging from 12% (for racing in ABC engines) to 33% (old "British" formula). Recently I saw up to 40% for breaking in the MP Jet engine, but that seems <really high! In the typical diesel, with an iron (Meehanite) piston and hardened steel cylinder, the minimum oil content recommended is 23%. It's a characteristic for proper lubrication of those two materials. A safe bet is 25% oil for any engine. That way there's lots of lubrication for the connecting rod.  

PAW recommends 30% for break-in, and it isn't going to do any harm to run higher oil (over 25%) all the time, but the surplus is going to go out the exhaust and may not contribute much to lubrication beyond 25%.   

Ether  

A typical ether content is 32%. This assures good atomization, and also gives a safety margin for loss due to evaporation, for the ether will evaporate quickly if the container is left unsealed.  Engines will run on lower percentages; apparently 25% is no problem, and probably one can go lower yet. But at some point the fuel will not atomize as finely and power can be expected to drop. There might also be some effect on combustion, and possibly it would be necessary to increase the compression to get the fuel to ignite early enough on the upstroke. This is not a good idea, for it will impose heavier loads on the moving parts. 

Some fuels are even higher than 32% in ether. They will burn cooler than "standard" fuel and will produce less power, but there may be some times when this is an advantage, as with a Davis head for a Cox engine; the lower temperature is not as likely to melt the Teflon disc that seals the top of the cylinder. This disc melts when "normal" fuel is used and the engine is tuned for full power.  

Higher ether content can apparently be used if an ignition improver chemical is not available, but I have no real information on this.  Ether used to be easy to obtain when it was still being used as an anesthetic. But it is not used for that purpose any more, so there is little incentive for drug stores to carry it. And ether is also used in processing some illegal drugs, which has made it difficult for legitimate users to obtain it.   

Ignition improver  

The ignition improver, like Ethyl's DII(3) (octyl nitrate), causes the ether to ignite at a lower compression ratio, and also smoothes the combustion process. Without it, a diesel has a cackly, rattly sound, and compression has to be set higher, which puts more load on the moving parts.  With it, a diesel runs smoothly and purrs like a tiger. A typical formulation is 2% for engines up to about .19 and 1.5% for larger engines.   

Kerosene  

Kerosene or jet fuel makes up the balance. Kerosene has higher energy per volume than ether, so a higher percentage of kerosene is desirable. But this is limited by considerations of ignition and lubrication.   

Commercial fuel  

The simplest way for most modellers to obtain diesel fuel it to obtain it from one of the commercial manufacturers/suppliers. Companies that I know of that sell diesel fuel in the USA are: -Eric Clutton (Dr. Diesel) -Aerodyne -Red Max -Ed Carlson -Davis Diesel 

Note: The following instructions begin with starting a diesel engine, then branch to instructions for adjusting it for full power output (for a broken-in engine) and for break-in (for a new engine).

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STARTING THE ENGINE  

Probably the most difficult part of running a diesel engine is getting it to fire the first time.  This may not seem like a particularly profound statement, but they do have to be set within a small range of adjustments to fire properly. One would think that it is only necessary to crank the compression high and it should fire. But it doesn't work that way. The engine can be just as reluctant to start if the compression is too high as it is if the compression is too low, maybe even more so.  The following method of starting a diesel is my own, which I immodestly call the "McQueen Method" since I have never seen the key part described anywhere else.

The key part? Whereas other starting instructions always seem to include the words "Fill the tank", I emphatically say "Do <not> fill the tank!!" Determine the correct starting settings by running the engine only on a prime. <Then> fill the tank, get the engine to run continuously, and adjust it for full power.  Why this way?  The reason is because a diesel is easy to flood, and a flooded diesel is very difficult to start. The idea is to put a controlled amount of fuel into the engine and to start the engine on that. But if there is fuel in the tank, it is very likely to dribble into the engine and interfere with your efforts to put in that right amount of fuel.  

Note that there <are ways around the problem of flooding if it occurs, and I'll include them below, but they require a lot of unscientific fiddling and tomfoolery that can be avoided if you follow these instructions carefully and understand why you are doing what you are doing. (Much of the information here can be usefully adopted for starting glow engines.). So, the most important instruction is: DO NOT FLOOD THE ENGINE!   

PREPARATIONS  

Mounting the engine  Mount the engine on a strong mount (or in a model). It is convenient if the engine can be removed without too much difficulty in case it is necessary to invert it and drain out excess fuel, though this will not be a problem if you are careful and don't flood it. And excess fuel <can be cleared if you just flip the prop long enough. (Reduce the compression initially to reduce load on the innards.)   

Fuel system  

The fuel tank should be positioned as with any model engine, as close as possible to the engine and with the center line of the tank no higher than the spraybar. (Some sources say to put the top of the tank level with the spraybar.) And, as touched on above, keep the spraybar higher than the tank (or the supply line pinched off) when the engine is not running, to keep fuel from dribbling into the engine.  For flexible fuel line, use only neoprene. Silicone tubing cannot be used. It swells up and may actually fall apart in contact with diesel fuel. Ordinary PVC (vinyl) tubing can be used where flexibility is not needed, but it will become stiff.   

Propeller  

Select a prop of suitable size from the manufacturer's instructions. For break-in, a "suitable size" is one of greater length and lower pitch within the mid-range of sizes suggested in the instructions (so that it will not place a heavy load on the engine). And the heavier the prop, such as nylon, the better, for greater flywheel action.  Mount the prop so that the piston comes up against compression at the "ten past eight" position   

Compression setting  

The screw in the head, and thus the position of the contrapiston, are usually in the right ballpark when the manufacturer packages the engine.  PAW test run every engine to make sure that it will start. You <did resist the temptation to turn that little screw, didn't you? If not, try to remember where it was and return it to that position.  Grasp the prop and try turning the engine over. The engine should turn over freely, though compression should be good. If it feels difficult to turn over TDC, compression may be too high. Back the compression screw out at least a quarter turn (it can be more if "little fingers" have been playing with the comp screw) and flip the prop. This should push the contrapiston up, and the resistance due to compression should be reduced.  If the engine turns over freely, it should be in the ballpark. If the engine turns over very easily, it is possible that the compression has been set too low. It is just as well to do nothing at this point, but keep in mind that it may be necessary to increase the compression later.  

As noted earlier, starting is not easier when compression is too high. It seems to make the engine actually harder to start. Often the engine may be undercompressed but a really hard flip will get it to fire anyway. It won't run properly, but it will still show some life.   Throttle setting  For an engine with a carb, set the carb wide open.   

Priming  

Obtain a small squeeze bottle that you can fill with fuel and then use to measure out fuel <drop by drop. This ability to measure drops accurately, without flooding, is very important!  I always prime an engine through the venturi or carburetor, not into the cylinder. Some modellers are successful with prime against the side of the raised piston. But if there's a muffler this becomes impossible anyway. Note that putting even a small amount of fuel into the cylinder decreases the volume, thereby raising the compression ratio. If the CR is just right with the prime, what happens when the fuel burns? The volume decreases, so does the CR, and the engine will likely stop. Or, if the CR is just right for running, the extra volume of fuel may be enough to raise the CR beyond the point where the engine will fire.  

A prime into the intake allows the fuel to be vaporized and carried into the cylinder in the same way as when the engine is running normally. A correct prime is literally only <a couple of drops. Literally! For small engines, .06 and smaller, it should be a <single drop, or even a partial drop into the venturi. (Make a drip on the end of the tube and then touch it to the venturi to make it drop into the intake.) For one over, say, .19, it could be two drops and maybe three for a big engine. But don't go over two drops at first. That should be enough to get the engine to fire and run briefly.  Putting the right, small, amount of fuel into the engine is the single most important part of getting your engine running!   
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STARTING THE ENGINE  

Prime the engine as just described. Hold the prop in your hand and turn the engine over slowly. This is to ensure that the engine is turning freely and that the dreaded hydraulic lock has not occurred. Bring the prop up against compression and then flip it as hard and as quickly as you can. A sharp snap is the key to good starting.  If you are really lucky, the engine will start and run for about a second with a good burst of power. This is the optimum response and your target with this exercise.  My engines usually do not fire until the second or third flip, so don't be discouraged if you are not successful on the first flip. Repeat the hard flip, several times if necessary.

The following responses are possible:  -If the engine starts with a good burst of power, perfect. The compression setting is good. -If the engine starts, but runs weakly with a "soft" sound and/or misfires (skipping), and soon dies, it is undercompressed. Turn the comp screw in a small amount, about 1/16 of a turn, and flip again a few times without re priming in case there is still fuel left in the crankcase. If the engine does not fire, prime and try again. Repeat these steps as necessary until there is that good burst of power, and then repeat once to make sure that the action is repeatable. (A slight complication is that running the engine several times will warm it up a little, and that has the same effect as increasing the CR a small amount.)  -If the engine starts, but runs with a harsh, metallic, rattly, clattery sound, and stops abruptly, it is overcompressed. Turn the comp screw out 1/8 - 1/4 turn and flip again a few times to push up the contrapiston and to clear out any fuel.

It may start while you are doing this. If not, reprime and try again. Repeat these actions until the engine fires and runs reliably with a good burst of power, then proceed to "RUNNING AND ADJUSTING THE ENGINE".   

Flooding  

If the engine does not start after several tries of these instructions, it may be flooded. If the engine can be inverted, do so to drain out any excess fuel through the exhaust posts and the venturi. Turn the prop so that the crankshaft inlet is open. Then right the engine and turn the prop slowly. If it is difficult to turn, there is excess liquid in the cylinder.  Reduce the compression by up to a turn, drain out the exhaust ports, and flip repeatedly to clear out all fuel. (This is your only option if the engine is solidly mounted in a stand.)  The engine may start at some time. This will either clear out the fuel, and the engine will run and then come to a soft stop. Or it could draw up more fuel, if the engine was really badly flooded. In this case the engine might stop abruptly. In this case, drain out any fuel possible, reduce the compression more, and flip again.  Eventually the excess fuel will be cleared out, compression will again feel soft, and you will need to increase the compression a little at a time until it is in the ballpark again.  

This business of clearing a flooded engine is a general pain, and the fiddling necessary to clear it can get aggravating. If you are careful not to flood the engine you should be able to avoid this monkey-foolery completely. But some time your engine is going to get flooded and you have to know how to clear the problem.  
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RUNNING AND ADJUSTING THE ENGINE  

At this point the engine has run out the prime with a good burst of power. Make a note of the position of the comp screw. (I make a scratch on the head to match a mark or feature on the screw.) That position is your starting point (unavoidable pun) in future. In colder weather, you may have to turn the screw in slightly from that point; likewise if you later fit a smaller prop (reduced load). And you may have to back the screw out slightly in hot weather or if you fit a larger prop (increased load).  At this point you are ready to run the engine.   

Needle valve and throttle positions  

The instructions with the engine should give an indication of a suitable starting position for the needle valve. If not, start with the needle valve open only about half a turn. (For break-in, this will be three to five turns open from fully closed.)  The throttle should be wide open. (With experience you may be able to start at reduced settings, and for engines converted from glow with a Davis head, set the throttle about half-open.)   

Filling the tank  

Fill the tank. Be sure that your fuel is fresh. If too much ether has evaporated from the fuel, you will be wasting your time! While filling the tank, clamp off or disconnect the fuel line to the engine so that no fuel can leak into the engine. And, when starting the engine, either pinch off the fuel line with a finger or hold the nose of the model high (if it's a small-enough model) so that gravity will keep fuel from leaking into the engine. When the engine fires, then release the line or return the nose to horizontal. The engine should run long enough on the prime to draw in fuel and keep running.  Do not draw fuel up to the spraybar. Fuel will be drawn to the engine when it starts.   

Starting the engine  

Clamp the fuel line against the engine or model with a finger (or keep the nose high) while you are flipping the prop. Prime the engine and flip it the same way that you did successfully above. As soon as the engine fires, remove your finger from the fuel line (or bring the nose to or below the horizontal). Fuel will be drawn to the engine. If the needle valve is open enough, the engine will keep running. If it stops after only a brief run, open the needle valve half a turn and repeat. (Keep the fuel line blocked or nose high while doing this.) Repeat this operation of starting the engine and opening the needle valve more until the engine continues running. [This method of going from too-lean to correct setting I credit to Dave Larkin, the only person who has written about it to my knowledge. I have previously always gone from a rich setting toward lean, but that carries more risk of flooding.]  When you get the engine to keep running, congratulations! You are well on the way now!   

Adjusting the engine  

At this point the engine is running and needs to be adjusted for full power. The compression screw and needle valve settings interact with each other. The main adjustment is the compression screw. But when the CR has been set properly, it may be necessary to lean the fuel mixture more. That will make the engine run hotter, which advances the ignition point, and that in turn may require reducing the CR to retard timing slightly. The following should enable you to adjust the right one at the right time and get the engine adjusted correctly. This may seem daunting the first time, but it becomes instinctive quickly when you understand why you are making an adjustment.  I'd like to be able to post a flowchart at this point, for it simplifies the rest of the instructions. But I can't, so just follow the words. Let the engine run for half a minute to warm up, then go through the following questions and actions.  Is it running softly, misfiring, skipping, loping, etc? If so, CR is too low. Turn the comp screw in 1/8 turn.  

Alternatively: Is it running harshly, sounding labored, rattly, cackly? If so, turn the comp screw out 1/8 turn.  Repeat these evaluations and adjustments until the engine is running smoothly, then continue to adjusting the needle valve.  

Note: We got to this point by gradually opening the needle valve until the engine would keep running, so it should be in the ballpark of the correct setting. However, at some other time you may be starting with the NV open more than this, as would be true while breaking in an engine on a rich setting, so the following will cover that eventuality as well.  Is the engine exhaust very oily? Is the engine four-stroking? If so, it is running too rich. Close the needle valve 1/8 to 1/4 turn, allow a few seconds for the change to take effect, and check again. Repeat this check until the engine is "singing" at a good speed.  

When the engine is tuned for nearly full power, it will heat up and this advances the ignition. Has the engine sound become labored, harsh, rattly, or cackly? If so, reduce the CR 1/8 to 1/4 turn and check the sound again.  Continue to close the needle valve in small steps. If the engine speeds up, you are going in the right direction. Repeat he procedure. If it begins to misfire, it is probably too lean. Open the needle valve enough to restore smooth operation.  If misfiring occurs, the compression may be set too low. Turn the comp screw in 1/8 to 1/4 turn. If the engine speeds up, you are going in the right direction. If the engine sound becomes labored and harsh, back the comp screw out to the former setting.  This procedure will have you close to the full power output. If the engine is in a model, try flying it.   

NOTES: -Once the model is released and it accelerates, it is possible that the engine will start to misfire, especially if the engine is fitted with a large, high-load prop. Do not try to fly if it is misfiring. It may not be developing enough power. The needle valve may be too lean, and probably the CR is a little bit too low. (As the plane moves forward, the load on the prop is reduced and the engine can turn faster, needing more fuel. And the ignition point may need to be advanced a little.) Turn the comp screw in about 1/16 turn and open the needle valve a little. Try again. Repeat the adjustments as required.  

In general it will probably be necessary to richen the mixture slightly and increase the compression slightly from the initial settings to develop maximum power in the air.  -You do not have to worry about a lean setting damaging a diesel the way it would a glow engine. If a diesel is set too lean it will misfire or simply stop.  -Once you have become familiar with your engine, particularly if it is .19 or larger, you may be able to prime the engine by choking the inlet and turning the prop enough to draw fuel to the inlet. But this can be tricky, for there is the risk of flooding, especially for small engines.  -Diesels run cool. You can put a piece of ordinary PVC tubing on the outlet of the muffler to guide exhaust away from the model. The tubing can be up to a foot long without detriment to engine operation.  -Except when the engine is running very rich, the exhaust has a color. With a light load, the exhaust will be a light tan. But the more heavily it is loaded with a large prop, the more the exhaust will tend toward black.  Hence the usefulness of the tubing to keep exhaust oil off the model. (Minimize the amount of this exhaust that gets onto your clothes, for it has a distinctive aroma that not everyone appreciates!)   
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BREAK-IN  

The instructions with your engine will probably cover this, but here are a few tips anyway.  For an iron-steel engine, the procedure is to run the engine for a couple of minutes, stop it and let it cool, then run it again. The engine is initially run in a very rich four-stroke, with just enough compression to keep it running smoothly.  I build up about 20 minutes at the richest setting, then close the needle valve 1/4 turn for each successive run until the engine is beginning to break into two-stroking. I give it short bursts of moderate-speed two-stroking, again with just enough compression for smooth running, followed by richening it again to keep the engine from running hot too soon. This two-stroking is gradually increased in duration and maximum speed.  

When the engine (any engine) is manufactured, the surfaces of the piston and cylinder are like microscopic mountains facing each other. The slow break-in allows removing the tips of the peaks an atom or two at a time instead of gouging out chunks of metal. The finished product is a pair of smoothly polished surfaces to run against each other. The reason for slow running and not letting the engine run hot at first is to prevent metal expansion that would push the peaks into hard contact with each other. 

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TROUBLESHOOTING  

Clearing flooding  You will flood an engine at some time. Here are some tips on clearing it.  Block the fuel line. Invert the engine and drain fuel out of the intake and the exhaust. Rock the prop back and forth several times to ensure that the ports open. Turn the engine upright, back off the comp screw 1/2 to 1 full turn. Hold the prop with your hand and turn it over against compression. If it turns easily, continue. However, if there is resistance, keep backing off the screw until it will turn freely. Begin flipping the prop. The engine may or may not fire during this flipping. Slowly turn in the comp screw as you flip and clear out liquid fuel.

When you have reached the normal starting setting, start the procedure from the beginning.  Note: If the engine was severely flooded, it is possible that inverting and draining still will not remove all the fuel. When you reduce the CR and flip, the engine may start and run for a moment, then come to an abrupt halt. The engine may then be difficult to turn over. What has happened is that a slug of liquid fuel has been drawn into the cylinder and caused hydraulic lock. Drain the engine again, reduce the CR further, and start flipping again.   

Poor compression  

The engine must have good compression if it is to start easily. There are ways around poor compression, and I have had to use them with several engines converted to diesel operation with Davis heads. This is not a reflection on the heads, but on the engines.  My problems came with an O.S. 25FSR and all three of my 10FSR engines. All three 10FSRs had poor compression from new, as did a replacement piston-sleeve set that I put into a 25 engine worn by fine dust at our flying field. I don't know if O.S. was making them so loose that users could not seize them up with lean runs, but the 10s were so loose that they were very difficult to start as glow engines.  If your engine has poor compression, it is better to repair it. 

However, there are ways around the problem.  If you have an electric starter, you can try using it, but <very> carefully. If the CR is too high, or if hydraulic lock occurs, you can quickly bend a con rod. Be sure that the CR is a little on the low side, and be sure that there is no fuel in the engine (not flooded). Set the throttle wide open and the needle valve out enough turns for running. Then apply the starter to the prop.  If the engine does not turn over easily, stop immediately and reduce the CR. Also check for excessive fuel.  I could always start the 10FSRs quickly with a starter.  If you don't have a starter, set the CR and needle valve to approximately the running settings. But this time you have to get enough liquid fuel into the crankcase that you can invert the engine and have liquid drain around the piston and seal it. Turn it right-side-up and immediately begin flipping.  (Don't use a starter this time!) With luck the fuel will give enough compression seal to get it going.  Sometimes I have had to do the flipping inverted and then right the model when the engine fired, but this was with small models that I could hold in one hand.  If you have a new engine you should never need to use these emergency methods.  

Good luck and good dieseling!    Ian McQueen  

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