Operating Fixed Compression Diesels

By Adrian Duncan, August 2007.

  Basic Concepts
  Running Characteristics
  Optimizing performance
  Notes On Specific Engines
      Micron 5 cc
      Owat 5 cc
      Mite .099


It scarcely needs to be said (but I'll say it anyway!) that the fundamental operating principle of a model "diesel" engine is the ignition of the fuel solely by the heat generated by compression of the fuel mixture within the upper cylinder, with no additional ignition source being employed. In all other respects, the model "diesel" engine is a bog-standard two-stroke engine just like any other.

The timing of the ignition in any internal combustion engine is one of the more critical operating parameters. Ideally, ignition should be initiated just prior to the piston reaching top dead centre so that the greatly increased pressure created in the cylinder by the heating of the trapped gas following combustion may be fully developed in time for the descending piston to take full advantage of it during the power stroke. Within limits, the higher the speed, the greater the amount of ignition lead time or "advance" is required.

In a "true" diesel engine, the correct timing of the ignition is achieved by injecting the fuel into the combustion chamber at exactly the right moment for ignition to commence. The model "diesel" has no injector, and hence should more properly be referred to as a "compression ignition" two-stroke engine. However, the term "diesel" is so well entrenched in modelling circles that it would be futile to attempt to amend this undeniably convenient term at this stage. I certainly don't intend to try!

It is crucial to understand that the main function of the compression control in a conventional model diesel is not simply to set the compression ratio high enough to generate the heat necessary for ignition. Rather, its chief function is to adjust the timing of the ignition so that the maximum possible use is made of the expansion of the heated gasses in the combustion chamber. The compression adjustment functions in this regard exactly like the timer arm in the old spark-ignition engines and takes over the timing function normally exercised in full-size "true" diesels by the injector. The higher the compression ratio, the earlier ignition will occur as the piston nears the top of the compression stroke and the greater the ignition "advance" will become. This is why higher compression settings are required for higher-speed operation of a particular model diesel engine.

During the early years of the development of the model diesel engine, a number of design issues were debated which we would take for granted today. Among these was the need for some means of varying the compression, and hence the ignition timing, while the engine was running. Pretty much every model diesel engine made since 1950 has had some means of varying the compression ratio with the engine running. This allows one to vary the ignition timing to suit different speeds and loads as well as accommodate different fuels and atmospheric conditions. It also assists in the clearance of a flooded engine by allowing one to burn off excess fuel by flicking over with the compression well backed off.

However, these advantages of variable compression were less than obvious to a number of designers in the mid and late 1940's. Simplicity was seen as the selling point of the diesel—no batteries, coils, timers or even any need for an external means of heating a glow-plug. The elimination of the contra-piston or equivalent was very much in keeping with this philosophy of extreme simplicity—one less control to mess about with! A substantial number of early model diesel engines appeared on the market during these years which featured fixed compression ratios which could not be varied while the engine was running. Examples included the French Morin (an early example of a "kit" motor intended for home construction) and Micron 5cc, the British Owat, the American Drone, Edco, Mite and Vivell diesels, the Canadian Strato .601 (the largest-ever fixed-compression diesel) and the Australian-made GB-50 Stunta Mota (more or less a clone of the Drone). A significant number of these motors survive today in the hands of collectors around the world, and there are doubtless a good few still resting in attics here and there, awaiting discovery by some lucky collector.

But very few of these hardy survivors ever get run!! Indeed, most diesel users today shudder at the idea of trying to run a diesel using fixed compression, and tend to assume that the concept was a decided failure. Well, that's true insofar as the concept of fixed compression did not survive the 1940's in commercial terms and has not been seen on any production model diesels introduced after that time. But it would be completely untrue to say that all engines built using this system were failures! On the contrary, a number of them compiled impressive contest records in their day. The Micron 5cc won everything in sight in the free-flight field in Europe during the years 1946 to 1947, and the Drone 5cc diesel cleaned up in late-1940's American control-line stunt contests before being supplanted by the lighter and more powerful Fox .35 and its glow-plug rivals.

Now it must surely be obvious that these successes could not have been achieved if the fixed compression feature had limited the performance and utility of these engines to the extent that is often assumed by modern diesel users. Clearly, the owners of these engines somehow managed not only to overcome the limitations imposed by the fixed-compression feature but to extract contest-winning levels of performance from them. How?! Well, mainly by understanding very clearly what they were dealing with and approaching the task of making the engines work in light of that understanding. The intent of this little dissertation is to help present-day owners of these engines who may be interested in trying them out do so with confidence.

Before proceeding any further, I wish to stress the fact that what follows is based purely upon my own experiences. Having no-one to advise me when extreme curiosity led me to try running a few of my own fixed-compression diesels, I had to learn everything from scratch, and it took a while! I have no doubt at all that others may have more practical approaches and clearer insights than I do. If so, I hope that this summary may encourage them to share their own experiences with the rest of us and set me straight where I've gone wrong.

Basic Concepts

To begin with, it's important to understand the exact nature of the limitations with which we are dealing. The key point is that the sole limitation of so-called fixed compression is our inability to alter the compression while the engine is running! We retain the ability to alter the compression at will at all other times! In that sense, the term "fixed compression" is a misnomer—in fact, we can change the compression ratio at all times when the engine is not running!

This is critically important. Everyone who has used model diesel engines a lot knows that a well run-in diesel with an appropriate load and suitable fuel for the intended application will usually start and run at the same compression setting as long as the load and fuel remain unchanged—it's just a matter of letting it warm up before launching. I often spend a day flying without ever touching the compression setting of my engines. In effect, that's fixed compression operation! Saves a lot of unnecessary wear on the contra piston.

But for this to work, the compression has to be set right to start with. In that regard, we have options with a fixed-compression engine just as we do with a unit that is fitted with a contra-piston. It's just a bit more involved, that's all.

There are actually two ways of varying the compression of a so-called fixed-compression diesel:

So in fact, we can readily adjust the compression ratio of our "fixed compression" diesel. It's just that once the engine is running on a given tankful of a given fuel, the effective compression ratio will remain constant throughout the run.

Since it's readily apparent from the above discussion that the fuel mixture is pretty critical for successful fixed-compression operation, it's appropriate at this point to spend some time looking more closely at this piece of the overall equation.


As noted at the outset, the factor which sets model diesel engines apart from other types is the absence of any specific ignition source. The fuel is ignited simply by the heat generated by compression of the gas trapped in the cylinder during the compression stroke. As a given mass of gaseous fuel mixture is compressed into a smaller volume, its temperature rises. When the temperature reaches the self-ignition point of the fuel mixture being used, combustion will occur spontaneously and the power stroke can begin. The trick is to time these events appropriately for best performance.

The fundamental base component of any model diesel engine fuel is ethyl ether. There are several reason for this:

But here the advantages end. In other respects, ether is a bad diesel fuel! It has a decided tendency to detonate (i.e., to ignite explosively rather than burn progressively), which imposes greatly increased stresses on already-stressed engine components. Furthermore, either has a considerably lower calorific value (the amount of energy released by burning a given amount) than other fuels such as kerosene and gasoline. In other words, you get substantially less energy from burning a given amount of ether than you do from burning the same amount of other fuels. For this reason, modern model diesel fuels tend to use only as much ether as is required for good starting, and to make up the rest of the combustible content of the fuel with a high-energy smooth-burning constituent such as kerosene.

However, we can turn some of the characteristics of ether to our advantage when looking at fixed-compression operation. One factor in model diesel operation that often gives rise to a need for a reduction in compression during running is that of over-heating. Since we can't respond in this manner to any overheating issues with our fixed-compression diesel, we need to do everything that we can to prevent an overheating situation from arising in the first place. This means that the use of hot-burning fuels is definitely out—do not attempt to run a fixed-compression diesel on conventional modern diesel fuel!! If you do, damage will likely result.

To keep temperatures under control, fixed-compression diesels are generally run on a fuel consisting of straight ether and mineral oil, with no power-enhancing additives such as kerosene (which burns far hotter than ether) or ignition improvers such as amyl nitrate. Ether has a high latent heat of evaporation, and the fuel mixture with an ether-based fuel thus enters the working parts of the engine at a very low temperature and does much to promote cooling of those parts. And the relatively low calorific value of ether reduces the tendency for the engine to run hot, although it also reduces the power potential of the fuel. But the imperative is to prevent overheating, and fixed-compression diesels run very cool on an all-ether fuel, so there is little chance of the overheating problem occurring.

Apart from its low calorific value, the main limitation of ether as a diesel fuel is its previously-mentioned tendency to detonate rather than burn smoothly under self-ignition conditions. For reasons which are unclear, the use of mineral oil in the fuel does much to counteract this tendency—in this respect, the more oil, the better! The point is that the oil content of the fuel does more than merely lubricate the engine—it also reduces the tendency of the fuel to detonate. For some reason, castor oil does not seem to have this same "dampening" effect, so fixed compression diesels are definitely best run on mineral-based fuels. I've had excellent results using SAE 20-50 multi-grade mineral-based motor oil. I've also had satisfactory results using straight SAE 30. You can buy mineral-based oils at most automotive supply stores. I've never tried the modern synthetic oils, and have no idea how they would work.

In his most interesting write-up on the Drone which appeared in MECA Bulletin no. 71, in January 1981 (over a quarter century ago—how time flies!!), long-time Drone Diesel user Gus Munich recommended the use of medicinal mineral oil. I can only say that I've tried this, with very indifferent results. I've had far better luck with conventional mineral-base motor oil. Perhaps the 1940's medicinal mineral oil recommended by Gus was a different substance from that sold in more recent years?!? The manufacturers of the Drone engine recommended "Heavy or Extra Heavy Mineral Oil (also known as Liquid Petrolatum)" or straight SAE 20 oil, with the addition of a little extra SAE 30 mineral oil during break-in. There are some clear inconsistencies in these recommendations, and I can only repeat that the oils that I have noted above work very well in my experience. I believe that the slightly thicker oil provides better wear protection at the high bearing stresses under which model diesel engines operate.

As far as ether goes, the ready availability of this material is becoming increasingly compromised, largely because of its use in the making of illicit drugs such as crystal meth and consequent crack-downs on its availability. The most accessible source at present is the automotive "starting fluid" available from many automotive supply stores. predominantly ether, and fuel made up using this material is perfectly satisfactory. The better grades of this material have a high ether content in the 60% plus range, with the bulk of the balance being heptane in most cases.

Heptane is actually a member of the ether family in a chemical sense, although it has a somewhat higher self-ignition temperature. Its combustion characteristics are however far closer to those of ether than those of kerosene in that it tends to burn more "explosively" than kerosene. Accordingly, the presence of a proportion of heptane in the mix does not appear to greatly affect the performance of an either-based diesel fuel as long as the ether predominates.

It's important to realize that not all starting fluids are created equal! Some in fact (like the appropriately-named Australian product "Start, Ya Bastard!") have relatively low either contents in the 25% range and are unsuitable for our purpose. The best by far appears to be John Deere starting fluid, which is 80% ether by weight. A portion of the balance is an upper cylinder lubricant, which doesn't seem to upset things to any meaningful extent. I've also had reasonable results with Gunk Liquid Fire, which has a guaranteed ether content of at least 60% with much of the balance being heptane and only a very small upper cylinder lubricant fraction which can be ignored when mixing.

Starting fluid is sold in an aerosol spray can, and my own method of extraction is very simple. I begin by cooling the can in the freezer for a while and then holding the can upside down and pressing the spray button to open the valve. The pressure bleeds off while the liquid contents remain in place inside the can. This process needs to be repeated several times with a pause between attempts, since part of the content consists of dissolved gasses which take time to come out of the ether as the pressure is relieved.

Once the pressure has been completely relieved and repeated presses produce no more hissing, I then take a sharp-ended camping can-opener and cut a small hole in the base of the de-pressurized aerosol can. This releases any remaining pressure in the system. I then cut a second slightly larger hole through which the contents can be drained into whatever container is to be used to store the ingredient. Needless to say, I do this outdoors well away from any ignition source! I don't claim that this is necessarily a safe technique—I can only say that it has worked well for me so far, and I'm still here to prove it! Others may or may not wish to try this technique themselves—if so, it's entirely at their own discretion and at their own risk. End of liability disclaimer .

OK, so we have our basic ingredients. How much oil should we use in our fuel mix?? Well, that depends, as we'll see when we get onto the actual operational issues.

Running Characteristics

Wait a minute, you say, we haven't even got the engine started yet!! True—but in this instance (as so often in life) it will help us to get there if we first know where we're going! So it's now the appropriate time to talk a little about how these engines actually run. Their characteristics are very different indeed from those of a conventional diesel.

Let's assume that we have a fixed compression diesel set up in our test stand and have somehow got it going on an ether-based fuel as described above. We have no control over the compression at this point, so the challenge facing us is to set the needle valve for best running.

What does the needle valve do?? Basically, it adjusts the percentage of fuel vapour in the air which reaches the combustion chamber through the induction and transfer process. We've already seen that ether will burn over an extraordinarily wide range of percentages of vapour, so we can confidently expect that the engine will keep running in some fashion over a very wide range of needle settings. And in fact, that's exactly what we find.

Most likely, our engine is running with a crackling exhaust note with plenty of missing, puffs of white exhaust smoke and probably some "knocking" indicative of detonation. In some ways it may look and sound like a conventional diesel that is running too rich. In reality, such a running condition with a fixed compression diesel running on straight ether indicates that it is running too lean!! You can turn the needle in quite a long way with the motor continuing to run like this. But there is a limit—if you keep going leaner, the lower explosive limit is finally reached and the engine stops without ever achieving a smooth-running state. .

OK, so much for going leaner—no cure for the erratic running there! What about going the other way?? Yup, that's the ticket! The cure is to open the needle valve progressively until smooth running is obtained and the engine stops missing and throwing out those puffs of white smoke. At this point, running becomes very smooth, with little if any detonation and little exhaust smoke. Nice!

Now, we know what indicates a too-lean condition. What about too rich? Well, here's one of the great challenges of fixed-compression operation! For the best power, best cooling and best lubrication, we want to put as much fuel through the engine as possible, right? So we should keep opening the needle until the engine shows signs of becoming too rich and slows down, right? Well .....great in theory! Go ahead and open the needle valve some more! The engine keeps right on running very smoothly, and probably picks up a few revs. The wide explosive limits of our fuel mean that you can keep this up for a while. But now comes the problem—the first indication that the engine will give that it's too rich is very simple and direct—it stops!! This happens with no warning whatsoever—you're turning the needle very slowly to open it, and the thing simply cuts dead without missing a single warning beat! And of course, maximum power is found just prior to that occurring—the motor runs perfectly smoothly right up to the point of cutting out. AAARRRGGGH!!

So to accommodate the inevitable changes in fuel draw which will take place under real operating conditions, we need to set the needle somewhere in the middle of the smooth-running range. Fortunately, the wide explosive limits of ether mean that this is by no means critical, and a reliable setting is easy to find. The trick is to make a couple of runs to establish the point at which missing begins to occur (bordering on too lean) and the point at which the engine cuts (too rich). The most dependable running setting is somewhere right in the middle.


Now that we know where we're going, let's see how we get there! We'll assume that our engine is set up in a test stand with some suitable ether-based fuel in a tank with its top more or less at spraybar level and a suitable prop fitted.

We already know that we're not going to be able to ease the task of clearing a flooded engine by the usual strategy of backing off on the compression and flicking until the thing fires and burns out the excess fuel. This leads to our first principle of good starting on a fixed-compression diesel—don't let it get flooded! I've found that turning the thing over with the intake finger-choked until the fuel line is full, then giving it one choked flick (or two at the most) usually gets enough fuel into the system for a start to be obtained. The ether is so volatile that the smallest amount of fuel getting into the crankcase sends a good whiff of ether up into the cylinder, and the very wide explosive limits of ether mean that the actual amount that gets up there isn't all that critical in any case—almost any amount of fuel will do! Accordingly, if you follow these procedures, these engines tend to be one or two-flick starters. I've never at any time had to prime a fixed-compression diesel and I don't recommend it—a flooded engine is to be avoided at all costs!

What about the needle setting?? Well, that's actually relatively non-critical for starting as long as it's somewhere within the quite wide range over which the engine will keep running. The critical thing is to ensure that we're not trying to start with the needle set too rich, because we've seen that the engine won't keep running at all under that condition and may tend to flood. I've found that it's far better to start with the needle set a little leaner than the best running setting. That way, there's far less chance of the engine "loading up" while starting, and smooth running is easily established by opening the needle once the engine starts.

Of course, this is all very well when you know the needle setting for best running! If you don't know that setting, it's best to start from a fairly well-closed needle setting and work up. On a too lean setting, the engine may fire and run erratically for a second, and then stop. If it does this, you simply open the needle half a turn and try again. Eventually, the engine will keep going, albeit running in the hit-and-miss lean condition, and it's then a simple matter to smooth things out by opening the needle progressively until smooth running is obtained. Then open it a little more, and you should be in the pocket!

It's well worthwhile finding the needle setting at which smooth running commences from the lean side of things and then finding the "maximum rich" setting at which the engine cuts out as described above. I find that if you set the needle right in the middle of that range, you'll get pretty dependable running in flight as long as the tank is appropriately located.

For cold re-starting, simply close down the needle about half a turn from the running setting and follow the above procedure. Using this procedure, you'll find that fixed-compression diesels are extremely easy to start—my Drone, Owat and Micron fixed-compers are one or two flick starters every time.

If you manage to get the engine flooded, you have little alternative other than to remove it from the stand and drain the excess oil out of it by way of the bypass and then the exhaust. The ether content evaporates very rapidly, but the excess oil has to be removed. A small bottle of straight ether may help to flush a flooded engine. However, I repeat—if you follow the recommended steps, there's very little chance of flooding the engine.

OK, what about hot restarting?? My advice for most fixed-comp diesels is—don't bother! Hot re-stating is not these engines' strong suit—the detonation issue becomes very problematic, and the engines pre-ignite and kick back fiercely, to the extent that stresses on the rod, gudgeon pin and crankpin must go through the roof. Best avoided—allowing complete cooling between runs is the only fix for this. The American Mite .099 diesel is an exception to this rule, to be discussed below.

If you follow the above advice, you'll find that the operation of these engines is if anything less challenging than it is with a more conventional later-model diesel!

Optimizing performance

Anyone who uses "modern" diesel engines knows that once the engine is running, the thing to do is to adjust both the compression and the needle valve to optimize the ignition timing and fuel mixture respectively to give the best running on the particular load and with the fuel being used. The use of a fixed-compression design doesn't alter that as far as the needle valve is concerned. However, we can't alter the compression once the engine is running.

That doesn't alter the fact that there will always be an optimum ignition timing, and hence an optimum compression setting, for any compression ignition engine operating at a given speed under a given load. In the case of our fixed compression engine, the ignition timing is essentially fixed. However, the load, and hence the operating speed, is not fixed—we can change that simply by trimming or changing the airscrew being employed.

Hence, the normal mode of operation of a model diesel engine has to be in effect reversed when running on fixed compression—rather than matching the ignition timing to the speed by varying the compression, we achieve the same objective by varying the applied load until the engine runs at the ideal speed dictated by the fixed compression ratio. The end result remains the same—we get the engine running as close to perfectly as possible.

The trick is to learn the operating speed for which the ignition timing is set at its optimum point by the fixed-compression head. To do this, one runs tests on a series of airscrews, noting the rpm and running characteristics achieved for each. One starts with a very large airscrew and then tests progressively smaller props until there are clear signs that the engine is reluctant to run any faster with the compression setting presently in place.

On the oversized props, the engine will run smoothly but in a rather laboured fashion, possibly with some indications of detonation to spice things up. It may also sag somewhat as it warms up. These are clear symptoms of premature ignition or over-compression, about which nothing can be done on a fixed-compression engine. Operating under these conditions is very bad for the motor—internal stresses will be greatly magnified and damage may result. Do not run the engine for any length of time in this condition!

Things will improve as the speed increases with smaller loads. At some point, the engine will run smoothly throughout the run with no sign of sagging and no trace of detonation. At this point, the engine is operating in harmony with its built-in ignition timing.

If we keep on reducing the load, a point will be reached at which smooth running can no longer be achieved at any setting of the needle. The engine will misfire despite our best efforts to smooth it out using the needle valve. Trying an even smaller prop will result in little if any increase in speed. These are the familiar symptoms of retarded ignition or under-compression.

The trick is to note the lowest speed at which the running becomes smooth and un-laboured and also the highest speed at which smooth running can be obtained. This gives us the range of operating speeds for which the fixed compression of the engine has been set. Using the test fuel, as long as we keep the engine within the identified speed range, all will be well and we will ensure smooth running and minimal internal stresses at all times. To minimize internal stresses on the engine, we should ideally be operating somewhere near the upper limit of this range to ensure that the engine is not lugging. All that is now necessary is to select a flight prop which will run in the air at or near that upper limit.

Now if we really want to operate at a speed above that at which the symptoms of under-compression become apparent, we may have an option—reduce the thickness of the shim or gasket which provides the seal between the head and the cylinder liner. This will slightly lower the head and marginally increase the compression ratio. Our ability to do this is entirely dependent upon whether or not the gasket or shim that is already in place offers sufficient scope for thickness reduction. If it is already minimally thin, we're out of luck!

This can of course only be carried so far in any case—any increase in the fixed compression ratio will tend to render starting more problematic by encouraging more of a tendency for the engine to kick back. Ultra-high compression ratios are incompatible with fixed compression operation! But we may be able to extend the optimal operating speed range upwards a little using this approach.

Conversely, if our engine shows signs of being over-compressed at our preferred speed, we may be able to improve matters by inserting a thicker shim or gasket between the head and the cylinder liner. This will raise the head slightly and reduce compression marginally. Once again, this can be overdone—at some point, starting may become compromised due to the reduced heat generated by the lower compression during flicking over to start. In addition, too thick a gasket may render the engine subject to gasket blow-out.

The most convenient manner in which effective fine-tuning of the compression and hence ignition timing can be achieved is by adjustment of the ether/oil mixture. This can be surprisingly effective. We noted earlier that an increase in the oil content of the fuel has the effect of increasing the operating compression ratio and advancing the ignition timing somewhat. So if we want to operate in a higher speed range than that indicated by the tests described above, the easy fix is simply to increase the percentage of oil in the fuel. Then run the tests again to establish the revised operating speed range.

A technique which I've found helpful is to make up a few small batches of fuel having differing oil contents—say, 25%, 30% and 35%—and try the engine on each prop using the different fuel mixtures. If running improves on a given prop as the oil content rises, that says that the geometric compression ratio as set is on the low side for that operating speed—the extra oil compensates for this in the manner described earlier. In such a case, the geometric compression ratio may be increased by re-shimming the head as noted above. If the reverse is true and the engine runs harder as the oil content increases, geometric compression is on the high side for that speed. That says that the engine should be operated in flight at a higher speed than the one being tested or that the geometric compression ratio should be reduced by re-shimming as described above.

However one goes about it, the key is to ensure that the engine operates in the air at a speed which is compatible with the built-in ignition timing. Like all model engines, the motor will pick up revs in the air. Therefore, it's generally no use using a flight prop which allows the engine to run at its optimum conditions on the bench—we want those conditions to be reached in flight, not on the bench!

Accordingly, once one has established the engine's ideal operating speed by running a series of tests as described above, the trick is to use a flight prop that pulls the speed on the bench (or on the ground) down to some 1,000 - 1,500 rpm below the speed for optimum running conditions. If this is done, the engine can be relied upon to pick up speed in the air to the point where it is operating at its optimum condition during flight.

It's obvious from the above discussion that the selection of the airscrew can have a very significant bearing on flight performance. These are not high-revving engines—they're all about high torque at low revs. To utilize that torque effectively, one should use a prop with plenty of blade area and a relatively high pitch. An 8 inch pitch seems to be the minimum advisable if a reasonable airspeed is to be realized.

This all seems very complicated, no doubt, and there's little question that the provision of some means of varying the compression is a godsend in that it eliminates the need for all this testing and tuning. But if one perseveres and establishes the optimum combination of geometric compression ratio, fuel mix and flight prop, the results can be impressive. My personal experiences with actually running and flying fixed compression diesels (notably the Drone) are that once you get things dialled in they are every bit as easy to use as their more modern counterparts and perform extremely dependably.


If the engine is kept scrupulously clean and is run on a fuel with at least 25% oil, and if the oil used is of good quality and adequate viscosity, these motors appear to run forever! There are no corrosive fuel residues to speak of since any residual ether quickly evaporates. The mineral oil does not gum up or bake on like castor oil. Wear rate is minimal, and all that is necessary is to use a good after-run oil to preserve the interior surfaces.

It is just as well that this is so, because now we must touch on another Achilles Heel of the fixed compression diesel. Any wear which takes place in the rod bearings or the main bearing will have the effect of slightly lowering the position of the piston at top dead centre and hence of lowering the geometric compression ratio. And of course, our ability to adjust the engine to compensate for this is limited as described earlier. For this reason among others, it is more than usually important that undue wear be avoided with these engines. They should be kept as free from dust or grit ingestion as possible—an air filter on the intake is probably a good idea. The engine should be wrapped in a plastic bag when not in use.

Despite one's best efforts, some degree of wear is inevitable over the long term. As wear takes place, it may be found that an increase in the oil content of the fuel is necessary to re-establish the ideal operating compression ratio. This has the added benefit of enhancing the lubrication of the engine and thus slowing the rate of further wear.

The other maintenance issue is the fuel. It is important to remember that ether is a rather hazardous substance in a number of ways. It is extremely volatile and inflammable, and must therefore be stored in a tightly sealed container well away from any ignition source. Furthermore, if exposed to light and air for any length of time it can form peroxides which are highly unstable and can actually explode with minimal provocation. For this reason, ether should be stored in a cool, dark environment in an airtight container (a screw-cap metal can, for instance) well away from any ignition source.

The other inconvenient characteristic of ether is its extreme volatility, which leads it to evaporate at the slightest provocation. An ether/oil fuel which is stored for any length of time can lose a surprising proportion of its ether content unless stored under cool conditions in a very well-sealed contained. Even at the flying field during a day's use, the ether content of such a fuel can drop significantly.

My own preference is not to store such fuels for any length of time, but rather to make up fresh fuel each time I plan to do any fixed-compression running. That way, I can be sure of my oil content. A single can of starting fluid will make up enough fuel for a day's flying, and I generally use up the entire batch at one session, thus eliminating the need to store any such fuel at all. If it is necessary to store some of this fuel, that's OK as long as it is kept in a well-sealed airtight container in a cool, dark place well away from any ignition source. It will safely keep for quite a while under such conditions.

Notes On Specific Engines

Here are a few comments on some fixed-compression engines with which I have actual operating experience. More details regarding the engines themselves may be found in The Finder.


In my part of the world, the most commonly-encountered fixed compression diesels are the American-made 5cc Drone units. These were made in two models—the plain bearing first model and the single ball-race second model. They are extremely well-made and very sturdy—a credit to their designer and manufacturer, in fact. About 10,000 examples of the first model were made, and perhaps 5,000 examples of the second model. A substantial number of these survive today.

The two models of the Drone are quite distinct—in fact, the only interchangeable parts are the prop driver and prop washer! The first model has a plain bearing crankshaft and was supplied with a fixed compression ratio of 18:1. This is on the low side for model diesel operation—high-speed diesels frequently require compression ratios of up to 20:1 for optimum performance. But the Drones are definitely not high-speed engines, so the compression ratio adopted is well suited to their normal operating speeds.

I've found that this version of the Drone actually operates best on a 30% oil mixture. The plain bearing benefits from the application of the extra oil, since wear in this bearing is devoutly to be avoided given the effect which any such wear would have on the geometric compression ratio. This is in fact a reported problem with this model of the Drone, and there's no easy fix—the bearing has to be re-bushed or a new shaft made. However, the bearing is very substantial and it will take tens of hours of running to wear the bearing to the point where a problem develops, as long as detonation is avoided and the oil content is of sufficient quantity and quality.

The engine seems to run very smoothly in the 5500 - 7500 rpm range, but starts to struggle with under-compression above that range. One tester reportedly rated it at 8,400 rpm on a 9 x 4 wooden prop and 4.400 rpm on a 14 x 8. The 9 x 4 is definitely operating past the engine's peak, which is probably in the region of 7000 rpm or so, and the 14 x 8 is definitely lugging the engine in my view. I obtain around 6,800 rpm on the bench on a 10 x 8 wood prop, and would recommend that or an 11 x 6 as a very suitable bench test prop for this engine. For control-line flying (for which these engines were primarily intended), one should aim for a ground speed of around 5,000 - 5500 rpm or so. The makers recommended 10 to 12 inch diameter props for control-line stunt, with pitches ranging from 8 inches to 10 inches. The purpose-built Drone stunt prop for this engine was an 11 x 10, which would probably work very well if one could be found. Failing that, a slightly cut-down 12 x 8 should work fine. Or you may be able to cut down a larger prop of higher pitch. The trick is not to under-prop the engine and to use a high pitch for good airspeed at the relatively low revs which the engine will deliver. Such large props will shift a lot of air—you'll be amazed at the static thrust developed! I don't recommend running a Drone below 5,000 rpm under any circumstances—at lower speeds the condition of premature ignition sets in, with potential harm to the engine.

The second model Drone has substantially stronger mounting lugs than the Mk. I. Even more significantly, it features a single ball-race located just in front of the crankweb in the usual place. This is a great step forward from the first model Drone, mainly because it essentially eliminates the issue of reduction in the geometric compression ratio due to main bearing wear. It also allows the engine to rev a bit more freely. For this reason, if you plan to do some serious fixed-compression running and/or flying, this is the motor with which to do it! In his previously-mentioned 1981 write-up, Gus Munich (whose all-time favourite engine this was) reported that he had a second model Drone which had over 500 running hours on it with no replacements! My own "flyer" second model Drone has around 25 hours in my hands, and who knows how many hours before that?!? It shows virtually no signs of wear.

The second model Drone also has a slightly higher geometric compression ratio of 18.5:1, which allows it to keep running evenly at slightly higher speeds than the earlier version or alternatively allows the use of a fuel with a slightly lesser oil content—I generally use 25% oil in the fuel for my Mk. II Drones, as recommended by the manufacturers. The same prop sizes as for the first model generally work, but it may be found advantageous to trim the blades slightly to allow the second model motor to reach higher revs in the air. It probably peaks at around 7,500 rpm as supplied.

Gus Munich recommended setting the engine a little lean for take-off and allowing centrifugal force to richen the mixture in flight to smooth out the running. Personally, I've found that the use of a well-located Uniflow or chicken-hopper tank eliminates any need for such measures since such tanks maintain a constant fuel head throughout the flight except insofar as manoeuvres may affect mixture. The trick is to locate the tank at or near the centre-line of the spraybar so that centrifugal force actually plays a minimal role in infuencing fuel head relative to the spraybar. If anything, the tank should be a little inboard of the spraybar centre, since (as Gus rightly pointed out) it's easier to adjust for a slight richening in flight than it is for a slight leaning-out. In a sidewinder mounting as used on a profile stunter (the recommended mounting position for the Drone), placing the tank in the usual position on the outside of the fuselage achieves this pretty well. The tank should also be tucked in as closely as possible behind the motor to minimize the effect of manoeuvres on the mixture.

The motor should run smoothly throughout the flight using this approach, although it will start to run lean just before the tank runs out, thus giving you plenty of warning that the flight is about to end. If it crackles momentarily in manoeuvres, this is a symptom of momentary leaning-out. Nothing to worry about—it won't stop and will pick up smoothly as soon as the momentary anomaly is passed! A slightly richer take-off setting may cure this as long as the setting stays within the rich-running limits of the engine. However, the opposite effect can also occur—the engine may suddenly cut out altogether if it is run too near the upper limit of fuel mixture richness and a temporary manoeuvre-induced fuel surge makes it go even richer. If this occurs, you'll have to adjust the fuel system or take off at a slightly leaner mixture. But the engine actually has pretty good suction, and troubles of this nature can generally be overcome by test-flying and appropriate adjustment. Once the system is fine-tuned, flying becomes smooth and trouble-free.

The Drones can best be summed up as powerful and dependable engines which are one or two-flick starters and will run forever if well looked after. If you follow the above procedures, they can be run with complete confidence!

Micron 5 cc

This French engine seems by "feel" to have a very similar compression ratio to that of the Drone, although it has been reported as having a higher compression ratio of 20:1. All I can say is that its performance suggests otherwise—it definitely likes the lower speeds, appearing to peak at around 6000 rpm or so. It also does well on a 25% oil fuel mixture, which was in fact the mix recommended by the manufacturer. These engines were designed for free flight and as such were used with larger diameter lower-pitch props than the Drone. A 14 inch diameter prop was specified, with pitches around the 6 inch range. I've never flown one of these, but can report that it starts very easily and runs flawlessly. I get around 6000 rpm on a 13x 6 prop, which probably duplicates airborne performance on a 14 x 6. The thrust generated is amazing—despite the low revs, the large airscrew means that the thing moves a lot of air!!

Owat 5 cc

This British engine is pretty much a clone of the Micron, and the same comments generally apply. The makers recommended a 14 x 9 prop, which they claimed would turn at around 5000 rpm. They also recommended a very low oil content of only 10%, which would have reduced the effective operating compression ratio somewhat to match the low recommended speeds. I find that in practise the Owat runs fine on the bench on a 25% mixture, with a performance very similar to that of the Micron. I've never flown one, though.

Mite .099

This little American-made motor is the one that stands out from the rest in terms of its operating characteristics. The reason for this is its remarkably low compression ratio—only 13.5:1!! This is excessively low for any model diesel and in my view represents a design error on the part of the manufacturers. The makers appear to have recognized the implications of the low compression ratio employed, because the recommended fuel mix is 50% oil, 50% ether! And you really do have to use such a mix—with less oil, the effective operating compression ratio is simply too low for smooth running at any speed.

For starting, this engine is unusual in that you need to get a lot of fuel into the cylinder to reduce the unused space in the combustion chamber and generate enough heat for starting. Thanks to the very wide explosive limits of ether, you can get away with this. However, another problem rears its ugly head at this point—the method of mounting the prop driver on the shaft is highly unsatisfactory, with an absurdly small tapered shoulder against which the prop driver rests—more of a slip joint than an actual fixture! As a result, the prop tends to slip while starting the engine from cold. You have to really tighten it down, and even then it's problematic.

For this reason, the Mite is one fixed-compression diesel which is actually far easier to start when hot! There's no tendency to detonate—the low compression ratio and high oil content see to that! When hot, you just follow normal fixed-compression procedure as outlined above, and the engine re-starts very easily.

As far as props go, I've found that an 8 x 8 prop is the lightest load on which the engine will run smoothly on the bench. Try an 8 x 6 and you get past the speed range at which the timing is optimized. The Mite turns an 8 x 8 at around 5500 rpm on the bench. Naturally, it moves a fair bit of air while doing so, and would undoubtedly fly a small model very well. But it will not run smoothly above about 6500 rpm on the standard 50% fuel mix, and it seems absurd to consider adding even more oil to such a mix. So I'd recommend an 8 x 8 as the ideal test prop for this engine. Indeed, this prop size was recommended by several contemporary testers of this engine.

The castings on the Mite are all magnesium, so you have to follow the usual precautions to prevent corrosion while the engine is in storage. That said, the Mite is a neat little engine which runs well but would undoubtedly have benefited from being supplied with a higher compression ratio and a better method of mounting the prop driver.

I also have a Vivell fixed-compression diesel, but have never run it since I wish to preserve its New condition. It feels as if it has a rather low compression ratio as well, and I suspect that its running characteristics would not be unlike those for the Mite.

Well, there it is! There's no reason to fear setting up that old fixed-comper in the test stand and giving it a run! You'll be surprised at how instantly they start and how well they do in fact run once everything is sorted out. And the contest records compiled by several of them will no longer seem so improbable!


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