Diesel Engines by Ian McQueen
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|The ED Story by Ron Reeves (in collaboration with Ron Chernich) This work is licensed under a Creative Commons Attribution-Non-Commercial-Share Alike 3.0 License|
|Refurbishing Old Diesel Fuel - by Jack Hiner|
|Diesel Engine Basics|
Elfin 2.49 diesel
2.49 cc Front Induction.
Made in England from 1947 to 1958
The Elfin was popular for all kinds of aeromodelling applications and it still is with SAM.
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
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
feature 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
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
In a glow engine, the fuel will begin to burn at a
fixed point on the upstroke (with some caveats). With a suitable, 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
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
The English Mills diesels in 1.3cc
and 0.75cc are remarkably efficient in turning really large propellers.
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.
The fuel consists of kerosene for power, ether
to ignite the kerosene, lubricant, and an ignition improver. Here
are some notes about the ingredients.
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!
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
(especially when the air is hot).
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. Starting spray for
full-size engines contains ether. One source is John Deere dealers.
One freezes the container and punctures it to release the ether, but
I have never done this, and these instructions are not enough to teach
you how to do it safely!
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.
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
The simplest way for most modellers to obtain diesel fuel is to obtain it from one of the commercial manufacturers/suppliers. Companies that I know of that sell diesel fuel in the USA are:
The following instructions begin with starting a diesel engine, then branch to adjusting it for full power output (for a broken-in engine) and for break-in (for a new 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" at the beginning, 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.).
Mounting the engine
Mount the engine on a strong mount (or in a model).
It is convenient if the engine can be removed from a mount 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.)
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. Partly this depends on the ability of the engine to draw fuel.) 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 should not be used because it swells up in contact with diesel fuel, though you should get by using it for a day if you are prepared to replace the tubing. Ordinary PVC (vinyl) tubing can be used where flexibility is not needed. It will become stiff after prolonged contact with fuel.
Select a prop of suitable size from the engine 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
The screw in the head, and thus the position of the
contrapiston, are usually in the right ballpark when the manufacturer
engine. You DID resist the temptation to turn that little screw,
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. These uncertainties are what make diesels so much fun!
Set the carb wide open.
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 is very important to avoid flooding.
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
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!
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 does not start after several tries of these instructions, it may be flooded. Full instructions for clearing a flooded engine are given at the end. Clearing a flooded engine is a general pain, and the fiddling necessary to clear it can get aggravating. Try to avoid flooding it.
Running and Adjusting the Engine - Top of this Page
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 for
a prop of that size. In colder weather, you may have to turn the screw
in slightly from that point; likewise if you later fit a prop of less
diameter and/or pitch (reduced load). And you may have to back the
screw out slightly in hot weather or if you fit a prop of larger diameter
and/or pitch (increased load). At this point you are ready to run the
Needle valve and throttle positions
Needle valve: The instructions with the engine should give an indication of a suitable starting position for the needle valve. For an unfamiliar engine being run for the first time, especially for break-in, I usually open the needle valve three to five turns from fully closed. (An alternative method used by diesel expert David Larkin is to start with the needle valve open only about half a turn and to open it a quarter turn with each unsuccessful attempt to keep the engine running, but the engine might then run at quite high speed when it catches, and this can be undesirable for an unrun engine.)
Throttle: The throttle should be wide open. (With
experience you can start at reduced settings; 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.
Usually there is no need to draw fuel up to the spraybar beforehand.
Fuel will usually be drawn to the engine when it starts.
Starting the engine
With the fuel line pinched against the engine or model with a finger (or the nose held high), 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 usually be drawn to the engine and the engine will keep running. If the engine doesn't keep running, immediately clamp the fuel line (or lift the nose), open the needle valve 1/4 to 1/2 turn and repeat.
When you get the engine to keep running, congratulations! You are well on the way now!
And if you are using the method of opening the needle
valve in steps from closed and if the engine doesn't keep running after
the prop has been flipped several times, immediately clamp the fuel
line (or lift the nose), open the needle valve 1/4 to 1/2 turn, prime,
and flip again. Repeat as necessary.
At this point the engine is running and needs to be adjusted. Instructions are given first for starting and setting an engine that has already been broken in. Instructions for a new engine are given later. 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, the next step may be 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.
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 adjusting the needle valve until the engine would keep running, so it should be in the ballpark of the correct setting.
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
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
the procedure. If it
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 or even beyond.
This procedure will have you close to the full power
output. If the engine is in a model, try flying it.
In general it will probably be necessary to richen
the mixture slightly and increase the compression slightly from the
initial settings to
Break-In - Top of this Page
The instructions with your engine will probably cover this, but here are a few tips anyway.
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
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 compresson 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 so they break off.
ABC engine: The procedure is completely different
for ABC engines. The principle here is to get the engine reasonably
hot as quickly as possible, for the fit between cylinder and piston
is usually very tight at TDC and one wants to expand the top of the
cylinder enough to provide the normal running clearance. One should
use a relatively small propeller so the engine can turn freely and
quickly find optimal CR and needle valve settings to run relatively
rich at high speed. (K&B, in their instructions for breaking
in a<glow> 6.5RIRE ABC pylon engine recommended a prop cut
down so the engine would four-stroke at.....20,000 rpm! Use this
information for guidance.)
Troubleshooting - Top of this Page
You will flood an engine at some time. Here are some tips on clearing it. They are based on an engine in a model so it can be inverted easily. If the engine is in a test stand, there are ways to clear flooding, but removing the engine for draining remains an option.
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.
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
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 one O.S. 25FSR and all three of my 10FSR engines. All of the 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 as glow engines, but the 10s were so loose that they were very difficult to start even as glow engines.
If your engine has poor compression, it is better to repair it (new internal parts). 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
If the engine does not turn over easily, stop immediately and reduce the CR. Also check for excessive fuel inside the engine.
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 just
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 is a regular contributor to SAM Talks Yahoo discussion group, webmaster )
GB500 Replica by Gordon Burford and David Owen, accepted by the Engine Committee as a pre 1950 diesel.
(Photo Model Engine News)
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