Conference vmszoo::rc

    I have used the Davis Diesel Conversion on a Cox TD .049. It uses
    a replacement for the glow-plug which is a variable compression
    head with a PTFE disc as the gas seal. The engine ran O.K. but was
    a little more fiddly to get the settings correct. As usual with
    a diesel the prop used was a slightly greater length than normal.
    
    The big disadvantage with the conversion was that the PTFE discs
    would only last a couple of flights before needing replacement.
    
    It was an interesting experiment to try, but I don't see any
    great advantages.
    
    I also remember seeing a diesel conversion for a Webra 61 that
    was installed in a Mannock Biplane. This did not seem to suffer
    unduly from vibration, but the flying was stopped abruptly when
    the crankshaft snapped!
    
    If you want to fly a diesel, buy a diesel engine. I have seen diesels
    up to 3.5 c.c. used in R/C models with no vibration problems. My
    father even tried a PAW 19 in a Club 20 racer.
    
    I think diesels are great characterful engines, much quieter than
    the same size glow engine if un-muffled. My all time favourite engine 
    is the Mills 0.75 c.c. diesel.
    					Andrew.

    re .1: can you give me the names of diesel engine manufacturers?
           how long ago did you try the cox .049?
    
    the latest issue of rc scale modeler (april) has an article
    written by mr. davis himself.  english is poor but content is
    very good; i forget the exact title but subject is schneurle ported
    engines but it migrates quickly into an advertisement for his
    product.
    
    anyways, despite "dont do it"s from multiple people, i have gone
    ahead and ordered a conversion head for an enya 45cx thats currently
    in my cap.  i regard this as an (expensive) experiment, if it works
    out i intend to dieselize other scale ships.  i will keep you
    informed.
    
    kevin

Al, 

Have a read of note 69.*.
    
>    Has anyone tried the Davis conversion and, if so, are the claims
>    to be believed?  How does it start/handle?  Where is the fuel ob-
>    tainable?  Is there a problem with evaporation of the ether in the
>    fuel, special considerations in the summer?

Ether evaporates very quickly, Diesel is best kept in glass of as a second 
choice metal containers never plastic. The container must be resealed as soon 
as you have removed the fuel to prevent excessive ether loss or you end up with 
a mixture of castor oil and kero.

If you add Amyl Nitrate to the fuel for improved performance it reduces the
shelf life to a few weeks. Today Iso Propol Nitrate (IPN) is being used as a 
performance additive due to the improved shelf life of the fuel, it is cheaper 
(ie; in Oz Amyl is $120/litre IPN is around $60) and finally and most important
safety, Amyl is a hart stimulant.

John.    

    Kevin,
    
    So did you ever follow through with the experiment?  If so, what
    were/are yer' conclusions??
    
    Adios,	Al

    well, where to begin...
    
    i bought a 40-size cap21 at an auction with a enya 45cx.  it flew
    like hell but also was loud as hell with a tatone in-cowl muffler
    (if you can call it that).  the diesel seemed like a way to quiet
    it down.  also diesel supposedly has lots of advantages which scale
    enthusiasts should be interested in, like
    
    more efficient which translates to lower fuel consumption and less
    back pressure.
    cooler exhaust.
    lower rpm range, able to swing bigger props.
    quieter than 2cycle glo.
    no glo plug, better idle on inverted engines.
    slow idle.
    
    so i bought a davis diesel conversion head for the 45cx.  its kinda
    neat.  there's a big allen screw on the head that varies static
    compression by moving a piston in the head up and down.  looks like
    very precise machining.  a much smaller allen screw also on the
    head is used to "lock in" settings.  installation is just as easy
    as a head change.  just remember to use a fuel pump made for gas,
    i tried to get away with a quicky tank fill but the dubro pump melted
    immediately.  anyways starting it is pretty wild, just prime and
    hit with starter (yes, its like a gun thats always loaded, ignition
    is always on so dont turn that prop with your finger.
    compression is real high so you need a good charge
    on your starter battery.  mine seems to run real rough at first,
    then gets better.  after starting, your supposed to set needle valve
    for max rpms (this is all from memory and its been a while), then
    start reducing compression till motor starts to miss and increase
    compression 1/8th turn till smooth again.  much like you set a needle
    valve.  i never felt confident that i was setting compression right,
    there was no point where motor suddenly ran rough, the more
    compression, the better it ran.  also much more power.  with higher
    compression settings it had about the same power as the glo version
    but thats a rough measurement.  i worry about engine longevity at
    higher settings and was very conservate when adjusting.
    i never got very good at starting it, the first
    start of the day was tough, the rest were easy.  i had 20+ flights
    in on the cap before the conversion but only got 5 or 6 with the
    diesel cause i dorked the cap in pretty good one day.  (another
    dual-rate related snap roll)  other projects and obligations caused
    the plane to sit in the hanger the rest of the flying season, however it
    has since been recovered (glass instead of monokote) and i'm anxious
    to continue the diesel experiments.  i would not recommend diesel
    to anyone unless they need an inverted engine or must swing a big
    (or multiblade) prop.  i didnt find the engine to be THAT much
    quieter than a glo altho with less backpressure, some interesting
    custom exhaust systems are possible.  i also disliked the smell
    and the black oily exhaust residue.  i can say id idled reliably
    with inverted engine, so slow that wings would rock back and forth.
    
    the bottom line is i dont yet know if i like the diesel or not.
    i still hope it works out, it is a pain.
    
    kevin

    forgot to mention that with glo i was turning a 10.5x6 or 10x7
    prop.  with the diesel i only tried an 11x7 prop.  maybe could have
    been bigger.

    Does this diesel conversion have a fuel injector in the head or
    does the old carburetor still get used?
    	
    If the carb. is still used, how does it manage to idle?  When you lower
    the amount of air inducted into a diesel, it doesn't get as hot on the
    compression stroke... which means that all of a sudden, it stops
    igniting.  No smooth ramp-down of power, just a repeated engine
    "miss".
    	
    Or am I the one who is "missing" ?  :-)
    	
    	-Bill
    
    

    the davis diesel conversion kit is just a replacement head.  the
    same carb is used but because the diesel is more efficient, it needs
    less air and fuel, so full throttle is effectively achieved while
    the throttle barrel is only 1/2 to 3/4 open.
    
    i guess with diesel auto engines fuel is injected directly into
    the cylinder and is a function of throttle position while air enters
    thru (begin hand-waving) intake valve and is ???  constant?  beats
    me.  another thing thats different is that car engines use a glow
    plug for cold-starting, right?  i'll try to remember to read the
    side of my fuel can to see whats in it.  also if interested i can
    probably decmail out a photocopy of directions supplied with head.
                                                
    kevin

    Yup, in 'real' diesels, the fuel is injected at many PSI into the
    cylinder head at the top of the compression stroke, and is ignited
    by the warmth of the air (at 23:1 compression or so it gets hot...),
    and the amount of fuel injected depends on throttle position.  There
    is nothing between the air cleaner and the intake valves to 'throttle'
    the air flow, you always get a full charge of air and a variable
    amount of fuel.  Even at full throttle, you have oxygen left over
    (this is called stochiometric or some such), so they tend to be
    reasonably efficient.  The glow plugs are used to help the air get
    up to temp during cold starts.
    
    I've always been mystified by model 'diesel' engines, as they don't
    seem to have many of the same features (except for compression setting
    off the fuel mixture, but virtually all model engines use this....).
    
    Willie
    

    	Most diesel engines are 4-strokes with compression ignition.
    The fuel burns during injection because the air temperature is
    already above the ignition temp of the fuel.  The fuel charge burns
    in excess air - thus no throttle on the air is used.  The air enters
    through the intake valve just like a gas 4-stroke, but the fuel
    is directly injected into the combustion chamber after the compression
    stroke.  Diesel model engines are 2-stroke diesels with port timing
    and no valve train.  Fuel and air are compressed together just as
    in a glow engine, and ignition takes place as soon as the temperature
    reaches the ignition temperature. (During the compression stroke.)
    Model diesel fuel contains either to make cold starting easier.
    Ignition timing is regulated by altering the compression ratio.
    This is done by changing the amount of clearance between the piston
    and the head at Top Dead Center.
    
    Charlie
    

    I'm still lost, don't model (glow?) engines ignite the mixture through
    compression also?  Or do model diesel engines not have glow plugs?
    So far (except that you can vary the compression) I don't see any
    difference between model diesel and model glow engines.  How would
    varying the compression help you, I would think that early compression
    would lead to backfiring (and the engine wouldn't run) while late
    compression would lead to missing (and the engine wouldn't run)....
    
    Willie
    

    Now, what'd REALLY be neat is to have a model diesel with a real
    injector (maybe triggered by a pin sticking out of the piston crown?)
    	
    The carburetor and throttle would go away, and the engine output
    would be regulated by how much fuel you injected on each power stroke.
    
    Or am I missing something still?
    

>    		......Or do model diesel engines not have glow plugs?

* Correct.  Model diesel engines have no glo-plug.

>    So far (except that you can vary the compression) I don't see any
>    difference between model diesel and model glow engines.  

* Exactly.  The glo-engine is just almost a diesel in that the fuel/air mixture
very nearly reaches combustion temperature by virtue of compression.  The glo-
plug merely adds the last coupla' degrees of temperature required to cause com-
bustion.  It is a fact that, on a given day when conditions are "just" right, a
glo-engine will start just by flipping the prop, "with no battery attached to 
the plug."  While relatively rare, this happens frequently enough to be mention-
ed occasionally in the safety columns.

>							.........How would
>    varying the compression help you, I would think that early compression
>    would lead to backfiring (and the engine wouldn't run) while late
>    compression would lead to missing (and the engine wouldn't run)....

* I'm only brainstorming here but I think the compression adjustment only varies
the compression ratio, not the timing.  More compression = more power and vice
versa, timing being a fixed value dependant upon porting.  I do recall, while
fooling with that (*^&^$@# O.K. Cub .049 diesel way back when, that you had to
start the engine at a lower compression setting; too little and no combustion 
occurred, too much and the piston would compression-lock, stopping the prop dead
while your finger slid down the length of the blade, lacerating the bejesus out
of it.  Once running ["if" you were lucky], the needle valve was adjusted for
best setting [just like a glo-engine], then the compression could be increased
`til the engine began to sag and backed off a little for optimum power.  The
little Cub diesel had buckets more power than the comparable glo-engine but the
aggravation of starting it was more than a 14-year old [me] could cope with at
the time.

In addition, I had a devil of a time keeping the fuel [ether content] fresh....
it seemed like, the moment you opened the fuel can, the ether evaporated and
the necessary volatility was lost.
    
I guess I'm kinda' responsible for rekindling interest in this topic as I was
sorta' hoping that things might've changed somewhat from the very negative ex-
perience I had with diesels.  So far, with Kevin being the only noter with any 
experience with the Davis-diesel conversion, it would seem that not enough has
changed for the better to interest me in giving diesels another try.

Adios,	Al

    I think that varying the compression is equivalent to varying the
    timing of injection because the mixture will ignite early if the
    compression ratio is too high.  Since the volume of the combustion
    chamber is so small to get the 23/1 or greater compression ratio
    required, adjustment is the only easy way to make an inexpensive
    model diesel.  On a 2-stroke diesel, you adjust injection timing
    the same way you adjust spark timing on a gas engine.  This is done
    on the injection pump which is driven off of the crank or camshaft.
    	One reason that our glow engines don't idle slowly is that the
    glowplug cools off too much between power strokes.  This is why
    an engine will idle much slower with the battery attached.  When
    you remove the battery, the plug cools off and the engine stops.
    This problem is worse on 4-strokes because they only fire every
    other revolution.  THis is equivalent to running at half the
    rpms of a 2-stroke.  THis gives the plug more time to cool off.
    On board glow systems give the plug juice when you are at low
    throttle settings.  This greatly improves idle reliability at lower
    rpm's.
    
    Charlie
    

I've really learned a lot in this discussion, thanks guys.  Stu
Richmond in Model Builder is a diesel advocate and has written
much on the subject in the last year (though I've learned more
here).  He has been virtually poetic about a small (.09 I think)
diesel made in the Soviet Union -- which can be had here for a
little over $50.

I've been tempted by diesels -- I remember a friend had a McCoy
.049 diesel back in the '50s; it ran rather well, but this guy
had a gift with engines.  The only things that have stopped me
from trying one now are 1) The Davis conversion is nearly as
expensive as the engine, and 2)Here in Coleraddy they speak glow
fuel only.  Only commies use diesel. Its bad enough that I bring
something other than a .40 powered trainer to the field.  If I had
access to fuel, I might try it, no matter what.

Did anyone ever explain how a 2-cycle diesel is throttled?

    John,
    
    A Davis diesel conversion retains the 2-cycle engine's stock/ standard
    carb.  I'm only speculating here but I'd assume a specifically designed
    diesel R/C engine (if there is such a thing) would utilize the same
    type carb arrangement.
    
    BTW, a boyhood flyin' buddy back in the `50's also had a McCoy .049
    diesel and [if memory serves] a larger one [maybe .099?] as well.
    Both of these seemed a bit friendlier that the miserable O.K. Cub
    [wonder what ever happened to them] I had.  I recall him using the
    .049 in a Baby Barnstormer 1/2A ukie-stunter and it worked fine
    but still required a special knack [not to mention a lot of fiddling]
    to start and adjust properly.  Mine finally exceeded my patience
    one hot summer afternoon when trying to fly at the local schoolyard.
    It resisted my every attempt to make it run and literally butchered
    the first two fingers on my right hand to the extent that I lost
    my cool and "wing-walked" the [innocent] little bipe it was mounted
    in `til only fragments remained.  Memory fails as to what ever happened
    to the engine but I never used it again!  I built an identical
    replacement airplane and went back to glo-engines forever...at least
    to the present.
    
    Adios,	Al

    I'd think a 'real' diesel would be incredibly difficult to build in a
    model size, as the injectors and injection pump are built to very close
    tolerances, (part of a tuneup involves setting the injection pump
    timing to within 0.02 inches....) work at incredibly high pressures (my
    Rabbit manual mentions that the injectors open between 1706 and 1849
    PSI).  Also these things tend to be rather massively built...  I
    don't know that there are any full scale airplane diesels at all,
    due to hp/lb considerations, but a 1/4 scale car or boat could probably
    be built around a diesel, but at that point you are probably into
    chainsaw and lawnmower engines....
    
    Willie 

    Metering fuel in the small quantities used by a model size engine
    would be impossible at reasonable cost.  This is one reason why
    any model diesel has a carb.  It's only my opinion, but as long
    as I can get glow fuel and a battery, I wouldn't touch a diesel
    and its high tolerances and smelly exhaust for anything.  That goes
    for an automobile also.  If the EPA was anything but useless, they
    would force the diesel cars and trucks off the road unless they
    could get the particulates out of their exhaust.  That sh** is just
    as unhealthy as the CO and NOX emissions that the gas cars have
    been forced to lower at great expense.  There are only two practical
    applications for diesels that I know: Trucks and boats.  Both of
    these applications don't care about weight.
    
    Charlie
    

    I kind of liked the idea of the pin at the top of the cylinder poking
    at the injector to make it open, though you would have to do some
    really close tolerance machining to make it come out right.  Also
    the fuel might need some work, conventional diesel fuel has a lot
    of energy in it, but it won't burn readily, and has to be injected
    as a "fine mist" in a very short period of time.
    
    I'm just glad Rudolf Diesel's first engine exploded, as his original
    fuel was coal dust....
    
    You would need a cutting torch to pry me away from my diesel Rabbit!
    At least my engine isn't hidden under the emissions controls...
     :+)
    
    Willie
    

    Willie,
    I'm just glad you can't keep up to me since I don't make it a practice
    to ride behind 'oil burners'.  (Just kidding, but it is a problem)
    I know too many people who have had bad diesel experiences. (You
    have one of the better diesels as far as reliability goes.)
    I have a little 2 cylinder diesel aux engine in my sailboat.  I
    have to scrub the transom a couple of times a summer to get rid
    of the carbon from the exhaust.  That little bugger always starts
    up when I need it though.
    
    Charlie
    
    

    dear converters,
    
    I think I have to help you.
    Some years ago I converted two glowengines to diesel. I wanted to
    use them for controlline goodyear racing.I modified the engines,
    a USE MKI and a Rossi 15 (both 2.5cc engines) using a concept of
    Robbie Metkemeyer, a guy I know fairly well and who's a multiple
    CL-teamrace champ. Both engines ran pretty well and I probably will
    be able to find a drawing. Also Rob wrote a story about how he designed
    his FMV teamrace diesel (Integral cylinder, AAC, 1st place EC 1984
    or '85) in a magazine called Aeromodeller. If you've read this report
    y'll probably understand something more about diesels and a lot
    more about engines in general.
    
    The difference between glow and diesel isn't that much. They are
    both of the 'hybrid' type, meaning that compared to 'real' engines
    they are not according to the diesel or the otto principle but a
    mix of them.
    A diesel engine needs some other fuel. I used 40% ether, 50% 
    paraffine, 7% oil and 3% isopropylnitrane (IPN) for my conversions.
    Of course if you don't have AAC piston/liner and bronze bearing
    in the pushrod's big end you will need some more (15 to 20%) oil.
    Never buy your diesel fuel from the shelve, it has a very limited
    shelve life. Mix it yourself than you know what you have. Limited
    lifetime is about 2 weeks, I prefer to mix my fuel the evening before
    I am going to use it! 
    Dieselfuel burns because of the ether in it. By compression of the
    mixure this is heated untill it reached its flaming point. Therefore
    compression adjustment is used to controll the timing. If you use
    not enough ether your engine won't start. Also if you did succeed
    in starting it will run too hot. The heatcapacitance of the ether
    is used to control thermal management during running. If you use
    too much ether your engine won't run very continuous because its
    temperature stays to low.
    
    And about the drawing to convert the engine, just let me know 
    where I have to sent it to. However one must be aware that 
    some machining has to be done, certainly a lathe is needed.
    
    Leo Voss
    NMGV06::VOSS OR JGO::ENG_RPT
    DTN 889-9350