Hi I know that quite a few people on the list fly R/C planes, so I was wondering if anyone has some advice:
I have a 2-stroke glowplug engine that I'm intending to convert to spark ignition. I've built an electronic ignition
unit using a magnet in the prop driver and a Hall sensor that will mount on the crankcase.
My question is how do I determine the position of the Hall sensor with respect to TDC of the engine?
I am only interested in single-speed (full throttle) running at the moment.
Has anybody got any practical experience or can they point me to a book or website - I've looked a lot myself
but without success.
Thanks
Alan
Embedded Systems Engineer
Microtima Ltd
Ouseburn Mews
3-7 Stepney Bank
Newcastle upon Tyne
NE1 2PW
TDC is at the top of the compression stroke - quite easy to find, put your finger over the hole in the top of the cylinder, turn the crankshaft in the right direction, when the engine stop pushing air past your finger you are close to TDC - if you can see the piston you'll see where is stops moving at the top of the stroke.
In a car you will see the spark pulse around 10 degrees before TDC - this is adjustable by turning the distributor (this is the equivilent of rotating the mount of your hall effect sensor) I suspect you may need less of an angle in this engine as the flame front will move across the combustion chamber in considerably less time than in a car due to size differences.
Hope this helps
Eoin
>>> .....alanKILLspam@spam@MICROTIMA.CO.UK 04/25/02 08:26AM >>>
Hi I know that quite a few people on the list fly R/C planes, so I was wondering if anyone has some advice:
I have a 2-stroke glowplug engine that I'm intending to convert to spark ignition. I've built an electronic ignition
unit using a magnet in the prop driver and a Hall sensor that will mount on the crankcase.
My question is how do I determine the position of the Hall sensor with respect to TDC of the engine?
I am only interested in single-speed (full throttle) running at the moment.
<snip>
Thanks
Did you rule out using a photo diode and led, encapsulated in the same
package? This could be mounted on the "Crankcase" of the motor, and a disk
with a notch cut out, mounted behind the boss of the prop. This "Disk" could
resemble a "Hole saw" style tool, but without the teeth, and a single notch
instead. This could be attached to the prop boss and rotated to adjust
timing with just loosening off the prop nut.
Just my 10c worth, but this may solve potential counterbalance problems
that you may have with a magnet!
Ignition timing is proportionate to engine speed and load, most engines
run with initial static timing of 10 Deg Before TDC, there is an Australian
PIC kit out there that is designed to control aftermarket ignition timing,
advance and retard. Just remember that optimum advance at full load will not
ensure easy starting (And poor part throttle fuel economy), as there is
generally too much advance on the ignition timing at cranking speeds.
Hope this helps!
Gareth Bennett
Thanks for the reply, it gives me a start if nothing else!
I have my own trick for finding TDC - remove the glowplug and insert a piece
of dowelling down the plughole until it touches the piston crown.
Turn the crankshaft until the dowel does not rise any higher and then put a
mark on the dowel where it exits the cylinder head. A tiny 'pop' or scratch
can be made on the end of the crankshaft denoting TDC.
Your suggestion seems to give a little bit of ignition advance. I was hoping
that someone had either heard of a reasonable ballpark amount of advance for
a model engine or had done it themselves. I will be the person flicking the
prop here and IIRC too much advance can lead to a nasty bite!
Thanks
Alan
>In a car you will see the spark pulse around 10 degrees before TDC - this
is adjustable by turning the distributor (this is the equivilent of
>rotating the mount of your hall effect sensor) I suspect you may need less
of an angle in this engine as the flame front will move across the
>combustion chamber in considerably less time than in a car due to size
differences.
>Did you rule out using a photo diode and led, encapsulated in the same
>package? This could be mounted on the "Crankcase" of the motor, and a disk
>with a notch cut out, mounted behind the boss of the prop. This "Disk"
could
>resemble a "Hole saw" style tool, but without the teeth, and a single notch
>instead. This could be attached to the prop boss and rotated to adjust
>timing with just loosening off the prop nut.
> Just my 10c worth, but this may solve potential counterbalance problems
>that you may have with a magnet!
Is this true? Surely a disk with a slot cut from it will also be out of
balance when it is rotated.
I actually think that my solution is pretty elegant becasue the magnet is
only 2mm dia. and
is flush-fitted in a hole in the propdriver. The material that gets drilled
out for the hole is more
or less replaced by the magnet.
Also, slotted IR devices are prone to get crudded-up on the oily front end
of a 2-stroke glow engine.
> Ignition timing is proportionate to engine speed and load, most engines
>run with initial static timing of 10 Deg Before TDC, there is an Australian
>PIC kit out there that is designed to control aftermarket ignition timing,
>advance and retard. Just remember that optimum advance at full load will
not
>ensure easy starting (And poor part throttle fuel economy), as there is
>generally too much advance on the ignition timing at cranking speeds.
Bingo, this does help. You have pre-empted me a bit! I want to get the
engine running at one
speed and then make my own PIC based advance and retard unit that plugs into
the throttle outlet on my R/C
receiver.
> Your suggestion seems to give a little bit of ignition advance. I was hoping
> that someone had either heard of a reasonable ballpark amount of advance for
> a model engine or had done it themselves. I will be the person flicking the
> prop here and IIRC too much advance can lead to a nasty bite!
As I sit here reflecting on the numerous scars remaining on my left thumb
from an incident in '75 -- a thumb/nylon prop/OS Max .35 interference
issue -- I feel I should remind you that's what they make "chicken sticks"
for. That, and electric starters. My poor thumb looked like hamburger
for weeks, and I had no sensation in it for many years. I don't recommend
it. YMMV.
>make my own PIC based advance and retard unit that plugs
>into the throttle outlet on my R/C receiver.
Sounds like a good way to have the thing fall out of the sky if you are not
real careful. I think I would prefer to have it do it automagically as the
throttle is adjusted. At least it should then be possible to have it set up
so that you are not accidentally setting it to one extreme or the other
while having the engine at the wrong revs, and stalling while over the
deepest part of the local boating lake or most expensive car in the car
park.
Hi!
I didn't explain THAT one very well.
I've seen commercial units that are basically a uC in a box that reads the
length of the throttle pulse
and alters the spark advance proportinate to the pulse length.
I've already designed and built a PIC based R/C boat speed controller and I
think that the pulse length
measuring part can be stolen from that.
>I think I would prefer to have it do it automagically as the
>throttle is adjusted.
>As I sit here reflecting on the numerous scars remaining on my left thumb
>from an incident in '75 -- a thumb/nylon prop/OS Max .35 interference
>issue -- I feel I should remind you that's what they make "chicken sticks"
>for. That, and electric starters. My poor thumb looked like hamburger
>for weeks, and I had no sensation in it for many years. I don't recommend
>it. YMMV.
>
>Dale
Hi Dale
This is going to sound horribly macho but I have to confess that I don't use
a chicken stick or an electric starter.
I do however wear a *very* thick leather gauntlet while flicking. I have to
say that I've been tangling (maybe not the best word) with model IC
engines for about 15 years and have never yet been bit while starting one
(frantically touching wood at this point).
My biggest concern with this is that if I get the ignition point too far
advanced then I get backfiring and a loose prop - no chicken stick or
starter
is going to help in that scenario, yes?
I appreciate your concern for me fingers, but believe me, I value them very
highly!
> >As I sit here reflecting on the numerous scars remaining on my left thumb
> >from an incident in '75 -- a thumb/nylon prop/OS Max .35 interference
> >issue -- I feel I should remind you that's what they make "chicken sticks"
> >for. That, and electric starters. My poor thumb looked like hamburger
> >for weeks, and I had no sensation in it for many years. I don't recommend
> >it. YMMV.
> >
> >Dale
>
> Hi Dale
>
> This is going to sound horribly macho but I have to confess that I don't use
> a chicken stick or an electric starter.
Oh, neither do I. I just don't reach around the prop to adjust the needle
valve any more! 8-) 'Course I haven't flown in years.
> My biggest concern with this is that if I get the ignition point too
> far advanced then I get backfiring and a loose prop - no chicken stick
> or starter is going to help in that scenario, yes?
Nope. That's what big rags are for, quickest way I've found to stop a
misbehaving engine without damage to self or equipment. Of course without
a prop it may not be of much use... then it's finger-over-carb time!
Pressure sensors seem to be the rage on the list at the moment- how about
mounting one in the top of the cylinder so you can detect exactly when the
pressure is optimal to fire the plug? :)
>Your suggestion seems to give a little bit of ignition advance. I was
>hoping that someone had either heard of a reasonable ballpark amount of
>advance for a model engine or had done it themselves. I will be the
>person flicking the prop here and IIRC too much advance can lead to a
>nasty bite!
Hehehe. I think it's called the RC finger (red, swollen index on right
hand).
When you tinker with this you do not want the engine to start so dry the
carb out and use a pipette to put just a small amount of fuel (3-4 drops)
on the exhaust ports and work it into the engine with the ignition off,
then make a start attempt. Repeat. High compression 2-stroke engines MUST
move the ignition point to negative advance at start because they will
start backwards otherwise. (negative a. is beyond TDC in the direction of
moving). Also beyond and not at because at TDC the detonation will produce
no torque and can dent the crank pin (remember the speed is zero and the
crank lever is inline with the piston). If you do this right the engine
will start with the first flick of the prop (unlike normal glowplugs).
I am tinkering with this when I have time exactly now!
You cannot have decent results if you do not take the RPM into account.
The normal setup for simple cases is a movable sensor on a lever that
allows -5 to +5 degrees of advance. Provide a lever or a hole for a lever
so you can actuate it without touching the cylinder or the prop. You can
also have an electronic delay.
To account for RPM there are several dozen ways. With only one sensor you
can measure the 'width' of the pulse from the Hall.
Arrange such that the falling edge from the Hall pulse triggers the firing
(not the rising edge). Build an ignition circuit that won't issue a last
deadly spark when you turn it off (since it is on the trailing edge).
good luck,
Peter
PS: BTW did you calculate the ignition power at 20,000 rpm and 20mJ/spark
assuming single spark ;-). Because I did and I will try to run the engine
very slowly ...
----
Hi=20
I know that quite a few people on the list fly R/C planes, so I was =
wondering if anyone has some advice:
I have a 2-stroke glowplug engine that I'm intending to convert to spark =
ignition. I've built an electronic ignition
unit using a magnet in the prop driver and a Hall sensor that will mount =
on the crankcase.
My question is how do I determine the position of the Hall sensor with =
respect to TDC of the engine?
I am only interested in single-speed (full throttle) running at the =
moment.
>You cannot have decent results if you do not take the RPM into account.
True enough - that is why I was trying to keep things simple and run at a
single speed for now.
>The normal setup for simple cases is a movable sensor on a lever that
>allows -5 to +5 degrees of advance. Provide a lever or a hole for a lever
>so you can actuate it without touching the cylinder or the prop. You can
>also have an electronic delay.
Yes, I've seen this kind of movable fixture on vintage ignition engines, the
thing that I was missing was the typical advance angle.
Now you seem to have answered my question. I can probably start and run the
engine with -5 degrees of advance?
>To account for RPM there are several dozen ways. With only one sensor you
>can measure the 'width' of the pulse from the Hall.
So you propose a 'spark firing delay' based on the length of the pulse from
the Hall sensor - this would mean a fixed, known position for
the Hall sensor - would this be at TDC?
>Build an ignition circuit that won't issue a last
>deadly spark when you turn it off (since it is on the trailing edge).
What do you mean here?
>PS: BTW did you calculate the ignition power at 20,000 rpm and 20mJ/spark
>assuming single spark ;-). Because I did and I will try to run the engine
>very slowly ...
Where did you get the figure for the spark energy? Do you have access to
some information on spark ignition or is this just an experienced
estimate?
>So you propose a 'spark firing delay' based on the length
>of the pulse from the Hall sensor - this would mean a fixed,
>known position for the Hall sensor - would this be at TDC?
If you are going to need -5 to +5 degrees of timing adjustment then this is
relative to TDC, so you are best to put the sensor at about 6 or 7 degrees
before TDC and delay from there, giving a maximum delay of 11 or 12 degrees
from the sensor position. This is a lot easier than trying to time a full
360 degrees minus the advance when the spark needs to be before TDC, but
then switching to less than 5 degrees delay as you want the delayed spark
after TDC.
It also has the advantage that when the engine revs are below some figure
(less than the idle speed, say half idle speed) then you set maximum delay
to get the start timing someone else mentioned.
Thanks to all who contributed on this. I reckon I have enough information to
make a start now, so I'm going to give it a try.
I'll let you know what happens.
Alan
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This is a post that belongs to another thread but I had email trouble so I
post this reply late:
Alan:
> start and run the engine with -5 degrees of advance?
Yes you can but you will need a very good starting arrangement, like
compressed air or a gas generator or a strong spring or such. Forget
ticking it over by hand. It will backfire on you every time and start
backwards given a chance (2 stroke).
Set up so you can set 0 to +3 degrees advance (3 deg. after TDC!) while
starting and you can move it back to -5 or more after starting (an extra
adjustment beyond throttle and mixture needle).
>To account for RPM there are several dozen ways. With only one sensor you
>can measure the 'width' of the pulse from the Hall.
>So you propose a 'spark firing delay' based on the length of the pulse
>from the Hall sensor - this would mean a fixed, known position for the
>Hall sensor - would this be at TDC?
As Alan Pearce said, it needs to be earlier than the earliest advance you
plan to use. Modern ignitions often use two sensors to detect both speed
and direction. The device refuses to work if the axle turns backwards at
any time. This is a very good idea. See below.
>Build an ignition circuit that won't issue a last
>deadly spark when you turn it off (since it is on the trailing edge).
>What do you mean here?
If your circuit is built improperly it will produce an extra unwanted (and
untimed) spark either when turning on the ignition or when turning it off.
Normal circuits with points & coil can and do that. If the distributor
rests in the closed points position when you turn the ignition off you
will have an extra spark. Normally this will do nothing but with an older
model engine it may cause a big booming backfire, unless the automatic
advance moves the firing point slightly beyond TDC with the motor at rest
(vacuum driven ones usually do this). You might remember old b/w movie
scenes with Oliver & Hardy and old automobiles that made big booms when
being started/stopped. The scenes were exagerated but relied on real
facts. It is your responsability to plan for this sort of thing. A
backfire on a fueled engine will start a carb fire in 3 out of 5 tries and
may destroy the carb, not to mention send it flying off and burning
you to a crisp ;-). Hint: small fires are as painful as big fires.
>PS: BTW did you calculate the ignition power at 20,000 rpm and 20mJ/spark
>assuming single spark ;-). Because I did and I will try to run the engine
>very slowly ...
>Where did you get the figure for the spark energy? Do you have access to
>some information on spark ignition or is this just an experienced
>estimate?
I have access to several old engineering books that cover such things as
combustion and ignition among other things. 20mJ is the input energy in
some small spark based igniters, like in a battery powered gas lighter or
very small gas engine. A car coil is more like described at:
and has some 500mJ per spark. High energy ones go to 6-8J per spark
(disregard what they say in that link about the voltage in high energy
coils - it has nothing to do with voltage in this case) and require
special plugs and special everything.
The power required to run the ignition increases very fast with rpm and
spark primary energy. 20000rpm and 20mJ on a single cylinder 2-stroke
require some 7W. This is a lot of power for a small engine. A untuned car
ignition with electronic switching (aka controlled saturation) requires
some 80W (4cyl 4 stroke 1 coil 5000rpm). Non-electronic ignitions consume
much much more power, especially at low speed.
sorry for the long post, but I felt that I owed an answer,
In the context of the thread the -5 degrees mentioned was taken as being 5 degrees AFTER TDC - the initial posts said 10 to 5 degrees BEFORE TDC (which has been taken as being a positive number)
Agreed on the timing wanting to be after TDC for starting. The frame of reference just wasn't the same :)
>>> RemoveMEplpEraseMEEraseMEACTCOM.CO.IL 04/29/02 04:33PM >>>
> start and run the engine with -5 degrees of advance?
Yes you can but you will need a very good starting arrangement, like
compressed air or a gas generator or a strong spring or such. Forget
ticking it over by hand. It will backfire on you every time and start
backwards given a chance (2 stroke).
<snip>
sorry for the long post, but I felt that I owed an answer,
"Peter L. Peres" wrote:
> You might remember old b/w movie
> scenes with Oliver & Hardy and old automobiles that made big booms when
> being started/stopped. The scenes were exagerated but relied on real
> facts.
I've seen some exagerated episodes such as blowing mufflers apart and
right off the exhaust system.
My ol' 48 Harley loved to backfire twice when the throttle was closed.
It did it so loud one day that I saw people jump more than 300 yards
away :-)
>"Peter L. Peres" wrote:
>> You might remember old b/w movie
>> scenes with Oliver & Hardy and old automobiles that made big booms when
>> being started/stopped. The scenes were exagerated but relied on real
>> facts.
>
>I've seen some exagerated episodes such as blowing mufflers apart and
>right off the exhaust system.
My father used to own a 1937 Plymouth that refused to start one day. After
much attempting to start and getting no-where fast, he decided to run the
car down the hill to get it to start. Eventually there was a very loud
"Boom" and still no start. He went investigating to find the rotor had
broken and eventually with all the cranking had made connection between the
coil lead and the spark plug lead of a cylinder that had the exhaust valve
open. The resulting flame down an exhaust system full of explosive mixture
did the rest. :))
AG>> start and run the engine with -5 degrees of advance?
>
>
PLP>Set up so you can set 0 to +3 degrees advance (3 deg. after TDC!) while
PLP>starting and you can move it back to -5 or more after starting (an extra
PLP>adjustment beyond throttle and mixture needle).
Ah! I have confused your terminology. I assumed that when you mentioned -5
degrees
of advance you meant that the ignition point would be *retarded* when in
fact you meant advanced.
It's all clear now!
PLP>If your circuit is built improperly it will produce an extra unwanted
(and
PLP>untimed) spark either when turning on the ignition or when turning it
off.
PLP>Normal circuits with points & coil can and do that. If the distributor
PLP>rests in the closed points position when you turn the ignition off you
PLP>will have an extra spark. Normally this will do nothing but with an
older
PLP>model engine it may cause a big booming backfire, unless the automatic
PLP>advance moves the firing point slightly beyond TDC with the motor at
rest
My basic ignition circuit (i.e. the Hall sensor, two transistors, three
resistors and a coil
only 'fires' when the magnetic field leaves the Hall sensor.
>The power required to run the ignition increases very fast with rpm and
>spark primary energy. 20000rpm and 20mJ on a single cylinder 2-stroke
>require some 7W. This is a lot of power for a small engine. A untuned car
>ignition with electronic switching (aka controlled saturation) requires
>some 80W (4cyl 4 stroke 1 coil 5000rpm). Non-electronic ignitions consume
>much much more power, especially at low speed.
I'm aware that electronic ignitions use less power, but it's nice to see
some figures.
>sorry for the long post, but I felt that I owed an answer,
No need to be sorry - and you certainly don't owe me an answer!
Thanks again for the advice, I'll let you know as soon as I get any joy!
>>In the context of the thread the -5 degrees mentioned was taken as being 5
degrees AFTER TDC - the initial posts said 10 to >>5 degrees BEFORE TDC
(which has been taken as being a positive number)
>>Agreed on the timing wanting to be after TDC for starting. The frame of
reference just wasn't the same :)
>My basic ignition circuit (i.e. the Hall sensor,
>two transistors, three resistors and a coil only
>'fires' when the magnetic field leaves the Hall sensor.
You may get away with this using a hall sensor, but do be aware that if you
use a pickup coil you may well get some automatic advance with change in
engine speed. The amount you get is dependant on the shape of the magnetic
pole pieces and the resultant rate of magnetic flux change in the coil.
I believe this effect is used in magnetos to give automatic advance.
> My ol' 48 Harley loved to backfire twice when the throttle was
> closed. It did it so loud one day that I saw people jump more
> than 300 yards away :-)
Aye caramba !!! A few feet is quite a jump - but wow, 300 yards.
Man, no wonder Aussies win so many gold medals - you've been
x-bred with 'roos, c'mon 'fess up !!
Jinx wrote:
>
> > My ol' 48 Harley loved to backfire twice when the throttle was
> > closed. It did it so loud one day that I saw people jump more
> > than 300 yards away :-)
>
> Aye caramba !!! A few feet is quite a jump - but wow, 300 yards.
> Man, no wonder Aussies win so many gold medals - you've been
> x-bred with 'roos, c'mon 'fess up !!
> -----Original Message-----
> From: Alan B. Pearce [SMTP:@spam@A.B.Pearce@spam@spam_OUTRL.AC.UK]
> Sent: Tuesday, April 30, 2002 11:22 AM
> To: spamBeGonePICLISTKILLspamMITVMA.MIT.EDU
> Subject: Re: [EE]: electronic spark ignition for model engines
>
> >My basic ignition circuit (i.e. the Hall sensor,
> >two transistors, three resistors and a coil only
> >'fires' when the magnetic field leaves the Hall sensor.
>
> You may get away with this using a hall sensor, but do be aware that if
> you
> use a pickup coil you may well get some automatic advance with change in
> engine speed. The amount you get is dependant on the shape of the magnetic
> pole pieces and the resultant rate of magnetic flux change in the coil.
>
> I believe this effect is used in magnetos to give automatic advance.
>
It is used on systems that have a variable reluctance sensor. As the engine
speed increases, the output from the sensor increases with amplitude. By
using a fixed decision point on the first edge of the pulse gives a crude
automatic advance. I've seen this used on many motorcycle engines.
I'm not very fammilar with hall effect devices, but I would imagine their
output wouldn't incraes with speed like a variable reluctance device would?
>I'm not very fammilar with hall effect devices, but
>I would imagine their output wouldn't incraes with
>speed like a variable reluctance device would?
That is what I figure as well, because they are sensitive to the intensity
of the magnetic field, not the rate of change.
Alan B. Pearce wrote:
>
> >I'm not very fammilar with hall effect devices, but
> >I would imagine their output wouldn't incraes with
> >speed like a variable reluctance device would?
>
> That is what I figure as well, because they are sensitive to the intensity
> of the magnetic field, not the rate of change.
Modern automotive hall devices almost always
have signal conditioning built in and provide
a fairly clean digital output as the shaft is
turned. The timing should not change with
shaft speed. :o)
-Roman
>>I'm not very fammilar with hall effect devices, but
>>I would imagine their output wouldn't incraes with
>>speed like a variable reluctance device would?
>
>That is what I figure as well, because they are sensitive to the intensity
>of the magnetic field, not the rate of change.
That's correct however in a linear sensor the slope mimics the shaft speed
and since it is connected to a coil (eventually) it changes its dU/dt and
hence the spark timing in conjunction with the capacitance in the circuit.
Think of it as a low Q RLC circuit with the spark across it. If the coil
voltage raises fast (high dU/dt) then the breakdown voltage is reached
early. Raise slowly and it comes late. Similar ideas apply to magneto
'pointless' ignitions. Getting these to work right is a very time
consuming and messy lab job. Usually there is a hysteresis element in the
circuit that triggers the ignition proper. It can be a voltage controlled
latching device (thyristor + zener works f.ex.).
Peter
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