tegwin
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| posted on 24/8/13 at 10:59 AM |
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Lighting gas with sparkplug
I am trying to build the igntiion system for my little jet engine. Using a 555 timer as an astable to excited an automotive coil at about 300Hz...
I can get the plug to buzz with a purple arc but it won't fire off the gas. The piezo ignitor in my blowtorch does a much better job with a
single white arc. Any thoughts on how I can improve the spark and get the gas to light off?
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dave_424
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| posted on 24/8/13 at 11:07 AM |
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reduce the frequency to give the coil more time to charge?
Dave
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JC
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| posted on 24/8/13 at 12:00 PM |
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Prime engine with damp start? Spark will light this which will light gas, jet should then be self sustaining?
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PhillipM
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| posted on 24/8/13 at 12:09 PM |
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What coil charge time are you getting there?
3ms isn't a whole lot of time to charge, trigger, discharge and start again unless you have a good coil and driver.
What's the signal duration you're getting out of your circuit for a single pulse?
[Edited on 24/8/13 by PhillipM]
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tegwin
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| posted on 24/8/13 at 12:10 PM |
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The issue is lack of energy in the spark to get it to light off the gas in the first place....
I wonder if my coil is knackered...
Looking at it I think I would be better off with a CDI and coil rather than the traditional coil I am using, would negate the back EMF issue I am
having with the switching transitor..
Anyone have a CDI and coilpack kicking about? I only need a single plug though.
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tegwin
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| posted on 24/8/13 at 12:26 PM |
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quote:
Originally posted by PhillipM
What coil charge time are you getting there?
3ms isn't a whole lot of time to charge, trigger, discharge and start again unless you have a good coil and driver.
What's the signal duration you're getting out of your circuit for a single pulse?
[Edited on 24/8/13 by PhillipM]
Hi.
(from theory)
Total period of 0.00311849 seconds
High time of 0.003049199 seconds
I have the 555 astable switching a cylindrical oil filled automotive coil via a large transistor.
There are videos on youtube of similar setups kicking out quite some impressive sparks at varying frequency ranges... Suspect something in my setup is
not right. Possibly the coil.... The transistor gets hot too as well which is not ideal
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skov
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| posted on 24/8/13 at 01:34 PM |
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quote:
Originally posted by tegwin
Suspect something in my setup is not right. Possibly the coil.... The transistor gets hot too as well which is not ideal
Is the transistor tuning fully on? If not, that could explain why it's getting hot and you have a poor spark.
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tegwin
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| posted on 24/8/13 at 01:36 PM |
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Good point. What's the best way of checking that it is? Measure the voltage across it?
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skov
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| posted on 24/8/13 at 01:46 PM |
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Best way is to stick a scope on there. A volt meter won't tell you much at 300Hz.
Can you post up a diagram of the circuit?
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tegwin
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| posted on 24/8/13 at 01:53 PM |
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Sadly I don't have a scope. Only a multimeter with a frequency counter on it.
This is the circuit I am using.
The actual astable part seems to work ok as I can measure the expected frequency at pin 3.
If I run with the 1K resistor between pin 3 and the transistor then it doesn't work...
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skov
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| posted on 24/8/13 at 02:51 PM |
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I like the retro transistor 
Not so keen on the circuit though... It might work on a good day, but you really want another gain stage between the 555 and transistor, or a perhaps
a different transistor with a higher gain (a darlington would be better).
Presumably it must still work (to some extent) with the 1k in there as it does spark a bit?
Without redesigning it, I think I'd try lowering the value of the 1k resistor to around 500R to see if that makes any difference.
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tegwin
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| posted on 24/8/13 at 02:53 PM |
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If I have the 1k in before the tranny it doesn't spark atall. It's removed at the moment. Would it be worth adding a smaller transistor
before the big one do you think? If you can see a better way of doing it I would be so grateful for a sketch
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skov
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| posted on 24/8/13 at 03:05 PM |
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So have you currently got the output of the 555 connected directly to the base of the transistor?
You want some resistance in there or you'll risk damaging the 555 and/or the transistor.
[Edited on 24/8/13 by skov]
[Edited on 24/8/13 by skov]
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paulf
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| posted on 24/8/13 at 03:40 PM |
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Have you tried running it at a lower frequency? it does not need to spark at that frequency to ignite so maybe 75 or 100 hz may give time for a
stronger spark output.If the coil drive is the problem then maybe it would be possible to use an ignition amp from a contactless distributor to drive
the coil and feed it the signal from the 555.
Paul
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gremlin1234
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| posted on 24/8/13 at 03:56 PM |
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quote:
is D2 in the right place? I would have expected it across the coil.
http://en.wikipedia.org/wiki/Flyback_diode
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designer
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| posted on 24/8/13 at 04:10 PM |
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Why not a glow plug?
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Peteff
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| posted on 24/8/13 at 05:31 PM |
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Use a piezo from a Calor burner, they light my blow heater, simple to fit and only cost a few quid with no complicated circuits.
Ignito
r and
lead for under a tenner.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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MikeRJ
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| posted on 24/8/13 at 07:57 PM |
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quote:
Originally posted by tegwin
Sadly I don't have a scope. Only a multimeter with a frequency counter on it.
This is the circuit I am using.
The actual astable part seems to work ok as I can measure the expected frequency at pin 3.
If I run with the 1K resistor between pin 3 and the transistor then it doesn't work...
There are several things wrong here:
1) A 2N3055 has very low current gain at high collector current. If you want to get a primary current of 5 Amps or so you will need to be putting at
least 0.5 Amps into the base to switch it on, and preferably a little more than that to be sure of driving it into saturation. Obviously a 555 timer
can not deliver that level of current (especially not with a 1k resistor in series!), so you will need a driver stage before the 2N3055.
2) The maximum permissible voltage across the collector and emitter of the 2N3055 is about 60 volts, making it inherently unsuitable for driving an
ignition coil. Above this voltage the transistor junction will start to break down. Since the secondary voltage is directly related to the primary,
limiting the primary voltage to such a low level means a low secondary voltage as well. Typically you want to allow the primary voltage to reach
200-300v. With a bipolar transistor you must provide suitable protection to prevent the voltage exceeding this level, which you don't have (D2
is doing nothing really, since the reverse breakdown of a 1N4007 is around 1000v).
3) The duty cycle of the 555 is way too high, you are allowing about 3ms for the coil to charge which is not unreasonable, but you are only giving the
coil 70 microseconds to discharge which is too short. About 1ms would be correct for an inductive system like this.
They way to fix most of this this in one fell swoop is to ditch the prehistoric 2N3055 and replace it with a suitable IGBT, rated for ignition systems
(e.g. BIP373). These have simple voltage drives to the gate and usually provide inbuilt clamp for voltage protection.
You could also use a simple ignition amplifier module, such as those used in the 80's on cars that came with electronic ignition from the
manufacturer.
[Edited on 24/8/13 by MikeRJ]
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tegwin
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| posted on 28/8/13 at 09:28 AM |
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Thanks for your thoughts all.
This looks like an interesting product http://www.maplin.co.uk/ignition-amplifier-kit-3162
If I used that I should be able to fire it off with a simple transistor feed from the 555!?
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BaileyPerformance
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| posted on 28/8/13 at 09:46 AM |
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It's the diode across the 2055 that's clamping the fly back and preventing the coil from releasing the charge to the secondary.
Replace the diode with a 1.0mf 250v poly cap.
A 2n3055 has quite a low C E breakdown voltage so no sure how long if will last, plus you need at least 150ma at the base to fully switch it on
assuming the coil is pulling 10amps. Another transistor configured in Darlington pair would help.
A just your 555 to 100hz with mark space of 20% ish to avoid overcharging the coil and frying the 2055.
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Not Anumber
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| posted on 28/8/13 at 11:23 AM |
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This sounds like the jet engine would have similar ignition requirements to an exhaust flamer kit such as this one on the bay:
http://www.ebay.co.uk/itm/FLAMER-KIT-FLAME-KIT-EXHAUST-FLAMER-FULL-KIT-/321050177765?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac014f0e5
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Mr Whippy
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| posted on 28/8/13 at 12:09 PM |
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And why is there no picture of this jet engine????
Lets see it, what you going to do with it? I'd like a jet engine they sound so cool
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scudderfish
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| posted on 28/8/13 at 01:44 PM |
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My BOE calcs say a 3ms spark is equivalent to a 4 cyl engine doing 10,000 rpm. If you're just trying to light a jet engine, do you need to be
banging away that fast? Is the coil charging enough in that time?
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Mr Whippy
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| posted on 28/8/13 at 02:47 PM |
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Chances are the mixture is wrong and the flash point to high for a simple spark to ignite. A car gets away with it because the compression stroke
heats the air mix up.
You only need a spark every half second or so but it's going to have to be very hot, even then it might need a blast of fresh air to fire up
I'm sure you can hear the spark ticking away on youtube vids of jets starting up, there not very rapid at all
[Edited on 28/8/13 by Mr Whippy]
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Dualist
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| posted on 28/8/13 at 03:31 PM |
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I had problems with the above circuit years ago and as I know nowt about electronics me and my mate just used a vacuum cleaner on blow and blowtorch
on the outlet, was a bit of a nack trying to get the flame to go back inside past the blades but it worked 
[youtube]http://youtu.be/xXKjUWMiwnc[/youtube]
[Edited on 28/8/13 by Dualist]
Rotary build coming soon...
quote:
Originally posted by RichardK
I recently discovered that pigs can p i s s sideways when being transported 
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