SPICE model for a spark plug?

I thought your dirt bike had pedals.

A spark plug breaks over at about 20kV. It's an avalanche breakdown in comp ressed gas, so you have to have a free charge carrier show up to initiate t he the avalanche. Car cylinders are dirty places, so it might be a chunk of soot.

The currents involved in the established avalanche would be high enough to sustain an arc if it went on for long enough, but you have an initial glow discharge with a voltage drop of a hundred volts or so for perhaps a micros econd before the ion bombardment gets one electrode surface hot enough to allow the arc mechanism to take over - which is to say to let the surface g et hot enough into distort under the influence of the applied electric fiel d to create lots of atomically sharp spikes that emit electrons by "warm" f ield emission.

Best of luck modelling that in Spice.

The presence and absence of fuel make a difference to the multiplication pr ocess that forms the initial spark, so the initial breakdown voltage is goi ng to be different, and it influences the glow discharge period when the di scharge moves from a glow to an arc. Once the arc is established the fuel c ontent doesn't make any difference.

A friend of mine set up a laser system for looking at the gas flows inside a single cylinder test engine for Shell in the later 1970's, but he was mor e worried about the combustion initiated by the spark than the spark itself .

Wouldn't be better to "just" run a test, and feed the voltage/current waveform into spice?

Cheers

Klaus

Yes, but this isn't an engine application and testing would not be easy at all. Or not really possible a priori.

It's for something way bigger :-)

It also works fine in clean air.

The current (100-200mA), duration and so on are well know. I could probably piece together a behavioral model but I am not very good at that and thought that I sure can't be the first one. But maybe I am.

Yes, different job.

Break-over depends on risetime as well as voltage. I may have a model. Cross fingers. ...Jim Thompson

Untested by JET. It has the odor of an LTspice model, so behavior and convergence are automatically suspect >:-)

.SUBCKT SPARK_GAP2 1 2 PARAMS: V_GLOW=1500 V_ARC=150 I_SUS=.002 V_BREAKDN=12000 I_ARC=.2

• SINCE THE STRIKING IS VERY FAST, IT IS STRONGLY ADVISED

• TO SET TRTOL TO 1 VIA: .OPTIONS TRTOL=1 and ITL4=1000. THIS WILL FORCE

• PSpice TO BE MORE VIGILANT IN THE VICINITY OF TRANSITIONS

• Documentation on the Sparkgap model can obtained from AEi Systems,
  formatting link
  310-216-1144, snipped-for-privacy@AENG.com

• D_D3 10 11 DZ_ARC X_SARC 9 0 3 10 SPARK_GAP2_SARC R_RT1 7 0 50 D_DT2 8 9 D1N4148 E_EBREAKDN_CONTROL 6 0 VALUE = {IF(ABS(V(1,2))>{V_BREAKDN} , 12 ,

• IF( ABS(I(V_VSENSE)) > {I_SUS} , 12 , 0 ))} D_D4 5 11 DZ_ARC E_EARC_CONTROL 8 0 VALUE = { IF( ABS(I(V_VSENSE)) > {I_ARC} , 12 , 0 )} D_D2 5 4 DZ_GLOW V_VSENSE 5 2 DC 0 C_CT2 9 0 0.1U R_RT2 9 0 50 C_CT1 7 0 0.25U X_SBREAKDN 7 0 1 3 SPARK_GAP2_SBREAKDN D_D1 3 4 DZ_GLOW D_DT1 6 7 D1N4148 .MODEL DZ_GLOW D (BV={V_GLOW} IS=1U RS=5 IBV=10U) .MODEL DZ_ARC D (BV={V_ARC} IS=1U RS=5 IBV=10U) .MODEL D1N4148 D (RS=.8 CJO=4PF IS=7E-09 N=2 VJ=.6V

• TT=6E-09 M=.45 BV=100V) .ENDS .subckt SPARK_GAP2_SARC 1 2 3 4 XS_SBREAKDN 3 4 1 2 SWhyste Params: Ron=5 Roff=50MEG VT=10 VH=0 RS_SARC 1 2 1G .ends .subckt SPARK_GAP2_SBREAKDN 1 2 3 4 XS_SBREAKDN 3 4 1 2 SWhyste Params: Ron=5 Roff=50MEG VT=10 VH=0 RS_SBREAKDN 1 2 1G .ends

...Jim Thompson

Thank you. LTSpice doesn't like this line though:

E_EARC_CONTROL 8 0 VALUE = { IF( ABS(I(V_VSENSE)) > {I_ARC} , 12 , 0 )}

It complains about the "extra" curly brace at the end which to me seems correct, not "extra". If I take it out LTPSICE, of course, complains about a missing curly brace. Software really isn't my thing. Or maybe there is a bug somewhere.

Meantime I found a nice article about spark gap modeling using the Rompe-Weizel formula but it's complicated:

Now I've got to take care of a production line stop situation. Those are always fun. Grumble.

mpressed gas, so you have to have a free charge carrier show up to initiate the the avalanche. Car cylinders are dirty places, so it might be a chunk of soot.

o sustain an arc if it went on for long enough, but you have an initial glo w discharge with a voltage drop of a hundred volts or so for perhaps a micr osecond before the ion bombardment gets one electrode surface hot enough t o allow the arc mechanism to take over - which is to say to let the surface get hot enough into distort under the influence of the applied electric fi eld to create lots of atomically sharp spikes that emit electrons by "warm" field emission.

process that forms the initial spark, so the initial breakdown voltage is g oing to be different, and it influences the glow discharge period when the discharge moves from a glow to an arc. Once the arc is established the fuel content doesn't make any difference.

e a single cylinder test engine for Shell in the later 1970's, but he was m ore worried about the combustion initiated by the spark than the spark itse lf.

One car I had with points & coil produced around 6kV for sparkplugs IIRC, n othing like 20kV

NT

the interesting real life behavior of spark plugs is...

if you have a "fouled" plug" that the car cannot fire....

sometimes adding another spark gap in series with the fouled plug will allow the car to fire the fouled plug.

It seems that if the coil is connected directly to the fouled plug, it can never develop enough voltage to fire.

But if a second gap is inserted in series, then the coil CAN develop the v oltage and when it fires, both gaps will arc.

I discovered this quite by accident. Back in the late 60's I had a car tha t was mis firing on one cylinder. The standard troubleshooting procedure c alled for removing the wire from one plug one at a time and the one that ma kes no difference is the bad one. I noticed as I was putting the wire back on, when I held the wire close to but not attached, an arc would jump and the car would run better. I latter found they actually made small inline gaps just for that purpose.

Nothing is ever simple.

Mark

What brand? In air, or under compression? ...Jim Thompson

ters

compressed gas, so you have to have a free charge carrier show up to initi ate the the avalanche. Car cylinders are dirty places, so it might be a chu nk of soot.

h to sustain an arc if it went on for long enough, but you have an initial glow discharge with a voltage drop of a hundred volts or so for perhaps a m icrosecond before the ion bombardment gets one electrode surface hot enoug h to allow the arc mechanism to take over - which is to say to let the surf ace get hot enough into distort under the influence of the applied electric field to create lots of atomically sharp spikes that emit electrons by "wa rm" field emission.

on process that forms the initial spark, so the initial breakdown voltage i s going to be different, and it influences the glow discharge period when t he discharge moves from a glow to an arc. Once the arc is established the f uel content doesn't make any difference.

side a single cylinder test engine for Shell in the later 1970's, but he wa s more worried about the combustion initiated by the spark than the spark i tself.

, nothing like 20kV

Lada, late 1960s design. 6-7kV at idle. AIUI figures like 20kV are more ass ociated with electronic ignition, but it's not an area of expertise for me. Measured by an adjustable spark gap in open air, that was measuring the ma x V_out the coil could produce. So it can't have delivered more to the comp ressed cylinder content - must have dlivered less in fact.

NT

ters

compressed gas, so you have to have a free charge carrier show up to initi ate the the avalanche. Car cylinders are dirty places, so it might be a chu nk of soot.

h to sustain an arc if it went on for long enough, but you have an initial glow discharge with a voltage drop of a hundred volts or so for perhaps a m icrosecond before the ion bombardment gets one electrode surface hot enoug h to allow the arc mechanism to take over - which is to say to let the surf ace get hot enough into distort under the influence of the applied electric field to create lots of atomically sharp spikes that emit electrons by "wa rm" field emission.

on process that forms the initial spark, so the initial breakdown voltage i s going to be different, and it influences the glow discharge period when t he discharge moves from a glow to an arc. Once the arc is established the f uel content doesn't make any difference.

side a single cylinder test engine for Shell in the later 1970's, but he wa s more worried about the combustion initiated by the spark than the spark i tself.

, nothing like 20kV

at idle and low loads there isn't much pressure

onsdag den 6. juni 2018 kl. 20.09.01 UTC+2 skrev snipped-for-privacy@yahoo.com:

an never develop enough voltage to fire.

voltage and when it fires, both gaps will arc.

hat was mis firing on one cylinder. The standard troubleshooting procedure called for removing the wire from one plug one at a time and the one that makes no difference is the bad one. I noticed as I was putting the wire ba ck on, when I held the wire close to but not attached, an arc would jump an d the car would run better. I latter found they actually made small inlin e gaps just for that purpose.

many cars run wasted spark with two plugs in series

The redneck solution would have been to strap a piece of wood across the area and lash that one plug connector to it so that it just fires, and call it a day. "Hey look, I fixed it!".

That's purely a trick to use a special ignition coil to serve two cylinders from a single CB wire using a floating HT winding that effectively puts the two spark plug gaps in series. It works best when the waste spark occurs on the exhaust stroke (on a 360 deg crank twin cylinder 4 stroke engine for example). The effect with old fashioned Kettering ignition (whether transistorised or not) on fouled plugs is merely an unexpected bonus.

If you want a more foulproof system, the obvious way is to remove the dependence on coil inductance as the HT generating mechanism (flyback pulse mode) and use the coil purely as a step up transformer to the instant application of a 400v charge from a capacitor connected in series with the LT winding across a quenchable dc-dc converter which can be briefly shorted out via a thyristor which arrangement allows one full cycle of ac energy to be applied to produce some 30 to 40 kilovolts from the bare coil, reduced by the actual cylinder pressure at the spark plug points (circa 6KV at tickover to 20 or so KV at full throttle maximum torque revs).

There isn't a flyback pulse mechanism for fouling induced leakage resistance to interfere with since the coil is just used as a voltage step up transformer in this case. The leakage due to plug fouling may knock a few kilovolts off the 40KV unloaded coil output but this won't stop the plug sparking at the lower breakdown voltage circa 20KV or so required to ignite the air fuel mixture.

The importance of the circuit topology, capacitor charged via the coil's LT winding by the output of a quenchable dc-dc converter which is shorted out by the thyristor, is that it allows the current in the capacitor to reverse via the converter's output rectifier bridge after it hits the positive peak, allowing for a negative voltage peak on the 2nd half of the full cycle which recharges the capacitor up to somewhere in the region of 70 to 80 percent of its initial voltage by which time the thyristor has unlatched and the converter restarts to top up the capacitor in plenty of time for the next sparking pulse.

Replacing the function of the CB points in the old Kettering setup with a high voltage switching transistor is so last century (and, oh so shit).

Q: Why should there be any difference? The compression ratio doesn't change. When you measure the cylinder compression, it is a static measurement at low rpm. The engine is cranked over by the starter. Variations between cylinders indicates leakage from intake or exhaust valves or worn rings, or a blown head gasket.

High loads means more fuel and air enters the cylinders. If the cylinder pressure increased, it would start acting as a diesel. This is called knock and can be very destructive. Most cars have a knock sensor to prevent damage. Also, diesel engines idle fine. This means the cylinder pressure is adequate to ignite the fuel. Why should it change at operating rpm?

because the throttle is closed so the manifold is at vacuum. When doing compression test you need the throttle wide open

diesel engines don't throttle the air they just meter the fuel