Deciphering the igniter...

Extrapolating from my experience with power transistors...

Junction type semiconductor devices invariably short when they fail. The junction develops a hot spot - a tiny area that is hotter than the surrounding junction and therefore has more current flow. That produces a nearly instantaneous runaway condition, melting the junction in that area. If the junction is back-biased, like the collector junction of a transistor is, the voltage ensures the PN junction is homogenized.

(Still assuming the power switcher is a bipolar transistor) The collector in power transistors is usually soldered to a heat sink. The base and emitter leads are fine wires. When the collector shorts, the fault current flows through emitter lead and often burns it out, leaving the device open circuited. Some power transistors have heavy emitter leads and will just stay shorted, even with fairly high currents, but it would not be good to have the ignitor fail in a shorted condition.

If the switcher is actually a power CMOS device, I don't know enough about them to say.

Mike

1. One could view the old contact points as a combined crank angle sensor and coil actuator. Some early electronic ignition systems actually still had contact points (and vacuum and centrifugal advance), but the points were only used as an input to the electronics. The actual switching of the coil was done by the electronics. The electronic box would simply ground the coil whenever the contact points were closed, and open-circuit the coil whenever the contact points were open. The only benefit of this vs. the non-electronic system was that the points saw very little current and virtually no inductive loading, thus there was no arching and the points would not wear.

The ignitor (ICM) does exactly what the electronic box did in the example above, it simply grounds and open-circuits the coil at the falling and rising edge of the signal from the ECU. Theoretically the ignitor could be mounted inside the ECU, but then you would have a very nasty signal (high voltage spikes etc.) running all over the place rather than being nicely tucked inside the distributor.

1. Yes. There are 3 sensors in the distributor, the CYP, or Cylinder Position Sensor, gives one pulse for every complete rotation of the distributor rotor (presumably around TDC prior to the power stroke on cylinder #1, but that is a guess on my part), the TDC, or Top Dead Center, sensor gives 4 pulses for each complete rotation of the distributor rotor, one pulse each time a piston is at TDC at the beginning of its power strike. Finally there is the CKP, or crank position sensor. This sensor gives a large number of pulses for each rotation of the distributor rotor.

The CKP gives the crank angle with very fine resolution, but it is a relative measurement only. I.e. by looking at the CKP output you can tell that the engine now has rotated e.g. 76.3° since you started counting, but you can not tell what specific position it is at. Then what you (well, the E-C-you, that is) do is you reset your counter every time you see the TDC signal. Now you know the exact number of degrees you are past the latest power-stroke TDC, and since you know the rotor goes 90° between each power-stroke TDC, you can easily do the subtraction and find out how many degrees /before/ TDC you are. The ECU calculates the desired ignition timing based on throttle position, RPM, manifold pressure etc., and then simply turn on or off the signal going to the ICM at the exact right point in the rotation as read by the CKP and the TDC signals.

The ECU really does not care about /what/ cylinder gets the spark, the finger in the distributor does that in the conventional way. The ECU /does/ need to know what injector to fire, however, and that is what the CYP sensor is for.

The above is relevant for my '94 Civic. Newer systems have gotten rid of the distributor finger as well, and then you need the CYP sensor to get the right ignition sequence.

I don't know about Honda, but some other newer cars have only one sensor, essentially the CKP. The absolute position information is indicated by missing pulses on the CKP signal. Imagine an ABS wheel sensor where you file away a couple of teeth in the right locations. E.g. you remove 4 teeth, each 90° from the next removed tooth. Then you remove a 5th tooth at some other location. now you can deduce the CKP, CYP and TDC information all from one sensor.

Incidentally, the CKP signal can also be used to detect misfires. During the power stroke, the crank will accelerate a bit, then retard a bit around TDC. This can be measured as a slight variation in the frequency of the CKP signal. If you see that there is no acceleration at the time when you know there is a power stroke, you conclude there was a misfire. This type of detection is required in OBDII equipped cars.

1. It is my understanding that the output transistor fails in the ignitors. I can not find any reliable reference on this, however. I don't think the main relays fail because of poor soldering from the factory. I believe the solder fatigues over the years, since solder is the only /mechanical/ fastening for a fairly heavy relay. Solder, if subjected to stresses and strains (like shaking a relay for mile after mile), will fatigue. If you have a stranded copper wire, tin the leads and clamp it down really good in a screw terminal, you can come back a few years later and see that the solder has been reduced to dust and that the wire now is loose. I do not believe the ignitor has any heavy components in it.

Michael Pardee wrote: | Extrapolating from my experience with power transistors...

Thanks for the insights.

| If the switcher is actually a power CMOS device, I don't know enough | about them to say.

Extrapolating from my experience with Power MOSFETs (not much), I would tend to agree. However, I have seen both happening, the MOSFET developing an open-circuit and "wire replacement" :) I wouldn't be surprised if short circuit between drain and source would eventually overheat something inside and cause an open circuit.

Don't know whether current igniters are MOSFET or bipolar. MOSFET would make sense due to the ease of driving them, the ridiculously low losses (perhaps not that much of a problem? I tinkered around with a MOSFET that takes 180A (amps, not milliamps!) and has something like 15 mOhm (milliohm) "on" resistance). On the other hand, car manufacturers tend to be rather conservative with electronics, so they might hold on to bipolar for another while?

From a page on Toyota igniters,

Now, if they use IGBTs, then someone else needs to give their input ;)

Lots of stuff can go wrong. It's a brutal environment.

Ozone can carbonize dirty surfaces near the HV and create a conductive path into sensitive circuits. The coil's winding insulation can fail and overheat the coil. Excessive voltages caused by cracked wires, worn points, or a failed capacitor results avalanching in the power transistor, the semiconductor equivalent of an arc-over, and accelerated aging of the coil insulation. Avalanching slowly degrades the silicon crystal into a more passive form, like a resistor, that will overheat.

Transistors usually fail as a short circuit or a low resistance. If the fried transistor heats enough to melt a lead wire, the flyback power released from the coil can lead to a small explosion at the break.

Typical ignition systems are flyback types. Inductors act as constant current devices. Applying 12V causes current flow to gradually increase. Break the current rapidly and voltage shoots in the opposite direction in an attempt to maintain the current flow. That's a few hundred volts on the primary and tens of thousands on the secondary. Flyback transformers have a problem with imperfect magnetic coupling between the two coils. Some of the power spikes back into the primary even if the secondary discharges into a spark plug. It's the capacitor's job to dampen the primary's voltage so it doesn't arc over the switch/transistor or rise faster than the switch/transistor can turn off. If the secondary doesn't discharge into a spark plug, the flyback voltage will rise until something gives way. Hopefully its a protective avalanche diode or spark gap.

Honda shows MOSFETs in their ignition schematics of the Civic but doesn't show what is used in the Accord.

MOSFETs are kind of tricky to use for flyback circuits. There's a strong capacitive coupling between the gate and the source. Several amps coming off the gate from that capacitive coupling isn't unusual. It can lead to nasty RF ringing or destruction of the driver. The solution is of course to use pair of power MOSFETs to drive the HV MOSFET. The driver is complex but it fits on a chip.

Bipolar transistors have less capacitive coupling but they need lots of holding current. The driver is trivial but it needs power resistors that run too hot to be on a chip.

MOSFETs can't handle much current when they're designed for high voltages. A small 30V MOSFET can handle 50A to 150A but a small 500V MOSFET handles 4 to 8 amps. Automotive ignition coils is pretty much their limit. Larger HV power supplies use bipolar transistors or IGBTs.

An IGBT has impressive current and voltage abilities but one might be too slow for an ignition coil. The IMA, A/C, and other 144V 3-phase motors are driven by IGBTs the Hybrid Accord.

I agree. Mounting of heavy components by solder is a dubious practice, and the vibration makes fatigue an "if, not when" proposition.

When I worked in avionics, we would often see a popular transponder with fuse failure. We replaced the fuse every time and only once saw a unit come back. The fuse was a slo-blo glass fuse, and we could see the solder attachment at the end had crumbled, leaving rough edges, instead of the melted ends of blown fuses. We never saw it in twin engine planes, only singles.

Mike

This brings up one mystery - how do electronic ignitions get away with no capacitor or equivalent energy shunt? Points needed a capacitor to give the contacts time to get a little air between them before the voltage peaked, so a capacitor isn't needed (strictly speaking) with an igniter. But if the HT wires open up, the energy has to go *somewhere*. I know in Integras it often goes into the coil with bad results, but many systems aren't damaged when the HT lead is open.

Mike

Power MOSFETs also get around the thermal runaway problem. My hunch is they are some sort of MOS devices.

Ditto - I barely know they exist!

Mike

practice, and

When my ignitor failed, I didn't take it apart but perhaps should have so see what's inside.

It could be that they wirebonded the dies and put thermally conductive gel around it all -- the same way they make hockey puck Solid State Relays, for instance. They make enough of these ignitors to justify the one time charges and would make them pretty cheap in quantity. Not sure if they actually did this, but mechanically and electrically it is a superior method of making a very reliable product.

Wow, what a response!

With the knowledge amply herein exhibited, maybe this group ought to be called rec.autos.makers.honda.electronics! I'm impressed. This is the sort of thing that makes a good FAQ possible. Thank you.

I'm responding in this particular message only because my original post has been pushed off the stack by my news provider.

With the wealth or information in everybody's responses, I've saved them all and will have to review them. I will post a draft once I figure I understand all this.

For a picture of the insides of an igniter, see here:

"TeGGeR®" wrote in news:Xns964BD70A3C96tegger@207.14.113.17:

I looked at this pix and brightened it up with IrfanView,and it appears that the transistor has a part number that could be researched IF one could read all the marking,which I couldn't.It starts with BU.Get me the whole PN,and maybe I can find what it is and it's specs.Knowing the manufacturer maker on it would help greatly,too.

I could not read the number on the transistor(xstr). It seems to be a standard BU-type semi.It might be a combo MOSFET/bipolar with a internal protection diode,like what's used in a TV's high voltage flyback,or just a plain bipolar PNP xstr.

Dunno.Japanese power supplies commonly use metal clips to hold a power semi to the heatsink;quicker and cheaper.

"flyback" is when a coil is energized,a magnetic field is built up,and when the current is quickly removed,the field rapidly collapses and induces a REVERSE current from the original charging current.The coil's magnetic field is an energy storage device.This is how a high voltage spike is generated. This also holds for transformers.It's how many PC power supplies work,too.

Ignition "dwell" time (for the older points-type ignitions) is the length of time the points stay closed to charge up the ignition coil,thus determining how much spark energy is generated with the flyback pulse.Modern electronic systems can control this much better,no points bounce,either.

True, but in this application the transistor (FET or BJT) is always either off or in saturation, so thermal runaway isn't an issue.

They usually still need a capacitor. The primary voltage shoots up to lethal voltages before the secondary can fire the spark plug. If that went into a protective avalanche diode, there'd be no power for a spark.

I have a gizmo that drives two old-school ignition coils out of phase. The MOSFETs (Two NTE2385 in parallel) are rated for >500V and can survive avalanche breakdown. I can get away without using a capacitor only because the coils are rather lossy and the MOSFETs are rather expensive. I once had a similar setup with better coils that would burn out instantly without a capacitor.