Dwell angle, electronic ignition

the more modern ones do indeed "go overboard" and sense current and time to threshold. i believe most of the time this is done in a local ignition module, bit some also do it at the ecu end since there's more computational horsepower depending on how much you want to manage the coil[s].

the advantage is, and this is very important, is that this has a material impact on coil heating, or more accurately, limiting heat build-up. modern potted coils can't dump heat like older oil bathed ones since there is no convection possible.

i should check, but iirc, there's some code for this on the megasquirt engine management project.

There are several different systems, Paul. As you stated, dwell angle means little or nothing unless you are using a Kettering system.

On electronic systems, traditional capacitive discharge systems used high voltage DC systems to charge the capacitor(s) to a high value, then the high voltage charged was dumped through a coil or coils to generate a strong spark. Studies showed that one good hot spark was all that was needed to ignite the fuel, but, even so, some companies engineered systems with multiple spark discharges (MSD). IMHO they are good, but no better than simple CD systems.

Since we dont know what your needs are, it is hard to comment further.

MSD systems are good for carbed vehicles that may not have real good mixture of fuel. The multiple spark serves to ignite overly rich pockets of fuel when the ignition pulse breaks them down and mixes them better. On a modern vehicle with port injection they don't work as well because the mix is much better and more evenly dispersed.

Modern ignitions have advanced a LONG way from the old Kettering system. Current vehicles use the crank/cam sensors to not only tell the ECM when to fire but also look for the power pulse and duration to determine IF the cylinder fired properly.

you're confused. most electronic ignition systems are not capacitive discharge, they're inductive discharge. two very different concepts in terms of spark energy and duration.

not exactly. what's needed is a spark of sufficient energy. just like trying to fire a muzzle-loader with an improperly primed firing cap, a low energy spark, regardless of voltage, will only achieve partial ignition. you need a high /energy/ spark to achieve good ignition.

inductive discharge can provide not only ignition voltage, but current over time as its magnetic flux decays, and thus the energy delivery required. indeed, that's why it's used on almost all oem ignition systems. cd ignition uses a transformer [not coil] to step up the capacitive voltage, but it has little energy and poor duration.

dumbed down explanation here:

most msd systems are indeed capacitive discharge. and while they offer the advantage of being able to offer multiple sparks per cycle at low rpm's, they suck at high rpms because their spark energy is low.

some people don't know their own needs for information, let alone have the knowledge to comment on the needs of others.

I recommend a copyu of Carroll A. Brant's _Transistor Ignition Systems_. Like all TAB books it's full of typos and errors (including some in schematics) but the wwaveforms seem okay. It is phenomenally dated but still useful.

Some of them do in fact use active current control. Constant angle is fine at low speeds, but becomes a problem at high speed. Some systems use constant angle but with a minimum time threshold.

Take a look at megasquirt, it does some interesting goofiness as I recall, and the source code is there for your perusal.

--scott

I'm building a new ignition module for a 33 year old vehicle whose old module is getting a bit flaky (intermittent).

Its a 1979 Toyota Landcruiser with a reluctor pickup and solid state ignition. Replacement (original) parts can occasionally be found for $350. So I figured I'd build one.

I've seen several DIY conversions for points systems, but since I've already got the pickup in the distributor, I'm one step ahead of those projects.

If I can keep the existing system running, I can throw an oscilloscope on it to see what it does, but that's sort of iffy. And I don't like hooking my (non automotive) electronics gear up to ignition systems. The inductive kick (even on the primary) can blow the input stage out of some expensive gear pretty fast.

at both ends of the plug. systems that can monitor the voltage decay of the coil can tell if the plug's not firing. this helps differentiate between plug and injector faults.

before you blame the module, just make sure it's not something like an issue on the h.t. end. that's where the vast majority of ignition reliability problems lie - hence the modern move to coil on plug systems.

i think all you need to do is scope the reluctor output, and use that signal to drive an off-the-shelf "igniter" unit. the igniter takes care of the coil current [and in some cases, dwell], so all you have to do now is figure out how to trigger it. junkyards are full of suitable candidates.

Understood. I assume that you are pretty sure the reluctor pickup is not involved in the intermittency? Also assume the the spark advance is mechanically generated?

You have a number of options and it should be a fun project.

That's why we have 10X probes, to deal with that. Or get yourself a nice reliable Tek 545. The probe will arc over before the nuvistor is damaged.

If it were me, I would just go with a conventional analogue capacitive discharge system like the Mark Ten or the like, and Brant's book has details on various different designs (including some complete schematics that can be used outright) as well as details on the stuff that should be coming off the various different types of pickup.

I have a Mark Ten retrofit unit in my 2002 and it's been running fine for the past 150,000 miles. I don't think I have replaced points in all that time although I have had to adjust them. It doesn't care about how the actual dwell is set; there's an internal RC network that sets the length of time the SCR is kept conducting no matter how wide the input pulse is. So it would not seem to be difficult to trigger it from a variable reluctance pickup or a phototransistor.

--scott

are you for real? really for real? i mean, the guy that doesn't know the difference between inductive and capacitive ignition discharge is really asking questions unrelated to the op's topic - that of dwell angle management - you know, like "Dwell angle, electronic ignition"???

of all the options you have available to NOT post completely meaningless drivel, you still go ahead and do it anyway.

but capacitive discharge is not "conventional" - inductive is. "electronic" != capacitive.

capacitive is a fad among after-market systems as some form of differentiation, and people whose electronics knowledge has not progressed since the 1970's. while it may offer benefits over kettering ignition, it's inferior to inductive due to lower spark ignition duration and energy delivery.

you have a 2002 vehicle with points????????????

but we already know that the vehicle has "reluctor pickup and solid state ignition"...

This would be one of the simpler options, Scott. There are so many ways to go that are interesting, but probably dont improve the system so much.

The dwell should not make much difference, if any, on a system like the one you describe. When points are used like this, there is usually a set current range through the points which will help retard burning the points.

The first plug firing voltage spike is enough to ignite the gas/air charge and there is not much to be gained by allowing the circuit to have multiple discharges. There has been a lot published about this, but many will still not believe it.

Timing is a lot easier to deal with if you retain the distributor, but even that is not necessary.

BMW model 2002.

Right, so should be no problem to trigger any one of the third-party ignition modules.

--scott

I believe Scott has a BMX 2002, as did I. My '72 was one great car! The '74 not so much - quite a bit heavier.

Pretty sure. The reluctor continuity is good (even during a failure condition) and it seems to be putting some AC signal out when cranking the engine at these times.

Yes. This is a centrifugal/vacuum advance system.

Pretty simple, actually. Once I figure out what the requirements are.

I'd just throw an old HEI module at at.

I dont see that you have any extreme ignition requirements for this type of car. You could spend a ton of money and engineer a sophisticated system, or you could take an aftermarket system and adapt it. Performance will likely be about the same either way.

It is, and even worse I have the Solex carbs. But it's still a fun car!

--scott