Times Have Changed: Cold Starting

How so?

I'm not familiar with this theory, but my EE degree is nearly 20 years old so maybe I just forgot. Can you explain or provide a reference?

Matt

And, your engine has WHAT type of fuel injection?

I seriously doubt you'd see as great a difference if it were a 93 Magnum

318 instead of an 89 TBI 318. As I said, TBI has a lot of the same cold-weather fuel distribution and vaporization issues that a carburetor does, and the thicker oil is going to aggravate it a lot more than it would an MPI setup that doesn't puddle fuel in the intake.

There's a danger to both of us in that: You'd eat all your profits, and I'd eat myself into a BBQ coma :-p

People who haven't been there to see it don't believe me when I tell them this, but under just the right conditions (warm weather, engine recently shut down, and crank landing so that one cylinder is just after TDC), I can occasionally start one of my cars just by turning the key ON (no starter at all). The spark from the electronic ignition powering-up lights off the mixture in that one cylinder, whcih kicks it over enough to start it. I can pretty reliably get it to "kick" enough to visibly move the crank any time I want, but it kicks backwards about half the time because the piston is BTDC and usually only turns the engine very slightly. But once in a blue moon (maybe twice in the past 20 years) the darn thing will kick forward hard enough to start and keep running.

Advanced computer controlled PNDEFI.

And the problem...?

But:

1. The voltage at the spark plug is limited by the breakdown voltage of the spark gap. Once you get a spark, the voltage required to achieve that is what you will measure.
2. Once you do get a spark, the total energy is what is really important. I don't know if you are measuring that in any way.

I have heard this -- even tried it once. But ask yourself, why don't they build (possibly additional) spark gaps into production ignition systems? I believe the air gap technique really serves as a band-aid for dirty systems: to explain, an air gap can cause a faster rise time of voltage. If the voltage rises slowly at the plug, energy can be lost through resistive (dirty) paths. I don't think a good system can be improved by a spark gap.

Another thought: you guys are comparing early 80s carbs. These had all kinds of add-ons to cope with emissions regulations -- stuffing too much fuel down the exhaust does not help the cat. In my experience, earlier carbs with manual chokes work reliably -- providing the driver knows how to use them.

So you are saying that well maintained US-built vehicles of the early '80 did not start well? Yet you (or rather Daniel) knock British cars (which I know from personal experiance start well in in damp and cold conditions provided they are well maintained).

They did. Called 'em "Series-gap spark plugs". All the major reputable makers made them, when there was a demand.

WAAAAAhahahahahahaha!

Sure, ace. \/\/hatever. Don't feel bad, just 'cause your country's automotive industry is an international laughingstock.

An ignition coil does not normally output maximum voltage, but will if the resistance in the secondary circuit is increased, creating a gap that can be jumped essentially is an increase in the circuit resistance, since there is a gap between the rotor and the cap contacts, the firing voltage realized will always be higher than if there is no gap such as with DIS.

I may also be wrong on this, but my observations on my ignition scope reveal that firing voltages on conventional ignition systems tend to be a few KV higher than firing voltages on DI systems.

The greatest gap theory was authored by Mac VandenBrink in 1965, Mac worked as an EE for many years for Allen Test Products. The only references I know of WRT Mac's theory are on i-ATN, easily found if you're a member. Unfortunately I can not copy them and post them here because all i-ATN posts are copyrighted and doing so would jeopardize my membership standing. I became familiar with Mac's teachings way back in the early 80s when I was a GM dealership mechanic during training on the Allen Smart Scope.

But the two voltages are different, it takes more voltage to jump the gap initially than it does to maintain it afterward.

More like energy over time, i.e., it is generally believed (taught) that a spark duration less than 1.5 milliseconds is an indication of a problem. 1.5ms on a breaker point system is not hard to achieve.

Because it creates an enormous amount of electrical noise which tends to upset radio reception, drive the on board computers nuts and cause airliners to plummet to the ground.

Yup, that's why the tow truck was summoned.

They will get the engine started, but drive off driveability still can not compare to modern EFI, especially PFI.

Not compared to well maintained US built vehicles of the 90s with PFI

Dunno, I've never owned a British car. I tried starting a Norton motorcycle once.

I gave up. ;-)

I don't see how. The voltage output of the coil is a direct function of the strength of the field around the coil that is collapsing. The strength of the field is a function of the current through the primary of the coil when the field is being established. The field doesn't have any "knowledge" of the resistance in the secondary circuit. Greater resistance in the circuit will result in less current (simple Ohm's law application given the EMF induced by the collapsing field) and thus a weaker spark, not a stronger one. I searched around last night and could find not reference whatsoever to either this theory.

That could well be. I haven't measurement them personally. However, that isn't what is usually claimed. In any event, the strength of the spark is not a huge factor in starting the car in cold weather. I proper fuel/air mixture will ignite with only a small amount of provocation. The really strong spark is needed in cases where the ratio isn't idea. That is why I can believe that the ignition system on a non-FI car might need to be more robust as it is much harder to ignite a mixture that isn't ideal.

Sorry, but this sounds like cold fusion to me. If this were true, then somebody else somewhere would have researched or commented on it. Thus far I've found zilch.

Matt

It's always both amusing and sad to watch Brits stand there and insist their cars' electrical systems work.

Only in England would they produce bike brakes that stop working when they get wet (and put Bobbies on them!), and car electrical systems that can't cope with more than 15% ambient humidity, let alone a drizzle...

...and then stand there and insist they're right and the rest of the world is wrong.

No, the *current* output from the coil is a direct function of the speed of the collapse. The voltage output is whatever is needed to get that current to flow. Hopefully, the lowest breakdown voltage in the system is the spark plug gap, so that when it's exceeded you get a spark in the combustion chamber. If it's fouled or something, the spark will happen someplace else -- through the plug wire insulation, inside the coil.... The gap in the rotor will cause a higher voltage in the coil, but not over in the spark plug where it's needed.

That surprises me. I wouldn't be at all surprised to hear that the voltage was higher at the coil for a conventional ignition system; greater at the plug... I don't see how.

I've heard of it, but I'd have to think a while to see if I believe it. Yes, you'd get a greater coil voltage this way, but the maximum voltage across the spark gap is determined by its breakdown voltage, not the gap over at the distributor.

I've heard the theory, though I also can't find a reference to it in a quick search -- at one time, there were products that you'd put on your coil that were basically a wire with a gap in it, based on just this theory.

Designed by: Sir John Lucas, original inventor of the concept of "Dark."

God Bless, Dan'L

("If I'm going to reach out to the the Democrats then I need a third hand.There's no way I'm letting go of my wallet or my gun while they're around.")

The system doesn't know or care *where* in the secondary the gap(s) are. Voltage across the plug gap is determined by the breakdown voltage of the highest-dielectric gap in the secondary. Ordinarily that's the spark plug, but if the plugs are wet and/or dirty, the moisture and/or dirt provides a voltage leakoff path from the center electrode to ground and the spark never occurs. By introducing a high-dielectric gap upstream of the plug gap, secondary voltage is increased such that such leakage paths become less parasitic in absolute terms, and the spark has a greater likelihood of happening.

I used to demonstrate this on an old '64 Dart parts car with a whipped engine. The plugs fouled fast on that engine, and cylinders would stop working. I would pull plug wires partway off to create a 1/4" to 1/2" gap between their terminal and the spark plug terminal, and one by one as I did this, those cylinders would come back online. It got noisy under the hood, though (Snap! Snap! Snap! Snap!)

The "Spark Intensifier" gadgets that used to be sold by JC Witless, in the back pages of Popular Mechanics, etc., were nothing more than enclosed spark gaps designed to be placed in series with the coil wire. Secondary component life suffered (especially the points!) but they did the same thing as described above. And as previously mentioned, all the reputable spark plug makers have offered series-gap spark plugs over the years. They fell out of favor as gasolines got cleaner and ignitions got stronger.

DS

No, the voltage available is defined by the turns ratio of the transformer (coil). I admit it has been a while since I studied and worked with transformers, but I believe the open circuit voltage will still follow the turns ratio rule when the field collapses. The voltage available depends on the turns ratio, not on the size of any gap in the output (secondary) circuit. If the gap is too large, then no current will flow, however, a very high potential will still exist across the secondary terminals even with zero current.

I've been thinking about it and researching it, and at this point I think it is snake oil.

Yes, outfits like J.C. Whitney sell all sorts of gimmick gadgets like this to an unsuspecting public, but that doesn't make them real. Kind of like the magnets for your fuel lines, etc. Every time they are legimately tested, the results are the same ... they don't work.

Matt

True. That's a cool phenomenon and one I've only got to work a couple of times. Big plug gaps and monster coil outputs have made that a thing of the past for the most part.

Toyota MDT in MO

Here's a link to a simple adjustable spark tester:

Notice the scale on the bottom, as the gap is increased, the KV requirement increases. If I'm wrong, then this tool does not work, but I own a version of this tool and have used it with my ignition scope connected and it's pretty much dead nuts on, probably because the dielectric of the air gap is pretty predictable.

What is usually claimed? For sure, a DI system will output a higher voltage when the demand is there, but firing voltages and spark line voltages are measurably lower.

Thank you. That is what I've been saying all along.

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Correct. The majority of the heat that ignites the mixture comes from compression, the last 5-10% comes from the spark across the spark plug gap. Ooops, I did it again, I threw out a number that I can't provide verification for yet is standard teaching.

A lean mixture is harder to light than a rich mixture, so it all depends on what exactly is keeping the cold engine from wanting to run, but for carbureted vehicles it usually amounted to gasoline washout inside the cylinder, the washout occurs because the old choke systems were very poor at controlling the amount of fuel they delivered.

I don't know, it isn't like the "greatest gap theory" has been on the front burner technology wise like putting a man on Mars might be. I can tell you that on i-ATN, Mac writes that he was scoffed at over his theory, one of his college professors attended one of his classes, this professor had authored some books on EE, the professor told Mac after the class that contained discussion on greatest gap that had he taken Mac's class before he had authored some books, the content of those books would be different in that regard. His theory is very easy to prove in person with the correct equipment, connect an engine analyzer and set it to measure secondary ignition voltage, using insulated pliers, remove a spark plug wire from the spark plug (engine running of course) and observe the firing voltage, the firing voltage will not increase even though you can hear the arcing until the gap you're creating exceeds the value of the spark plug. The other observation one can make is to perform this same exercise on a DI equipped vehicle with an engine analyzer capable of measuring firing voltage during the waste spark event, the voltage on waste spark will begin to increase before the voltage for compression spark will increase.

Now, not that what I'm saying gets confused, the rotor air gap is not always the dominant (greatest) gap, but during cranking on a cold engine with a carburetor with very little throttle opening, the choke closed and very little pressure being developed in the cylinder, the rotor air gap probably will exceed the value over the spark plug gap.

Lots of different things in play here...

Bottom line, DI systems are utilized because they have many fewer parts and because splitting the ignition load amongst multiple coils increases the working life of those coils, not because there is some requirement for lightning bolts to get a cold engine running.

Not universally, but we're talking about starting a cold engine, slow cranking speed, rich mixture, closed throttle and choke which results in very little pressure inside the cylinder, under those conditions, the .100" (or so) rotor air gap is likely to be dominant over the .035"-.045" spark plug gap.

Sorry if what I've said so far lead to any confusion, but the subject is cold starting.

Matt, what do you suppose the voltage on the primary is when the field collapses?

This is the step I'm not seeing in the discussion: voltage across the plug gap should be determined by the breakdown voltage of the plug gap (which is an almost, but not quite, circular statement!). I'm not seeing how increading the breakdown voltage someplace else will increase it across the plug gap.

I think I need one step more detail in the explanation to see how this happens.

OK...

I remember the gadget, but I don't remember it working.