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Marmite!!

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As all threads in Clive's Corner are closed & members can't reply or start new threads here so that the info doesn't get lost in hundreds of posts/threads, I thought I would post an Open thread where members may ask questions or make comments on his papers.

 

I know members have done so on the open forum in the past but it only makes sense to keep all the comments on the same board.

Edited by Marmite!!
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Hello,

I note Part 3 has been superseded, is there a replacement document?

 

Alec.

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Alec it has been superseded by the new version of Part 4.

 

The various updates meant that there was quite a lot of duplication anyway. So I lifted what residue there was & vamped it up so that it would blend into Part 4. I thought we ought to put 'superseded' in case people thought there was a forum fault if there was no longer a reference to Part 3.

 

Funnily enough it is exactly this same Sunday 10 years ago when I started a series of weekly visits to sort out someone's 90A system. With all the EMERs in hand I thought it would be a doddle, but after 3 Sundays of work I realised the EMERs were of very little help, either because they missed so much information or confused Mks or had errors or assumed one had all the facilities of a base workshop. Then that started me thinking about finding better ways of sorting it & to try to find out how it was all meant to work.

 

Incidentally I did find the fault. There were 3 dry joints & a defective earth.

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I've just read Clive's excellent paper "Bright Sparks for Land Rovers" and it answered a lot of my nagging questions about ignition systems. One thing still puzzles me. The secondary circuit has two air gaps in series, rotor-to-post in the distributor cap, and the spark plug. I understand the rotor-to-post gap is usually quite large relative to the plug gap, but its "resistance" is less than the plug gap "resistance" due to it being at ambient air pressure. So the plug gap, and conditions in the cylinder, are what primarily dictate the height of the firing line voltage.

 

But air gaps don't behave like normal fixed-value resistances in series. They both initially have very high resistance and then when a spark is generated across them, much lower resistance. So what is the physical process of generating sparks across two gaps in series? When HT builds up at the rotor-to-post gap, what makes the rotor spark across to a post so that HT can then build up at the plug? The post isn't connected to ground because the plug hasn't fired so what makes current flow? Does enough potential difference build up across the rotor-post gap due to "charging" of the downstream cap and wire?

 

Malcolm

Edited by sexton

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Malcolm that's a very good point. I think the initial jump across the rotor arm gap is due to charging further down the line due to the capacitance of the sparking plug.

 

"Sparking plugs both screened and unscreened are said to have a capacitance of about 10 pf, I have taken measurements of many types and found this to be incorrect. Unscreened plugs have a capacitance in the range 15-20 pf and screened plugs 30-40 pf."

 

I have no text book nor could find any internet source that actually addresses your point. The initiation of the spark & its switching to the plugs by the rotor arm is not explained, not even in a Lucas training manual, which in all other respects is detailed & covers aspects not really covered by most other sources.

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Indeed an interesting question. However, from the point of view of the current flow, is the actual initial resistance of any component of any significance as long as it is low enough that the flow can start? In which case, is the actual initial resistance of any number of such components in series, whether they be copper or air, irrelevant as long as the road is open? Presumably once the first air gap is breached and its resistance drops then the next gap will then be ready to be jumped?

 

I always think of the hosepipe example: the number of kinks is not important, as they can be each defeated in turn, or those pictures of lightning where the path that meets the ground carries the main charge while those branches that do not merely fade away.

 

trevor

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Clive, So it may be charging the "capacitor" that is the wire and plug downstream creates enough potential difference across the rotor gap to cause a spark. Hmm, I'll have to try that. I could generate HT across a stationary rotor gap with the wire and plug isolated from ground to if if there is a spark across the rotor gap with no spark at the plug.

 

Malcolm

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Malcolm, so I suppose the thing to do is generate your HT with a "closed" rotor arm, take the lead for that plug not to a plug but say a 25pf capacitor that has the other end earthed. Then see if you get a spark at the rotor arm.

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I did a quick bench test tonight. The setup was:

- contact breaker points dizzy

- 12V coil, supplied with 6.4V, primary current 3.5A with points closed

- graphite core wires from coil to dizzy and dizzy to plug

- plug NOT grounded

- HT wires, plug, and cap held 3" off bench on inverted plastic parts trays

- piece of flat bar resting on one post inside cap and 1/8" from centre carbon button (representing rotor gap)

- dizzy was hand-rotated

 

First test:

- wire and plug connected to "shorted" post in cap

- strong sparks seen across "rotor" gap, i.e. from button to flat bar

- no sparks at plug (checked in darkness)

 

Second test:

- removed plug, so just wire connected to cap post

- still strong sparks across rotor gap

 

Third test:

- removed plug wire, so nothing connected to cap post

- sparks across rotor gap noticeably weaker but still steady and consistent.

 

So the potential difference building up across the rotor gap (see later for some values) is eventually enough to ionize the air and discharge some of the pent-up charge to the cap and wire, or as a minimum, the cap. I would guess the cap and wire build up a small static charge each time. It reminds me of when I worked in an office with a nylon carpet and dreaded using the steel filing cabinets on a dry day. It also reminds me of the electrostatic discharge of lightning that Trevor mentioned. Good job this happens or we'd all be driving diesels.

 

I hooked up a scope for some of this testing to look at the secondary voltages. The highlights are:

- the sparks are very short duration, with no noticeable burn time, and lots of ringing oscillations afterwards, which maybe supports the low energy electrostatic discharge theory

- peak voltages in the coil to cap wire were in the 20-27 kV range, even though the gap was only 1/8" in air

- the peak voltage in the cap to plug wire was 20 kV and it decayed very quickly. Is this actually measuring the static charge (and discharge)?

 

Malcolm

Edited by sexton

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Excellent bit of home research Malcolm with some interesting results. I'm rather jealous as I would like to have tried something similar, unfortunately all my spares & test equipment are in storage in preparation for a house move. Amongst that I do have some electrostatic kvoltmeters. Thank you for following that up, which adds to the knowledge base for us all.

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Yes, I agree, an excellent test!

 

My engineering experience was focussed at the other end of the wire, at the plug gap, but I have always wondered what really goes on under the distributor cap. With the rotor turning rapidly inside an enclosed space does it produce a ring of ionized air, on the one hand being dragged around by the friction between the air and the rotor blade, yet stirred at the same time like milk in tea?

 

I wonder if the rotor creates a fluctuating magnetic field as the current flows and ceases?

 

trevor

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Thank you for the kind words.

 

I forgot to mention, after this testing, using the same setup, I grounded the plug to represent the normal situation on an engine and looked at the HT on the scope. The firing (peak) voltage dropped from 20-27 kV in the coil-to-cap wire with the plug not grounded to a more normal 6 kV, with the expected ~ 1ms burn time. I was wondering why the HT voltage in the wire is so much lower with the plug grounded and sparking. It occurred to me, with the plug not grounded, the rotor gap spark may initiate at 6 kV or less, but as the cap/wire/plug assembly builds charge, the potential difference across the gap drops rapidly so the coil keeps ramping up the kV to maintain the spark. I suspect it actually "runs out" of voltage rather than energy, based on the large ringing oscillations that occur after the spark extinguishes.

 

When the plug is grounded, and the spark occurs across the rotor gap at say 6 kV, the cap/wire/plug instantly charge up to now create a potential difference between the plug central electrode and ground of 6 kV, which creates a spark, which grounds the coil and allows maximum energy transfer from coil to ground.

 

Trevor, at least at fast idle, there's no ring of ionized air, based on this video! Probably just as well because I've seen the rust and plastic degradation the ozone created can cause.

 

 

Malcolm

Edited by sexton

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I have tested the coils on our M24 Chaffee and found something I don't understand. Every other coil I have tested, the secondary coil resistance is measured between one of the primary coil terminals and the HT post, usually measuring in the 7-11 kohm range. And measuring between the HT post and the casing shows infinite resistance. Clive Elliott specifically says this is normal in his ignition system paper.

 

But the coils on the Chaffee are quite different. The resistance between the primary terminals is 2.4 ohms, which is normal. But the resistance between one of the primary terminals and the HT post is infinite, so there is no connection between the primary and secondary coil. I can only measure the secondary coil resistance by measuring between the HT post and the coil casing. One coil shows 8.8 kohms, the other 10.0 kohms. I thought the coils were bad, but I bench tested both coils and they both work well, making strong sparks on a tester set at 3/8" gap.

 

From what I have read, having separated primary and secondary coils does not take advantage of the back-EMF amplification effect.

 

What am I missing here? Did coil design change in the 1940's?

 

Malcolm

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What am I missing here? Did coil design change in the 1940's?

 

Malcolm

 

Malcom where the primary & secondary are not connected inside the coil it is for use in an Insulated Return system.

 

Where the primary & secondary are connected inside the coil it is for use in an Earth Return system. This is by far the most common system nowadays.

 

I have used an Insulated Return coil on a Land Rover (Earth Return) by connecting the end of the primary to the end of the secondary. It is important to connect them to add the back EMF of the primary to the secondary, which way round will depend on polarity of the vehicle's system & which system the coil was designed for.

 

I have touched on IR & ER coils on page 26 of the Bright Sparks for Land Rovers article & the wiring arrangements for positive & negative earth systems on page 3.

Edited by fv1609
Page reference

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Dear All,

 

Can Malcolm, Clive or anyone else tell me what adaptor I need to enable my scope to read KVs? This is something that I have wanted to be able to do for some time.

 

John

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John I made a potential divider from a string of resistors & tapped off the low voltage for the scope from the earthy end. Don't know where my device is at the moment as not fully unpacked after move, but there are diagrams around the internet.

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Clive, thanks for the info.

 

I confirmed by bench testing both coils give a strong spark so I'll use them as-is.

 

I read Bright Sparks for Land Rovers before I posted here thinking you may have addressed this, and I just read through it again, and I can't find where you discussed the insulated return and earth return for coils.

 

Malcolm

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Malcolm I wasn't claiming to have discussed IR & ER coils, I just said "I have touched on IR & ER coils" by way of defining the difference in the list of coils & a photo on an IR coil. I assumed people knew of their differences but there is scope for expanding on that in a newer version.

 

But as it is people complain that I have deviated too much from the original subject which is meant to be about Land Rovers, so it is difficult to please everyone. Incidentally I assume you are reading V1.03 which has 6 more pages than V1.00? (I have no way of removing the old version).

 

Funnily enough I was at a stall last week & bought this 1943 book by SP Smith CBE The Electrical Equipment of Automobiles. This gives a commentary on the two systems.

 

ER IR Coils.jpg

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Clive,

Yes, including some info on IR and ER coils would be useful as I had never come across a coil like this before.

 

The secondary coil grounds through the frame mount, it doesn't have a separate grounding terminal, so it seems to me it was not designed for use in an IR system. The design for some reason is conventional transformer, rather than auto transformer. And maybe this design still benefits from the primary coil back-emf amplification effect as they share the same core.

 

Anyway, I take your point about getting too far off topic so I'll say no more.

 

Thanks again for the information.

 

Malcolm

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As all threads in Clive's Corner are closed & members can't reply or start new threads here so that the info doesn't get lost in hundreds of posts/threads, I thought I would post an Open thread where members may ask questions or make comments on his papers.

 

I know members have done so on the open forum in the past but it only makes sense to keep all the comments on the same board.

 

Just a reminder that if anyone would like to comment on Clive's latest or any of his other papers they may do so in this thread.

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Just read the article about paint DBG to Nato green.

I once looked for info about IRR paint which was OK, but this article is way better.

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Thank you it took a long time, in fact decades, to string together enough references to put together some coherent story of the various interests at work.

 

I'm grateful to have a platform on here to share the info with other enthusiasts, it is a shame that it was not considered suitable as it stood for my club magazine to publish it. Striving for "professionalism" in magazines is all very well, but it saddens me at club level there seems to be no place for first hand research if there is too much text & not enough colour photos. :-(

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Thank you it took a long time, in fact decades, to string together enough references to put together some coherent story of the various interests at work.

 

I'm grateful to have a platform on here to share the info with other enthusiasts, it is a shame that it was not considered suitable as it stood for my club magazine to publish it. Striving for "professionalism" in magazines is all very well, but it saddens me at club level there seems to be no place for first hand research if there is too much text & not enough colour photos. :-(

 

Frustrating, well at least you have a captive audience here.

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