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[Marks DTM Calib]

If you fed two plugs off a single coil only one plug would fire most of the time 

That's not strictly true. As long as there is a circuit from one terminal of the coil back to the other terminal, it'll fire. The second terminal doesn't have to be a chassis/ground potential.

I can name at least one case where the circuit goes coil+ve -> spark plug1 tip - >spark plug1 threads -> cylinder head ->spark plug2 threads -> spark plug2 tip -> coil-ve.

Both spark plugs 1&2 fire every time at the same time. One cylinder is at the top of the exhaust stroke, whilst the other is at the top of the compression stroke. Although the exhaust stroke spark is wasted (hence the term wasted spark), it takes very little energy to fire it when it's surrounded by hot combustion gasses, so the energy loss is minimal.

It makes spark timing simpler - you can do the timing off the crank (rather than the cam) since you don't need to know if the piston is in the compression or exhaust phase of the 4 strokes.

No idea if that's how a Smart car works though.

You missed this by a country mile   

This is two plugs on the same cylinder, not a wasted spark setup. In this case if you had a single coil the plug with the smallest gap or most optimum conditions will fire, once it has the ionised air/mix in the gap will reduce in resistance and the spark remains on the one plug.....hence two plugs per cylinder nees two coils (well at least two secondary coils)

[LC0112G]

If you fed two plugs off a single coil only one plug would fire most of the time 

That's not strictly true. As long as there is a circuit from one terminal of the coil back to the other terminal, it'll fire. The second terminal doesn't have to be a chassis/ground potential.

I can name at least one case where the circuit goes coil+ve -> spark plug1 tip - >spark plug1 threads -> cylinder head ->spark plug2 threads -> spark plug2 tip -> coil-ve.

Both spark plugs 1&2 fire every time at the same time. One cylinder is at the top of the exhaust stroke, whilst the other is at the top of the compression stroke. Although the exhaust stroke spark is wasted (hence the term wasted spark), it takes very little energy to fire it when it's surrounded by hot combustion gasses, so the energy loss is minimal.

It makes spark timing simpler - you can do the timing off the crank (rather than the cam) since you don't need to know if the piston is in the compression or exhaust phase of the 4 strokes.

No idea if that's how a Smart car works though.

You missed this by a country mile   

This is two plugs on the same cylinder, not a wasted spark setup. In this case if you had a single coil the plug with the smallest gap or most optimum conditions will fire, once it has the ionised air/mix in the gap will reduce in resistance and the spark remains on the one plug.....hence two plugs per cylinder nees two coils (well at least two secondary coils)

Yes - if the two plugs are in a parallel combination. However, nothing to stop you wiring them in series, even if both plugs are in the same cylinder.

I'm struggling to see the reason for firing two plugs separately in parallel in the same cylinder, unless the timing is such that they are firing them at slightly different times?

[Doctor Gollum]

It's a Mercedes thing 

The M112/113 engine uses coils as Mark pictured, which have two short leads running to individual plugs. In turn there are two spark plugs per cylinder.

Incidentally, they're all 3 valve engines so there's a plug for every inlet valve. Presumably making combustion more efficient...

So my S280 V6 has 6 coil 'packs' (paired coils), 12 leads and 12 plugs. The V8 has 8,16 and 16 respectively, and the V12 M156/275 has 2 coil packs, no leads and 24 plugs

[TheBoy]

It's a Mercedes thing 

No it ain't. A fair few manufacturers do in on "normal" cars, and quite a lot on performance cars.

For LG - fired at the same time, idea being a better burn from 2 flame fronts.

[Marks DTM Calib]

If you fed two plugs off a single coil only one plug would fire most of the time 

That's not strictly true. As long as there is a circuit from one terminal of the coil back to the other terminal, it'll fire. The second terminal doesn't have to be a chassis/ground potential.

I can name at least one case where the circuit goes coil+ve -> spark plug1 tip - >spark plug1 threads -> cylinder head ->spark plug2 threads -> spark plug2 tip -> coil-ve.

Both spark plugs 1&2 fire every time at the same time. One cylinder is at the top of the exhaust stroke, whilst the other is at the top of the compression stroke. Although the exhaust stroke spark is wasted (hence the term wasted spark), it takes very little energy to fire it when it's surrounded by hot combustion gasses, so the energy loss is minimal.

It makes spark timing simpler - you can do the timing off the crank (rather than the cam) since you don't need to know if the piston is in the compression or exhaust phase of the 4 strokes.

No idea if that's how a Smart car works though.

You missed this by a country mile   

This is two plugs on the same cylinder, not a wasted spark setup. In this case if you had a single coil the plug with the smallest gap or most optimum conditions will fire, once it has the ionised air/mix in the gap will reduce in resistance and the spark remains on the one plug.....hence two plugs per cylinder nees two coils (well at least two secondary coils)

Yes - if the two plugs are in a parallel combination. However, nothing to stop you wiring them in series, even if both plugs are in the same cylinder.

I'm struggling to see the reason for firing two plugs separately in parallel in the same cylinder, unless the timing is such that they are firing them at slightly different times?

Thats not actually a single coil, its two secondary windings with the centre grounded and a single primary 

[LC0112G]

Thats not actually a single coil, its two secondary windings with the centre grounded and a single primary 

So remove the centre ground tap, and it then becomes a series spark setup with one primary and one secondary

Would be interesting to continuity check from the plug lead terminals to chassis/ground.

[Lizzie Zoom]

God, my Omega DIS Pack, or whatever it is,is a damn sight easier to understand than all that!

I must remember never to buy a Smart car! Is that the car that needs the engine out to replace the exhaust pipe?  One of my friends had one of those and she explained how the Mercedes mechanic complained about having to work on such a design of car!

How I miss the simplicity of past cars with a coil, distributor, contact, leads to each plug witch had a simple connector that was easily changed!  My A40 was a doddle to work on compared to these modern machines!  Are the electrics going to be easier?

[Doctor Gollum]

It's a Mercedes thing 

No it ain't. A fair few manufacturers do in on "normal" cars, and quite a lot on performance cars.

For LG - fired at the same time, idea being a better burn from 2 flame fronts.

OK, my experience of it twin plugs is the Merc application. Fiat/Alfa with their twin spark could well be a similar set up.

Incidentally, the M104 straight six developed from distributor ignition to three double coils each feeding two single plugs per cylinder.

[TheBoy]

Thats not actually a single coil, its two secondary windings with the centre grounded and a single primary 

So remove the centre ground tap, and it then becomes a series spark setup with one primary and one secondary

Would be interesting to continuity check from the plug lead terminals to chassis/ground.

I think the coil would have to be enormous to generate 2 sparks on the compression stroke.

[LC0112G]

Thats not actually a single coil, its two secondary windings with the centre grounded and a single primary 

So remove the centre ground tap, and it then becomes a series spark setup with one primary and one secondary

Would be interesting to continuity check from the plug lead terminals to chassis/ground.

I think the coil would have to be enormous to generate 2 sparks on the compression stroke.

It's the same primary coil either way and therefore the energy stored in the magnetic field is the same since there is only one primary. The energy discharged per spark event is therefore also the same.

If there are two secondaries then presumably half the energy stored in the core gets discharged through each of the two coils/plugs. Doesn't matter if the centre connection between the two coils is grounded or not. If it is then you end up with +V on one terminal and -V on t'other. If it isn't then you end up with 2*V between the terminals, but it's floating wrt ground. The ground comes then from the fact that the two plugs are screwed into the cylinder head, so the threads will (probably) be grounded - but they don't have to be.

I don't know that thats how it wired on the Smart - it's possible that both coil terminals go to +V wrt ground (rather than +V and -V). But that gives you a DC current in the head as the return path for the spark which isn't a good thing generally.

[Doctor Gollum]

Interestingly, I have discovered that the Audi Totally Frigging Stupid Idea lump has two plugs per cylinder... As does the 2008 ish 5.7 Hemi lump

[Andy B]

......

How I miss the simplicity of past cars with a coil, distributor, contact, leads to each plug witch had a simple connector that was easily changed!  My A40 was a doddle to work on compared to these modern machines!  Are the electrics going to be easier?

I miss points, condensers & dizzys like a hole in the head .... they were a pain in the harris!!

And though I've yet to replace an exhaust on either Smart (different engines), I doubt that's correct. They're not the easiest of cars to work on engine wise

[Nick W]

......

How I miss the simplicity of past cars with a coil, distributor, contact, leads to each plug witch had a simple connector that was easily changed!  My A40 was a doddle to work on compared to these modern machines!  Are the electrics going to be easier?

I miss points, condensers & dizzys like a hole in the head .... they were a pain in the harris!!

Yes, they are the first thing to get rid of on an old car. Especially now that high quality points and condensers are hard to find.

[Doctor Gollum]

Interestingly, I have discovered that the Audi Totally Frigging Stupid Idea lump has two plugs per cylinder... As does the 2008 ish 5.7 Hemi lump

And the Continental IO 520... From the late '60s

So no, apparently not just a Mercedes thing 

[Kevin Wood]

Interestingly, I have discovered that the Audi Totally Frigging Stupid Idea lump has two plugs per cylinder... As does the 2008 ish 5.7 Hemi lump

And the Continental IO 520... From the late '60s

So no, apparently not just a Mercedes thing 

Pretty much all aero engines do.. for redundancy.