What it is the purpose of twin spark?
Discussion
I've just taken on my Dad's old Alfa 147 2.0 Twin Spark to use as a daily and it got me thinking.
I've heard of some WW2 aero/tank engines having 2 spark plugs per cylinder as the piston/squish area is so big, but in a little 2.0 car engine, what's the purpose if it?
It's an unremarkable 2.0 16V engine making 150hp. I can think of plenty of other nasp 2.0 16V engines that make 150hp (and higher) with just 1 plug per cylinder. And as far as 2.0 valvers go, it's no smoother or any more economical than other engines in it's class.
Just thinking for experiment's sake, pulling the second HT leads and see if I notice a difference in economy / performance
I've heard of some WW2 aero/tank engines having 2 spark plugs per cylinder as the piston/squish area is so big, but in a little 2.0 car engine, what's the purpose if it?
It's an unremarkable 2.0 16V engine making 150hp. I can think of plenty of other nasp 2.0 16V engines that make 150hp (and higher) with just 1 plug per cylinder. And as far as 2.0 valvers go, it's no smoother or any more economical than other engines in it's class.
Just thinking for experiment's sake, pulling the second HT leads and see if I notice a difference in economy / performance
Very simple. After the spark, combustion takes a period of time. This means it is less controlled than it could be and you need to run a set level of spark advance to take account of this time.
So the more control we can have over the burn the better. Igniting the mixture from 2 points speeds up the burn process, it means less spark advance is needed which is a good thing.
In general it just allows the engine to run more efficiently, but due to packaging it is a difficult thing to achieve without spending lots of money. Which is why few manufacturers do it.
Although modern Porches and Mercedes now offer twin spark engines too.
So the more control we can have over the burn the better. Igniting the mixture from 2 points speeds up the burn process, it means less spark advance is needed which is a good thing.
In general it just allows the engine to run more efficiently, but due to packaging it is a difficult thing to achieve without spending lots of money. Which is why few manufacturers do it.
Although modern Porches and Mercedes now offer twin spark engines too.
If I hadn't seen with my own eyes 2 plugs in each cylinder (or seen the bill for £160 to replace them!), I wouldn't have known it had twin plugs. For Alfa to have gone to the trouble of implementing it, it would be nice to feel something tangible behind the wheel There's certainly no performance or economy benefits that I can see, compared to similar engines.
I would have thought better injectors, ECUs and head / piston designs would negate the need for 2 plugs these days?
There's certainly no performance or economy benefits that I can see, compared to similar engines.ian_uk1975 said:
I suspect it's mainly a marketing differentiator.
Probably supposed to give a more complete and more even burn?
Yeah, you certainly can't miss the big "Twin Spark" badge on the boot!Probably supposed to give a more complete and more even burn?
I would have thought better injectors, ECUs and head / piston designs would negate the need for 2 plugs these days?
C.F Taylor summarised the work that had been done over the years on multiple spark plug heads in 2v chambers (mainly by Ricardo if I recall) in his books on engine design. I think they went as far as having 15 miniature plugs in a single cylinder test engine. Adding more plugs led to very rapidly decreasing extra benefit but certainly going from 1 to 2 or even 2 to 3 gave measurable results.
Reducing flame propagation time and hence the need for ignition advance is clearly beneficial (up to a point) as it reduces the energy wasted on the rising piston prior to TDC and concentrates more of the burn on the power stroke after TDC. In theory an instantaneous burn right at TDC would produce the most power but would create unmanageable cylinder pressures.
The burn speed of 2v chambers depends very much on chamber design. Small compact bathtub chambers as in the Leyland A series burn quickly and might require only 30 degrees maximum ignition advance at WOT. More open hemi chambers need low to mid 30s degrees and really bad designs like the Heron head Ford Crossflow with a flat head face, all the chamber in the piston crown and no squish band might need high 30s or more. A faster burn could help here but you're still only talking a small number of percentage points in terms of extra bhp per litre between the best and worst head designs. Other factors like bore/stroke ratio, valve size and port flow swamp the gains or losses from different burn speeds.
However 4v heads tend to be very similar in shape of necessity and benefit from a central plug and high amounts of mixture agitation (tumble) from the twin incoming air streams which produces a fast burn and as little as mid 20s degrees maximum ignition advance at WOT. Adding a second plug to one of these will give barely measurable results although in a world driven by ever tighter emission constraints if you can measure anything I suppose you can claim a benefit from it.
So yes it's a marketing ploy more than anything else in a 4v (or even a 2v) with cost and complexity downsides which vastly outweigh any benefit IMO.
Reducing flame propagation time and hence the need for ignition advance is clearly beneficial (up to a point) as it reduces the energy wasted on the rising piston prior to TDC and concentrates more of the burn on the power stroke after TDC. In theory an instantaneous burn right at TDC would produce the most power but would create unmanageable cylinder pressures.
The burn speed of 2v chambers depends very much on chamber design. Small compact bathtub chambers as in the Leyland A series burn quickly and might require only 30 degrees maximum ignition advance at WOT. More open hemi chambers need low to mid 30s degrees and really bad designs like the Heron head Ford Crossflow with a flat head face, all the chamber in the piston crown and no squish band might need high 30s or more. A faster burn could help here but you're still only talking a small number of percentage points in terms of extra bhp per litre between the best and worst head designs. Other factors like bore/stroke ratio, valve size and port flow swamp the gains or losses from different burn speeds.
However 4v heads tend to be very similar in shape of necessity and benefit from a central plug and high amounts of mixture agitation (tumble) from the twin incoming air streams which produces a fast burn and as little as mid 20s degrees maximum ignition advance at WOT. Adding a second plug to one of these will give barely measurable results although in a world driven by ever tighter emission constraints if you can measure anything I suppose you can claim a benefit from it.
So yes it's a marketing ploy more than anything else in a 4v (or even a 2v) with cost and complexity downsides which vastly outweigh any benefit IMO.
I'm afraid I can't add anything to the technical argument. However, I'd argue that the Alfa TS engine is anything but a run of the mill engine.
The problem is that it's old hat now, but 148 PS back in 1987 when the twin spark engine was launched in the 75 was quite something. With the twin balancer shafts of the later engine it's silky smooth and a typically characterful Alfa lump. I've had a couple and love them.
The problem is that it's old hat now, but 148 PS back in 1987 when the twin spark engine was launched in the 75 was quite something. With the twin balancer shafts of the later engine it's silky smooth and a typically characterful Alfa lump. I've had a couple and love them.
Mave said:
The engine in the 75, and the later FIAT based engines are completedly different though, right? Did the 75 have 2 similar plugs, or did it share the same arrangement as the late rengines where there is a definite primary and secondary plug?
75s have 2 identical plugs and a pair of distributors.FIAT modular based TS engines have one big plug and one smaller one. Not sure how it all works, though.
148bhp from an 8v 2lt in 1987 was indeed very impressive - it was also one of the first (mass produced) engines with variable cam timing, as I recall.
150-155bhp from the modular lump wasn't quite as impressive - but it was still a lot more than most manufacturers were getting when it was launched in the early 90s - I think only Peugeot were making a 2lt 16v engine with more BHP at the time.
Bitofbully said:
Mave said:
The engine in the 75, and the later FIAT based engines are completedly different though, right? Did the 75 have 2 similar plugs, or did it share the same arrangement as the late rengines where there is a definite primary and secondary plug?
75s have 2 identical plugs and a pair of distributors.FIAT modular based TS engines have one big plug and one smaller one. Not sure how it all works, though.
148bhp from an 8v 2lt in 1987 was indeed very impressive - it was also one of the first (mass produced) engines with variable cam timing, as I recall.
150-155bhp from the modular lump wasn't quite as impressive - but it was still a lot more than most manufacturers were getting when it was launched in the early 90s - I think only Peugeot were making a 2lt 16v engine with more BHP at the time.
Bitofbully said:
150-155bhp from the modular lump wasn't quite as impressive - but it was still a lot more than most manufacturers were getting when it was launched in the early 90s - I think only Peugeot were making a 2lt 16v engine with more BHP at the time.
Honda were making a 1.6 with more power than that, and their 2.0L was pushing out 200bhp.Dave Walker of Emerald fame made a twin spark Pinto some years back and managed to extract quite a bit of extra power from it compared to the same engine with a single plug. It means you need to run less advance to get peak cylinder pressure at the same point, which is always an advantage for power.
My early Aprilia Mille has two plug cylinders,recently the side plug on one cylinder stopped working and the engine felt harsh and lost performance. When fixed the difference was very noticeable so it is worthwhile having two plugs on this engine, even though they both fire simultaneously. Later models employ a single plug in a redesigned head and perform as well if not better.
Bitofbully said:
75s have 2 identical plugs and a pair of distributors.
FIAT modular based TS engines have one big plug and one smaller one. Not sure how it all works, though.
148bhp from an 8v 2lt in 1987 was indeed very impressive - it was also one of the first (mass produced) engines with variable cam timing, as I recall.
150-155bhp from the modular lump wasn't quite as impressive - but it was still a lot more than most manufacturers were getting when it was launched in the early 90s - I think only Peugeot were making a 2lt 16v engine with more BHP at the time.
Thanks for the info!FIAT modular based TS engines have one big plug and one smaller one. Not sure how it all works, though.
148bhp from an 8v 2lt in 1987 was indeed very impressive - it was also one of the first (mass produced) engines with variable cam timing, as I recall.
150-155bhp from the modular lump wasn't quite as impressive - but it was still a lot more than most manufacturers were getting when it was launched in the early 90s - I think only Peugeot were making a 2lt 16v engine with more BHP at the time.
Mine must be the FIAT engine then as it has 4 big plugs and 4 small ones. Under the rocker cover it's a standard coil on plug affair, but each one has a little HT lead coming off it to feed the second plug.
60K service intervals on the plugs. The 4 little ones had no electrodes left at all and the big ones had gaps you could measure in actual mm, rather than fractions of!
I'm amazed it even ran at all. No noticable improvement with new plugs though!
Yeah 148hp in 1987 was quite good. I think the Astra 16V red top engine was only making 150hp in the 90s, and that's a well known engine in tuning circles.
Mr2Mike said:
Honda were making a 1.6 with more power than that, and their 2.0L was pushing out 200bhp.
Sorry, what's your point?There were one or two engines around at the time doing that kind of output - but if you look at a lot of the mainstream brands, they were wheezing on with anywhere between 100 and 130bhp from a 2.0 16v lump at the time - so anyone doing more than 150bhp was near the top of the pack.
Fiat group always make pretty powerfull engines. The current range of engines are pretty small CC wise but are getting pretty high BHP figures.
I work within the group and all our company cars are obv Fiat or Alfa. They all go really well even the bog std bottom of the range things. Ive not driven a 500 but presume they are the same.
I work within the group and all our company cars are obv Fiat or Alfa. They all go really well even the bog std bottom of the range things. Ive not driven a 500 but presume they are the same.
Stewart330ci said:
Honda Jazz has 2 plus per cylinder too
Yes they do and if you leave the rear pencil coils unplugged (i.e just 1 plug per cylinder) the Jazz develops a flat spot/hesitation on pick up f
king main stealer's it don't throw the engine light on either 
still I gained a customer 
Edited by Simon says on Wednesday 24th October 09:55
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