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This just came up in my curious mind. In a typical four-stroke engine with an even number of cylinders, spark plugs fire in pairs. This means there is a wasted spark as one cylinder is in exhaust, while the other actually fires at the top of compression.

I understand this is probably done to save space under the hood since it would double the number of required coil packs. It also saves money on initial purchase and replacement. However, they don't take up much space and don't cost that much (at least for my 4-cyl car).

My question: Why not double the number of coil packs to avoid the wasted spark? Though this would take up more space and cost more for the packs, surly it would reduce wear and tear on the spark plugs to some extent as well as the spark plug wires and perhaps the alternator. This would half the number of sparks on each plug since they could be set to only fire on a compression stroke.

I'm sure the space and cost considerations answers the question, but I was hoping for some other thoughts on the advantages/disadvantages of a system that avoids the wasted spark.


Bonus question: Do two-cylinder engines typically spark both cylinders at the same time? Or do they tend to spark separately?


Bonus question 2: Is there a wasted spark in engines with an odd number of cylinders? I wouldn't think so.

  • Not sure what you're referring to as "wasted spark" here. Do you mean to say that two cylinders should be firing at the same time? – Zaid Aug 29 '15 at 19:44
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    Even if this were true, the actual amount of "waste" you're talking about is extremely small, and the spark is not the most important component in spark plug wear. Adding more coils would just mean adding more parts that can wear out and need to be replaced. – barbecue Aug 30 '15 at 0:38
  • @Zaid, no. I mean that two spark plugs spark at the same time, but one sparks the fuel/air mixture immediately following the compression stroke and the other sparks into the exhaust (being wasted). – Poisson Fish Aug 30 '15 at 20:17
  • If you consider that a coilpack on a Mercedes S-class costs more than $1000, it would be silly to double the number. – Captain Kenpachi Aug 31 '15 at 13:27
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    @JuannStrauss, yes that would be pretty significant, but my research suggests that S-class coil packs should be around $50 each. You might want to check on your parts guy... – Poisson Fish Aug 31 '15 at 16:07
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I'm sorry if this ends up being a lengthy answer. The answer to this question is more historical than anything else but first a little background.

In a waste spark system an ignition coil has two spark plug outputs unlike every other system only having one. Each ignition coil is hooked up to two spark plugs. These spark plugs reside in two opposing cylinders, meaning the pistons move up and down at the same time. This puts one cylinder in the compression stroke and the other in the exhaust stroke at the same time. When the coil fires, both spark plugs spark at the same time, one in the cylinder with fuel and the other in the cylinder with exhaust. The spark in the cylinder with exhaust is called the waste spark.

When waste spark first came out it was on the cutting edge. It was an excellent replacement for a distributor but ultimately it was a compromise. The biggest reason for waste spark is that it required less computational power. Automotive computers were in their infancy at the time and just couldn't crunch the numbers to fire a set of individual coils. This is evident from the fact that at the same time multi point fuel injection systems were all the rage. The injectors were fired all together like one big injector unlike sequential fuel injection that followed that fired the injectors individually in the firing order. This is further evident from the use of ignition modules like in GM vehicles and EDIS in Ford vehicles. These modules performed some of the needed calculations to relieve the PCM from having to do them.

The wear and tear on the spark plugs you mention is over exaggerated because the waste plug always fires in the exhaust stream which is full of hot ionized gas that is really easy to fire through. In reality the spark plug that is firing backwards (side electrode to center electrode) incurres the most wear.

Most automotive manufacturers did eventually go to individual coils per cylinder, this is called coil on plug or coil near plug. It's a more advantageous system because dwell and timing can be controlled per individual cylinder. Even with the fact that they are so modern these systems still retain some the the roots of where they came from. For example in a Ford V8 engine with coil on plug, if the camshaft position sensor stops working the engine will use only the crankshaft position sensor and regress to using waste spark and firing the injectors in banks instead of sequentially. This is a drive me to a garage limp mode. Also some manufacturers still retain waste spark even on engine manufactured today. Take the GM Ecotec 4 cylinder engine for example, it still uses waste spark.

Finally your two cylinder question. It really depends on the design of the engine. if the cylinders are opposing like Vtwin engine then yes they use waste spark, but also these engines normally use a magneto system which always fires the same spark plug in the exhaust, it can't help it. If it's a flat engine like in some motorcycles then it does not use that system because the stroke of the cylinders is opposite.

P.S. odd number cylinder engines (greater than one that is) are so rare that it would be more of an exception that a rule as to what ignition systems they used.

  • Thank you for the lengthy answer! It answered my questions directly and confirmed my suspicions that the wasted spark is mostly a compromise complexity and cost. – Poisson Fish Aug 30 '15 at 20:20
  • I'll add, there is a flat-twin engine that uses wasted-spark, the classic Citroen 2CV: en.wikipedia.org/wiki/Citro%C3%ABn_2CV#Engines Since the car was designed exclusively for mechanical simplicity and reliability to appeal to the mass market, the system suited it well. Horizontally-opposed engines work fine with wasted-spark systems because the pistons move in opposite directions, strokes 180' apart. – Gargravarr Apr 5 '17 at 12:03
  • @Gargravarr I guess i have to make a distinction. Flat V type engines can not use waste spark. Flat opposing engines like the Subaru boxer can use waste spark. The distinction is whether two pistons ride on the same crank throw (flat V) or all the pistons have their own throw (boxer). – vini_i Apr 5 '17 at 12:36
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Some four-stroke engines did have wasted spark ignition. The basic advantage was not needing a distributor, which was one of the less reliable parts of the ignition system. Wasted spark systems were used long before plug-top coil packs came into common use.

The reliability issues are not so important with modern ignition components.

Wasted spark ignition is more or less standard on small single-cylinder four-strokes that use magneto ignition, because of its simple and reliable design.

In motorcycle engines wasted spark has the advantage of using fewer coils, each of which can be physically bigger (and more powerful) in the limited space available.

Wikipedia has a list of wasted spark ignition engines here: https://en.wikipedia.org/wiki/Wasted_spark#Practical_examples_of_.27wasted_spark.27

Aero piston engines with two spark plugs per cylinder are designed that way for extra reliability. Each plug has a completely separate ignition system, and if one plug's system fails it has a negligible effect on the engine performance. Each ignition system can be disabled independently from the cockpit if it develops intermittent faults in flight, etc. One of the pre-flight checks is to ensure that the engine will run correctly on each ignition system with the other one disabled. This is unrelated to wasted spark ignition.

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