Coil On Plug Ignition For Tiny Engines

The spark plug was a key invention in the history of the internal combustion engine, allowing combustion to be easily controlled and engines to rev faster than messy earlier designs. Mid-century cars tended to rely on points ignition with a distributor and coil, however more modern designs place a coil on top of each individual spark plug. [Roger Moore] decided to build a similar setup for a small model engine on his workbench.

The rig is built with an Arduino, a flyback transformer, a smattering of MOSFETs and passives, an IGBT and a capacitor. The Arduino outputs PWM through a MOSFET which is stepped up through the transformer, and then charges the capacitor. The capacitor is then discharged into a coil mounted on top of the sparkplug of the single-cylinder engine, which fires the spark. The timing of the spark is determined by a Hall effect sensor reading a magnet placed on the flywheel.

Later development aims to shrink the system further to fit on a V10 design [Roger] is planning to make. It’s been done on a small scale before, and we’d love to see another tiny engine with way too many cylinders. Video after the break.

[Thanks to Andy Pugh for the tip!]

38 thoughts on “Coil On Plug Ignition For Tiny Engines

  1. @Lewin Day said: “Mid-century cars tended to rely on [a] points [based] ignition with a distributor and coil, however more modern designs place a coil on top of each individual spark plug.”

    Oh really? To this day on a planet called Earth where I live, using one coil per cylinder is rare indeed. Think about it, which of the following two designs would you choose?

    1. Build many coils (one per cylinder) and connect them to the distributor with thick expensive copper wires that have to overcome high losses due to low voltage, high current circuits.

    2. Build only one identical coil and connect it to the cylinders via the distributor using inexpensive thin steel wires that do not have to overcome high losses due to the much higher voltage, lower current circuits.

      1. I have a 20 year old V8 and : distributor and one coil. Works fine but would like to have a system like Saab and Ford , one coil per cylinder. Where and evt how to make?

    1. You’re joking right….. Might need to take a look at some more vehicles hasn’t been a distributor in sight for almost 20 years. COP is usually about 14awg wires going to each coil.

    2. I have a 2004 Nissan and a 2008 Mini. Both use one coil per cylinder. It’s not a particularly exotic arrangement these days.

      Now that I’m used to what once felt like ridiculous overkill, I can appreciate the benefits.

      My third vehicle does use one coil per 8 cylinders, and breaker points ignition: indeed a common configuration for 1966.

    3. Yup, most modern cars use coil over plug (COP) and there are many retrofits to provide COP setup to the single coil setup. Why you say? Well, for one thing you can now get rid of the high voltage (HV) wires which can rob the transferred energy from the single coil to a spark plug. Also, the HV wires act as pretty remarkable antennas which creates EMI issues. Thirdly, a COP setup does not have a single point of failure than a single distributor setup, Fourthly, the ECU has more granular control of firing the cylinders which can be used to improve efficiency and power. Fifthy, the the COP setup can have more time to energize the primary windings and thus have a higher output secondary voltage… There are lots of reason to move away from single coil systems… you just need to dig a little.

      1. Plug wires, no matter how they are made, are a weak point on an engine and will crack and fail, leaving intermittent spark, bad/no combustion, and eventually they will kill the cat converter with poor maintenance. The more cylinders you have, the easier it is to ignore “one” bad wire.

        The poor maintenance that many people subject their cars to has always amazed me.

        Distributors have an enormous amount of geartrain slop (by today’s standards), not even repeatable, and a very approximate advance curve. Not good for efficiency. They needed to go.

        Chrysler supposedly discovered the efficiency improvements of crank-triggered ignition on the drag strip during the 1960’s.

    4. You don’t have a clue what you are talking about.

      Even my 2000 Ford V10 F250 had coil on plug ignition. It existed even earlier than that in trucks as early as 1998. And that is not a high end sports car or anything. Just an old work truck.

      Why don’t you spend 30 seconds on Google, no even better, real life. Pop the hood of your vehicle and take a look. If it is 20 years old or newer it probably has a coil on plug.

    5. Every car I’ve ever owned has had individual coils for each cylinder. My 1994 Oldsmobile did, my 1999 and 2002 Silverado with the 5.3L both did and my current Cruze 1.4L does. The Silverados and Oldsmobile both had typical spark plug wires with caps though whereas the Cruze has the coil-on-plug setup.

      Even motorcycles have gone that way. My TL1000R had individual coils with plugs and my MT-10 has coil-on-plug units like my Cruze.

      Coil on plug has the benefit of not having the high voltage lead snaking around the engine compartment connecting the spark plug and coil which reduces electrical interference.

    6. Drone,

      My 1996 Isuzu V6 uses Coil on Plug the thing has been utterly reliable and in fact is still ticking on the original set of coils and driver. The valve train ticks too, but that is another story.

    7. My Volkswagen van has tubes from the distrubutor to each cylinder for the magic smoke to come out of the high pressure Bosch spark plugs and make the wheels turn !!!! If I paint them red will I have more power ? What do you think Mr Drone ?

  2. Coil-on-plug is very common for cars built in the last 15(ish) years. Distributors largely disappeared even earlier in the 2000s. Timing is controlled directly by the ECU in pretty much every car these days.

    I agree that [Lewin Day]’s statement is misleading – there’s a big gap between “mid century” and this century, where vehicles stopped using points but were still using distributors. Perhaps even the majority of vehicles on the road fall into this category. But he’s not really wrong either.

    Spark plug wires are a major point of failure – they’re way more complicated than “thin steel wires” due to the shielding, reinforcement, and fatigue resistance required. The amount of low-voltage current needed to generate a spark is also not nearly as much as you might think – look at the size of the 12v wires going into an ignition coil. It draws 3-5 amps, less than a single headlight. It’s simpler and more reliable to just use ordinary wiring to connect multiple coils.

    1. “Arduino just for pwm to feed a transformer seems like overkill. Why not a 555?”
      Pay attention to the yellow trace on the scope and the “kicks it up the arse” comment.
      There are hundreds of fly back circuits out there using 555s
      BUT, you can’t, to the best of my knowledge, change the duty cycle and frequency on the fly. Its fixed by the value of capacitors and resistors. The Arduino is changing both the frequency and duty cycle of the PWM on the fly and within a few microseconds by altering the timing registers on the processor.
      Once the capacitor is charged, the frequency and duty cycle change so that the capacitor is just being trickle charged / kept topped up. That’s why the circuit only draws 100 or so milliamps when the engine is not running , but the ignition is on, despite the cap being at 400V. After the cap is discharged into to coil, for a time determined by the Arduino, (600 microseconds), the PWM is changed to fast charge the cap, it then reverts back to trickle charge the cap until the next ignition event. The engine is running at 10K to 12K rpm at full throttle (i haven’t measured it yet) but firing on every revolution of the engine, so every other spark is wasted. thats effectively 20-24K RPM and there’s still significant headroom as the cap is charged and ready way before the next ignition, even at an effect RPM of over 20K.
      And as Andy Pugh correctly points out, the overall object of the exercise is to test both the concept of COP but mainly fuel injection which is where the Arduino will come much more into play.

      1. This has little to do with the original project, more of a side-quest thing, but what if one were to modify the typical 555 oscillator circuit by replacing the fixed-value discharge resistor with something adjustable? There’s someone out there making logic out of LEDs coupled into CdS cells, something like that maybe? I know, I know, if you’re starting from code you’d still need the pwm to adjust the LED to adjust the CdS resistance to modify the 555 duty cycle. But what if you weren’t using code at all?

        Not even saying this is a good idea, just trying to think out the box, not everything needs code.

  3. Coil on plug design is theoretically an improvement over the classic main coil / HT distributor but some problems appears when you ignore some basic knowledge : the engine compartment is the worst place on earth for electronics due to cold / heat variations, moisture, vibrations, etc. And the worst place inside it is on the engine head, where the temperature is the higher.
    First CoP implementations (late 90) where very poorly designed, using cheap coils with standard insulated copper wires / non-heat resistant enameled wires, which means they failed early due to arcing/cracking or because of the faulty connectors / loose location on the engine head.
    More recent designs (early 2000) fixed that on most car brands and some designs even include the switching power transistor. At least this is the case on my 2005′ Mitsubishi…

    1. “ the engine compartment is the worst place on earth for electronics”

      You have clearly not seen inside the control cabinets in ageing physics experiments. When the asbestos roof leaks, it gets wet. When it catches fire, it gets toasty. When the building shakes it wobbles. And worst of all are the decades of “fixes” made by gremlins :)

      1. Yeah, sure I’ve not seen this, but I use my car every day, like most of people, and every day the engine get wet, toasty and vibrate !
        On the good side : no gremlin allowed around. ;)

  4. My utility vehicle is a 1964 DKW (now Audi), has a 2 stroke engine with 3 cylinders, 3 coils, 3 sets of points. Then, if one point or one coil burns, you still have 2 cylinders, and you can come back home at slow speed. This is a military design, to remove single points of failure and moving parts. 56 years after, still running.

  5. “Mid Century” is the 1950’s and 1960’s. A distributor with breaker points was the system used by all but a very few internal combustion engines. Widespread use of electronic ignition didn’t happen until 1975 when it was made mandatory in the USA, excepting some models where 1975 was their final year. (Same as was done with front disc brakes the prior year, the 1974 Ford Maverick and Mercury Comet still had drum front brakes as standard equipment.)

    Distributorless ignition systems started in the late 1980’s with multiple coils with each coil firing two spark plugs at a time, with one on the power stroke and the other on the exhaust stroke. Some had two coils per module with four terminals.

  6. If you could put together or help guide me on material list and how to buil the circuit for 1994 Chevrolet Camaro Z-28 equipped with the LT1 engine from the Corvettes that era, id appreciate it more then you could fathom. This engine has the worst ignition system design with the stupid optispark. Now GM and ACDelco stopped manufacturing replacements and about all the aftermarket units are from what ife read hit or miss but usually junk. So really if i could figure out a way for the hall effect sensor to work to get the correct timing, i could just use the LS1 coil packs. There are a few companys that make similar systems but you cant touch them under about 1000 bucks. So if you could design the Ignition Control Module from the Arduino or whatever, and sell them for say 2-300 dollars, man i could see you makikg some good dough. There alot more people in similar shoes with failing optisparks who cant afford thousand to repair but would love to do away with it.

  7. Mr Winterburn, I installed a branded Crane Cams CD ignition module in my 1971 Dodge Charger way back in 1974. Made a world of difference having a charged capacitor dump into the primary side of my stock distributor. Would fire fouled plugs with no problem. Truly innovative.

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