So where exactly is the high voltage return in this automotive ignition circuit?

[Ben321]

 Why isn't the circuit done like this in actual cars?

Many new cars these days have the secondary ground of the high tension side either connected directly to ground or through a circuit to ground to measure ionization current.

Older systems had the return path either on coil positive or coil negative. For practical purposes it does not make any difference in the car.

When did ignition coils ever get a separate HV return connector? Can you show a picture of what such a coil looks like?

[HighVoltage]

When did ignition coils ever get a separate HV return connector? Can you show a picture of what such a coil looks like?

BMW started already very early on with the 12 cyl. engines.
Almost all car manufactures followed.
Since the last 20 years at least, almost all manufacturers have a coil with a return path directly to engine ground.

Here are some sample pictures.
All of these modern coils have one pin in the wiring harness connector that goes directly to engine ground.

Some modern coils even have 3 separate ground connections:
- High Voltage engine ground (Spark Return)
- Battery ground (For high current)
- Logic ground, just for the ECU input signal

Some modern coils have very high primary winding currents of almost 30A peak,
resulting in about 300 mA peak current in the spark. These coils also have some
clever processing inside the coil with lots of software. Knowing how, you can even
update the firmware inside a coil.

[Circlotron]

Internal diagram of a GM LS1 coil.

[Ben321]

When did ignition coils ever get a separate HV return connector? Can you show a picture of what such a coil looks like?

BMW started already very early on with the 12 cyl. engines.
Almost all car manufactures followed.
Since the last 20 years at least, almost all manufacturers have a coil with a return path directly to engine ground.

Here are some sample pictures.
All of these modern coils have one pin in the wiring harness connector that goes directly to engine ground.

Some modern coils even have 3 separate ground connections:
- High Voltage engine ground (Spark Return)
- Battery ground (For high current)
- Logic ground, just for the ECU input signal

Some modern coils have very high primary winding currents of almost 30A peak,
resulting in about 300 mA peak current in the spark. These coils also have some
clever processing inside the coil with lots of software. Knowing how, you can even
update the firmware inside a coil.

You mean the coil itself has an embedded microcontroller? Or do you mean the car's computer simply has connections to sensors in the coil to monitor its performance?

[james_s]

This fairly energetic coil I pulled out of my junk box has one end of the secondary available for grounding via the black wire.

can't imaging there is many cars left that use coils like that, isn't almost everything now wasted spark, coil on plug, or a combo?

New cars from the dealer? No, probably not any at all, they're all coil-on-plug now but there are LOTS of cars out there still using setups like that. Pretty much any American V8 made up into the early-mid 2000's. That particular coil is an aftermarket unit people put on hotrods and vehicles they wish were hotrods.

[xavier60]

When I bench tested some Falcon ignition coils, I noticed that they could randomly give weak spark. I soon realized that the primary is polarity sensitive. I'm assuming that the core is magnetically biased.
https://www.supercheapauto.com.au/p/goss-goss-ignition-coil-c198/631721.html?gclid=CjwKCAjw8JKbBhBYEiwAs3sxN3xBojCWDzflm-hFUgF4A2PUKFCCGwI_I9AqYciuCuAddPVHB8DFnhoCoskQAvD_BwE&gclsrc=aw.ds

[SeanB]

Plenty of vehicles now integrate the coil driver into the coil. Makes design of the ECU easier, as there are no high current high energy circuits to route through it, and you only need a relatively low level signal to provide the drive. Also they integrated this into a coil per pair of plugs as well, and also made the whole assembly a single unit, so that for MB you have a large unit that sits above the engine, containing not only the 2 plug per coil units, but also a DC Dc converter to provide a 400VDC bus to drive the coils, so they can use standard off the shelf IGBT transistors to drive them, but use low inductance coils, few turns of heavy wire, and still get both multiple firing pulse per cylinder, up to around 6 pulses per firing period at max RPM, enough to start even the weakest of mixtures reliably in stratified burn engines. So now you have both 2 coil packs, a pair of DC DC converters, around 400uF at 450V of capacitor, and a CAN bus microcontroller, all sitting exposed to 120C on top of the engine.  And these units will typically fail shortly out of warranty, they are that well designed even though the components are running on the edge of their temperature envelope in normal operation.

[HighVoltage]

You mean the coil itself has an embedded microcontroller? Or do you mean the car's computer simply has connections to sensors in the coil to monitor its performance?

Yes, some modern coils have a microcontroller inside the coil and a totally clever feedback system.
The voltage pulse width of the signal tells the coil what it should do: (Normal spark Multispark, Ion feedback, Slight retard ...)
And the current of the input signal gives feedback to the engine computer if this mode was successful by adjusting the current in 5mA steps from 5mA to 35mA for instance.

[HighVoltage]

Plenty of vehicles now integrate the coil driver into the coil. Makes design of the ECU easier, as there are no high current high energy circuits to route through it, and you only need a relatively low level signal to provide the drive. Also they integrated this into a coil per pair of plugs as well, and also made the whole assembly a single unit, so that for MB you have a large unit that sits above the engine, containing not only the 2 plug per coil units, but also a DC Dc converter to provide a 400VDC bus to drive the coils, so they can use standard off the shelf IGBT transistors to drive them, but use low inductance coils, few turns of heavy wire, and still get both multiple firing pulse per cylinder, up to around 6 pulses per firing period at max RPM, enough to start even the weakest of mixtures reliably in stratified burn engines. So now you have both 2 coil packs, a pair of DC DC converters, around 400uF at 450V of capacitor, and a CAN bus microcontroller, all sitting exposed to 120C on top of the engine.  And these units will typically fail shortly out of warranty, they are that well designed even though the components are running on the edge of their temperature envelope in normal operation.

The DC/DC converter system has been abandoned for a while already, even on expensive cars. It was far too costly and had too many problems.

These days the most reliable coil system has the IGBT and driver circuit and microcontroller inside the coil, to reach 30A primary current, for the goal to get the maximum spark current for best combustion results.

[Circlotron]

so that for MB you have a large unit that sits above the engine, containing not only the 2 plug per coil units, but also a DC Dc converter to provide a 400VDC bus to drive the coils, so they can use standard off the shelf IGBT transistors to drive them, but use low inductance coils, few turns of heavy wire, and still get both multiple firing pulse per cylinder, up to around 6 pulses per firing period at max RPM,

What car had that setup?

[HighVoltage]

so that for MB you have a large unit that sits above the engine, containing not only the 2 plug per coil units, but also a DC Dc converter to provide a 400VDC bus to drive the coils, so they can use standard off the shelf IGBT transistors to drive them, but use low inductance coils, few turns of heavy wire, and still get both multiple firing pulse per cylinder, up to around 6 pulses per firing period at max RPM,

What car had that setup?

Maybach comes to mind and SAAB