Maths is cool - Part 7
by Steve Smith

I know the following may not be for everyone, but I thought it worthy of sharing based on the events discovered during these captures and if nothing else, to revisit ignition theory.

We are often asked about dwell angle by classic car enthusiasts, racing teams and restorers and there is frequent disbelief and dismay when users are informed there is no dwell feature in PicoScope.

I fully understand their response, as technicians of a certain age (I include myself here) would measure dwell angle daily either via “The Crypton Machine” or a multimeter. So why is it not included in PicoScope?

To be completely accurate, it is. However, it requires math to reveal the dwell event of the primary ignition circuit. The downside is the techniques required to obtain the dwell and interpretation of the results.

An average Dwell angle at cranking, idle and high engine speeds measured by early generation engine analysers would have been exactly that, an average over time with a questionable refresh and sample rate.

PicoScope, however, will measure every single primary ignition dwell event and present the results to the user in graph or table form. This may sound like a bit of an overkill, but it does reveal some characteristics that, once again, I was not aware of. 

I must refer to the following forum post and thank Richard, STC, and Kim, for their invaluable contributions. 

Dwell angle is defined as the amount of time the contact breaker points are closed (current flowing through the primary ignition circuit) and it is measured in degrees of distributor shaft rotation.

If we concentrate on a 4-cylinder engine, the total cycle time (expressed in degrees of distributor shaft rotation) for each ignition dwell event would be:

360° of distributor shaft rotation / 4 cylinders = 90° cycle time

Cycle time refers to the total amount of time between each ignition event, or to express this another way, the total amount of time the primary ignition current is switched on and off between each ignition event, better known as the Duty cycle. 

Follow this link to learn more about duty cycle with math channels.

In order to calculate the dwell angle for this 4-cylinder engine, we use the following formula:
duty(B) / 100*90. 
Duty B indicates the percentage of time that our signal is positive during our cycle time. 
Dividing by 100 converts our positive duty percentage into a decimal value.
Multiplying by 90 allows the software to display the dwell period, expressed in degrees of distributor shaft rotation. 

For 6 Cylinder engines (360° / 6 = 60°)
duty(B) / 100*60

For 8 cylinder engines (360° / 8 = 45°)
duty(B) / 100*45

This is one of those rare occasions where the formulas use a positive duty cycle, given we are measuring current and not voltage, as it provides a more stable signal for the math to function. 

If you wish to use the negative duty of the primary voltage, I have included the primary voltage on channel A. To do this, you have to use the formula: duty(-A)/100*90. (To view Channel A, right-click on the scope grid and select Channels.)

I mentioned earlier about the techniques required when using maths to calculate the dwell angle. 

These include:

1. AC Coupling the primary ignition current (Channel B) to define a clear crossing point at zero amps.
2. Low pass filtering of channel B (4 kHz) to remove excess noise so improving the outcome of the math channel (this is essential). 
3. An increase in the number of samples (6 MS) improves the performance of the math channel to ensure accurate results.

Looking at the waveform above, we can see a deviation in the dwell angle from approximately 46° at idle speed to approximately 36° at WOT (a deviation of approximately 10°). This would initially suggest a worn distributor shaft bearing/bush, creating a deviation in dwell at higher engine speeds (during the transition from idle to WOT).

In the waveform below, I have included the rpm math channel 60 / 2 * freq(B). This is to clearly indicate the change in engine speed and so the effect on the dwell angle.

At this stage, we cannot confirm if the variation in dwell is due to wear in the distributor or activity of the distributor base plate centrifugal advance mechanism. What I do know, is that the vehicle performs fine without any running or timing issues and that a mechanical inspection for lateral movement of the disturber shaft confirms no significant wear. 

What I find intriguing, is that the deviation in dwell angle (approximately 10°) at high rpm is approximately half that of the 20° ignition timing advance specified at the crankshaft! This is far too coincidental and so I shall return to this vehicle to confirm the advance in ignition timing, while also investigating the effect on dwell angle as a result of distributer base plate rotation/advancement.

Theoretically, the frequency of our dwell event should change but the duty/dwell should remain the same. With that said, when the distributor base plate changes the relationship between the heel of the contact breaker points and distributor shaft, the dwell angle will momentarily change. 

Given that we can now measure each and every ignition cycle with extreme accuracy, we may be revealing an event that has always been present but not captured due to average dwell values.

Using the Deep Measure feature of PicoScope (read more about Deep Measure here) we can drill deeper into every ignition cycle to reveal how the duty cycle (and so dwell angle) alters at higher speeds. The waveforms below highlight the change in the negative duty at idle speed and then WOT.

To obtain the positive duty from Deep Measure, subtract the values highlighted below from 100.

Cycle 83 highlighted at idle speed below:

At idle speed, the duty cycle (low) is given as approximately 48% (52% high or positive).

Cycle 187 highlighted at WOT:

At WOT, the duty cycle (low) is reported as approximately 59% (41% high or positive) indicating a difference of approximately 11%.

The Deep Measure feature certainly helps to confirm the behaviour of the math channel during WOT and of course, further investigation will be required. It would be great to carry out a similar test to a vehicle with a worn distributor shaft bearing/bush, as this would highlight how the results compare to the capture above.

Moving onto dwell angle expressed as a percentage:

You may often see dwell angle stated as 46° or 51%! Both refer to the same event but use different units:

Here is how to convert a dwell angle of 46° to 51% for this 4-cylinder engine:

We know that 360° / 4 cylinders = 90°
We also know that for 46°out of the 90° of distributor shaft rotation our points are closed (dwell angle).

The following formula converts the dwell angle to a percentage value: 46° / 90° x 100 = 51% 
51% also happens to be the number we have for the positive duty. The waveform below has the additional math channel duty(B) included.
 

Cycle 187 qt high speed

Food for thought:
Here is a 1985 Vauxhall Astra Van with the 1.3 S OHC engine utilising a traditional distributor but with electronic ignition (the engine performs fine). 

The Deep Measure function measures the change in duty (low) of our primary ignition current between the time rulers from approximately 82% at idle speed (18% high) to 65% at WOT (35% high).

Notice the behavioural pattern of the dwell angle math channel, which is extremely low at idle speed (16.55°) and increases to 27.08° during WOT. This is the complete reverse of our mechanical contact breaker points and demonstrates how far ignition systems had advanced even during the ’70s and ’80s. 

A big thank you to Kevin Ives for digging up these vehicles and the use of his workshop, which no doubt, I will be returning to discover more!
 

Duty low from the Astra

You can find the .psdata file for the Sherpa Van (with contact breaker points) below.

Remember to activate a 4 kHz lowpass filter to Channel B and please be patient when loading the file as there is a lot of data for the PC to crunch through with maths, filtering and Deep Measure applied.

Freight Rover_Sherpa_Idle to WOT.psdata