Maths is cool - part 12
by Steve Smith

Following on from our Pico Practical engine evaluation Q&A which you can watch here, I wanted to dig a little deeper into the benefits of 48 V MHEV in comparison to 12 V systems.

The current Audi A8 proved to be the perfect vehicle for such an experiment as it has the ability to crank either by a conventional 12 V starter motor or a 48 V Motor/Generator (MG) system.

From a cranking perspective, the 48 V system offers more torque and power without the accompanying lag of the 12 V system during the Stop/Start phase.

With this in mind, I thought it would be great to do these comparisons with maths and incorporate “Time” into our math channel equations.

Starting with the 12 V system, below we have crank and start of the V6 3.0 TDI engine using a conventional starter motor.

Above, I have calculated the power of the starter motor using the math channel voltage x current and the engine RPM via the frequency of the crankshaft sensor signal (incorporating a filter). Read more about how to do this here.

Below we have added multiple math channels now looking at Torque (Nm) using power and engine speed, Energy in Watt-hours (Wh) using power and time, and energy again but in Watt-seconds (Ws) using the integral of power (Integral being the area under the power math channel).

Looking at the included measurements above, we can paint a picture of the events taking place during the 460 ms of cranking required to start the engine.

Now, let’s look at the same engine starting via the 48 V MG system.

Once again, looking at Torque (Nm), Energy in Watt-hour (Wh) and finally Energy in Watt-seconds (Ws) in order to draw a direct comparison between both systems.

The first item that stands out to me is the cranking time of the 48 V system at 277 ms compared to the 460 ms of the 12 V system. Let’s be honest, 460 ms is good but note the dramatic increase in cranking speed from zero rpm to idle speed with the 48 V system. It achieves over 600 rpm in a quarter of a second!


To summarize


I need to revisit the results for Energy via time for the 12 V system as they do not appear true! This may be due to the fact that the capture contains three crank and start events in one waveform buffer, where the 48 V system contains just one crank and start event. I will feedback ASAP.

All in all these additional benefits of power, torque and speed originate from a totally independent power source (48 V system) resulting in the absolute minimal impact on the 12 V system during the Stop/Start phase.

Couple this to the fact that fuel consumption, emissions, energy recuperation and starting torque all improve I can now see the benefits.

Another interesting fact is that the 48 V MG system has the ability to compensate for the effects of typical engine vibration orders such as E3 with this V6 engine which is mind-boggling!

Another BIG thank you to our math guru Pico Martyn for all his support on this adventure.


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