ICE vs Battery at Highway Speed

That correlates well with my observations as well. And, I've found that because the computer's attention doesn't wander as mine does that it generally gets better mileage than I can.

In town, I can easily beat the cc computer if the terrain is rolling or if there are stop lights and the like.

 

This is a reasonable simplification of what is happening.

As it is, in addition to supplementing the ICE with the battery and electric motors/generators, the CC will also increase and decrease the ICE RPM as needed for optimal generation of motive power to meet the requirements of the vehicle at any given moment. Using my Scangauge, I've found that the Prius definitely does not "maintain a constant ICE RPM" while using CC.

Additionally, while the ICE will continue to spin at all times (at a minimum of bit less than 1000 RPM) at speeds above 41 MPH, it will cut fuel and spark to the ICE when the ECU determines that it is inefficient to use the ICE.

As I understand it, when operating CC the ECU takes into consideration:

The current state of charge of the HV battery
The temperature of the HV battery
The current speed of the vehicle
The current RPM of the ICE
The increased/decreased amount of torque needed to maintain the set speed of the vehicle.

It then determines the best combination of ICE RPM and electricity generation or use at each of the 2 motor/generators to create the required torque.

If you are using CC and traveling enough downhill for gravity to accelerate the car, CC will use the torque created by the wheels to generate electricity and charge the battery to keep the car from accelerating.

You are correct that the accelerator pedal does not move as the ECU increases or decreases the torque.

 

Cruise control is irrelevant to this discussion. As far as the ECU is concerned, it doesn't matter if cruise control or your foot provides the input; either way the control actions will be the same. The balance between battery and ICE is done to maximize efficiency. One of the ways to do this is to run the ICE only at efficient RPMs and loadings, which is why the ICE tends to vary its speed in stepwise increments, stopping only at efficient points.

Cruise control becomes an issue only in that it agressively tries to maintain a set speed. A human driver will often back off a bit on uphills and accelerate a bit on downhills, often without thinking about it. We judge speed changes by sensing g loads. Being pushed back into the seat makes us think we are accelerating, while being pulled forward does the oppisite. When going uphill, gravity pulls you back into your seat, so you feel like you are going faster. The natural reaction is to back off on the accelerator. The converse is true for going downhill. Cruise control doesn't do this, which is why it feels agressive and sometimes out of control on hills.

Tom

 

The cruise control is irrelevant from a purely control input perspective.

It is very much relevant in maximizing MPG because it is capable of much smaller inputs into the system than I, at least, am capable of. So, on level terrain it is much better at avoiding the demand of more power than is necessary and thus doesn't cause the ICE to increase RPM as high.