Power surge on highway?

Yes, I have noticed this also. I'm new to the Prius, only have had it a few weeks. Maybe it is the battery cycling...giving the electrical assist for the acceleration? I'm sure one of the Prius "experts" on the board should be able to explain the surging sensation. There are so many things that feel and sound different in this car that I just wrote it off as "normal". I hope it is anyway!

 

Welcome to the boards. Your name indicates you're probably from Wisconsin or Michigan, right? :wink:

Now as far as this Prius thing goes. I can't give you a tech explanation. However, you have to keep in mind that nothing about this car is going to be like any other car you have driven.

I have had mine almost 2 months and now I know what to expect. This makes my car 'normal' and everyone else's car 'weird.'

This car will definitely change your perception of cars in general.

 

The onboard computer systems are constantly evaluating and reevaluating the driving conditions it senses. Are you going uphill, downhill, flat, slight incline up or down. And then it will optimize the combination of power from the ICE and electric motors to give you the ground speed you are requesting. Surges, or I suppose I could reclassify them as shudders are very normal. I'll try to explain further.

I basically live in the Cascade foothills where the landscape is very young, relative to a lot of other parts of the U.S. and lots of highway driving out here is done up and down mountains, ridges and river valleys. If you are descending from a plateau above the Columbia at 70mph, the vehicle will go all electric and do a combination of things, coast, regenerate power or possible use electric power to keep up your road speed. But the ICE will probably not be operating. When you get to the bottom of the descent and you are about at the river level the ICE will kick back in to provide power. Now, at 70, the ICE was spinning as you were going downhill, because if not the smaller motor/generator( I believe it is MG1), would have been overspun, which is a bad thing. So, the ICE is spun without ignition to "use up" some of the kinetic energy generated by the downhill drive at highway speeds. At the bottom of the descent the computers would have noticed the change in movement, especially the tendency of the vehicle to start slowing down and the ICE would have reengaged. I have generally found that ICE reengagement at highway speeds is usually very smooth, however all ICE reengagement is noticeable. The degree to which you notice it depends on ground speed, uphill or downhill driving and how warm the engine is when the computers command it to start.

I also want to add that when going up hills, the ICE will tach up pretty quickly and distinctly. That may also seem like a power surge. Rest assured that the computers do know what they are doing. Compared to other 4-cylinders out there, the rpm limit on this one is pretty low and the computers will no go beyond its programmed RPM limit.

Without droning on further, the vehicle has jolt, surge or shudder tendencies when the computers decided to fire the ICE back up. How distinct it feels depends on many factors.

 

I usually describe ICE powering up as feeling like a bad shift in a conventional transmission. OK, maybe bad might not be the word, but bit harsh shift.

Also, don't worry about the engine appearing to race when you request full acceleration, as if the transmission were slipping. I assure you, it isn't. Whatever power is not going directly (mechanically) to the wheels is diverted to the generator (MG1), which in turn is fed back to the motor MG2 to also turn the wheels.

 

When I was noticing the surging feeling, I was going slightly uphill at highway speeds 65-70. It is not the shudder that I feel when the ICE turns on/off when it is warming up. It definitely feels more like a surge in power. I assumed it was the battery just kicking in and out to give some extra power to keep it up to speed. It feels as if I were depressing the accelerator and then taking my foot off repeatedly, but slightly more subtle. I wasn't too worried about it, but it definitely is one of those new Prius feelings! I didn't notice it much on flatter ground.

 

The surge is probably the computer trying to figure out the most efficient balance of power from the various powerplants. Again, going uphill on the interstate is very interesting because the vehicle will tach up the ICE, but for the most part it will not loose speed. The car is absolutly amazing in cruise control, it maintains the set speed better than any other vehicle I have ever driven. Most other vehicles, when navigating a hill under cruise, will usually bog down a little until the tranny shifts down, if it ever shifts down on its own.

 

I installed a Sears aftermarket universal cruise control to my 87 corolla. That thing was solid on the speed, since it referenced the tach, assuming fixed gear ratio. It was numbingly bland, it was so steady, humming out this one note song.