Why does the ICE start running again when battery charged?

Hi all,

I am wondering about a phenomenon that I have observed in 3 of the 5 days this week that I have driven my new Prius to work.

My commute is 6 miles and takes about 10 minutes. The first mile features a 40 MPH limit 2-lane road with a big hill to go up and then back down. Then it's 4 miles on a 45 MPH limit road with a couple traffic lights, fairly flat, and fairly heavy traffic. Finally it's 1 mile on a 30 MPH road, which is flat or slightly downhill, with traffic generally moving at 40+ MPH. And finally, I turn in to work and go up two levels in the parking garage.

As expected, the ICE runs for about the first 5 minutes. Even when coasting down the hill the ICE is on and MFD registers about 50 MPG while regenerative brakeing. After about 5 minutes I can do a fair amount of gliding on the 45 MPH road. By the time I'm done on that road, I have 6 bars on the battery and coast or stealth the entire mile on the 30 MPH road.

My question concerns when I get into the parking garage. If I'm sufficiently lucky not to have anyone tailgating me, I can advance to my parking spot in all electric mode. The problem is, the ICE kicks in, and even when in stealth or coasting, MFD reports 30-50 MPG. It seems pointless for the ICE to come on here, as I have 5-6 bars on the battery and the car is warmed up at this point.

Any insights? Would the EV switch help here (activate EV when in the parking ramp to keep the ICE off)?

 

When the ICE comes on, there is no transfer of energy (i.e., no orange arrows) from the ICE. The only way I know the ICE is on is because I hear it. And, it's the phenomenon of getting only 50 MPG (or less) during regenerative braking or stealth?

Yes, I know the ICE can (and occasionally does) come on going up a ramp, and there's energy transfer then. It's when I'm going slightly downhill (on the straightaway) either coasting or in stealth where I am wondering why it would come on.

Also in case it is not clear, the car has been on and driving for 10-12 minutes by the time I hit the parking garage.

 

Thanks for the suggestions. I did read the last linked article with interest, but based on the ICE being rough when shutting off, that might not be it. I'll try to do a little more experimenting with this, if I'm on-time enough and don't have anyone behind me. I just find it strange that it happens at the same point in the commute, seemingly regardless of how the rest of the commute has gone.

I'd buy the explanation about the catalytic converter, or anything else that the computer is trying to keep warm, cooling off during 4-5 minutes of little gasoline use. Tomorrow I'll try to drive it more aggressively between home and the parking garage (meaning, try to keep the ICE on). I'd figure that this explanation could be ruled out if I keep the ICE on most of the way and have the battery charge the same as it usually is (5-6 bars), and still observe the same effect.

 

Sorry -- My previous post wasn't clear - what I had intended to say was something along these lines: the article says that the engine shutoff will be rough, but when the engine shuts off on me it's smooth, so it's probably something else. In any case I will take the most inefficient Prius commute I can muster tomorrow and see if the phenomenon repeats itself in the parking garage, and report back.

 

efusco probably nailed it - but I'll post the result of my experiment anyway. I was successful in having the ICE run for the majority of my trip (I'd say 90%). At the end this got to be a challenge - I was at 6 bars (suspect I was "going on 7") and under 35 MPH, and the car wanted so badly to stealth. Anyway, when I got to my parking location, I was still at 6 bars and observed the same (ICE on, stealth mode indicated my MFD, mileage at 40-50 MPG during that). When I got to my parking spot I sat there on the brake until the ICE shut off, which was about 5 seconds, and was rough.

Since the ICE invariably kicks on as I ascend from one level to the next, and I don't stop when reaching the level, the part about the ICE continuing to run until I come to a stop for several seconds is probably the explanation. If I happen to arrive at work and have nobody behind me, I'll stop at the end of the incline until the ICE turns off, and see if my results are reproducible.

After my experiment today - granted one drive isn't enough to establish or deny conclusions - I must say the Prius is a pretty amazing machine. Normally my commute has a 5-minute bar at around 35 MPG and then one at 75 MPG, with my "best effort" at coasting and such. Today, the bars were 35 MPG and probably about 60 MPG, intentionally driving "badly." So even driving badly, it still performs!