ICE cycling off and on repeatedly in traffic

I noticed yesterday in traffic, looking for a parking space the ICE was constantly cycling off and on. The battery was only at 2 bars so I understand why it was doing that. How does the Prius get the mileage it does if the gas engine is turning off and on all the time in traffic? Yes it was hot and I had the A/C on too, which of course draws more current. We had driven 130 miles and I was amazed at how short the SOC lasted.

Suffice to say that once we got out of the congested area and I could brake more effectively the off and on went away.

Also on the way back I used the normal mode for windy hilly areas and the eco mode for the highway with cruise on. Oddly enough at the end of the trip the CONS was showing 57.6mpg. Didn't seem like changing modes hurt the mileage at all if any.

Thanks

Because in heavy traffic like that your mpgs will fall sharply, but the good news is that they won't fall anywhere near as badly as the guy you're behind or in front of. The HV battery will only allow you to travel about 1.5 miles tops before needing to be recharged and in heavy bumper to bumper traffic the only way that will happen is with the engine switching on and off. It'll top you up from 2 bars to about 3 bars before switching off again, but that will still allow you to sit in traffic with the engine off for a considerably longer time than the other cars. Also, with the HV battery at 3 bars, it'll still allow you to travel some distance with the engine off before it needs charging again.

So in the conditions you describe, your real time mpg's will be about 30 mpg, but the guy in front will be getting about 8 mpg.

Don't worry about it or the HV battery - my Prius spent the last 65k miles in those conditions every day.

The different modes will not have a large effect on fuel economy once the car interior has cooled down. So if you spend 30min driving around in normal mode with the AC on then hit the highway and put Eco Mode on the difference in AC consumption will likely be quite small. I've watched the consumption wattage via the Torque app.

Grumpy, you don't live where it's hot. I clocked it, and get 16.7 mpg on my mfd once, and I know that over estimates I think they just don't have the right programming for 100 degrees + air conditioning + traffic jam. It does fine if its only in the 80s, or if you are moving, but the battery needs to charge up more in real heat to be effective.

If your consumption is 57, all is good though. You could not have been over cycling

OK, so you got 16 mpg in a very hot traffic jam. The guy next to you with his engine running all the time with his a/c on will have been getting nearer 2 mpg then.

But you're choosing an extreme condition here. I believe the Prius gets poor mpg's when you drive quickly up a hill too, but for 80% of the time, it'll get you significantly better mpg's than any other car of similar size. Even in Europe where we have manual diesels that get near to the Prius on economy; they still can't beat it.

As was already eluded, the fact that the engine is not running constantly is how. Those conditions you encountered qualify as extreme too. The EPA testing takes place when the temperature is between 68 and 86°F. You won't see "cycling" like that then. When in extreme hot and extreme cold, it's normal. Stuck in heavy winter traffic, the same thing happens to me due to the heater. Fortunately, it can be several minutes between the on & off.

Not at all, it was cycling too fast. I get good mileage on long trips, but hot days heavy traffic it underperforms relatively. The op brought up the point. Short trips, hills, also relatively under performs.

I was commenting on the condition the OP was asking about. It depends on how hot it is whether the cycling is proper for gas mileage. It does great on 70 degree traffic jams.

I know what you meant. The a/c will have been working overtime as it was so hot. The a/c gets its power from the HV battery but as it draws significant kwh's when on full, the battery will go down very quickly and thus the engine will have to come on more often. I believe the a/c draws 4kwh on full and the HV battery is 1.5 kwh. I work that out as about 15 mins give or take the traffic.

I'm sure that even in 105 degree heat, if the op turned the a/c off the engine wouldn't cycle anywhere near as much; in fact, probably as much as on a 70 degree day.

I measure the wattage draw to be between 1500w and 1900w when the A/C is going full tilt.

You are correct. In 105F-110F temps the car does not cycle on and off nearly as much as it does with the A/C on and working hard.

The HV battery really isn't all that big and will that kind of load, it won't last long. The battery is a nice balance between capacity and weight/size.

I'm sure if the driver turned off the AC when it was hot outside, the engine would not cycle so much, I don't think that would be good for the humans or battery in the car. The problem is the car will only charge the battery a little, then cycle off, then cycle it on. It would be better for it to stay on longer in an efficient range, and charge the battery more. One thing I did not think of was to turn off eco, lower the temperature in the car to draw more current, then cool the cabin down lower, to fool the ECU You could then raise the temperature set point. I have tried force charging, but don't think that really helps.

Its just one of those things the prius does not do well. It does great running the air conditioning efficiently when stopped, and I'm sure the system helps reduce losses do to air conditioning parasitic losses at higher speeds.

Thanks to all of you for the replies.

I know I saw Wayne in one of his demos leave the car in gear, put the brake on and hold the gas pedal down to charge the battery. He said " You hardly ever see the battery at 2 bars." Maybe he doesn't but I do and nothing happens if I try to do what he did with the gas pedal. I thought of this as the ICE was cycling off and on.

But all noted, it's normal and I won't bother with worrying about it!

Regards

Yes, I have been caught in a traffic accident for about 30 minutes in 100F heat. Once the HV battery drops to 2 bars and the ICE comes on it only charges up to 4 bars. So, it continues to cycle quite often between 2 and 4 bars.

As F8L pointed out the AC running hard puts a large draw on the battery. I tried turning the AC off and opening windows but between me overheating and the HV battery overheating I turned the AC back on.

As soon as we were moving again and the ICE was running I noticed it did not immediately recharge the HV battery. It was providing power to the wheels. After a couple of times of regen braking the HV battery climbed back up to normal. All was fine again.

You have to put your foot firmly on the brake and depress the throttle completely to do a force charge.

kevin

I agree. I have asthma and have to have the A/C no matter what it does to the overall mileage. Case in point, it can't be all that bad, I filled the tank and measured 53.5 mpg so I'm not complaining! I've had less driving locally.

In that situation, how much more efficient (if at all) would it be to turn off the heat and turn on the heated seats and/or use one of those 12V heaters?

Heated seats would be an idea as would a heated steering wheel which some cars now have, but those 12v heaters that you plug in the cig lighter sockets are a waste of time. You'd be better just rubbing your hands together for the amount of use they are.

The heated seats work well as they don't cause the ICE cycling at a stop that running the CC does. However, 50mpg isn't worth much if you can't see so not running the defroster is not an option.

In such slow traffic, couldn't you wipe off the windshield with a towel when you're stopped or run the defroster just enough to keep it clear?

If it was that cold I'd run the heat. No way am I going to be uncomfortable to save a couple of quid in fuel.

The reason the Prius is popular is that you can save money without compromising your lifestyle or comfort. I know there are some committed environmentalists out there who might enjoy wearing sandals and hair shirts, but that's not for the other 99.9% of the population.

Turning off heat and defrosters is a step too far.