After how much time is it better to turn off the vehicle, rather than idle?

The vehicle will turn itself off when you stop That is to say, it shuts off the ICE when stopped unless the engine is cold.

It happens Auto-magically

 

It won't idle the engine. The system is designed to turn off the engine when stopped.

 

They never idle for 5 or 10 minutes (maybe Alaska winters on startup) they shut down automatically. Just drive it, the computers in this car are very good at their job.

 

There is still a benefit, from cutting the whole car 'idle' electric load that must be replaced to the traction battery later. But it is very small compared to the fuel of traditional non-hybrids, or to efficient hybrid driving technique. Just don't turn it to Accessory mode for extended periods for listening to music, as this will put the small-ish 12V battery at risk of early failure.

 

Sometimes when I'm idling the ICE will shut off, then it will come on again. Why is this? Is it charging the hybrid battery?

 

For me, I shut down the entire car if I think I'll be standing still for more than 3-5 minutes (except in stop-n-go traffic). It's always better to conserve energy than waste it slowly at EV idle.

 

It could be any of several reasons: hybrid battery getting too low, or engine coolant getting too cool (threshold varies with climate control and ECO/Normal settings), or exhaust catalyst getting cold, or some other things I don't know about.

 

Yes, this is the most common case. It could also do this to keep the engine warm, especially if it's during winter in a cold climate, and the heat is on.

As for answering the OP's question, I'd say that less than 2 minutes you're probably better off leaving it on; more than 10 minutes, you're probably better off turning it off. In between it's a bit of a grey area. If you're at 2 bars on the battery, you might as well turn it off, since the car will turn the engine on to charge the battery soon; if it has to be on to charge the battery, it might as well be while you're moving. Turning the car off and back on will usually incur a warmup penalty - even if the engine is totally warm, it usually runs at idle for a little while after powering on. Avoiding that warmup by leaving the car on is why you might be better off leaving it running for a few minutes.

Some things really change the math, though; if the A/C is on at all, it'll drain your battery very quickly. If you're going to leave the car on, you might want to open the windows and turn the A/C off. Or if it's very cold and the heat is on, the engine might need to run to produce more heat. And if the battery is already low when you stop, the engine will start soon as well.

Or, you can just drive it. (Errr, just leave it in park.) The car will take care of itself, running as necessary to keep you comfortable and keep the battery charged. The only things you SHOULDN'T do in this area are 1. leave it in neutral for a long time (the engine can't run to charge the traction battery), and 2. leave it in ACC or IGN-ON mode (the READY light is off) for a long time (you're running the car off the 12V battery, which isn't very powerful).

 

PerfectTimingLV - Here is Catgic's "Best Practice, Rule Of Thumb" on how to manage the ICE to maximize incremental MPG-Fuel Economy, and, it follows, to maximize Per-Tank Full MPG-Fuel Economy delivered by your Petrol Warfighter:

If The ICE Is Running, And You Are Sitting Still, You Are Getting Negative Incremental Contribution To MPGs. If The ICE Is Not Running, And You Are Moving, You Are Getting Positive Incremental Contribution To MPGs.

Yes, the "Hybrid Gremlins" in the Hybrid Synergy Drive (HSD) control software will eventually shut the ICE off-down if it is deemed by them to not be required to keep it running to charge the Traction Battery or to meet some other "Hybrid System Energy/Temperature Housekeeping" demands, needs, or requirements. However, there is no "Free Energy Lunch."

Even if you rely on the HSD Gremlins to shut down the ICE, if you opt to keep the POWER BUTTON ON with the HVAC, Radio, Entune, and et al other accessory stuff cookin' away, your Traction and/or Accessory Batteries will be slowly discharging. This electrical energy depletion will have to be replaced/recharged later by running the ICE "To Pay The Energy Cost" for your time sitting stationary draining the batteries, resulting in negative impact on Per-Tank Full MPG.

Therefore, if conditions and safety permit it, and everything else being equal, POWERING OFF MANUALLY will assure that you are not eating into your fuel economy because POWER OFF means NO ENERGY USE OR DRAIN.

 

Hmm....
I don't see much benefit in turning off The Prius.
As already stated it will turn itself off...and on as needed.
For me? I also want to keep the engine warm. The engine will kick in if it cools too much. If I turn it off at an extended idle I risk not only running the battery low, but also the engine could cool to the point where I'm repeating an entire warm up sequence.

For some MPG benefit in my Honda Fit, I sometimes turned the vehicle off in a long Drive-Thru lane. But a real benefit of The Prius is I don't have to do that anymore. The vehicle itself handles the idle as efficiently as possible.

 

I leave my Prius idle most of the time because I want to keep the trip calculation running. Not the trip A/B, the one that you get when you turn off the car. If I think my idle time will be more than 5 minutes, I would turn everything (such as headlights and A/C) off except for the radio. In winter time, I'd also lower the temperature set point so that the ICE wouldn't try to heat up the cabin.

 

Ok, think I handle that question...: Immediately?

 

Technically yes, regarding fuel use. But whether it's always wise to do depends what the OP is doing when sitting in the car. There is always the problem of discharging 12V battery if not in Ready.

 

If you leave the vehicle in READY mode, it will tend to itself. It will run the ICE, if needed.
IMO! After a complete warmup, the engine shouldn't come back on within 15 min. unless it's
cold outside. If your running the heater to stay warm, the system will function to keep the car warm.
It's really a fool proof design and I would just leave it on so it can operate as designed

 

OTOH, there's no penalty for turning the system off, no reason to leave it on in the first place. No point in draining the battery if you don't need to.

 

I agree with this.

I disagree with this. While their may be no "penalty"...as stated above, the Prius as a machine is designed to operate efficiently while it is on and active, even if that "on" and "active" state includes extended periods of idling.

Turning the vehicle Off while idling, will prohibit the vehicle from starting and running the gas engine (obviously) BUT it is in that state is where the danger of draining the battery actually is greater. Depending on how long you are "sitting" the vehicle can cool and need a complete warm up cycle upon restart, and if you are using any accessories while powered down, you risk strain on your battery.

Why not let the amazing engineering of the HSD system do what it is designed to do? To me? You want to turn your vehicle off and walk away from it? Turn it off and walk away from it. If you are forced to "idle" in it? Traffic Jam or Drive-Thru line? Just let the system operate as designed.

 

Here's a reference point: After Hurricane Sandy last year I had my Prius c in Ready mode for 54 hours straight, running an inverter to keep electrical essentials going in the house (a very light load averaging about 60 watts). When I shut the car down, it reported under a gallon of gas used for that period. At $4 per gallon, that's less than eight cents an hour to keep the car in Ready.

Of course using A/C or heat would cause significantly higher consumption, but this shows that the always-on car systems don't use all that much energy.

 

I totally agree with you. That's why I usually leave my Prius in idle unless I'm walking away from my car for an extended period of time. Besides your inverter, the 12v battery has to keep the onboard computer powered up but the power draw is very little.