Maybe idling to warm up is beneficial for MPG?

I have seen in some of the Japanese data from Ken@Japan that is one of their practices. I've also helped a local couple go from 50 MPG to 58 MPG by suggesting they let the car 'warm-up' in place. But I agree it needs more study and there may be an ambient temperature variable.

Bob Wilson

 

I've been doing this for three months.

I have several intersections on the way to work that I use as checkpoints for displayed fuel consumption. Once I'm past the first 3 miles or so I'm doing as well or better than driving immediately on the cold start.

It's also a much better feeling taking off with the engine contributing to the propulsion (and the battery freshly topped up).

 

I've identified these specific, cold-start, warm-up phases:

• catalytic converter light-off - ~50 seconds, the engine runs "open loop" using the stored fuel trim. The car uses traction battery at moderate acceleration. It can reach 40 mph and shifting into "N" results in high, instant MPG numbers.
• engine coolant warm-up to 60/70C - ~3 minutes, the engine is running "closed loop" but uses higher than normal fuel consumption rates. With each 10C increase in coolant, the Scangauge II shows a decrease in the gallons/hour. The car will shutoff the engine at a stop but otherwise runs. EV mode is possible but only to 9 mph.
• efficient power operation - the engine consumes 0.60-1.20 gallons/hour at urban speeds, 50 mph or less, and the engine will turn off automatically. This happens up to 46 mph but more reliably at lower speeds.
  • 70F and higher
    • cruise control speeds of 25-27 mph -> approaches 99.9 MPG
    • cruise control speeds of 28-32 mph -> closer to 85-95 MPG
    • cruise control speeds of 33-35 mph -> closer to 75-85 MPG

During colder weather operation, I can see the instant MPG scale peg during the engine coolant warm-up by shifting into "N". But lately, I'm not seeing the same jump over keeping the car in gear and on level road, cruise control, speed maintained.

During the catalytic converter light-off, I'm also not seeing the same instant MPG pegging.

It is an area that begs further investigation and metrics. Near as I can tell, the warm-up costs consume the bulk of the fuel. This is where the PiP has the advantage of letting the engine warm-up at minimum fuel consumption while EV provides motive power . . . the short trip penalty.

Bob Wilson

 

I don't think more than a minute of warmup (enough to exit stage 1a) is worthwhile. You can force the engine to start immediately after the Ready light by giving the accelerator a tap. I then fasten my belt, tune the radio, adjust the mirrors, etc. to spend the time.

 

...that would be nice but I am under impression PiP does not really solve the low MPG at ICE start-up problem.

 

If the ICE warm up while you are moving 10-15 mph, you can easily get 50 MPG. Above 25 mph, you can accelerate and still get instantaneous 50+ MPG.

The PHV battery is more aggressive in assisting during the warmup.

 

Someone will certainly correct me if I'm wrong.....

I was under the impression that the computer does not calculate mileage unless the car is moving. Thus, any gas burned while parked is just ignored. Seems like this would negate any assumptions or calculations made regarding warm up effecting MPG displayed.

 

The instant MPG calculation is turned off when the car is just sitting but it is my understanding that the trip A and B mileage are still being calculated accordingly.

 

How far did you travel when you took the reading of 67 mpg?

 

The Trip A/B mileage is calculated strictly by dividing the miles traveled by the gallons consumed while the car is in Ready mode. Very early in a trip you can see the ave. consump. value decay as you sit at a stop sign with the engine idling.

 

You must have a different display than I do. Unless you're talking about the fuel consumption display on the NAV screen. The MPG display on my dash is "----" when sitting still.

 

I'm talking about the trip consumption, not the instantaneous. I'm also not driving a C, FWIW.

 

Actually it recalculates when any gas is burned. In my UPS studies, I needed to know how much gas was burned at different power levels. So I would park the car with a value on the trip meter and note it. Then I would put the load on. After a suitable interval, an hour or so, I would get the decreased MPG and use the first and last measurements to calculate the fuel burned.

Bob Wilson

 

We park in an underground garage at our apartment. Would we be better off sitting in place for the ~50 second S1a period or rolling slowly (<10 mph) while driving out of the parking garage during the S1a period? Is there any way to force the Prius to skip S1a? In our Fusion Hybrid I can force the car to skip its equivalent of S1a and go right to S1b which I've found greatly increases the efficiency in the FFH.