Short Trip Math and Idle from Cold-soak

So this afternoon, I decided to gather some metrics from our 2010 Prius:

	Column 1
0	[tr][th]mi[th]gal[th]MPG[th]tot mi[th]tot gal[th]tot MPG
1	[tr][td2]0.0[td2]0.0000[td2]0.0[td2]0.0[td2]0.0000[td2]0.0
2	[tr][td2]0.0[td2]0.0090[td2]0.0[td2]0.0[td2]0.0090[td2]0.0
3	[tr][td2]0.7[td2]0.0223[td2]31.3[td2]0.7[td2]0.0313[td2]22.4
4	[tr][td2]0.6[td2]0.0136[td2]44.1[td2]1.3[td2]0.0449[td2]29.0
5	[tr][td2]1.3[td2]0.0230[td2]56.5[td2]2.6[td2]0.0679[td2]38.3

This data is gathered from the following screen shots that any Prius owner can replicate:


These two photos show the fuel consumed in an initial, startup that ends when the catalytic converter reaches operating temperature.


This is the second segment at neighborhood speeds, 25 mph, to 60C engine coolant.


Here is the third segment at neighborhood speeds to 70C engine coolant.


This is the fourth segment when the engine coolant reached 85C and began to 'auto-stop.' The car was already in the 40 mph speed range.

The duration of trip is the sum of the segments. In this case, the car was sitting on the driveway, cold-soak. The engine ran for two minutes and burned 0.009 gallons. Since it did not move, 0 MPG. This is the extreme limit of a short trip. But as the engine warms up, the efficiency significantly increases:

• 31.3 MPG - miles 0-0.7
• 44.1 MPG - miles 0.7-1.3
• 56.5 MPG - miles 1.3-2.6 hb

The end of trip is when the car sits long enough to lose operating heat:

• catalytic converter - a couple of minutes
• engine coolant - a couple of hours

The short trip penalty is the warm-up cost. So the rule of thumb still works:

1. order errands with the longest first for maximum warm-up

For those of us who actually pay for our gas and have a Prius, knowing how short trips and cold-soak works can lead to fuel savings . . . often with little or no impact on our lives. Just efficient scheduling and driving to maximize our return on investment.

Bob Wilson

 

Good points!

I wanted to also want to lay out a basic methodology, a way, we can approach getting some hard numbers for Prius warm-up. I've been and remain an advocate of:

• start car
• in first 45 seconds accelerate in EV to speed
• shift into "N"

But with the warm weather, 81F yesterday, it isn't clear this is as effective as it seemed in colder weather 30-40F.

Bob Wilson

 

You might try setting the histogram display to 1 minute chunks, higher resolution. But what I'm seeing in my data:

• ~45 seconds (call it first minute) - catalytic converter light-off, engine runs 'open loop' using the stored 'trim' for mixture control. If you are gentle on the throttle, you can accelerate up to ~40 mph on traction battery energy. Any heavier and the engine sucks fuel.
• ~2-3 minutes - to get the engine coolant warm enough, ~60-70C, to allow engine off or hybrid mode
• ~20 minutes - to get the transmission fully warmed and reduce viscosity losses

I captured some more data this morning and will add it to the original to compare and contrast.

Bob Wilson

 

It is not easy to quantify in our vehicles because of the other variables. However, there is a good paper, "ORNL/TM-2004/247" that shows the effect of temperature on transmission drag. Every BTU of transmission loss is three BTUs of gasoline energy.

Bob Wilson