Warm-up: Stage-1A

Many of us have noticed that the early startup of our Prius leads to an MPG deficit that takes awhile, 4-6 miles, to clear. This is due in part to open-loop fuel burn and warm-up fuel consumption as well as putting a charge back into the traction battery. But I think I'm seeing an approach for rolling in Stage-1A that appears to improve the situation.

Ken@Japan defined the term, Stage-1A. None of this data has quantitative numbers, yet, but I wanted to share my impressions. First, my understanding of "Stage-1A:"

Stage-1A is the time it takes for the O{2} sensors to begin working and controlling the fuel-air mix and typically lasts 45-60 seconds. During this time the engine runs a rich mixture and if the Prius accelerates, most of the motive power comes from the traction battery. Different models of Prius behave differently during this phase:

ZVW30 in "P" will run for about a minute and then auto-stop the engine. I understand the NHW20 does the same.

NHW11 in "P" will run until the engine coolant reaches 70C and then auto-stop the engine.
​

The advantage of staying in "P" for the ZVW30 is this keeps the engine power demand at a minimum. The overly rich fuel burn is not compounded by asking for more ICE power in this open-loop mode. However, the NHW11 does not auto-stop the engine at the end of Stage-1A.

With the NHW11, I found that a better strategy in Stage-1A is:

1. Gently accelerate to a speed as close to 42 mph as possible using electric power. The goal is electric acceleration without asking for more ICE power.
2. Once Stage-1A ends and closed-loop O{2} begins, use "N" to minimize ICE power of the still cold engine. Using "N" keeps the ICE in a minimum power level and fuel consumption. But it also allows the Prius to 'glide' at whatever speed was obtained under battery power. Often this leaves the NHW11 with an indicated 35-40 MPG depending upon the velocity reached before the end of Stage-1A.
3. After the coolant reaches 70C, transition to Stage-4, full hybrid mode, as opportunity arrives and/or run the car at speeds above 42 mph to let the engine recharge the battery as a fraction of the fuel burn. This preserves the initial MPG.

I have observed that the ZVW30 appears to benefit from this strategy IF the driver can accelerate to a high speed. Certainly when at a lower speed, use of "N" dampens the fuel burn when moving until the engine warms up enough. Sad to say, I don't drive my wife's Prius often enough to gather good data.

Designing a good experiment will take a little time but I noticed the effect on our ZVW30 and early use of "N" during the first minute of operation.

Bob Wilson