"motor assist" obscure warm-up EV mode

This past spring, I was testing a ScanGauge when I noticed our 2003 Prius would hold the fuel consumption to 0.60 gal/hr regardless of the accelerator and speed. I already knew the minimum, idle fuel consumption when 'cold' was 0.30 gal/hr but this fixed fuel consumption was something new. I soon verified our 2010 Prius has the same behavior.

Upon further research, there is a Toyota SAE paper that explained before the catalytic converters reach operating temperature, the Prius tries to maximize, EV mode while letting the engine run in a minimal fuel consumption mode. This means the car can accelerate to speeds of

45-50 mph using just traction battery power for the ~45 seconds it takes for the catalytic converter to become operational. My detailed write up is here:

The practical effect is maximized by:

1) Don't start car until you are ready to drive, no stationary, cold idle. So try to park so you can see traffic before having to start the car. 2) Accelerate to 38-40 mph without pushing the accelerator so the engine spins faster. Watch the energy flow display and make sure the power arrow shows from the traction battery. It should take about 30 seconds to reach 38-40 mph. 3) Shift into "N" to coast in minimum fuel consumption mode, 0.30 gal/ hr, for as long as possible. 4) Shift into "D" if you need to stop or slow down for traffic. 5) Until the coolant reaches 70C (you'll need a OBD scanner like a ScanGauge) shift into "N" whenever practical and safe. Use "D" when slowing or coming to a stop.

Because of this revelation, I'm changing my warm-up protocol including parking practices. Initial data indicates this is making a significant impact but we won't really know until winter weather arrives.

Bob Wilson