These driving techniques can reduce but not eliminate the warm-up fuel burn of the 2001-03 Prius, NHW11. Unique to North America, the 2001-03 Prius has a hydrocarbon (HC) emission system that requires a 70C coolant temperature before fuel efficient, hybrid operation can begin.

ROLLING WARM-UP

There is no advantage to sitting in "P" to warm-up the engine. The ICE will warm-up quite nicely on the road and this will improve mileage. But during warm-up, the ICE will run a rich mixture and idle with a significant rate of fuel burn. This fuel burn can be cut in half by putting the car in "N" at most opportunities.

Putting the car in "N" on the driveway with the parking brake set will reduce some of the earliest fuel burn. Unfortunately, it also eliminates the engine auto-shutdown when it reaches 70C. If you do warm-up the car on the driveway (due to snow or cabin warm-up,) use "P" and let the engine auto-shutdown five minutes later when it reaches 70C.

The best approach is to drive 1-2 miles through a neighborhood at speeds of 25 miles per hour or less. When practical, slip the car into "N" to cut the fuel burn in half and coast to a slower speed, traffic and road conditions permitting. If you need to stop, slip back into "D" to use regenerative braking and then back into "N" when halted.

USING "N" AT STOPS

If you are stopped at a light or by traffic during the warm-up, slip into "N" and within about 3-4 seconds, the rate of MPG loss will cut in half. You can monitor this by watching the MPG count down and see that within 3-4 seconds, the rate slows down. If you put the car back into "D," the MPG count down rate will increase even though the engine noise does not change.

USING "N" TO MONITOR WARM-UP PROGRESS

Once the blue "cold" light goes out, the ICE still needs to continue warming up to hybrid mode. But there are two critical speeds, 23 miles per hour and 17 miles per hour, that can be used with the instantaneous MPG to monitor progress.

At 23 miles per hour and in "N," getting 100 MPG on the instantaneous display means the engine is still too cold to go into hybrid mode. The ICE will not auto-shutdown and fuel consumption in "D" and "P" will be almost twice that of "N". Keep driving and coasting in "N" as much as is safe and practical.

At 17 miles per hour and in "N," getting 100 MPG on the instantaneous display means the engine is warm enough to go into hybrid mode. Thereafter, do not use "N" because it inhibits completing warm-up. Just drive the car.

35 miles per hour AUTO-STOP; STOPPED AUTO-STOP; AND BATTERY CHARGE

Once the engine is warmed up enough, the 17 miles per hour "N" test confirmed, driving to at least 35 miles per hour will enable engine auto-stop until the next physical stop. Higher speeds will now be fuel efficient but you need to reach 35 miles per hour to enable engine auto-stop, a partial hybrid mode.

Should you come to a stop while the engine has auto-stopped in this mode, only reaching the 35 miles per hour threshold will let the engine again auto-stop. If you go less than 35 miles per hour, the engine will continue to run and run and run, not a good thing.

The final and best threshold into full hybrid mode is to pull out from a stop to get the engine running and without going faster than 35 miles per hour, come to a complete stop and wait 4-5 seconds for the engine to auto-stop. This last threshold puts the Prius in fully automatic hybrid mode.

Sometimes, I've pulled into a parking lot not only to let following traffic pass but to come to a full stop for the 4-5 seconds it takes for this state-change to occur. Ken@Japan described these two warm-up threshold events and they work in the North American, 2001-03 Prius after it reaches the 100 MPG indicated, 17 miles per hour "N" state.

In cold weather or after a long shutdown, the battery state of charge may require engine charging before letting the auto-stop, full hybrid mode work. After the 17 miles per hour "N" test, it is perfectly fine, even desirable to drive at moderate speeds, 30-68 miles per hour, to let the engine charge the battery to the optimal 60% state. Moderate speeds reduce the battery charging overhead to a fraction of the total MPG.

Never forget that the full-stop, auto-shutdown only works if the car comes to a stop with the ICE running. If you've reached speeds above 35 miles per hour, a temporary auto-shutdown can let the engine stop before halting. The ICE must be running with the car stopped for 4-5 seconds to fully enter the hybrid mode.

IMPACT OF THESE DRIVING TECHNIQUES

These techniques bring my morning commute MPG from 51-53 MPG to 56-58 MPG. However, they are somewhat involved and distracting. Mastery takes practice in controlled environments, not when in a rush to get somewhere. Drivers who are used to manual transmissions seem to handle this technique best. Take your time to master the technique by working on just one part at a time. It needs to be easy, not some distracting menu taped to the visor (or windshield!)

A better solution is to use an engine block heater or thermistor hack to shorten the warm-up time to 17 miles per hour "N" mode. But when I forget to plug-in the heater or don't run with the thermistor hack, these backup driving techniques help.

These techniques have not been tested with the 2004-current and non-North American Prius so we don't know if they are effective in the other models. However, the other models have the advantage of allowing auto-shutdown at lower coolant temperatures (estimated to be 40-50C.)

Comments? Questions?

Bob Wilson

 

I'm guessing that a lot of the general principles should apply to the TCH, if not the specifics.

 

I have no way of testing a TCH. I learned these tricks from others and my own testing and testing is how I'd approach the TCH.

Bob Wilson

 

One note as this has come up alot in the FEH forum, in some states driving in neutral is illegal...

...rapidly approaching 100K in our 2002 Prius...