Application of Prius driving technique

[bwilson4web]

Hi,

One of the things a Prius is very sensitive about is vehicle load during the initial 5 minute warm-up. To minimize fuel burn during warm-up, I adopted the technique of driving through the neighborhood, posted 25 mph, and shifting to "N" at every opportunity. So I tried that technique with our BMW i3-REx this morning using the 'instantaneous mi/kWh'.

The first observation is shifting to "N" while rolling, the maximum 'mi/kWh' is 19.9 mi/kWh. It doesn't matter the speed but this is largest value ever shown. So this afternoon with the AC running, I shifted into "N" and saw values in the 9-13 mi/kWh, not the 19.9 mi/kWh. What this means is rolling in "N" while monitoring the 'mi/kWh' may provide a coarse metric for overhead functions. It should also work for looking at cabin vs seat heat energy consumption.

I don't know how useful this will be but anytime we stumble across another way to get vehicle metrics ... 'carpe diem.'

BTW, I've noticed the cruise control will often 'trip-out' in the first 5 minutes of driving. I'm thinking turning it on while using my neighborhood coast in "N" technique that might avoid the trip-out by the time I reach the cross-town, divided road.

Bob Wilson

 

[alohart]

One of the things a Prius is very sensitive about is vehicle load during the initial 5 minute warm-up. To minimize fuel burn during warm-up, I adopted the technique of driving through the neighborhood, posted 25 mph, and shifting to "N" at every opportunity. So I tried that technique with our BMW i3-REx this morning using the 'instantaneous mi/kWh'.

The REx engine doesn't warm up whenever an i3 enters Ready state, so why do you think that it would be more efficient to shift into 'N' in an i3 when first driving in the morning? Coasting to decelerate is more efficient than regenerating to decelerate unless one misjudges so much that friction braking is required. One can coast in an i3 without shifting to ''N' by adjusting the accelerator pedal so that the power meter is at 0.

The first observation is shifting to "N" while rolling, the maximum 'mi/kWh' is 19.9 mi/kWh. It doesn't matter the speed but this is largest value ever shown.

I can't recall for certain, but I think an i3 can be set to display metric units. If so, the metric unit for efficiency is kWh/100 km, the inverse of the English unit mi/kWh. The instantaneous efficiency in kWh/100 km would decrease as efficiency increases, so a maximum value would not be reached.

Did you notice that regenerative braking is still available in 'N' when one presses the brake pedal?

BTW, I've noticed the cruise control will often 'trip-out' in the first 5 minutes of driving.

By "trip-out", do you mean that ACC suddenly returns control to the driver (i.e., shuts off)? If so, might the sun be blinding the ACC camera which is in the rear view camera housing?

I always turn on ACC when I first begin driving and it has never suddenly shut off during that time (although it has when its camera is blinded by the sun or fooled by entering a dark underpass under a bright bridge).

 

[jadnashuanh]

An electric motor has minimal friction if it is in the 'coast' mode on the i3 (as controlled by the foot, not the transmission). In reality, there should be almost no difference between neutral and coasting if your foot is well calibrated!

I agree on the ACC...the operator's manual describes the situations where the camera can be fooled, and it will shut off. My guess is that early morning, you're more likely to see the sun at a low angle, and shadows could be darker than they might be at other times...there should be no reason for it to be finicky unless the lens is covered with dew.

 

[bwilson4web]

When I use coasting in "N", I am trying to find out if the instrumentation tells us something new, something that might be useful. In particular it looks like we might be able to quantify the lights, AC and heating loads. In effect we are taking out the traction load so these others can be observed.

In "N" there is no regeneration yet the brakes feel exactly the same. That is a remarkable accomplishment.

Bob Wilson

 

[alohart]

In "N" there is no regeneration yet the brakes feel exactly the same.

Hmm, the last time I tried braking in 'N', the power meter moved into the regeneration region. I can't recall whether that was before or after the November, 2015, software update was installed. Maybe this software update eliminated regeneration when in 'N'.

 

[jadnashuanh]

I have the impression that the motor is always connected to the wheels, and that neutral doesn't really do all that much. Park inserts a locking pawl to prevent things from turning, but if I'm correct, all of those bits are always turning. As opposed to an ICE, reverse does not need a new set of gears, only to reverse the spin of the motor. I've never noticed any gear clunk or other noises when shifting from park to either D or R implying that they are always engaged with no clutches or torque converter. Has anyone?

 

[alohart]

I have the impression that the motor is always connected to the wheels, and that neutral doesn't really do all that much.

That is correct. Much simpler drivetrain than in an ICE vehicle.

 

[bwilson4web]

I didn't think to check the charge-in/charge-out. Will try it later this afternoon. My original testing was on flat land. But later, I came on a Stop sign and the brakes felt normal even in "N" and I was paying attention to traffic.

In the Prius, "N" turns off the power electronics so both motive and regen are disabled.

Bob Wilson

 

[jadnashuanh]

Unless I'm all wet, the Prius in neutral also disconnects the transmission, so you lose the drag from turning most of the gears and engine braking. That is totally not the case with the i3...neutral should be essentially the same as coasting if your foot is well calibrated.

In most states, moving down the road in N is illegal, and can be unsafe. The risk just doesn't seem to me to be worth any marginal increment in range. On an ICE, it also puts extra wear on the clutch(es), whether it is a manual or an automatic.

 

[bwilson4web]

Unless I'm all wet, the Prius in neutral also disconnects the transmission, so you lose the drag from turning most of the gears and engine braking.

Not really. There are two motor-generators when when MG1 has no current flow, it is the same as "N". There is a 'torque limiter' but the clamping ring is bolted in place. It only 'slips' when something bad happens.

. . . That is totally not the case with the i3...neutral should be essentially the same as coasting if your foot is well calibrated.

I use "N" anytime I want a ballistics speed curve. Regardless, I notice initially no change in the 19.9 mi/kWh but heavier braking seems to bring in some regeneration at the end. I'll try to characterize more details later but the charging numbers were small.

In most states, moving down the road in N is illegal, and can be unsafe. The risk just doesn't seem to me to be worth any marginal increment in range. On an ICE, it also puts extra wear on the clutch(es), whether it is a manual or an automatic.

I understand the risks but also grew up driving manual transmission cars that have a clutch.

Bob Wilson