Above 46 mph, downgrade, in "N" vs "D"

So I was curious Saturday evening and descended Brindly Mountain 525 ft. (160M.) 8% grade. I started parked and accelerated to 25 mph. While still accelerating, I shifted into "D" which left the engine running at 0.20 gal/hr. Within 100 yards, the crest of the hill began and car started picking up speed, pretty quickly.

Once the car exceeded 50 mph, the engine continued to run. So I started shifting between "D" and "N" and observing the gal/hr and instant MPG:

• "N" - ~1,100 rpm, 0.20 gal/hr, 300+ MPG
• "D" - ~1,100 rpm, 0.02 gal/hr, 9999 MPG
• 50-75 mph speed range - it was starting to get a little too exciting as there is a curve at the bottom with freshly replaced guard rail.

When in "D", some power from MG2 is used to spin the engine without fuel. There is a little drag but nothing compared to the energy available descending an 8% grade. When in "N", the engine runs at idle with less drag on the car but burning 0.20 gal/hr. In either mode, the fuel burn or kinetic/potential energy is pretty much 'in the noise.'

What surprised me was the engine speed stayed in the 1,100 rpm range regardless of speed. I've seen our older, 2003 Prius, engine increase the rpm descending the same hill in "N" as the speed builds up.

Bob Wilson

 

Yes, this is a hill where normally "B" is and should be used. But the primary goal was observing the fuel flow and instant MPG on the Scangauge which has four digits and fuel flow per hour.

I was surprised that the ICE rpm did not change with velocity on a downgrade when above the hybrid threshold speed. Our 2003 changes the engine speed as a function of velocity on the downgrade even when in "N". The engine speed increase keeps MG1 rpm under 6,500 rpm, the maximum rpm. I was expecting similar behavior with the 2010 at a higher speed but that never happened.

I need to repeat the test with a data scanner recording MG1 rpm to how fast it spins. The NHW20 had a limit of 10,000 rpm but I've not seen a limit for the ZVW30.

Bob Wilson

 

Above 42 mph, there is a tradeoff:

• "D" and speed decreases - shift into "N" as the loss in kinetic energy is to be avoided. The small amount of fuel burned in an engine at idle going down a grade is not enough to get the lost kinetic energy back (aka., the speed loss.)
• "N" and speed increases - shift into "D" or "B" as the higher speed increases drag losses. There are grades too steep to coast down in "N" either safely or otherwise.

Below 42 mph:

• Engine running - shift into "D" long enough for engine to stop and back to "N". A stopped engine does not burn fuel.
• "N" and speed increases - at over 42 mph, shift into "D" and follow the "Above 42 mph" rules.
• "D" and engine running - either in warm-up mode or traction battery low or something wrong.

The highest priority should be constant velocity (aka., kinetic energy). The only exception is on a downgrade if the speed limit is about to be busted, shift into "D" and/or even "B" to keep the speeds safe. Running off the road or getting a traffic ticket wastes way too much gas.

Bob Wilson

 

Actually I think the "0.02gal/hr" is an artifact of the Prius engine control laws. I suspect Toyota uses that value to avoid a divide by zero fault when trying to calculate "miles / gallon." But it looks like in the ZVW30, they are perfectly happy to report "0.00gal/hr" when:

• speed > 42 mph
• engine rpm > 0
• regeneration_energy > 0

This is an obscure area of the control laws and I suspect (have not done the experiment!) that regeneration at high speed uses some of the captured kinetic energy to spin the engine but I find it hard to believe no fuel. The reason why is ambient air passing through even a closed throttle plate will have a quenching effect on the exhaust manifold, O{2} sensors, and catalytic converter. I find it difficult to believe that these necessarily short duration (unless starting at the top of a tall mountain) events really do just turn off the fuel and pass cooling, ambient air through the engine.

But we're talking about fairly rare and short duration events. Perhaps if I lived in the mountains or commuted over mountains, I might have a different point of view. But I have to go out of my way to find even a 500 ft change of altitude. I burn more gas just getting over and back, not to mention the time, for what in my case is about a minute of test data.

Now if we could just find someone who lives in West Virginia, Colorado, or parts of California, Oregon, Washington, or Idaho with adequate instrumentation and curiosity.

Bob Wilson