Sharp MPG drop at higher speed

[cyclopathic]

Car: Prius C.

this is cross-post from PC, so bare with being fragmented

Original:
was traveling last weekend. After stopping for gas and pulling back to interstate, "from the start" MPG was holding on at steady 38-39MPG. This is on flat surface with cruise set at 73mph, ECO on and instant consumption staying in green zone.

and it was like that for 1.5-2hr, until we pulled to lower speed limit zone and dropped speed. Gen3 in such situation would get 6-8MPG more.

It normally gets 58-60MPG in commute.

[cyclopathic]

So I hooked TorquePro and looked what is going on.

at 73-75MPH RPM fluctuate in 2,200-2,600 range
at 62-63MPH RPMs are in 1,100-1,900 range.

Looks like Toyota cut costs by simplifying Prius C intake valvetrain, and it is not optimized for higher RPMs.

So increased aerodrag is getting ICE out of optimum range and causing sharp MPG drop. I suppose aeromods and better LRR tires are needed if you want to improve MPG.

EDIT:
From read up: Prius C is equipped with VVT-i on intake, which is similar to Honda VTEC in the sense that it switches from one cam profile to another and it looks like at higher lobe ICE isn't as efficient. From observing RPMs engine likes to hang onto one or another RPM range, so ECU is probably programmed to minimize switching.

[Daox]

I don't think this has to do with cams or intake stuff. VVT-i doesn't have separate lobes, all it does is does is alter cam timing.



Looking at the 1NZ-FXE bsfc chart (from a 2nd gen Prius, I know it was updated for the Prius C but it should be similar), the engine probably runs more efficiently at the higher rpm. The lowest fuel consumption per amount of power developed is from 2500-3500 rpm. However, that higher rpm almost directly correlates to more power use. So, we can fairly safely say that going from 1100 to 2200 rpm really means you're using close to twice the horsepower to push your car down the road. This could be due to the higher speed, temperature, wind, inclines, road conditions, accessory use, etc.

A Gen 3 would definitely get better mileage simply due to being more aerodynamic. I'm not sure about 6-8 mpg better though. Plugging the numbers into our aero & rr calculator, it looks to be a difference is more like 1-2 mpg.

The best way to increase mileage at those kind of speeds is aeromods hands down.

[cyclopathic]

Originally Posted by Daox

Looking at the 1NZ-FXE bsfc chart (from a 2nd gen Prius, I know it was updated for the Prius C but it should be similar), the engine probably runs more efficiently at the higher rpm. The lowest fuel consumption per amount of power developed is from 2500-3500 rpm. However, that higher rpm almost directly correlates to more power use. So, we can fairly safely say that going from 1100 to 2200 rpm really means you're using close to twice the horsepower to push your car down the road. This could be due to the higher speed, temperature, wind, inclines, road conditions, accessory use, etc.

A Gen 3 would definitely get better mileage simply due to being more aerodynamic. I'm not sure about 6-8 mpg better though. Plugging the numbers into our aero & rr calculator, it looks to be a difference is more like 1-2 mpg.

The best way to increase mileage at those kind of speeds is aeromods hands down.

Gen3 is definitely getting higher MPG at that speeds, speaking from 1st hand experience. Though it is more of the anecdotal evidence, never ran back to back tests to prove it. BUT many hours at +5MPH on 70/75/85mph interstates.

I suspect that Toyota was too optimistic and the actual Cd of Prius C is higher than 0.28, or C ICE was optimized for different range. The compression ratio is different, 1:13 vs 1:13.4

here is more on difference btw Gen2 and C variants:
https://en.wikipedia.org/wiki/Toyota_NZ_engine#1NZ-FXE

[jamesqf]

Originally Posted by cyclopathic

at 73-75MPH RPM fluctuate in 2,200-2,600 range
at 62-63MPH RPMs are in 1,100-1,900 range.

Huh? Unless it's downshifting to make more power at higher revs, in order to overcome the extra drag of the higher speed?

[cyclopathic]

Originally Posted by jamesqf

Huh? Unless it's downshifting to make more power at higher revs, in order to overcome the extra drag of the higher speed?

It is eCVT so it is not really downshifting. According to calculator above the load should have gone up by 25%, and if you consider torque to be constant RPM (load) had gone up by ~50%. So the possible explanation that either ICE is less efficient at higher RPMs or Cd is higher than 0.28.

[cyclopathic]

@Daox
I was checking RPMs on a way home (took secondary roads) 53mi 40mph average, 64.3mpg. RPM got over 2,000 only handful of times. For 1hr20min drive maybe 40-60sec in total. There is no doubt C engine optimized for lower RPMs than Gen2.

[jamesqf]

Originally Posted by cyclopathic

It is eCVT so it is not really downshifting.

I'd say it's downshifting, just with infinitely-variable gear ratios. Really no different than when I used to drop the Insight into 4th to keep up with 70+ mph traffic on the long hills on I680 in the East Bay.

[cyclopathic]

Originally Posted by jamesqf

I'd say it's downshifting, just with infinitely-variable gear ratios. Really no different than when I used to drop the Insight into 4th to keep up with 70+ mph traffic on the long hills on I680 in the East Bay.

If you mean it is changing ratio then it is. Question is why so much loss in MPG? Eco calculator estimates ~25% loss in MPG. Real life data and RPM suggests more by higher margin.

[cRiPpLe_rOoStEr]

A simple rear spoiler might already lead to a good improvement in highway mileage.