Sharp MPG drop at higher speed
 

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.

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.

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.

http://ecomodder.com/wiki/images/d/d...prius_bsfc.jpg

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

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.

@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.

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.

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

It already has one.

I would say Toyota optimized it for the EPA test, real world be dammed. The EPA test doesn't cruise at freeway speeds. I really wish they had a EPA test that showed what a car used on flat ground, standard atmosphere pressure and temp, at a steady 70 mph. I guess it hard to believe some people actually do just get on the open freeways of America and drive for hours until they have to pee.

Hi cyclopathic,
What's the problem? If you take a load (power) increase of 25% and a speed increase of about 20% You get a work increase of 50% which should be the same as the rate of increase of fuel consumption, if the engine efficiency is unchanged.
-mort

I punched the numbers into recommended calculator 2800lbs, ,28 Cd, .31 engine, .92 transmission efficiency, etc and calculator shows 53 at 65 and 42 at 75. Test snows 53 at 65 and 38 at 75 consumption. Calculated drop is 21% and actual is 28%, 1/3 worse.

Clean MPG'S steady state MPG test on the Prius C-3. Pretty cool stuff.


CleanMPG Forums - View Single Post - Steady State Speed vs Fuel Economy results

Uhh...the Prius C uses the 1NZ-FXE, exactly the same powertrain as the 2nd gen Prius but with worse aerodynamics. If I am not mistaken about my Prius generations, the 3rd gen uses a 2ZR-FXE which is a bit more efficient.

The 2ZR has lower valvetrain friction than the 1NZ, dual VVT-i, and they were probably more aggressive with the cooled EGR flow. It's only ~4% more efficient though.

The only reasonable explanation is that your car was simply consuming more power. Maybe there was a bit of a headwind. As you can see from the BSFC chart that was posted, the mid 2000rpm range is where the efficiency peak occurs.

I don't own a C, but the MPG sounds in the ballpark for the speed traveled considering we don't have enough info on the other variables (elevation gain, wind speed, temperature...). My gen III probably averages 45 MPG at that speed.

My Gen3 had done ~48 there but this is with Energy Savers.

There is no elevation gain on that road, and trees along to block most of the wind if there were any.

Issue isn't that C is getting bad mileage, it is worse than it should be when explained by aerodrag alone.

The fact that a CVT does the thinking for you doesn't change what it's doing.

Changing ratio is... shifting.
Changing to a lower ratio is... downshifting.

People get so wound up about scary things like clutch pedals that they forget what's actually happening.

numbers
 

There will be a normal seasonal variability to mpg with any vehicle.
There will be 'cold-start' mpg variability vs 'hot-start'.
Steady-speed mpg is evaluated with a fully-warmed vehicle,after 30-miles at 50-mph.
Journalists who've done long-term tests with the Prius C report up to 58-mpg.

Well I guess we can rule out bad gas since you got the better mpg when you slowed down..
If it wasn't the wind, then it must be something mechanical..

Or wait a second.. Did the mpg go back to normal when you went into the slow zone?

Found this graph posted by Bob:
http://hiwaay.net/~bzwilson/prius/edmunds_020.jpg
the Gen2 seems to be showing exactly the same pattern. Most likely the same combination of increased aerodrag due to increased speed and reduced engine efficiency at higher RPMs

The EPA could easily require automakers to pull steady state MPG numbers off their dynamic runs used to get sticker info. They could get them for 10 mph increments from 50-80 to help show people the advantages of slowing down and also what cars might be better suited for long distance, high speed interstate travel. Old Mechanic posted a paper from 1975 showing that info, with today's computer programs they could do it with every car with little extra effort.

I'd expect car manufacturers would resist it on the grounds of making certification more expensive or in case of vw cheating cover-up. Interesting to see to what extent gearing/aerodynamics are optimized for epa test procedures, prius is no exception.

That one-size-fits-all test standards are ridiculous. It seems to have been there to give automatic transmissions an advantage.

I don't think it was intended that way back in 70s when procedures were established, but current generation of drive by wire software controled cars takes advantage of it.

You could argue that CVT equipped car has taller final ratio and can be programmed to lag engine at low RPM/low intake vacuum to reduce friction and pumping losses which would help economy.

MPG is not linear, and the actual rate of consumption over a set distance must be used. The calculated drops are much worse than you realize.

For example, a drop of 5 MPG is meaningless without knowing what the start and end MPGs are. A 5 MPG change from 25 MPG to 20 MPG is an increase of 1 gallon of gas per 100 miles (A change from 4 to 5 gallons = 25%). Another 5 MPG change from 10 MPG to 5 MPG is an increase of 10 gallons of gas per 100 miles (A change from 10 to 20 gallons = 100%).


For your numbers:
53 MPG drop to 42 MPG increases fuel consumption by 0.494 gallons per 100 miles (26%)
53 MPG drop to 38 MPG increases fuel consumption by 0.745 gallons per 100 miles (39%)

Gallons per 100 miles = 100 / MPG

53 MPG = 1.887g /100m
42 MPG = 2.381g /100m
38 MPG = 2.632g /100m

So got to travel on the same road in very similar conditions (load, temperature, wind, etc). Once again using cruise control to take driver out of consideration.

Computer calculated MPG was 41.4 vs 38.7 last time, this is average over almost an hour (had to abandon test after ~50mi as we ran into construction zone with lower speed limit). 7% improvement.

Difference? A set of EVO vortex generators installed alone the roof trailing edge.

I am building the underbelly panels when it warms up.