Ravigneaux Gearset

WV mountains are a different animal than Florida hills.

I understand why they don't (world car where sub 2.0L makes more sense from an efficiency and tax perspective), but in a perfect world for me, both the v and the CT would have the 2.5HSD. It is such a flexible, pleasant drivetrain. The cost difference between 40 and 45 mpg is negligible for my yearly mileage and cost.


Tandy 400

 

Quentin – I recognize that in WV, and elsewhere in and along the Appalachian mountain range, hybrid vehicles must operate in cooler outside air temperatures riding on roadways traversing steep terrain. That makes WV very much less “Hybrid Heaven” than driving over the comparatively flatter topography in Florida’s very moderate and mild, “Almost Hybrid Heaven…” year-around outside ambient temperatures.

Considering your WV driving environment, I understand your “but in a perfect world for me, both the v and the CT would have the 2.5HSD” desire. However, since, compared to you, everyone living in Florida, along with the vast majority of drivers across the U.S., are flatlanders, which puts the “world car where sub 2.0L makes more sense from an efficiency… perspective” right on the hybrid money for “Flatlanders.”

As regards a 2.5HSD-powered v-wagon or CT as a hybrid family-hauler transportation appliance, the CT is nice, but a bit on the small side, which causes it to fail my wife’s “Must Be Able To Haul A New Washer/Dryer Home From The Big Box Store” Cargo-Hauling Test. I have already shared my 2¢ worth on how I feel about a thirstier 2.5HSD-powered v-wagon.

 

Yea, I understand why they did what they did. You don't become a profitable company by catering to the fringe and my needs are certainly fringe.

The CT is in fact too small. It was cut off the list after we realized it wasn't much larger inside than the MINI that we drove there for the test drive. We liked it otherwise.


Tandy 400

 

What mpg do you get at 70 mph (GPS mph, not dash) and actual mpg (not dash)? I've been getting ~36-38 in the winter.

As Quentin points out, you live in Florida, so I doubt you see a fraction of the hills I see, and I see a fraction of the hills he sees. Realistically, the 2.5 probably doesn't even fit in the Prius, but I do enjoy a good hypothetical discussion!
You can see the from the BSFC map that the 1.8 hits the 'high torque' zone at ~3650 rpm, leaves the 230 g/kWh zone at 3850 rpm, and leaves the 240 (I guess) g/kWh zone at 4250 rpm.

For the record, you're incorrect. The mpg, both city and highway are due to the v's increased weight and larger frontal area + worse Cd - as I showed earlier. It amuses me when people complain about the v's rated fuel economy compared to the liftback when physics answers the question quite well.

 

JeremyB I think you hit the nail on the head here. The weight and Cd changes don't tell the whole story, the cross section area is also quite a bit bigger resulting in the significant CdA penalty you calculated. This will tend to dominate highway mpgs.

I'm not saying it couldn't be a factor, but the HSD is a very different beast than the engines in the land yachts of yore and their modern descendants. One of the unique features of the HSD arrangement is a much broader peak efficiency range on the efficiency map, and the fact that it can stay in that peak band under a very wide range of operating conditions.

Looking at the BSFC map and standard operating line for the Gen 3 2ZR-FXE engine, it maintains stays within ~5% of peak engine efficiency from idle to >4000 rpms or from <10kW to >50kW output. Because of the the variable gear ratio inherent to the HSD, the operating point is a simple line that you move back and forth depending on load / power. IE if you press down on the gas to get more power, the rpms are increased to generate the amount of power requested.

Here's another version that includes the power curves.


The situation is significantly more complicated with a traditional engine / transmission. For example:


So, first thing to note is that peak efficiency is lower, and the peak efficiency range is much narrower. In this case 1000 to 4000 rpm at the most efficient torque varies more than 11% from peak efficiency, with the 5% window only being about a 1500 rpm window "if" you could just simply move left to right on the chart near the peak efficiency torque as the Prius mostly does. Second, you have fixed gear ratios so there is a fixed relationship between speed and rpm in a given gear. At a given speed you can only move left to right on the chart by changing gears in fixed steps. As you press on the gas, you can only move up and down the chart vertically. I believe all of the points on the chart refer to a single speed condition, at various gears and throttle levels. So you can see trying to hit the sweet spot is a lot more tricky. You can also see that with light loading, even at steady flat highway speeds, if your final drive ratio is too high you tend to end up in the bottom right quadrant of the chart. This is a very inefficient place to operate, and can put you at 50% of peak efficiency or worse.

This is why overdrives made such a difference on the land yachts, pulling the highway cruising operating point back up and to the left toward peak efficiency. You'll notice that on the Prius, this area does not really exist as a valid operating region on the BSFC map. Unless you are driving under conditions that routinely put you over 4000 rpms on the ICE, the impact on engine efficiency is going to be pretty small.

I think you can really see how all this comes together when you look at the following plot, which shows the toque / rpm location on the bsfc map for a Saturn 1.9L through the duration of the city driving cycle test. As you can see, in a real life-ish driving condition the engine gets to spend very little time near peak efficiency.


While in a different format, this data that Bob Wilson measured in his 2003 / Gen 1 on a long trip shows just how much time the Prius spends near peak efficiency in real world operation. The units are upsidedown relative to the BSFC, so in this case higher numbers are more efficient. The Gen 3 should look even better.


Sorry for the length, hope it helps

Rob

 

FWIW here are the flat steady state speed numbers I get when I plug the weight, CdA, and tires on the liftback vs. v into evcalc:

Speed : HP required liftback : HP required v : v mpg penalty (assuming constant efficiency)
40mph : 7.995 : 9.277 : 13.8%
50mph : 12.36 : 14.54 : 15.0%
60mph : 18.29 : 21.76 : 15.9%
70mph : 26.11 : 31.35 : 16.7%
80mph : 36.14 : 43.67 : 17.2%

So as was stated above, simple physics can easily account for most of the 20% difference in highway mpgs. The remaining 1-2mpg may be related to other factors, or may just be that the EPA highway cycle is still more complex than just simple steady state speed and accelerating a heavier car is going to burn more fuel. So it would seem that the drive ratio can't be having more than this 1-2 mpg imapct, and is likely less.

Its also perhaps worth noting that at 80mph the v needs 43.67 HP cruising power, which translates to 32.56 kW @ 4023 rpm which looks to be just on the verge of leaving the peak efficiency zone of the BSFC chart. The liftback at 80mph requires 27kW @ 3650 rpm, so at 80mph and above the final drive ratio may start to have a more significant imapct on engine efficiency.

Rob

 

Quentin, JeremyB & miscrms – Thanks for the interesting and thought provoking discussion, opinions, calculations, and information.

I agree “it's just physics,” but so, presumably, are the details of the causal factors purported to be producing the “Global Warming” phenomena, now renamed to “Climate Change” to follow and match the uncooperative actual global temperature and weather data trending and facts. The question of “How many angels can dance on a head of a pin while juggling RR, CdA, Curb Weight, Final Drive Ratio along with a 1.8HSD?” will remain unanswered opinion and conjecture, until what is known by those in the know at Toyota Town, Tsutsumi decide to go “Open Kimono.” Quote Sonny & Cher “… And the beat goes on… the beat goes on… La de da de de, la de da de da… The beat goes on and on and on…”.

Hope springs eternal…I hope Toyota works some more “Techno-Design Magic” to bump up the Next Gen/Gen2 v-Station Wagon’s fuel economy a skoshi bit more. They are “Whispering” an 8% bump up in fuel economy for the Next Gen/Gen4 Prius HB Sedan running with the 1.8-Liter I4 HSD. A 5%-10% increase would be very welcomed in the Next Gen/Gen2 v-Station Wagon. However, until then, I expect Toyota is happy with the current 44/42/40 EPA MPG numbers, and thinks the MPG-FE numbers are high enough to keep the v-Station Wagon marketable to the typical “I just drive it” driver-buyer who reads the Monroney Sticker when they make their buying decision, but than does not actively track their actual, running fuel economy after driving their new v-wagon off the lot.

I actively used the OEM performance readouts and instrumentation in my 2012 v-wagon to milk MAX MPG out of it. I like the 2012-2014’s Multi-Information Display, find its functionality to be “Quick Glance & Easily Readable While Driving,” very useful, and I use it to measure, monitor, match, and compete my “Hybrid $mart” driving performance against myself in maintaining, pursuing, and achieving ever increasing fuel economy that continues to surpass Official EPA MPG by upwards of 25%.

Therefore, I was very disappointed to see the direction Toyota took with the 2015 v-Station Wagon “Refresh” in the way of consolidating various displays into the new 4.2-in. TFT Multi-Information Display (MID). They reduced what information can be viewed simultaneously, how information is viewed-displayed, and what driver action must be taken in order to display information on the MID instrumentation. Display readouts that are important to me were moved into the 4.2-in. TFT MID. Whereas, the Hybrid System Indicator (HSI) and Dynamic Radar Cruise Control (DRCC) in the 2012-2014 v-wagon were dedicated, full-time displays, in the 2015 v-wagon they were rolled into the 4.2-in. TFT MID, whose size makes it a bit of an “Eye Test.” The 2015’s HSI, DRCC, LDA readouts are shared, sequentially on the 4.2-in. TFT MID, as sequenced and manually selected by the driver on the steering wheel button controls.