How would Toyota improve highway consumption?

The Prius doesn't use the battery much during highway driving unless you are braking a lot. Increasing the size of the battery and using it for warp stealth won't improve highway fuel economy for the average driver, since all power comes from the ICE and burning gasoline.

At low speeds we use Pulse & Glide to make the ICE run more efficiently. At highway speeds, the Prius ICE should be running very efficiently, so we do not need to use P&G -- although occasionally implementing P&G according to traffic and terrain will bring a benefit. The dominant factor is rolling resistance and air resistance.

The four main ways of engineering greater highway fuel economy will be increasing aerodynamics, decreasing weight, increasing thermal efficiency, and decreasing rolling resistance.

There isn't a single Prius to date that has active shutters, so I'm thinking Toyota is reserving that feature in the future and not implementing as long as it is the MPG king.

 

At highway speeds, any battery is dead weight, unless it is charged from the grid and contributes enough sustained power to keep the engine running at optimal RPM. Compare Gen 1 Honda Insight to Gen 1 Prius, where the 2x larger battery in the Prius does not compensate for its added weight.

It will be difficult to make a marked improvement on aerodynamics or rolling resistance. The two areas for improvement would be engine efficiency and extra energy storage (ala Prius PHV)

 

I'm sure the engineers would love to see active aerodynamics used in their creations, but it's a matter of cost. Perhaps there are ideas 'in reserve', but not likely because Toyota is 'sitting on its laurels' in any way.

On the straight and level, maybe, but coasting down long hills makes a huge difference to real-world mileage. Where I live, the hills are long and steep enough that the car would benefit from whatever battery capacity it could be stuffed with.

 

For normal highway driving, a larger battery would hurt mileage. At highway speeds aerodynamic drag consumes the majority of the energy, so the ICE can run steadily at a reasonable power level. The extra battery would just be dead weight.

Using the current Prius HSD as the baseline, highway mileage could be improved by the following:

1) Improved aerodynamics. Most likely this would include adjustable suspension that would lower the car at highway speeds.

2) Direct coupling of the ICE to the drive wheels. The electrical path, including MG1, MG2, and the HV battery, is inefficient. For general conditions it is more efficient than the alternatives, but at steady highway speeds it is wasteful.

3) Higher glide speed. At highway speeds the current HSD must spin the ICE whether needed or not. Gliding efficiency would be improved if this was not necessary.

As noted by Hyo, it is different if you are driving through the mountains or doing "city" type highway driving, where traffic speeds up and slows down.

Tom

 

Going to direct injection would offer ways to increase thermal efficiency of the engine, which would help everywhere.

 

But 'normal' for me IS lots of up and down. Defined as 'flat', then your statement is perfectly true.

And, not to quibble, but it's the extra weight of more batteries that would be the problem, not necessarily the increased capacity. Lighter cells, and more of them, would be a good thing.

I agree with you on the adjustable suspension. More ground clearance at times would also be very useful, in addition to lower ride height for improved aerodynamics. Again, this is probably a cost consideration, and possibly a warranty issue as well. As for weight, coil springs are fairly heavy, so an air suspension with an on-board compressor and the requisite 'plumbing' may not end up being a weight penalty.

 

The Gen III uses a larger ICE and gets better highway MPG. Where the "sweet" spot for Gen II might have been 55 mph it's moved to 65 for the Gen III.


The biggest factor for high-speed highway driving is aerodynamic drag. Drag is a function of the square of the speed.

 

Because my commute consists of a 1200ft. elevation gain I can agree with ystastino and Hyo. On my long downhill section of highway I often drop below the 57.5% SOC threshold and the ICE comes on and mpg goes from infinite to about 300mpg. So a higher capacity battery would allow for longer glides in hilly terrain. I understand there is no free lunch but it seems like the longer gliding potential would trump the energy required to recharge the battery.

I acknowledge that not everyone has a commute like mine.

 

The current ICE max efficiency is about 38%; whatever the average efficiency is in highway driving defines the possible improvement gap using current ICE technology.

BTW, Toyota has said that direct injection (DI) is the next step in ICE efficiency. IIRC they are talking about 45% at best.

So far as aero drag goes, it is going to be hard to improve on the Prius shape. Perhaps a more strreamlined underbelly ? Of course a smaller car might have less cross sectional area.

 

When traveling uphill, the ICE expends extra energy to lift the weight of the battery. Seriously guys, the ONLY benefit to having a traction battery NOT charged from the grid is to capture braking energy.

 

I thought a test was done with a GII Prius where they doubled the battery capacity. It resulted in no gains and perhaps a loss, at least as I recall.

It would be difficult to improve aerodynamics without making the car less "nice" (duck to enter, etc).
Weight loss is possible, at a cost. How much do you want to spend?
Thermal efficiency gains are also possible. Again, how much do you want to spend?

 

I think there's another benefit in that it allows the use of a smaller engine and allows it to be used in Atkinson mode. Without the battery to kick in the engine doesn't have enough power to accelerate to highway speeds fast enough to be safe. The battery has allowed them to fit an engine for the average power requirement instead of the peak.

 

For one the Gen4 is going to have more efficient ICE; stage 1 is supposed to have thermal efficiency of 42% (actual test results 43%) and stage 2 efficiency 45%. Current Gen3 has efficiency 38%

At present the electric output is limited by battery, but not by capacity by discharge rate. Higher discharge rate will allow to P&G at higher speeds (current ceiling ~47MPH) and could improve MPG at 65MPH.

 

disagree; if you look at new Sonata, Camry hybrid, new SkyActiv Mazda3 they have Cd 0.27 w/o going to Prius shape or active shutters. The EV1 had Cd=0.16, GM Precept Cd=0.19.

There is room for improvement: active shutters, removing side mirrors, hiding wipers, etc. Below some info on low Cd cars

Top to bottom: Ford Probe V, 1985 (Cd .137), DaimlerChrysler ESX3 (Cd .20), Ford Prodigy (Cd .199), GM Precept, 2000 (Cd .16), DaimlerChrysler Bionic Car, 2005 (Cd .19)

 

In my best Crocodile Dundee voice, that's not streamlined. THIS is streamlined...

...a little impractical for the average commute, maybe, but it also shows there's always room for improvement. This particular bike is the current world record holder, at 82 mph. Yes, I said bicycle.