Toyota Prius Plug-in – Two Weeks of Test Driving

Toyota Prius PHVBy Tom Bartley (3/24/11)

My two weeks were up yesterday and I had to pass the car to the next Clean Fuels Coalition board member. I liked the experience and it saved me money. This dual fuel (electricity and gasoline) Prius is all about having a nice ride and being energy efficient. The car I drove was a Toyota factory prototype that was essentially a 2010 basic Prius modified to accommodate a 5kW Lithium ion battery that could be charged through a power cord from an external 110 VAC standard 20 amp circuit.

I liked the car. If you are not a numbers person, skip to the Nice Ride section.

When I plugged the charging cable into my external 110 VAC house socket it tested out ok, but after 5 minutes of charging it tripped the GFI breaker and it wouldn’t reset without tripping.  I tried a different circuit and everything was fine. I have not yet diagnosed the problem. The current draw was 12 amps leading to 3 hours for a full charge (3.96 kWh). This is less than the full battery 5 kW capacity and is probably part of the battery management strategy that stays away from the top and bottom SoC (State of Charge) to assure a long battery life before replacement. The dash board display indicated 14 miles as an estimated average full usable charge EV range. (The 14 mile EV range estimate could vary significantly depending on elevation change, speed, and driver style.) This works out to 3.54 miles/kWh or 283 Wh/mile. The larger battery capacity in the Plug-in Prius makes the normal hybrid mode even more efficient by providing more storage to recycle deceleration energy while going down grades or slowing from high speed driving.

This compares to manufacturer estimates of 240 Wh/mile for the Nissan Leaf and 400 Wh/mile for the Chevy Volt. While the larger capacity battery packs in the Leaf and the Volt qualify for a $7500 federal tax credit, this Prius is projected to have a $3000 tax credit.  I estimate the price of the Plug-in to be somewhere in the $33,000 price range to compete nicely with the Volt.

Energy Efficiency

I managed to drive 423.1 miles on the 6.762 gallons of 87 octane E10 that I pumped into the gasoline tank. With the current high prices, I paid a total of $26.67. So, here are the petroleum numbers:

  • 67.52 mpg
  • 6.3 ¢/mile

The electric numbers will only show up integrated into my overall utility bill at a cost of 15.5 ¢/kWh. I estimate the efficiency of the car, including the battery losses, at an average 300 Wh/mile leading to an electric cost of 4.65 ¢/mile. How does this add to my total cost/mile? Most of my trips were less than 15 miles. I had one long roundtrip over 75 miles of mostly high speed freeway driving. I can only estimate that 20% of my miles were electric. The onboard display indicated that 12% of the previously driven 12,000 miles were driven in EV mode. Adding (20% of 423.1 miles) 84.62 miles @ 4.65 ¢/mile results in $3.93 for electricity yielding a total cost of $30.60 for 423.1 miles or 7.2 ¢/mile. In comparison, my Toyota Sequoia SUV at about 29 ¢/mile ($4.00/gal / 13.6 mpg). That makes for a whopping $92.00 fuel savings over two weeks! That’s some nice extra pocket money. True, this is not the whole picture and did not include purchase price and maintenance costs, like the cost of the batteries, but the immediate impact is significant. Also, knowing that my cost was less I drove more miles than I otherwise would have.

Side Note: I found the fuel economy displayed and calculated by the on-board computer for each car was optimistically overstated as 77.7 mpg for the Prius and 15.6 mpg for the Sequoia. One mitigating factor is that I may not have received a full tank of gas with the Prius. The odometer should be accurate because, according to the owner’s manual, the Prius automatically calibrates the odometer (using the GPS navigation system?) to compensate for tire wear.

Nice Ride

Overall, I liked the Plug-in Prius and didn’t want to give it back and I love the power of my 2004 Toyota Sequoia Limited and a 1970 classic big block Chevy Corvette, both with their high power V8s and plenty of “go-power” torque throughout the driving speed range. The Prius Plug-in compares favorably with my many ride-n-drives in electric vehicles, hybrids, and high priced hydrogen fuel cell hybrids.

For the driver’s pleasure the Prius has push button selection of three different driving modes, ECOnomy, normal, and PoWeR. At first these different settings seemed to be scaling the accelerator pedal movement to better match the drive style, i.e., more push to get smaller acceleration in ECO, and less push to get greater acceleration in PWR. However, the actual driving experience felt like the PWR mode actually allowed the drive system to put more torque into the drive wheels to the point of spinning the tires with engine and electric motor combined. Knowing that electric motors produce max torque right from the “get go”, I expected more performance of the line without much push on the accelerator, but, because I wanted to stay in EV mode without starting the engine, I didn’t ask for more start up acceleration by jamming the pedal to the floor.

The PWR mode was fun to drive in urban traffic congestion at all speeds. The normal mode was comfortable in almost all driving environments easily keeping pace with other standard 4 cylinder and 6 cylinder gasoline engine cars, especially in the 25 to 50 mph speed range.  I wouldn’t recommend the ECO mode in heavy traffic. The ECO mode was there to sooth my energy conservation conscience when I was sharing the road with only a few other drivers. Cruise control is available to make it even easier.

This car has a sports car feel and it’s easy to parallel park. The ride is smooth, the suspension is tight, and the steering is responsive.  Braking is very responsive.

The gear shift lever has a D for normal forward movement and a B to select more coasting drag. I think the B should be the default position because it felt like I was able to capture more of the deceleration and braking energy into the batteries for recycling. I also liked the B more positive control feel of the car before I had to use the brake pedal.

Rather than a “gear shift” lever it is really a joy stick that always returns to a fixed center position. The position of R and D was a safety problem for me. The position of R at the top or forward and D at the bottom or back is traditional for many transmissions’ gear selection. However, because the control lever is on a raised center console deck and almost horizontal it felt more like a joy stick where DRIVE would be forward and REVERSE would be back. Several times I selected the wrong direction while backing out or parking. Fortunately, it was at low speed and the annoying repetitive beep of reverse provided a helpful alert

Driver Displays and Steering Wheel Button Controls

The main display panel looks like it is digital and programmed for the subject matter information. It is sunk into the middle of the dash for, what looks like, the best viewing of all the vehicle occupants, not just the driver. I had two problems with that positioning. With everything offset to the right side of the driver’s view, the separation between the left and right turn signal indicators is too small to see clearly without taking my eyes of the road to look at the actual arrow. Also, one of the information displays gives immediate feedback to the driver about the torque demands and the EV and hybrid modes of charging and discharging the batteries. Again, the information was not easily seen, down and off to the right, without taking my eyes off the road. Placement in front of the driver would be a nice improvement and a heads up display would be superb.

The standard parts of the driver display included gasoline fuel level, digital speed (analog and digital would be even better), turn signal indicators, odometer, real time mpg, and various mode indicator lights. Being somewhat of a techie, I would have liked to see the engine rpm and temperature; and the electric motor rpm and some appropriate critical temperature. Battery SoC in addition to the estimated EV miles would have been nice too. The display has five selections stepped through by one of the steering wheel buttons:

1. Battery level and number of EV miles, and horizontal bar graph showing real time torque demanded by the accelerator pedal. This display was useful in raising my awareness of the energy effects of changing elevations up and down hils, and the effects of air drag at higher highway speeds.

2. A graphic of the car showing the real time energy flow between the engine, electric motor and battery.

3. Longer term averages of fuel economy

4. Percentage of miles driven in EV mode.

5. Settings that could be cycled through and changed with the same steering wheel display button.

Additionally, when I put my finger on one of the steering wheel buttons there are two cool looking pop up displays that illuminate the button functions. However, if I have to take my eyes off the road to see the pop ups, I can just as easily look at the steering wheel. The left hand steering wheel button was like my Sequoia, operating the radio modes, presets, and volume. The right hand steering wheel button operated the display selection, trip odometers and resets, temp up and down for heating and A/C, and recirculate control for ventilation. I found the temp and recirculation controls redundant to the other same controls close by on the dashboard.

Lights, Wipers, Mirrors, and Visibility

I am spoiled by the light sensing automatic turn-on head light control in the Sequoia. This basic Prius did not have them and several times I had to return to the car to turn off the lights. As in many other cars, these controls were located on the turn signal lever. The emergency 4-way flasher control was a nice big button on the console.

The interior lighting was superior with several different automatic modes that anticipated the entry and exit of the vehicle. It took a while to discover, but just pushing on the light lens is a nice switch feature. Also a nice touch is an indirect beam of light out of the ceiling that illuminates the console while driving at night.

One place that could use a light is the electric charging compartment to illuminate the socket and cover. It would have made it easier when I was trying to insert the charging plug at night in the dark.

I had one wiper and lights problem that turned out to be pilot error. California law requires the headlights to be on any time the windshield wipers are on. While driving in the rain one day I did this only to have the driver display go dim and unreadable. The automatic dimming control expects a reduced ambient light level if the head lights are turned on. The manual dimming control is a thumbwheel on the left side of the dash. If the thumbwheel is advanced into the maximum brightness détente, the display will stay bright even if the headlights are turned on.

The electric positioning left and right external mirrors are the same as my Sequoia, but the control was located on the left side lower dash panel next to the display brightness thumbwheel instead of the center console. From my best position setting of the outside rear view mirror without repositioning the mirror I couldn’t see the curb position when parallel parking. Ok, it is a little picky.

The visibility out the rear window from looking at the inside rear view mirror has an irritation shared by almost all Prius drivers. The rear spoiler required for that nice low aerodynamic drag coefficient puts a horizontal bar across the rear vision.  For the extra mileage at highway speeds it was acceptable, but I never got used to it. I heard that some late model Prius’ may be offered with a camera and a screen that I hope eliminates the bar in the view. My recent experience driving a Volt exposed a similar bar. Adding additional irritation to me was the manual lever for day/night viewing of the mirror. The automatic transitioning of my Sequoia mirror is another nice thing to have.

Driver visibility is good except for two blind spots at the rear corners. I solved this problem on the Sequoia with the addition of small round stick-on wide-angle mirrors to the standard outside mirrors. I elevated the driver’s seat to its highest level for my best outside viewing angle.

Navigation and Entertainment

Your children and grandchildren will love the screen and controls. I liked the large navigation screen, which automatically shifted from a light background to a dark background when the headlights were turned on. After reading the navigation manual for over an hour I decided that I was not going to become a proficient operator during the time I had the car. To prevent driver distraction, several of the functions were not available while the car was moving.

The angle of display has a cute little shift control to help minimize glare reflections. The whole display tilts forward to expose the CD insertion slot. It looked like only a single CD at a time. I prefer the 6-CD changer I have in the Sequoia. The sound system was great when using the radio.

Other Amenities and Comments

Keyless entry, locking, and start-up were new and enjoyable for me. I now look at my other keys as archaic. Three people could fit in the back seat with reasonable comfort, but only two could be there to have the cup holders that were in the center pull down armrest.

The center console cover had a latch that slid back to expose a cup holder in addition to the one with it’s own cover door in front of it. More pulling on the latch exposed a compartment with a tray and a 12 VDC 120 W power socket and an aux port for and iPod. What at first confused me was that the latch had to be depressed again to slide the cover forward for complete closure. I forgot and left my Bluetooth wireless phone earpiece in the tray because it was not completely visible.

In front of the console under the joystick platform is an open tray that has another 12 VDC socket along with switches for the heated seats.  I found the switch placement inconvenient for the driver but ok for the front passenger. The heated seats get hot fast! While more pleasing for the leather seats in the Sequoia, the seat heaters may not really be necessary for the soft cloth seats in the Prius.

Normally, the car was ready to go in total EV mode after putting my foot on the brake and pressing the START button. However, I found that leaving the front defroster selected from prior driving caused the engine to start after pushing the START button.

There are upper and lower glove boxes for added flexibility.

All four side windows have automatic up and down modes. I quickly closed the windows after trying them down during highway driving. The car body aerodynamic airflow is sensitive to window position and my ear drums were taking a beating.

The steering wheel has a “scope” adjustment in and out for comfortable arm positioning.

Fortunately, I had my Sequoia experience to understand the Toyota HVAC (heating, ventilation, air conditioning) control strategy. The only difference was that the Sequoia has a knob for setting the desired temperature rather than the up and down buttons of the Prius.

Best Electric Car or Hybrid Car

In 2012 there will be several plug-in car models to choose from, each slightly different to match the driver’s needs and wallet. From the ones I have driven, going from low to high purchase cost:

  • Mitsubishi iMiEV – All electric (no petroleum) short range no frills basic gets the job done, smallest, lowest weight and most efficient. Low cost because of credits and grants.
  • Nissan Leaf – All electric (no petroleum) medium range, excellent feel and performance, virtually no service required, perfect for urban commutes and short trips, some longer trip options. Lower cost because of credits and grants.
  • Toyota Prius – The standard with great mpg and long range, but doesn’t utilize grid energy
  • Toyota Prius Plug-in – Better mpg and hybrid efficiency, long range and short EV range
  • Chevrolet Volt – High end luxury quality feel, performance, and features, serial hybrid design has lower efficiency and mpg.

Top 10 Electric Cars including Plug-in Hybird



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About Author: Tom Bartley

Tom Bartley is an expert about vehicles, drive systems, and energy storage. Tom Bartley has a BSEE and MSEE from Stanford University. He is a life member of the IEEE and a member of SAE. He is a director on the board of the San Diego Clean Fuel Coalition and is working with Transpower of the development of heavy-duty electric drive systems.

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