Introduction

This web page describes an approach to achieving, in a reproducible, safe and affordable way, 99.9 MPG in an NHW11, 01-03 Prius:
The goal is to achieve 30 minutes of maximum MPG performance in Prius MFD so all six bars are completely filled. Depending upon country, this works out to be 40 km/l in Japan (~95 MPG) and 99.9 MPG in the USA.

The methodology is to optimize:

Background

There have been three widely reported, hybrid mileage tests. Two of them, the 2005 Prius and 2006 Insight, were endurance tests of 48 and 112 hours. The impressive 2006 Prius test by TeddyGirl was over a daily commute route. But the problem is the expense and availability of the unique route and conditions needed to achieve 100 MPG or 40 km/liter.

Getting Max Bars MFD

This procedure maxes out the Prius MFD in a period of 30 minutes by using the 6 mph threshold for MFD recording. The trick is to make a single, instanious, 100 MPG bar every 5 minutes, or after the last one was captured. With six of them, the bars are a solid 100 mph across the board:

DISTANCE: 0.3 miles
MODIFICATION: thermistor hack (critical in cool weather!)
METHODOLOGY:
(1) fully warm up car and charge battery (~20 minutes above 42 mph)
(2) regen slow down to enter a flat, empty parking lot
(3) put car in "N" and coast to a stop with minimum electrical load and reset MFD
(4) after 'mileage bar' updates, put car in "D" and gently accelerate to 6 mph to get 99.9 'current' bar and then put in "N" and coast to a stop
(6) repeat step #4 until all bars show 99.9 MPG
(7) take photos and have a happy

Stable 100 MPG

The trick here is to hold a steady speed, well within the 100 MPG performance speed range for as long as desired:
DISTANCE: 10.3 miles
MODIFICATION: thermister hack (critical in cool weather!)
METHODOLOGY:
(1) fully warm up car and charge battery (~20 minutes above 42 mph)
(2) do one full lap in a flat, empty parking lot at 15 mph
(3) reset MFD display
(4) hold 15 mph until all bars bars have updated, ~40 minutes
(5) take photo and have a happy

Predicting MPG vs MPH

A steady speed of 15 mph is well within the predicted, 100 MPG operating range of the NHW11 Prius:

No form of 'pulse-and-glide' can exceed the MPG of the prediced, drag based, NHW11 performance. To exceed this curve, either the engine has to be operated in a range that increases the thermodynamic efficiency above 31% (possible) or a more energetic fuel (possible) or a substantial drag reduction (possible.) Sad to say, 'pulse-and-glide' can easily abort investigation into these other areas to the detriment of the community.

To date, all high mileage claims have asserted that "pulse and glide" is the key to their success. In the case of an IMA Insight, this makes sense because it has no credible, battery-only mode. Curiously, the recent test was run at an average speed of 18 mph and closer to 15 mph on the last couple of days:

This speed range is well within efficient, steady-state, Prius operation.

Prius MPG Tests

The following data comes from the multi-function display over either a flat route without acceleration and regenerative braking effects or a test loop track:
Obviously, temperature plays a big part in vehicle performance.

Test Track Energy at 15 MPH

The track was entered and exited from the highest access road, the eastern access road:

Test Track Energy at 24 MPH

Test Track Energy at 30 MPH

15-18 MPH Test

The following test used a speed boost to 18 mph to trigger ICE startup prior to the first slope:
Notice there are some periods where the battery power generated is not proportional to the ICE power. This suggests monitoring battery charge levels may be key to higher efficiency.

The following scatter chart shows the ratio of MG1 Nm, directly correlated to ICE torque and power, versus the generated battery current:

This data suggests a 15-20A range is optimum. Also, we need to avoid cases of the ICE running with auxillary battery power. Not shown is the 94% efficiency between generated battery power and used battery power: the round-trip efficiency in and out of the battery.

Test Track Analysis

By combining the energy data for 15, 24 and 30 mph and scaled over the route, we get the following energy graph:

The best mileage occurs at 15 mph which is unique by having both dual-ICE and triple-ICE operation. Also unique is the nearly level, ~5.2 KW (7 hp) battery recharge rate compared to variable rates at 24 and 30 mph. It looks as if starting ICE operation prior to the first 'slope triggered', ICE operation may be key to managing ICE operation time over the track.

Prius Test Track

While looking for a driving test route in an industrial park, I found an interesting loop:



Speed

Built-in to the Prius is a total energy management system that switches in ICE power only as needed to supplement the battery system. This means at steady-speeds, the vehicle will maintain speed on battery alone. The engine will pulse on only as needed to recharge the battery and sustain speed during the charging. In effect, the ICE pulses on and off automatically while the battery maintains speed.