Dec. 25, 2006 - Added improved efficiency chart.


This miniscanner data was captured using the fast rate possible for the following values: The rest of the vehical characteristics are embedded in the spreadsheet.

ICE Power and Fuel Injector Time

The goal is to translate injector timing into fuel flow. So far, it looks like the 55-60 psi fuel pump pressure is keeping the fuel injection rate proportional to the injector timing. Also, Yoshi reports the injector delay is 0.5 msec, which shows up in some of the data inflection points. This chart reflects a 0.5 msec delay.

It is fairly easy to divide the ICE power by the fuel injector time to determine the watts per unit of fuel:

Recently, there was some question about the efficiency of the Prius ICE and MG1 configuration. The following chart plots the fuel/ICE watt as a percentage of peak fuel efficiency across different ICE rpm and ICE torque ranges:

Unlike the area efficiency charts of the Dept. of Energy report "Evaluation of 2004 Toyota Prius Hybrid Electric Drive System" (Staunton, Ayers, Marlino, Chaisson, Burress, ORNL/TM-2006/423, Dept. of Energy, May 2006), this chart comes from operational data captured from a Graham miniscanner. The vehicle and engine control units dictate the allowable operational range. Several things standout:

The following chart plots the air mass flow rate as a function of the fuel flow rate based upon rpm and injector timing. The injector times, not adjusted for the 0.5 msec delay, in the following plot are color coded in groups:

This data comes from ordinary driving, not maximum power hill climbing. The low end of the mass flow rate never really sits at zero but from 4 g/sec and above, it looks linear:
The inflection point at 0.5 msec corresponds to Yoshi's report of a 0.5 msec injector delay.

Cruise Control versus Overpasses

Although we do not have the altitude changes and mechanical braking losses, we do have ICE power, battery power and the change in kinetic energy:

Warp Stealth

On a recent trip to and from Washington DC, I was able to record a full-tank, 562 miles, of interstate highway cruising from Toms Brook VA to Stevenson AL. The cruise control was set to 62-63 mph and reduced only to deal with local traffic events such as speed zones, construction areas and traffic congestion. The temperatures ranges from 48-68F with about 25% in light to moderate rain. The block-to-block speed, not counting breaks, was 59.4 mph for an average MPG of 53.1 (NOTE: only able to add 10.58 gal. and .6-.7 gal blocked by bladder effect.)

During the test, we recorded a number of "warp stealth" events.

Curious, I've never seen a report of ICE braking while the battery was providing motive power. However, you can see the battery voltage take a dive.

Using energy parameters suggested by Hobbit, I selected records for speeds above 55 mph and low ICE torque (aka., MG1 torque of 8 Nm. or less) to get less than 4,000 records, the excel plotting limit. These records show 'coasting' events where the cruise control managed speed on the downgrade side of hills put the NHW11 03 Prius in a low energy, coasting mode. The data revealed two forms of coasting: (1) low-power ICE idling, and (2) low-power ICE braking:

Low-power ICE idle

In this mode, the ICE consumes a higher rate of fuel but the traction battery power is very low too. This maintains the battery SOC so less energy is needed later to charge the battery. The advantage is avoiding energy conversion losses on the subsequent battery recharge versus drawing power at an inefficient ICE range.

Low-power ICE braking

This mode has the traction battery providing power to maintain ICE rpm and the fuel cut back to the bare bones minimum and not enough power to keep the ICE spinning fast enough. This is the minimum fuel burn rate but the traction battery power requires future ICE power to charge the battery. What we don't know is if the future ICE power mode is efficient enough to outweigh the energy conversion losses.

Trip Record Data

The following 4.5 MB file Excel spreadsheet holds the recorded mini-scaner data from a 562.1 mile trip from Washington. Let me know if you have any trouble handling the file.

MPG vs. mph

Using the drag formula reported by our Japanese friends, I've plotted the amount of energy versus speed. Then I added some MPG vs. mph data previously recorded:

There is good agreement between the standard day test runs on a 2.5 mile track. The other results suggest specific inefficiencies that need further investigation . . . and improvements. For example, the "engine thermal inefficiencies" occur because the NHW11 tries to keep the coolant temperature above 60C. In cold weather, the ICE will run just to keep the coolant warmed even if the vehicle otherwise has enough energy for hybrid-electric cycling.

Misc. Graphs


Graham's mini-scanner requests and reports up to six data items from the NHW11 Onboard Diagnostic Bus. A fine product, the following state-chart shows how to program the device:

One problem with Macintosh serial interfaces like the Wallstreet, is they prefer to work with 5 V. RS-232 but the mini-scanner transmits a higher voltage. This was solved by using a 500 ohm and 1 kohm resistor network as a voltage divider. The Wallstreet Rx goes across the 1 kohm resistor while the mini-scanner feeds both resistors.