May 24, 2005 - Provide DoE energy report data and analysis.
May 22, 2005 - Baseline release


This page shows the transaxle oil temperature, Amsoil ATF, measured at the drain plug in an NHW11, 03 Prius. A modified drain plug has a thermister in direct contact with the oil whose resistance is measured with a DVM. Data was recorded manually and time using the vehicle clock. An Excel spreadsheet converts ohms to degrees F.

Commute Routes

Due to stop signs and lights, the actual speed is always less than the target speed. The operator target speeds on the current commute route uses:
The rate of temperature increase is about 0.9 (F)/min.

This route is a little faster: 1.1 mile @25 mph warm-up neighborhood streets; 5 miles @55 mph; 2.0 miles @35 mph; 1 mile @55 mph; 1 mile @25 mph cool-down.

The rate of temperature over the whole interval is 1.0 (F)/min.

Highway Warm Up

The route was relatively level, divided highway, I-565 west of Huntsville, from Research Blvd. to I-65 and back. Three cruise control maintained speeds: 65, 70 and 60 mph.
The rate of temperature increase was 1.6 (F)/min. during the first 18-19 minutes.

Transaxle Energy Loss

The November 2004 report "Evaluation of 2004 Toyota Prius Hybrid Electric Drive System Interim Report" by C. W. Ayers, J. S. Hsu, L. D. Marlino, C. W. Miller, G. W. Ott, Jr., and C.B. Oland, from Oak Ridge National Laboratory (ORNL/TM-2004/247) has a series of tables, 3.8-3.13, that show the energy loss of the 2004 transaxle at temperatures from 28-80 (C) at different rpms. The following chart shows a series of parametric curves at different rpms showing the energy loss as a function of temperature within each rpm:
Two lines, one at the 28 (C) value and one at the 70 (C) value were drawn to give an idea of energy loss at rpms between any two rpm curves. Not the highest value, the 70 (C), 158 (F), line is just above the highest temperatures so far measured in high-speed cruise tests. To keep the rpm curves separated, a 'null' entry was put in that displaces each curve to the right. The report 500 rpm data was dropped since the Prius ICE never runs at this speed and rpm above 5,500 since the ICE is speed limited below this value.

Transaxle oil temperature is controlled by the heat from mechanical energy generated within the gearing and cooling from case cooling and some inverter coolant loop. As the oil warms up, the viscosity goes down and the amount of heat generated goes down.

Unfortunately, there is no oil cooling and heating loop in the radiator as would be found in a regular, automatic transmission vehicle. In other cars, this missing heat exchanger accellerates heating to 80 (C) when the ICE warms up. Later, the missing radiator heat exchanger helps cool the transaxle oil so it won't suffer thermal runaway. Both heating and cooling are important to efficient transmission operation.

Although the absolute energy losses are modest, all of this heating comes from mechanical energy. So a 750 W. load requires 1 hp from the ICE which assuming a 33% efficency, requires three times as much energy to generate. Meant only as a rough rule of thumb, 1 kW of inverter energy from the Prius takes about 0.25 gal/hr. of fuel. In normal cruise, 55-65 mph, the NHW11 Prius fuel burn is ~1.1 gal./hr. The expected fuel savings will be much less than 0.25 gal/hr. along with longer transaxle oil life due to less stress and a controlled thermal environment.

If we can use engine waste heat and radiator cooling to keep the transaxle oil in a narrower temperature range, say 80 (C), we can avoid some of the mechanical heat loss and keep the transaxle in a temperature range less subject to thermal stress. But this require knowing exactly what is going on in the transaxle during normal operation. To that end, a modified drain plug was installed with a thermister to record transaxle oil temperature:

Getting the thermister resistance values into the cabin for measurement is solved by installation of a data cable.

Data Cable Installation

These photos show how the Cat-5 cable is routed from the engine compartment into the cabin:

The following photo shows the MG2 power box with the small vent tube located on the top-right. The end of the tube has a .25 inch gap with the plate under it. It is very important that cleaning the engine never put water in this area because it could cool the box and cause water to be injested onto the MG2 power cables. This could cause a ground-fault failure.