Hi,
As I read more and more of the article, I was more and more disappointed with the claimed expertise. They really didn't understand how the Prius transaxle works and mixing terms and making unsubstantiated claims made it harder and harder to read. Remember the Indian tale about the seven blind men and the elephant, I felt like two or three of the blind men had just finished their examination of the Prius transaxle. For example, using terms "generator" and "MG2" wrong really makes it difficult to take the rest of their descriptions seriously. Then their claimed efficiencies, "20%" or "30%" without engineering units . . . this is not something that inspires confidence. But that is no reason to not address the question about how the Prius transaxle works.

As for their discussion of the Atkinson cycle, well that is easy to fix. The Prius engine has an 8 to 1 compression stroke and a 13 to 1 expansion stroke. This means the Prius does not spend much energy compressing the charge before ignition and gets an extended power stroke.

I don't have an opinion about "belt and pulley" CVTs because I've not read any engineering evaluations. But I will ask.

Again, I have a hard time talking about performance 15 miles per hour above the local legal speed limits. Are you sure this is an area of great interest?

What I'd like to do is offer some source material that will provide a cornucopia of engineering data about the Prius transaxle before describing how it works in lay terms. I'm giving the references first so I can make ready references in my write-up. My Google findable references are:

1. "Report on Toyota Prius Motor Thermal Management" ORNL/TM-2005/33<
2. "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" 2004-01-0064<
3. "Evaluation of 2004 Toyota Prius Hybrid Electric Drive System Interim Report" ORNL/TM-2004/247<

The first step is to understand how motor generator 1 (MG1), motor generator 2 (MG2), the planetary gear called power split device (PSD) and internal combustion engine (ICE) work together. Start with Figure 2.4, pp. 9 of (3), which shows:

• sun gear is connected to MG1, the low power motor generator that balances the ICE to generate torque; starts and stops the ICE; and generates electricity as needed when the ICE is running and the vehicle is stopped.<
• planetary gear carrier is connected to the ICE and has a ratio of 72:28 to the sun gear. Rather than get lost in the details, just think of the ICE requiring a balancing force of ~1/3 generate by MG1 to transfer power. What isn't clear is the power generated by MG1 is IN THE DIRECTION OF FORWARD MOTION. That MG1 torque and power is trying to move the car forward with the ICE.<
• ring gear is connected to MG2, the high power motor generator that handles regenerative braking, assists the ICE and MG2 in forward motion, provides all reverse power and at cruise, generates the electricity needed to power MG1 to get the ICE power out of the ring gear.<

For a layman's approximation, the electrical efficiency of MG1 and MG2 whether working as a generator or motor is going to be about the same, ~93% pp. 11-15 (1). This means the electrical power losses crusing down the highway from MG2 to MG1 will be ~86% or ~15% loss. But the power required by MG1 to balance the ICE is only ~1/3 of the ICE power, so we're looking at an actual 5% loss, independent of speed. That means to get 100% of the ICE power delivered to the drive gears, the ICE generates 105% power and 5% is lost as heat.

Now there are also mechanical losses from the gears. This loss is a function of RPM and transaxle oil temperature. At maximum 5,000 rpm, the mechanical energy loss is 1.1 kW or about 1.3 hp, pp. 26-28 (3). The axle rpm can be converted to miles per hour but I don't have that handy. Regardless, we can calculate the hp based upon the rpm and torque. Divide the 1.3 hp by the power, 6.6 hp and you'll have the Prius transaxle mechanical efficiency at maximum RPM of ~20%. At a more sedate, 2,000 rpm typical of highway cruising, the efficiency improves to ~10%.

So let's review:

• 25% efficiency at engine redline (HOLY CRAP! ARE YOU CRAZY!)<
• 15% efficiency at ordinary cruise speeds, ~55 mpg<

OK, we've done the number for the Prius, where are the Honda CVT numbers?

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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