About the advocacy, I was confused by the "better than my car (MT HCH)" comment. But as for comparing vehicles, engineering needs to maximize the common elements and the CVT transmissions of the Prius and HCH models share the lack of a clutch.

The HCH with manual transmission is a fine car although not everyone would be able to drive it. However, the manual transmission means it has significantly different performance characteristics. It makes a head-to-head comparison all but impossible.

It is more accurate to say some non-linear effects in the Prius makes it subject to significant differences from the EPA numbers. These non-linear effects are different and once understood, EPA results and better can be achieved.

This is the introduction I was looking for. It turns out we have a fairly extensive and detailed list of the changes between the Prius I and the Prius II. We can list them and look for something that would only "maximize the EPA tests."

- Cd 0.30 -> 0.26
- Electronic controlled brakes with extended regenerative braking range
- More powerful MG2 with higher redline speed, 6,700 rpm
- Faster MG1 redline, 10,000 rpm, for extended electric range
- Use of bearings for bushings for one transaxle gear
- Higher voltage, more efficient inverter
- ICE speed increased from 4,500 to 5,000 for more power
- Thermos heat storage for faster warm-up replaces a hydrocarbon emission system
- More drive shaft HP

Of these changes, the coefficient of drag, Cd, electronicicly controlled brakes, faster MG1 speed and thermos would most impact the EPA numbers. But they also improve the delivered MPG. We know the fleet average is up 3 MPG over the earlier NHW11. So we have this conundrum; the NHW20 fleet vehicle performance is up over the NHW11 yet the EPA performance numbers show an even greater improvement than the fleet performance.

We know the EPA performance test represents a driving pattern last seen 30 years ago. In Huntsville, every mile I drive outside the city limits seems to turn the clock back one year. When I travel down highway 72 from Huntsville to Memphis, I pass through many small towns where the downtown posted speed limits of 25 miles per hour match the EPA profile and the same is true in Kansas and Oklahoma. These downtown speed limits of 25 miles per hour are enforced! Furthermore, many of the smaller, two lane highways off of the divided four lane, have speed limits of 55 miles per hour and in some curvy areas, even 50 miles per hour. But I also remember my year in Okinawa Japan.

The Japanese streets in Okinawa were smaller and speeds much slower than in the USA. In 1972-73, they did not have a super highway and the frequent, unbanked curves and climate kept speeds much more moderate . . . just like many small Alabama and Oklahoma rural towns where main street has an enforced speed limit of 25 miles per hour. BTW, a sample of 78, Japanese Prius owners reveals an average of 58 MPG versus the USA 47.7 MPG.

It is in this rural environment where the slow speed capabilities of an NHW20 Prius (and the NHW11 model) really shine because they can fully exploit the limited EV modes and engineering changes. What was good in the NHW11, slow speed performance of 52 MPG versus highway 45 miles per hour, has gotten better in the NHW20.

And another introduction to compare and contrast the EPA test protocol vs. 'real world' numbers. This is what I expect to demonstrate:

- 42 miles per hour is a critical transition speed
- Starting and Stopping the ICE impacts MPG
- the EPA city protocol stays below the 42 miles per hour critical speed
- the EPA highway protocol is above and transits the 42 miles per hour critical speed
- US urban patterns encourage a lot of 42 miles per hour transitions
- non-linear airflow pattern exists in the NHW11

MG1 Protection Transition Speed - 42 miles per hour

The Prius MG1 has a maximum RPM speed limit. It has to rotate very rapidly in the reverse direction when the engine is off. At speeds above 42 miles per hour, MG1 could fly apart if the engine is not running. This is unique to the Prius CVT and well documented by Ken's posting of an miles per hour vs MPH chart, scan bus data from NHW11 and various engineering papers. It is the functional equivalent of a shift-point but transition through this speed impacts MPG.

Starting and Stopping the ICE Impacts MPG

One of my earliest experiments was to monitor the magnitude of current through MG1 and MG2. What I found is MG1 has a significant current to start the ICE and there is a significant MG1 current needed to stop it! Automatic starting and stopping of the ICE is the enemy of good Prius MPG at speeds around 42 miles per hour.

To start the ICE, energy is needed to spin up MG1 from a negitive, several thousand RPM to a positive couple of thousand RPM. This also rotates the ICE crankshaft, pistons and valves, which takes a significant amount of power. Although done without the sound and fury of a regular starter, the energy demands are significant. But stopping the ICE takes a simular amount of energy in the reverse direction!

When cruising in urban city traffic at 42 miles per hour, the normal speed variations around 42 miles per hour will trigger more ICE on-off transitions than going 4 miles per hour slower or 8 miles per hour faster.

EPA City Protocol

One of the best summaries of the EPA protocol comes from a USENET posting:

Toyota replaced a hydrocarbon emission system with a thermos of hot engine coolant to speed up ICE warm-up. The ICE can be cold while the thermos coolant is closer to 170-180 (F). Is this 'cheating' or replacing one emission control system with one that also improves MPG?

But where besides rural main streets and neighborhoods do you find an average speed of 20 miles per hour?

EPA Highway Protocol

An average speed of "48 miles per hour" means it is too close to the 42 miles per hour critical transition speed. This is right in the range where highway MPG takes a hit in the Prius.

US Urban Traffic Patterns

Most metropolitan areas have a mix of arterial flow streets with speeds of 35, 40 and 45 miles per hour. Worse, most US drivers are happy to run over these speeds which moves the 35 and 40 miles per hour speeds right into the range of 42 miles per hour. Unenlightened Prius drivers "going with the flow" are driven into the worst speed range for optimum MPG!

Non-linear Airflow Pattern

Although the coefficient of drag is often assumed to be linear, there can be critical speeds where laminar and non-laminar effects can change the coefficent of drag. This has been observed with my NHW11 Prius using ribbon 'tuft' testing and later a ping pong ball on a string. I have observed greater turbulence in the air behind the trunk at speeds of 65 miles per hour and higher. The result is a rapid fall-off of MPG much greater than the V**2 effect.

UPDATE: Subsequent analysis of the Prius ICE efficiency revealed that above 2,400-3,300 rpm, the fuel efficiency begins to decrease. This becomes more pronounced above 3,300 rpm and higher (NHW11.) It turns out that the engine power and rpm needed to sustain speeds above 70 miles per hour corresponds to this decrease in the ICE efficiency.

I suspect the NHW20 Prius has a similar characteristic BUT I have yet to repeat these test with an NHW20. In the meanwhile, I have to treat 65 miles per hour as the maximum, efficient cruise speed.

Conclusions

There are two non-linear effects in Prius operation centered around 42 miles per hour and speeds above 65 miles per hour. The EPA tests skirt these inefficient ranges but USA urban driving and interstate travel above 65 miles per hour seems to pound on these non-linear areas. This is why there is a significant difference between the NHW20 Prius II EPA specifications and the GreenHybrid.com fleet performance. By one way or another, the clever Prius pilot avoids these non-linear areas and achieves EPA and better results.

This does not make the Prius "better" than the HCH-MT but different. If the Prius pilot can maximize their travel in Prius efficient speed ranges, they will get better performance. If not, then the HCH-MT, which has no documented non-linear performance areas, is the better choice. Pick your poison.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

As the EPA is a government agency, they should divulge that info if requested in accordance with the Freedom of Information Act. It's not like they can hide behind a banner of "national security"..... or can they?

(Mulder, are you there?)

A great discussion. Since I have not driven an HCH I can't say it is a better car or better technology than the Prius. I cansay that after 40,000 plus miles I have done better than EPA combined mileage by about 3.2% after adjusting the EPA Combined for my driving pattern of 35% City and 65% Highway.


Methodology:

EPA combined MPG figures assume 45% City at 60 MPG and 55% Highway at 51 MPG for a combined 55 MPG

My own overall figures are 35% City and 65% Highway for a combined 55.9 MPG

The adjusted EPA figures for 35% City and 65% Highway would yield a combined (.35 * 60 mpg = 21) + (.65 * 51 mpg = 33.15) = 54.15 MPG



This makes me very happy of course. I am certain there are HCH owners who are just as pleased, if not more so, with their autos. What I try to keep in mind is that we are all cards in the same deck. Use less oil, produce less pollution, and save money doing it. While none of the current offerings are perfect they each have their place. What could be better right now, today? Better, cleaner, more efficient technology is certainly on the way and I for one won't mind who introduces it, just as long as it happens. And the sooner the better.

Alden Bowles
2005 Toyota Prius II HSD