Is HSD a variable ratio transmission? - Page 2 - GreenHybrid

bwilson4web · #11 (**permalink**) 06-29-2006, 02:26 PM

THANK YOU!!! This is what I needed to see:

Quote:

Originally Posted by nbalthaser

http://www.evworld.com/view.cfm?sect...le&storyid=887

the information comes from what i can glean from the user comment section at the bottom, not the article itself. the discussion is lively and interesting however as i said, i'm not a mechanical engineer so i'd be interested in hearing your comments (presuming your a mechanical engineer)

here is the particular quote from the discussion that references this:

Chris Ellis (author of the article that makes the claim):
Roger, as I said in an earlier comment, what may be irrelevant at Japanese speeds and acceptable at American speeds can be a problem at (still legal) European speeds. My calcs show a Prius needing a NET 21+ kW at the wheels to sustain 80 miles per hour on the level, implying a requirement for a lot more kWs out of the engine and into the transmission. As you say, 20% losses probably grow to 30% at higher speeds. You've shown that the use of a continuous electrical path doesn't matter much at 60, but it certainly does at 80, which was my original point. However, I absolutely accept your excellent point that the generator needs to generate (and therefore spin) most of the time (because there is a heavier than normal ancillary electrical load), which makes the solution more challenging. So, if you accept there is clearly a significant problem at high speeds, I will concede that a simple lockup on the generator/sun gear probably isn't enough to fix it. Which could explain why Toyota didn't put it in, and why GM/DC and others are working on more efficient solutions for higher speeds.
29/Aug/2005
[ 9136]

Roger Pham (previously rebutting):
Chris, yes, we have perfect agreement that the loss due to the electrical path will be much higher at higher speeds. At higher speed, due to the higher drag involved, the engine will be in lower gear ratio (figurative speaking) meaning that the engine will have to turn more rpm for every turn at the wheels in comparison to cruising at lower speed, due to the continously variable feature of the transmission coming into play. This means that the generator will turn proportionally much faster than the engine, and proportionally, more of the total torque and horsepower at the driving wheels will have to go through the generator-motor pathway which will now be subjected to ~30% loss of efficiency and not 20% because of higher current going through the copper winding meeting more internal resistance instead of impedance. I would predict that the total efficiency loss at 85 miles per hour would be in the order of 15-20 %, and I'm sure that you have calculated a more accurate number based on actual testing. That's why technical review articles such as yours are important in order for the prospective buyer to be awared of problems not mentioned otherwise. The Prius of course was designed for the American and Japanese market, where the typical cruising speed would be a lot slower than in the European autobahn. Direct injection turbo diesel (TDI) would be more suitable for the European market, where diesel fuel costs half the cost of gasoline due to much less tax on diesel fuel. Daimler-Chryler has just revealed a four seat TDI capable of 70mpg freeway miles, having a cd (drag) of merely .16 (very slippery). Horses for different Courses, indeed. Roger
29/Aug/2005
[9152]

My initial impression from what you've posted is they are discussing the loading "at 85 miles per hour", a very important qualification. It is not uncommon to find the efficiency of any system varies as a function of the load. In this case, 85 miles per hour is a speed that except for parts of Montana and the Atlanta beltway outside of rush-hour would result in a speeding ticket with reckless driving citation.

I'll do a little more research and independent confirmation. It so happens, I've got an NHW11 03 Prius with instrumentation. Knowing my lead-footed wife, I will take some measurements (between the occasional "Holy SH*T! Do you know what the speed limit is??")

BTW, I was a mechanical engineering student at Oklahoma State University before doing four years in the US Marine Corps (there used to be a draft.) This means I not only know how to figure it out but I also have unusual skills to defend my opinion. <GRINS>

Bob Wilson

ps. As I was reading the comments, some obvious errors made my vision go blurry. This will take a little longer and the detailed answer more than expected. However, it will be backed by vehicle measurements and hopefully a description that will clear up some of the FUD (fear, uncertanty, doubt.)

nbalthaser · #12 (**permalink**) 06-29-2006, 03:02 PM

Quote:

Originally Posted by bwilson4web

THANK YOU!!! This is what I needed to see:

My initial impression from what you've posted is they are discussing the loading "at 85 miles per hour", a very important qualification. It is not uncommon to find the efficiency of any system varies as a function of the load. In this case, 85 miles per hour is a speed that except for parts of Montana and the Atlanta beltway outside of rush-hour would result in a speeding ticket with reckless driving citation.

I'll do a little more research and independent confirmation. It so happens, I've got an NHW11 03 Prius with instrumentation. Knowing my lead-footed wife, I will take some measurements (between the occasional "Holy SH*T! Do you know what the speed limit is??")

BTW, I was a mechanical engineering student at Oklahoma State University before doing four years in the US Marine Corps (there used to be a draft.) This means I not only know how to figure it out but I also have unusual skills to defend my opinion. <GRINS>

Bob Wilson

you're welcome! i definitely want to hear your take on the article and discussions. like i said, i'm trying to understand the interesting intricacies of hsd. i think it is an ingenious system and frankly i agree no system is going to be operating as its peak efficiency during all load conditions.

having said that, i'm still leaning toward saying that psd is a true cvt at lower speeds (under say 85mph) but acts less like a true cvt at higher speeds (say above 85mph) than would a belt and pulley cvt. i say that at speeds above 85mph, the fixed gear nature of psd begins to introduce non-significant losses (from the electric motor needing to be running and running more the higher the speed) and that the cvt-like behavior/benefit of the system begins to slip away as the ice revs higher and higher to make up for the inefficiencies of these losses. this behavior would not be present in a belt and pulley cvt as the gear ratios exist to allow the ice to operate in a comparably more efficient range. i say this without qualifying the need for traveling at such speeds and just purely looking at the question posed.

and as a final disclaimer, the prius is *excellent* at what it does and meets 95% of our needs here in the states. by understanding hsd better, we may shed light on how to improve the system for the last 5%.

bwilson4web · #13 (**permalink**) 06-29-2006, 07:38 PM

Hi,

Quote:

Originally Posted by nbalthaser

. . . i definitely want to hear your take on the article and discussions. like i said, i'm trying to understand the interesting intricacies of hsd. i think it is an ingenious system and frankly i agree no system is going to be operating as its peak efficiency during all load conditions.

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.

Quote:

Originally Posted by nbalthaser

having said that, i'm still leaning toward saying that psd is a true cvt at lower speeds (under say 85mph) but acts less like a true cvt at higher speeds (say above 85mph) than would a belt and pulley cvt.

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

Quote:

Originally Posted by nbalthaser

i say that at speeds above 85mph, the fixed gear nature of psd begins to introduce non-significant losses . . .

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:

"Report on Toyota Prius Motor Thermal Management" ORNL/TM-2005/33<
"Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" 2004-01-0064<
"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

blinkard · #14 (**permalink**) 06-29-2006, 07:48 PM

Couldn't we have had a 4th choice for "It makes my head hurt"?

It's impressive, it's clever, and when I concentrate, I can understand it. But part of me still thinks it has no right to work, sort of like people who still think the world is flat.

But as to "is it a variable ratio transmission," that depends on which ratio you're referring to. ICE speed vs. drive shaft speed? Yes.

foo monkey · #15 (**permalink**) 06-29-2006, 08:24 PM

My brain asplode.

/just wants to get past the ****ed warm-up penalty.

nash · #16 (**permalink**) 06-29-2006, 08:43 PM

Quote:

Originally Posted by bwilson4web

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?

Are the numbers the same for the TCH CVT?

I've heard references to the TCH as the "third" generation HSD. Does anyone have any links discussing the Camry Hybird or know if this is so? I'm curious as to if and how the TCH HSD is different from the Prius HSD.

All I really know at this time is the inverter has been redesigned and smaller, that MG1 is rated 140HP, of which 40HP can come from the 244V battery pack with hard acceleration.

I've also noticed the TCH will drop into EV mode in literally just a second after removing my foot from the gas if my speed is under 44mph. I'm wondering how much wear and tear I'm subjecting the PSD and MG1 to when I'm coasting/maintaining speeds around 35 to 40mph on EV only power?

bwilson4web · #17 (**permalink**) 06-30-2006, 03:46 AM

Quote:

Originally Posted by nash

Are the numbers the same for the TCH CVT?
. . .

I don't know if anyone has any data to report. The Prius was lucky in that the Dept. of Energy took a special interest in the technology. They took the parts to the lab, ran the experiments and reported the results. Toyota engineering has also published a number of very insightful papers. Then too, I have an NHW11 03 Prius and seek these sources. But you raise an interesting question, how do we evaluate the ICE-to-wheel efficiency of other vehicles?

If I were doing it, I'd mount strain gages or put optical strain marks on the ICE shaft to the transmission and simular strain gages on a drive wheel. This would provide the torque and along with RPM, the power-in and power-out . . . piece of cake. A great assignment for undergraduate engineering students.

Bob Wilson

gonavy · #18 (**permalink**) 06-30-2006, 04:41 AM

Not having a HSD vehicle (or any CVT for that matter), I never thought of it much.

A planetary system with 3 independent and driven in/ouputs, one for the sun, one for the carrier, one for the ring, is DEFINITELY capable of providing a continuously variable gear ratio by varying the relative speeds of the three parts.

As Bob summarized, MG1 more or less dictates where the engine power goes, and in what proportion. Because there is no fixed, linear relationship between engine rpm and output rpm, globally speaking it is definitely CVT. If MG1 was not capable of changing speed to affect the motion of the engine/carrier relative to the ring/output, this would be a fixed reduction.

The gears themselves are fixed; it is the presence of the ability to change their relative rotation rates that makes it useful. The "transmission" is the entire system, not just the gears; therefore the "transmisison" is a CVT. Similarly, a normal transmisison includes all the hydraulic channeling required to engage clutches, all the idlers, etc.

Nowhere in the "definition" of CVT does it require that the mechanical parts be solely responsible for the ratio changes. Honda does it by sliding the belt up and down the conical pulley to affect the ratio; Toyota does it by using an electric motor (MG1) to affect the ratio.

The lay versions:
http://auto.howstuffworks.com/hybrid-car2.htm
http://www.ecrostech.com/prius/origi...plitDevice.htm

bwilson4web · #19 (**permalink**) 06-30-2006, 05:49 AM

Hi,

While walking the dog, I realized I made a mistake:

Quote:

Originally Posted by bwilson4web

. . .
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

The gear losses are a combination of friction and thrashing the transaxle oil losses. We've made an assumption that the friction losses will track with the power-in and power-out. However, thrashing the transaxle oil losses are independent of the power-in and power-out. I don't know how to factor this out except by repeating the test at different power levels. In simple terms, we are looking at the worst possible, highest values.

One other observation is the Prius MG1 is effectively a torque and power meter for the ICE. This means if I want to map a maximum ICE efficiency range, a combination of fuel flow and MG1 data would allow us to map it out.

LATE EDIT

In gear losses, we are used to having two fixed axles that see a lateral force proportional to the applied torque. However, the power split device is a planetary gear set. This means the MG1 and ICE torque are co-axial and not subject to the ordinary lateral forces of regular gears. The silent chain power take-off from the ring gear would induce some lateral force, the chain tension, but compared to a friction drive, this would be very small. Bottom line, PDS friction losses are even lower than ordinary gears.

Bob Wilson

pcflorida · #20 (**permalink**) 06-30-2006, 06:09 AM

I read this whole thread, I pretty much understand, and I love it all. Great topic.