This has been discussed in the past but we have some new members and more knowledge so let the debate begin!

I could fit the entire choice in the poll so here are the entire choices:

1) No! HSD is locked in top gear all the time....The torque to start comes from the motor(s).

2) Yes! The action of the motor in concert with the ICE acts as gear reduction/overdrive....depending on which way the motor spins.

3) What is HSD? / Don't know /Don't care / Can't decide.

It is an elecrtic continuously variable transmission, not a mechanical CVT, mechanically it is always in top gear. It uses a single planetary gear set that "splits" the engine power between the mechanical pathway (in top gear), and the electrical pathways, which take advantage of the torque characteristics of the motors to act as a low gear. Blending the pathways results in continuously varying the torque. I voted "yes" by the way.

Weird, by your reasoning I would have thought that you would have picked "no" because you say " mechanically it is always in top gear" like the "no" option.

Perhaps I've not worded the question correctly. Based on my incomplete understanding of HSD I'm voting "yes". The reason is simple. The ICE spins faster (reguardless of the power path) when the car is going slower and then the ICE slows down relative to the cars speed as the car speeds up just like a regular CVT.

It's a tough pick, it is always in top gear along the mechanical path, however, the amount of power that follows that path is continuously variable, so it does vary the torque that is transmitted just as if there were real gears, or a pulley/cone system.

Another thing which confuses things even more is the fact that GM has a similar transmission in their prototype models that works in a similar way -- an elecrtic CVT, but with two different mechanical gear-ratios as well, and slightly smaller motors, which should (in theory) allow for better top-end efficiency as not as much power would be lost along the elecrtic pathway at top-speed.

i'm by no means an expert on hsd but technically, isn't the torque ratio fixed at about 72% to sun gear 38% to ring gear? it's the power that is variable right?

also, i don't understand how you're defining "variable ratio" and "transmission". some would argue that psd isn't really even a transmission.

one challenge i see in trying to categorize psd as cvt is that at higher road speeds (lets say greater than 80mph), the motor/generator attached to the sun gear (mg2?) is going to consume more and more power as speed increases thus requiring more power from the ice at maybe 30% inefficiency. that means the ice is going to have to work harder to provide the power. this quirk gives me pause when considering psd a true cvt. obviously a belt and pulley cvt doesn't have this problem.

so at lower speeds, psd appears to act like a traditional cvt but at higher highway speeds it starts acting less and less like one. i would imagine there is a speed limit where mpg drops precipitously b/c the ice is working so hard to overcome the inefficiencies of the mechanical->electrical->chemical->electrical->mechanical energy conversions.

like i said, i'm not fully an expert on psd but this latter point merits discussion i would think.

Sorry abuot that, you're right, I meant power, and elecrtical transmission from the sun generator to the ring motor (or vice versa).

As far as I know there is not electrical->chemical->electrcal phase, unless power is being stored into the battery. Most of the time, it is simply transferred from one motor directly to the other without passing through the battery.

It is true that the system acts like a CVT at slower speeds, but runs into the problem of possibly over-revving the components at faster speeds. The solution to this would be to implement the ability to have a separate true gear. Some eCVT designs (which still go by the name elecrtic CVT) have multiple gears for this reason.

If a gear is something with a finite number of teeth, then it is a fixed ratio transmission with two motor generators that provide a continiously variable transmission function.

If a gear is a type of lever, then it is a continiously variable transmission with MG1 acting as an electronic, variable fulcrum lever to the ICE and MG2 helping to power and operate MG1.

To paraphrase the Bard, "A CVT by any other name would still perform so sweet."

Bob Wilson

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got it, thanks!


i may be mistaken about the battery being in the loop. even so, at higher road speeds my understanding is that the electrical inefficiencies would be around 30%, mainly due to impedence at those higher voltage levels.

yeah, i understand that daimler/gmc are working on a system that basically has two power split devices thereby offering a second fixed gear optimized for higher highway cruising speeds. the problem i see with this is the added complexity of the system. i suppose an easier solution would be to do what they did with the camry and put in a larger ice that could handle the power requirements at higher cruising speeds - of course now you're on a slippery slope...

i guess i'm inclined to say that psd is not a true cvt b/c of this speed limitation problem. i mean the whole purpose of cvt is to optimize the rpms of the engine at any cruising speed. obviously a belt and pulley system doesn't have this limitation at 80mph or even 100mph (provided the proper gear ratios exist). if hsd is having to artificially rev the ice more at higher speeds just to make up for the inefficiencies of the electrical system then i can't seem to get myself to consider it a true cvt...

not withstanding this, hsd is a beautiful system.

Either you are mistaken or a Prius getting 55+ MPG at 65 miles per hour is breaking several Laws of Thermodynamics. But I am very interested in the source for:

Sometimes folks cite something out of context not realizing that the caveats are important. Give me a pointer to the source and I'll be happy to review this claim. This is not the first time I've read it.

Bob Wilson

Operation Iraqi Oil Freedom:

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My other 1500 cc car:

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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
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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
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