Power Split Device

There are 2 ways in which the Prius has been modified over the generations to allow the vehicle to go faster speeds without the ICE needing to spin.

1. Redesign MG1 so it has a higher max rpm

2. Change the "final drive ratio" so that MG1 spins slower at vehicle speed

Toyota has done a combination of both over the years, as I recall.

 

Right, yes, the final drive—the gearing after the PSD and before the differential—also plays a role in the relationship between vehicle speed and MG1 speed. That went from 3.905 in Gen 1 to 4.113 in Gen 2, which does spin MG1 a bit slower at a given vehicle speed. (Out of techinfo days to look up the ratio for Gen 3 or 4.)

But that's entirely apart from the reduction gear that first appears in the Gen 3 transaxle, which was the subject of post 37 and also of the video in post 39. The video never shows the final drive components at all (though he does sort of describe them). That added reduction gear only sits between MG2 and the PSD, so MG2 can spin faster.

-Chap

 

I think those ratios are correct (or at least they look familiar) for gen 1 and gen 2. However, gen 2's higher 4.113 ratio actually makes MG1 spin a little faster than gen 1, not slower.

Things changed a lot in gen 3 with its 3.268 ratio between the PSD ring gear and the wheels when the ICE isn't turning. So, MG1 in gen 2 had to spin 26% faster than in gen 3 for the same vehicle speed.

A similar but less dramatic story applies in gen 4 because the ratio went down again to 2.834. So, gen 3 MG1 spins 15% faster than in gen 4 at the same vehicle speed.

Taken together, the MG1 in gen 2 spins 45% faster than the MG1 in gen 4 with the ICE not turning. The ratio between MG1 and the PSD ring gear has always been 2.6 in all Prius generations when the ICE isn't turning so that part isn't an issue.

To dig into the details, see this thread:
P610 transaxle | Page 4 | PriusChat

 

Thanks for catching me. Yup, had my numerators and denominators flipped for the final drive ratio.

Interesting that they keep being able to make the final drive ratio lower since Gen 2, while preserving good performance. I guess moving off the line relies mostly on torque from MG2, which they can keep delivering by further increasing the MG2 reduction ratio, in step with the decreasing final drive ratio. So in that sense, indirectly, the MG2 reduction gear is part of the story of how MG1 revs are kept down ... just not in the way the guy with the lab coat purported to show.

-Chap

 

Wow, this thread became interesting while I wasn't looking.

Right, Chap, there's nothing hard to understand or scientifically heretical about the so-called "heretical mode." It's a perfectly logical extrapolation of the way the system functions in other circumstances.

From the gear tooth counts (third-generation numbers) and tire size, I made a simple spreadsheet that computes car speed, MG1 speed, or engine speed when the other two of those are known. It isn't pretty, but works.

Does anybody know whether the speed limits on the motor-generators are based more on mechanical stresses, or on keeping back-emf's within safe limits? One clue is that the speed limit for MG1 spinning backwards is apparently lower than its forward speed limit. (Or perhaps instead it's programmed for a more conservative factor of safety for the more frequent case of coasting with engine off?)

 

I read somewhere that, unless the car is stopped, the engine is being turned, regardless of whether it's supplying power or not. Is that true?

 

No. At speeds of 42 mph and above, the engine must turn. Below that speed, the engine only runs if needed. However, the Prius Prime conditionally moved that speed to 84 mph.

Bob Wilson

 

Isn't it higher on the standard Gen 4 as well?

 

I've gotten the EV indicator as high as 73 MPH, at least.

 

While off, the ICE is spun at higher speeds to avoid over revving M/G1. IIRC, 42mph was the tip point on the gen2. I believe that speed limit has increased with each successive generation.

 

No.
Gen 1 and 2 engine must turn above 42 MPH
Gen 3 engine must turn above 62 MPH
Gen 4 engine must turn above 83 MPH

 

I like it! The Volt is a very nice car, but a little expensive. I always try to park near the one where I work so I can look at it when I walk by.

 

Yes, they brought it out here - at the time put a price of well over $60,000 - put it into Benz C, BMW 3 series territory. Consequently, most were still in GM dealerships 2-3 yrs later. At a more reasonable price, I could have been tempted.

 

Over $60,000? Yeouch, that's some serious early adopter pricing right there. I looked up current prices, the Prius starts at 24,6 and the Volt at 33,2. Hmm, and just for fun, did some cost comparison with an EV calculator, and it says that the Volt costs more to operate under any condition. Well not *any*, but at $3.10/gal and 11c/kWh, it's not even close. Steady loser. Apparently the problem is the crappy mileage off of its engine... just how the heck does a steady-state generator manage to charge the car and get only 40mpg, when the prius is topping 60 quite often? It is a mystery.

Edit: Sorry for semi-derailing. Thank you to the folks in this thread providing such cool information about the Priawesomeus.

 

It's not a steady-state generator, though. And, that wouldn't be the most efficient way to operate anyway - even though engine efficiency varies with RPM, it's sometimes more efficient to use engine power directly to the wheels, even at a slight efficiency penalty, than to charge a battery earlier, and use that power later. (Conversely, sometimes (usually at very light load), it's more efficient to charge the battery and then shut off the engine, rather than run the engine at light load.)

In any case, the Prius's engine has higher thermal efficiency (40% versus the new Volt's 36.5% thermal efficiency), and the Prius has better aerodynamics and less weight (so less power required to move the vehicle).

 

I suppose steady state was a poor term. I would expect it to be able to operate closer to steady state than the Prius though, as it is less reliant on the engine to provide power at any given moment, and therefore should be operated closer to optimal. Guess the problem is in your last sentence, the Volt's engine simply doesn't work as well. (Of course, it's a bit surreal to be complaining about an efficiency of 36.5% as inferior...)

 

The 2017 Prius Prime has an EPA rating of 54 mpg, and the non-plug mainstream Prius gets 52. The 2017 Volt has an EPA rating of 42 mpg.

Of course, many Volt owners are using relatively little gas since they have 53 miles of EPA electric range. GM spent their money on batteries and the electric aspect of the vehicle and chose to minimize costs on the gas engine side as part of further bringing down the car's MSRP. They could have spend more money to use the Malibu hybrid's new and more efficient gas engine, exhaust heat recovery module, and more expensive smaller motor-generator that uses rare earth metals and gets 46 mpg as a midsize car. Instead, GM chose to go with a cheaper and older engine, skip the exhaust heat recovery, and use a cheaper generator motor that uses iron permanent magnets instead.

I think this was the right choice. Folks who will be driving more on gas will tend to self-sort into buying a Prime and folks who can largely drive within the Volt's battery range will tend to lean towards a Volt. Choice is good.

 

A choice of "nicely under $30,000" was the price goal set by GM.

The reason for setting that was simple. A configuration able to achieve that with low enough production cost would be both competitive & profitable with other choices available on the dealer's showroom floor. Otherwise, those shoppers will just buy a different GM vehicle instead.

We still don't see that choice of GM plug-in hybrid yet.

True, taking the tax-credit subsidy from the MSRP would achieve that goal. But it limits inventory to well below competitive volume and is extremely short-term... hence the close look at Toyota's design. People want to know how Prius & Prime are able to overcome both engineering & business obstacles.

 

Yes, check out FUELLY data for VOLT - is substantially better, so most VOLT drivers must drive a fair bit on EV.

Though it's interesting - the range of averages is from 29MPG(US) to 437MPG - I would assume the 2 guys getting 437MPG always drive within the EV range limit.

 

... or tell factual inexactitudes. Just sayin'