One of the main differences between the Honda Hybrid and Toyota's is how the "CVT" part is implemented: Honda's is a true CVT, allowing both the electric and gas engine to have their gear ratio varied for optimal performance from both. Accellerating in the Honda, you can hear the gas engine rev up briefly, then sit at its optimal RPM continuously through 85mph.

Toyota's is an "eCVT," and I know I'll stir up trouble when I suggest this, but it basically allows the gas engine to run near-optimal RPMs, by making up for it with the electric engine. This basically means the electric engine falls outside of its optimal range easily. In practice it also means the gas engine isn't running at a constant optimal speed like it does in the Honda.

So, what if the Toyota model was a real CVT - if it had a mechanical CVT transmission tied in?

1) You could tow it in Neutral without concern - the mechanical CVT could modify the gear ratio to protect both motors.

2) The gas engine really could run at its optimal RPM at all times - in the eCVT design you can hear the gas engine rev low at 15mph and high at 70mph.

3) You could Glide at any speed, even 85mph if you like, because the mechanical CVT could protect the electric motor from revving over 10000rpm despite wheel speed.

4) I'm not sure of this, but you could probably get more optimal power out of the electric engine - because you could place a more optimal load on it at all times.

Ah, if only Honda and Toyota could work together.

 

"Honda's is a true CVT"
"what if the Toyota model was a real CVT"

Yeah, I can't wait to see the responses to this one... Should be fun. Kinda like walking into any restaurant in the South carrying a book entitled "The Encyclopedia of Biblical Errancy."

 

Yes - I'm not trying to be overly critical of the Prius - I love it so much that I bought it over the HCH afterall! Just giving credit where it's due.

 

If the patent dispute goes against Toyota, you may see a mechanical CVT in their hybrids.

Bob Wilson

 

I really didn't mean this as a religious issue... .

What I'm honestly saying is imagine the possibilities if that gear ratio could be varied? I'd love to glide at 55mph, which tends to be the cruising speed on local roads around here. I suppose it would significantly increase the cost of the Prius though, and would certainly increase the weight, though I don't know how much.

 

I think you are missing one of the critical engine differences, the Atkinson cycle versus the Otto cycle of the Honda. Although both vehicles have variable valve timing, the Atkinson cycle provides:

1) low-ratio compression, high-ratio expansion - this leads to high thermal dynamic efficiency
2) low-ratio compression - leads to easy starting so the ICE can easily go ON/OFF
3) nearly flat, thermodynamic efficiency - the Atkinson cycle has a broad, high efficiency energy range that avoids the narrower Otto cycle efficiency band

So you'll find the Atkinson engine in the Prius has a computer controlled red-line of 4,500, NHW11, or 5,000 rpm, NHW20. My understanding is the Honda engines typically redline at 6,000+ rpm.

Bob Wilson

 

Hi,

I do need to help clarify some of these statements:

I'm assuming you mean behind an RV. This limitation is also shared by most hydro-mechanical transmissions. That is why the two-wheel dolly tow sets are so common.

Actually, the towing limitation is a speed limit, 42 mph. We are planning to investigate the cause but the leading hypothsis is a voltage issue risks the inverter electronics. If the car is towed at speeds below 42 mph, it should be perfectly OK.

BTW, the NHW11 Prius has a mechanical gear selector. My understanding is the NHW20 Prius has an electronic version that may make leaving "P" without a driver difficult to impossible.

The Atkinson cycle has a broader range of optimal rpm compared to the Otto cycle.

With the exception of going down a hill, you can drop the transmission into "N" at any speed. Driving down the interstate at high speeds, dropping into "N" is perfectly safe. The only thing to avoid, which is also against the motor vehicle code, is to coast down a hill in "N."

The only way to over-spin MG1 is to start at a speed below 42 mph, coasting down a hill, with the ICE off and then putting the Prius in "N". As the car accellerates above 42 mph, MG1 will be spinning fast enough to generate high voltages that put the inverter at risk. But coasting down a grade is illegal in just about every US motor vehicle code.

This part has me scratching my head. I've gone up Brindley Mountain, ~160m rise, at 80 mph operating the ICE and MG1 at maximum power. Could you explain this a little better?

Bob Wilson

 

My opinion is that the Prius eCVT is inherently more robust than the band CVT used by Honda, Nissan, etc. It has the advantage that there is no dependence on friction surfaces to transmit power, means that there really isn't anything in there that would wear out. A regular automatic transmission uses clutches to change gears, and those tend to be the first thing to die. Same goes for a manual transmission, which depends on friction to synchronize the gears. (Granted, you can still drive a manual transmission with bad synchros but it's a PITA.) Like a manual transmission, a band CVT has a clutch that is used to get the car going. The band also depends on friction to engage the spindles. Now, don't get me wrong: I don't think these things would be around if there was a real problem. But I like the idea of gears that are always meshed and simple, reliable electric motors to vary the gear ratio.

I have faith that Toyota will find a way to improve highway fuel economy with their next generation. In fact, I'm banking on it .

 

Automatic transmissions do not have a clutch, they have torque converter.

 

Auto tranny's have a torque converter to effectively double the gearing, plus they have a clutch (wet pack, typically) to engage and disengage drive and reverse, plus they have individual clutches (bands, typically) on each gear to select which gear is engaged. That is, a typical 4 speed auto has a wet pack clutch, 5 bands (including one for reverse), and a torque converter. All seven of these items are frictional slippage devices, and only the torque converter does not wear some with each use (the torque converter is a fluid flow device).

Manual tranny's have a master clutch plus (typically) a syncro mesh to help engage each gear. That would be seven total frictional slippage devices for a typical 5 speed manual.

The Toyota HSD power-split device mechanically couples via planetary gear-sets the engine, the two motor/generators, and the wheels (via the differential), without any frictional slippage devices. The two motors, engine, and wheels are always FULLY engaged, nothing ever slipping. The effective gear ratio between the engine and the wheels is a function of MG1's speed relative to the engine.

In case you are interested, neutral is achieved by letting MG1 spin freely (open circuit). Similarly, by "shorting" MG1, it will be locked up, letting the engine directly drive the wheels (with or without the help of MG2).

-- Alan