CR says 60 MPH = 3495 RPM

MG2 is turning at 3575RPM however the ICE can be turning anywhere from 685RPM under no load to about 4370RPM under high load. On flat level ground the ICE will be quite slow. Have a look here http://eahart.com/prius/psd/

 

Which issue?

Bob Wilson

 

This just shows a lack of CR's understanding of the Prius power train. The ICE RPM constantly changes. It is low when there is no need for HP (coasting or going downhill) and high (yes, it can get even higher than the number quoted by CR) when accelerating or going up hill. There is no fixed relationship to speed.

JeffD

 

Hi tornado...,

Yes, CR is only giving you half the truth on this issue. Here is a video which demonstrates the CVT in action.


http://video.google.com/videoplay?d...=12&start=0&num=10&so=0&type=search&plindex=1

 

The only time I see ICE RPM that high at any speed is during very brisk acceleration or maintaining speed on a steep uphill. As others suggest, cruising at 60 MPH won't do it. Cruising on level terrain generally puts it in the range of 1500-2000.

The lowest I ever see it is about 980 RPM, a figure validated by many others.

 

Great simulation!

Fascinating demo, thanks for the link!

http://eahart.com/prius/psd/

 

It's a CVT, so the common RPM vs. MPH maps don't exist for a Prius. If you need more power the engine runs faster. A level cruise at 60mph with no wind, and I see barely north of 1500RPM. Climbing an insanely steep hill, maybe it would get over 3,000. The only circumstances under which I ever see 3500RPM+ is when I get on the throttle for whatever reason.

 

when going downhill and the ICE is essentially just coasting, the RPMS will be around 975 AT ANY SPEED... so CR is right... at least in one circumstance...it is possible with the right conditions for the RPM to be that high at that speed but it is much more likely to be higher at much lower speeds... brisk acceleration from a standstill will easily get RPM's that high

 

Could they have performed the test in "B"? I'll head over to the book store and see if I can find the article.

Bob Wilson

 

Thanks for correcting me, I thought it was about 990RPM.

I only used the tool from the site I linked. I was looking for a number about 990RPM but the tool only showed a minimum of 685 but now it is behaving differently, oh well. Would it go that low on a trailing throttle?

You get the idea anyway.

 

at 60 mph the figures should read depending on how level the road is and at sea level the numbers I see are
MG2 3400 RPM
ICE 990-1800 RPM
MG1 5200 to 2300 RPM
depending on the hill I've see the RPM all the way from 990 to 4990 RPM
Here on the west coast we have some hills and if you set it on cruise and let it do the controlling of the ICE there are some pretty wild RPM swings.

 

This is close enough that I think what may have happened is CR incorrectly identified this as the engine RPM and not an internal to transaxle, motor generator rpm. There are idlers in transmissions whose rpm really doesn't matter unless you're designing a modification.

Without seeing the original text or quoted material, I don't know that it makes sense to send CR a correction on what non-technical owners might consider to be 'techno-babble'. After all, our Prius do not come with engine tachometers so it isn't like a CR reader and Prius owner will be comparing the CR article and their non-existent tachometer reading.

Bob Wilson

 

Do any hybrids have tachs. I think it would be hard to have a car without a tach.

 

The problem is the engine rpm is not fixed but in normal operation can vary at 60 mph from 1,000-5,000 rpm and each is perfectly viable. Unlike all other vehicles, there is no fixed ratio gearing so the rpm is all over the place. That is what continuously variable transmission means. But you could try to calculate probable values if you have a graph of power required as a function of speed and power generated as a function of rpm:

So at 60 mph, we need 17.5 hp to sustain speed. Next we need the engine power as a function of rpm, which looks like:

So with this chart we find 17.5 hp can be generated from 1,550-1,700 rpm. The difference is how much torque is applied at the different speeds. However, there is a powerful motor-generator set and battery that may need recharging or be discharging to provide motive power . . . which is not part of an ordinary car.

Without seeing the original CR quote, the reported number is within spittin' distance of the powerful, MG2, motor generator rpm. It is reasonable to assume they used this number instead of the more flighty, rapidly changing, ICE rpm.

Bob Wilson

 

Nope, not hard at all. Press the accelerator pedal, and the car goes. Press the brake pedal, and the car stops. What do I need a tach for? The engine speed tells me nothing useful and is likely to change constantly as I'm driving down the road. It would get annoying to see the needle jumping up and down constantly.

 

Personally, I only read CR for their reliability report. Nothing else.
I have found on many occasions that their testing/reviews are flawed
For example, recently I have been researching Mazda CX-9 for my wife. Sat in
one at SF Autoshow last month.
CR listed its 60mph-to-0 braking at 159ft, which is as worse as a Ford Excursion.
No way! CX-9 has four wheel ventilated disc brake (can you find another SUV/CUV has that kind of braking setup?).
Other magzines such as MotorTrend and Automobile praised CX-9's outstanding
braking at 127ft (60mph-0).
Gee, who am I gonna believe, CR or MotorTrend?

Sorry for going off topic a little. There are people who are more professional in
testing vehicles than CR. Forgive their mistakes because there are lots of them.