64 % of Prius Power gets to wheels during max acceleration?

What HSI position would that correspond to? I noticed that top of ECO zone (where i try to accelerate all the time) corresponds ~2000 RPM (+/- 20%), lower at lower speeds and higher at higher speeds. So, that should be similar to your 2500 RPM at 60 MPH, right?

 

I think it would be cheaper for everyone to buy a hybrid (coming soon with the 50+ MPG CAFE standards).

 

No, I wasn't discussing 0-60 mph acceleration, only overall efficiency. The 74% figure was from Indy John, but it's nothing I've verified. The 64% might be more accurate. Sources will vary and I'm fine with that. My only point was that today's vehicles aren't very energy efficient. For the Prius engine alone, usbseawolf2000's estimate of 38% sounds plausible.

XS650 has done a great job of discussing the limits of CVT technology above.

Flywheels are being used to store energy in hybrid race cars like Porsche. Capacitors are being used, too. Flywheels are supposedly 70% efficient or more, but I've heard opposite things about their energy storage capacity - whether it's low or high.

 

Per the operating line, eCVT enables optimum BSFC (220 g/kWh) from 1,000 to 3,000 rpm. And then 230 g/kWh from 3,000 to 4,000 rpm. Beyond that to redline is 240 to 250 g/kWh.

Majority of the time would be spend at or near optimum BSFC.

 

Yes, but the argument is when the RPMs go higher than say 2500, the eCVT efficiency is dropping rapidly. Bob Wilson posted a paper from GM that hinted the highly variable efficiency of Prius' eCVT.

 

I monitored it when I had Gen2 Prius. Since I upgraded to PiP, I have not had enough chance to monitor and compare with HSI.

I'll keep an eye on it and will let you know. From Gen2 experience, as you exceed 60 mph, gas engine RPM gets so high to keep MG1 near zero. I think the engine would be outside the optimal BSFC. So it should be a balancing act between gas engine BSFC and minimizing conversion loss through electrical path. HSI bar may be doing that balancing act and providing us the information to stay at the most efficient load.

 

Link would be nice.

It depends on the vehicle speed. 2,500 rpm at 15 mph would not be good. From the ICE point of view, it'll be doing optimum efficiency. However, majority of the power would be going out to MG1 and out to wheels through MG2, hence conversion loss. It is a necessary evil because that's how eCVT multiple torque from ICE -- equivalent of a low gear but without shifting gears. As I have always said, Prius doesn't shift gear but instead it shifts energy sources. Motor = high torque / engine = low torque and how both blends in various infinite combination makes it an eCVT.

However, at 60 mph, all the power from ICE at 2,500 rpm would go out to the wheels. Very little energy would split through ICE->MG1->MG2 electric route because MG1 rpm would be near zero. eCVT would be very efficient during acceleration in this situation.

 

When you are talking about what power or efficiency The Prius "should" produce with a eCVT transmission and an Electric motor on acceleration are you factoring in the reality that The Prius has an Atkinson Cycle engine?

My feebly limited understanding is that this engine is more efficient but less powerful, especially when we are talking initial acceleration.

Just wondering, because I'm NOT a chart and graph guy. Infact, that one chart in this thread? I thought it was a joke until I realized it was real.

 

Both the Prius and Matrix have 1.8 L gas engine. Prius ICE runs on Atkinson cycle and peaks at 98 hp while Matrix runs on Otto cycle peaks at 132 hp. However, Prius has 36 hp battery to handle peak power situation giving it a combined power of 134 hp.

Matrix has step gear transmission so as it accelerates and changes gears, rpm will bounce up and down. It's peak power is at 6,000 rpm but due to the nature of step gear, it won't stay there for long and bounce up and down with the rpm. You'll get fluctuating (inconsistent) power output and that's what you feel as you change gears (some people actually like this!). The same applies to efficiency. There will be the most efficient rpm/load range and the step gear will move past that sweet spot as the car gains speed.

Prius is different and none of that exists, as the ICE can be held at certain rpm, as long as you don't move your foot on the accelerator. Atkinson cycle ICE is about 40% more efficient than Otto cycle. However, Prius has conversion loss due to the electrical nature of the transmission.

The conversion loss is 14-27% (90-95% for MG1, Inverter and MG2), lets say 20% but not all the power goes through the conversion. The power split varies on the vehicle speed and ICE rpm. Judging from the power rating of the MG1 and split behavior, I would say about 25% of all power goes through the conversion loss. 20% loss of 25% of ICE 98 hp is 5 hp or about 5% loss.

40% efficiency gain from Atkinson cycle with 5% penalty from eCVT is a good trade off. That's not counting the opportunity to stay near ideal BSFC offered by eCVT or the cooled gas recycled from the exhaust (to prevent fuel enrichment).

 

There are multiple apps that run on iPhone, iPad, and similar devices that use the accelerometers to record performance and with vehicle weight and aerodynamic coefficients, they report the effective HP. You have to add these other drag forces to properly adjust for the effective power. For example,

These HP lines are derived from the EPA roll-down test data for the Volt and PiP. Inertial acceleration data needs to include these drag forces to properly calculate the wheels-to-road power.

Bob Wilson

 

Bob, what do those HP lines represent? Are they only rolling resistance? 27.5 HP at 100 mph couldn't include aero drag for a Prius at 100 MPH.

 

Based upon the EPA roll-down coefficients, it would. But you bring up a good point. It is possible the EPA roll-down coefficients are for a restricted speed range.

Bob Wilson

 

Hummmm, maybe I need to conduct my own "roll-down" test:

• Find a LONG flat road, not well patrolled (over near Guntersville)
• Start a data recording OBD scanner and parallel GPS mouse
• Reach a speed of "a whole bunch"
• Shift into "N" and not touch the brake until the last minute (more exciting in the 2010 Prius that has not had the latest brake recall done, yet!!)

Take the velocity data and run it through the forumlas to come up with the roll-down parameters.

Bob Wilson

 

Opps, I see the mistake:

drag_lbs = (A + (B * v) + (C * v * v) ) :: "v" is mph, A, B, and C are roll-down parameters
drag_hp = drag_lbs * (v * 1.4667) :: I forgot to multiple the drag force by velocity in ft/sec.

The drag force curves are wrong scale but the proportions would be right. Regardless, I'll recalculate and update the graph images.

Bob Wilson

 

Good find, the resulting 59 rwhp to go 100 mph in a Prius is entirely believable.

It makes me really thankful that we don't use the metric system in the US, if we did we wouldn't have nearly as many silly conversions to keep our minds sharp.

 

Opps, there were two faults but let's go over basics:

• Test Car List Data Files | Cars and Light Trucks | US EPA - source of EPA rolldown parameters
• drag_lbs = (A + (B * v) + (C * v * v) ) :: "v" is mph, A, B, and C are roll-down parameters
• drag_lbs-ft/sec = drag_lbs * (v * 1.46667) :: "v" is converted to ft/sec
• drag_hp = drag_lbs-ft/sec / 505 :: scale to hp

Updated Volt_PiP charts:






So I've attached a spreadsheet, poorly documented, but usable for generating charts like these.

Four Prius models:

Notice how the 'Gen II' and 'Gen III' have nearly identical, drag-power requirements. Any difference in actual MPG will be: (1) more efficient transmission, and (2) improved engine efficiency.

Here we see the efficiency improvement between the 'Gen I' and 'Gen II' in drag-power:

Note that the drag-power does incorporate transmission and engine efficiencies. However, our 2003 Prius has: (1) Type WS transmission oil; (2) 6% larger diameter drive tires (aka., over-drive); (3) a well mapped, knee in the engine/transmission efficiency curve, and; Sumitomo T4s running max sidewall pressure, 52 psi.

Here we compare the drag-power required for the ZVW30 Prius and Prius v:

With nearly identical transmissions, the relative efficiency pretty well matches the relative EPA and user reported mileage.

Bob Wilson

 

That isn't a valid comparison. The M5 is traction limited a good share of the way to 60 so either the driver or engine control system, depending on mode selected, is limiting power to avoid wheelspin a good share of the way to 60.

 

So how do drag racer efficiencies compare?

Bob Wilson