Highway Misconceptions

Minor nit ... "MPG plenty" on page 3 probably wanted to be "MPG
penalty".
.
Major nit ... and I feel really funny correcting someone who's been
a recognized Prius authority for so long, but need to: The battery
doesn't stay full on the highway. It always seeks 60% SOC, or six
blue bars on the NHW20 MFD. Current bobbles in and out of it at a
very low level in steady-state travel, not enough to matter, until
something more radical happens like warp-stealth or decel. And both
motors must [per the nomograph] be spinning, one way or the other...
.
_H*

 

<div class='quotetop'>QUOTE(john1701a @ Jun 18 2007, 12:26 AM) [snapback]463779[/snapback]</div>

Well in that case,

Thanks for the suggestions but to be honest I was hoping that you would give advice on how to gain highway efficiency without instrumentation. I think a list of practical tips with and without explanation would be helpful. For example I was recently told that my MPH should never double my instant MPG as advice for higher efficiency ICE operation without a tachometer when accelerating or going uphill.

I understand the inclination of those technically gifted to help but as I'm not one of them I wish for a two line per point redux, call it the Prius decalogue.

The people who make these incorrect assumptions and those who are reading them without objection are newbies and get totally lost in the explanations. I don't think the current doc is too detailed, I'm referring to many others. Those who are further interested will slowly catch up.

But if you are looking to educate those who are most likely to have these misconceptions you speak of please make a list for Prius newbies and keep it simple. And I'm speaking as someone who recently went through this procedure of trying to understand how a planetary gearshift affects my mpg.

Thanks for your contributions we all appreciate them

 

Nice work, John, as always. Nothing jumps out at me beyond what others have pointed out.

<div class='quotetop'>QUOTE(ystasino @ Jun 18 2007, 01:53 AM) [snapback]463828[/snapback]</div>

I agree that would be nice, but I don't see John doing that. He is a proponent of the "just drive it" philosophy. As he likely will assert, the Prius is a fuel mileage champ when driven "normally." There is certainly nothing wrong with that approach. It probably suits the needs and driving style of the masses, whom he is among the best at educating. For us nerds who want more, we'll probably have to look elsewhere. (John, jump in to correct if needed.)

 

In the second sentence of the article, you state that, "a combustion engine is inefficient when accelerating."

I always thought that an ICE was at its most efficient when operating near its peak power output, and inefficient when operating at a small fraction of its peak output; and that since it spends most of its time operating at a low output, and only putting out peak power for a few seconds at a time, during acceleration, the hybrid gains its advantage by using a smaller ICE and arranging for it to be able to operate near peak output much of the time, since acceleration can be assisted by the motor, thus eliminating the need for a larger engine for those very brief periods when acceleration is needed.

If the principal inefficiency of a conventional ICE were during acceleration, the loss would be minor, since it is accelerating so seldom. But when the inefficiency is manifested at LOW power output, the loss is great, as this is where it is operating most of the time, and the use of a smaller engine makes a large difference.

The electric motor allows the engine to be smaller, and therefore more efficient. And the "full" hybrid arrangement allows the ICE to avoid inefficient low-output operation by alternately charging the battery (while the ICE operates in the more-efficient higher-output mode) and shutting off (letting the electric motor push the car for a while.)

In the ENGINE section, you state that the tradeoff in the Atkinson engine is reduced horsepower. This doesn't seem right to me. I thought that the tradeoff is reduced TORQUE for a given weight, in exchange for greater efficiency (greater horsepower for a given amount of fuel consumed). The reduced torque is compensated for by the electric motor, which has more torque per unit of horsepower than an ICE, making the Atkinson engine ideal for a hybrid, but a poor choice for a conventional car.

This, in any case, is what I understood of the HSD system.

 

<div class='quotetop'>QUOTE(ystasino @ Jun 18 2007, 01:53 AM) [snapback]463828[/snapback]</div>

FWIW, I heed Hobbit's much earlier suggestion to keep instanteous MPG between 25 and 75 for best efficiency at highway speed. If the ICE is on, try to keep it in that range. Practically speaking, within limits, that means I'll pick up speed on modest downhills rather than let the engine loaf above the 75 mpg level. Got 58.1 mpg yesterday over roughly 30 miles of secondary (55 mph) and 60 of interstate (65 mph), AC on, Prius packed with family of four plus camping gear. By a modest margin, that's the best highway mileage I've ever gotten. Lot of variables factor into that, but I think Hobbit's limits are a useful guide for the tradeoff between constant speed versus constant throttle.

 

<div class='quotetop'>QUOTE(daniel @ Jun 18 2007, 09:31 AM) [snapback]463921[/snapback]</div>

In the old days of carburators it was very true that Gasoline Engines were most efficient at WOT (Wide Open Throttle). I don't know whether that still applies to all modern Gasoline Engine configurations or not but I have not seen anything that indicates that it isn't still true.

So, I agree with your analysis.

 

<div class='quotetop'>QUOTE(chogan @ Jun 18 2007, 09:37 AM) [snapback]463922[/snapback]</div>

I guess I'm not breaking any news but would it not be clever for Toyota to have a "highway efficiency button" which would force the engine at a specific RPM. I mean, I read that many people get mpgs in the 60s with the current car on the highway at 60-65mph.

 

John,

Why do you say two-mode hybrid will be more efficient than HSD on the highway?

Although two-mode can have pure mechanical power flow at the highway cruising speed, the ICE will remain Otto cycle. I have not see anything from GM that would use Atkinson-Miller cycle ICE.

From my understanding, the electrical path for the HSD on the highway cruising speed is minimal and the efficiency loss would be very small.

I would expect both HSD and two-mode highway mpg to be comparable. If you have any supporting document, enlighten us.

 

John, I don't have a comment about the highway misconceptions document to add, but I do want to say thank you for the material you have posted on your site. It, and the approach of offering explanations from the angle of simplicity and clarity, were instrumental in my decision to look at a Prius in the first place, since otherwise the misconceptions regarding the technologies used and the operation of the car would have continued to put me off.

So, thank you!

 

<div class='quotetop'>QUOTE(john1701a @ Jun 17 2007, 09:26 PM) [snapback]463779[/snapback]</div>

Hi John!
Thank you for putting so much time and effort into these user guides
I would suggest, if you have information, adding something about the use of cruise control on highways, and the effect of that on MPG, and the hybrid efficiency. Since the cruise maintains a constant speed, is there enough of a variable to cause the battery charging that you attribute to the decline of 1 mph in normal driving?

Thanks,

 

<div class='quotetop'>QUOTE(Tempus @ Jun 18 2007, 05:35 PM) [snapback]463987[/snapback]</div>

This is still true, but some gasoline engines manage to run with wide open throttle under the right circumstances. The VW FSI engines over here do it, with high octane fuel and under steady, light load (highway driving). The gas pedal is drive by wire, and the computer will open the throttle wide and regulate the engine using the injectors only as I understand it. BWM achieves it using Valvetronic.

 

No, please don't say that and mislead the readers. The 1NZ-FXE is
not a diesel, it does not regulate fuel intake solely with injectors.
The engine runs under a stoichiometric mix at all times -- or to put
it another way that might fit in better with a public document, the
fuel mixture is kept as close to "perfect" as possible. That involves
careful coordination between injectors, throttle, valve and ignition
timing, and even how much load is placed on it by the electric motors.
There really aren't so many "all or nothing" conditions as people
tend to think in this car -- it's a smooth and fairly optimized
continuous blend of many parameters at once. Fast computers can
do that. The old early EFI klunkers of the eighties and early
nineties have left a bad taste in many mouths, but we're past that.
.
_H*

 

Hi Evan, etal,

From what I remember of talk on that ORNL paper on another group, the issue is not the PSD itself, most automatic transmission cars have planetary gear sets. Its the two sets of step-down gearing and silent chain from the high PSD speed to the axle.

A manual trans usually has the top gear as a straight through coupling of shafts, with no gearing. Which of course is very efficient. "Overdrive" is the exception, where the top gear speeds up the output shaft with relation to the engine/input shaft. So, when at highway speed, a standard manual trans has very little loss. I might be all confused on that - I have not worked on front wheel drive manual transmissions, except to change a clutch. So, maybe they are different.

Hmm, does that mean if the Prius had 12 inch tires and one less step down gear it would be more efficient? Not too practical with all the pot-holes around I guess. Unless you have 12 inch tires of the type on the cars in the "I Robot" movie, and a Boze suspension.

Also, they saw significant differences in efficiencies when the trans-oil was at various temperatures. The best being quite warm. Might be the reason for the 60.8 mpg tank this past two weeks here, while Chicago-land was trapped in the heat island.

Wheel motors makes sense too. If MG2 was taken out of the trans, and made a MGRight and MGLeft wheel motors. That way the electric path losses wont see the gear reduction loss. Or, put MG2 on the crown gear of the axle - same thing, but incurs axle joint loss.

 

<div class='quotetop'>QUOTE(john1701a @ Jun 17 2007, 11:26 PM) [snapback]463779[/snapback]</div>

I love this post! I recently bought my first Prius and as soon as I read your info, I started trying for the optimum MPG according to your suggestions! This morning alone, my overall average went up from 50.0 to 52.7 and that was only about 35 miles added to an already 435 miles on this tank! Thanks very much.

 

<div class='quotetop'>QUOTE(john1701a @ Jun 19 2007, 04:18 PM) [snapback]464943[/snapback]</div>

Thank you, John. I agree 100%. Any car will get better mileage if driven extremely conservatively. The only difference with the Prius is that the immediate feedback encourages the mpg video game, which can become a form of distracted driving.

Like John says: Just drive it.

And drive safely, especially if you live near me.