How good mileage could the Prius get

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 03:47 PM) [snapback]460353[/snapback]</div>

This is just a matter of shifting the argument to something else every time you face rebuttal, thus avoiding the very issues you raised yourself.

There are a number of potential sources of energy outside the vehicle that can impact on it's performance or fuel economy and which you don't consider. It may well be that these are largely self-cancelling over time, such as driving up the same degree of slope for the same distance as driven down, or facing the same headwinds at the same speeds as tailwinds, but it cannot be assumed to be so. One also has to factor in the varying efficiency of energy use, allowing for different distances to be traversed dependent on the source of that energy and how it is being fed to the wheels.

To take the most absurd example for the sheer sake of rebuttal: if one were to haul a Prius to the top of a tower block and drop it off, not one ounce of it's energy would come from the gas tank in the trip to the ground

 

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 04:09 PM) [snapback]460363[/snapback]</div>

So you don't know the figures, but you assume your stance on the subject is right even so? Yes, one can push a Prius on the flat, but it won't keep rolling when you stop. Yes, it's possible to pull a Jumbo, but it takes a fairly strong guy expending a lot of energy to do so, and not only doesn't the plane keep rolling when he lets go the tow rope, but it's a very long way from the mathematics of driving a car at highway speed for prolonged periods.

And yes, despite your assertion, weight does factor into straight/flat fuel expenditure. It works the same as lift does for aircraft (except the other way around of course!). The heavier the vehicle, the more effort is required to move it at at any speed in comparison to a light one, let alone to get it up to that speed to begin with, and stop it from that speed afterwards. In any event, since the real world doesn't feature many such straight, flat, surfaces to drive on it's a spurious argument in the first place. Normal driving requires on-going and pretty constant speed changes, all of which will use more energy to effect in a heavier vehicle than a light one.

As to the larger battery - you ignore the physics here too. More weight, more energy required to move it, longer periods required to charge it taking more gas to drive the ICE.

You may be right about the future development of alternative engines, but since these aren't available to Toyota today, they aren't a factor in the debate about whether the Prius is getting the fuel efficiency levels it really should.

You seem very selective in the assumptions you make and the arguments you draw from them. There is nothing, for example, to suggest that Toyota really did design the hybrid systems to meet the needs of Tokyo traffic, or that even if they did, it is not also well suited to the driving of many users here in the US too. You talk about 'optimizing' it, assuming that it is not optimized as much as possible, within the confines of the technology it uses and which is both available to the manufacturer and economically viable to include.

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 04:18 PM) [snapback]460363[/snapback]</div>

I wasn't thinking of using mine so much as one or other from the occasional dissatisfied owner who posts here from time to time!

 

Per,

You do dismiss things quickly without reasonable justification.

It sounds like you saw the Prius simulator that showed how much energy is used to propel the car. It broke out Aerodynamic drag, rolling resistance etc. I think the only factors are;

1. aero drag
2. Rolling resistance
3. Mechanical friction.

The CRX has a significant advantage in all categories. So, that explains why it gets better mileage.

You still state that you think that the Prius should be able to get better highway mileage than it does. Well, I don't think so. Can you tell me one reason why it should?

 

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 02:54 PM) [snapback]460422[/snapback]</div>

This is also basic physics. Its called INERTIA. A heavier object has more of it, and will take more energy to get moving. It'll also take more energy to STOP, but in the case of cars that's usually negated by friction and air resistance.

It doesn't happen with your bowling balls because heavier objects also have more gravity. That happens to exactly match the extra effort needed to get them moving, so they fall at the same speed. (everything has its own gravity well, but with most objects its so tiny you don't notice)

In the case of your "level ground" example...it doesn't matter because most of a car's effort is used to accelerate. The extra weight does add a bit more rolling resistance as well.

 

Per,

You claim you are an engineer, but you state that rolling resistance is not proportional to the weight of the vehicle. Then you use the Airplane example?!?! Sheesh.

Here is the formula from SKF bearings for bearing friction. Note that it is directly proportional to the load.

http://www.skf.com/portal/skf/home/product...;newlink=1_0_36

I could not find a formula for rolling resistance, but lets look at the physics of rolling resistance. The losses are mainly due to the hysteresis losses from flexing the tire. Those hysteresis losses will be proportional to the vehicle weight, since the rubber will flex more due to a heavier vehicle.

Now don't go and change the argument on me again. I explained the physics as to why it takes more energy to roll a heavier vehicle than a light one. Show me some data to contradict me.

 

<div class='quotetop'>QUOTE(Winston @ Jun 12 2007, 06:40 PM) [snapback]460474[/snapback]</div>

Well I am an engineer and I KNOW that weight has an effect on rolling resistance. The formula is F=CRF*FN
F: force required to move the vehicle
CRF: coefficient of rolling friction
FN: Force Normal (Force normal is WEIGHT X Acceleration of gravity or 9.81 m·s−2 on Earth)

It is a linear relationship where something that weighs twice as much will take twice the force to move. Try this out for yourself Per if you like with a wheelbarrow and some stones.

Now I think Per's point is that at highway speeds, rolling resistance is a small fraction of the total resistance of the vehicle. Rolling resistance of the tires increases at a linear rate while aerodynamic resistance increases exponentially with speed. So the aerodynamic resistance overtakes rolling resistance quite quickly. Usually this is somewhere around 35 to 45 mph. This is why land-speed record cars don't really worry too much about weight. When your going 300 mph the rolling resistance may only be a couple percent of the total resistance. At typical highway speeds for a car it is more like 20%/80%. (Wild Educated Guess)

I believe everyone else's point is that though it may be small it still exists.

Michelin has an interesting article on tractor trailers. For their typical 80,000 lb truck at 55 mph they put the split at 40% aero / 38% rolling / 14% drivetrain losses / 8 % engine accessories
http://www.goodyear.com/truck/pdf/radialre...etread_S9_V.pdf

 

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 11:49 PM) [snapback]460113[/snapback]</div>

OK. It is physics.
We are talking about on the flat and constant highway driving and no plug-in, right?
- Bigger battery does not work here. The Prius's battery becomes smaller and smaller, but both performance and fuel economy were improving on the Prius history.
- Regenerative energy does not work here.
- The tank-to-wheel efficiency of Prius's Atkinson cycle engine is already 37%. Yes, the diesel engine is more effective and diesel fuel has more energy than gasoline.
Please refer to following article...
http://www.greencarcongress.com/2006/05/fuel_consumptio.html
The best one is about 40 mpg at 70 mph.
We need 75% increase to achieve your 70 mpg proposal, but I think you need some magic.
I believe there is no magic here and we can't achieve 70 mpg by mid-sized sedan in the near term.
Of course, small and light weight vehicle like Honda CRX/Insight could achieve it, I think.
Maybe, we'll see a teardrop shape cars...


Ken@Japan

 

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 05:54 PM) [snapback]460422[/snapback]</div>

You mischaracterize the responses you are getting because I don't see much defensiveness here at all. What I see are rebuttals to your assertions that nothing counts except what you elect to include in your conclusions. Characterizing those rebuttals as defensiveness certainly makes it easier to dismiss them, but doesn't make the characterization an accurate one.

I think if you read the thread again, you'll see just about everyone who has responded to you has agreed that there are potential areas where the Prius could be better, they (we) just don't agree with some, or indeed many, of your suppositions about exactly how - particularly drawn from your initial comparison to the 80s CRX since as we have discussed, it's not comparing like with like as you assert.

What seems very clear to me is that IF car manufacturers in general (and even Toyota in particular) had retained their focus on fuel efficiency that came out of the oil crisis in the 70s, instead of returning to the staid and unimaginative gas-guzzling legacy of cheap fuel that had gone before, once the oil market had calmed down again, today's Prius would likely be getting 70 or 80 mpg or even more because we'd be 20 years ahead in development of alternative fuels and fuel-efficient powertrains than we are.

As such, I think there are plenty of ways in which the Prius can (and will) be improved to get better fuel efficiency. I don't think Toyota are, at this point, in a position to make much in the way of different compromises in creating a marketable and economically viable production vehicle however, because as much as it may be technically possible to do something such as 'optimize' it for different markets for example, doing so would likely create an upwards swing in cost (thus price) and a downward swing is sales.

What it really boils down to is a matter of what technologies are available at this time, and of those, which combination at viable cost produces what viable vehicle design options, and of those, which vehicle(s) become an economic proposition to put into production. The result is far from perfect, but it is, in a real sense, just the first step or two on an evolutionary ladder, not an end result of that evolution. As such, the 2009 model will undoubtedly bring several improvements which may address your interests rather better than the current model does - or indeed allow Toyota to take a different approach and 'optimize' further for fuel economy at the cost of compromising other factors. Ultimately, their engineers will design and develop the products their marketing people say they can sell and which their accountants say they can make best profits from. Our choice is to buy those products if their decisions fit our needs, or not if they don't.

 

<div class='quotetop'>QUOTE(ZA_Andy @ Jun 12 2007, 01:35 PM) [snapback]460380[/snapback]</div>

I agree that weight is a critical aspect of fuel economy that cannot be ignored. It probably doesn't matter if you're trying to measure the FE on a dyno, but the real world is a bit different. If the Prius went on a crash diet of weight reduction, and could weigh as little as a Metro, you'd probably see some pretty high MPG numbers.

 

<div class='quotetop'>QUOTE(Per @ Jun 12 2007, 07:49 AM) [snapback]460113[/snapback]</div>

HSD turns the ICE on and off frequently. The Atkinson design allows this to happen with a very low energy cost. Diesels, with their high compression, "cost" more to start, and are therefore less suitable for a frequent stop-start application.

In 2004, battery capacity was more expensive than it is now. You are correct that more battery results in better mileage, but in 2004 it was a cost:benefit compromise. The 2009 Prius will have more battery capacity (probably lithium) and will benefit from that.

MPG will be significantly higher in the MY 2009 than it is in the present generation.

 

<div class='quotetop'>QUOTE(Per @ Jun 13 2007, 12:22 PM) [snapback]460910[/snapback]</div>

No, the gist of responses here have been rebuttals to your assertions and notions, not to the possibility of the Prius being capable of being more fuel efficient of itself - more to the fact that your suppositions as to how that might be achieved were being challenged.

Neither is it interesting in any way that I make the case that mileage improvements could be made to the vehicle in the context of the discussion as it stands and for the reasons I have already given. It would be crass to suggest that as we start on the road to genuine fuel-efficiency in the face of certain oil supply constrictions and cost increases (which launched the fuel efficiency movement in the first oil crisis but was sadly not sustained at that time) that we have reached the zenith of achievement in just a couple of steps, and that Toyota have all the variables right. This is just the start. In a little over a year, the next edition of the Prius will move us further along, and after that... well, who knows.

This is really as much to do with attitudes in the marketplace as anything to do with the technologies themselves. As alluded to previously, in Europe and other places where fuel costs have been much higher, and perhaps where traffic density has also added other pressures, small, light, fuel efficient cars have been commonplace for many years. I was getting 45mph from my sub-1L engined mini in 1982, without much sacrifice in speed and performance, though it was hardly suited to a trans-continental trip! Given that I lived and worked in a big city in noerthern England, that wasn't part of my need, so the compromise was a good fit.

Meanwhile here, where bigger is better and fuel much cheaper, the emphasis has, aside from during the oil crisis, been firmly on large vehicles consuming far more fuel because the cost of fuel wasn't an imperative. Introducing a fuel efficient vehicle with those market influences would not have resulted in high popular sales, but would have resulted in smaller and less 'glamorous' transportation. And without the potential market, the imperative for manufacturers to develop fuel-efficient technologies is simply not there, or at least, not economic.

It may be that thanks to the pressure of traffic density in Tokyo, and the high cost of fuel in Europe that Toyota began development of what is now the HSD. I recall it being described on a science program on TV in the early 90s where the claimed fuel economy of potentially 70mpg (imperial gallons are 25% larger than gallons here, so mpg appears 25% higher when in fact it's the same actual distance per 'unit' of fuel) created a distinct stir, and lots of interest. The same claim here would have gone pretty much unnoticed.

Now, we're beginning to reap the benefits here of Toyota exploiting the market for fuel efficiency which existed elsewhere and justified the development of the Prius. Now that fuel costs are rising here, greater interest is being shown in the market for fuel-efficient vehicles (as can be seen from May 2007 record Prius sales in the US) and thus with the largest car market now stirring, Toyota will be in a position to justify increasing ongoing development of the technology because the economy of scale makes it increasingly viable.

It would therefore be an amazingly silly thing to claim that there was no way to make a Prius more fuel efficient - we just have to wait while technologies develop that make it possible.

 

<div class='quotetop'>QUOTE(daniel @ Jun 13 2007, 11:22 AM) [snapback]460909[/snapback]</div>

New diesel engines used in European cars turn their engines on and off at a stop like the Prius and other hybrids. This has been introduced in the latest cars from Mini, BMW, Mercedes, etc. While diesels have higher compression and thus are harder to start it also marks them more efficient.

 

I don't think people are just fixated on the weight. All the posts about weight is just for your information since you said that weight does not affect fuel economy. There are a lot more variables than weight and drag that will affect fuel economy.