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Sunday, Nov 8, 2009
c5kolb, I hope you get a star for that post.
you wrote:
I only drive maybe 7000 miles per year. At that rate, even if I was to buy a new car, it would take me years to make up the $10,000 price difference between a Prius and a Ford Focus (or something similar).
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the main reason I bought a prius was to reduce the amount of pollutants my vehicle was spewing into the air. I'd like to have an electric car and spew *no* direct pollutants at all, but the Prius is the best I can do in the here and now.
Cost is not the only reason to buy a Prius...your calculations comparing to your Saturn go into a cocked hat if you include toxic emissions.
The two major causes of high gas consumption at cruising speed are wind drag and pumping losses. If an engine is perfectly sized so that it requires maximum HP to cruise at some speed, the throttle will be wide open and pumping losses will be minimized. However, no one would want such an underpowered car. There are two ways to reduce pumping losses, the first being used by some cars is to shut off some cylinders. This requires a wider throttle opening to produce the required HP, reducing pumping losses. The other way, which is superior, is to have additional ratios in the transmission to reduce RPM's to a low level. In order to maintain the required HP the throttle has to be open more. Many cars now have 6-speed or even 7-speed transmissions for this purpose. So, by having variable valve timing, which allows the engine have lots of torque over a wide RPM range, an engine can comfortably operate at a very low RPM, where the maximum HP is minimized, and the throttle is open quite far. So despite your high school physics expertise, you really can have lots of power and good gas mileage. We'll talk about engine friction losses some other time.
"According to the news a week or two ago, Priuses are now available off the lot, without a backlog, and Toyota has expressed surprise and disappointment that they aren't selling faster. That's why I expressed dismay."
Hmmm....Toyota may be disappointed, but according to the numbers released today, Toyota has sold twice as many Prius this year as they did last. If it's anything like the way they market other products (Nintendo WII for example), the Japanese are experts at creating demand through limiting supply.(and they don't seem to have a problem meeting stricter CAFE standards.)
Detroit just hasn't made a product in decades that people care enough about to create that kind of buzz. Ignoring the coming gas price increases and inevitable shortages to follow will surely be the undoing of the American car industry. They are digging their own grave I'm afraid.
>The two major causes of high gas consumption at cruising speed are wind drag and pumping losses.
Yep. You're talking cruising speed now, and are quite correct. But before you were talking about acceleration. The faster drivers accelerate from a stop, the poorer their gas mileage.
You might be right on the acceleration, but consider this: rapid acceleration implies wide open throttle with its reduced pumping losses. Over-acceleration which would require excess braking would consume more fuel, but rapidly accelerating to a steady speed might be more fuel efficient than slower acceleration. Ideally, the way to accelerate is to get in the highest possible gear so that large throttle openings are required for adequate acceleration. Most automatics shift down under those conditions, but I have noticed that my Odyssey is reluctant to downshift unless I give it lots of throttle. And I have never gotten less than 20 MPG, even in town driving.
My approach to getting good mileage is to treat the brake pedal as forbidden territory, and to drive so as touch it as little as possible. After all the frictional heating of the brakes is directly equivalent to wasted gasoline.
>the frictional heating of the brakes is directly equivalent to wasted gasoline.
You're exactly right about this on normal cars. Hybrids use this energy to recharge the battery--that's why hybrids don't need to be charged conventionally.
as a physics ignoramus, I have to ask:
is it even possible to trade horsepower for efficiency? Or is that not a sensible question?
I thought that if one were making engines more efficient (that is, if engines produced more useable energy for a given amount of gasoline, less energy wasted as heat), one could have less horsepower, and greater gas mileage. Is this a completely incorrect notion?
Of course, this does not take into consideration the weight of the chassis/body/everything else. I assume if a smaller, lighter engine of greater efficiency is mated to a very light chassis/body one can get quite high gas mileage for a given horsepower..
More correctly, torque is a measure of efficiency and horsepower is an extrapolation of torque.
If there are 2 engines of a given identical displacement burning fuel at an identical rate, the one producing more horsepower is more efficient. And that is entirely possible in a modern ICE - the computers are most of the difference between a 2007 ICE and a 1990 ICE.
What can be done is to use this increased efficiency to allow for smaller displacement engines that use less fuel to produce the same horsepower. In Europe they used to (maybe still do) have outrageous taxes on engine displacement - a much better approach than CAFE and gas guzzler taxes because it encourages efficiency. Taxes on turbo and superchargers would be a good idea as well, since they allow an engine of a given displacement to burn more fuel.
And you are absolutely right about weight. Work is a measure of mass moved over time. Torque is a measure of work. And torque in an ICE is also, to a large degree, a function of the amount of fuel burned. Less weight means less work, which means less fuel burned to cover a given distance in a given time in a car of a given weight. Weight is the hidden enemy here, as subcompact cars have pretty much doubled in mass over the last 20 years. If we went back to 1985 weights for small cars,through the use of advanced materials and manufacturing techniques, fleet fuel efficiency would be greatly enhanced.
Much of the initial coverage about Fort Hood turned out to be wrong. Is there anything wrong with that?
The accountability imposed by another country for the CIA's kidnapping and torture reveals much about our own.
Fox News' morning show plays to type, talking about whether Muslims in the Army should face "special debriefings"
219 Democrats and one Republican join in favor of the legislation, which passed by a narrow margin
The survivor and author is upset about comparisons some on the right are making to genocide
Salon headlines in your mailbox
