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Coupes & Convertibles Scion tC Scion Owners: MPG-Real World Numbers
494 messages, Last post on Nov 26, 2009 at 1:58 AM
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| Just bought a new xD manual trans. Getting 34 MPG in SF Bay Area, mix of Hwy and city driving. Nice car. | |
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Replying to: kipk (Feb 21, 2008 8:36 am) This is not correct. You are thinking that when the rpms decrease when the car shifts into OD, that this means the engine is developing less power. The engine is actually developing the same power. The power being developed by the engine is determined by the vehicle speed, wind, slope of road, and not the engine speed in rpms. When OD kicks in the fuel flow in wt or vol per unit time (say gal/hr) stays the same, but the amount of fuel injected into each cylinder for each power stroke increases in inverse proportion to the rpms. At the lower rpm value the parasitic losses in the engine (friction between piston rings and walls, rod and main bearing friction, etc.) are reduced, which is the source of the increase in efficiency. The trade-off is that this increases the stress on the rods and bearings, but if they are designed to take it, then there's a net benefit. You get lower fuel consumption per unit distance travelled without engine damage that would ever come into play. That is, the engine will still last the lifetime of the body, electrical system, etc., or at least as long as the manufacturer designed it to.
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Replying to: jim314 (Feb 22, 2008 5:37 am) But if you did this and suddenly had to accelerate, then you would have to downshift to 2nd, which you might not have time to do in an emergency, and you would have to remove one hand from the steering wheel reducing steering control. Even today the operation of a standard manual tranny (as opposed to a clutchless with paddle shifters) can be problematic if say the driver is approaching an intersection in a relatively high gear (for fuel efficiency) and suddenly needs to accelerate to avoid an accident. There is a trade-off between keeping the rpms up for controllability with the accelerator pedal and with keeping the rpms down for fuel efficiency. I like manual trannys, but automatics are in my opinion safer, especially in city driving. I have not tried the new automatic dual clutch trannys with paddle shifters giving virtually instantaneous shifting. I'm sure I'd like it, but they must be expensive. |
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Replying to: dbecker6 (Feb 20, 2008 8:06 am) |
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Replying to: jim314 (Feb 22, 2008 5:37 am) Hey Jim, Remember when I wrote that I don't care about the why's and wherefores? I still don't! The only thing that interest me is results! If you wish to believe that an engine running at lower RPM develops the same power as the same engine running at higher RPM, that is fine with me. Apparently, in your thinking, the charts that show engines developing very low power at idle and a lot more power at high rpm are wrong. To my way of thinking, If you were right, there would be no reason for a throttle. Just let it idle and shift through the gears.
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Replying to: kipk (Feb 23, 2008 5:00 am) Look at it this way. Suppose you are cruising with speed control set on a level highway at 70 mph when you enter a gradually steepening hill. Just before starting the climb the rpms will be a certain value and the throttle position will be a certain value, the fuel flow rate will be a certain value, and the engine power will be a certain value. (I have said I think the engine power might be about 25 hp for a compact car, but the exact value doesn't matter.) As you begin climbing the cruise control will gradually open the throttle to maintain speed, but the rpms will not change if the auto tranny doesn't downshift or if the driver doesn't downshift a manual. But the power developed by the engine will increase because the fuel flow (wt or vol per unit time) is increasing as it must to work against gravity in climbing the hill. I am not trying to catch any body in a mistake, just trying to explain the matter as I see it. I am not an engineer. I am refining this idea in my own mind as we have this discussion. An engineer might find fault will some of my assertions or might say about what I have written, "Duh! . . . What Jim is saying is so obviously the case that we engineers don't bother to state it." |
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Replying to: jim314 (Feb 23, 2008 6:31 am) You seem to be contradicting yourself. You write: >"The power being developed by the engine is determined by the vehicle speed, wind, slope of road, and not the engine speed in rpms. " Here you are saying the RPM has nothing to do with Power. (Now before you go off in another tangent, It makes no difference to me whether someone refers to "Power" as torque or horsepower. To me it is the output of the engine. Period!) Then you write: >"This must be the case because they are showing the maximum torque and horsepower that the engine is capable of producing at each value of engine speed (rpm). The torque does not just depend on the rpms, but also on the throttle position. " Here you are saying the engine speed does make a difference. That is a contradiction to the quote above it. You come up with nice symbols and pieces of formulas, and you most likely understand them. As I said earlier, they go way over my head. I do understand that an engine at a given RPM does not develop as much power at a closed throttle as it does at wide open throttle. However, I do believe that at most any throttle setting the higher RPM will allow that engine to develop more "Power", than at a lower RPM. >"You are thinking that when the rpms decrease when the car shifts into OD, that this means the engine is developing less power." In real world applications, YES I DO! Remember? The torque does not just depend on the rpms, but also on the throttle position. " Here is a test for ya! No books involved or necessary. Just real world. Get on a long fairly steep up hill grade with a car with Manual tranny in 5th gear. When it gets to the point that with near wide open throttle the car is barely able to maintain it's speed, make a "Speed" shift to 4th gear, WITHOUT LIFTING YOUR FOOT OR MOVING THE THROTTLE. You will find your speed will increase. On a flat road, With the same MT car, be at 25-30 MPH in 5th gear and floor the throttle. Now do the same at 60 mph. Notice any difference in the engines ability to gain speed? Same car same throttle usage. But even with the increased resistances of air, tires, etc., the higher rpm at 60 vs 25-30 allowed the engine to develop more power quicker, to my way of thinking. Drag racers with the exception of rails, fuelers and funny cars, gear their cars to be approaching the RPM red line in top gear when they cross the finish line. NASCAR, gear their cars to have them pretty much topped out on RPM for the track they are running on. Why? More Power is produced at higher RPM. NOTE: Some of the dragsters develop so much power that gears are not necessary. Their main concern is harnessing that power to the asphalt. The others use gears to keep the RPM high for Power. Now back on Topic of real world mileage. Any way we spin it, wind resistance produced by speed plays a large part in fuel mileage. Manufacturers design their vehicles to be as aerodynamically as they can to minimize that resistance. Whatever vehicle we drive will likely get better mileage at 65 MPH than at 75 MPH. How much difference that makes will depend, somewhat, on the frontal area and shape of the car. That includes the front, side, and rear of the car. People drive fast for various reasons. Some just want to get the trip over with. Some just can't stand with being passed. Some like the speed. Some are intimidated by other driver. And they are willing to pay the higher price for fuel.  Kip
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Replying to: kipk (Feb 24, 2008 3:57 am) The fact that the motor mags show curves of both torque and horsepower reflects the fact that the motor guys know that torque and horsepower are different things. Thinking that they are more or less the same thing will lead to false conclusions about how to achieve best mpg and best acceleration. Power and torque are different, but are related by a very simple equation: Horsepower = torque (in lb*ft) x RPM / 5252. What determines the fuel use rate (in wt or vol per unit time) is the horsepower being generated by the engine, and not the torque value. The transmission is a torque changing device just like a lever is a force changing device. But the power coming out of it is equal to power going in minus a small loss in internal friction. The torque at the output shaft of the tranny is in general very different from the torque at the output shaft of the engine. See http://en.wikipedia.org/wiki/Horsepower . If you are interested, I can give an analogy to the problem of selecting the best pusher for a four-man bobsled. What I am referring to is, of course, the much discussed question in the fantasy bobsled community: Should you try for Jimmy Brown or Jimmy Tayor as your pusher? Jim |
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Replying to: kipk (Feb 24, 2008 3:57 am) Jim's on target, so you might want to carefully review his posts. He's not saying a car develops the same horsepower regardless of rpm. He's saying (correctly) that it takes a certain amount of power to move a vehicle at a set constant speed, regardless of rpm. Will more power be available if you suddenly choose to accelerate if you're in 2nd instead of 5th? Yes. But that's not what we're talking about here.
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Replying to: texases (Feb 24, 2008 1:19 pm) I have re read his post. As I've stated a couple of times above, a lot of that simply goes over my head. I've also expressed disagreement with some of his statements. My only concern is results. My present cars are achieving 30-35% better than the EPA ratings. I know from trial and error what works and what does not work. Results! I'm more than happy to share that knowledge with others. I feel confident that most folks on this forum are concerned with the mileage others are getting and how they can possibly improve their's. Formulas are not going to help most folks, as much as Real World experience. I've seen a formula that says a Bumble Bee can't fly! But real world says different, doesn't it!. As far as making "Power", again experience is a great teacher. In both practical and observation I "Know" that higher RPM are more capable of increased power than the same engine at lower RPM. As stated earlier, that is why drag racers gear their cars to near red line when they cross the finish line. The idea is to keep the RPM high and develop as much power as possible, with the engine restrictions that are imposed by rules. Of course there are always exceptions. Enough time has been spent on this subject. We need to stick to the topic of the forum. Jim can start a new forum "The Care and Feeding of Engines" ! Regards, Kip |
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