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#11
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>> Over the years I've owned my current ride, I have observed that it is
>> most >> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also >> observed that, at 3000 RPM or less, the mileage on my vehicle drops to >> 35MPG >> or lower. My experience with my current vehicle is similar to several >> other >> vehicles I've owned. In every case, I've found that the best fuel >> economy >> is achieved somewhere above 70MPH. In the case of my current vehicle, >> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will >> state >> the obvious, which is how do I know that my tachometer and speedometer >> are >> accurate? I don't. But the bottom line is, in top gear, I can achieve >> BETTER fuel economy at slightly higher than normal highway speeds. >> Actually, the better fuel economy is due to slightly higher engine RPM, >> and >> the higher ground speed is COINCIDENTAL. > > Your experience seems to be the opposite of most advice given to > maximize milage. I would like to find out more about how you conducted > your test. > > If I understand you correctly you are saying that your car gets better > milage at 78mph (3500rpm) in high gear than it got at lower increments > say: 70mph, 60mph and 50mph? > YES. And that is EXACTLY how my particular Otto Cycle internal combustion engine SHOULD perform. All cars are different obviously, but if they use an Otto Cycle internal combustion engine, maximum fuel economy will be achieved at close to 40% of the engine's redline. The only "difference" from one car to the next (assuming they both use an Otto Cycle engine) will be what ground speed that most fuel-efficient RPM equates to. In many cars, that most fuel-efficient ground speed happens to be above 70MPH. My car is one of them. IF the engine RPM drops below 3000, the engine itself is less fuel-efficient. This means it takes more fuel to travel a certain distance, and COINCIDENTALLY, this means that it is less fuel-efficient at a lower ground speed. In your terms, this means that at 60MPH (for example), fuel efficiency is LOWER. For an automobile to be most fuel-efficient at say, 50MPH, the Otto Cycle engine would need to be running near 40% of redline at 50MPH. IF that theoretical Otto Cycle engine was running at 40% of redline at a ground speed of 50MPH, then you would see a DECREASE in fuel economy at either 60MPH or 40MPH. Not because your ground speed changed up or down, but because the Otto Cycle engine was operating outside of it's most fuel-efficient RPM. (The ground speed change is coincidental) I don't understand why you are questioning my test methods, when my results exactly agree with how Otto Cycle engines are supposed to work. If I said that I've found that water freezes at Zero Celsius, would you question how I measured that? Measuring MPG at certain speeds is so simple the average 2nd-grader could do the math accurately. Some of them could even do it in their head. I've performed the same measurements many times, with similar results. All of this was done before I even knew what results I should EXPECT to see. The fact is, my Otto Cycle engine should be most fuel-efficient at around 3200RPM. BEFORE I KNEW THAT, I'd noticed, through several years of driving experience, that the most fuel-efficient speed for the engine was 3500RPM. That means my particular Otto Cycle engine is performing EXACTLY as it should. If my tachometer is dead-on balls accurate, my Otto Cycle engine is most fuel efficient at about 40% of redline, EXACTLY AS IT SHOULD BE. Yes, this means 78MPH is most fuel-efficient, and 70, 60 and 50 are less fuel-efficient. -Dave |
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#12
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How did you calculate the 40% bit and so forth in your original post
using the Otto cycle? I'm not disagreeing, just curious how you went about it :-) |
#13
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Ted B. wrote: > >> Over the years I've owned my current ride, I have observed that it is > >> most > >> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also > >> observed that, at 3000 RPM or less, the mileage on my vehicle drops to > >> 35MPG > >> or lower. My experience with my current vehicle is similar to several > >> other > >> vehicles I've owned. In every case, I've found that the best fuel > >> economy > >> is achieved somewhere above 70MPH. In the case of my current vehicle, > >> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will > >> state > >> the obvious, which is how do I know that my tachometer and speedometer > >> are > >> accurate? I don't. But the bottom line is, in top gear, I can achieve > >> BETTER fuel economy at slightly higher than normal highway speeds. > >> Actually, the better fuel economy is due to slightly higher engine RPM, > >> and > >> the higher ground speed is COINCIDENTAL. > > > > Your experience seems to be the opposite of most advice given to > > maximize milage. I would like to find out more about how you conducted > > your test. > > > > If I understand you correctly you are saying that your car gets better > > milage at 78mph (3500rpm) in high gear than it got at lower increments > > say: 70mph, 60mph and 50mph? > > > > YES. And that is EXACTLY how my particular Otto Cycle internal combustion > engine SHOULD perform. All cars are different obviously, but if they use an > Otto Cycle internal combustion engine, maximum fuel economy will be achieved > at close to 40% of the engine's redline. The only "difference" from one car > to the next (assuming they both use an Otto Cycle engine) will be what > ground speed that most fuel-efficient RPM equates to. In many cars, that > most fuel-efficient ground speed happens to be above 70MPH. My car is one > of them. IF the engine RPM drops below 3000, the engine itself is less > fuel-efficient. This means it takes more fuel to travel a certain distance, > and COINCIDENTALLY, this means that it is less fuel-efficient at a lower > ground speed. In your terms, this means that at 60MPH (for example), fuel > efficiency is LOWER. > > For an automobile to be most fuel-efficient at say, 50MPH, the Otto Cycle > engine would need to be running near 40% of redline at 50MPH. IF that > theoretical Otto Cycle engine was running at 40% of redline at a ground > speed of 50MPH, then you would see a DECREASE in fuel economy at either > 60MPH or 40MPH. Not because your ground speed changed up or down, but > because the Otto Cycle engine was operating outside of it's most > fuel-efficient RPM. (The ground speed change is coincidental) > > I don't understand why you are questioning my test methods, when my results > exactly agree with how Otto Cycle engines are supposed to work. If I said > that I've found that water freezes at Zero Celsius, would you question how I > measured that? Measuring MPG at certain speeds is so simple the average > 2nd-grader could do the math accurately. Some of them could even do it in > their head. I've performed the same measurements many times, with similar > results. All of this was done before I even knew what results I should > EXPECT to see. The fact is, my Otto Cycle engine should be most > fuel-efficient at around 3200RPM. BEFORE I KNEW THAT, I'd noticed, through > several years of driving experience, that the most fuel-efficient speed for > the engine was 3500RPM. That means my particular Otto Cycle engine is > performing EXACTLY as it should. If my tachometer is dead-on balls > accurate, my Otto Cycle engine is most fuel efficient at about 40% of > redline, EXACTLY AS IT SHOULD BE. > > Yes, this means 78MPH is most fuel-efficient, and 70, 60 and 50 are less > fuel-efficient. -Dave Interesting...apparently Dave is signing for Ted B. or vice versa. Anyway... Why don't you tell us in detail how you measured the mileage at various increments. A meaningful test would be one that does the following for each speed increment: 1. Uses the same car. 2. Drive the same route. 3. The car starts with a full tank of gas. 4. Covers a meaningful distance, at least 100 miles but preferably a full tank. 5. The car is held at a constant speed as much as possible. 6. The milage is computed the same way each time, either with the car's averaging computer or by a complete fillup at the end. The reason I'm questioning your statement is that the effect of wind resistance on milage increases faster than speed. (It isn't arithmetic) I can see this effect on my cars during a 375 mile trip that is repeated several times a year to North Carolina. Driving at 60mph results in 33mpg while driving at 80mph results in 26mpg for the same distance. I'm puzzled about that you can have such different results. |
#14
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> wrote in message ups.com... > How did you calculate the 40% bit and so forth in your original post > using the Otto cycle? I'm not disagreeing, just curious how you went > about it :-) > I didn't calculate it. Through research, I learned that the Otto Cycle engine is most fuel-efficient at about 40% of redline RPM. That's not my calculation, it's just how the Otto Cycle engine works. -Dave |
#15
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Dave C. wrote: > > wrote in message > ups.com... > > How did you calculate the 40% bit and so forth in your original post > > using the Otto cycle? I'm not disagreeing, just curious how you went > > about it :-) > > > > I didn't calculate it. Through research, I learned that the Otto Cycle > engine is most fuel-efficient at about 40% of redline RPM. That's not my > calculation, it's just how the Otto Cycle engine works. -Dave >From your original post: "During research on various engine types, I happened to discover that Otto Cycle engines are most fuel-efficient at RPMs near (not necessarily at, but near) 40% of redline. I had no reason to disbelieve that particular scientific fact. I did the math for my own car, and found that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT 3200RPM, with my own Otto Cycle engine. " Was curious what math you said you did there for your own car. No big deal really, was just curious. I've written a couple of engine simulations and it hasn't yet dawned on me how one would use the Otto Cycle to derive any conclusions like this. I'm not arguing or saying it's wrong, after all, the best fuel efficiency isn't going to be at 1 mph, nor 300 mph, so it'd have to be somewhere in between :P Todd Wasson Performance Simulations http://www.PerformanceSimulations.com http://www.PerformanceSimulations.co.../ToddSim9a.wmv |
#16
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> The reason I'm questioning your statement is that the effect of wind
> resistance on milage increases faster than speed. (It isn't arithmetic) > I can see this effect on my cars during a 375 mile trip that is > repeated several times a year to North Carolina. Driving at 60mph > results in 33mpg while driving at 80mph results in 26mpg for the same > distance. I'm puzzled about that you can have such different results. > You shouldn't be puzzled at all. You are missing certain facts that would clear things up quickly for you. Does your car have a tachometer? If so, what is the highest number that the tachometer reads? Multiply that times ..4. For example, if your tach goes up to 9000, multiply by .4 to get 3600. NOW, assuming your tach goes up to 9 grand, and your 40% level is 3600, you will probably find that 3600RPM is somewhere below 80MPH in your car. 33MPG at 60MPH is not bad, as far as fuel economy goes, and I don't even KNOW what kind of car you drive. BUT, based on the fact that you get 33MPG at 60 but only 26MPG at 80, it's easy to (LOGICALLY) conclude that, YOUR engine is running closer to 40% at 60MPH than it is at 80MPH. This is just a total guess, but 80MPH for your engine might be 50% or higher, but definitely higher than the optimum 40% RPM speed. -Dave |
#17
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How do the manufacturers calculate redline anyway? I suspect that it
can be limited by a variety of factors, such as valve float, piston speed, vibrations, or a business manager thinking about warranty claims on those two-sigma outliers on the left side of the quality distribution. Only some of these factors would directly relate to efficiency aspects of engine theory. I've got a further hunch that the lowest limiting factor gets rounded down to the nearest couple hundred rpm. Thoughts? --Joe |
#18
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Valve float is typically the limiting factor. To keep friction low you
want as light a spring as you can, but that lowers the red line. Piston speed relates more to connecting rod strength, but they can take considerable rpm, well above the redline or the typical spot where you'll want to put the horsepower peak. Basically the power game is to get the horsepower peak where you want it, then use as light a valve spring as you can. The result is you usually end up with the redline not too far over the horsepower peak. Indeed, there's bound to be a safety factor built in. They'll lower the redline a bit from what will probably be fine. I ran my engine a good 500-600 rpm past the redline and the whole engine sound changed. Most likely valve float there. :-) I don't recommend trying that though of course.. Todd Wasson Performance Simulations http://www.PerformanceSimulations.com http://www.PerformanceSimulations.co.../ToddSim9a.wmv |
#20
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> > > > I didn't calculate it. Through research, I learned that the Otto Cycle > > engine is most fuel-efficient at about 40% of redline RPM. That's not my > > calculation, it's just how the Otto Cycle engine works. -Dave > > > >From your original post: > > "During research on various engine types, I happened to discover > that Otto Cycle engines are most fuel-efficient at RPMs near (not > necessarily at, but near) 40% of redline. I had no reason to > disbelieve > that particular scientific fact. I did the math for my own car, and > found > that (surprise, surprise) I SHOULD achieve maximum fuel economy at > ABOUT > 3200RPM, with my own Otto Cycle engine. " > > Was curious what math you said you did there for your own car. No big > deal really, was just curious. I've written a couple of engine > simulations and it hasn't yet dawned on me how one would use the Otto > Cycle to derive any conclusions like this. I'm not arguing or saying > it's wrong, after all, the best fuel efficiency isn't going to be at 1 > mph, nor 300 mph, so it'd have to be somewhere in between :P > > Todd Wasson > Performance Simulations OH! I guess I misunderstood the question. My own car tops out at 8000 RPM, so if the maximum fuel efficiency RPM is ABOUT 40% (for any Otto Cycle engine, not just mine) of that, then 8000 X .4 equals 3200. So, 3200RPM is ABOUT where my own engine should be most fuel efficient. I already knew that 3500RPM was the most fuel-efficient speed for my engine, so when I learned that it should be about 3200RPM, I wasn't surprised, at all. Yes, that computes. -Dave |
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