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Car Forum / Driving, Maintenance, Tuning / Driving / April 2008Sloth Kills Fuel Economy!
Articles like this one from CNN don't identify the true root causes of poor fuel economy. http://money.cnn.com/2008/03/26/autos/slow_down_save_gas/index.htm?section=money _latest When it comes to driving, SMOOTH is the watchword. When all your moves are SMOOTH, you can have both reasonable speed AND reasonable economy. The gas and brake pedals are not binary input devices; if you SQUEEZE on the brakes and EASE off, if you aren't always nailing the gas pedal to the floorboard, if you aren't making all kinds of jarring steering moves that upset the balance of the car, then your overall speed will increase and your overall fuel consumption will decrease. This is why everyone following the rules of the road is so important: when everyone is doing what they are supposed to, it's easier to predict and plan your own maneuvers and execute them smoothly. It's when people do unexpected (== stupid) sh.t that you have sharp steering input, heavy braking, followed by hard acceleration to get away from the fucktard that is endangering everyone around him. Sloth Kills fuel economy!  Signature"Dave's not here, man!" - Tommy Chong > Articles like this one from CNN don't identify the true root causes of > poor fuel economy. http://money.cnn.com/2008/03/26/autos/slow_down_save_gas/index.htm?section=money _latest > When it comes to driving, SMOOTH is the watchword. When all your moves > are SMOOTH, you can have both reasonable speed AND reasonable economy. [quoted text clipped - 12 lines] > > Sloth Kills fuel economy! So does driving over 3250 RPM in any gear when driving a fuel efficient four-cylinder car, based on my observations of driving several four cylinder cars including rental cars. It becomes a balance between driving too slow and driving too fast. Smooth doesn't have to mean sloth but it doesn't also have to mean brisk either. >So does driving over 3250 RPM in any gear when driving a fuel efficient >four-cylinder car, based on my observations of driving several four cylinder >cars including rental cars. Wow, that's quite a conclusion there! What, do you suppose, accounts for this 3250 RPM barrier, and why is it the same for every fuel-efficient 4-cylinder engine? What RPM is best for a non-fuel-efficient 4-cylinder engine (like the one in the 1973 Chevy Vega I used to have)? What about 6-, 8-, 10-, and 12-cylinder engines? Signature"Dave's not here, man!" - Tommy Chong >> So does driving over 3250 RPM in any gear when driving a fuel efficient >> four-cylinder car, based on my observations of driving several four cylinder [quoted text clipped - 7 lines] > 1973 Chevy Vega I used to have)? What about 6-, 8-, 10-, and > 12-cylinder engines? I can tell you right now, 3250 RPMs would put my V6 Mustang at almost 88 MPH, not its most efficient speed. The Mustang gets its best mileage at around 65 MPH and 2300 RPMs, and that's with a stone-age carb. 3250 RPMs in the Jeep which is a somewhat modern MPI 4 banger would put it at about 73 in 4th and 92 in overdrive, also not realistic speeds for fuel efficiency. 60 MPH (which is the slowest I can use overdrive) returns the highest MPG, and that's roughly 2100 RPMs. So if you're just going to base it all on RPMs, my observations of my vehicles place the best point at an average 2200 RPMs or so. BSFC is a much more important number than peak torque, IMO. Signature~/Garth |"I believe that it is better to tell the truth than a lie. Almgren | I believe it is better to be free than to be a slave. ******* | And I believe it is better to know than to be ignorant." --H.L. Mencken (1880-1956) >>> So does driving over 3250 RPM in any gear when driving a fuel efficient >>> four-cylinder car, based on my observations of driving several four [quoted text clipped - 22 lines] > > BSFC is a much more important number than peak torque, IMO. Sometimes - not always, but often enough that there seems to be some correlation - the lowest BSFC is at or near the torque peak. nate Signaturereplace "roosters" with "cox" to reply. http://members.cox.net/njnagel >>>> So does driving over 3250 RPM in any gear when driving a fuel >>>> efficient four-cylinder car, based on my observations of driving [quoted text clipped - 28 lines] > > nate Uh,Rouse DID say "not go **OVER** 3250 RPMs". (I wonder why the extra 50 RPMs? does his tach read that close? ) He didn't say "drive -at- 3250 RPMs". It's kinda like not depressing the gas pedal enough to open the second set of butterflies on a carburator,when it REALLY sucks gas. although for a FI system,fuel usage is probably a bit more linear than the old carbs,and they only have the single butterfly plate in the airflow. Rouse's RPM figure probably derives from the final gearing on his autos. economy cars will have a higher final gearing than a Jeep or Mustang. Such as a 2.73 final instead of a 4.11 gearing (numbers from my memory of the old US autos....probably not applicable to todays FWD cars.) SignatureJim Yanik jyanik at kua.net > Rouse's RPM figure probably derives from the final gearing on his autos. > economy cars will have a higher final gearing than a Jeep or Mustang. > > Such as a 2.73 final instead of a 4.11 gearing > (numbers from my memory of the old US autos....probably not applicable to > todays FWD cars.) Heh, you nailed both of them. :) In my case, the Mustang does have 2.73 and the Jeep has 4.11. Since the Mustang doesn't have any overdrive, the 2.73 is a must if I expect to hit freeway speeds. Signature~/Garth |"I believe that it is better to tell the truth than a lie. Almgren | I believe it is better to be free than to be a slave. ******* | And I believe it is better to know than to be ignorant." --H.L. Mencken (1880-1956) >Uh,Rouse DID say "not go **OVER** 3250 RPMs". >(I wonder why the extra 50 RPMs? does his tach read that close? ) > >He didn't say "drive -at- 3250 RPMs". I guess I just don't understand what shift points have to do with anything, since the way most cars are geared the VAST majority of your cruising time is spent down in the neighborhood of 2200 RPM. Those few seconds you spend getting up to speed are pretty negligible, and I bet you couldn't even tell the difference in fuel consumption between shifting at redline and shifting at 3250 RPM. Signature"Dave's not here, man!" - Tommy Chong > >Uh,Rouse DID say "not go **OVER** 3250 RPMs". > >(I wonder why the extra 50 RPMs? does his tach read that close? ) [quoted text clipped - 10 lines] > "Dave's not here, man!" > - Tommy Chong My Jeep must have a weird setup: the shift idiot light glows when the tach gets to around 1500, and goes to 1000 after shifting (at 30 mph in 5th). 2000 on the tach in 5th is at 60, and 2500 is at 75 (about as fast as I'm comfortable going on Missouri's POS road system). 3000 rpm would have me at 90, which would add to the accumulation of gray in my hair. Because my Jeep is so low-geared, the highway mileage isn't much different from the city mileage. It appears that its best fuel economy is with the tach at around 2000. Letting it drop below 1000 is downshift time (it idles at around 800). IIRC it redlines at about 5k, and I haven't come close to that. > >>>> So does driving over 3250 RPM in any gear when driving a fuel > >>>> efficient four-cylinder car, based on my observations of driving [quoted text clipped - 33 lines] > > He didn't say "drive -at- 3250 RPMs". (snip just for this post's purpose) His tach is probably graduated in 250-RPM increments (4 per 1000 RPM). That is the case with mine. >>>> So does driving over 3250 RPM in any gear when driving a fuel >>>> efficient four-cylinder car, based on my observations of >>>> driving several four cylinder cars including rental cars. >>> Wow, that's quite a conclusion there! >>> What, do you suppose, accounts for this 3250 RPM barrier, and >>> why is it the same for every fuel-efficient 4-cylinder engine? >>> What RPM is best for a non-fuel-efficient 4-cylinder engine >>> (like the one in the 1973 Chevy Vega I used to have)? What about >>> 6-, 8-, 10-, and 12-cylinder engines? >> I can tell you right now, 3250 RPMs would put my V6 Mustang at >> almost 88 MPH, not its most efficient speed. The Mustang gets its >> best mileage at around 65 MPH and 2300 RPMs, and that's with a >> stone-age carb. >> 3250 RPMs in the Jeep which is a somewhat modern MPI 4 banger >> would put it at about 73 in 4th and 92 in overdrive, also not >> realistic speeds for fuel efficiency. 60 MPH (which is the >> slowest I can use overdrive) returns the highest MPG, and that's >> roughly 2100 RPMs. >> So if you're just going to base it all on RPMs, my observations >> of my vehicles place the best point at an average 2200 RPMs or >> so. >> BSFC is a much more important number than peak torque, IMO. >Sometimes - not always, but often enough that there seems to be some >correlation - the lowest BSFC is at or near the torque peak. But it depends on the _load_. The load is the desired torque. A car requires power to move at speed. The torque at the engine can be varied by selecting a different gear. Spark ignition (gasoline, LPG, etc) are far more prone to high fuel consumption under part-load conditions. And they are sensitive to engine speed at which that load is applied. A compression-ignition engine has fairly uniform fuel consumption; with it only increasing sharply under very high (typically >90%) or low (<20%) load; fairly independently of crank speed. The Golf TDI which I drive regularly seems to work quite efficiently between 1700 and 3800 rpm, though I wouldn't usually spin the engine that fast except in top gear, where 1900 rpm matches the freeway limit. The Golf GTI (1990) which I drive occasionally seems to work efficiently between about 2400 and 3000 rpm. 3000 rpm is good for 100 km/h in top gear of the GTI. Driving either efficiently requires the same strategy. Looking ahead as far as possible and assessing the traffic conditions to be in the right gear, at the right time; minimising the application of brakes. When accelerating, I do so briskly; but short of wide-open throttle, getting into the next higher gear as soon as it would be in the engine's optimum operating range. It is possible for most countries to cut their fuel consumption by 20% (or more) simply by all drivers exercising some common sense in this way; and by not trying to own the road. Share and enjoy. :-) Signature/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia \ / ASCII ribbon campaign | Great minds discuss ideas; X against HTML mail | Average minds discuss events; / \ and postings | Small minds discuss people. -- Eleanor Roosevelt CNN has discovered something Dad taught me decades ago, after a lifetime of putting it in practice. Easier on the machinery too. And it jibed with what they were telling me in Driver's Ed about how the actions of individual cars can make traffic flow better. So with all the electronics that they put on cars for frivolous reasons, where's the display of instantaneous and recent-average MPG so we can actually learn rather than theorize or guess what driving style makes our cars, with our load on our terrain, get the best mileage? LCDs have gotten so cheap you could even do it graphically as a function of various parameters such as throttle position if you wanted to go deeper rather than just calibrating your neural net to your right foot. Imagine instant feedback on how much it cost you to cowboy it on that last hill -- or whether turtling away from a light is saving you or costing you. There are times out in the boonies when a calculation of best speed for stretching maximum range out of the remaining fuel could be a very good thing to know too. With a fuel-injected engine under closed-loop computer control in most cars these days, seems simple as falling off a data logger to implement -- come to think of it, the advent of such things was about when you saw the first built-in "trip computers." But it's never caught on, appearing only here and there in fancy cars and a few technoid ones that emphasize mileage such as the Prius. Given the societal and personal-finance cost of wasting fuel, it should be a standard-equipment secondary instrument, no? --Joe > CNN has discovered something Dad taught me decades ago, after a > lifetime of putting it in practice. Easier on the machinery too. [quoted text clipped - 26 lines] > > --Joe I'm surprised that you can't see instantaneous fuel economy on most built in trip computers; I haven't driven enough newer cars to know whether this is common or not. It'd just be a simple calculation requiring injector pulse width, fuel pressure (if it varies,) RPM, road speed, and a calibration constant (that takes into account the properties of the fuel injectors,) all of which should be available from the PCM. nate Signaturereplace "roosters" with "cox" to reply. http://members.cox.net/njnagel >I'm surprised that you can't see instantaneous fuel economy on most >built in trip computers; I haven't driven enough newer cars to know [quoted text clipped - 3 lines] >properties of the fuel injectors,) all of which should be available from >the PCM. At the micro level, a fuel injector is really just a solenoid-operated valve that allows the pressurezed fuel rail to pump a little bit of fuel through each time it opens. Once the injector value is fully open the flow rate is presumably known (or calculable based on the fuel rail pressure), but what about the periods when it is opening and closing? Is the flow rate linear WRT how open the valve is, i.e. if the valve is half way open, is the flow rate exactly half of the "open" flow rate? Or are these some nonlinearities that come into play? And what if the injectors are clogged or dirty? How does that affect the accuracy of the flow calculations? Signature"Dave's not here, man!" - Tommy Chong >So with all the electronics that they put on cars for frivolous >reasons, where's the display of instantaneous and recent-average MPG >so we can actually learn rather than theorize or guess what driving >style makes our cars, with our load on our terrain, get the best >mileage? Every car I've purchased within the last 10 years has had an instantaneous MPG display. My IS-300 actually had an analog gauge for it, but the others had fluorescent or LCD displays. The information is available; it's just that gas is so cheap that few people really care. >LCDs have gotten so cheap you could even do it graphically >as a function of various parameters such as throttle position if you >wanted to go deeper rather than just calibrating your neural net to >your right foot. Imagine instant feedback on how much it cost you to >cowboy it on that last hill -- or whether turtling away from a light >is saving you or costing you. And yet no car manufacturer wastes their time providing such displays. Why? Probably because none of their customers are asking for it. >With a fuel-injected engine under closed-loop computer control in most >cars these days, seems simple as falling off a data logger to [quoted text clipped - 4 lines] >societal and personal-finance cost of wasting fuel, it should be a >standard-equipment secondary instrument, no? It's only worthwhile if people will use it. Imagine someone gave you a rickshaw and asked you to carry a group of 8 people and their luggage over to the airport, which is 10 miles away. How much would you charge them for all that work? Now consider that you can pile those same 8 people and all their luggage into a Ford Excursion and drive them 10 miles for roughly the cost of a gallon of gasoline, i.e. < $4.00. Even at current prices, gasoline is still dirt cheap, and most people just don't give a damn about fuel economy. The time when they'll start caring is right about the same time that the oil is running out. Signature"Dave's not here, man!" - Tommy Chong >So with all the electronics that they put on cars for frivolous >reasons, where's the display of instantaneous and recent-average MPG >so we can actually learn rather than theorize or guess what driving >style makes our cars, with our load on our terrain, get the best >mileage? It's already on quite a few cars, not all that recent. >when you saw the first built-in "trip computers." But it's never >caught on, appearing only here and there in fancy cars and a few >technoid ones that emphasize mileage such as the Prius. Given the >societal and personal-finance cost of wasting fuel, it should be a >standard-equipment secondary instrument, no? No. The last thing we need is more people playing video games with the mileage computer instead of paying attention to their driving. SignatureThere's no such thing as a free lunch, but certain accounting practices can result in a fully-depreciated one. >>So with all the electronics that they put on cars for frivolous >>reasons, where's the display of instantaneous and recent-average MPG >>so we can actually learn rather than theorize or guess what driving >>style makes our cars, with our load on our terrain, get the best >>mileage? >It's already on quite a few cars, not all that recent. My 1990 Golf GTi could be cajoled into displaying "instananeous" fuel consumption. As could earlier ones with similar trip computers. They used manifold pressure, engine speed and distance clicks to figure it out; fairly accurately on average. >>when you saw the first built-in "trip computers." But it's never >>caught on, appearing only here and there in fancy cars and a few >>technoid ones that emphasize mileage such as the Prius. Given the >>societal and personal-finance cost of wasting fuel, it should be a >>standard-equipment secondary instrument, no? >No. The last thing we need is more people playing video games with >the mileage computer instead of paying attention to their driving. Ahhh... so you've been behind a the proud owner of a new Toyota Playstation as well. Signature/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia \ / ASCII ribbon campaign | Great minds discuss ideas; X against HTML mail | Average minds discuss events; / \ and postings | Small minds discuss people. -- Eleanor Roosevelt > >No. The last thing we need is more people playing video games with > >the mileage computer instead of paying attention to their driving. > Ahhh... so you've been behind a the proud owner of a new Toyota > Playstation as well. Been *in* there with 'em, not quite relaxing until I realized that that particular Prius owner wasn't in a complete heads-down mode trying to figure out how much power was going where at the moment. (How useful this knowledge is, I dunno. It'd be different if that screen screen enabled you to *do* anything about anything -- one can readily imagine being able to lock-in electric mode for getting in or out of the driveway when everybody's asleep, for instance -- but just seeing it makes you feel like Geordie LaForge for about five minutes. (Remember the Turbo Trans Ams with the row of lights in the hood to indicate boost?) Toyota may have either predicted or concluded that a typical Prius buyer likes being able to see that it's busily hybridizing away down there, even though the functions could be perfectly well hidden from the user. The rate at which they're selling the things probably makes them think this is the right approach.) I think an MPG display should be a *simple* (instantaneous and recent average) small secondary instrument somewhere off in the corners of your main scan -- not an obscure function you have to know how to call up, and certainly not something that tempts your eyes off the road, where all too many drivers' eyes are most of the time anyway -- just a ubiquitous, always available bit of information on the edges of your awareness. The gas gauge is probably about the right location and level of emphasis as well as a logical category. Cars often seem to give me either too much or too little information, and not always in the right places at the right times... --Joe >I think an MPG display should be a *simple* (instantaneous and recent >average) small secondary instrument somewhere off in the corners of >your main scan -- not an obscure function you have to know how to call >up, and certainly not something that tempts your eyes off the road You'd love the IS-300 then. :) http://homepage.mac.com/chutson/iblog/C390284789/E20050820162408/Media/retro_dial.jpg Signature"Dave's not here, man!" - Tommy Chong > You'd love the IS-300 then. :) Well, I suspect that I would, although an obsolescing mental reflex still thinks anything in that price range ought to have a gas meter, not a gas gauge. > http://homepage.mac.com/chutson/iblog/C390284789/E20050820162408/Medi... It's certainly *pretty*, and clever, but seems a bit too busy and Rolexy for practicality. I wonder what it's like to live with. OTOH, an MPG meter that goes to 60 is a bit of a gesture of optimism, considering that the car's rated mileage is about a third of that. Maybe they meant to use the same cluster on a hybrid or somesuch? --Joe >> You'd love the IS-300 then. :) > [quoted text clipped - 6 lines] >It's certainly *pretty*, and clever, but seems a bit too busy and >Rolexy for practicality. I wonder what it's like to live with. It was no problem for me. >OTOH, an MPG meter that goes to 60 is a bit of a gesture of >optimism, considering that the car's rated mileage is about a third of >that. Not at all. I have pegged the instantaneous MPG meter on that car as well as several others. The easiest way to do it is to take your foot off the gas when going down a big hill. >Maybe they meant to use the same cluster on a hybrid or somesuch? That "chronograph" layout was an IS-300 signature. AFAIK no other car used it (except maybe the Toyota Altezza, the Japanese-market car upon which the IS-300 was based). Signature"Dave's not here, man!" - Tommy Chong |
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