We don't need hybrids,.. we need 55MPH! - a long winded ramble.

[AyashiKaibutsu]

Oh, I see, you just wanted to troll his response. Probably why everyone else ignored you. I will also.



[Troll]The vehicles of today do not time travel, so faster cars do not do it faster.[/Troll]

Just leave the discussion of efficiency at different speeds to the big boys you can go play somewhere else since it's clearly over your head.

 

[jteef]

if they removed some traffic lights, or put some effort into timing them to optimize fuel economy it would save more gas than 55mph speed limits. I don't have any facts to back this up.

 

[SabaII]

How about if shitty greedy car companies put more effort into overall efficiency so we could go 75 and get the same results you see by going 55? Is that too much trouble?

Fuck yes it is. There's no money in efficiency.

Alright skipped over some posts so this might have been covered. But cars could get better fuel efficiency. Not knocking you man because most people think that its the car companies holding back and it may be a small bit. The main problem is the tree huggers. You can get better fuel economy by changing air/fuel ratio but you increase NOx in the exhaust thus causing a vehicle to not pass emissions. Engines are tuned to run as clean as possible and not necessarily as efficient as they have potential for.

If you want better fuel efficiency then tune the car for it and drive at optimum rpm for your specific vehicle. Easy on acceleration and find the sweet spot to be running on the highway.

Also 55mph is not a true number for all cars. Not saying its not average but you have to figure in a LOT of factors to find when an engine is running at peak efficiency. Calculated load % at X mph and engine RPM vs Fuel consumption at that engine load and a ton more that are probably way over my head.

As stated in a reply they need to work on stop light timing I think too. Also putting more of these ECON buttons in cars that limit the engine when engaged should help. Honda is doing it in everything but the Pilot, Odyssey and Accord now. Not sure who else might be.

End ramble on mixed thoughts late at night when I should be sleeping.

 

[IcePickFreak]

Yeah... 4.10 gears aren't going to like cruising at high speeds. Big surprise.

In the G8, with the stock 2.92 rear end (yeah, seriously) I'm only hitting 2k rpm at 80mph in 6th and it's still got plenty of power without downshifting at all (3rd runs up to just under 110 at WOT.)

 

[Doppel]

Top Gear tested this before. They had a Prius go around the track as fast as possible. Clarkson was following in a BMW. It's not overly difficult to follow a Prius, so he's driving the BMW fairly easy. The BMW got better gas mileage

That test doesn't have much to do with what I said A Prius at 75 will beat virtually any car going at 55 in MPG, not to mention it's going 20 mph faster.

Anyway, highways are clogged enough. Until we get automated cars increasing efficiency the last thing we need is to depress speeds even further.

 

[Railgun]

My $.02/£.012...

Regarding speed limits...

There`s nothing wrong with a speed limit higher than what it is. You don`t need the government to tell you how fast/slow to drive to get the best economy. If you want to drive 55, or 45 for that matter, that`s what the right lane is for. If you follow the rules of the road, stay right, pass on the left, etc, then there`s no reason anyone should push for a lower speed limit.

Second to that, as some have touched on, the lowest RPM for a speed (sans lugging) isn`t necessarily the best. What`s your throttle position at that speed/gear? The 8-speed ZF has two overdrive gears. 8th may not be as good as 7th if you`re 15% more into the throttle.

Second, the Cd for your particular car, in particular at speed, will also dictate a lot of things. Overcoming wind and rolling resistance for example. Having a decently powerful car will more than likely dictate you less in the throttle than a 1L 3 banger that can barely do 60. Point is, you don`t have to have anemic power levels to have high effeciency.

Now, as far as engine types available in the US. The government shouldn`t mandate what`s available. It`s a simple matter of demand. There is such a stigma towards diesels that no one will buy them. I`ve now driven several out here since I`ve moved to the UK. BMW 116D and 118Ds, 320Ds, a 530D GT and some VW Golf TDIs. I`ve been wanting to drive a 335D for a while, but have not had the opporunity yet.

I was quite impressed, in particular the 2l+ BMW variants. For a city such as London, they`re perfect. Nice and torquey, but lose everything up top. In the US, other than more congested cities, there`s no want for that. We want fun, decently powerful cars. A Prius doesn`t cut it and no American make is cranking out any diesels save for trucks, which no one wants either.

My issue there is with the manufacturers. There`s nothing that is stopping them from bringing one over. If it`s a matter of an EPA test procedure, then do it. Most European makes are, or can be made to order. Going back to BMW for example, they already make the motor, in LHD, and there`s no chassis differences between a European and US chassis. It`s mainly bodywork and other options. The government doesn`t need to mandate squat as that`s not going to change any consumer minds. But, I would almost guarantee that there are many consumers out there that wouldn`t be able to tell if they were driving a new diesel over any gas variant if they didn`t know ahead of time. They`ve come a long way.

Next, as some have touched on...congestion! Traffic jams are horrible for efficiency. I don`t care how many stop/start features are installed, in stop and go traffic, it`s not going to apply all that much. Fix the jam problem and watch in amazement how mileage goes up. That means fast pass toll boths, more lanes, longer on/off ramps, maybe car pool lanes (that actually make sense unlike California). Build better bypasses around cities. Or build roads better the first time around to cut down construction.

There are so many other things that we can do to improve economy. Forcing us to slow down is just going to piss everyone off. Again, if you want to drive slower, knock yourselves out. Just stay off to the right (or left for me now) and we`re all ok.

 

[Zenmervolt]

Second to that, as some have touched on, the lowest RPM for a speed (sans lugging) isn`t necessarily the best. What`s your throttle position at that speed/gear? The 8-speed ZF has two overdrive gears. 8th may not be as good as 7th if you`re 15% more into the throttle.

Actually, if you're 15% more into the throttle at a lower RPM, you are better off.

RPM is the 800 pound gorilla in the fuel economy equation, throttle position is almost meaningless. In fact, larger throttle openings are actually slightly more efficient because the engine isn't having to work as hard to pull air past a restriction (the throttle plate) and therefore less energy is being wasted on pumping losses.

Now, if you're lugging the engine, that's not good. And if the highest gear means you need to be going much faster the additional drag (remember that drag increases with the square of speed) may make it less efficient in the higher gear, but that's because of drag, not because of anything dealing with throttle opening.

ZV

 

[skyking]

Actually, if you're 15% more into the throttle at a lower RPM, you are better off.

RPM is the 800 pound gorilla in the fuel economy equation, throttle position is almost meaningless. In fact, larger throttle openings are actually slightly more efficient because the engine isn't having to work as hard to pull air past a restriction (the throttle plate) and therefore less energy is being wasted on pumping losses.

Now, if you're lugging the engine, that's not good. And if the highest gear means you need to be going much faster the additional drag (remember that drag increases with the square of speed) may make it less efficient in the higher gear, but that's because of drag, not because of anything dealing with throttle opening.

ZV

Exactly! That is the game I (happily) play with my truck. It has a lockup torque converter, and I have a manual/automatic lockup system installed so I can use an aftermarket exhaust brake.
From the factory the converter locks in 3rd under load if 'OD off' is selected, and in 4th otherwise. It freewheels to a great extent when off the throttle; typically I see ~1200 rpm coasting on an engine that idles near 1000 in neutral.
Dodge did this to increase fuel economy, at the expense of the brakes.
I loathe using brakes, so I will engage TQ lockup to use engine braking, and engage the exhaust brake when needed.
This increases fuel consumption due to the higher RPM. My mechanical injector pump delivers a minimum amount of fuel per rev, unless it is shut down. I look at conditions and disengage the lock if I don't have to brake too much.

 

[Railgun]

Actually, if you're 15% more into the throttle at a lower RPM, you are better off.

RPM is the 800 pound gorilla in the fuel economy equation, throttle position is almost meaningless. In fact, larger throttle openings are actually slightly more efficient because the engine isn't having to work as hard to pull air past a restriction (the throttle plate) and therefore less energy is being wasted on pumping losses.

Now, if you're lugging the engine, that's not good. And if the highest gear means you need to be going much faster the additional drag (remember that drag increases with the square of speed) may make it less efficient in the higher gear, but that's because of drag, not because of anything dealing with throttle opening.

ZV

More air=more fuel. More RPM=more fuel. But it's not necessarily as black and white as more/less RPM= more/less fuel consumption.

Your duty cycle at 1000 RPM is less than at 1500 RPM. Your load at 1000 RPM will be higher than at 1500 RPM (given constant road speed). Look at torque/power curves. It all comes down to BSFC, which is a PITA to calculate across the board. Throttle position is directly related to air intake as you point out, which will needless to say change fuel consumption to hit your target AFR. It's not meaningless.

I'm not saying at 60, you should be in 4th at a 1:1, of course you should be in some overdrive gear, but it's not as black and white as being in the highest gear as often as possible.

 

[ShawnD1]

Actually, if you're 15% more into the throttle at a lower RPM, you are better off.

RPM is the 800 pound gorilla in the fuel economy equation, throttle position is almost meaningless. In fact, larger throttle openings are actually slightly more efficient because the engine isn't having to work as hard to pull air past a restriction (the throttle plate) and therefore less energy is being wasted on pumping losses.

Now, if you're lugging the engine, that's not good. And if the highest gear means you need to be going much faster the additional drag (remember that drag increases with the square of speed) may make it less efficient in the higher gear, but that's because of drag, not because of anything dealing with throttle opening.

ZV

Better keep those thoughts to yourself. I said the same exact thing in the AT Garage and people freaked the hell out.
http://forums.anandtech.com/showthread.php?t=2149845

"There's no way open throttle is more efficient. Herp derp, my great grant pappy said put a light foot on the throttle and rev the shit out of the engine instead of flooring it in the next highest gear. Engineer? No, he was a janitor, but he knew cars I tell you what."

I even posted a picture of the mpg meter in my Impreza showing that I was crushing the EPA estimates by at least 30% in city driving. Nope. Still not good enough. There's no way flooring it at low rpm is more efficient than feathering it in first gear up to 6k rpm, even though Shawn's car clearly gets better gas mileage than every other Impreza on the road. That has nothing to do with Shawn's driving style or the bsfc graphs he stole from some website. It's Jesus making the gas mileage that good. :whiste:

 

[jlee]

Better keep those thoughts to yourself. I said the same exact thing in the AT Garage and people freaked the hell out.
http://forums.anandtech.com/showthread.php?t=2149845

"There's no way open throttle is more efficient. Herp derp, my great grant pappy said put a light foot on the throttle and rev the shit out of the engine instead of flooring it in the next highest gear. Engineer? No, he was a janitor, but he knew cars I tell you what."

I even posted a picture of the mpg meter in my Impreza showing that I was crushing the EPA estimates by at least 30% in city driving. Nope. Still not good enough. There's no way flooring it at low rpm is more efficient than feathering it in first gear up to 6k rpm, even though Shawn's car clearly gets better gas mileage than every other Impreza on the road. That has nothing to do with Shawn's driving style or the bsfc graphs he stole from some website. It's Jesus making the gas mileage that good. :whiste:

I don't think you know what "exact same thing" means.

 

[JCH13]

More air=more fuel. More RPM=more fuel. But it's not necessarily as black and white as more/less RPM= more/less fuel consumption.

Your duty cycle at 1000 RPM is less than at 1500 RPM. Your load at 1000 RPM will be higher than at 1500 RPM (given constant road speed). Look at torque/power curves. It all comes down to BSFC, which is a PITA to calculate across the board. Throttle position is directly related to air intake as you point out, which will needless to say change fuel consumption to hit your target AFR. It's not meaningless.

I'm not saying at 60, you should be in 4th at a 1:1, of course you should be in some overdrive gear, but it's not as black and white as being in the highest gear as often as possible.

Your analysis is reasonable, but simplistic. You are ignoring throttle losses as ZV points out, which do matter. You're also ignoring accessory losses also. An engine spinning faster loses more power to the alternator, water pump, oil pump, friction (gears, pistons, bearings), engine pumping (intake and exhaust strokes), viscous drag in the transmission, and the valve train, than an engine spinning slower.

Better keep those thoughts to yourself. I said the same exact thing in the AT Garage and people freaked the hell out.
http://forums.anandtech.com/showthread.php?t=2149845

"There's no way open throttle is more efficient. Herp derp, my great grant pappy said put a light foot on the throttle and rev the shit out of the engine instead of flooring it in the next highest gear. Engineer? No, he was a janitor, but he knew cars I tell you what."

I even posted a picture of the mpg meter in my Impreza showing that I was crushing the EPA estimates by at least 30% in city driving. Nope. Still not good enough. There's no way flooring it at low rpm is more efficient than feathering it in first gear up to 6k rpm, even though Shawn's car clearly gets better gas mileage than every other Impreza on the road. That has nothing to do with Shawn's driving style or the bsfc graphs he stole from some website. It's Jesus making the gas mileage that good. :whiste:

EPA estimates are woefully conservative for many people's driving styles. I also see 30% or more over the EPA estimate for my car with my spirited "heavy right foot" driving. Those intentionally hyper-miling MS3s see in the realm of 50%+ over the EPA mileage. 30% over EPA is not very impressive if I can do it without trying and others can far surpass it. I know an MS3 is not an Impreza, but it is a solid example.

Frankly I still don't think you really understand how to correctly interpret a BSFC chart, and regardless of that you're also analyzing your fuel economy in a simplistic manner. It may be more efficient to generate power at certain RPM/throttle positions, but those BSFC charts do not always account for the parasitic losses I mentioned earlier.

 

[Zenmervolt]

More air=more fuel. More RPM=more fuel. But it's not necessarily as black and white as more/less RPM= more/less fuel consumption.

Your duty cycle at 1000 RPM is less than at 1500 RPM. Your load at 1000 RPM will be higher than at 1500 RPM (given constant road speed). Look at torque/power curves. It all comes down to BSFC, which is a PITA to calculate across the board. Throttle position is directly related to air intake as you point out, which will needless to say change fuel consumption to hit your target AFR. It's not meaningless.

I'm not saying at 60, you should be in 4th at a 1:1, of course you should be in some overdrive gear, but it's not as black and white as being in the highest gear as often as possible.

(1) "Almost meaningless" != "meaningless." Words are important, especially qualifying words like "almost."

(2) If the speed is fixed (and holding all other conditions like incline, vehicle weight, wind conditions, etc. constant), the amount of power at the wheels is fixed. If you were making more power by opening up the throttle with lower RPM, you would accelerate. BSFC decreases as pumping losses decrease because less power is being "wasted" by overcoming the throttle restriction.

(3) RPM is far and away the single largest player in fuel economy. Period. End of story. Yes, other factors come into play, but RPM overshadows them all and the gains in vehicle mileage from lowering RPM will outstrip the tiny losses from using a larger throttle opening.

(4) You can't just "look at torque/power curves." The curves are only valid at WOT. Unless you're running around foot to the floor, the torque/power curves are completely inaccurate. A car only needs 25-35 hp to cruise at freeway speeds, yet my engine spins at 2,100 RPM at 55 mph. According to the power/torque curves the engine is capable of putting out just around 100 hp at that RPM (~250 ft-lbs * 2,100 RPM / 5252). The car is NOT using 100 hp to cruise at 55 mph.

(5) Yes, it is as simple as being in the highest gear possible, without lugging the engine, when you're cruising at a steady speed. Acceleration is different, of course, and accelerating in too tall of a gear can be bad for economy but that's a different scenario.

ZV

 

[ShawnD1]

I don't think you know what "exact same thing" means.

My thread was about explaining why automatics are so bad when it comes to gas mileage. They literally cannot do what it takes to be the most efficient. In my car with a manual, I can stomp the gas at 2k rpm and it gets great fuel economy. My automatic Corolla simply does not allow that. It will only stay at 2k if the pedal is lightly touched, which is inefficient. Stomping on it causing it to drop gears and rev really high, and that too is inefficient. There's no way to win. This is why my Impreza and my Corolla get roughly the same gas mileage even though the Impreza is 300 pounds heavier, has AWD, and has a lot more power with a bigger engine. I drive both of them the same (really hard), and the only major difference is the transmission type.

30% over EPA is not very impressive if I can do it without trying and others can far surpass it

AT Garage seems to disagree with you. I said what mileage I was getting in my Impreza and people straight up called bullshit on it. Actual real world city mileage I was getting when I first got that car and mentioned the mileage was around 28-29mpg (that's 8.2L/100km). US gallons, not Canadian gallons. Nobody believed it. Even with a picture, nobody believed it.

Frankly I still don't think you really understand how to correctly interpret a BSFC chart

It's simple - lower is better. The points where the fuel consumption per unit of power is lowest is when the engine is most efficient. On the graph showing different levels of throttle openness (25%, 50%, 75%, 100%), the 100% line (full throttle) is lower than all of the other lines at every position on the graph. You can look at full throttle 2000rpm and compare it to limited throttle at 3000rpm, and the 2k rpm is better. Flooring it in a high gear is how you get good gas mileage. Feathering it in first gear up to 4k burns a hell of a lot more gas than stomping it in second gear. Then someone in that bsfc thread said they hope my car breaks a rod. Are people really concerned that their car will explode at full throttle? Maybe it will when it's -30 and hasn't warmed up yet, but this is just silly in any other case. That Subaru wasn't made in China. It says made in Japan on it, and it even has some Japanese writing under the hood. If it were made in China, it would have Engrish writing under the hood like "please to refill motor oil only 5w-30 prevent engine suicide"

but those BSFC charts do not always account for the parasitic losses I mentioned earlier.

Parasitic losses (at the same velocity) are often tied to rpm, and higher rpm has more losses. In that case, the bfsc chart and the parasitic losses both say the same thing - low rpm is efficient while high rpm is inefficient. Flooring is better than feathering, but flooring at 2k rpm is more efficient than flooring at 5k rpm. Of course that seems painfully obvious even when driving an automatic. If 5k rpm feathering burned less gas, they would have it hold up to 5k rpm instead of having the transmission always attempt to use the highest gear

 

[jhu]

With the advent of VVT, I don't understand why manufacturers don't make the vehicle enter an Atkinson cycle when driving on the highway. That's one of the reasons hybrids do so well on the highway.

 

[hanoverphist]

We have these 1000cc cars in India which can do 50 MPH(Can take ages to get there though) and are rated to give about 20KMPL or 75MPG.

There is also this car known as Chevrolet Beat.. the diesel variant of this car is rated for 87MPG. On top of that it costs about $8k. Needless to say, almost all of the cars will fail crash tests here in the US.

from that link:
"Starting from Rs.
3,59,394

The Chevrolet Beat has muscle car styling, twin-barrel headlamps, most powerful engine and interiors inspired by the Corvette. This tough, sexy, smart machine with signature front grille is sure to set your heart racing!
"

haha, i love it! its actually not that bad looking either.

my truck sees little benefit from 70 to 55. hell, a tank in the city only gives the same mileage that a complete highway trip does. within .3 mpg. my little civic, on the other hand, gets about 33 with my normal city/ highway use but drops to 29.8 or so when i take it on long hauls down to the border. i average about 70 the whole way, as most of the trip is a 75mph trip.

i think its the engine rpm that governs how much savings or efficiency you get. if i drive 300 miles at 65 in 5th gear i wont have to stop for gas. if i leave it in 4th and do 65, i may not make it to the gas station in 300 miles.

 

[JTsyo]

I'll take on a different part of the title and say hybrids excel in city driving. So increasing your fuel economy on the highway doesn't replace the increased efficiency of a hybrid. If you driving is mostly highway, yea, a hybrid is not to your advantage. But for city driving, you're not going to be doing 55mph nonstop.

 

[exdeath]

from that link:
"Starting from Rs.
3,59,394

The Chevrolet Beat has muscle car styling, twin-barrel headlamps, most powerful engine and interiors inspired by the Corvette. This tough, sexy, smart machine with signature front grille is sure to set your heart racing!
"

haha, i love it! its actually not that bad looking either.

my truck sees little benefit from 70 to 55. hell, a tank in the city only gives the same mileage that a complete highway trip does. within .3 mpg. my little civic, on the other hand, gets about 33 with my normal city/ highway use but drops to 29.8 or so when i take it on long hauls down to the border. i average about 70 the whole way, as most of the trip is a 75mph trip.

i think its the engine rpm that governs how much savings or efficiency you get. if i drive 300 miles at 65 in 5th gear i wont have to stop for gas. if i leave it in 4th and do 65, i may not make it to the gas station in 300 miles.

I'd drive that....


... in India where everyone else is driving the same thing.

No way I would drive a 80 HP car in the US where the average commuter has a 300 HP V6 and would want to road rage me for blocking traffic or act smug at every light.

 

[LTC8K6]

With the advent of VVT, I don't understand why manufacturers don't make the vehicle enter an Atkinson cycle when driving on the highway. That's one of the reasons hybrids do so well on the highway.

Hybrids do much better in city driving than highway driving. They typically aren't much better on the highway than their regular counterparts.

 

[thescreensavers]

55mph would make my commute 10min longer, 20min a day and 1hour 40min of wasted time a week. Nah Ill keep going 75

 

[ThatsABigOne]

I read somewhere that for many engines it is more efficient if the RPM is in the range of 1850-2150 RPM when driving at high speeds.

At 65mph, my 1998 Acura 3.5RL is at 2700rpm. So it is possible that if I reduce speed and get astronomical amount of savings. This is 4 speed auto tranny.

My 2nd car, 1972 Jaguar XJ6 does 3100rpm at 65mph. 3 speed transmission. The savings going to be minimal if I drop my speed, so I do not practice my hypermiling techniques.

 

[desy]

Right, in the Jag optimum would 45

 

[hanoverphist]

my dodge ram truck cruises 68 at about 2000 rpm in 5th gear. if i keep it at 65 ill have to downshift to make it up an onramp hill. 75 doesnt take it much over, hovering about 2200 rpm. it cruises well, but for some reason my mileage is always the same, whether im driving around town for 300 miles or cruising the freeway to nogales.

 

[LTC8K6]

Generally, vehicles that are less aerodynamic, see less of a difference in mileage. They still get worse mileage at the higher speeds, though.

Something brick-like, is already very hard to push through the air at 55mph. Like a Jeep Commander.

We are only talking about steady state cruise.

If you start to factor in other variables, things can get skewed.

 

[m0r1san]

As many have stated, this all just comes down to science.

Physics:
net force = engine force - drivetrain loss - wind resistance
force = mass*acceleration
-> engine force is a function of vehicle+cargo+occupant mass and
-> drivetrain loss is a function of drivetrain mass and rpm
-> wind resistance is a function of coefficient of drag and speed

*Weigh less, accelerate slower, drive slower, drive cars with simple drivetrains (not AWD/4x4), lower rpm, remove roof racks/spoiler/other sources of drag.

Chemistry:
Stoichiometric ratio of Air/Fuel is 14.7:1 you and your oxygen sensor hit at part-throttle cruising. Under WOT, protection is needed, so your engine dumps more fuel to push the A/F ratio down to ~13:1, or lower depending on your engine's aspiration (~12:1 for forced induction).

*Lugging engine is basically WOT at low RPM. Don't do this... you will inject more fuel.

Basically, satisfy the Physics while satisfying the Chemistry and you get best gas mileage. Basically go slow and don't lug the engine. Oh, and apply some common sense and know that this does not apply to a vehicle idling at rest

disclaimer: I'm missing a million things in my equations.

TL;DR - http://youtu.be/mWoUCDTfWqA