Gas Mileage 202: Good News for the pedal mashers (long)
 

I am an engineering student studying vehicle design @ WWU. Here is a little gas mileage info I just learned: One of the standards that the industry professionals use to measure fuel efficiency is something called BSFC, (brake specific fuel consuption) which is measured in g/kw/hr.... grams (of fuel) used, per kilowatt of power made, per hour. It can also be measured in lbs/hp/hr.....pounds/horsepower/hour. The basic principal is that the engine runs most efficiently at wide open throttle. Check out the attached graph to see what a rough example of the curve is. This one is based on a small 4cyl engine. So the most efficient range is somewhere between 1600 and 3800 with the throttle wide open (.....sorry one can only do some much with windows paint) The info in the graph seems at times counter-intuitive, but can be quite informative on how to drive for fuel efficiency & plan for gear changes and stuff like that.

Hope you all enjoy



Also if the attachment doesn't work, could someone help me out, or post it as a pic in a reply, because I couldn't figure out how to do it. Thanks

cool... so im really saving gas most of the time ;)

SO THAT MENAS foot mashed to the floor is best right?

:pat:

Yes, even at lower rpm's and road speeds. I argued with my prof about low speeds and the age old thinking behing "leadfoots" but he assured me that underload (normal energy required to move car), wide open is best.

That don't work with the sports cars I've driven. My Eclipse went through nearly a 1/4 tank of 105 octane at the dragstrip (obviously WOT all the time) during an event and it's mpg would have dropped to around 12mpg and under easy driving I could get close to 20mpg.

its probably the acceleration that it takes to get to that speed that uses up the gas...but if you drive at a constant 90 mph the gas usage would be less then a stop and go to 40 or so

????

Yes,it's the acceleration that uses the gas but just mashing the pedal is going to suck more gas and when you get to 90 and stay constant then you're not mashing the pedal anymore. Any constant speed over 60 or 90mph probably burns the same amount of gas unless steep inclines are a part of the trip since trying to saty at 90 verses 60 will use more fuel.

I would like to know more about the test vehicle...carbed, EFI, TBI, TPI, Chevy, Honda, coefficient of drag, weight, tire size and weight, etc.??

Personally I don't think we have enough information to accept or reject the null.

(I am in statistics right now, so forgive me :) )

Christian,good points brought up;ideal car could support this MPG but real world driving is the true measure here since you can't just drive around mashing the pedal.

We do in our Accord...no choice! :laugh:

It sounds good in a lab, but in the real world there are things like wind resistance to deal with. I heard once in Click and Clack about ever x mph increase in speed increases the amount of power required to overcome wind resistance geometrically.

I have proven this to myself by driving from Washington to Montana at 75 mph and recording my fuel mileage. It is actually worse than it is when I drive around town by 3 miles per gallon. All due to the increased resistance encountered at higher speeds.

you forget that while the engine might be most efficient at WOT, the 2tons of steel it has to haul through the wind and it is shaped like a brick. If not I would get better milage at 80 than at 50mph. And how do you keep it at WOT but keep the RPMs to 1600?

More info definitely needed. Something tells me this wont apply to my F150 Lightning :D

:pissed: ...and I thought I could use this to get out of my latest photo radar ticket :chair: :pat:

Oh the wonders of a turbo.... :bigok:

there's a bunch of good info in this thread and i wanted to join in and give my observations:

1) it's not too surprising that an engine is going to be more efficient at WOT, the engine doesn't have to do extra work pumping a vacuum down after the throttle plate. so more work goes to the wheels.

2) the curve will be different for different engines or even different intakes and exhausts. the engineers design things to be optimal at a certain rpm, outside that, things go down hill

3) the engine may be more efficient at WOT in the lab, but when you need to push air around and overcome friction, then slower is better. generally friction increases linearly with speed and air resistance increases with the square of speed. slow==good mpg.

4) the plot that NMcruiser gave us is normalized by horsepower: lb/hp/hr. at WOT (and generally at higher rpms) you're making more horsepower so you're dividing by a bigger number which keeps the fuel consumption ratio smaller. what would be a better plot is to plot lb/hr as a function of rpm, or even better lb/mph. this way you can see at what speeds your car is the most efficient.

5) even if driving at WOT is more efficient, you dont want to go around just mashing the pedal from a stop. remember that it takes fuel to both overcome friction and wind and to accelerate the vehicle. the slower you accelerate the less fuel you'll need. it's all about being smooth.

IIRC, I read a book on how to drive and get the best MPG. I think it was printed in the mid 80's. As I recall the idea was to accelerate to approx. 35mph quickly and then gently apply brake or gas to attain the desired speed or stop. The reason behind that is that it takes the most power ... work ... to attain the 35mph and once there gently accelerating, or braking to retain your momentum, is the best way to increase the fuel economy. I'll have to go back and see if I can find the reference for this.

good old WWU... they do this stuff all the time. NMcruser would you happen to be a junior?

2 weeks ago had all of my camping gear roughly 300lbs incl firewood. 2 kegs of beer (full) and pulling a light generator trailer (1k lbs) avg speed was about 70-85 for distance of about 95 miles i was probably still getting about 22 MPG. It was for our off road event so no I did not drink the two kegs myself it was for everyone.

yeah...why?