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Old 07-17-2009, 09:34 AM   #1 (permalink)
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Lower RPM for higher MPG?

Hi everyone,

First time as a poster, but i've been looking around for quite a few month... well ever since I bought my new truck, a Jeep Wrangler 2004.

I know that it's a brick and that it isn't made to be fuel efficient, but I can have a dream if I want to

Well anyway, I just found something yesterday that made me think about my driving habit.

Help and Tech Articles | Interco Tire

With this information here, I compared myself with the graphic.

Usualy, on city road I get to my highest gear possible with the lowest rpm possible, which is 5th gear at 1100 rpm, I acheive 35 mph easily. I usualy shift at about 2000rpm

On the Highway I get 60 mph at 2100 rpm in fifth gear.

Now I'm wondering, Am I driving in a rpm not high enough so much that I'm wasting fuel somewhere?
With those rpm, will I get higher MPG in town?

Still quite confused here...

Could someone point me the light with his infinite knowledge of car mechanics


-Josua

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Old 07-17-2009, 10:57 AM   #2 (permalink)
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Just got to be carefull that your note putting too much load on the engine (granted it'll be a biggy I'm guess) but if your exhaust not is substantially deeper than it would be than in a lower and your carrying more throttle just to sustain mph then you may be shifting too early - some times a cople hundred more rpm and a lighter throttle is better than knocking the bottom end out of the engine
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Old 07-17-2009, 11:06 AM   #3 (permalink)
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No, keeping rpms low is a good thing. Lower rpm means higher engine loads which uses gas more efficiently.

Check out the wiki BSFC charts to see why.

http://ecomodder.com/wiki/index.php/...on_(BSFC)_Maps
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Old 07-17-2009, 11:20 AM   #4 (permalink)
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Josua - the "infinite mechanics knowledge" answer to this question is a tough one to wrap your head around. The answer lies in the BSFC chart. That's Brake Specific Fuel Consumption (the name won't help you understand, i'm just saying). What this chart describes for you is the power-generated-per-flow-rate for any given RPM and Load. That is to say that it tells you how much energy is being extracted out of that gallon of gas. There is a BSFC for every engine.

What you'd need to know is the load and the RPM for cruising at 35 in each gear. Unless you have feedback instrumentation or an intimate knowledge of the mathematical model of your vehicle then you can't get the load.

One point of confusion is that load is NOT proportional to throttle position. I believe that it is possible to have 100% load at 50% throttle if the engine could not put out more power by pushing the pedal down farther.

The one-size-fits-all average seems to be 80-85% load between 1500-2000 rpm makes for the most efficient use of the engine. However, that is only useful in acceleration because you can't cruise at 80% load (you'd speed up!).

I think what you're doing is good.
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Old 07-17-2009, 12:30 PM   #5 (permalink)
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sorry lads, I know load isn't proportional to throttle, i think my brain wasn't fully connected to my fingers when typing out my reasoning . from the Bgraphs you can see the sweet spot for each graph around round the 1500-2500 range. what I was trying to explain was its not ALWAYS great to grab the next cog as early as poss. Take the 2.0l zetec graph



the way i was taught to read Bgraphs is that at 245@2000you produce 145nm-ish of torque but if you're running 1000rpm you need to be putting in 320g to create the same 145 nm-ish of torque. Now its been a few years since doing thermodynamics so if I've got that wrong please tell me so i don't look too much of a idiot!
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Old 07-17-2009, 01:28 PM   #6 (permalink)
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Thats the gist of it.
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Old 07-17-2009, 01:31 PM   #7 (permalink)
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Quote:
Originally Posted by robchalmers View Post
sorry lads, I know load isn't proportional to throttle, i think my brain wasn't fully connected to my fingers when typing out my reasoning . from the Bgraphs you can see the sweet spot for each graph around round the 1500-2500 range. what I was trying to explain was its not ALWAYS great to grab the next cog as early as poss. Take the 2.0l zetec graph



the way i was taught to read Bgraphs is that at 245@2000you produce 145nm-ish of torque but if you're running 1000rpm you need to be putting in 320g to create the same 145 nm-ish of torque. Now its been a few years since doing thermodynamics so if I've got that wrong please tell me so i don't look too much of a idiot!
Yeah, you're reading it right, except that it's also proportional to gearing. You don't need 145NM of TQ to keep going down the road at 35MPH, so you're still burning less fuel.

Just being more efficient doesn't necessarily mean burning less fuel. It just means getting more work generated out of the fuel you're burning.
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Old 07-17-2009, 02:56 PM   #8 (permalink)
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Here's two BSFC plots with horsepower isolines:




Any of the points on an isoline produce the same horsepower and get the job done just as well. Suppose you need 27HP (20KW). The best way to get it is with lots of throttle and low revs.

These plots, especially the ones with HP isolines overlaid, explain why P&G saves gas. Suppose you have a manual tranny and the 1.5L Prius engine from the second image, and it needs 10kW to cruise down the highway. You're probably going to do that at 2000 RPM and half throttle. If you have enough gears, you can save some more gas by cruising at 1500 RPM and 3/4 throttle. But if you can efficiently make use of 30kW (40HP) at full throttle, then switch the engine off and coast for a while, you're producing the same average power with less gas.

Here's an extreme engine whose sweet spot happens to produce quite a bit of power. P&G ought to use fully 10% less fuel:

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