Weight, where is the loss?
 

I realize that this sounds silly, but hear me out:

My usual driving these days is a 320 mile high round trip across WV and Maryland. I've been pretty consistently getting around 35~ mpg round trip with this.

I usually get better mileage east than west, but I realized this is because the station I fill up there isn't level. There is a 500 foot elevation change from where I start to where I go but over 160 miles that's not an issue. When I get back it averages out so it's not that big of a deal.

Anyways, scangauge is usually correctly calibrated each tank fill. (Off by less than .1 gallon)

Recently though, my car was loaded up like this:

http://lesbaru.net/stuff/censuspack.jpg

And I could really feel all that extra weight in my glides. I've driven this route so many times I know where to glide and where I can keep coasting down the next mountain, I know where to neutral coast versus in gear coast for DFCO, etc etc

But this changed it all. I could coast much much further than I could normally, and I could even DFCO on parts that before I couldn't maintain speed in neutral.

And to top it all, the engine didn't seem to notice, as at 55 or 60 (what I'm usually doing up the mountains) the engine still would maintain 16~20 mpg up. Then I could glide all the way back down. And then some.

Scangauge was actually reporting 38 mpg until I hit traffic near the city, it usually is reporting 36 until I hit traffic.

I understand that removing weight helps with the city, but how much does it really hurt on the highway, through the mountains?

:turtle:

I might do a mini experiment and throw some weights in the car and see how much farther it will coast.

Rolling resistance increases linearly with weight, so it takes more energy to move the car down the road, even with no braking or accelerating. Check out the Calculator to find out exactly how much.

By P&G'ing, you've increased your engine efficiency, and perhaps by enough to compensate for the extra rolling resistance.

If you had to engine brake down the hills, though, then the extra mass would be a huge disadvantage.

brucey -

I've always imagined an uphill/downhill commute where I had jugs of water on the downhill run. The water or ballast or whatever would be "delivered" at the bottom of the hill. On the uphill run the car would be empty. That would make for a net MPG gain.

The trick is having a situation where the cargo is "useful", aka doing something important like making $ for me or ???

CarloSW2

I was also surprised that extra weight didn't kill my fe on last year's trip to Romania. The car had 5 people and a bunch of cargo, filled to its weight limit (over 2 tons). The coasts were long, but the acceleration was noticibly slower. Lots of steep climbs with multiple switchbacks and I still managed 4.2 l/100km, much less than I got in everyday driving.

I am a sube nut..:p
the boxer likes the weight.

keep good rear and front bearings, change oem out yet?
the federal mogul replacemnts..nice.

I am awaiting the ring nut tool subes use to change one that is just starting to have a wiggle in the cold.(163 foot pounds and 23 years.. I best get the real tool to remove it):rolleyes:


with the weight in the back, take a peek at the exhaust pipes distance from the body while loaded, and then while empty...

I found something sube fans hate me mentioning...
stiffening what I found was another amazing thing to do for these physics.
subys have a squishy spot...they stay elusive for years and years..


I am at 150+ hours in weld and fabrucation saving a ten geared sube..to learn the new ones did something even squishier. The car can even do better than it is now...it is a nice engine.:thumbup:

:confused: What?

might as well be pops and clicks... I dont get what he is saying either.

As for going into DFCO going downhill, I only do that if my speed is picking up above the speed limit, since otherwise I'm going to have to brake anyway.

Even in DFCO I will still have to brake sometimes on some of the steeper parts. I'll turn the headlights on for the extra drag (minimal) and the A/C compressor even (you can actually feel the drag when it comes on) but sometimes doing that will kick the car out of DFCO. Silly thing. But Once I'm nearing the bottom of a pass I'll knock it back into neutral and hold my speed till a few miles under the speed limit.

As I said, climbing it will get 16~20 mpg IF I can keep the torque converter locked (35 TPS on scangauge) and if not, it will hit 11's or 12's, so I try my best keep it locked.

Anywho, on the stretches of the journey where I normally would have it in neutral, I was in DFCO to keep my speed at the limit. At the bottom of the grades where I'd normally have to start gassing again, I was still coasting and not even losing speed. For having an extra 400 lbs in the car, the engine really didn't seem bothered by it.

I'll do a test this weekend, I might be able to squeeze my entire weekend travel from one tank (hoping)

I have given up trying to understand his posts.

Increased mass just makes the effects of inertia more apparent. Good for highway cruising, bad for stop-and-go in the city.

I have to say the increased rolling resistance due to increased mass is bad for highway cruising; bad for any cruising for that matter.

Usually. You are right under most circumstances, but not all.

Let's say you live at sea level. And you drive to a place that is 100 or more miles away that is at a much higher elevation, more than 1000 feet. You will burn more fuel going there than coming back, assuming that all other factors are equal.

Now, if you are fortunate enough that you leave with your vehicle empty (including a half-empty gas tank) and return with a full tank of gas and a 1000 pound load in it from that higher elevation, it will work to your advantage. It's all downhill from there, essentially, no matter how you slice it.

But if you have to leave with a full load, driving to a higher elevation, and you must return with it empty, the prevailing forces will be working against you.

Now, is the increase in rolling resistance enough to counteract inertia? Or does the increased resistance to change speed overcome the rolling resistance?

I'm pretty sure that the slight increase in rolling resistance is not enough to overcome inertia.



Thymeclock: Of course, increased mass means that the pull of gravity will be stronger. The added weight comes at a disadvantage for uphills. However, my initial post assumed level roads.

http://ecomodder.com/forum/tool-aero...ToStep=5-200-5

About a third of the load at 60mph is rolling resistance for the above-linked car. So a 3% increase in mass (here, 90lbs) should bring a 1% increase in fuel consumption on a flat road with no stops or hills. It's a small impact, but every little bit helps (or hinders).

Also, inertia is bad for fuel economy. It makes DWB harder, it moves your RPMs upward during acceleration, raises the stakes every time you brake, and it makes you downshift on too steep of a hill climb. The only good thing about it is that it makes P&G easier with longer pulses and longer glides.

I use my 96 Geo 3 banger as a delivery truck. I usually carry from a min of 100 lbs to a max of 400 lbs. Over nearly 3 years I have found that every 100 lbs of extra wt in my car at the normal 60-65 mph costs me an extra one mpg.

I think that's what is happening on this route. Since it's all either uphill or downhill. It somehow averages out.

Or maybe I'm just silly and that was a fluke.

Right there shows the added mass overcomes the added rolling resistance and makes it easier to keep it moving. However, added weight does make it harder to get it moving from a stop, as you said.

Hence, You also need to take in consideration the placement of the load, how much and where it compresses the suspension. Driving with the front lifted and the rear lowered will increase drag by forcing more air under the car, and compressing it as it moves to the rear.

So in brief the argument is that the gain in potential energy difference may offset the loss due to increased rolling resistance. I suppose that may be possible in steep grades. I would be interested in seeing a calculation of the example of 1000 feet drop over 100 miles. :)

Remember the "inertia", or more precisely, the kinetic energy, comes from burning fuel. More fuel will be burnt to reach the same speed with more mass. There is no "free lunch", just net loss due to increased rolling resistance. On a flat road at least. :)

Nothing beats hard data. :thumbup:

Sure, that's real easy to do.

A car with 300lbs extra cargo converts .4MJ of extra gravitational potential energy from a 1000 foot descent. 300lb*9.81m/s*1000ft - Wolfram|Alpha

With some fair LRR tires, CRR=0.010, the cargo burns up that much energy through rolling resistance every 20 miles. .01*300lb*9.81m/s*20mi - Wolfram|Alpha

If you were descending an endless slope, it must be a 1% grade for extra cargo to pull its weight at CRR=0.01. This is independent of factors like the weight of the car, cargo, aerodynamics. It depends solely on CRR.

My experience is a bit different, but I dont have much in the way of instrumentation so don't swear by it. When moving a few years ago we moved about 150 miles with about a 1000 foot climb in altitude. I would load up 1000lbs+ of stuff, and didn't notice a difference in FE, I'm pretty sure I would have noticed anything greater than a 1mpg loss, which would have been about 4%.

I was adding about 30% extra weight, which is about the same you added at 400lbs (guess). What % loss in FE is yours?

From your description, it appears you are driving a big, heavy vehicle to begin with, seeing that you had 1000 lbs. in it. With larger, more fuel thirsty vehicles that have bigger engines, they tend to burn approximately the same amount of fuel whether empty or fully loaded. If there is any difference, it will be slight, or at least harder to discern.

Then of course, you are correct. Sometimes gravity is your friend, and sometimes it's your enemy. :)

Just looked at the weight of my truck, 2380lbs, GVWR a little over 4000 and it runs a 2.4L 4 banger. And I was getting about 26mpg regularly at the time.

Car weighs 4000 lbs with a full tank of gas and me in it. I added roughly 150 lbs of weights, my toolbox (75 lbs), and a hydraulic jack (100 lbs) and went about my trip. This is an extra 8% of dead weight.

This weight could be felt on the downhill glides, but not the uphill pulses. I averaged 37 mpg for an entire tank of 600 miles. Which is roughly what I normally get.

I don't doubt it is making a difference. But in this car, in this terrain, with this driving style, it really seems to average itself out and make no noticeable difference.

This is quite interesting. I seem to remember you having a 2.5L, and frankly, I find the 2.2's to be over powered for a Legacy. So your engine should have no problem with the load difference when climbing.

One thing no ones brought up is the aerodynamics of a loaded car. If we're talking high way speeds and flat ground, and not considering acceleration at all, then the variables are: the amount of tire touching the road, and how high the car is sitting.
The aerodynamic change would be quite minimal, but it would likely favor the loaded car. But how it's loaded could make a difference.
The tire resistance is quite obviously greater when loaded, normal people could compensate by uping the pressure, but eco modders like to kill the middle tread first :p


I used to commute 27miles with more than a 1000 foot elevation change (which was really only in the first 10 miles). I was planing on getting some water tanks. Fill them at home, and then dump them at work, or better yet, wile driving once i got into the valley. But that was not a winter project, and I've moved into the valley since.

I think I'm going to do a more scientific test when I have some time off on Pulse and Glide runs with/without extra mass.

Yeah, I think the motor don't seem to mind the extra weight at all either, climbing.

http://www.submariner.org/thepno95/P...0vs%20DOHC.jpg

(Wheel figures, and mine is the SOHC)

Lucky you; that looks like the better torque curve.

It's always frustrating to see how high most dyno plots start. I used to upshift my Subaru at 2000RPM, so this plot captures none of what I care about.

so true, I also up shift at about 2k (with an automatic, so with minimal throttle)
But once your on the highway in top gear this chart becomes useful (well not so much if you have a 2.2 like me)

With the automatic in the subie: I've found that accelerating quickly(1/2~3/4 throttle) (at least to 30 mph or so) is no more harmful than accelerating slowly. It gets you into the wonderful top gear much quicker that, at least according to the scan gauge, it makes no real difference.

interesting, I'll have to try that. I don't have a scan gauge, so I'll take your word for it. You do get better mileage than me.

I'd also only recommend it if you know it's clear up ahead, of course.

For city driving (where I can only do 24 or so during daytime traffic.. at night I can get 30 assuming I can catch the lights right) my usual method is:

From a stop, make sure it's clear, accelerate somewhat briskly (The trans will shift about 3000~3500 rpm) to about 32 mph and let the transmission shift into 4th and lock the torque converter. Try to hold that as gently as possible if it's a long stretch... OR Put it in neutral if I know I can coast to the next stop, if there is traffic I'll try not to get too low under the limit for courtesy sake.

If you give it too much gas (nearly floored) it will change the fuel trim and go into a more rich burn for about a minute before going back to its normal mixture.

We've both got the 4EAT, so I assume yours is very similar. The gearing (Mine is 4.444) is probably the only difference. So just try and find out how low you can get the torque converter to lock up in 4th. (The rpms will stop varying with throttle and the car will buck a bit more while accelerating, like in a manual car)

RobertSmalls: Yeah, I know what you mean. At my normal cruising speed (55~60) I'm around 2500 rpm, so between that and the 4.444 final drive I think I have plenty of power to make up a few extra pounds.

Well, when you look at the average intent when dynoing a car, you can see why such low RPM is never looked at. But one reason simply is setup, trying to not "bog" the motor from such a low RPM to read a very small amount of power.

However, there are other ways to measure HP off a dyno. :thumbup:

MPG2 = MPG1*(WT1/WT2)^(0.72)

MPG2 = 26*(3000/4000)^(0.72) = 21

Not even close to what I observed. If I had dropped below 24mpg I would have to get gas at both ends of the trip. Only once did I have to get gas midway in the trip, and that was when I had beds hanging out the sides and over the top.

If this applies to weight of the car at the design level, it includes an upsized engine as well. So 3000lb becomes 4000lb, 26mpg becomes 21mpg, your 2.2L becomes a 3.0L, and the car is no peppier.

Ok that makes better sense, in fact it fits pretty well with if I had gotten the 4X4 version with the V6.

It wouldn't relate well to whats being discussed here though because we are compairing loaded/unloaded weight.

This topic came up in another forum (sorry, no link) and one of the things that was brought up is how much torque the vehicle produces.

Vehicles that produce more torque seem to be effected less by a weight increase then one with less torque.

I just got my best tank ever, and I had the car loaded with about 500lbs of stuff on the return trip (tank was for both ways). Granted, everything when really well as far as driving conditions. But I also had the CEL on the whole trip (O2 and knock).
I have found it almost pointless to P&G at 60 with my car. It has to be one heck of a hill, and then to get it back into gear, and get the torque converter to lock seems like it balances out most of the benefit. But with a heavy load it drives like a aerodynamic Legacy!

I have found it nearly impossible to get it to lock under 41, but when it does I can holed it down to 30.

That's the hysteresis for the TCC logic. There is often a minimum vehicle speed, RPM, coolant temp, max engine load, max TPS %, and a couple of qualifications timers for the lock function.

To unlock, basically the reverse is true with some additional factors such as delta-TPS, brake-pedal switch, park-neutral switch, delta-MAP, and of course the MPH hysteresis which gives the delay so you don't have constant locking and unlocking.

You can get better mileage by adjusting most of these calibrations.