Glides like a Brick Wall (Jeep Cherokee, GMC Pick-up)

[DonEaston]

How large of a factor is vehicle aerodynamics as compared to engine size, driving style, gear ratio and all other factors that influence fuel economy?

I am thinking that Aerodynamics is indeed THE largest factor; ESPECIALLY when the vehicle is grossly underpowered as most 'fuel efficient' vehicles seem to be these days.

Between my wife and I, we have two vehicles:

1) a 1999 Jeep Cherokee Sport 4x4 w/ 4.0L inline 6 cylinder engine - 70k miles

2) a 1992 GMC Pickup, 4x4, 5.7L engine - 160k miles

The Jeep gets around 17mpg, whereas the old truck gets about 10.

It's tough out here in the sticks, because the nearest plowed road is about 3 miles away, creating some serious snow between us and town. I had to get rid of my precious Monte Carlo, that I had up to 32mpg (sticker only said 29 when it was new, AND even with 278,000 miles, I got 32mpg!).

Anyway, aside from those three miles, these vehicles suffer from "pushing a wall of wind" syndrome. The truck plows through it better I think, as it's engine is more appropriately scaled for the weight / size of the vehicle. The Jeep, though it has a lot of torque and power, has a tough time pushing the air efficiently when you get near freeway speeds (70-80mph in Michigan), and their fuel economy suffers. I think the last time I checked, the GMC pickup gets about 7mpg at those speeds.

Okay, I look forward to your responses!

Don

[newtonsfirstlaw]

Brick walls actually glide a lot better than you'd think... the problem is it takes a lot of fuel to regain the speed after you've lost it.

Barn doors glide poorly, but you use less fuel on the pulse.

With the comments about vehicles being "underpowered", I have to ask if you are trolling. If you aren't, I'd ask if you consider a semi-trailer to be "underpowered", and your reasoning behind that.

[Lazarus]

It depends on the type of driving you do. If you're doing city driving and never get over 35 mph then aero mods won't help as much as driving 70 mph on the freeway. I think most people don't necessary spend most of there time on the highway so the biggest bank for the buck is driving style and gearing then aero. Some of the aero stuff should be standard on all cars though IMO. Belly pans, some form of grill blocks and wheel deflectors. Oh and something that all cars should have is some kind of mandatory FE instrumentations.

[DonEaston]

Sorry for the confusion, Mr. Firstlaw- not trolling at all.

I don't have too much experience with sub-compacts, but my father in law is an enthusiast of Geo and Kia, and when I drove them, I felt like I really had to push the things to get them to do what I wanted.

So, not "underpowered" in the sense of "my vehicle is more powerful than yours", simply in the sense of "is it really economical to try to push a (relatively) heavy vehicle with the smallest engine possible? Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"

For example, I know a guy who put a large V6 on a small tricycle type frame, and the unit itself got extremely high mileage. I do not recall the numbers, but I remember wondering if perhaps the unit would not have gotten such good mileage had the engineer chosen a smaller engine.

Okay, all for now.

Pat

[SVOboy]

Welcome to the site. I will chip in that I think the balance of what's most important depends on the car, but certainly for a truck (as far as physical mods go) I think aeromodding would give you the most bang for your buck.

I actually just downgraded the engine power on my car (in hopes of greater fuel economy). Sure, I can't accelerate as fast, but I can still go down the highway as fast as I need to with quite a bit extra power if I feel like downshifting,

Again, welcome. Would be interesting to see some crazy aeromods on the truck.

[newtonsfirstlaw]

Originally Posted by DonEaston

Sorry for the confusion, Mr. Firstlaw- not trolling at all.

Ok then, welcome to ecomodder.com!

So, not "underpowered" in the sense of "my vehicle is more powerful than yours", simply in the sense of "is it really economical to try to push a (relatively) heavy vehicle with the smallest engine possible? Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"

The answer is "it depends on how you drive it".

If driven correctly, a smaller engine will ALWAYS be more economical, provided that it is correctly geared and that power produced at peak BSFC is more than the load on the engine at constant desired cruising speed on level ground. This is the case with any car you care to name, geo and kia included, and including my car (660cc engine). The exception might be a 105cc unfaired postie bike, but only if driven at top speed.

For top fuel economy, giving it more than about 65% of available power in an Otto cycle engine at that rpm is wasting fuel. At about that power level, you get more bang for buck, i.e. more power at the wheels for a given quantity of fuel. Diesel is different, wide open throttle (WOT) is the most efficient, so as long as your engine is capable of cruising, it is not undersized wrt fuel economy.

Also, for hill climbing, even the smallest engine can move a truck up a hill, given the correct gearing. (I just know there has to be someone more nitpicky than I am who is about to bring up static friction, but I beat you to it. ) "Give me a place to stand and I will move the earth" - Archimedes. It's all about leverage, not power. Getting there at the same pace you would cruise on the level is a different story however. But this is a site about fuel economy.

Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"

No. The reason for that is the engine map of a petrol engine. They are most efficient at load, and get less and less efficient the closer you get to idle. The reason for this is that in a larger engine, at idle, you have more mass to rotate, more surface to rub against, more air to pump, a more closed throttle plate that is choking off the flow through the engine.

You could get a good idea of what it's like to be an Otto cycle engine by forcing yourself to only breathe through a drinking straw every time you slowed down, and a smaller drinking straw when you stop. It's hard work, and you are putting out next to no power. If your aim was to go somewhere, it would be easier to walk or jog along.

It's not a great analogy, but it might help?

Remember not to confuse it with gearing issues. Low power at low rpm is more efficient than the same power at higher rpm, but it is still inefficient compared to a smaller engine at low rpm with a more moderate power loading.

[DonEaston]

WOW... this is fascinating...

Thank you guys, for the information- really.

My passion and obsession lies far from driving a fuel efficient vehicle, (to me, fuel efficiency is "miles per tank", therefor, my GMC is more efficient because I end up filling it up less than the Jeep), but I am VERY interested in this subject. I cringe at the thought of a country filled with tiny cars, as driving, to me, is an art. I do enjoy feathering my pedal, especially when I had the Monte, as I would try to top my record with each tank, but I ALWAYS have days where I'm all about hammering into and out of corners...

Anyway, I have company and it's very rude for me to be in here typing... I'd better head out!

Guys, I plan on coming back. I love all things mechanical, and I love not the man who goes out and buys a Prius with the notion in mind that he is saving the world, but the man, like you, who appears to be meticulously striving to get the most out of the machine.

Congrats to you, sir. I look forward to hearing more of what you have to say.

Pat

[trebuchet03]

Originally Posted by DonEaston

So, not "underpowered" in the sense of "my vehicle is more powerful than yours", simply in the sense of "is it really economical to try to push a (relatively) heavy vehicle with the smallest engine possible? Would not make more sense to give a very light vehicle an engine that could essentially 'idle' down the road?"

Well, I was going to chime in before reading the entire thread... But I just checked and someone else mentioned it Higher load is the way to go for a gasser

I don't have too much experience with sub-compacts, but my father in law is an enthusiast of Geo and Kia, and when I drove them, I felt like I really had to push the things to get them to do what I wanted.

Here's a nugget to think about... Given a car's shape etc. lets say it takes 20hp to stay at speed on the highway. Weather or not your engine can make 30 or 300hp - you can only make 20hp to maintain a steady speed. So on an engine that makes less power, yes, you do need to push on the pedal a little further... But you're still going to make the same amount of power

Now think about this - the largest source of restriction in your intake is your throttle plate. If you have to keep your throttle open 30% in the smaller car versus 10% in the larger car - which one has more restriction (and therefore more pumping losses to overcome)? <-- this is a bit of an oversimplification, but a quick/easy thing to think about



For my last bit... I wouldn't call any of the modern fuel sipper ecoboxes underpowered by any means. For what they are - they're grossly overpowered (my 20hp example above really isn't too far off for cruising at highway speeds ).

[DifferentPointofView]

Okay, so your saying my car shifting into it's highest gear while I glide with the pedal off is actually hurting my fuel economy? If It drops the RPM's from 1600 to 750 at 40mph I'm getting worse than pushing the pedal? That's confusing. Also, In MO, I drove my moms Dodge Caliber R/T which has the largest engine available for 07 and is rated at 29mpg highway. Well MO has lots of hills, and going up a hill, with the speed limit at 75 me going 80, you'd go up these hills that are over 2 miles up and the engine would do 3xxx rpm keeping it going 60, I would have to floor it to go atleast 70, whereas in my Jeep I could keep it at 2200 RPM and keep trudging up the hill with no problem. There's where the Underpowered part comes in. If you frequently drive in MO, your gonna have some problems keeping up with the flow without getting rear ended by a cell phone talker.

1) a 1999 Jeep Cherokee Sport 4x4 w/ 4.0L inline 6 cylinder engine - 70k miles

2) a 1992 GMC Pickup, 4x4, 5.7L engine - 160k miles

The Jeep gets around 17mpg, whereas the old truck gets about 10.

Do a grille block, I did one on my ZJ, and the results are feel-able. also if you do an belly pan, then it will still look like a Jeep without the other Jeeper's laughing at you. Also do as thin an oil as you can. I just switched from Mobil 1 10w-30 full syn High Mileage to Mobil 1 5w-30 full syn Truck and SUV and noticed, with the grille block, have so far done approx 26mpg this tank ^_^ really good results up from 21mpg! that's only judging of going 78 miles on aprox 3 gallons.

[trebuchet03]

Is the Caliber's aero really that bad? I was able to climb the mountains through Albuquerque in my Jetta @70mph - smallest engine available (110hp versus the Caliber's 14?) - both cars are about the same weight I believe (awd?). Don't know if I could go faster - I just had my cruise control set the whole time. When I had an HPV on my roof - those same climbs were between 55 and 60mph floored (I was also loaded with a summer's worth of gear ). I hung out in the semi-truck lane Going back down was awesome though

I know that's a comparison between a sedan and hatch - but that makes me even that much more angry at GM

In any case, I personally don't classify a car as a fuel sipper unless it gets >30mpg (my car is on the fence, morso not, without good technique ). It's almost as if the mfr's break 100hp just for the sake of breaking 100 :/

Okay, so your saying my car shifting into it's highest gear while I glide with the pedal off is actually hurting my fuel economy? If It drops the RPM's from 1600 to 750 at 40mph I'm getting worse than pushing the pedal?

Gliding is very different than cruising.... But, and I should have made this more clear (my fault), my comparison was between two different engines. One that can make more power versus one that makes less