Introducing myself
 

I am a retired engineer /physicist. I fill my time these days by working on ideas that I think might have some practical value. My strengths are in understanding the science behind things and applying math to problems.

Six years ago, about, I decided to work on some ideas for improving automotive fuel economy. I bought a 2003 Beetle TDI diesel for my "test bed." Frankly, I haven't accomplished nearly as much as I would have liked--which means I have a ton of things yet to try. To start off, I made some small adjustments to the Beetle to improve its mileage.

1) I designed a drag reducer for my car. You can see pictures at www.ernsblog.com and at Welcome to www.max-mpg.com Basically, it's a large "wing" on the back to correct the downflow on the backside of the beatle. My drag coefficient is thought to be about Cd = 0.31. The result is an increase of 3 mpg, roughly independent of speed.

2) Tire diameter was increased by 3/4-inch, going from 205/55-16 to 205/60-16. I also raised tire pressure, using 40 to 44 psi depending on conditions and how serious my mileage goal is.

3) I tested a wide range of motor oils and found that some synthetic oils do improve mileage by quite a lot. Bottom line: lowest factory-approved viscosity gives best mileage.

4) I add lubricity enhancers to my diesel fuel: bout 3% biodiesel, and 1/4-bottle of Power Service (silver bottle) to a tank of fuel. I think there is a modest improvement in mileage from this, and a definite extending of engine life. In highway driving, I think my engine is about 35% efficient. (This is based largely on theoretical calculations.)

From the factory, my car would get about 49 mpg at 60 mph. I now get 65 mpg at 60 mph, driving here at 5000 ft elevation with good summer fuel.

My special interests are tire and wheel design and development of a more efficient engine. I am also involved in aerodynamics, having worked in that field before.

I have been working on the theoretical side of engine efficiency for about five years. I think I know how to build a clean-burning engine that is 60% efficient; a patent was filed in December. I hope to find a source of funding to build the engine and try it out.

As for tires, I am still working on ideas here. Some rules I believe to be true--

1) Soft rubber makes more efficient tires
2) Big wheels with low-profile tires are really bad for fuel economy
3) Narrow, large-diameter tires are best
4) The second number on the tire (profile) should be 60 or higher
5) High tire pressure improves fuel economy AND tire life
6) Pavement matters. Smooth (or lightly grooved) concrete can raise mileage by as much as 10% over smooth asphalt, 15% for rough asphalt.

By the way, larger-diameter tires allow your engine to run at lower RPM where it's more efficient, but I think most of you already knew that. Be careful to correct all data for the change in tire diameter. Watch your speed.

My best mileage ever on one tankful was obtained in a friendly competition within the TDI Club. ( Fred's TDI Page. TDIClub.com. VW TDI Enthusiast Community ) I filled my tank at Park City, Utah and headed east on I-80, driving about 55 mph. I reached Davenport, Iowa, before getting fuel-- a distance of 1,209 miles on 15.5 gallons of biodiesel blend.

When people ask how to get better mileage, my standard answer is, "drive slower and keep your foot off the pedals as much as possible."

Ernie Rogers

Welcome to the site Ernie. Glad to have you here. I've been to your maxmpg site more than a couple times. You've got good stuff there. :) Care to share your future plans for the Beetle?

Ernie -

Welcome to EM! You're work has been read and cited a lot on this site.

CarloSW2

Hey Ernie, glad to finally see you around this neck of the woods, :thumbup:

Hi Ernie -

Welcome to EcoModder. Your Beetle wing has been mentioned in discussions here several times, most recently here: http://ecomodder.com/forum/showthrea...lper-7444.html

Looking forward to finding out more about what you're up to these days.

Darin

Mind if I ask what temperature the engine exhaust is, for the theoretical engine?

Welcome to the forum

Some things to note about tires.

A harder rubber compound makes a more efficient tire. It improves mileage in the same way as higher tire pressure does. Typically tires rated to last the longest have a harder compound. A harder rubber compound won't give you as good of grip as a softer one so you loose traction.

Also an over inflated tire tends to reduce the life of the tire because it causes more pressure and wear on the center of the tread.

Generally the low profile tires have less side wall flex which reduces rolling resistance. Unfortunately going to a larger rim to accommodate a lower profile tire may increase the rotational inertia of the wheel. Which way to go depends on if you do a lot of stop and go driving or if you do a lot of driving at constant speeds for extended periods of time.

Also its important to note if you go to a larger diameter wheel with a larger profile this will make your car handle worse and make it more prone to rolling over.

Changing tire diameter doesn't always help. Some cars have their gear ratios set to optimize the engines maximum efficiency rpm range to the typical speed limits.

In your situation with a diesel, peak efficiency typically occurs at the rpm where peak engine torque is generated.

Hello, and thanks for the welcome. Pardon my bruskness at times, I don't mean to offend, but--

The energy loss in a tire comes from the work it takes to deform the rubber as the tire adjusts to make its contact patch. A hard rubber takes more work to deform it so it loses more energy, and is less efficient. A corollary is that a well-worn tire is more efficient than a new tire--less rubber to bend.

By the same logic, you should conclude that a low-profile tire is less efficient because more deformation in the wall is required to form the contact patch. This is verified by experiments, and common experience by drivers.

High tire pressure improves efficiency because the contact patch is proportionately smaller, and deformation is less.

About high pressure and tire life-- it's my experience that a good tire with a solid radial belt won't wear unevenly in the middle. Many police and highway patrol cars have tire pressures above 50 psi (for improved safety) with no such wear problems.

About wheel moment of inertia-- that's mostly an old myth too. Wheels and tires have double the effect on car total inertia because they both spin and move linearly. If you increase the weight of wheels and tires by 15 lb each, the total effect on the car is the same as adding 120 pounds in the trunk, or a small mother-in-law in the back seat. Smile, you will get by just fine.

Your comment near the bottom, about larger tires not always working for some cars, is probably true, especially with some automatics. For my car, it's speed for best mileage appears to be somewhere near 50 mph in fifth gear. And, I think you should only make small changes in tire diameter, less than two inches unless somebody has already tried it.

Sorry about the lecture.
Ernie Rogers

Ooops, I'm embarrassed
 

Ooops, now that I have read all the posts in this thread-- a person can be rude on the internet, thinking that nobody knows who he is--but it seems everybody knows me here. Now, I wish I hadn't been so hard on a couple of guys.

Hey, unchosen, you sound like an automotive engineer, very astute question. Let me look that up for you.

The calculated exhaust temperatures are--

For my Beetle engine......... 900 deg. K at 1.1 atm.
For the new engine........... 580 deg. K at 1.1 atm.

Now, if you wanted to verify engine efficiency, you would need to have the approximate combustion temperature....that's 2,000 deg K for my engine. You could apply a mechanical efficiency factor of 0.85, and the Carnot efficiency formula, to estimate brake efficiency for the "theoretical engine."

Ernie Rogers

That sounds interesting. Must be using something other than standard motor oil as it breaks-apart and or ignites above 773 K. Yeah I am an ME student, but I used to be a ChE senior, why I asked about the temp(thermo and organic).

Hello and welcome, Ernie. This forum is filled with the intelligent, the "crazy", and the advocates of trial and error...and now we have a physicist :p. Although you'd probably be labeled as all three in the "real world", you've arrived at ecomodder where my initial vote would be to put you in the intelligent category. I've no doubt your voice will be a valuable addition to this forum.

Excellent point about thermal stability of motor oil. That's why engine makers are very concerned about keeping cylinder wall temperature cool enough, where the oil film is. I use 453 deg. K for wall temperature in my calculations.

Ernie Rogers

No offense taken. :D

You are looking at the right relationship in force x distance and what you say would be true if the a harder rubber deformed by the same amount. In this case the force to do the deforming (the weight of the car) is held constant not the deformation. All other things equal, a tire made from a harder compound has a smaller contact patch as well. This is why trains use steel wheels instead of rubber ones.

A well worn tire has a higher rolling resistance because it flexes more. Its side walls are weaker and its belts are less stiff. The same force applied causes more flex and therefore is applied over a greater distance requiring more work. New tires have their least rolling resistance within about 5000 miles. This is because they have had their flashes and tread corners worn off making them more round. This is the reason that the SAE and ASTM standard tests for rolling resistance of tires are conducted over 5000 miles. After 5000 miles the rolling resistance increases due to increased flex.

I dispute this for the same reasons listed above. All other things equal a shorter side wall tire will have a smaller contact patch. Most often however short sidewall tires are used in performance/racing applications and are wider and are made from softer compounds to give more grip. This all adds to their rolling resistance which quickly can offset any gains from having a shorter side wall.

Police cars receive much more maintenance than a regular car and have their tires checked quite often. Also the abusive driving conditions they go through means the tires get replaced quite often. Due to these factors, tire wear on a police car is not a valid comparison.

A good belt helps, but eventually it wears. As inflation pressure increases the contact patch shape gets rounder and rounder. This increases wear on the center of the tread.

I agree its not a lot but it does play a roll. It has effects beyond just acceleration as well. Every time you turn a corner it takes energy to over come the gyroscopic forces.
I don't take things as a lecture. Its more like a debate. :)

ConnClark,

It's the other way around. RRC is mostly dependent on how much material you are flexing. When a tire wear, there is less material to flex (lower hysteresis losses), and for a fixed pressure, RR goes down.

The same thing happens when you raise the pressure. The tire is being held by the air pressure inside it, up the pressure and less material is flexing while the tire is rolling.

Same thing again when you are driving on a rough road.

I do not think harder compound reduces RR. Because the material is still going to flex, as air pressure is the predominant factor here, and since it's harder to flex, hysteresis losses are just probably gonna be greater.

Welcome Ernie
 

I'm always glad to see another TDI on Ecomodder. I wonder when one of us is going to get really brave and Basjoos a VW to see what the diesel can really do when optimized for aero with the right gearing.

I do have to side with ConnClark on tire wear. I have been running higher-than-suggested-on-the-sidewall tire pressures for decades. Up to a point you get better treadwear because you don't scrub off the shoulders as much when cornering, especially in a front wheel drive car. However, if you are doing a lot of straight line driving, the belts will bulge from the pressure and the middle wears faster.

Californian Study of Rolling Resistance:

http://www.energy.ca.gov/2006publica...0-2006-001.PDF

Page 27:
Page 63 - Has some interesting charts regarding vehicle energy consumption

Page 64:
Page 67:
Page 68:
I could not find anything in the article regarding the "hardness" of a tire, but it stands to reason that the harder the material the less it flex and thus will experience less hysteresis. The above example of a steel train wheel is a great example of an extremely low rolling resistance wheel.

The article does discuss to a large extent the relationship of the performance of the tire compared to the efficiency (rolling resistance) of the tire. It generally states that their is probably somewhat of an inverse relationship but that tires could be designed to improve their RR without adversely effecting its other properties.

I've read that study before, and I have a problem with the following part:

It's a good thing they say "should" because the Lambilotte study does not agree with this, at least for the tire model tested. From the limited data in that slideshow, we can still see the following:

- The lowest RRC regardless of width for Diameter/Aspect ratio are: 13/80, 14/75, 15/70. Above or below those values the RRC usually increases.
- The lowest RRC size for a given tire diameter is usually the width, regardless of aspect ratio, closest to the tire diameter times 14.

Are we really having a discussion about Tire Rolling Resistance in a thread where Ernie introduced himself?

Wouldn't a separate thread be a better forum - particularly considering that folks might be interested - and even join in - if they could identify the thread as such?

Thanks CapriRacer, you're right. For the concerned about tire rolling resistance, start up a new thread and you guys can post up your info there and figure it all out. :) I can even move over posts for you guys.

Now back to Ernie's intro!

Notice: As suggested, I will start a new thread on tire selection. For that purpose, I will post a "lesson" I did some years back on the topic of tire efficiency.

Ernie Rogers

Here's a link to the new discussion:

http://ecomodder.com/forum/showthrea...ency-7811.html