“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, October 29, 2006

New car considerations

I've had the 2000 Jeep Grand Cherokee Limited that has been the subject of the experimentation described (ad nauseum) in this blog since August of 2000. It has about 142,000 miles on it and has been a truly wonderful car. I know some have had lots of trouble with their Jeeps, but I've changed the oil in mine, and the brake pads and rotors once. Other than that, it's been maintenance free. But it's getting a little long in the tooth, with no navigation system, the cd changer in the back of the truck rather than in dash, etc. I decided to start looking for a replacement.

Most who are aware of what I've been doing to minimize fuel consumption in the Jeep assumed I'd look at Prius, Insight, Civic Hybrid, or other mileage maximizing vehicles. Today I looked at the Acura RL, the Acura RDX, and the Lexus RX350. Huh? The best of these achieves E.P.A. ratings of 20 city, 26 highway. Isn't this hypocritical?

Well, for starters, they all are rated higher than my Jeep (15/20). But the fact of the matter is, I like big engines, acceleration, and luxury. The fact that I never utilize the horsepower available to me in the Jeep doesn't mean that I couldn't (and haven't). And there are those who say "you've squeezed all the blood out of that turnip" in reference to my experiment. Occasionally, I do think it would be nice just to drive, enjoy the performance of a nice car, and not concern myself with trying to extract the last possible foot out of each milliliter of gasoline.

But the fact is, none of the vehicles I looked at today excited me. The RL is nice, high tech, has real time traffic through XM satellite radio, is comfortable, and handles well. The RDX just didn't thrill me at all, nor did the RX350. In fact, my experience today led me to realize how much I really do like my Jeep. It's exceptionally comfortable, has power in case I should ever decide to go back to using it, and has demonstrated flawless reliability. I'd like to have satellite navigation, but that can be achieved with a Garmin portable unit or similar. Blue tooth hands free calling would be nice, but I can get an earpiece. An iPod port would be great but I can get an adapter. XM satellite radio is cool, but I can get a portable unit. All in all, I'm just still very pleased with my Jeep.

One of my business partners got a BMW X5 last week, though I haven't been in it yet. I'm not a BMW kind of guy. They seem like a "look what I can buy" kind of vehicle, though Brian assures me that he isn't a "look what I can buy" kind of guy. It's been suggested that I look at the Infiniti M class. I don't know much about the car, either in terms of features or gas mileage. But unless it knocks my socks off, I'm thinking very seriously of staying with old faithful.

Sunday, October 22, 2006

Effects of weight

I've scoured the internet for the last year (plus) looking for mileage and fuel economy related sites. One of the "rules of thumb" I've seen quoted is that "you will lose 1% to 2% of your gas mileage for every 100 pounds of excess weight you carry" (see here for example). Since I tend to be a pack rat and that tendency extends to the cargo area of my Grand Cherokee, it's one of the areas where further savings may be possible.

First, suppose it is true. In that case if I removed, say, 200 pounds of stuff from the car I could expect to improve from the 23.6 m.p.g. I am currently averaging to about 24.1 m.p.g. In the course of the approximately 20,250 miles I drive per year, I could expect to save about 17.8 gallons. At current Southern California prices, that represents a savings of a little over $43. Maybe dinner at Islands for two, but no movie afterward. Of course, long-term readers of my blog (lol) will realize that this likely exceeds the savings realized by eschewing the drive through window. That means I MUST do it if I can demonstrate that it's a plausible number. Let's see what we can do.

A couple of posts back I discussed mass as it relates to mileage. As related there, I think there are probably three detrimental effects of a more massive vehicle on gas mileage. Only two can be controlled by eliminating weight from a given vehicle: the energy cost of lifting mass up hills and not receiving full repayment on downhills; and tire rolling friction.

I will make an educated guess that increased dissipative losses on hills due to increased weight are a so-called "second order effect" and that the primary effect of increased weight on fuel economy is based on the increased rolling friction. I have cited a web site several times where the author discusses the physics of automobiles, and on that site the author contends that rolling friction is approximately 1.5% of vehicle weight at freeway speed. His discussion is actually more detailed, but that's my estimate based on the information he provided. In another post I've shown that his calculations agree with the ones I've made based on fuel consumption, so I think it's reasonable to use his figures.

Thus, I can estimate that 200 extra pounds would result in 3 extra pounds of rolling friction. Since fuel expended to maintain speed is proportional to the total resistive force, which I calculated using the rate of fuel consumption in a previous post as approximately 139 pounds, and that I will calculate in a subsequent post using a different method as 170 pounds, 3 pounds represents somewhere between 2.2% and 1.8% of total resistive force. The "second order effect" mentioned above will only add to the savings, though likely by a minor amount. But that's pretty close to the 2% to 4% predicted by the rule of thumb, so, out comes the junk.

Sunday, October 15, 2006

Efficient speed

It's been well over a year since I began my experiment to increase gasoline mileage in my Jeep Grand Cherokee Limited. Without any doubt, huge increases can be achieved. At the outset of the experiment, in August, 2005, my average mileage indicator on the display was at 14.9 m.p.g. It is currently at 23.6 m.p.g., a whopping 58.4% increase, and an estimated 31.1% above the EPA estimate for the vehicle (18 m.p.h. combined). I should add that the information on the average mileage indicator is confirmed by an extremely detailed, tank full by tank full spreadsheet. I calculate mileage by tank full, five and ten tank full moving averages, standard deviation, and estimated annual savings in gallons and in dollars.

In an earlier post (More on acceleration) I estimated that about 14% of my savings come from reduced rate of acceleration. It might be wondered where the rest comes from. As loyal readers may recall, the other steps I've taken are to utilize cruise control at 55 m.p.h. on highways and freeways; anticipate stops and slowdowns to enable coasting to stops and speed reductions so as not to waste energy by braking; minimize use of "appliances" (air conditioning, headlights, seat heaters, defroster, etc.); coasting downhill out of gear (the savings here are controversial - some maintain that modern computerized cars are more efficient coasting in gear); filling the tires to 2 p.s.i. above recommended maximum; avoidance of drive through windows; and turning the engine off on long downhills and at long stoplights.

Of these, I think it's very clear, based on both theory and the evidence of the instant mileage indicator, that the main contributor to my increased fuel efficiency comes from my reduced highway speeds. My understanding of the physics involved leads me to conclude that the reason for the dramatic decrease in mileage per gallon at speeds above 55 m.p.h. is that aerodynamic drag increases as the square of speed (as noted previously, others say cube, which would make it even more dominant).

I have done a lot of "googling" using search terms involving fuel efficiency, minimizing fuel consumption, etc. and there are many people on forums and blogs who contend that their vehicles are much more efficient at 70 m.p.h., and even 80 m.p.h. than at 55 m.p.h. As I noted in my original article on acceleration (To floor it or not to floor it) there is general agreement that fuel efficiency (m.p.g.) increases as speed increases up to a point where the aerodynamic drag increase overrides the increase in efficiency from utilizing fuel for motion rather than merely running the engine. A further complication is that the gearing and engine parameters for a particular vehicle may make it utilize fuel to develop power more efficiently at some relatively high engine speed.

So is it possible that the above-mentioned posters are correct? It would imply an extremely low coefficient of drag, combined with an engine and drivetrain combination that would lead to terrible low speed performance. Since I don't have such a vehicle, I can't do any experimentation, but I suspect it's wishful thinking on the part of the drivers of those vehicles in an effort to rationalize their behavior. I'm not a psychologist, so I have no comment on why they would have a need to engage in such rationalization.

Saturday, October 07, 2006


It's been almost two months since my last post due to some surgery that made writing and typing difficult. I hope I haven't lost my devoted readers. Right. In any case, onward and upward.

The trend in my mileage has been a significant increase in standard deviation, together with a slight decreasing trend in mileage. This, despite the installation of the K&N high flow air filter noted in my last post. Starting June 19, there was a major downtrend in my mileage from which I've never really recovered, followed by a trendless few tank fulls with large variation.

A comment was left in my post about Dr. Steven Dutch and his article about the 200 mile per gallon car. At the end of that post, I stated that it was my belief that "in order to achieve major reductions in oil consumption without going to vehicles such as the scooter I discussed a couple of posts back, large-scale changes must be made in the technology of internal combustion engines or other propulsion methods must be employed."

Bill Anderson, host of the blog entitled "mental radiation," commented that large gains can be made in automotive gas mileage by reducing the weight of vehicles. He stated that two thirds of the energy used at the wheels is used to overcome weight, and concluded that by reducing weight the amount of energy required to get from point A to point B can be reduced. Dr. Dutch implied a similar conclusion.

What about this? Well, obviously, since F=ma, that is, Force equals mass times acceleration, it takes more force to get a heavier (more massive) vehicle up to a given speed. But at speed, on level road, acceleration is zero and hence, the sum of forces acting on the vehicle must be zero. These forces are dissipative (drag, rolling friction, driveline friction, engine friction) and force applied to the road by the engine. With the likely exception of rolling friction, seemingly none of these are a function of mass, though engine friction must increase with engine size, which in turn typically increases with vehicle weight. Though this is probably not necessary by the laws of physics, a certain capacity for acceleration must be provided by its manufacturer to make the vehicle saleable.

And since I concluded in a series of earlier posts that engine friction is a very significant component of energy usage, heavier vehicles must use more fuel even in unaccelerated travel, though it isn't a direct correlation. Added to this, it takes more fuel energy to lift a heavier vehicle up a hill, energy which is not fully recovered in the descent due to disspative forces. Further, it seems very likely that heavier vehicles produce higher tire rolling resistance. In fact, this is almost certainly the largest contributor to increasing fuel consumption with increasing weight. Finally, unaccelerated travel on level roads for long periods is not the norm.

Thus, I agree that weight reduction is an effective means of increasing fuel economy, but at freeway speeds a large percentage of the force the engine must overcome is produced by drag. Reduction in "flat plate area" can be achieved by making cars smaller as well, but there is a limit - we still want a driver's seat and a passenger seat. I doubt we'll see tandem seating anytime soon.