“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 21, 2007


I live in Southern California, and we experience a phenomenon known as "Santa Ana Winds." These winds can literally reach hurricane speeds, I heard that a gust reached 108 m.p.h. today. These winds are invariably extremely dry and typically result in a rash of wildfires. Such has been the case today.

I was driving back to my house from Lakewood, an easterly trip down the 91 freeway. Santa Ana Winds originate in the Great Basin and thus are typically northeasterly. As I tooled down the freeway at my usual 55 m.p.h., I noted sand, pebbles, leaves, etc. blowing into my windshield as my car experienced significant buffeting. Looking at my miles per gallon display, on a level stretch where I expect to see 21.5 m.p.g. I noted 17.2 m.p.g. I had heard that the sustained winds in this area were on the order of 30 to 40 m.p.h. with possibly something like a 22 to 29 m.p.h. headwind component (it was off my nose by maybe 45 degrees), and so presumably that's the sort of gas mileage I could expect in the range of 80 m.p.h. (using the average of 22 and 29 as the headwind component).

Needless to say, I wasn't pleased but I wasn't willing to slow down to, oh, say, 30 m.p.h. to attempt to save fuel. Even my compulsiveness has limits. Besides, I didn't want a crash or a ticket. I've been pulled over for driving 55 m.p.h. in the right lane. Though the officer wouldn't say why he pulled me over, I'm sure he thought I'd been drinking (I've been sober for 28 years) and that I was trying to avoid being pulled over. He didn't even ask for my license and registration, he just shined his light in my eyes, told me to drive carefully, and left.

So what's to be learned from this headwind experience? There's not much to be done about it, but I can at least plug the numbers into my fuel consumption versus speed equation and see if they fit. Maybe my mileage gauge can serve double duty as an anemometer.

Sunday, October 14, 2007

More on A/C

With the Scan Gauge 2 I can find out a lot about what my engine is doing. I have it set to provide a continuous display of speed (it's about 2 m.p.h. slower than the dashboard speedometer display), r.p.m., instant mileage, and absolute manifold pressure. The manifold pressure is a very sensitive indication of throttle position, since throttling is accomplished by restricting the flow of air through the throttle body.

I've found a few stretches of freeway where it seems like the road is level, or at least its slope is constant (constant slope will suffice for this). Thus, with cruise control on, manifold pressure will remain quite constant on these stretches. This is an ideal time to experiment with turning the air conditioning on and off to see if there is an effect on manifold pressure, indicating that throttle position is increased in order to operate the compressor while still maintaining the selected speed.

So what happens? Well, the manifold pressure increases by approximately 0.4 p.s.i., typically from 9.1 p.s.i. to 9.5 p.s.i. I found this to be a consistent and repeatable result. What does it mean in terms of fuel consumption? I can run a few calculations and come up with a number, but I'm not extremely confident in the accuracy because the indications on the instant gas mileage display are not as dramatic, consistent, or repeatable.

But let's proceed anyway. PV=nRT in an ideal gas, we'll assume (inaccurately) that that's what we have. Since, for a given length of time the volume, V, and the temperature, T, are fixed, and R is the universal gas constant and thus never changes, a change in P means that n, the quantity of the gas (number of moles), must change by the same proportion. A change from 9.1 p.s.i. to 9.5 p.s.i. represents an increase of about 4.4%, so fuel consumption should increase by a similar amount. Since I'm typically looking at about 21.7 m.p.g. or so, I should see a decrease to something like 20.8 m.p.g. It doesn't seem like I see this much of a decrease, but I'm going to be doing some more checking.

In another post I determined the LR3 uses something like 24.5 horsepower to cruise on a level highway at 55 m.p.h. Since burning fuel provides this horsepower, the additional fuel burn should reflect the increased power required to run the air conditioner. How much power? It works out to be just a tiny bit over 1 horsepower, and as I concluded in my previous post on air conditioning, I find that to be a number that squares nicely with my intuition.

Tire pressure and the last 1%

Regular followers of my blog (mythical creatures though they might be) will have noted that I have compulsive tendencies. This character trait has expressed itself in various ways through my life, some destructive and others not. I consider my pursuit of maximum mileage from gasoline in my vehicle to be in the latter category. For that reason, I'm glad to continue my activities and analysis in this area.

I've had my Land Rover LR3 HSE for just shy of a year (324 days to be precise). I haven't spent a lot of time checking tire pressure, how important might this be? Various sites (see no. 4 here for example) give percentages of 2% to 4% as the excess consumption caused by under-inflation. Edmunds conducted some testing of various "tips" to save fuel, reported in a column entitled "We Test the Tips." Tire pressure is number 5 in their list of tested tips. They were unable to find consistent savings, though they did find what they termed "modest savings" in two vehicles.

What about the physics? Well, it's quite a complex topic to solve analytically, but I utilized a tool called "Dimensional Analysis" to make an approach to the problem. I concluded that rolling resistance is inversely proportional to the square root of tire pressure. This would mean that there might be approximately a 5% difference between overfilling by 2 p.s.i. versus being under-inflated by 4 p.s.i. Keep in mind that rolling resistance is only one of the external forces acting on the vehicle and that it decreases in relative importance as speed increases, since aerodynamic drag increases with the square of speed. Interestingly, tire rolling resistance is independent of vehicle speed, at least insofar as the depth of the analysis I performed.

So at highway speeds, it's likely that an increase in rolling resistance of 5% might contribute about a 2% increase to overall resistive forces, exactly in line with what many of the fuel saving sites indicate. I'd better get that gauge out of the glove box.