Desert Driving Dilemma: Newsletter Challenge (08/29/06)

In most newer vehicles the air conditioner is as efficient as the windows open and feels better. So no savings there. The heat of the closed up vehicle is not good for the driver mostly due to the fact that the wife is going to kill him. Also with the vents closed the vehicle is not exporting the heat of the motor as well as it might. The engine could possibly over heat.

Put the car on cruise at a speed that gives optinum mileage. Usually around 60 to 70 miles an hr. I'd say keep the air on, walking the extra 400ft. to the gas station would be good for you after riding in a car for a 99.9922 miles.

Yaep thats right....

As someone who does a lot of dsert driving in the Sahara - i am afraid your cruise would not be much use!!

Jazzy1oh1 wrote: "Also with the vents closed the vehicle is not exporting the heat of the motor as well as it might. The engine could possibly over heat." The ONLY means for the vent system to extract heat from the engine, or otherwise affect engine heating is to run the HEATER; the vents will NOT affect the engine temp... so jazzy's statement was incorrect and misleading.

The redneck geek explanation: Since wind resistance increases exponentially to the 3rd (or is it the 4th?) power, the faster you go, the higher resistance your beat up Dodge needs to fight against to git you where yer going. This is outweighed on the slow side by the need to run the engine to git er done and git anywheres. I think most automobiles are tuned, aerodynamically, to function best in the 50-65MPH range. [PS. According to mythbusters, you actually create more resistance and get worse gas mileage by dropping the tailgate although if you happen to be hauling a large roadkill deer or alligator, it may fill up enough of the pickup bed to act like a tonneau cover, which will actually create a better slipstream and better gas mileage]. Getting back to the real dirt, oh, which reminds me having a clean pickup will also increase your gas mileage (less drag from a smaller boundry layer against any shear area). Having a smooth, 'slippery' waxed finish can actually allow you to go faster :) The moral, if you were the one bagging the roadkill deer or gator, it's probably best to hose off the blood or at least wipe it off with an old pair of dirty underwear. You spouses' shirt may also work and if you're in dire straits, maybe even your dog. Which brings me to the next point, closing the windows and vents. This may be fine for a little while but what happens when the old Dodge heats up in about 3.5 minutes? It's ok to let the woman bitch about how yer stupid and how hot it is but ol Rufis will be getting pretty hot too. Since the AC isn't blowing on him anymore, he'll have to at least stick his head out the window and let his tongue flop in the wind. I don't see a humane way around this. Now you have another problem. The window is down, blowing the engineered aerodynamics of your old beat up Dodge out the window. Rufis isn't helping by making a drag car chute with his big slobbery cheeks in the wind, creating an unaccounted turbulent zone on one side of the truck which carries back and also messes up the slipstream bonus you get from the roadkill deer in the back. Solution to the problem: Keep the deer since even if you run out of gas and get stranded, you at least have something to eat. The likelihood of getting stranded is greater because you have also been hauling around more weight, which also lowers your MPG (but we didn't get into that). On the bright side, your woman has also been bitching about the dog farting so if you do run out of gas, she can go outside and cool off in the fresh air, and while she's at it, she can start pushing the truck to the next gas station. Just to be nice though, you suggest that she can take off her shirt and wipe the blood and dirt off the truck to make it clean and increase your MPG. Note: make sure she also gets the tobacco juice stains off the driver side from you spitting out your dip. Also on the bright side while she's pushing the truck you can tell her that she's getting all this great exercise she needed. Win win situation. Let the AC blow...

Shutting down the AC will relieve the engine of a few HP. Closing the windows reduces aerodynamic drag (although, it has a diminishing effect as speed goes down). The vents really make no impact and should be left open. Anything that draws power (mechanical or electrical) that can be turned off should be turned off.

Your speed should be lower, too, and select the highest gear to maximize MPG.

As a final straw, reducing vehicle weight will also help increase travel distance with available fuel. It is advisable to jettison as much mass as possible. You could throw the wife out of the car, but it is better to do this while rolling since stopping and starting uses up more fuel than maintaining present speed.

If you really feel like you are in a good mood you could slow down a little before having her punch out.

In addition to the above mentioned items, one should not only slow down but use the pulse-and-glide technique: with the engine turned off (may as well leave the windows open!) let the car glide to a speed of ~10mph; [jump] (with manual tranny) start the engine and engage a reasonable gear and mat the gas until getting up to ~30mph then immediately cut the engine. (Insert fine-print here:) Of course, all of this is on a controlled route with a professional driver, etc...

Most newer cars are designed for maximum MPG around a certain speed (i.e. 55 MPG). The reduction in drag force obtained by traveling slower than this speed is offset by the decreased efficiency of the drive train. It is the speed at which the overdrive operates at its lowest RPM. So as long as the car is new enough to have overdrive then you should use it and drive at the lowest RPM (1000-1500). This may be hot but a couple hours in a hot car beet a couple days stuck in the desert :)

Correction:
Most newer cars are designed for maximum MPG around a certain speed (i.e. 55 MPH). The reduction in drag force...

Actually, not. The physics of the problem always ends up with a speed slower than 55. Ideally, close to idle point of the engine in the highest gear.

I believe you got it all correct here engineering wise. Socially the simplist is to get out of the car have the wife drive on (saves weight less drag). Hitch a ride with the first fun love gal that comes along. If the wifes car runs out of gas you don't care, she can push it the rest of the way. Wait at the motel with the girl for awhile and see if the wife comes to pick you up. If she doesn't, good you got a girl and if she does at least you get your car back.

Nobody, clued in to the fact that flipping the switch to turn of the air conditioner inside the vehicle, does not totally disable the air conditioner. to properly turn it OFF, one must get under the hood and remove the connector that links to the solenoid clutch that drives the air conditioner. When a person disconnects inside the car, the compressor solenoid clutch, still comes on,(to circulate the fluid, to make startup quicker). Considerable savings occur, from my experience, when the air conditioner is totally disabled. It is important to reconnect the clutch ocassionally, to prevent the fluids from coagulating.

Ok,I saw that on Myth Busters. The engine did consume more fuel with the air conditioning on as the compressor did put a load on the engine. But wind turbulence with the windows open or closed did not affect fuel milege. Hence, you should turn off the air conditioning for better fuel milege but roll the windows down and tell the wife to stick her head out the window and see if you can hear her screamimg at you. Wait, that's another topic air flow vs souund

Mythbusters also revisited that one in a later show because they received so many negative replies. In the original show they drove at about 45mph. At that speed the mechanical losses from the air conditioner were larger than the aerodynamic losses. When the speed was increased to a normal highway speed, aerodynamic losses with the windows open became larger than the mechanical losses and they reversed thier decision. Even with older autos the most efficient speed was usually quoted as 45mph if memory serves (which nowadays is problematical)

Is there any norm that most efficient speed with autos as 45 mph? If this pertains to normal highway speed,is there similar norm for Desert driving? If so,on what basis the efficient speed is arrived at? Fuel economics? Can anyone clarify?

Get a divorce and a hybrid then you don't have to worry about this. Remember in 30 years time, our grandchildren will laugh at the fact that we used fossil fuels to get around the place.

Spoken like a true liberal.

Yeah, and in 1940 everyone thought we would all have flying cars by the 1980's. Instead we had lines at the gas stations.

Have you not heard the news? There is supposed to be a new oil field off the coast of Russia that will make it the new "Saudi Arabia"!

If that is the case, look for long-term oil prices to drop in a few years time as cash poor Russia floods the market with cheap oil and the Arabs and Latin Americans have to drop their prices to compete. Don't sell that monster SUV or luxo gas-guzzler. Put it in mothballs and bring it back out in a few years as a "classic"!

Of course by then we'll be fighting Global Warming, and it may be unpatriotic to be producing large quantities of "greenhouse gases", no matter how low the cost of gasoline. Technology will be focused on ways to convert CO2 to solid carbon and oxygen at the source, the tailpipe. Then, whenever we fill up with cheap gasoline, we can drop our carbon cake into a disposal unit, which collects them for shipment to a processor who compresses them and ships them to the third world to burn as fuel!

One equivalent developed for auto fuel is Ethonol. Can someone clarify why this fuel has not yet been in wide use ?

Cause the oil companies would lose money.

I have heard it said that it takes more energy to produce a gallon of ethanol than what is available in the ethanol when you burn it. This is because firmentation can only produce a 15% or so concentration of ethanol and the fuel required to distill that to concentration that can be used for a fuel is more than the fuel that is produced. Can anyone confirm this?

No - there is a net energy gain in producing ethanol as a fuel. The problem is that there is not enough arable land in the world to produce the fuel we use AND the food we need.

As ever, the problem is first the internal combustion engine - it just isn't feasible for everyone to have one - and second the burning of any fossil fuels at all: there is plenty of the stuff, but it all locks up carbon that was removed from the atmosphere to give us the climate we have now...

Etholol has a low return on the energy side. From corn to fuel the return is 1%.

Oil has a greater return from the well to the gas station... nearly 10%.

If you want to make it and stay cool, then turn off the A/C which makes the engine work harder and consume gasoline, close the windows to reduce drag and turn on the cooled, Climate Controlled Seat (CCS), which are offered in most cars and trucks. The CCS system runs off the battery and doesn't reduce the MPG.

Agree with the first part of this answer, but if the CCS is running off the battery then what's charging that? The alternator is going to put more load on the engine to supply the seat! (We don't have those here in the UK anyway) paul

WAIT till night fall car runs better on cold air switch to methane you can get this straight from the wife

If my HS geography is correct, you don't have deserts either.:-)

Jesus has nothing to do with it, and would not help anyway.

You can't use energy from the battery without recharging via the alternator. The alternator is driven by the engine, and as soon as the alternator is switched to deliver energy to the battery the MPG is reduced.

At least GlobalSpec had the class to use the word "spouse" but since you all have turned this into a battle of the sexes (since the technical answer to this week's challenge is glaringly obvious). Here's the solution: Go ahead and crank the AC up full blast and open up any dang window or vent that you please! The said "nagging wife" happens to be a engineer herself and has calculated the statistical basis for which her husband will fail to plan accordingly for road trips. She has packed an extra 10 gallons of gas!

Pretty funny, but I must say If you are in the desert 100 miles away from the nearest gas station... why are you in a climate controlled car? Wouldn't a mercedes benz flat nose semi be more fitting? what else it made to take that kind of torture. I mean lets put your nice 1500 in that heat and abrasive sand and you can hold your breath for a failure! and jetissoning thigs is risky... "Lets dump the water we can get there in a day" Crash crack glug glug (waters gone) (engine sputters then dies) "We need to make decisions as a family" "not now edna!"

You are right: The wife had ten gallons of gas, but the car was a 1961 Ford "T" Bird and it gets just under ten miles to the gallon no matter speed or load conditions of car, or for that matter how the Ac or windows are set. The car had to travel 120 miles after tank went dry and the ten extra gallons was added to the tank. Now the guy has to deal with the frustration of the wife because she outsmarted the husband, but her outsmarting was deficient, so she is really irrated with him, and twice as much because she did not show him up, and he refused to carry the can to the next gas station, and the start walk time was noon in the High Desert, and the direction was up a mountain. If he lives he is lucky. Fris.

Most of the replies refer to on road driving. In actual desert conditions, the road if there is one, will be rough. The losses will due to ground conditions rather than air resistance. When forced to use lower gears to cross the many obstacles the airconditioning compressor will absorb plenty of power. Don't expect to get into overdrive at all and if you do select it expect to use even more fuel. At the off road speeds open windows and vents will make minimal difference to drag and fresh air is necessary to think and react rationally. The hot dry air is bearable in a moving vehicle, but when you get closer to the coast the rise in humidity will make both of you very uncomfortable because the previously dry sweat buildup liquifies and runs. Believe me it isn't at all comfortable. If you want to chuck anything away, it isn't the water. If you run out of fuel, you don't repeat don't, leave the vehicle, or you die. Every year people die in Australia from exactly this scenario.

Well, in the USA, if you're in a desert and the next gas station is 100 miles away, just turn the car around and go to the previous gas station which must be closer. I defy anyone to find a stretch of road, paved or otherwise, in the USA that is not within 100 miles of a gas station. To rephrase that, there are no gas stations in the continental USA more than 200 miles distant from another station. I live in the Chihuahuan desert, and large towns are only 75 miles apart. Small towns with gas stations are closer together than that.

Gas mileage would be irrelevent as I would never survive the trip after turning off the air conditioner on my wife. She's lke an Ice Cream cake- sweet but must remain refrigerated!

I've heard that running with the A/C off, and the car windows open, saves fuel only if your driving under 40 mph. Faster than that, the air resistance becomes a major factory, and maintaining the aerodynamics of the car, with windows closed, and A/C on, is most efficient. Info From Naked Scientist 8-06. Steve

In my experience driving with the windows down vs. A/C on, you will al ways get better gas mileages with the A/C off? I do a lot of interstate driving the posted speed limit is 75 MPH and I often drive 80 MPH. If I have to drive 40 MPH or less to make driving with the windows down fuel efficient then why do I see an increase in fuel mileage when I turn off the A/C? This statement is likely put out by someone who wants people to buy more fuel. IF Naked Scientist says drive with the A/C on they ether have wrong data or are biased in there views. I have kept track of my fuel mileage on all of the vehicles that I drive and it is my experience that turning off the A/C saves fuel. Rolling down the windows does not use more gas then the A/C. I own three vehicles and they all get better gas mileage with the A/C off; they are 98, 99, and 2000 year models. If you do not think I am correct then do your own test. Test: When you fill up with gas record the amount of gas and the amount of miles since the last fueling (you can use your trip odometer instead of the total miles). Now take the miles driven divided by the gallons of fuel put in and you get your mileage. Keep a record for each time you fill up your car. To know your difference in mileage for A/C vs. windows down set a time frame for observation (first week of July no A/C windows down, second week of July A/C on and windows up). To make the test fare you have to keep track of your mileage at all times. Also you can't use highway vs. city driving so the best is to combine both highway and city by lengthening your time frame (I always check my fuel mileage when I fuel up; so I get both highway and city combined thus the only difference there is is A/C vs. windows down.

I am a test engineer for diesel vehicles and I, myself was astonished by the AC effect on fuel consumption. During idle, the consumption was about 4-5 mm^3/stroke when engine hot. But, as I switched on the AC, it became as much as 12 mm^3/st. This means the AC spending more than the engine would otherwise spend on idling.

In order to know if this is a good idea, we first must get some information about the situation. The question says that you are driving in the desert and that the nearest gas station is 100 miles away. This tells us that whoever is driving is driving on a marked road. Now knowing that this is a marked road we can assume that the condition of the road is in good condition. If you are driving on a 100 mile stretch of road with your A/C on then you are most likely in a country that has good roads. Now weather to drive with the A/C on or off? By turning The A/C off on any vehicle equipped with one you will experience an increase in fuel efficiency. This was proven by the myth busters and by my own experience with my Chevy S-10 pick-up (1998 model), Mercury Sable (1999 model), and a Chrysler Town and Country mini van (2000 model). On the mini van I experienced a two MPG increase by turning off the A/C. Now let's assume an average gas mileage of 19 MPG. It will take 5.3 Gal. of gas to make it to the gas station. Now if we turn of the A/C we will get an extra 2 MPG now it only takes 4.8 Gal. to get there, we saved a half (.5) Gal. of fuel. Now the question says that the driver is not sure if he/she has enough gas to get there. So far we saved a half gallon of gas but we still are not sure so lets do the next best thing SLOW down. This will save some 15 to 45% of your fuel consumption (according to the EPA web site. Going from 55 to 65 MPH increases fuel consumption by 15% and increasing to 75 MPH increases the consumption by another 25%). Now assume a 22.5% decrease in consumption at 21 MPG this will save some 4.7 Gal. of gas. This would be 25.7 MPG. Now originally it was going to take 5.3 Gal. to get to the gas station. So we turned of the A/C and saved .5 Gal. now we slowed down and the total gallons to get there are 3.9 you saved 1.4 from the original 5.3 gallons. Roll down or keep windows up? If you turn off the A/C you will have to roll the windows down to keep from killing your self of excessive heat. No need to worry about your spouse or any passengers causing you harm. Now the issue of the windows down verses fuel consumption. The amount of fuel consumed from the wind resistance will not make enough of a difference (in my own observation there is not enough fuel saved by decreasing wind resistance to warrant keeping the windows up). Also the fact that you will get to hot with out the A/C on means the windows will have to be down. If any of the passengers do not like the A/C off can walk for gas (I keep a gas can in my car for jest such occasions). Now you are sure that you have enough gas to get there by turning off the A/C and slowing down (45 to 55 seams to be a good choice of speed as this range of speed has the most pros vs. cons). The windows have to be down so as to not die of heat exhaustion. By turning off the A/C and slowing down you have done the most fuel saving options for this scenario and any other things most likely will not help, they will jest be done with comfort in mind.

I'm sorry folks, but I was over exposed to trick questions in my youth.

Is WHAT a good idea?
Driving with not enough fuel - not good.
Spouse getting very upset - not good.
Turning off AC and closing windows - good. Running the air conditioner while the windows are open is contraindicated when wishing to conserve fuel.

Given the perverse nature of trick questions, I vote "yes" it is a good idea.

I'm glad that came to a quick conclusion. I didn't have to go into problems with our lack of data. For example:
Is it hot outside? It could be night or winter.

Driving any vehicle with all the windows closed and no outside air getting in is not a good idea even if the temperature is below zero. With one person in the car the carbon dioxide builds up at a surprising rate and can seriously affect your judgment. As to the A/C many modern cars have a throttle offset that opens the throttle further whenever you turn the A/C on and hence even if the thing is disconnected it will use more fuel because the throttle is open further. Finally don't forget the tires, increasing the pressure in the tires reduces the distortion as they roll and hence reduces the drag. So in summary, windows down, A/C off, tires pumped up and keep the speed down.

You sort of missed the point that it's a trick question and that the MPG is irrelevant. Otherwise I'd wait for a tail wind and then follow a semi. Regarding the CO2 buildup, can you imagine the death toll and lawsuits if cars were actually sealed when you closed the windows and vents? Even a VW bug doesn't float forever.

What type of desert? "Deserts cover about one fifth of the Earth's land surface. Most Hot and Dry Deserts are near the Tropic of Cancer or the Tropic of Capricorn. Cold Deserts are near the Arctic part of the world." - ok, either way, to improve the gas milage of the vehicle, reduce the speed to be between 45-60 mph, Hopefully the tires are propperly inflated, use cruse control, and/or overdrive gears, and keep windows closed. Turn of the A/C. Watch the temp gauge and if it starts to overheat, roll down the windows and run the heater and the vents this will take some of the heat from the engine, stop the car if necessary. If the wife does not like the temperature, tell her that you need to lighten the load, and so anything that can be left behind should be discarded. This may include her ginormous purse, clothes, and if she complains more, this may include her.

No, it is not a fesable idea to close up the cab. Most autos from the mid 70's have a built in environmental air circulating system, otherwise refered to as the climate control system. Yes, shut down the a/c and switch to the vent, or economizer mode. it allows for fresh air to be circulated with less strain on the engine. Then you have time to explain to your wife the laws of physics and economics in the way of load factor to the co-efficiency of your auto.

Given that this is an engineering site, I am somewhat concerned about the lack of physics knowledge.

Wind resistance rises with the square of speed. Hp requirement, therefore, rises with about the cube. For instance, if you double the speed, then drag quadruples. Let's say it goes from 10 lb to 40. Then, the engine must then produce four times the tractive effort, twice as quickly, meaning eight times the horsepower. This explains, in part, why vehicles with 50cc displacement can go 80kmh but ones with 6 liter engines (120 times as big)can only go 2 - 3 times as fast.

Fuel consumption is more or less directly related to hp, with .5 lb of gasoline used per hp per hour. The differences in efficiency between one rpm and another are relatively slight and nothing remotely close to the cubing effect of speed.

So... it is always more efficient to go slower. Although most automotive engines are "tuned" by the engineers to do well at 55 (because, in the US that's the test speed) that speed is anything but the most fuel-efficient speed. Even with an automatic transmission, you are far better off operating even at speeds well below the torque converter lock up speed (which is usally 75 kmh or so) because the gain in efficiency (huge) from doing so far outweighs the slight gain (a couple percent) from lockup.

To demonstrate this effect I once drove a 1986 Lincoln Continental gas guzzler at a constant 20 mph on a flat road. Got about 80 mpg, per the quite accurate trip computer.

There is no one speed at which aero drag predominates, nor is ther any one speed at which the aero disturbance from opening windows balances the extra load on the engine. It depends on the size of the vehicle, its degree of streamlining, etc., etc. Mythbusters, notorious for bad science, had to retract their claim that you are better off opening the windows (and turning AC off), partly because they picked just one of many vehicle types (one in which you'd expect to see little influence on aero drag, because the vehicle was basically a box on wheels) and partly because they tested at subhighway speeds.

So if you're worried about fuel usage, go as slow as you can tolerate. Roll up the windows. Turn the ac off. Turn off any lights or other accesories that use eletricity. Pump up the tires till they are like rocks. Jettison any excess weight. Drive like they do in the Shell Eco marathon, where people get thousands of MPG.

Of course, for this particular puzzle, first ask "What's the temperature?" At night in the desert, it can be pretty chilly.

You could also do a coast down technique. Run up the speed to 60-70ish and turn the engine off. Let the car coast down to nearly a stop before restarting and regaining speed to the next coast down. Also, fold in your mirrors for less wind resistance...

The coast down technique is similar to a switching converter that has a "burst" mode which fires multiple switching cycles and then coasts. If you look in most any buck converter datasheet, you will notice that the highest efficiency is not at light loads when you are deeply in burst mode. Generally, the highest efficiency is when you are at mid loads where you are switching continuously and near the boarder of continuous and discontinuous conduction. For a car, that would probably be a fairly slow, CONSTANT speed. Okay, there is one difference in the coast down technique, and that is turning off the engine while coasting. (You will note that in burst mode, a switching converter turns off as many blocks as it can, but it cannot turn off everything so there remains some idle "quiescent" current.) However, there is also some energy loss in turning back on the car's engine -- either with the starter or by popping the clutch, which will remove some kinetic energy from the car. For this reason, the coast down technique will never beat operating at a slow, constant speed where the efficiency is optimized. Furthermore, in the coast down technique, the throttle opening when accelerating from a slow speed to a high speed would need to be optimized for efficiency, as well, which may not be obvious. In a switching converter in burst mode, the switching cycles during the burst are designed to occur at the most efficient inductor current level (near continuous/discontinuous conduction), rather than at the current limit of the converter (full throttle), for example. The reason is that the average efficiency at light load can never exceed the efficiency during the burst pulse-group because the idle time will always throw away a little extra energy (i.e. the quiescent current or the energy used to start the engine back up). Another poster also stated that slower is always better, but I don't know of any regulator that gets anything but 0% efficiency at no-load, which means the mpg will go to zero as you approach zero miles/hr. Again, this is due to quiescent losses (just keeping the motor on, even with no load). There is always an optimum speed for efficiency, and it is always where the quiescent and switching losses (motor running and electrical system losses) are exactly equal to the resistive losses (rolling resistance, drive-train resistance, wind resistance, etc.). That is the highest point in the efficiency curve of the converter (and, if done correctly, it is also the operating point during burst mode pulse groups when at lighter loads).

Since it is not a purely scientific question one is limited to solutions that take into account the human factor. 1. You are not sure if you have enough fuel so you are also not sure you don't. 2. You are in the desert so you need the AC. 3. Going slower reduces the effects of drag and you may save as much there as you would have by shutting off the AC plus you will get extra points from the spouse for finally driving like you have some sense. 4. Finally if you fall a little short your spouse will not have to endure the heat too long and you won't have made him/her sweat for a hundred miles so you have some time before he/she gets upset now. Now you have a proper use for your cell phone and road service good luck you can be sure you will need it.

Nobody mentioned pumping up the tyres, rolling resistance is lowered. and could give 5% or more better mpg. The air conditioner should be disconected, but half open windows should not pose too much problem. 'Mexican Overdrive' on any downhill stretches, and keep the speed to 50 mph or less.