Gas has broken the $4.00 per gallon mark around here, and with that comes a plethora of advice on how to increase fuel economy. The suggestions range from over inflating tires, to pumping gas in the morning when it is cooler. I read a good article over the weekend that debunks most of those suggestions as ineffective.
The most common suggestions include:
- Change the air filter. While this is an important maintenance item, most newer car’s on-board computers can compensate for dirty air filters. If you have a dirty air filter, it may effect your overall performance, but will likely have little effect on the gas mileage (consumer reports)
- Over inflate the tires. Bad idea. This may save a little bit of gas by reducing friction between the tire and the road, however, friction is an important part of keeping your car on the road instead of in a ditch, especially in wet or icy conditions. Tires should always be properly inflated.
- Fuel additives. (increase octane, etc etc) Snake oil.
- Crank case additives. (decrease friction, etc etc) More snake oil.
- Pump gas early in the day when the fluid is cooler. Most motor fuel is stored in underground storage tanks, thus the temperature does not fluctuate to the same degree as atmospheric temperatures do, thus time of day gas pumping will have little difference in the fuel density.
- Turn the AC off and drive with the windows open -or- turn the AC on and drive with the windows closed. Makes no difference in most cars, especially at highway speeds. Now, if you can turn the AC off and drive with the windows closed, you would save some gas.
- Use premium grade fuels. Makes little or no difference. Unless your car specifically call for premium grade gas, the regular unleaded will work just fine.
What happens when you slow down
Vehicles have three main forces to overcome: Gravity, Friction and Wind Resistance. Gravity and friction in a well maintained car are constants, there is little that can be done about either unless you know of someway of always driving down hill. Wind Resistance, on the other hand, can be reduced.
A mini physics lesson is required. The engine in a car uses energy produced from fuel to provide power. The power is used to overcome all of the forces that are acting on the vehicle and cause it to move. Basic Newtonian Physics. The power of wind is defined by the following equation:
P=½ρ AV3
Where:
P is the power in watts
ρ (pronounced “rho”) is the air density (1.21kg/m3)
A is the area in square meters
V is the velocity in meters per second
Notice the V is cubed. That means for every increment of increase speed, the wind resistance increases exponentially by a factor of three. Here is an example:
Lets say I am driving an average mid sized car. The total frontal area adds up to 1 square meter (to make things easy). I am driving down the highway at the speed limit, 55 MPH when I notice that I am late for an important meeting. I then break the law and speed up to 65 MPH to shave a few minutes off of the drive time. First, we need to convert this stuff to metric. Sorry Todd, it just works better for me.
55 MPH=89 KPH=24.7 Meters/Second -and- 65 MPH=105 KPH=29.2 M/s
At 55 MPH I am using:
P=0.605kg/m3 x 1M2 x 24.7M/S3=9116.8 Watts to overcome wind resistance.
Convert that back to Horsepower 9116.8 Watts÷746 Watts per Horsepower=12.2 HP
At 65 MPH I am using:
P=0.605kg/m3 x 1M2 x 29.2M/S3=15062.7 Watts to overcome wind resistance.
Convert that back to Horsepower 15062.7 Watts ÷ 746 Watts per HP = 20.2 HP
Carry that one further, 70 MPH I am using:
P=0.605kg/m3 x 1M2 x 31.3M/S3=18551.9 Watts to overcome wind resistance.
Convert that back to Horsepower 18551.9 Watts ÷ 746 Watts per HP = 24.9 HP
Therefore, to increase speed from 55 MPH to 70 MPH requires double the power. And that is in a medium sized car, a large truck with a huge frontal surface area would be even worse.
Something to think about. I love physics.
I now return you to your regularly scheduled blogging.
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Paul,
When it comes to physics problems such as you described I have to agree with you that Metric is better. English units are the only way for me to deal with construction distances and forces (easy for me to relate to).
I love your example and I love that you break things down to the basic core of physics! Ahh……a true engineer! Sorry folks..but I too love physics and love it when people can take the basics and show real world examples.
One more thing to think about for your readers….and it relates directly to your example. Driving 70 mph and having an accident has the same result…..a MUCH worse collision and much worse transfer of energy. Seems there are many good reasons to slow down a bit.
You could always push your car which uses no fuel and gives you a workout, which a large percentage of this country needs more than lower gas prices……
OR
You could go 130 MPH and get to your destination faster, obviously using less fuel……
and if you couldn’t tell I am a sarcastic person.
What needs to be looked at here is how much gas are you really saving? I see the horsepower decrease, but how much fuel is used to create 1hp? Is it worth being late to a meeting or annoying every driver on the road during rush hour to save a few cents? Like all great physics problems there are always outside factors that skew the results, but A+ for the physics lesson, been awhile for me and I loved that stuff in high school.
You know, I started driving 65 or less a few weeks ago… and have realized SIGNIFICANT improvements in mileage. I have a Subaru wagon (4WD), and was getting 24 to 28 mpg… I’ve been getting 30 to 32 consistently for the past few weeks.
Slow starts, and coasting to a stop help a lot, too… just watch traffic, and keep a big enough buffer that you don’t have to use your brakes!
Ok, let me have a wack at this.
This past spring I actually conducted an experiment of sorts. I had a 700 mile trip from Alabama back to Indiana. I had heard that changing the air filter and slowing down made a difference. I can disprove the air filter trick works and prove that slowing speed works.
1997 Nissan Maxima, 215,000 miles, tank size = 15 gallons, and I am not sure of the weights. One difference between the trip down and the trip back. I left in an ice storm so I was running the heater. I returned in shorts, t-shirt and sandals so I was running the A/C.
Now the easy first. Clean air filter trip down at rough average speed of 75 mph = and average 25 mpg.
Trip back:
The plan, with a full tank of gas at the start point, I would drive at a speed of 65mph or less until the tank was empty then get a room for the night. The next day, i would fill up and drive the normal 75 mph other than for safety or in metro areas.
60mph = 34mpg with a 15 gallon tank this is a total miles of 510/tank.
15 gallons x $4.00 = $60/510 = $0.12 per mile
NEXT
75mph = 25mpg with a 15 gallon tank this is a total miles of 375/tank
15 gallons x $4.00 = $60/374 = $0.16
Total savings/loss $0.04/mile
Total trip 700 miles – 510 = 190 miles x $0.04 = savings/loss $7.60
Total trip driving time as driven = 11.03
Trip time at 75/mph = 9.33
Trip time saving/loss = 1.7
Hotel cost = $65.00
Get home safely? Priceless.
AND, it was a business trip so I was reimbursed.
Hotel cost = 0
reimbursement/mile = $0.55
You can do the math from there.
Just a little fun.
Fred
Todd, you are exactly right, the more energy into a crash, the worse the results will be
Joe, what Fred said. I’ll go one further, saving $0.04 per mile adds up to $480-600 per year (12-15K miles driven). That is not bad just for slowing down a little bit, just a little bit Joe.
Jen, You are also right, driving habits, such as jack rabbit starts and excessive braking can reduce mileage. Good point.
BTW, I hate using brakes. I do it as little as possible, but it drives my wife crazy sitting over there in the passenger seat stomping on the floor with her left foot and all.
Fred that was great, you completed Paul’s formula!
I’ll give it a shot and see. Right now at 70mph on a trip to Albany and back home I get 31mpg in the summer and 34 during the winter (A/C does make a difference versus heat with Windows always open in both scenarios).
I’ll try just one trip (I HATE driving slowly, if I could still afford tickets I would do more than 5mph over the speed limit) to go 65mph the whole way and average out my mpg’s.
I too use my breaks as little as possible, I down shift and let the car roll, but at the same time I do take off at a light like a jack rabbit leaving everyone behind….
Excellent post and comments so far. Really relevant to current times.
Joe,
Thanks for the vote of confidence but Paul is W A Y smarter than I. I could never compete with him on this stuff.
Paul,
Looking at your power equation, it is kind of close to the Lift or Drag Equation
L=Lift
ρ (pronounced «rho») is the air density (1.21kg/m3)
Cl = coefficient of lift (This is a product of airfoil shape and angle of attack)
V is the velocity in meters per second
L = ½ρ Cl V^2
This is important to me because the helicopter I fly the tail rotor speed is very critical. Its speed is 7.1 times the main rotor speed. If you droop the main rotor speed you droop the tail rotor speed by a factor of 7 squared, thereby severely reducing its lift capabilities. This results is the aircraft spinning in the opposite direction of the main rotor. Y I K E S!
Just for you Paul I know you like this math stuff. And I would have never figure this stuff out unless I was taught over and over again and tested on it every single year of my flying career. (I just took my checkrides so it is fresh in my mind).
Fred
Fred,
Both formulas are deal with mass flow rate, in your case rho factors much more importantly than in mine because your altitude changes. In my formula the V is cubed because we are talking about power, which is defined as energy use over time, which is another formula.
My guess is that they speed up the tail rotor with respects to the main rotor to give the aircraft more directional stability. Just a guess though, as I know very little about helicopters.
Most of my helicopter time was in CH-46’s and a few HH-65’s. I did manage to fly on one CH-53E and was somewhat taken aback by the hydraulic fluid running down the inside of the aircraft. The young Marine crew chief must have seen the look on my face, so he shouted in my ear something about “thats how we know it still has hydraulic fluid.” Comforting.
Paul,
I flew the CH-47D for a while in the Army. The CH-46 can losely be said to be a smaller version of the -47. Them there are some hydraulics and aerodynamics that are tough. But if you are a testosterone type of person the CH-47 or H-53 are the way to go.
I hear the H-65 is a sweet machine but underpowered.
I was thinking about this again while I was driving around below E for 20+ miles.
Does RPM effect the calculations? The higher the revolutions the faster the gas is burned for ignition. So 55 MPH at 6,000 RPM’s has to burn faster than 55 MPH at 2,000 RPM right? Same speed, same horsepower.
Let me know what you think.
Joe, Sort of. BMW did a study on how to best drive and conserve fuel. What they found in a standard car was to keep it in the highest gear as long as possible. That is generally why standards get better gas mileage than automatics.
Once you are in the highest gear, standard or automatic, then it is the wind resistance which most effects mileage.
Thats what I thought but I hate those high gears, they take too long to react to me stomping on the pedal. Cars were meant to be driven almost to redline and then quickly changing gears with the sound of the engine roaring. If they weren’t, then why do we make them able to reach such speeds? I love cars and love working on them.
As you can tell I don’t get good gas mileage unless I’m on the highway =P
Hey Fred, great idea to post that math, but unless I’m wrong (which I admit is possible) your calculation for savings/loss is wrong.
On a 700 mile trip the Total Savings/Loss should be: 700 x $0.04 = $28.
Drive slower and save $28 and get home 1.7hrs later.
Drive faster and lose $28, but get home 1.7hrs earlier.