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Constant speed car efficiency

  1. Oct 3, 2011 #1
    I've read (I think it was on slashdot) that using constant velocity cruise control on your car is much more energy efficient than letting your speed vary. To the point where they said you would want to control your speed to much less than 1 kph.

    What is the general consensus on this? I'm a little skeptical. If you're constantly fighting the effects of wind, friction, road topology, etc. what is so special about constant speed? I would think constant throttle may be more efficient. Or even a more complex throttle pattern. Maybe one that is inverse with the road resistance.

    I'd expect it's a throttle pattern, near the speed you'd like to travel, would keep you at peak engine efficiency, rather than constant velocity. I guess my question is, can you predict behind the driver's wheel what is the optimal throttle or is constant velocity the safest approach.

    Anyone have any comments?

    P.S. To add to the discussion and also partly my confusion. I also read somewhere (I think it was a BMW test) that your peak efficiency is at 60% of full throttle. So maybe an ideal driving pattern would include throttling at 60% for the longest time?
  2. jcsd
  3. Oct 3, 2011 #2
    To go up a hill you need more throttle and down a hill less throttle.
    With constant throttle, you would be slowing down when climbing a hill and then racing down the other side.
  4. Oct 3, 2011 #3

    Andrew Mason

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    The ideal would be to keep the engine running at constant base load where it is most efficient and to use an electric motor/generator run from/to a battery to handle the changes in load presented by uphill/downhill, acceleration, deceleration.

  5. Oct 3, 2011 #4
    Without having thought too much about it, I think the problem at hand is larger than the engine alone...sure, the engine has its optimal point of operation, but the 'machine' at large is the car...

    ...also, we are riding in it...I for one would hate to be jerked around!

    So, maybe, this constant speed is more of a convenience kind of thing than gas efficiency thing. If you do not have available a constant speed, you will never know how long is going to take to get there.

    On the flip side, maybe it requires less work to keep the car at constant inertia than having to accelerate it when it falls too much behind...and I am talking from the engine's efficiency point of view...
  6. Oct 4, 2011 #5


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    I believe that the engine works most efficiently under two conditions: RPM in some range (like 3000-4000), and the throttle in some range (like 25%-75%). You should just try to stay within this range driving on highest gear and without using brakes.

    Automatic cruise control tends to either reduce throttle to 0 on downhills or to reduce gear on uphills, so they are less effective than car controlled by an intelligent driver.

    The driver may start to accelerate before the hill - to gain enough kinetic energy to climb it without changing gears nor even without pressing throttle to max. If you speed up from 100km/h to 120km/h just pressing throttle from 50% to 75% on the last kilometer before the hill, it gives you energy to climb 20m at lower expense than maintaining constant speed. Driver may reduce throttle before downhill slope starts to prevent braking later. At lowland areas, where slopes are rarely higher than few tens of meters, such style of driving works fine.
  7. Oct 4, 2011 #6


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    Hrmm. I might have to disagree with you xts. I don't see how automatic cruise control is less effective than a person. It constantly monitors the speed of the vehicle and will be able to keep a steady speed/throttle much more effectively than any person can. Is a car more or less efficient (in general) at going up a hill if you shift to a lower gear instead of accelerating on a flat? Not to mention that accelerating before the hill can get you speeding tickets!!
  8. Oct 4, 2011 #7


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    Automatic cruise control maintains the speed in a short timescale, in effect it pushes throttle to max even on a short slope up, and reduces throttle to 0 (or shifts to neutral) on even short slope down. It also often reduces gear on uphills, and cars are less effective on low gear than on highest one.

    Back to example of 30m uphill: you need some additional energy to climb. CC provides this energy by maintaining the constant speed: reducing gear and pushing throttle close to max, while rotation out of optimal range.
    Human driver may accumulate part of that energy in much longer time (accelerating before hill), without leaving the efficient ranges of RPM and throttle, provide only part while climbing (still with throttle at 75% or so and top gear - thus RPM in effective range), and borrow rest of that energy - finishing the climbing at lower speed (let's say 80km/h) than average.

    The same extra energy is generated by human driver in a longer time (thus at small extra power - engine runs in its optimal ranges), while CC generates it in short time, stepping out of the optimal range.

    Yes, higher the gear - more effectively engine works (at reasonable RPM). If your car is able to monitor fuel consumption, make a simple test. While on a flat motorway, drive at the constant speed of 100km/h at 5th (or 6th if your car have 6 gears) which for most cars is about 3000 RPM (my Peugeot makes 100km/h at 2800 RPM 5th gear). Then switch to 3rd gear - RPM jumps to about 5000. Maintain the same speed for half a minute. Compare fuel consumption reported in both cases.
    I did such test once - for my car the difference is dramatic: on a flat asphalt road, no wind, at 5th gear it consumes about 5.5 l/100km at 100km/h, at 3rd gear (and still 100km/h) it consumes about 9 l/100km.
    Last edited: Oct 4, 2011
  9. Oct 4, 2011 #8
    Cruise control is designed to be used on relatively flat roads, highway speeds, and with relatively low traffic volume. In these conditions, I believe it is more effective than any regular driver would be in maintaining a constant velocity. On top of that, if anyone is using CC in hilly terrain, city driving, or in heavy traffic they are just asking for an accident to happen. Any hill short and steep enough to cause CC to full throttle of shift down, would be a hill where the average driver not on cruise control would most likely necessarily do the same thing.

    The difference, as you say though, is that the driver can see ahead when coming to the top of a hill and release the pressure on the accelerator before, wheras CC doesn't have the ability to think ahead and keeps the engine fast running all the way to the top. so you do lose some Litres/ km in that situation.
  10. Oct 4, 2011 #9


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    I'm still going to disagree with you xts.(About cruise control being less effective, not the fuel efficiency per gear) I seriously doubt the situations you are suggesting are what the article meant when it said to keep the speed the same. But unless someone finds the article then we can't know for sure.
  11. Oct 4, 2011 #10


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    Some cars have "adaptive" cruise control (Mercedes, Infinity), that allows the driver to set a follow distance from any car ahead. These adaptive cruise control systems will apply the brakes if needed if the car ahead slows, (not full brake, but about 1/3 to 1/2 g of braking decleration).

    Most cars are overpowered for cruising speeds, and if the only goal is maximum fuel milage, absent any other traffic, and on a flat road, depending on the average speed, it's better to moderately accelerate in quick bursts then coast for long periods in neutral. This wouldn't work in a real world situation where traffic flows at a nearly constant speed. I doubt this method would help with a hybrid car either.
  12. Oct 5, 2011 #11
    The SAE runs a competition for teams from University and high school from North America to design a car and to run the car on a track or course. The team finishing by using the least amount of fuel wins. Any team getting less than 2000mpg might as well not enter to win.

    One such competition is called the Super Mileage competition, where the reigning champion is from University Laval, Quebec City,Quebec, Canada. I would have given a link but the sites are a little mumbled are not current nor totally factual. University of Saskatchewan, Saskatoon, Sk, Canada won some tittles in row several years back. Here's wikepedia anyways:http://en.wikipedia.org/wiki/SAE_Supermileage_Competition.

    The burst of speed and coasting is a tactic used to achieve such fuel economy in these types of competition.
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