Dr. Coggan effectively made his case that the small speed variations of the bicycle that occur because of pedal torque variations result in extremely small losses. I had expected them to be larger because of my understanding that automobile cruise control can increase fuel economy about 10% over manual speed control. Yet, in another thread, the case was made that the highest fuel economies are obtained when speed variation is increased, running the engine at maximum efficiency during a power phase then turning the engine off and coasting down. Such that it would appear that speed variation is not a consideration even in the automobile.
Why is it that cruise control offers substantial fuel economy savings?
Speed variation without cruise control is greater than the speed variation during pedalling
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Speed variation without cruise control is greater than the speed variation during pedalling
Yes, but speed variation when trying to maximize fuel efficiency is greater than speed variation during manual control of the automobile. That is the point.
Cruise control offers savings over standard driving (on flat roads) due to lower speed variance… that is the point…
Whether it does or doesn’t offer savings over some other driving style is a different point…
I think you are actually starting with a relatively faulty assumption. You say that cruise control offers a 10% fuel savings. Where are you getting this number? You can’t just start with “a” number.
I’ve found that manual adjustment with a manual transmission offers the greatest fuel economy.
However, if you had a car where cruise control did offer an improvement in fuel economy, it would likely be because all cars have central computers, which could balance power requirements and fuel economy, effectively metering throttle based on a simple feedback loop.
Also, some cars turn off a number of cylinders when in cruise control mode, so that substantially alters the equation.
Bottom line, considering cruise-control in a CPU controlled automobile is really a control systems equation, and it’s probably not all that relevant to the discussion you really want to have.
I think you are actually starting with a relatively faulty assumption. You say that cruise control offers a 10% fuel savings. Where are you getting this number? You can’t just start with “a” number.
I’ve found that manual adjustment with a manual transmission offers the greatest fuel economy.
However, if you had a car where cruise control did offer an improvement in fuel economy, it would likely be because all cars have central computers, which could balance power requirements and fuel economy, effectively metering throttle based on a simple feedback loop.
Also, some cars turn off a number of cylinders when in cruise control mode, so that substantially alters the equation.
Bottom line, considering cruise-control in a CPU controlled automobile is really a control systems equation, and it’s probably not all that relevant to the discussion you really want to have.
That was my understanding. It may be completely faulty. And, I am not talking about turning off cylinders, that is a relatively new innovation to save fuel. If so, I am sure someone here actually knows. Even if it resulted in zero fuel savings I would not get rid of it for the right leg cramping it prevents on long drives. 
I am simply trying to get a grasp on what the significant differences are (or are not) between these two problems.
Cruise control “in general” will offer better fuel economy because people don’t pay attention when driving and will speed up and slow down significantly on a “constant speed” highway drive. But, a skilled driver can always beat cruise control for MPG, because the driver can see changes in terrain and “preempt” the worst fuel economy situations. They actually have contests to see who can do the best job. Google ultramilers and honda insight. some people are able to get >100mpg when they take control of the regenerative braking.
There are massive differences. One is a problem that is a feedback control problem that takes into account a number of pre-defined variables. For example, you’d end up with a different control equation if you set out to optimize fuel economy vs. minimizing speed variance. Speed variance is, largely, a comfort issue. You don’t want to be speeding up and slowing down for comfort reasons. There’s also the issue of how long a period you control over. Do you speed up if the car slows down for more than 1sec? 5sec? Etc. And of course, you have to consider optimal horsepower and torque curves. All things you can map and program. I.e., going uphill, you may actually not be able to maintain a given speed at the most optimal fuel economy due to horsepower limitations. In that case, you’d be better off, from an economy standpoint, to go slower and keep the engine at a lower power. But if making a certain average speed is your goal, then you will have to punch the engine. This is generally why cruise control is more finicky on underpowered cars and is often less practical for them.
Simply put, you are wasting your time trying to compare the two. At least until cyclists are replaced by four-stroke motors run by computers.
Cruise control “in general” will offer better fuel economy because people don’t pay attention when driving and will speed up and slow down significantly on a “constant speed” highway drive. But, a skilled driver can always beat cruise control for MPG, because the driver can see changes in terrain and “preempt” the worst fuel economy situations. They actually have contests to see who can do the best job. Google ultramilers and honda insight. some people are able to get >100mpg when they take control of the regenerative braking.
If you will search this site you will see that some can get over 2000 mpg when they “maximize” the speed variation. What is the key to maximizing efficiency? Engine efficiency, speed variation, or something else. That is the question I am asking. I do not know the answer.
Please take this to the Lavendar Room, folks - it doesn’t belong here.
Please take this to the Lavendar Room, folks - it doesn’t belong here.
Sure it does. We are talking what is the key to energy efficiency. I do not know the answer but it could affect how one races. Why don’t you give us your answer to this conundrum?
Please take this to the Lavendar Room, folks - it doesn’t belong here.
Sure it does. We are talking what is the key to energy efficiency.
Pardon my French, but WTF does the effect of cruise control in an automobile have to do with cycling effeciency??
Why don’t you give us your answer to this conundrum?
Been there, done that, proved that you were wrong - NEXT!
Please take this to the Lavendar Room, folks - it doesn’t belong here.
Sure it does. We are talking what is the key to energy efficiency.
Pardon my French, but WTF does the effect of cruise control in an automobile have to do with cycling effeciency??
Well, it was my understanding of the relationship of cruise control to automobile fuel efficiency that cuased me to mistakenly think that the speed variations of the bicycle from an uneven pedaal stroke might have a similar effect. If both of these were true it would be a simple problem of “magnitude” in explaining the difference. However, it is the problem of the “maximum” fuel efficient car that seemingly maximized the speed variation that makes me think there is a problem in this “simple” understanding. There is a conundrum here, it seems to me, that has yet to be explained satisfactorily. It seems you don’t even understand the conundrum, let alone able to explain it.
Why don’t you give us your answer to this conundrum?
Been there, done that, proved that you were wrong - NEXT!
No, as I said above, it seems you don’t even understand the conundrum.
It depends on your metric. And on the course. When you look at the super efficiency races, they are races. Time IS a factor.
So efficiency is not just mpg. Rather it’s a combination of mpg & mph.
The same is true with cycling. The RAAM rider’s definition of efficiency would clearly be different than a pursuit rider’s definition.
It depends on your metric. And on the course. When you look at the super efficiency races, they are races. Time IS a factor.
So efficiency is not just mpg. Rather it’s a combination of mpg & mph.
The same is true with cycling. The RAAM rider’s definition of efficiency would clearly be different than a pursuit rider’s definition.
What you are saying is gibberish to me. I assume those trying to maximize efficiency at expected to do so at some specified average speed. I am trying to reconcile why, in one situation, constant speed is seemingly more efficient and, in another, it is not. In view of these differing outcomes it is not clear to me what is the most important factor in these determinations.
Gibberish? You are the one who decided to assume something without telling anyone about it…
Also, what situation are you talking about where constant speed is the most efficient? Where was that proved? And for what example?
Gibberish? You are the one who decided to assume something without telling anyone about it…
Also, what situation are you talking about where constant speed is the most efficient? Where was that proved? And for what example?
http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html
Did you notice the last sentence: “One thing that’s important to note: if you are in a mountainous area you should turn off cruise. It will try to keep you up to the speed you’ve set and will use a lot of extra gas downshifting to lower gears to accomplish this.”
Same thing is true with headwinds. And they admit it smoothes out “nervous” acceleration. And they compare cruise control to +/-5mph swings. Or constant speed vs. speed variance of +/-7%. I wouldn’t exactly call that relevant to the discussion at hand.
Did you notice the last sentence: “One thing that’s important to note: if you are in a mountainous area you should turn off cruise. It will try to keep you up to the speed you’ve set and will use a lot of extra gas downshifting to lower gears to accomplish this.”
Same thing is true with headwinds. And they admit it smoothes out “nervous” acceleration. And they compare cruise control to +/-5mph swings. Or constant speed vs. speed variance of +/-7%. I wouldn’t exactly call that relevant to the discussion at hand.
Nervous acceleration is the same acceleration we were talking about with the cyclist as far as I am concerned. In the hills they seem to be suggesting that the “problem” is the tendency to “downshift” on the downhill portions. The reason for this is the uphill downshifts to a less efficient RPM and, of course, the downhill would also tend downshifting to slow the car to the set speed, which would also tend to increase gas consumption. This is why using cruise control can hurt gas consumption on hilly courses. But, that wasn’t the question. We were examining flat courses and “constant” speeds. It is simply “well accepted” that cruise control, or a smoother speed" improves fuel economy on flat straight courses about 10% over what one can do manually, despite the fact that one may try to drive “smoothly”.
Except there is no study that supports your claim that cruise control can save 10% on flat courses. In fact, the “studies” our there show the exact opposite. If you do some googling, as suggested by someone in this thread, for ultra-distance economy with a Honda Insight, you’ll see that manual speed control ALWAYS yields the best results for a driver with any experience. You cannot, in any way, infer that it is “well accepted” that cruise control offers the 10% savings you are claiming. That isn’t at all the conclusion of the edmunds “study.” Basically, you are pulling a number out of your ass and then claiming it is a “well accepted fact,” when nothing could be further from the truth. Nothing new there…