Actually, there is one more way to lower the amount of energy needed to accelerate to your cruising speed…lower your weight. It’s just one more reason I need to go on that diet…
I have lost 40 pounds since last winter, not so much to get better gas mileage but because being overweight took a lot of the fun out of last winter’s ski trip. The only thing that kept me off the hard blues and easy blacks was my lack of stamina.
The biggest problem with this test is that slow acceleration is not the only thing conservative drivers do, they also obey speed limits, try to time lights, and refuse to accelerate towards red lights.
I personally drive very conservatively, only instead of accelerating like the world’s most overloaded 18 wheeler, I accelerate at a average to higher than average rate to a lower cruising speed achieving nearly the same average speed as the conservatively driving traffic. 43-45 mpg out of a car EPA rated at 29/36 mpg, the results speak for themselves.
Okay,okay,okay…
but no matter. As you so eloquently stated in the early posts, it takes the same amount of energy to get a Caddy to 60 mph slowly as it does to get it to 60 mph quickly. And as I described, the real difference is that when doing it quickly you change the amount of energy used by the engine to run itself. That, I believe, is why it takes more gas to accelerate quickly to 60 mph than it does to accelerate slowly to 60 mph. Even if the total distance traveled remains the same, the amount of gas used is still greater. It’s the losses internal to the engine that change.
I think the mistake we’re all making is in looking at the masses and resistances of the car. We need to look at teh masses and resistances of the engine. The car still ends up at the same speed, but the engine operating parameters change dramatically.
“I can tell you what I have observed in my Prius: if I accelerate slowly, the engine uses 100% batttery; if more quickly, the gas kicks in. Does that tell anyone anything?”
The obvious, thanks ! :=)
I can’t help it, I have to reply to the OP’s following question. "The real question is, do you use more fuel laboring the engine for a shorter time but using more torque or do you use more by laboring the engine for a longer period of time but with less torque? "
Answer: While taking the longer time to reach the desired speed, you won’t be laboring the engine.
Billicard the following statement of your’s is false, the fact is that slower acceleration allows the transmission to upshift earlier. “My personal observations indicate that accelerating too slowly causes the transmission to linger in lower gears longer”
Most of the hypermiling fanatics on the Cleanmpg forum say that the Prius gets the best fuel economy when you minimize the use of the battery. The energy provided by the battery is not free, it’s stored energy provided by the engine. You are better off driving so you don’t need regenerative braking than using that feature, although it’s better than throwing all the energy away with friction brakes.
If the motor is 90% efficient as a motor, it is also 90% efficient as a generator so turning 90 percent of the energy into electricity and then turning 90% of the electricity back into kinetic energy means you only get back 81% of the original energy, but wait, there’s more, the battery is not 100% efficient either, if the battery is also 90% efficient then the energy return is 90 percent of 81 percent or 73%.
About any other car besides the Prius? No.
When you increase acceleration, you simultaneously decrease distance to reach a given speed. Thus, the ideal amount of work is the same for the same final speed. That is what you are missing.
You are all over looking the most obvious culprit, HEAT! I have tested this and can tell you that accelerating slowly uses less gas. When you accelerate quickly you produce much more heat and every good mechanic knows that a hot engine burns more fuel than a cool one. Accelerating slowly when possible make the car run cooler and more efficiently even though it still has to put out the same amount of power. Not to mention that accelerating quickly puts more stress on every part of the car, even the frame or even the antenna due to the momentum of the drive wheels. The wheels want to move forward and the frame wants to stay in place.
So, you’re saying a cold engine is more efficient than one that’s thoroughly warmed up?
Confusing temperature with heat is just as bad as confusing energy with power.
Temperature is measured in degrees Celsius or Fahrenheit. There are also two absolute temperature scales, Kelvin based on Celsius degrees and Rankine based on Fahrenheit degrees.
Heat, on the other hand, is measured in British Thermal Units (BTU) or calories.
I think there is a factor missing, total distance traveled.
http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html
The biggest problem in the test that you linked to was that in the comparison of aggressive verses gentle driving, the aggressive driver not only accelerated faster but also accelerated to a higher top speed. This is changing two variables at once.
Anyone with any scientific training whatsoever will tell you that when conducting experiments, changing only one variable at a time is critically important.
No one has ever argued that aggressive driving is good for gas mileage. But, did the rate of acceleration or the higher speed accelerated to cause the increase in fuel used? It is unknowable in this test.
Shucks, they could also have inflated the slower driver’s car tires with nitrogen instead of air and concluded that nitrogen inflated tires yield better gas mileage.
"The more power you demand from a given engine, the more HEAT it generates and wastes."
The heat won’t be wasted in very cold weather, HU?
CR does not stipulate what speed you accelerate to. They just say;
“Drive smoothly”
Avoid hard acceleration and braking whenever possible. In our tests, frequent bursts of acceleration and braking reduced the Camry’s mileage by 2 to 3 mpg. Once up to speed on the highway, maintain a steady pace in top gear. Smooth acceleration, cornering, and braking also extend the life of the engine, transmission, brakes, and tires."
There are enough engineers in their stable to provide a footnote to assertion that brisk acceleration w/o harder braking is OK; they don’t.
the same mountainbike, You are absolutely correct.
“When you slow down and stop, you really lose fuel economy.” It is not the slowing down and stopping that uses fuel, it is the accelerating back up to speed.
…every good mechanic knows that a hot engine burns more fuel than a cool one.
I think you have that backwards. Every good mechanic knows a cold engine uses more fuel than one at operating temperature. If your mechanic ever contradicts that statement, don’t let him/her touch your car.