“Work on the waste part of the cycle by re-using energy, such as using the engine as a brake in a manual transmission car (downshifting instead of braking)”
I don’t get it.
Downshifting a manual transmission enables you to use the energy that went into getting you up to speed to slow the car without using the brakes, except for the last few feet. By dropping into a lower gear, the wheels are turning too fast for the engine, and will slow down, then you drop it into the next lower gear. Without downshifting, you are braking the wheels against the engine to slow it down instead. The waste comes in the form of heat from the brakes. A hybrid uses this same principle when it slows your car by driving a dynamo. The effort to drive the dynamo slows the car and stores energy in the form of electricity. The electricity is then used to drive an electric motor during the acceleration phase and then turns the drivetrain back over to your gasoline engine which has been idling this whole time. Electric motors are very effective at instantaneous torque, whereas gasoline engines need to spool up to peak torque, but are better at higher speeds.
Downshifting doesn’t do make a radical improvement in your mileage, but it will extend the life of your brakes by more than a hundred percent, so it is making use of energy that would otherwise be wasted in heat. The technique is very popular in Europe, because of the type of driving in mountains and curvy roads, which requires a lot of torque changes to maintain engine speed. Downshifting in the U.S. is not always common, but it should be, if only because it gives you much greater control of your car. The more gears you have, the better, and in recent years car companies have increased the number to six forward speeds. Your automatic transmission is doing the same thing, but not nearly as efficiently when it slows because of its fluid clutch. Watch your tachometer when the shifting takes place and you will see a jump in the RPMs as it shifts down. Automotive companies have come to rely on automatic transmissions to offset bad driving and get their CAFE (Corporate Average Fuel Efficiency) numbers up, so manual transmissions are becoming high end options.
Trucks use the technique constantly, since they have as many as eighteen forward gears. They also have a device called a “jake brake” that uses engine power to slow the truck quickly. You may have seen signs that prohibit their use: “No Jake Brake”. That’s because they’re quite noisy.
Yes, even small changes in elevation matter, and load as well, but I was talking about a single trip where most things remain the same. Not only that, but direction matters because of the wind direction at the time. But my post was getting over-long, so I left that stuff out. But I live in Northwest Ohio. The saying here is that it’s so flat you can watch your dog run away for three days. Also, our roads are straight and flat.
But it doesn’t matter how you measure it, ethanol will not deliver near the energy of regular gasoline or they would offer it in pure quantity. In Brazil, for instance, you can buy either straight gasoline or straight ethanol, but the price difference is quite large and most people understand the difference in effectiveness. When I asked why that was, a taxi driver told me that in town, ethanol is okay because the traffic is so bad that you can never really get above 40 KPH, and most of the time you’re just sitting in a traffic jam, so why waste good fuel? Furthermore, Brazil used to destroy part of its sugar crop in order to keep sugar prices high, so distilling cheap fuel is better than burning up sugar cane.
I have done the experiment a number of times, and the difference is real. Most recently I did it with a rented SUV. I will not put ethanol in my own car because it has other problems, not the least of which is its affinity for water. Ethanol is never distilled better than 180 proof, which is 90% alcohol and 10% water, so the energy content is really 54% by volume. Ethanol people will claim 66% by weight, but we don’t buy gasoline by weight. The water going through your engine won’t do any real harm in operation (doesn’t do any good either), but is corrosive when you shut down and as the engine cools it condenses into liquid water throughout your fuel system. Ethanol also deteriorates seals and hoses.
I first noticed the difference in 1994 after I had been transferred from Ohio to Wisconsin as an automotive engineer. Wisconsin had then, and I’m sure they do now, a mandated 10% ethanol (some states, Oregon and Minnesota and probably others plan to increase it to 20%). Since I always check my fuel consumption as a means of looking for problems, I noticed the drop right away. On a trip home, mileage was way down. I was worried about it (I had a Porsche—you worry about everything), but after getting to Ohio and tanking up with normal gas, my mileage returned. I remembered from my college days that alcohol fuels had never gained widespread use because of the poor energy content, so I did my research and confirmed the chemical reaction. One of the things I remembered was that the Germans, who are the absolute experts on all things chemical, chose NOT to use ethanol during the war, even with dire fuel shortages, because of the loss of performance and range for their combat vehicles. Their thinking was that they already had a shortage, distilling ethanol would only add to it.
My big gripe is that it is bad science and that it is being used to sell something to people who don’t realize it’s bad science, and the people behind it are spending Billions of dollars each year to push its use. It also makes people feel like they don’t have to conserve because it’s renewable, when in fact, conservation through efficiency is the only proper direction for our scientific resources to be working.
“Downshifting a manual transmission enables you to use the energy that went into getting you up to speed to slow the car without using the brakes”
True, but the energy is still being discarded as pumping losses. Not re-used in any way.
The rest of your post seems to drift from topic to topic, but doesn’t explain how engine braking by itself could save fuel.
If it’s stopping the car; that’s re-use of energy that is otherwise totally lost in brake heat and pad wear. Only a small portion is pumping loss if you downshift properly (by keeping engine speed above wheel speed). But there’s no need for debate here. Look at fuel consumption claims for any vehicle that offers both automatic and manual transmissions. The manual transmission mileage will be slightly better. When submitting the data for CAFE mileage on manual transmissions, it’s based on downshifting. i said the “downshifting won’t make a radical improvement in your mileage” but will make substantial savings in brake material. That’s conservation.
When you use only your brakes, the engine is still running and consuming fuel as it decelerates. When you downshift, it is working against additional intake of air and fuel. That’s where the fuel savings come in.
Real fuel savings come when you learn to drive so that you don’t need to brake all the time. Engine braking throws away energy just as much as using the brakes does. On many engines, particularly those with carburettors, downshifting for braking actually makes the engine use more fuel.
On EFI engines that feature deceleration fuel cut off, you don’t need to downshift to activate it, leaving it in high gear pretty much cuts off the fuel until you get below about 20 mph in my car.
Manuals don’t always get better EPA gas mileage figures than automatics.
Downshifting does not “reuse” energy in any way or form. An engine that turns that absorbed energy back into fuel to return to the gas tank is still a pipe dream.
“Read a physics book. It will tell you that it will ALWAYS take more energy to accelerate a given amount than you regain by decelerating that same amount”. No they don’t. In physics terms, the energy to accelerate and energy from deceleration are equal.
But, mainly, internal combustion engines are not some idealized power source. I'm not going to suggest pulse and glide is a great idea, in any kind of traffic it's not, but engines do run at higher efficiency under heavier load, so accelerating a bit then letting it coast can be more efficient than just maintaining a constant speed.
I don't know why you're being so bitter about this, he merely asked about using the instantaneous MPG meter to save fuel. I don't advise "pulse and glide" but I have been able to save fuel with the MPG meter I'm sure -- I've been in several vehicles where letting up the throttle the tiniest bit, maybe losing 1MPH up a hill (nobody behind me) makes the instantaneous mileage go up 5MPG or more. The engine didn't sound different, I wouldn't know this fuel savings were happening if the gage didn't tell me.I see we have left the area of engineering and physics and are now into semantics. I never said it “created fuel to return to the gas tank” and I was using English the whole time. At no point have I suggested anything about “creating” energy, only getting more out of it. As in “wasting less of it”. I said exactly this: You have utilized a certain amount of energy to get the car up to speed. You can get your brakes hot and leave a lot of brake material on the road in losing that energy (which is wasteful), or you can use it to stop or slow the car instead, which is less wasteful. That is a re-use of energy whether you like it or not. Re-using energy doesn’t require that it be converted back into gasoline, which is stored energy. If you convert the kinetic energy that is propelling your car forward into energy for stopping it, that is an energy conversion. If you don’t think it takes energy to stop a car, stand in front of one. It also takes energy to manufacture brake pads. A turbocharger uses the exhaust gases to drive a turbine that compresses air in the intake. We already converted some amount of gasoline to get that energy. We can expel all of it out the exhaust, or try to recapture some of it to do additional work. Stopping a car with the engine is using the wasted energy to do additional work, not attempting to “create” energy. It’s less wasteful.
I haven’t seen anyone build a car with a carburetor for quite some time, but I challenge that assertion as well. I also haven’t seen anyone drive so they don’t have to stop the car for traffic lights, turns, etc. This discussion has nothing to do with driving so you don’t have to slow or stop a car—there’s no such driving. Good drivers downshift when they drive a manual transmissions because it improves mileage and control. If you don’t think downshifting saves fuel, I recommend that you forget your dreams of a career in racing, where there is one hell of a lot of downshifting going on.
Conservation (which does not mean creating energy, but reducing consumption of it) begins by reversing the trend of consumption in small steps. Americans need to understand that distinction.
And your wrong. There are NO cars that get better CAFE mileage with an automatic transmission than the same model gets with a manual transmission.
And your wrong. There are NO cars that get better CAFE mileage with an automatic transmission than the same model gets with a manual transmission.
Really? Let’s see.
2010 Honda Element, 2wd automatic 20/25
manual 18/23
2010 Hyundai Santa Fe 2wd Automatic 20/28
manual 19/26
Well that’s two vehicles so far and I could keep going down the list finding more I’m sure. These vehicles had the same engine for both transmissions by the way.
I haven’t seen anyone build a car with a carburetor for quite some time, but I challenge that assertion as well. I also haven’t seen anyone drive so they don’t have to stop the car for traffic lights, turns, etc. This discussion has nothing to do with driving so you don’t have to slow or stop a car—there’s no such driving. Good drivers downshift when they drive a manual transmissions because it improves mileage and control. If you don’t think downshifting saves fuel, I recommend that you forget your dreams of a career in racing, where there is one hell of a lot of downshifting going on.
Race drivers downshift mostly so they will be in a gear for maximum acceleration after the apex of the turn. In F-1 and Indy cars, the transmissions shift sequentially, like motorcycle transmissions so they have to go through the gears to get to first or second. The lion’s share of the braking is done by the brakes.
Yes, you sometimes have to stop, but you don’t have to accelerate towards red lights and turn what would have been a 15 mph stop into a 40 mph stop.
My motorcycle has carburettors and adopting the habit of pulling in the clutch when coasting to a stop instead of downshifting and using engine braking increased my gas mileage from the mid to high forties to the lower fifties.
Combining a gentle touch on the accelerator and cylinder deactivation can result in some insane fuel economy. I carpool, it’s 90 miles R/T and when its my turn we take my '08 Jeep Grand Cherokee with the Hemi. Combining Hypermile technics, keeping top speed under 60, I can average almost 22mpg. Great, then my wife uses it the next day, drives 4 miles R/T, idles, sits in traffic, guns it at lights and the combined average falls to 15mpg two days later. I checked…she gets about 10mpg in our congessed town and using her heavy foot!
Yes, when you’re wrong, it’s usually a good idea to change the argument. How do you get “racing toward red lights at 40 mph” out of any of my posts? And downshifting IS sequentially dropping into lower gears. If you would read what I wrote instead of hopping on the keyboard to put your very poor interpretations on what was written, you might be able to comprehend some of it. And why do you suppose Formula ONe and Indy car transmissions downshift sequentially, as does your motorcycle? To save on the clutch while dropping into one lower gear after another—While it is DOWNSHIFTING TO SLOW THE VEHICLE. If F-1 and Indy cars used their brakes “for the lion’s share of the braking” they wouldn’t make it through the race.
But that’s enough of this. Go argue with yourself.