Hello Everyone,
With gas prices as high as they are I was just wondering, is it better for fuel economy to quickly (not flooring it) accelerate up to cruising speed or to accelerate very slowly?
I’m torn, because while using more throttle you’re obviously using more gas but for a much shorter period than if you slowly accelerate.
Any thoughts?
The harder you accelerate, the more gas you burn. Avoid frequent acceleration and braking bursts. Accelerate slowly and smoothly.
Or so says Consumer Reports, which resembles what I learned in Driver’s Ed oh so many decades ago.
Slower acceleration. Wide-open throttle acceleration in a vacuum is technically most efficient, but frictional losses at very high engine speeds eliminates any efficiency gains. Especially with an automatic transmission.
What if you only wide open throttle it up to somewhat reasonable RPM’s, like 3500-4000?
Anyway, this is why something like a range extender on an EV gives better gas mileage than an ICE directly powering the wheels. The range extender can be designed to run at its most efficient throttle and RPM to run the generator, which then goes to the electric wheels which are more or less the same efficiency at any power, and their inefficiency due to high RPMs doesn’t ramp up as fast as that of an engine. This is opposed to your ICE engine, which needs to operate across a number of throttle ranges and RPMs, most of which will be well out of the most efficient operating regime of the engine.
If you do have to accelerate, get it over with quickly. Fuel efficient vehicles actually generate better gas mileage when a “brisk” acceleration method is used. Slow acceleration will yield less mileage on such highly efficient cars. But any acceleration makes for terrible mileage, so get the acceleration out of the way so that you can cruise (excellent mileage!)
From this article on hypermiling.
That’s in the context of doing everything else they recommend, of course. Fast acceleration and then going to fast or having to break right away is not recommended.
Does that equation change with hybrids? If I accelerate gently, my gas motor doesn’t kick in until much later, and it seems like it gets better mileage, but I may be losing it later with more of the gas kicking in anyway to keep the battery charged.
I remember reading that back in the days of BIG V-8 engines, the goal was to get the vehicle up to speed as quickly as possible because there was a high “overhead” cost for every minute the engine was running. For small engines - the recommendation was to accelerate slowly.
The gist of the article was that the best mileage may be counterintuitive - and it would be worth experimenting with different driving techniques.
By the way - if you are trying to wring every mile out of fuel - your mantra is correct tire pressure, aligned wheels, and good oil. And then plan ahead as you drive. (I have to shake my head at the drivers I see speeding from one red light to the next.)
Probably. I don’t think batteries have the same resting inefficiencies that ICEs have.
If you could keep the engine RPM within a relatively narrow range - for example, by having a 10-speed transmission, or even a CVT - then yes, a relatively high engine load tends to provide best efficiency.
For reference, here’s the brake-specific fuel consumption (BSFC) map for a Saturn 1.9-liter gasoline engine, which appears to have come from the Saturn S series of compact cars. The contour numbers on the map tell you how many pounds of fuel you have to burn to deliver one horsepower-hour of mechanical work to the flywheel. Lower numbers are better - in other words, BSFC bears an inverse relationship to fuel efficiency. Note the yellow, green, aquamarine, and red curves denoting 25%, 50%, 75%, and 100% of max load (torque). You can see that the red island corresponding to best efficiency (lowest BSFC) is at a relatively modest RPM (about 1500-3500 RPM) and a relatively high percentage of max load (75%).
If you stomp your foot down to the floor for 100% load, not only will you be above this island of best efficiency, you’ll also be to the right of it, because the driveline computer will be selecting lower gears so as to keep the RPMs high and give you the high horsepower you seem to want (note the blue hyperbolic curves of constant horsepower).
If you apply very gentle throttle - say, 25% of max load - you’ll be operating below the island of best efficiency.
Each type of engine has its own BSFC map, but most gasoline engines will have this general shape. Diesel engines will have a smaller RPM range, somewhat better peak efficiency (occurring typically at a lower RPM than for a gasoline engine of similar displacement), and better part-load efficiency.
So yes, some middling load on the engine is likely to give you the best fuel efficiency out of the engine. It should be noted though that best overall fuel economy also depends on what you do with the mechanical work that the the engine delivers to the driveline. The short answer is conserve that energy:
keep your highway speeds down. This is less of an issue for city driving, because your cruising fuel economy actually gets worse at very low speeds. Passenger cars typically get their absolute best cruising fuel economy somewhere around 40-50 MPH; the engine is less heavily loaded here than at highway speeds (so less efficient), but your aerodynamic drag and tire rolling resistance are lower, and they kind of hit a crossing point in this speed range.
keep your tires properly inflated.
keep your braking to a minimum by looking way down the road to see what’s going to happen soon and let off the throttle sooner or change lanes before you have to hit the brakes and piss away precious energy.
minimize use of the air conditioning.
In my auto transmission car, slow acceleration is absolutely better.
In my manual transmission cars, the best economy comes from not-quite-wide open throttle (I’m guessing something like 80-90% throttle*) but short shifting, generally around 2100 RPM. The next gear picks up around 1500 RPM, and then its back to widely opening throttle. This style allows me to keep up with traffic around town and still get great fuel economy.
The problem here is we’re not defining “slow acceleration.” Yes slowER acceleration than flooring it is better, but at some point you’re accelerating TOO slowly and that also hurts efficiency. If you’re pissing off the motorist(s) behind you then you’re probably accelerating too slowly for best efficiency too. My approach has generally been to accelerate like I’m not in a hurry to get anywhere, while also not dawdling. That gets the engine up to its sweet spot in RPMs without also staying at lower gears. If you’re driving an automatic and it’s revving/not upshifting to maintain power then you’re accelerating too fast. If you’re going so slow that the engine is dragging then you’re not accelerating fast enough. I’d say 0-35mph should be in the 7 second range (I’m just playing that out in my mind based on my own experience, but I’ll time it out next time I go driving).
All that said, probably the best way to improve fuel economy is to drive like you have no brakes. Anticipate what’s ahead so you don’t have to slow down unnecessarily, and you can take advantage of dips and rises in the road. Downshift when you need to slow down instead of brake, and watch traffic ahead so you can coast, or don’t accelerate as fast if you see that you’ll need to slow or stop in a block or two. That also affects how you accelerate, so you’ll find that sweet spot naturally.
0-30mph in 7 seconds seems to be a bit more reasonable.
I think my '02 Prius was either the first or second year they were brought into the US.
For a couple weeks, I was a mileage geek. Quickly, though, I realized how easy it was to lose track of the bigger – much more important picture: everything going on around me.
So while it isn’t necessarily an answer to the FQ, and while I do not mean to assume that you’ll be guilty of this … do be aware of it. Early Prius drivers quickly became infamous for things like … driving 52mph in the HOV lane (with a 65+mph limit), blocking traffic.
They, too, were praying to the Great God, MPG.
But … yeah … checking your tire PSI as much as weekly (at least early on, until you establish the rate at which they lose air), maybe going a couple pounds higher than the placard in the driver’s door ‘frame ,’ and ensuring that your car is running as well as it possibly can are rock solid ways that will (probably) never take your mind off of your driving environment.
Does anyone else downshift in an automatic to slow down? I do, but I never see anyone I’m riding with do it. And I’ve had passengers ask why I’m doing that.
I do it all the time. Paddle shifters make it a lot easier, but I did it in my last car too which only had an auto/manual shifter.
Only in the mountains on long downhills, but generally not in normal city driving. The better option is to start slowing earlier, so downshifting isn’t necessary, but that isn’t always practical in traffic.