Ok, so it’s like the drifeshaft of a car when you pick the wheels up and spin each in the opposite direction? Not moving, hut connected.
Good to know. I stand corrected.
I stand by fuel cut-off, in gear, wasting fuel…particularly so if top gear is very short (i.e no overdrive).
Joe, I am not questioning the techniques used by hypermilers. I am only questioning whether it is better to leave the vehicle in gear or take it out of gear when coasting and the engine is still running. If the engine is still running, it is better to leave it in gear as it will use less fuel than coasting in neutral with the engine on.
This summer, my son and I visited many of our national parks. We drove 6897 miles in 15 days. I had intended to do all the driving, my car 2002 Saturn SL 5sp. My son always puts the car in neutral when coasting, either downhill or coasting to a stop, the exception being when he has to use engine braking to control his speed on a steep downhill. I always leave it in gear and only push in the clutch at the last minute.
I drove the first 5495 miles and averaged 40.33 mpg. Then I got to sick to drive (doc thinks it might have been West Nile but my immune system had killed whatever it was by the time I got checked, but I did get pneumonia in the process). My son drove the last 1402 and averaged 34.75. Thats my evidence.
BTW, normal speeds, no pulse and glide or other hypermiler techniques.
Fuel cutoff only comes into play if the downhill is so steep that you can maintain cruising speed with the throttle completely closed and gravity is capable of propelling the car plus overcome engine braking. If the downgrade is gentle and gravity is only capable of maintaining speed in neutral, in order for the engine to not slow the car, you are giving it just enough gas to make zero horsepower at 2200 rpm. Essentially, the engine is burning the same amout of fuel as it would be if you were racing it at 2200 rpm in neutral. Now it stands to reason that an engine idling at 600 rpm burns less fuel than a engine that’s racing at 2200 rpm without a load.
Fuel cutoff is determined by vacuum, not by how steep the hill is. It cuts off when you are decelerating on level ground.
Yeah, but you can coast a much shallower hill in N than you can in gear. A lot (most?) of the time, you are descending a hill steep enough to brake, or you get off the gas to come to a stop. Here, cut-off is an unmitigated good–you save gas, and however much brake lining you’d need to overcome the added energy of the idling engine.
But, in the specialized case of coasting down a slight hill, you can either spin the engine with gasoline, or by “borrowing” from the car’s momentum (that will eventually need to be repaid.) It stands to reason, it takes less energy to spin an engine at 600 RPM, for one minute, than to spin it at 2200 RPM (among other reasons, 1600 fewer total engine revolutions).
“It stands to reason, it takes less energy to spin an engine at 600 RPM, for one minute, than to spin it at 2200 RPM (among other reasons, 1600 fewer total engine revolutions).”
But not less gas. In the shallow downhill, gravity is supplying part of the energy. The question becomes how much energy is being supplied by gravity and how much is being supplied by gas.
While friction is pretty linear as far as engine RPM is concerned, it has a starting plateau called start up friction which can be pretty significant. From the moment the engine starts to turn, then the friction rises pretty linear with RPM. Of course with a moving vehicle, air resistance comes into play and that is exponential with speed of the vehicle.
This is starting to get too complicated. I think I answered the original question correctly, I have not seen anyone challenging that, yet. I think we are getting into the “cattle call” area and are maybe providing more evidence for “The Andy Letter”
keith
Fuel cutoff is determined by vacuum, not by how steep the hill is.
How steep the downgrade is determines how high the manifold vacuum is. Vacuum is higher than idle when the engine is braking.
Speaking from personal experience of driving my Charger into the mountains west of Denver then driving back: Put it in cruise control whenever possible. This includes allowing the cruise to limit my downhill speed. Driving at interstate speeds on relatively flat ground my MPG is usually around 22 MPG. But when I drive as described above in the mountains, by the time I get back to my starting altitude I am getting upwards of 24 or 25 MPG.
This works on my Hemi Charger which shuts off half the cylinders under light loading, so at least half the time I’m operating as a 4 cylinder car. It may be different for cars that operate all cylinders all the time.
The most efficient way to cycle in hills is to use gravity to your advantage and race down one hill to accumulate enough momentum to help ascend the next hill. That same tactic is necessary when operating a heavily loaded and under powered vehicle. It seems logical that the same technique would be the most efficient way to deal with hills in an automobile. Why fight gravity? Go with the flow… Up to the speed limit, that is.
Red Knox, you erroneously say that is the most efficient. Actually what you have described is the QUICKEST way for an underpowered vehicle (cycles included) to get through hills. But we all know that the quickest is rarely the most efficient.
Rod Knox is correct. Once you learn how to drive efficiently, you will get better gas mileage not using the cruise control that you will by using it.
@HuskerPower: Any time you hit the brakes (in a non-hybrid car), you convert kinetic energy (motion) into heat…that is then dissipated into the atmosphere. Short version, you’re “wasting” it. (Granted that there may be legitimate reasons for wasting it, like not dying, but still.)
How could “wasting energy” ever be more efficient that “not wasting energy”? This point has been tossed around enough and “He who brakes the least and has the fewest engine revolutions, burns the least fuel” is a working first-order approximation. (There is a dissenting opinion, but I’ve resolved to let that be.)
And for all the talk about braking, remember that the OP mentioned a 1% road gradient. In any streetable vehicle, 1% will result in a slow deceleration at highway speeds, no braking required.
No production car is capable of maintaining highway speed on 5hp, so any car would slow down descending a 1% hill.
"He who brakes the least and has the fewest engine revolutions, burns the least fuel" is a working first-order approximation. (There is a dissenting opinion, but I've resolved to let that be.)
No dissent on that as long as revolutions under fuel cutoff don’t count.
Meanjoe75fan I never mentioned hitting the brakes once. Perhaps you should reread what I wrote so you understand before you reply?
@HuskerPower: “You” didn’t mention brakes, true. You did, however, reference a post by RodKnox, where he mentions “using gravity to your advantage” [read: BY NOT BRAKING] as a means to obtain max MPG. You disagreed with his (implied) not braking strategy. I then posted that 1) braking never saves energy, and 2) braking is a moot point W/R/T the original question, because the gradient is too shallow.
I find your most recent post to be needlessly argumentative, and I humbly request that you edit it for tone.
As RodKnox said, “why fight gravity?” How could “fighting gravity” possibly be interpreted to mean ANYTHING OTHER than “braking?”
Foot OFF the gas pedal on the down hill is not braking per-se’ yet is still fighting gravity.
if only…
if only all the drivers would work the hills in the same manner it could be a lot safer and result in better mpg for all.
But there’s the caveat…as I mentioned before , the right way to get maximum mpg on hills is actually the wrong way to drive.
– unless you’re solo on the roadway or others drive in the same manner.
In recent years I have taken up bike riding and most days pedal an hour or more and whether conserving gasoline or calories taking the best advantage of gravity is an obvious benefit. And yes, doing so certainly results in traveling more miles in a given time, but not taking advantage of the obvious would leave me walking and pushing the bike up long steep hills, using drastically more energy and taking much of the fun out of the ride.
And, years ago, driving trucks for a dairy, it was just as obvious that the under powered Freightliners could only find there way up the hills in Alabama when pushed to gain as much momentum as possible going down the preceding hill. I feel sure that the engine was using drastically more gasoline when operating at WOT in 2d gear struggling to go 10 mph compared to accelerating to 70 on a descent and downshifting as necessary as the momentum was lost while cresting the grade at 30 mph in 4th gear. A Continental flat head 6 cylinder pulling 70,000 lbs is beyond its limits on hills.
@RodKnox: Wow. Hats off to you, man. 35 tons with an engine “underpowered” in a taxicab. Must have taken real skills just to keep things going even when everything was going right!
I don’t think any taxicabs had room under the hood for the engine in those trucks.
Duh! I guess Conti made more than one flathead six…[walks away sheepishly]