Gassing Downhill, Coasting uphill. A fuel efficiency question.

[OrtSurfer]

I regularly drive on a route with a lot of medium length, high-grade hills. One day I had the idea that instead of maintaining a constant speed through out the route, it might be more efficient to over-accelerate on the downhill, and coast in neutral on the uphill.

Would this technique, in this setting, actually achieve a higher fuel efficiency? I realize the difference is most likely negligible, especially in my "flying-brick" of a jeep wrangler. But I'm curious if there is anyone learned in this field who could give me an idea.

Thanks in advanced!

 

[Urwumpe]

Not really, since you waste a lot of fuel on friction then. since you have a Jeep Wrangler, this will even be much more extreme, since you have a very friction-rich car. (4WD, poor aerodynamics, larger than necessary tires)

Better would be the opposite: Coasting downhill and permit some minimal slow down during uphill, instead of keeping speed at all costs. Especially with modern hybrid vehicles, which use the kinetic energy for charging the battery during such phases, you can gain a lot that way.

 

[Izack]

Better would be the opposite: Coasting downhill and permit some minimal slow down during uphill, instead of keeping speed at all costs. Especially with modern hybrid vehicles, which use the kinetic energy for charging the battery during such phases, you can gain a lot that way.

I've a question on this - on top of coasting downhill, would there be any advantage to placing the car in neutral while coasting down long slopes? Would the transmission wear be worth it, if the vehicle is automatic?

 

[Urwumpe]

I've a question on this - on top of coasting downhill, would there be any advantage to placing the car in neutral while coasting down long slopes? Would the transmission wear be worth it, if the vehicle is automatic?

Depends on the transmission. On most modern cars with automatic transmission, that I know (The DSG double-clutch type of VW), the transmission is effectively in neutral position if you coast and you don't need to manually select neutral.

I usually also place my manual transmission car into neutral during long slopes, but that has another reason... it would be better if I would only do so during shallow slopes, so I reduce the wear on the brakes in steeper slopes, but my car has an old dirty PCV valve and ingests a relative lot of oil when running down longer slopes in engine brake. (I am not sure if I should replace it... the car is now 13 years old, still driving well, but the replacement is rather expensive)

 

[Linguofreak]

I usually also place my manual transmission car into neutral during long slopes, but that has another reason... it would be better if I would only do so during shallow slopes, so I reduce the wear on the brakes in steeper slopes, but my car has an old dirty PCV valve and ingests a relative lot of oil when running down longer slopes in engine brake.

In mountainous terrain it can be quite dangerous not to engine brake (especially when driving a truck or other heavy vehicle). In such conditions you can completely burn out your brakes and be left with no way of stopping. I've not done a lot of mountain driving myself, but according to my dad you should shift down if you need to use the brakes to keep a legal/reasonable speed for more than about a mile (in a car, trucks have to be even more careful).

 

[Urwumpe]

In mountainous terrain it can be quite dangerous not to engine brake (especially when driving a truck or other heavy vehicle). In such conditions you can completely burn out your brakes and be left with no way of stopping. I've not done a lot of mountain driving myself, but according to my dad you should shift down if you need to use the brakes to keep a legal/reasonable speed for more than about a mile (in a car, trucks have to be even more careful).

I tested it on the last trip and it wasn't that bad. I just travel down from 900 meters to 250 meters in 15 minutes and about 20 kilometers of distance. That is not that steep compared to the Alps and you only need engine brake for the final steep section of the road. No signs of brake overheating afterwards.

The difference is quite critical: It is only a small bit of visible exhaust because of moisture and oil when reaching level ground again and throttling up vs. 30 seconds of visible light gray smoke, when spending the full 15 minutes downhill in engine brake.

 

[tparvel]

Yes and No.
Some modern cars will shut down a cylinder on down grades to save fuel.

Basically with any vehicle it is a weight,RPM, and resistance wind, friction in all its many forms.
The higher the RPM the more fuel you are burning.
The higher the weight the more fuel you will need to keep it at a certain speed this is off set by horsepower but the greater the horsepower the more fuel you tend to burn.
There is also gear ratio's and engine efficiency, and the list goes on and on.

One of the easiest ways to get better fuel efficiency is simply using the cruise control as much as possible. This to be honest takes your lead foot out of the mix. People are not as good as a computer at maintaining a constant speed and as such will burn more fuel.

Automatics in the quest for better fuel efficiency will try and keep the RPM's very low and shift gears too soon on hilly terrain.
So even though down shifting raises your RPM and as such burns more fuel it can be a lot less then then putting your foot to the floor cause you are in too high a gear.

So basically the lower the RPM and the the less the RPM's have to vary, the more fuel you will save (don't forget the offset of foot to the floor)
Every vehicle is different so you have to find the sweet spot for each vehicle based on the terrain you are driving.

And lets not forget sometimes a little fuel savings is not worth the ticket or going off the road.
Both can make your savings null in void.

 

[OrtSurfer]

Just to clarify, I keep it in 2WD unless I'm in the mud or snow/ice, and my tires are stock. I've often considered getting good mud tires but they are too expensive and, as aforementioned, inefficient. Also, it is a manual transmission so switching to neutral is no big deal, and its a 1994, so probably no fancy cylinder shutdown.

By over accelerate, i mean I accelerate evenly so that when i reach the trough, I'm going 65-70mph (in a 45mph :lol By the time i reach the crest, I am going around 35mph

I did take wind resistance into consideration. I suppose my way of thinking was whether it would be better to fight the wind when i have gravity working for me, or gravity working against me. I assumed it was better to accelerate when it was easier, and use my momentum to conquer wind and gravity.

 

[Urwumpe]

Yes, but then, lets say you are getting close to maximal speed downhill and 90% of your propulsion power are lost for friction. only 10% + the gravity can still accelerate you. Only a few km/h more, and gravity is also consumed.

uphill, gravity works against you, friction is initially the same. You slow down, even if you would keep the throttle up.

In the opposite scenario, you drive slower, and use the gravity downhill for coasting. You don't need any gas at all for keeping a modest speed (I actually still accelerate a bit on a shallow slope when going neutral on a 2% slope at 100 km/h). I am slower when it goes uphill again, but I used almost no gas for this part for countering friction. Uphill, the slower speed means lower friction to fight, and I will consume less gas for fighting friction, I have more power for fighting gravity.

After all, net gravity losses will be zero when you reach the initial altitude after starting on a crest again. Up and down compensate.

Drag depends on speed, twice the speed = four times the drag, regardless if you drive up or down.

The propulsion also has friction inside, this depends mostly on speed again and does not depend if you drive down- or uphill again.

So, for saving gas, it is better, to invest your energy, when you are slow, and accelerate only with small investments. You would be slower on the average then, but also save a lot of fuel.

 

[Proximus]

When in neutral, an engine will burn fuel in order to remain at idle rpm.
When in gear most modern engines burn NO fuel at all when engine braking.

Cruise control doesn't work so well where I live as the gradients tend to be a little on the harsh side, basically the electronics don't make use of momentum to help out the engine. :facepalm:

 

[C3PO]

Isn't worrying about a Wrangler's fuel economy a bit like choosing the correct sun-lotion to prevent nuclear radiation? :lol:

If you need an All-terrain vehicle, you just have to pay the price.
If you occasionally need it, buy a more economic car for "regular" travel.
If you just really like your 4x4, you have to decide if it's worth the extra cost.
PS: If your 4x4 has low-profile tires, your driver's licence should be revoked ASAP!

 

[n72.75]

My method is: shift at 4k so the next gear hit's just before maximum brake torque, brake and corner at the limit.

 

[Quick_Nick]

Starting to read this, I thought he was going to throw in Oberth Effect. :lol:

 

[OrtSurfer]

Isn't worrying about a Wrangler's fuel economy a bit like choosing the correct sun-lotion to prevent nuclear radiation? :lol:
PS: If your 4x4 has low-profile tires, your driver's licence should be revoked ASAP!

:rofl: yes.

I already get above average mileage for it. I easily get 18mpg, but never actually did an official fill,drive,fill test to see. Infact, I have never filled her all the way up

 

[N_Molson]

On my (small and old) car manual, they say that, to optimize fuel consumption, you should keep the same level of acceleration regardless of the slope. So you just go slower uphill and faster downhill (as long as the engine don't lose or gain unreasonable RPM). Manual gearbox BTW.

Oh and obviously, don't forget to inflate your tires well and check regularly. As Urwumpe said, friction is critical, and pressure inside the tires can make an huge difference.

 

[RisingFury]

If you want to drive efficiently, keep the engine at its optimal RPM and shift the gear to keep the engine there. Your car's manual should tell you what the most efficient RPM and speed is, but typically the most efficient speed is about 90 to 110 km/h in top gear. The RPM efficiency varies with the engine (gas or diesel) and with the particular build.

One other thing you can do if you're an experienced driver is to shift up a bit earlier. If you typically drive in 3rd gear at 50 km/h, it's ok to shift up to 4th already. The engine won't have much power so if you need to accelerate you'll do that more slowly, but for coasting at constant speed, it's more fuel efficient.

 

[Fabri91]

Regarding switching to neutral or not on downhill roads (considering only efficiency, and not whether engine brake is required or not), as far as I have seen while driving myself, it should depend on the type of throttle and thegrade.

Most, if not all, modern cars don't have a mechanical connection to the engine's throttle body, but a potentiometer on which the gas pedal is mounted. This allows the engine management unit to "know" when the driver is indeed commanding idle, while an engine with a mechanical throttle body can only read the mass flow through the intake, but can't determine whether a low flow is due to the driver effectively commanding idle at a low altitude or the driver commanding low power at a slightly higher altitude (less dense air).

This should boil down to the following: a modern engine will cut-off fuel if it senses that the driver isn't depressing the accelerator while a gear is selected, while an older won't.

Thus if the slope is steep enough to enable the car to maintain speed without depressing the gas pedal, a modern car is driven more efficiently by keeping the gear selected, since fuel cut-off will mean zero fuel consumption, while on an older car it would be more efficient to select neutral, since otherwise the engine would be forced to a higher rpm, producing an increased air flow compared to idle rpm and forcing the engine management to inject more fuel.

I hope this makes sense. It's what I noticed when comparing the instantaneous fuel economy reading from the trip computer of our older first generation Ford Focus 1.8 (mechanical throttle body) to the newer Focus 1.0 with which we replaced it.

 

[C3PO]

If you want to drive efficiently, keep the engine at its optimal RPM and shift the gear to keep the engine there. Your car's manual should tell you what the most efficient RPM and speed is, but typically the most efficient speed is about 90 to 110 km/h in top gear. The RPM efficiency varies with the engine (gas or diesel) and with the particular build.

It's much more complicated then that. The fuel efficiency isn't only dependant on RPM. The actual load on the engine plays a huge role on the mileage. And one factor that's almost always forgotten is the mass of the stuff you're carrying around.

One other thing you can do if you're an experienced driver is to shift up a bit earlier. If you typically drive in 3rd gear at 50 km/h, it's ok to shift up to 4th already. The engine won't have much power so if you need to accelerate you'll do that more slowly, but for coasting at constant speed, it's more fuel efficient.

You have to be very careful when driving in a high gear at low speeds. You can be driving at 50 km/h in 4'th without any trouble, but you can hit an almost imperceptible incline that triples your fuel consumption if you are inexperienced. It's hard to numerically calculate the best strategy in such a complex system. A neural network is much better at those things, and most drivers already have one of those. :lol:

 

[Urwumpe]

Also, not all car engines like operating at lower RPMs, you should always keep that in mind when selecting a higher gear.

My car can for example still drive well at 1800-2000 RPM, but does not produce enough electricity then for powering lights, heating and sparkplugs = you drain the battery. You need at least 3000 in the winter months, while you can easily drive at 2500 in the summer. Turn the steering wheel too fast while driving at less than 2500 RPM, and the engine stalls...

Still, I need an alternative to my car for the way to my workplace... the distance is too short now. I am constantly driving with cold oil, unless I drive a longer route to my home.

 

[C3PO]

Also, not all car engines like operating at lower RPMs, you should always keep that in mind when selecting a higher gear.

Particularly if it's a modern highly strung low cc engine.