[pamiboy]

Hi Guys, I have always thought that coasting (driving in neutral or with clutch depressed) especially when going downhill tends to save fuel. As I understand the general perception among drivers is the same. However I recently found an article which says otherwise. In modern cars coasting burns more fuel than driving in gear .


When Coasting Downhill, Leave the Car in Gear.


There are those who refuse to be shaken from the practice of coasting downhill in neutral to save gas. This is a bad idea no matter how you look at it. Let's set aside fuel economy for a moment. Coasting downhill in neutral is illegal in most states. And it's dangerous in all states. In neutral, you have no way to accelerate to avoid a hazard, and if the engine stalls, you have no power steering or vacuum boost for the brakes. If the hill is steep enough to call for hitting the brakes to keep you from gaining speed, they're more likely to overheat—and overheated brakes lose effectiveness until they cool off. They'll probably do that right around the time the police show up to take the accident report.

Here's the surprise: There's no trade-off between safety and fuel economy in this case. Leaving the car in gear while coasting downhill actually is more efficient. Why?

Most fuel-injected engines today use computer-controlled Deceleration Fuel Cut Off: When you lift your foot from the gas while leaving the car in gear, injectors shut off automatically, and the car's rotating tires—which are connected to the engine via the transmission—keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

In contrast, the fuel-consumption rate for an engine idling in neutral falls between 0.2 and 0.4 gallons per hour (gph). Splitting the difference and using 0.3 gph for our example, idling in neutral down a ½-mile-long hill consumes fuel for 30 seconds, for a total of about 0.32 ounces of gas. Popping the car into neutral actually wastes gas.

This may seem counter-intuitive, but that's what data are for—replacing good guesses with solid facts. Watch the data, and over time the savings will take care of itself.

Courtesy: 6 Driving Tactics to Save Gas This Summer - Yahoo! Autos

[sachinj12]

Originally Posted by pamiboy

Hi Guys, I have always thought that coasting (driving in neutral or with clutch depressed) especially when going downhill tends to save fuel.

You do that, you will meet your creator for sure.

My uncle's friend's distant relative did the same and he paid with his life for doing that. He just switched off the engine while going down hill.

[pranavt]

When using engine braking while doing downhill, the ECU goes into fuelcut mode and no fuel is injected. If you drive in neutral, the ECU has to inject fuel to keep the engine idling while the car is going downhill.

[pamiboy]

Originally Posted by sachinj12

You do that, you will meet your creator for sure.

My uncle's friend's distant relative did the same and he paid with his life for doing that. He just switched off the engine while going down hill.

I didn't say that I do it. But I've seen some drivers (esp taxi operators) doing it.

[dhanushs]

The prime rule while going down a hill: Drive in the same gear you came up.

As pranav mentioned cars with ECU's are more fuel efficient whist in gear, than in idling. Plug in an OBD and you'll know.

[Mr.Boss]

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[dot]

Originally Posted by pamiboy

Most fuel-injected engines today use computer-controlled Deceleration Fuel Cut Off: When you lift your foot from the gas while leaving the car in gear, injectors shut off automatically, and the car's rotating tires—which are connected to the engine via the transmission—keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

Originally Posted by dhanushs

As pranav mentioned cars with ECU's are more fuel efficient whist in gear, than in idling. Plug in an OBD and you'll know.

This is totally true. Releasing the foot off the accelerator enables the ECU to cut off fuel supply. Here is a video of a 8th gen Civic AT where the scangauge (OBD tool) records the same. Have a look.

YouTube - ‪8th Gen Civic FD Fuel Injection Shut-off‬‏

p.s. Congrats @dhanushs for the four figure mark.

[rahul4640]

Originally Posted by dot

This is totally true. Releasing the foot off the accelerator enables the ECU to cut off fuel supply.

Please excuse my ignorance for asking a silly question, does the fuel supply remain cut off by the ECU for the cars having manual transmission as well, if I keep the car in gear but press the clutch pedal, while going downhill?
Regards,
Rahul Biswas
Calcutta

[wild child]

Originally Posted by rahul4640

Please excuse my ignorance for asking a silly question, does the fuel supply remain cut off by the ECU for the cars having manual transmission as well, if I keep the car in gear but press the clutch pedal, while going downhill?
Regards,
Rahul Biswas
Calcutta

Dear Rahul

Pressing the clutch while coasting down in gear is equal to coasting down in neutral cause in both cases the engine starts to idle.

Experts, correct me if i'm wrong.

- cheers

[dot]

Originally Posted by rahul4640

Please excuse my ignorance for asking a silly question, does the fuel supply remain cut off by the ECU for the cars having manual transmission as well, if I keep the car in gear but press the clutch pedal, while going downhill?

@rahul, it is not a silly question!! and the answer is yes, modern cars with manual transmission will cut off fuel supply when foot is taken off accelerator.

Edit: Forgot to add that when clutch is fully depressed while foot is off the accelerator, engine will idle. In that case fuel will be consumed.


However while going downhill, if clutch is depressed, then engine braking is lost. A dangerous move. One can control speed by brake till you realize that the gear cannot take it anymore, then one can upshift. Normally when I drive on deep downhill roads on a manual car, I leave the car in second or third gear as appropriate; use clutch only to change gears and utilize the brake when needed.

[vina]

Originally Posted by dot

@rahul, it is not a silly question!! and the answer is yes, modern cars with manual transmission will cut off fuel supply when foot is taken off accelerator.

Edit: Forgot to add that when clutch is fully depressed while foot is off the accelerator, engine will idle. In that case fuel will be consumed.


However while going downhill, if clutch is depressed, then engine braking is lost. A dangerous move. One can control speed by brake till you realize that the gear cannot take it anymore, then one can upshift. Normally when I drive on deep downhill roads on a manual car, I leave the car in second or third gear as appropriate; use clutch only to change gears and utilize the brake when needed.

I'd like to add - pressing the clutch is still better than putting the car in neutral. In case you need the power back urgently, you can gain it back quickly by releasing the clutch, that will not be that easy with the car in neutral.


Barring some exception (e.g. anticipating a gear shift or actually shifting the gear) pressing the clutch is a bad idea - even in regular cars (as against hybrids) at the very least a coasting car can cut off fuel and use the enrgy to run your AC etc. rather than use fuel for the same purpose

[sgiitk]

The only safe way is the keep the car always in gear. Try the coating/neutral trick and you will fail your UK driving test pronto!

[vina]

Originally Posted by sgiitk

The only safe way is the keep the car always in gear. Try the coating/neutral trick and you will fail your UK driving test pronto!

Sir

I think by coasting here people mean - in gear, clutch engaged (i.e foot off), foot off the pedal (may be on the brake to slow down in emergency)

[alpha1]

If the fuel is cut-off during deceleration - then how do the pistons keep moving?
Because of vehicle intertia?

Doesn't look possible - because we all know that we can park the vehicle on slope by keeping it in gear.

[pranavt]

Originally Posted by alpha1

If the fuel is cut-off during deceleration - then how do the pistons keep moving?
Because of vehicle intertia?

Since the clutch is engaged, the wheels move the pistons through the tranny. Basically the reverse of what happens when throttle is applied while the car is in gear.