Do you save fuel if......?

Question

you put the clutch in and coast down all the hills you come across instead of keeping your foot on the accelerator?

I must hasten to add that I already know the answer to this question but my friend and I have had a heated discussion on the subject and I just want to show him that he is wrong.

Conversely, of course, your answers may be the opposite of mine and I will have to eat some serious Humble Pie!

creviazuk... if the fuel consumption stops COMPLETELY in overrun... then why doesn't the engine stall?

Answer 1

You certainly have a lot of misinformation for answers! Let's dispell some myths...

Brakes: Unless you are driving a semi-truck with air brakes, modern passenger cars use hydraulics to apply the brakes. Power assisted hydraulics use a vacuum and diaphragm booster to apply the force, normally applied by your foot, to the master cylinder which merely squeezes hydraulic fluid down the brake lines to the wheel cylinders or calipers to apply the brakes. No servos at all. As far as brake control goes, you have the most vacuum with the throttle plates CLOSED which occurs when your foot is OFF the gas whether the engine is at idle speed or not.

At average speed, when you crest a hill gravity will assist in your downward motion causing you to accelerate. You compensate by taking your foot off the gas and the drivetrain then acts on the engine keeping it at the same rpm for that speed. Sometimes, the hill is so steep the drivetrain causes the engine speed to increase and the brakes must still be applied to slow down.

So, what happens when you put in the clutch... it simply disconnects the engine from the drivetrain. With no driving force from your foot on the gas or from the drivetrain, the engine slows to idle and you coast down the hill. Just because the engine is disconnected doesn't mean you are out of control! You merely do not have the drivetrain connected to assist in slowing you down as the engine is a big load on the drivetrain and gets to be an even bigger load if you were to downshift.

While the car does slow with the engine acting as a load, the brakes were designed to slow the car and the engine/drivetrain was designed to make the car go. Not the other way around. You aren't losing any control at all by disconnecting the engine coasting downhill. You are fully capable of putting the engine in gear at any time you so choose. Ultimately, THAT is control.

Let's go a bit further with this... Pushing in the clutch activates the throwout bearing which spins against the spinning clutch springs as it holds the clutch disengaged. Why not put the transmission in neutral and let that clutch back out when going downhill? The transmission could care less if you are in neutral and the clutch will be reingaged so no forces are on it anymore either! The engine will still be disconnected from the drivetrain and idling instead of racing.

At this point, the acceleration caused by gravity on the downward side of the hill, can only be stopped with the brakes. The brakes are designed to do just that and are far cheaper to replace than the clutch! Depending on how good your brakes are and the size of the hill, you may have to be alert for brake fade as they heat up but some people install cross drilled rotors that let the gasses extremely hot brake pads produce escape which all but eliminates brake fade due to heat. Your brakes will slow you down as they are designed to do.

So, how does all this relate to mileage? Mileage is merely math... miles traveled divided by gallons used equal miles per gallon. To increase gas mileage all you have to do is increase the mileage traveled while decreasing the gallons used! Simple enough. When does your engine use the least fuel? AT IDLE. It's at idle when your foot is completely off the gas and the drivetrain is not keeping the engine at a higher rpm (for you carbureted chaps). The more road you can put under the car with this reduced fuel consumption, the better your gas mileage will be.

So, in summary, disconnecting your engine by putting the transmission in neutral and coasting downhill will definitely increase your gas mileage and not hurt your car nor take away any control.

Answer 2

You may save a little. It depends how you normally drive.

However, with the clutch disengaged, the engine requires a small amount of fuel to keep it turning at idle speed.
With the clutch engaged and the car in a high gear, the momentum of the car keeps the engine turning without needing fuel so this will be more economical.

If you have a hybrid car, you can actually gain rather than lose energy in this situation, as the car will automatically shut down the internal-combustion engine, and will use the momentum of the car to generate electricity (slowing the car slightly, using the electric motor(s) as generator(s)). If you cut the engine in a normal car you will lose any power-assistance to brakes and/or steering, so this is not recommended. The brakes and steering systems in a hybrid are adapted to remain fully operational in this situation.

Answer 3

OK, forget all of the above 'educated' guesswork........

this is how a modern engine works....and how I think you KNOW it works!!

To maintain constant speed or accelerate uses normal running amounts of fuel.

Depressing the clutch & coasting will return the engine to idle speed - using a small amount of fuel.

Leaving the clutch - and lifting off the gas puts the engine on 'over-run' when going down hills. This is where the fuel delivery is STOPPED entirely, as the 'car' is moving faster than the engine - can visibly be demonstrated if you have a fuel computer with an instant consumption display.

Answer 4

You save fuel as the engine is not working as hard. There fore not using so much fuel. It's not rocket science is it.

Not sure how it can be bad for you engine though as it means that you engine is doing nothing more than ticking over , So some one explain how this can damage your engine.

As for the people that say if you do it down hill then you lose control . I take it then that these fools race down a hill with out taking their foot off the accelerator pedal . Because to slow down you take you foot off it. There fore Idling the engine. And most cars have disc brakes. Servo assisted are not used to much these days. Unless you live in Cuba

Answer 5

Newer cars are computer controlled, so no there is no savings in fuel since the same amount of fuel is pumped in. On older cars not computer controlled there would be a minimal savings since the fuel would not be controlled as precisely. A diesel engine would be a measured amount of savings.

The practice would/could be unsafe though and more expensive in the long run for the obvious reasons. Modern autos are engineered with many considersations including grades.

Answer 6

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Answer 7

Have been led to believe if you de-accelerate down a hill, in gear, without accelerating a car engine will use less fuel than if you roll out of gear with the engine ticking over??

Answer 8

An alternative way to be certain your car is often using fuel efficiently may be to check and clean a oxygen filters. A dirty air conditioner filter reduces air pass into your engine which will decreases effectiveness and petrol economy.
Tnx..

Answer 9

you do save fuel but this is a very dangerous practice as it gives you less control over the braking when needed so would advise against.
Just for qwerty the guy at the end yes your brakes tend to be disc brakes but they are also servo assisted and abs in most cases.

Answer 10

Fuel is always going into the engine or the car would stop. When you press the accelorator more fuel is injected into the engine.
So if you don't press it you save fuel.
Pressing the cluch, I don't think will make any difference.