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Thread: Deceleration using gears down a slope - Fuel Consumption?

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    Default Deceleration using gears down a slope - Fuel Consumption?

    I've read that excessive use of brakes down slopes e.g. from Damn-e-Koh in Islamabad cause extra brake wear but if I use lower gears to decelerate along with some amount of braking, what affect would that have on the following two things:

    1. Fuel consumption (even though I'm not pressing the accelerator)
    2. Engine wear and tear due to higher RPMs

    I read somewhere that some cars are equipped with a feature called DFCO (I think it means Deceleration Fuel Cut Off) which automatically cuts off fuel in certain cases but I'm not sure if mine has it. My vehicle is a 1.0L Toyota Vitz 1999.

    But regardless of DFCO, the question is do vehicles still consume fuel when engine is revving hard down a slope without the gasoline pedal being pressed?

    I've always been confused about this and would appreciate some information on cost(s) vs. benefit(s) of the said approach.

    Thank you.

    This is as much of a test for me as it's for them

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    Many a time while coming down form Pir Sohawa, I have stopped in the way to check my wheels for the stench, but thanks God it was not from my car, but other motorists. Many motorists do not know how to drive down hill. Every car manual mentions how to do it. As somebody said above, climb down the hill in same gear in which you climbed up using the breaks to control the speed. Safety first, CBA later.
    Be polite on the road. Always give way to others, particularly ambulances and pedestrians.

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    Quote Originally Posted by qwertypoiu View Post
    This is not relevant to topic but ever tried going down in neutral its extremely fun
    Use to do it on my 80cc bike and then Datsun 120Y. I now feel it was stupid to do that, but did it as a habit without losing breaks, traction. Old cars were so fun. Today's cars are just toys.
    Be polite on the road. Always give way to others, particularly ambulances and pedestrians.

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    expert drivers use this techniqe when breaks are dead. i was told this when i wal learning to drive

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    the most advisable thing is to never go above 50 and keep it at second gear it will give good climbing performance and will keep speed low when going downhill

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    @Lonely_Puma: If your car don't have CNG, I am afraid you can't drive with my technique. Yes one should be on petrol first before going to neutral. Car on CNG -> Shift it to Petrol -> Immediately Shift it to Neutral...You are done!

    @autophile: Yes the engine will be runing. It is not possible with 'engine shut' as you mentioned. I wrote DON'T SWITCH OFF YOUR ENGINE. So the engine is not switched off.

    @2c: It will not even consume water in the radiator. Offcourse it is great technique.

    @cracked: the engine will never shut off. This is my brain where this amazing idea was generated.

    @ALL: I will now explain it. Most senior drivers don't believe my this invention and the secret I revealed to them.
    Energy can be produced from many things and we can transform energy from its one form to another. When we run car on plain road, the engine requires energy to run against gravity. Gasoline is burnt in combustion chamber and energy is produced which forces pistons and ultimately our car moves. If we cut-off fuel supply then car will stop, because the engine has no force left to go against gravity to produce motion.
    On downslide why your car runs without pressing accelerator? This is because you are already going with the force of gravity which is not in the case when you are driving on plain surface. At this stage car pistons don't require fuel to produce force. You are already using gravity for moving the car. Here I introduce my amazing technique. Your car is initially on CNG when you start downsliding, turn your car on petrol and immediately turn it to neutral. You disengage fuel supply now. The engine will not shut down, it will keep working with the force produced by its own motion due to gravity. About gear shifting, you can drive car neutral, but it is very dangerous in car handling and braking. So it is advised to shift gears according to speed.

    No one asked that how AC, brakes, power steering, headlamps, hi-end stereo component will work without fuel

    Believe me this technique is not only for Bilal's Mehran. Come to me and I will prove my words and your car, headlamps, AC, Power Steering, power brakes, hi-end stereo component, all these things will be working without any fuel.
    Bhai sab kuch tu aap ki garri ka chala dia hay muft mein, aub bhe yakeen na aiey tu chalao petrol par mujhay kia
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    Quote Originally Posted by wittycranium View Post
    Yes engine and gears are more precious, but you will not harm them as along as you are keeping below the red line and you are doing it smoothly, not jerking the system too hard. In local cars i'd suggest at least 1,500 RPm below the redline would be ok.





    Completely wrong assumption. The revving of engine without pressing the accelerator pedal does not equal more fuel consumption.

    Actually, it might be even less than idling.

    You see, when you are accelerating at a high RPM, the power of the engine is being delivered to the wheels through the clutch and transmission, the power of the engine comes when you burn fuel, hence more fuel consumption.

    On the contrary, when you are going down hill, while the clutch is engaged (pedal not pressed, car in gear, not neutral), it is not the burning of fuel that is revving the engine, it is the force of gravity and your forward momentum that is moving your wheels which are connected to your crank shaft through the clutch and transmission. So here the gravity and forward momentum is causing high RPM, not fuel.

    This is why we have better fuel consumption while going downhill and on the highways, even with high RPMs.

    Correct me if I am wrong.
    ++1 (we reduce the speed by useing
    Compression of engine) no more fuel consumption

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    I don’t understand that what is the fuss about de accelerating your car on gears. I have always been using gears to slow down my cars even once I am not driving on the hills. My previous car which I sold was Khyber and I have always slowed it down using gears and my friends at times used to say that your excessive de-accelerating your car on gear may damage your car engine but I sold that car after driving it for two lac and twenty four thousand kilometers and never faced any issues with the engine.

    I believe that most of the good drives uses gears a lot to de-accelerate their cars at higher speeds and on slopes. It’s a matter of an habit which puts you in more control over your cars. Now it’s my habit to use a gears a lot in order to slow down my car but never faced any issues and never noticed an increase in fuel consumption. Even while driving back from murree I had always used my gears to control my car but never saw that in any way that has an impact on my fuel economy. I can assure you this for cars such as corolla 2007, Liana and Honda City 2007.

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    Lucky bilal you are always running your car on the petrol which is retained inside the carburator but remember whenever the fuel runout inside the carburator and you have to overtake some vehicle in a hurry you will have to push the gas padal because though the engine parts ie pistons c/r rods and crank shaft moving with the tyre's momentum but not giving you boost and to get you boost you need fuel and that moment running short of the fuel may cause problems so i suggest safety first.
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    Quote Originally Posted by qwertypoiu View Post
    This is not relevant to topic but ever tried going down in neutral its extremely fun
    ill bet you 1k if you do that in LDA parking plaza and make a video

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    Bilallucky your technik is hard to buy, power steering need hydraulic pressure & brake booster need vaccum from intake manifold if engine is not 'IDLING on whatever fuel' how would vehicle be conveniently controlled.

    I drive from KHI to Muzzaffarabad round trip twice a year for down hill journey using 'engine' at mostly idling revs to manage control with minimum of brakes,

    anyone else tested such tech to support bilal????

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    Common guys! Using engine braking power downhill your car will not consume any more fuel than it will consume while idling. And as long as you don't jerk it wont have impact on engine wear. Make sure you don't press too much clutch!

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    When i was learning driving my teacher told me that at high speeds use engine to stop the car and use brakes less and also my father used to tell me the quote couple of times that a good driver never uses brakes
    "Women - always in trouble with them, but can`t live without them."
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    All of the latest vehicles are equipped with decel fuel cut board their ecus whenever some one decelerate for a long distance ie says 100 meter down a slop and decelerate the ecu will not supply the fuel to the cylinders unless driver press the throttle or ths car loses it decel motion. And for the theory which bilal is talking about to decel on complete fuel cut. Ie. Leaving the ignition switch on and just cut of the fuel what ever the fuel is an unwise action as you can need power of the engine any time for overtaking an other car or a sudden turn like on pirsohawa road. The accesories will keep running with the engine rotational power which comes from the transmission while deceleration on hilly areas. I do adopt this nasty technique on my old school diesel engine with manual tranny during down climbing from murree road on the older one and decel with the fuel solenoid switched off and the ignition switch still on while all of the accessories are working perfectly till the decel motion is continuous but it is unsafe practice and dont attempt it it is risky and insecure.

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    ^^though your input is appreciated, it seems to be a DIG, thread is 4 years old
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    Quote Originally Posted by arzaam.bhatti View Post
    ^^though your input is appreciated, it seems to be a DIG, thread is 4 years old
    Digging is best thing to follow the old threads which are dead but are useful for the new members and site guests who don't know about some different questions which arouse in daily routine.

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    Nice dig

    DFCO depends upon a lot of parameters:

    1- The engine must be within the operating temperature range. So if you are running without a thermostat/ direct fan, say goodbye to DFCO.

    2-The engine must be above a certain RPM, which varies by manufacturer. For i-Vtec Hondas, it is roughly 1100 RPM. This RPM requirement may be as high as 2k-2.5k or above (Scion XD for example). A/C use may increase this RPM range even further.

    3-For Toyotas, DFCO may not work below 40 km/h.

    4-It doesn't kick in as soon as you let off the accelerator; there is a time delay of a few hundred milliseconds.

    There might be more parameters; you will have to experiment in your car. A Scangauge / Ultragauge might help.

    Overall, DFCO uses more fuel than simply coasting in neutral. While there is no fuel injected during a DFCO event, you will slow down much faster. You will have to use more fuel to get back up to speed.

    Neutral coasting uses some fuel to spin the engine at idle, but you will coast much farther. The net fuel consumption is lower than DFCO.

    DFCO is useful only when you want to slow down/come to a stop anyways. It is also useful for coming down from Murree etc.

    Neutral coasting is better for all other scenarios where you might want to conserve or gain speed.

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