Is it true an engine uses NO fuel when you take your foot off the accelerator?

Hello there - I'm trying to do various things to increase fuel economy - tyre pressures, gentle acceleration, emptying the boot, half filling with petrol etc.

However I've read a couple of websites advising not to coast - saying that modern engines use no fuel when you just take your foot off the accelerator completely - for example when going down hill. I drive a 10 plate peugeot 107 so I assume I would have one of these "newer engines" but - is this true? And if so - how does it work? How can an engine work if there is no fuel? I thought it was suck,squeeze,bang, blow? Are they saying the wheels then start just spinning the engine? Sorry if this sounds daft, I don't know as much about engines as I'd like!

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  • 9 years ago
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    this mainly is for cars with manual transmissions, when you are going down a big hill rather than slip it into 'angel gear' and ride the brakes you leave it in 5th or whatever gear and use the wheels to turn the engine over. there is no need for it to inject fuel as it doesn't need to make power. if you are getting that tight about fuel life must be getting rather dull.

    the half filling this just seems false economy to me, you then have to make many more trips to the servo to get fuel, where as if you just did one big fill you wouldn't need to travel all the extra kms to go get fuel. unless you have long range fuel tanks causing you to carry over 100lts of fuel the extra weight is just not worth worrying about.

    if you are getting that picky with things, don't use the heater/air con. the fan causes the alternator to load up using more fuel. don't use the radio, that also will use fuel. if driving at night use battery torches, head lights use a heap of fuel.

    personally, i drive a big car (5.3mt long) which has a high performance straight 6 4.0lt engine, the climate control is always used, meaning the air conditioner is always used, i always drive with my headlights on (safety) and i always fill the tank to the top - 70lts of fuel. my car has a full sized spare wheel, full of leather and woodgrain and sound deadening. around town it uses a fair amount of fuel, i am lucky to see 250kms to 60lts of fuel, however out on the highway it really shines to be the long distance cruiser it is, my fuel economy AVERAGES out to be around 7-8lts per 100kms. my 1.6lt 4cyl mazda uses about the same amount of fuel, little cars are good around town but on the open road the little engines are working so much harder causing them to use so much fuel. little cars are not always the best option.

    if you are serious about saving fuel, put about 35-37 PSI in the tyres, don't use climate control if you have it, the air conditioner will kick on and off causing you to use more fuel, don't use your head lights when you don't need to ( however the extra in fuel is cheaper than a crash) and also use a high octane fuel, i get much better economy by using 98 octane. Have you thought of converting to LPG? however the weight and size of the gas tank is a disadvantage.

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  • 4 years ago

    Open QuestionShow me another » 1. The clutch pedal is found in cars with manual and automatic transmissions.? B. False only in manual transmissions 2. Choose the correct names for the car symbols shown in this picture. larger image A. Low beam; safety belt B. Low beam; head restraint system C. High beam; safety belt Can't answer, don't have the picture 3. With manual transmission, use your right foot for the brake and accelerator and left foot for the clutch. A. True 4. If the temperature warning light/gauge goes on, what should you do? A. Stop as soon as you can at a safe place and shut the engine off 5. Which gauge indicates the total number of miles your car has been driven since it was manufactured? C. The odometer

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  • 9 years ago

    On a modern fuel injected engine yes. If you put the car in nuetral to coast how does it keep from stalling? It has to open the fuel injectors to keep the engine at idle speed. That burns fuel. Its also stupid because you have less control of your vehicle if it is not in gear. Saving a few pennies in fuel isn't worth your life.

    If you are cruising in gear and take your foot off the gas pedal the injectors close because the engine revs are enough to keep it running without stalling since it is still connected to the transmission and wheels. I've watched this on a diagnostic scan tool which has a readout of how much fuel the injectors are delivering.

    The real key to good fuel economy isn't any of that stuff though. The real key is to avoid using your brakes. Brakes convert money into heat. Think about it. When you are at speed it cost you money (petrol) to get to and maintain that speed. If you use your brakes to reduce your speed you are wasting that money by converting it into heat. Yes you do need to stop and not crash into things so I am not advising you never use your brakes, just don't use them wastefully. You need to change your driving style so that unlike all the other sheep on the road you are looking far ahead at what is coming up, not just at the car directly in front of you. That means anticipating curves, intersections, terrain and traffic well ahead of where you actually are at the time. No sense having your foot to the floor just to slam on your brakes for that traffic light when you could take your foot off the gas and slowly bleed off speed long before you get to that light. If you time it correctly it will probably go green before you get there and you won't even have to stop - which is of course much more efficient that sitting there idle.

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  • 9 years ago

    It depends on the driving conditions. The exact numbers vary from one vehicle to another but all modern systems cut off injection completely above a certain rpm when you lift the accelerator. In my previous work truck, a Silverado, I could feel the transitions when I used the engine for braking on steep hills - it fired back up around 2500 rpm. Modern automatic transmissions have lock-up torque converters so the principle is still the same: when the momentum is turning the engine it it pointless to feed it fuel.

    The whole reason for this behavior is emission control. It is very hard to get clean combustion when you don't want the engine to produce power and it is just being dragged along. The exhaust is much cleaner when nothing is being burned. Of course, those conditions of engine braking are pretty rare, so in everyday driving non-hybrid engines just keep running.

    Source(s): 35 years maintaining my own cars
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  • Anonymous
    9 years ago

    The fuel is cut off when you take your foot off the accelerator.

    When you take your foot off the acelerator the engine basically isn't working, it's being driven by the wheels as you thought.

    It's advisable not to coast as you have no engine control which can be dangerous in certain situations.

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  • Anonymous
    9 years ago

    not true at all, fuel is being burned from the time you turn the key until you turn the key again

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  • 9 years ago

    as long as the engine is running it will use gas.

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  • 9 years ago

    when you, let your foot off the gas peddle it, does not burn as much but, it still is burning.

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