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    Thread: Downshifting!! Does it waste Gas??

    1. 10-24-2006 11:19 PM #1
      i know its not a very interesting or important topic, but i was having this conversation with a buddy of mine who says downshifting instead of braking wastes more gas. I feel as it does not, seeing as how i'm not pressing the throttle at all. I have tiptronic, so engaging the clutch at a proper rpm (which would invovle pressing the throttle) does not apply here.

      My friends arguement is that in order for the engine to slow the car down (due to the sudden increase in RPM's), there must be an input of energy. The slowing of the car expels energy, so according to physics, there must be some sort of input of energy, i.e. from the fuel.

      What do u guys think??


    2. 10-25-2006 12:37 AM #2
      Interesting theory.

      I know that higher RPM's = higher fuel consumption, but that's usually on the the way UP.....not so sure about on the way DOWN.

      Any experts out there?


    3. Member KO-R32's Avatar
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      10-25-2006 10:51 AM #3
      Quote, originally posted by evoic »
      higher RPM's = higher fuel consumption

      Seems reasonable to me.

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    4. Member intonation's Avatar
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      10-25-2006 11:09 AM #4
      It is my understanding that fuel delivery to the engine is cut off during engine braking. Can soneone confirm?

    5. 10-25-2006 11:27 AM #5
      when you downshift, ur using the engine rpms to limit how fast the wheels spin. therefore, ur in higher rpms. higher rpms means more gas. when you brake, i usually press the clutch, so itz back to idling rpms.

      which uses more gas? 800rpms or 4000 rpms?
      simple as that. the car does expel energy, but that's just in heat only using the breaks. the kinetic energy of the car is being trasnferred to the engine as the rpms slow down. basically instead of breaking, ur using the momentum of the cylinders to slow you down.


    6. 10-25-2006 12:36 PM #6
      Quote, originally posted by T.A4 »
      The slowing of the car expels energy, so according to physics, there must be some sort of input of energy, i.e. from the fuel.

      This isn't a correct argument. Conservation of energy says that since the car is losing kinetic energy, this energy can't just disappear, it has to go somewhere. In this case it is either transferred into heat in the brakes or heat in the engine, or both, depending on your method of braking. There is no physics argument saying the fuel has to provide an input. it IS a correct argument to say that higher RPM's means higher fuel consumption.

      That being said, looking at this from a Total Cost of Ownership perspective, we would need to analyze whether the incremental change in gas mileage offsets the cost to replace brake pads and rotors more frequently. I don't know the answer to this.


    7. 10-25-2006 12:38 PM #7
      Also, here's an interesting discussion of engine braking at wikipedia.

      http://en.wikipedia.org/wiki/Engine_braking


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      10-25-2006 02:47 PM #8
      But downshifting and hammering the gas/ car is different from down shifting and being steady on the gas or lifting of the pedal all together...

      So it depends downshifting to accelerate vs. downshifting to slow down..One definenatly uses more gas the other I am unsure exactly about...

      I don't think it is solely about RPMs.....just my 2 cents


    9. 10-25-2006 03:09 PM #9
      Personally I don’t agree with the assumption that higher rpm = higher consumption. In my opinion, throttle position = higher rpm = increased fuel consumption. So I think higher rpm, without pressing the gas pedal, won’t cause higher consumption. I believe during engine braking there is no fuel being supplied to the engine regardless of what the rpm is so the fuel consumption is zero. If you press the clutch during braking, the engine is in idle mode and fuel is being consumed. So in my opinion advantage: engine braking

      Here is another question: If you are rolling down a hill which is more fuel efficient putting the car in neutral or leaving it in gear?

      If you are in neutral, your engine will be in idle with an rpm of around 900. If you leave it in gear, your rpm will be higher, but there will be no fuel supplied to the engine. Because of the engine braking, your car won’t reach as high of speed as in the first scenario and won’t roll as far either, but most people would agree that the idling engine in neutral would use more gas than what you would need to make up for the lost distance.


    10. 10-25-2006 03:18 PM #10
      Not really on topic, but here's a recent discussion about rpm vs. fuel consumption:

      http://forums.audiworld.com/a4b7/msgs/62466.phtml


    11. 10-25-2006 05:29 PM #11
      I guess one could argue that if there was any gas being pulled into the engine during engine-braking, you'd assume that there would be combustion (assuming there was air and a spark) and the car would actually accelerate (based on gearing of course).

      I'm no expert, but just my two cents...

      It should be noted that you will have to use some gas to blip the throttle to match RPMs before finishing your down shift otherwise you'll be using your clutch every time to match the road speed to your RPMs. I guess it depends on how fast you want to slow down too.


    12. 10-25-2006 10:09 PM #12
      definitely an interesting theory; however, lets be reasonable... when you down-shift; what level of RPMs are you reaching? Also, what is the duration? I know when i down-shift i'm at around 3k for a second or two. This is usually when i corner or approach moving traffic; in my daily commute it may happen 5+ times. Is it really a concern to someone that you "might" be wasting fuel at that small duration?

      Also keep in mind, your flow rate through the injectors is also based on load (simply put), you dont really require a load when tapping the accelerator pedal in neutral. As you accelerate forward in gear, yes of course you require quite a bit of flow and that is based on your throttle position sensor (MAF to keep stoich).




      Modified by drexplode1 at 7:13 PM 10/25/2006


    13. 10-25-2006 11:50 PM #13
      The thing is, i downshift alot, on a regular basis even when there is a red light, or say a car in front of me slows down a bit, i would rather downshift then hit the brake. This is because i have an automatic, its just easier to tap the shifter. I know its not that good for the engine, but its more fun, seeing as how automatic gets boring sometimes .
      I noticed that on the computer display screen (where it displays how far you can go with current gas level), you can change it to display "L/100km", and when i did that, it showed me the current consumption of gas, depending on how hard i pressed the throttle. When downshifting to slow down, the display said "0L/100km", so according to the computer, downshifting doesnt waste gas.

    14. 10-26-2006 12:02 AM #14
      Wow, way too much misinformation in this thread. During engine braking (or compression braking etc...) the car is not delivering fuel to the engine. Thus no fuel is used during downshifting, unless you are applying throttle to match the revolution's of the lower gear. But the act of engine braking while not depressing the throttle uses no fuel whatsoever.

    15. Member Tanner74's Avatar
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      10-26-2006 08:47 AM #15
      There's still fuel being used if you're de-accelereating while in gear, but obviously the amount is much lower. I had a discussion with a friend about that 0L/100KM and it's actually not the case, it's still consuming gas but a very small amount.

      I think in the end, the amount of fuel used is negligible. Plus I rather downshift to the right gear while I'm slowing down, in case I need to get going fast to avoid a collision or something.


    16. 10-26-2006 09:33 AM #16
      Quote, originally posted by Tanner74 »
      ... I rather downshift to the right gear while I'm slowing down, in case I need to get going fast to avoid a collision or something.

      *Ding Ding Ding* WinnAr! Glad I am not the only one to see the danger of being caught in a high gear & low rpm, especially on a manual transmission. This is also why I usually don't go beyond 4th gear while in city traffic around 45 mph.


    17. 10-27-2006 05:11 PM #17
      An engine uses the least amount of fuel while engine "braking". It uses even less fuel than when idling.

      Remember, while in gear and slowing down, it is the cars momentum that is keeping the engine running NOT the combustion process, which requires fuel, air, and compression.
      So, if you're at 70mph in 4th at 4000rpm and you down shift to 2nd and engine rpm jumps to
      5500rpm you are NOT using more fuel just because the rpm goes up.

      Higher revs can cost more fuel, but you are NOT acclerating at this point, you are using the cars momentum to rev that engine to that speed. It is using little to NO fuel at that point.
      There is vacuum being created in the cylinders and the throttle being nearly closed is creating a "braking" effect slowing the car even more.

      As far as does rpm cost in more fuel? That's an interesting question because even though it seems to make sense that 2000rpm will use less gas than 4000rpm, you have to also consider what gear you are in, what load is on the engine such as up or down hill, and at what rpm does the engine work it's most efficient?
      CVT transmissions work on the idea that engines have a "sweet spot" where they are the most efficient, where throttle opening and engine pumping are providing the best power output being the most efficient with the fuel being used. That is part of the reason whey CVT trans cars get better MPG even though the engines tend to rev to a higher rpm and want to stay there longer.

      Lugging the engine in a high gear while at low rpm can actually waste more fuel due to extreme pumping loss as the engine is wasting power just trying to "breathe" or pump air in and out of itself. A combustion engine is, afterall, an air pump.

      Larger throttle openings, in cars with throttle valves, allow the engine to breath easier as there is less power being used to draw in that air compared to smaller throttle openings that give a greater restriction to air coming in. One suggestion for better MPG is to start off in 1st normally without big throttle openings and "jack rabbit" starts, but then shift sooner and use BIG throttle pedal pushes giving a larger intake opening the cylinders by which to breathe easier. So, you would start in 1st with normal throttle, once under way you bigger throttle to open the plate more but shift sooner, say at 2500-3000rpm, then quick shift to 2nd and give big throttle again till about 2500-3000rpm, and repeat.

      It's hard to change how we drive, so for me it's hard to do that technique for a whole tank of fuel to test it out. However, from my initial tests there may be something to it afterall.
      Give it a try.


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