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  1. #81
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    Default Re: For the clever guys!

    Kay...got the answer. ..One big 5 liter single piston, long stroke engine. ..problem solved...and maybe a 2 stroke ..
    Last edited by Kelpie; 2017/01/14 at 04:43 AM.

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    Default Re: For the clever guys!

    Quote Originally Posted by Eggie View Post
    No, it has mistakes in it.
    If you say so...

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    Default For the clever guys!

    A simple test in a vehicle with an onboard computer displaying the fuel consumption will give the answer for that specific vehicle.
    Last edited by Nielbd; 2017/01/14 at 06:51 AM.
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  5. #84
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    Default Re: For the clever guys!

    Quote Originally Posted by JBX View Post
    If both vehicle are travelling at exactly 4000 rpm in exactly the same road conditions, then the fuel consumption will be the same.

    The amount of fuel used is determined by the number of injections and hence the revolutions. It may vary a bit pending throttle opening.

    If both vehicle were travelling at the same speed (say i.e 100kmph) then the 5th gear vehicle will have lower rpm and less injections of fuel, hence less fuel will be consumed.

    The lower the rpm, the better fuel consumption provided the vehicle is driven within the torque band. If the rpm / throttle ratio is to low, then the amount of fuel injected will rise.
    This is an over-simplification of a question which is complicated by many factors. The main fuel guzzler at those speeds is wind resistance, hence the vehicle traveling at 120 km/h will tend to use more fuel than the one traveling at a lower speed even though it is traveling in a higher gear. Gearing and other design features of the specific vehicles will also play a role so the answer is not as cut-and-dried as it may appear to be.

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    Default Re: For the clever guys!

    See this as a journey in time. An engine at 4000 in 4th at say 100kmh uses less fuel per minute than an engine at 3000 in 5th at say 120kmh, but in 5th less minutes are spent on journey. Will fuel consumption journeywise not be roughly the same? Less fuel per more minutes versus more fuel per less minutes? Imho
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  7. #86
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    Default Re: For the clever guys!

    "All else being equal, having a vehicle that does 100 in 4th at 4000 rpm and 120 in 5th at 4000 rpm it should use less fuel in 4th."

    Yes but to reach your destination you are longer on the road so the fuel saved is used up again, iykwim.
    Ok I see Julius has the same thoughts.
    Last edited by Damdan; 2017/01/16 at 08:09 AM.
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  8. #87
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    Default Re: For the clever guys!

    I once did a calculation based on engine load, rpm, Air fuel ratio, engine capacity and it worked out surprisingly accurate compared to the real consumption.
    The point I got to was if you drive a brick you're better of the slower you go <120kmh
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  9. #88
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    Default Re: For the clever guys!

    Quote Originally Posted by 3bar View Post
    it will haveto do with the accelerator pedal position

    if 4th gear is 30% throttle
    5th gear is 50% throttle

    then 4th gear will be more economical

    its all about LOAD
    Above answer with wind resistance which gets piled on with the velocity squared !

    Then you have basically the answer (without being too technical)
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  10. #89
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    Default Re: For the clever guys!

    The fuel consumption in 5th gear at 4000 min/-1 will always be higher, since this results in a higher speed and subsequently at least about 40% more drag due to the wind resistance, which increases exponentially with speed. So your engine needs to develop more power at the same revs. Typically, the consumption of e.g. a Golf Gti is around 135g/kwh at 100 km/h and increases to about 177 g/kwh at 120 km/h.
    Also take it that way: If you drive 100 on a level road your accelerator pedal is kept constant. The moment you get a slight incline you have to push the pedal a bit harder. That increase in the road incline will need more power from the engine. Same happens when you drive at a higher speed, you need more power to maintain the speed.
    The secret is to drive in the most economical band - if you drive at the same speed at a higher gear you will most likely come closer to this band.
    Maybe this specific consumption graph can illustrate this a bit.
    Name:  specific consumption.png
Views: 443
Size:  351.6 KB

  11. #90
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    Default Re: For the clever guys!

    The fuel consumption in 5th gear at 4000 min/-1 will always be higher, since this results in a higher speed and subsequently at least about 40% more drag due to the wind resistance, which increases exponentially with speed. So your engine needs to develop more power at the same revs. Typically, the consumption of e.g. a Golf Gti is around 145g/kwh at 100 km/h and increases to about 177 g/kwh at 120 km/h.
    Also take it that way: If you drive 100 on a level road your accelerator pedal is kept constant. The moment you get a slight incline you have to push the pedal a bit harder. That increase in the road incline will need more power from the engine. Same happens when you drive at a higher speed, you need more power to maintain the speed.
    The secret is to drive in the most economical band - if you drive at the same speed at a higher gear you will most likely come closer to this band.
    Maybe this specific consumption graph can illustrate this a bit.
    Name:  specific consumption.png
Views: 443
Size:  351.6 KB

  12. #91
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    Default Re: For the clever guys!

    I tested this over the weekend & conclusion is that Fuel usage at 4000rpm is zero ...as the autobox keeps on shifting to 5th before I reach 4000rpm
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  13. #92
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    Default Re: For the clever guys!

    When I looked at the equation as stipulated, I concluded the following:

    4th - 4000rpm - 100kph => 1 hour traveled = 100km => 4000rpm*60mins =>240000 detonations for 100km
    5th - 4000rpm - 120kph => 1 hour traveled = 120km => 4000rpm*60mins =>240000 detonations for 120km [200000 detonations @100km mark]

    Thus, to travel 100km in 4th vs 5th, you'll get there quicker [50mins], and use less detonations. If the road was completely flat and in a complete vacuum, faster would be better!! However, the ECU calculates your fuel injection based on your throttle input and vacuum sensor, which will increase as the resistance becomes bigger.

    This will result in much more fuel required to maintain 120kph vs 100kph - the question here is, how much?? This will clearly depend on the motor, torque curves, aero, etc., but when one looks at an EGT gauge and how big an impact a small throttle movement has on temps, my guess is that many will be shocked at how big the impact of air resistance really is - just hook up a caravan...

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  14. #93
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    Default Re: For the clever guys!

    Let's add an additional question to make it clearer to the professors here:

    Q1: "One drives in 1st gear 4000rpm, and the other 5th gear 4000rpm. Will the fuel consumption be the same?"
    Q2: "One drives in 4th gear 4000rpm, and the other 5th gear 4000rpm. Will the fuel consumption be the same?"

    A1 and A2: The vehicle in the lower gear will consume the most fuel as it's covering less distance at the same rpm.

    This is the reason we have gears, so you can go faster, further, more efficiently.



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  15. #94
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    Default Re: For the clever guys!

    Hi all - I normally do not post on any forums, but this question got me thinking, not that I am a clever guy and maybe my logic is wrong.

    Question : 2 identical petrol cars at 4000rpm, one doing120km/h in 5th and other in 4th doing 100km/h: (assuming 4000rpmis in the torque range and probably a lot more other assumptions to simplify)

    Difference in fuel consuming?

    Trying to simplify what already been said and my logic, withoutgetting technical:

    Mostly depends onresistance at play.

    That being wind, tyres, internal friction, ect.

    Think we can agree that:

    Both cars in a vacuum and minimal other resistance, should beeasy that 5th would consume less.

    Same fuel gets consumed for same period of time but at120km/h you’ll drive further. This less fuel per kilometre travelled.

    Depending with an increase in resistance, whicheverresistance, the fuel consumption will start getting closer to each other.

    Thus a platcar with minimal internal or external resistanceswill be more economical in 5th gear than 4th, other than a LDV or SUV.

    With more resistance introduced, being wind, tyres orwhatever internal resistance, the differential between 4
    th
    and 5th will become less, since more throttle input would be required to maintain 4000rpm at 120km/h with increased resistance.

  16. #95
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    Default Re: For the clever guys!

    Quote Originally Posted by xdoomx View Post
    Let's add an additional question to make it clearer to the professors here:

    Q1: "One drives in 1st gear 4000rpm, and the other 5th gear 4000rpm. Will the fuel consumption be the same?"
    Q2: "One drives in 4th gear 4000rpm, and the other 5th gear 4000rpm. Will the fuel consumption be the same?"

    A1 and A2: The vehicle in the lower gear will consume the most fuel as it's covering less distance at the same rpm.

    This is the reason we have gears, so you can go faster, further, more efficiently.




    A1 Correct
    A2 4th gear

    It is unfortunately not a question with a " set in stone" answer once you start comparing all the gears at all speeds. I everybody that disagrees that 4th is heavier on fuel than 5th gets a level piece of road and compare 4th and 5th(or 5th and 6th) at 100km/h and 120 km/h. Use 4000rpm if you have a petrol and 2500rpm if you have a diesel and report back.
    Most of you will be surprised and may even not report back.
    P.S leave the math at work and report back honest actual figures.
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    Default Re: For the clever guys!

    Quote Originally Posted by Fishbj View Post
    Hi all - I normally do not post on any forums, but this question got me thinking, not that I am a clever guy and maybe my logic is wrong.

    Question : 2 identical petrol cars at 4000rpm, one doing120km/h in 5th and other in 4th doing 100km/h: (assuming 4000rpmis in the torque range and probably a lot more other assumptions to simplify)

    Difference in fuel consuming?

    Trying to simplify what already been said and my logic, withoutgetting technical:

    Mostly depends onresistance at play.

    That being wind, tyres, internal friction, ect.

    Think we can agree that:

    Both cars in a vacuum and minimal other resistance, should beeasy that 5th would consume less.

    Same fuel gets consumed for same period of time but at120km/h you’ll drive further. This less fuel per kilometre travelled.

    Depending with an increase in resistance, whicheverresistance, the fuel consumption will start getting closer to each other.

    Thus a platcar with minimal internal or external resistanceswill be more economical in 5th gear than 4th, other than a LDV or SUV.

    With more resistance introduced, being wind, tyres orwhatever internal resistance, the differential between 4
    th
    and 5th will become less, since more throttle input would be required to maintain 4000rpm at 120km/h with increased resistance.
    You forgot about the throttle opeing being smaller in 4th than in 5th. So you are not using the same amount of fuel but actually more. So might be traveling further for the same duration but it comes a a price.
    Last edited by JuanB; 2017/01/16 at 11:29 AM.
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  18. #97
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    Default Re: For the clever guys!

    Quote Originally Posted by jelo View Post
    sort of, yes

    to achieve 120km/h regardless of gearing will always require more fuel than 100km/h

    F = MA

    Hence if Acceleration (meters / sec squared) increases, and Mass is constant, then Force increases too.

    It's called physics.
    Except that when you're doing a constant 100km/h or 120km/h, the acceleration is? Zero. You need to apply the static principle, not the dynamic one, where the sum of all forces is null. And then you can say that the most important force, the drag, increases with the (square of the) speed, therefore the driving force must increase too.

    I haven't read all posts, but the question's context was a bit too vague. If you stick to the question, and simplify the parameters greatly, same RPM = same fuel consumption but in l/hr, or any unit of volume over time.
    Then again you can assume that the vehicle in 5th gear will be going faster, then when you look at the consumption in l/100km or km/l (volume over distance) then yes, it will be more economical.

    But in the end, it's much more complicated, depending on your own hypothesis.

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    Default Re: For the clever guys!

    Just had a good laugh here.. Happy new year too all (first post of the year..)

    Now lets make it more complex like Fluffy suggested it too be.. Looking at the post by 3Bar with regards to throttle opening..

    Now you are running 4000rpm with a motor using electronic fuel injection.. RPM is constant, where are you on the timing advance, what is your air speed in the intake (Biltong touched on this), is the motor struggling in general at this rpm and is it adjusting AFR (enriching) to keep the rpms high.. Is it a VVTI motor that wants to rev and is happy at higher rpm.. Number of valves will also play a role as the naturally aspirated motors will show the difference between new and old tech here.. As a rule less valves means power lower down on the rpm range.. Hmm, what about reciprocal mass?

    When I did the feul injection on the Hilux I played with vacuum (intake pressure) as it was mentioned that economy in 4th will beat that of 5th at same speed (unlike this problem).. The difference was way smaller than expected, sure I had higher rpm for a prolonged period, but the throttle opening saw me burn less air, being closer to the kW peak also so the motor content to rev.. Fuel consumption was within 10%, it was lighter in 4th, but a more pleasant drive in 5th.


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  20. #99
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    Default Re: For the clever guys!

    Quote Originally Posted by jelo View Post
    sort of, yes

    to achieve 120km/h regardless of gearing will always require more fuel than 100km/h

    F = MA

    Hence if Acceleration (meters / sec squared) increases, and Mass is constant, then Force increases too.

    It's called physics.

    If the speed stays constant, you don't have acceleration (A =0), I think the answer is in the fact that the energy needed to maintain constant speed is a function of the speed (in m/s) squared ?

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    Default Re: For the clever guys!

    Quote Originally Posted by Willied View Post
    Just had a good laugh here.. Happy new year too all (first post of the year..)

    Now lets make it more complex like Fluffy suggested it too be.. Looking at the post by 3Bar with regards to throttle opening..

    Now you are running 4000rpm with a motor using electronic fuel injection.. RPM is constant, where are you on the timing advance, what is your air speed in the intake (Biltong touched on this), is the motor struggling in general at this rpm and is it adjusting AFR (enriching) to keep the rpms high.. Is it a VVTI motor that wants to rev and is happy at higher rpm.. Number of valves will also play a role as the naturally aspirated motors will show the difference between new and old tech here.. As a rule less valves means power lower down on the rpm range.. Hmm, what about reciprocal mass?

    When I did the feul injection on the Hilux I played with vacuum (intake pressure) as it was mentioned that economy in 4th will beat that of 5th at same speed (unlike this problem).. The difference was way smaller than expected, sure I had higher rpm for a prolonged period, but the throttle opening saw me burn less air, being closer to the kW peak also so the motor content to rev.. Fuel consumption was within 10%, it was lighter in 4th, but a more pleasant drive in 5th.
    The vehicle in question is a Jimny. 4000rpm is it's happy place. The VVT has switched in and it is at peak torque.
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