best fuel mileage

Discussion in 'Technical' started by 88Ayrton90Senna91, Nov 16, 2011.

  1. part 1. will you get the best fuel mileage (MPGS) at lower rpms, ignoring factors such as the speed of the car and aerodynamics. lets just say the car is kept in first gear. so will your engine get the most MPGs at lower rpms? does the gear your in effect MPGs? i know the higher the gear the faster the speed of the car so more drag and less MPGs but what if you were to put the car on a dyno and have the wheels moving but not the car, would 1000 RPMs in 1st give you the same fuel consumption as 1000 RPMs in 5th. also please note this is about Miles per Gallon, not fuel efficiency in terms of gasoline being converted into power.

    part 2. if an engine will get the most MPGs running at lower rpms what happens when your vehicle is put under a load and struggles at said rpms. for example assume your engine is most efficient at 1000 RPMS (lets just say this was found out when your vehicle was in neutral or drive wheels had no load on them) but at this RPMs in first gear your car struggles and sputters to keep this RPMS. even though the engine is at 1000 RPMS its struggling, will this return the same MPGs, again ignoring vehicle speed.
     
  2. the fuel mileage will be the same along all gears at the same RPM.

    if the car is put under load (towing something) it requires more revs to move said load at speed thereby using more fuel. if the car is struggling at a low rpm it can cause damage and therefore is better to speed up a bit.
     
  3. #3 Fletzu, Mar 20, 2012
    Last edited by a moderator: Apr 25, 2016
    Ideal fuel economy is achieved between max HP rpm and max Torque rpm.
    In therms of mpg fuel economy gets at it`s best at around 90 kph/55 mph.

    Dunno how much of engineering do you understand, but this is how a average fuel graph looks like http://expha.com/articles/images/10_graph_2.gif
     
  4. low revs definately helps. as does breaking as little as possible; anticipate and just let the car roll, try too maintain your speed as much if you can, so you dont have to accelerate back to the speed you were doing.

    i dont know its correct, but i think the slower youre going (at constant speed) the less fuel you use.

    ive been trying to put all this in practice and see what i can do. ive currently dropped to a little over 5 litres per 100 km, but think i can do better. the factory gives it 5,8 per 100 km <A BORDER="0" HREF="http://www.supercars.net/PitLane?displayFAQ=y"><IMG BORDER="0" SRC="pitlane/emoticons/smile.gif"></A>

    honda is amazing for fuell efficiency. im driving a 4 cilinder 1,4 autobox on petrol. thought i would do way worse than this.
     
  5. Part 1. Will you get the best fuel mileage (MPGS) at lower rpms, ignoring factors such as the speed of the car and aerodynamics lets just say the car is kept in first gear. So will your engine get the most MPGs at lower rpms? - you will get the best fuel mileage when you are at the highest BSFC (Brake specific fuel consumption) at which you are at the minimum power needed to move the vehicle (this is engine specific, see an example map below). To determine what rpm will yield the best mileage, simply draw a horizontal line across the BSFC map below at the power required, where ever the BSFC is the highest will yield the best mileage.

    Does the gear your in effect MPGs? I know the higher the gear the faster the speed of the car so more drag and less MPGs but what if you were to put the car on a dyno and have the wheels moving but not the car, would 1000 RPMs in 1st give you the same fuel consumption as 1000 RPMs in 5th. - No, there are two main things in play here, one is rotational inertia and the other is gearing efficiency. Rotational inertia only effects dynamic situations (accelerating), but there is higher rotational inertia with the lower the gearing. Gear efficiency for the most part increases at the ratio increases towards 1:1, as most vehicle have over drive, the highest (or higher) gears are usually more efficient.

    part 2. if an engine will get the most MPGs running at lower rpms what happens when your vehicle is put under a load and struggles at said rpms. for example assume your engine is most efficient at 1000 RPMS (lets just say this was found out when your vehicle was in neutral or drive wheels had no load on them) but at this RPMs in first gear your car struggles and sputters to keep this RPMS. even though the engine is at 1000 RPMS its struggling, will this return the same MPGs, again ignoring vehicle speed. - Well first it will not necessarily get the best mileage at lower rpm's as I stated about. Looking at the example BSFC map again, if you stay at the same rpm, and increase the load on the vehicle, this will move up the spot that your engine is operating at on the map. The change in BSFC is engine specific, however usually going from no load to some load increases BSFC. Now even though you increased efficiency here, you are using more power, and because power is directly related to fuel consumption, you will use more fuel as you increase load by any manner.
     
  6. this, though to add to your first point, as long as the engine load is the same (or at least the fuel setting is the same at identical RPMs)
     
  7. but how does it do if im not hauling around topographical maps
     
  8. You dont.
     
  9. are you sure gear multiplication and friction arent factors at that point?

    1st gear at 4000rpm 2.66 gear ratio , 3.42 final drive

    vs

    5th gear at 4000rpm .78 gear ratio, 3.42 final drive.



     
  10. if you start off in first and work your way through the gears the difference in friction is so minute it barely registers.
     
  11. I will agree. There is not really a reason for the gearbox to have any more friction as long as the rpm is the same (on the input side). On the output side and differential, it will increase, but this will be small for the system overall.
     
  12. Fuel consumption. Alrighty! There are a few factors that affect fuel consumption when excluding maintenance, differences in models, etc, so on a single vehicle..

    Part 1

    1. Parasitic losses- This is your friction in bearings, gears, your tires, etc. For the most part, you want lower rpms to eliminate this. It also lowers part inertia the engine has to expend power to overcome. In most cases, the lower speed means less friction including friction internal to fluids, and better economy. However when it comes to fuel economy, won't mean much overall.

    2. Loads- There are an assortment of different ways the engine is loaded, from aerodynamics to your tire inflation. Fuel efficency at rpm all comes down to your torque curve. The peak of the torque curve is where your engine is most efficent. For driving, the closer you are to this torque curve, the more efficent your fuel combustion will be, and as such, the less air/fuel intake that is needed (ie more closed throttle position, etc).

    Example, my Astra, I cruise around the city. at about 2500 rpm in my 1.8L Ecotec with 4v/cyl engine. At 50km/h 3rd is my most efficent gear. I can feel the engine isn't working hard and with fuel range readout etc, I can empyrically see that I get better mileage than in 4th which bogs the engine. Unless I speed up a bit to say 65 to get back into the powerband. Comparatively, my trans am gets almost the same fuel economy in 3rd gear 50km/h with it's LS1 in city (this is cruising and does not include acceleration) gets about similar, not quite as good, but with a 1600rpm cruising speed and lower end torque curve, it runs efficently just fine.

    This is due to when cruising, you are only producing the power needed to overcome your load and friction. The TA is only a bit heavier, but the FR powertrain is a bit more efficient, and the engine producing needed power at lower rpm means less parasitic losses. Bad fuel economy is during the acceleration where you are loading the engine outside of efficiency and power is being wasted.

    Part 2.

    Well, guess I answered that in part 1. It all comes down to producing power to overcome your load. In overdrive instead of drive, you first off might be off your torque curve, too low of rpm and your engine is pumping fuel in like crazy to overcome the load. In a lower gear, you might end up having the power but by reving faster, in the end you are burning the same fuel quantity. If you wanted better fuel economy under a struggling load, you would actually want to reduce speed a little to run in a lower gear. Not for air resistance etc, but simply to get yourself into the powerband.

    There are two parts to powerband as well however, and that is gearing torque multiplication. In Drive, the straight 1:1 ratio will be putting the power to the road needed to keep moving. If in overdrive, there is a torque loss going to the read diffy, this torque loss alone might bring you out of an efficent zone and the engine will end up running inefficient just to overcome any loads.

    So to sum up, ideal fuel efficieny isn't so much rpm related but torque curve related. The most fuel efficient place is where you are needing to give it minimum throttle. More closed throttle means a lower air/fuel intake and higher fuel efficiency over time.
     

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