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Idle fuel usage
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Thread: Idle fuel usage

  1. #1
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    Idle fuel usage

    I've always thought the fuel economy readouts being MPG or L/100km to be rather pointless, and would honestly prefer it in terms of mL/min, or something similar... I think its a more useful measure, but hey I'm a bit strange.

    It got me thinking about the fuel usage of our little cars at idle... Since the meter only goes to a minimum of 2.0L/100km for us in Canada, its nearly impossible to calculate. However, once the engine is fully warmed, below 31km/h the meter starts to rise again. Granted its not proper science to use such inaccurate measures, such as looking down while driving or even using two data points.. it came up with some interesting numbers.

    A bit of fun math:
    2.0L/100km = 50km/L
    speed = 31km/h
    to get mL/h, simple division of speed by fuel consumption.
    (31km/h)/(50km/L) = 0.62L/h, or 620mL/h

    Data point 2:
    2.9L/100km = 34.48km/L
    speed = 20km/h
    mL/h: (20km/h)/(34.48km/L) = 0.58L/h, or 580mL/h

    From that and being REALLY lazy, I've come to the conclusion that our little engines pull ~600mL fuel per hour at idle. Or just about 10mL per minute. That's a coke bottle full of gas, for an hour (granted, doing nothing).

    That's also about 2 teaspoons of gas for approximately 1600 explosions (idle RPM = ~800, 1 explosion per cylinder, every two revolutions).

    Its crazy how efficient engines have become.

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    Senior Member bryanintowson's Avatar
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    *goes crosseyed from math allergy*

    I agree. When my Jeep (4.0L) idles, I'm pretty sure I can actually watch the gas needle go down.

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    Here's my data points;
    - Relying on the cars computer to be accurate is not advised.
    - MPG or L/Km or Km/L is used because distance traveled can be controlled and measured whereas time spent idling in heavy traffic cannot.
    - You are assuming that the car is using twice as much fuel when rpm doubles which is not always the case.

    As to efficiencies being high, my BMW 5 series wagon that weighs ~3700 lbs with a 2.8L six-cylinder engine and a 5 speed automatic has achieved over 30mpg(12.75 km/L) at 70mph(110KPH) on the freeway on flat, straight roads. Its platform dates back to 1997.
    2014 Abarth, Rosso/Nero, basically stock w/a few trinkets.

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    Quote Originally Posted by JimmyTestarossa View Post
    Here's my data points;
    - Relying on the cars computer to be accurate is not advised.
    - MPG or L/Km or Km/L is used because distance traveled can be controlled and measured whereas time spent idling in heavy traffic cannot.
    - You are assuming that the car is using twice as much fuel when rpm doubles which is not always the case.

    As to efficiencies being high, my BMW 5 series wagon that weighs ~3700 lbs with a 2.8L six-cylinder engine and a 5 speed automatic has achieved over 30mpg(12.75 km/L) at 70mph(110KPH) on the freeway on flat, straight roads. Its platform dates back to 1997.
    - I can understand the computer being inaccurate, but the same can be said in terms of MPG measure; it can vary greatly in heavy traffic as well. The distance may be the same, but the fuel consumed won't be. If you're speaking about external devices (such as GPS) then yes that may as well be more accurate for distances, but obviously that has zero tie in to the vehicle's fuel system. You'd also be calculating end goal fuel usage anyway by going to the pumps and checking what amounts to full. That however, doesn't solve the question of idle fuel consumption. Its also why at 0mph or km/h, you have infinite MPG. Miles are zero, while the engine is still burning gas. Throw in a bit of speed, and now we have something to calculate. Precise reason why some of the big engine/luxury cars now have start/stop systems when the vehicle stops.

    In this case I'm assuming fuel consumption is reasonably accurate considering most ECUs are designed to operate at near ideal combustion levels whenever possible and again we're going with zero engine load. Accuracy IS needed here, otherwise emissions start going all over the place.

    - I have no assumptions in terms of changing RPM or fuel consumption. The thread's stated as idle fuel usage; the variable speed is me coasting in neutral down a mild grade, and applying brakes. There's zero engine usage aside from it preventing itself stalling.

    Reason being that regardless of driving at 32km/h, 50 km/h, or 100km/h, the bare minimum readout will be 2.0L/100km when in neutral or not using the throttle (Fuel consumption is insignificant relative to the distance traveled). The number rises below 31km/h, meaning that you're now consuming enough fuel per 100km traveled to be significant enough for it to display. I could grab another data point at 10km/h, but now we're hitting the lower limits of speed accuracy, as the car stops measuring speed around 5-7km/h.

    - moving efficiencies with fuel usage is not something I'm going to bother recalculating, we have an entire stickied thread for that. There's also far too many variables to consider for calculating actual fuel flow levels without grabbing it directly from the ECU (throttle level, which cam profile the multi-air's using, speed, engine RPMs, torque load....).

    - This was entirely out of curiosity to figure out what the fuel usage is, when you're not actually asking the engine to do anything.
    40L of fuel? your engine can run nearly 3 days continuously before the tank runs dry (2 days, 18 hours, 40 minutes).
    Idling for 20 minutes? you just burned 200mL of fuel (in Toronto prices, that's ~25 cents on premium).
    Last edited by Surnia; 11-08-2014 at 01:15 AM.

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    Senior Member E Rossi's Avatar
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    OK, so in 5th gear, give us the MPG/MPL going 35, 45, 55, 65 miles-per-hour.


    Rosso 2013 Fiat 500 Abarth from Bettenhausen Fiat, Tinley Park, Illinois. Madness Rear-Seat Delete. P-Flo Intake by Neuspeed. Competizione Upper Front Brace Bar. Dunlap Direzza DZ102, 205/45R16, 87WXL, Summer Tires.

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    Quote Originally Posted by E Rossi View Post
    OK, so in 5th gear, give us the MPG/MPL going 35, 45, 55, 65 miles-per-hour.
    I don't understand what purpose that would serve, if the US measure is the equivalent to the Canadian one, it'll cap at ~117MPG (2.0L/100km) with zero throttle. And maybe a few accidents since you're engine braking at that point on a highway or street... The engine's also no longer idling.

    Again the intent was to derive IDLE condition fuel usage. Engaging the transmission puts load on the engine, and its no longer in an "idle" state.

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