Quote:
Originally Posted by Draigflag
Would 5th be too high? Revs too low? I find if the revs are too low and the engine labours, then you waste more fuel. In 4th I guess you would need less throttle which would help. But the speed is far too low to be practical for long journeys, I would need to do at least 100 kph.
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Disclaimer: It's been a while since I've had to put this into words but so far I for myself am satisfied with my opinion. I would need facts and proof to sway.
Info: I ride around on my Fabia with a obdb comp. giving me a nice readout constantly of several things.
Ok let's get into the details. The turbo diesel engines have a restriction in their airway and exhaust. The turbo. When you're not using it to make power beyond what the engine would be able to put out sans turbo it's a restriction. This is what gives the turbo diesels the typical specific fuel consumption chart. With a sweet spot around 1900/2200 (IF you're using the power needed you hit that spot (Around 70 to 85% of max torque @ specific rpm). Below a certain power level and rpm (When the turbo becomes a drag) fuel consumption per effective kw goes up.
Ideally you'd run a modern turbo diesel on the rpm and power level where the gain/drag is 0 of the turbo.
Given your 100kph that you need you can check the amount of fuel it's using then and use that as the amount of fuel/power you're using to get that speed. Recalculating the fuel usage from liters/100km to liters per hour takes in effect the speed/drag and will give you a nice indication of power you're asking of the car.
Example A. Using 3 liters per 100km while riding 33kph means you're only using 1 liter per hour. Using 3 liters per 100km while riding 100kph means you're using 3 liters per hour. So putting it bluntly you're putting 3 times more load on the engine. You can keep this in mind and once you have established your max fuel usage under full throttle at a certain speed and rpm you can get a feel for how much % power it is of the total available.
Example B: When i accelerate @ 100kph in fifth gear the fuel consumption is a maximum of 10 liters per 100km and per hour (rounded number).
When i accelerate @ 33kph full throttle in 2nd gear the fuel consumption is a maximum of 30ish liters per 100km meaning 10 liters per hour. same as the one @ 100kph. Because i'm asking the same power.
Now if i factor in the normal idle load fuel usage and how much it's using when i floor it in neutral (controls @ 3000rpm) 0.5 to 1 liter per hour, we may use this as a basis for own use and see a line there indicating increased drag of the turbo and resistance in the engine at higher velocities inside.
But it's only that little per hour. So
Example A again: So using 1 liter per hour total means i effectively put 0.5 liters of fuel into power to move. When i use 3 liters per hour at 100kph i'm using 2.5 liters of fuel to move. 5 times more than at 33kph. Keep in mind i'm rounding numbers.
Sweet diesel.
You can accelerate al be it very slowly at little over the fuel use needed to maintain speed. As you accelerate and keep the liters per hour at the same level you are in fact also increasing the power. Because the speed is increased and the same amount of liters per 100km is spend quicker.
So slow acceleration with a modern diesel will lead to better results. More drastically than a gas/benzine/petrol/peut engine. Where you can not even out the restriction (gas valve) at low power levels.
When you add a air valve in the inlet things get a little different.
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