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Old 03-29-2012, 08:15 PM   #1 (permalink)
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need a hyper lesson

I finally got a OBD and connected it via Bluetooth to my Tablet. driving my regular commute today was a new experience. I always assumed that a car driving at 2000rpm was burning the same amount of fuel whenever it was at 2000 rpm. now I see that rpm has nothing to do with fuel consumption. Iwas amazed at Tbe jump in fuel consumption when climbing hills, and disappointed b y how little a difference coasting downhill or driving downhill actually make.

I am wondering now if I should be accelerating down hills and coasting as much as possible uphill.

my commute is constant ascent and descent one after another with very little flat ground.

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Old 03-29-2012, 08:33 PM   #2 (permalink)
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I generally let the speed increase and then bleed it off climbing the hill. The extent to which you do this depends on the max elevation of the hills. You could also try coasting downhill even if your speed increases. Think of the roller coaster and how it climbs and descends without any additional power. The closer you can come to that type of movement, the less fuel you will use.
What you are experiencing is changes in load independent of throttle position. Every hill gives you the opportunity to use the mass of your vehicle to store energy. While the mileage climbing the hill may seem terrible, remember you can coast downhill with the engine idling or even shut off (if its a manual transmission) and use no fuel whatsoever going downhill.

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Old 03-29-2012, 08:44 PM   #3 (permalink)
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I know the average fuel use is the one I should focus on. I'll keep working on my driving. thanks.
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Old 03-30-2012, 02:07 PM   #4 (permalink)
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another thing I have noticed is that 105kmh seems to be more efficient than 90. even neutral coasting gets more efficiency at higher speed. I need more time to actually understand y OBD but it seems That way to me.
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Old 03-30-2012, 02:21 PM   #5 (permalink)
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Quote:
Originally Posted by redorchestra View Post
I always assumed that a car driving at 2000rpm was burning the same amount of fuel whenever it was at 2000 rpm. now I see that rpm has nothing to do with fuel consumption.
I see you are ready to enter the secret BSFC chamber...
In short, the amount of fuel your engine requires depends not only on rpms, but also on load (how much of its power-producing potential is needed at given rpm). Each engine has its own BSFC chart, ie how much fuel is used per unit of energy, usually in g/kWh, depending on engine speed and load. Here is an example:
Notice the little '250 island' in the middle? That's where you want to keep that engine as much as possible, since it uses the least amount of fuel for what it's doing. In other words, that engine is most efficient around 2600-3400 rpm and 70% load. The "sweet spot" is different for each engine, but it's usually around the max torque rpm and 60%-90% of load. Load depends on more than just how much your gas pedal is pressed, so something like a ScanGauge is worth its weight in gold if you're hoping to keep your engine in its most efficient area. A BSFC chart, like the one above, for your specific engine is also very valuable, but often hard to find. See this thread for more info.

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I am wondering now if I should be accelerating down hills and coasting as much as possible uphill.
The method Old Mech mentioned works well, but a few months ago someone here posted his test results which suggest that pulse&glide works well going uphill, even though the pulses are longer and the glides shorter.

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my commute is constant ascent and descent one after another with very little flat ground.
Are the hills long? Steep? Coasting down is the best, engine off if possible, then accelerating back up at most efficient BSFC (yes, that again). Or slowing down as you climb, then coasting back down. You'll have to compare.
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Old 03-30-2012, 02:26 PM   #6 (permalink)
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another thing I have noticed is that 105kmh seems to be more efficient than 90. even neutral coasting gets more efficiency at higher speed. I need more time to actually understand y OBD but it seems That way to me.
The faster you go while coasting the more mpg you will get because you are using a constant amount of fuel (or no fuel if EOC), thus you cover more distance in the same time while using the same amount of fuel.

All cars have different points of efficiency over the RPM range. Using a computer to see exactly which RPM amount gives you the most efficiency is the key.

If I were the OP I would use pulse and glide over the terrain. Accelerate briskly going up the hills and then turn the engine off and coast down the hill until you reach the bottom or near. You will then have to accelerate quickly to your target speed up the next hill and so on. This may seem to be bad for mpg because of the acceleration up hills but depending on your coasting distances and speed range the average mpg over your trip will be much higher than just driving at a steady speed.
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Old 03-30-2012, 02:34 PM   #7 (permalink)
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When driving on hills, I try to avoid situation where car has to pull or 'drag' car up to hill, instead I try to gain speed from downhill that will carry me up, usually it is possible to do this by coasting at neutral.

Usually speed decreases to 60kph on uphill, some hills even lower, but at downhill speed is gained plenty more than enough.

Goal of 'game' is to allow speed to decrease enough that when you coast downhill police will not give you ticket.

However if there are other cars behind, then this is not quite possible to do as they (other drivers behind you) will react emotionally and pass when it is not safe etc.

105km/h can be more efficient only if gearing is such that one is well below powerband at 90km/h, but with better gearing it might be possible to get better economy at slower speed as aero drag increases lot when going from 90km/h to 105km/h, so smaller diameter tires might be worth to consider.

I know that my winter tires are too large diameter for hilly ground near me, that is part reason why I got so horrible fuel economy over winter.
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Old 03-30-2012, 05:58 PM   #8 (permalink)
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Er, smaller tires better fuel economy? I think it's the other way around...
The reason 90 could be worse than 105 is because despite the extra power required, your engine efficiency could be going up a lot from 90 to 105, possibly enough to provide a net benefit. A simplified way of thinking about it is the engine has some amount of friction that you can't remove, and higher load only increases this friction by a relatively small amount. If you're only asking the engine to give you barely any torque, then most of the power made is actually going to overcome the engine's friction. If you ask the engine for say twice the torque, then you don't need twice the fuel, because most of the fuel was going to waste in the first place. If enough fuel was going to waste in the first place, then it's possible that the increased load could improve your mpgs.
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Old 03-30-2012, 07:22 PM   #9 (permalink)
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Er, smaller tires better fuel economy? I think it's the other way around...
Often it is, but when driving on hilly ground it can be working against you as rpm goes too low, so you need to shift down. With smaller wheels my car manages to climb hills much better, engine stays on powerband and with diesel less load is always better than less rpm, when you just keep it off the boost, which means there is limit to how much rpm you can have. Boost means lot of enrichment.

On level ground bigger wheels are really helping my economy, but not when there is hills.

As was posted previously, one can see that 250 area, with bigger wheels it is not possible stay in there at all on hilly area. (We have warning signs of 7% hills here for example).

With diesels at least higher load increases fuel usage a lot, you will start to get black smoke from end of tailpipe telling there is lot of unburnt fuel, efficiency is not optimal, better to stay at maximum torque with smaller wheels, better speed on 5th than with bigger wheels and 4th at maximum torque rpm so economy is better too.

Below 1800rpm for me is where there is nothing happening in motor, even I push pedal to floor there is very little happening except that black cloud and above 2200 boost enrichment adds lot of fuel, so that is also causing issue in my case, speed with bigger wheels 5th gear and max torque (around 2000) would be illegal by expensive amount, so that is also why bigger wheels will cause more fuel consumption.

Perfectly possible, such has happened many times, there is certain maximum that is going to be optimal, go above and there is impact to economy, go smaller and again there is impact to economy, finding that optimal is not always easy.
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Old 03-30-2012, 08:45 PM   #10 (permalink)
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Ah I see you're talking about a turbo diesel. I'm not too clear on how different a diesel's powerband looks, but if you shift down, it doesn't hurt economy as long as you're operating the engine near peak efficiency right? Shifting down reduces the load requirement anyways.

I guess the turbocharger + diesel combination changes the rules a little since diesels pass a larger volume of air through at low load than gas engines, and would build up boost fairly quickly. I suppose I didn't think of the fact that higher rpm on a diesel means a lot more gas going through the turbine.

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