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Hybrid Cars, Fuel Efficiency (MPG)
#500 of 519 Re: General MPG question [bobw3]
Dec 21, 2005 (8:52 am)
if the RPM is a constant 1500 then the gas used would be the same...correct?
I don't believe so. It requires more fuel as the load increases. You can maintain the same RPM and use more fuel climbing a hill than idling at the same RPM.
#501 of 519 Re: General MPG question [gagrice]
Dec 21, 2005 (9:55 am)
"It requires more fuel as the load increases."
Sitting in your driveway, you would need to apply very little throttle to rev the engine to 2k rpm in neutral. With really tall gearing and overdrive, one could be traveling in excess of 80 mph (in some cars) at 2k rpm. Obviously, the rate of gas consumption (measured in gallons per hour) would be higher than just sitting in the driveway revving the engine in neutral (no load) at 2k rpm. Gas consumption is based more on throttle position than engine rpm.
#502 of 519 Re: General MPG question [rorr]
Dec 21, 2005 (11:33 am)
"Gas consumption is based more on throttle position than engine rpm. "
Why? All that the throttle position does is increase RPM. That's why I'm confused. Regardless of the gearing, for ever revolution of the engine, the gas is sprayed into the cylinder, and spark plugs fire. So for any given revolution of the engine, the same amount of gas is sprayed into the cylinders, regardless of whether you're at idle or climbing a hill.
Or does the fuel injectors spray a greater quantity of gas into the pistons if the engine is under a load?? That would be the only way that the extra load would mean more gas is used for a constant RPM.
My understanding is that it's only the increased RPM that makes gas usage higher. And I'm not talking about MPG...miles per gallon, but gallons used per revolution.
The mpg at idle is zero, but I'm trying to calculate how much gas is actually expended at idle, so I'm calculating a gallon per RPM.
#503 of 519 Re: General MPG question [bobw3]
Dec 21, 2005 (12:24 pm)
"All that the throttle position does is increase RPM."
But how do you think it increases rpm?
Increased throttle position increases the amount of fuel/air injected into the cylinder, hence more torque is generated, and the rpm increases (assuming there is no load on the engine keeping it from increasing in rpm).
"So for any given revolution of the engine, the same amount of gas is sprayed into the cylinders, regardless of whether you're at idle or climbing a hill."
Forget hills/idle/etc. Picture two identical cars, identical gearing, identical EVERYTHING, both driving down a level road at the same exact vehicle speed and engine rpm. Since both cars have the same engine rpm, they should both be getting the same mileage, right?
But what if car 'A' is pulling a 5000 lb. trailer? Under your theory, they are identical vehicles with identical engines turning identical engine rpm so they should get the same mileage.....do they?
#504 of 519 Re: General MPG question [rorr]
Dec 21, 2005 (3:04 pm)
You're right. Good example. You'd have to press the gas more to get the same 2000rpm when towing, putting more gas into the cylinders to get the same power to get the same rpm.
So how do you calculate how much gas is spent when idling?
#505 of 519 Re: General MPG question [bobw3]
Dec 21, 2005 (3:51 pm)
"...putting more gas into the cylinders to get the same power to get the same rpm."
You put more gas into the cylinders to get MORE power at the same rpm.
Torque (and by extension, horsepower) is not fixed at a particular rpm. When you look at a dyno sheet which plots torque (or hp) vs. rpm, these curves are always based on WOT (Wide Open Throttle) conditions. A dyno sheet of power vs. rpm would look much different (same basic shape but lower power/torque) at part-throttle conditions.
In other words, a particular engine at 2000 rpm is NOT always developing 'x' amount of torque (or hp); it varies depending on throttle position. The reason why car 'A' must dip further into the gas when towing that 5000 lb. trailor is because, at 2000 rpm, more torque is needed to lug that trailer around.
"So how do you calculate how much gas is spent when idling?"
I've absolutely no idea. You would have to get your hands on the data used to program the engine management computer in your particular car to determine how the programmed fuel flow rate was set for idle conditions.
You could approximate this (if you just really REALLY had to know):
1. Fill your tank and drive home.
2. With the car in the driveway, top off the tank from a gas can.
3. Idle the car for some set length of time (the longer the time, the more accurate the calculation).
4. At the end of the time interval, shut off the car and re-top off the tank keeping track of exactly how much fuel used. This should give you an idea of the gallons/hr fuel flow rate at idle.
If you do this in a garage, make sure you leave the garage door open.
Of course, this will all prove somewhat difficult to do with a hybrid for obvious reasons.....
#506 of 519 Re: General MPG question [rorr]
Dec 22, 2005 (7:22 am)
Thanks for the info. I don't know why I'm so curious, but I just am. One more thing. Why does the RPM in the engine increase when you step on the gas? You said, "You put more gas into the cylinders to get MORE power at the same rpm." So that makes me think that when you press the gas, the engine RPS would remain the same, but they don't?
#507 of 519 Re: Hybrid Gas Mileage: Good? Bad? As Expected? [Sylvia]
Dec 22, 2005 (11:59 am)
My 2005 Prius bought last October was getting around 40mpg going up a mountain, but that has dropped to between 35 to 38.4mpg lately. Should I take it to the dealer? I am rather disappointed.
#508 of 519 RE: Hybrid Gas Mileage: Good? Bad? (Silvia)
Dec 22, 2005 (12:07 pm)
On a trip yesterday on level ground going 200 miles round trip, I got 48.6mpg's, so I guess that going up the mountain cuts down alot. A friend has a 3 year old Prius and says she get around 40 going up the same mountain?
#509 of 519 Re: General MPG question [bobw3]
Dec 22, 2005 (12:20 pm)
" So that makes me think that when you press the gas, the engine RPS would remain the same, but they don't?"
Back to the car going down the road at 2000 rpm (at, say, 50 mph). The car will require SOME amount of hp to just maintain 50 mph (say 25hp). Which means that, AT THAT throttle position, at 2000 rpm, the engine is developing 25hp.
Then you step on the gas. More fuel/air goes into the cylinder and AT 2000 rpm, the engine is now making more torque, and hence more power (perhaps now it is making 75hp).
That additional 50hp must DO something. The added force applied to the top of the piston is translated into additional torque by the crankshaft. That additional torque is transmitted via the tranny/axles to the wheels where the added torque accelerates the vehicle (F=ma or Force = mass x acceleration).
If the car was on level road and the power requirements to maintain speed was 25hp and now the engine is developing 75hp, then the excess power is used to accelerate the vehicle. RPMs increase.
If the car has started up a hill, then the power requirements to maintain speed may increase from 25hp to 75hp. So, the gas pedal goes down, until the engine is now making 75hp. Since the energy produced is balanced by the energy requirements, there is no excess power and the speed remains the same.
I hope this helps.