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519 messages, Last post on Jan 16, 2007 at 6:14 AM
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Replying to: robert47 (Dec 20, 2005 11:28 am) |
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Replying to: robert47 (Dec 20, 2005 11:28 am) |
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Maybe someone here can help me with my question. If an engine is going a constant RPM, say 1500, then regardless if it's idling, up a hill, down a hill, or on flat ground, if the RPM is a constant 1500 then the gas used would be the same...correct? So if at 60mph on cruise it's at a constant 2000rpm on flat highway and asuming I get 30mpg at 60mph, that means I used 2 gallons in one hour of driving at 2000rpm...correct? So if my idling RPM is 1000rpm, then am I correct to say that I'll use 1 gal for every hour I'm idling. So if I idle my car for 10min every morning, then every 6 days I've used a gallon of gas.
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Replying to: bobw3 (Dec 21, 2005 7:49 am) 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.
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Replying to: gagrice (Dec 21, 2005 7:52 am) Correct. 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.
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Replying to: rorr (Dec 21, 2005 8:55 am) 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.
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Replying to: bobw3 (Dec 21, 2005 10:33 am) 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." No. 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?
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Replying to: rorr (Dec 21, 2005 11:24 am) So how do you calculate how much gas is spent when idling?
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Replying to: bobw3 (Dec 21, 2005 2:04 pm) Almost there. 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..... 
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Replying to: rorr (Dec 21, 2005 2:51 pm)
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