Hybrid Gas Mileage Good? Bad? As Expected?

519 messages, Last post on Jan 16, 2007 at 6:14 AM

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What is this discussion about? Hybrid Cars, Fuel Efficiency (MPG)

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#504 of 519 Re: General MPG question [rorr] by bobw3 Dec 21, 2005 (2:04 pm)
Replying to: rorr (Dec 21, 2005 11:24 am) 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? Replies to this message: rorr (Dec 21, 2005 2:51 pm) bobw3 (Dec 22, 2005 6:22 am) rorr (Dec 22, 2005 11:20 am)
#505 of 519 Re: General MPG question [bobw3] by rorr Dec 21, 2005 (2:51 pm)
Replying to: bobw3 (Dec 21, 2005 2:04 pm) "...putting more gas into the cylinders to get the same power to get the same rpm." 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..... Replies to this message: bobw3 (Dec 22, 2005 6:22 am) rorr (Dec 22, 2005 11:20 am)
#506 of 519 Re: General MPG question [rorr] by bobw3 Dec 22, 2005 (6:22 am)
Replying to: rorr (Dec 21, 2005 2:51 pm) 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? Replies to this message: rorr (Dec 22, 2005 11:20 am)
#507 of 519 Re: Hybrid Gas Mileage: Good? Bad? As Expected? [Sylvia] by punawele Dec 22, 2005 (10:59 am)
Replying to: Sylvia (Jun 29, 2004 10:34 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) by punawele Dec 22, 2005 (11:07 am)
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] by rorr Dec 22, 2005 (11:20 am)
Replying to: bobw3 (Dec 22, 2005 6:22 am) " So that makes me think that when you press the gas, the engine RPS would remain the same, but they don't?" Good question. 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.
#510 of 519 56 mpg by robert47 Dec 22, 2005 (7:42 pm)
Hi folks, I hope you are all enjoying your hybrids. I could made 56 mpg on 87 miles today.
#512 of 519 Re: I can explain that CR thing completely so we can leave it alone... [lar by stevedebi Feb 17, 2006 (12:43 pm)
Replying to: larsb (Nov 21, 2005 12:00 pm) "As for the 35 MPG figure, that's one or a few drivers who are not trained for hypermiling and in fact could probably CARE LESS what MPG they get driving the car - it's not their car or their money paying for the gas. They probably drove the car without even knowing there is a way to keep in in EV mode for as long as possible. That one little test of MPG has no bearing on how real drivers who own the car and learn the car over time can perform MPG-wise. Any uneducated driver can get into a Hybrid and produce bad results. But if they OWN the car and are paying for the gas out of their OWN POCKETS and the car payment out of their own pockets, they are smart to learn to maxmize the MPG of the car using the tools provided by Toyota." Pardon me if this is an obvious question, but if a driver can achieve good numbers by following Toyota guidance (as perceived by the efficiency displays in the Prius), why can't Toyota program the Prius computer to make those same efficiency decisions? I realize that some of the techniques are driver only (timing the stoplights, for example), but EV mode? Why not at least put in a button to allow the user to force the car into better MPG?
#513 of 519 Re: Hybrid Gas Mileage: Good? Bad? As Expected? [Sylvia] by hybriddriver1 Feb 28, 2006 (8:06 am)
Replying to: Sylvia (Jun 29, 2004 10:34 am) Careful driving - no hard acceleration in city and under 65mph highway: over 30mpg Normal driving - some hard acceleration in city, under 75mph highway: 27 - 28 mpg Extreme driving - Aggressive acceleration in city, 80mph and up highway: 22mpg Conclusion: even driving this vehicle very hard, my Ford Escape Hybrid can give fuel economy better than the Ford Escape XLT, which according to Ford is a similar driving experience from the point of view of acceleration etc. The Escape Hybrid is probably a little less sporty, in truth, but it does feel to me much quicker than the Honda CRV AWD I drove previously (which gave me under 20mpg in hard driving, by the way). My full review ...">link title Replies to this message: hot_georgia (Mar 19, 2006 11:38 am)