LTC8K6
Lifer
- Mar 10, 2004
- 28,520
- 1,576
- 126
No, I don't. There is only one way to calculate mpg, how many gallons of gas did you use to travel x amoung of miles. MT used that method. If they had somehow invented a new way to caculate mpg then I would have an issue.
As has already been stated, that would be accurate, and there is nothing ludicrous about it. Assigning a mpg figure to an EV would be like assigning an rpm rating to a solid state drive. It's not ludicrous, it just doesn't make any sense.
The EPA has proposed the standard of 33.7kWh/1 gal for comparison purposes of electric cars to gasoline cars, so your proposal isn't the only the way. My electric bills using kWh, not joules, making cost analysis easier with kWh. It makes more sense to use standards people are already familiar with to aid in adoptions of new ones.
Indeed, thanx for the correction.
No it isn't. That car only drove 40 miles. The other 250 miles is wasn't driving, it got a ride.
Quick grade 10 physics problem. Which uses more energy: accelerating a car up to 60mph in 20 seconds or accelerating in just 10 seconds? The answer absolutely will not shock you, but I'm asking anyway
I don't see how that would make any difference. Regardless of how you drive the car, when the battery depletes it starts the gas motor. If no miles were run on the gas motor as a few of MT's trips didn't, then it doesn't matter how they were driving, they hadn't depleted the battery when the trip was over.
"The Detroit News reported an all-electric range of 32 miles (51 km) driving in a "very un-eco-friendly manner" and a total fuel consumption of 0.9 US gallons (3.4 l; 0.75 imp gal) for the 75 miles (121 km) trip. USA Today reported an EV range of 40 miles (64 km) driving more than an hour of stop-and-go through Detroit suburbs and the rolling Michigan countryside. Detroit Free Press reported an all-electric range of 46.2 miles (74.4 km) driving normally and using the air conditioning. Motor Trend first test review reports a combined fuel economy of 126.7 miles per US gallon (1.856 L/100 km; 152.2 mpg-imp)."
sdifox
No Lifer
- Sep 30, 2005
- 100,692
- 18,029
- 126
And in the case of Volt it got a ride off the battery. Bottom line is MPG is not a good unit to use for electric and Volt type hybrid.
LTC8K6
Lifer
- Mar 10, 2004
- 28,520
- 1,576
- 126
Quick grade 10 physics problem. Which uses more energy: accelerating a car up to 60mph in 20 seconds or accelerating in just 10 seconds? The answer absolutely will not shock you, but I'm asking anyway
Which weighs more? A ton of Volt or a ton of LEAF?
No. Any distance when the vehicle is not propelling itself is not included in calculations of fuel consumption.
Duh. The ton of leaves. How do you weigh a volt?
Good point.
Anyway, the reason flooring it doesn't ruin mileage is because electric motors are most efficient when they are loaded. Efficiency drops like a rock once the thing is up to speed. You use 2W of energy and only 1W of that goes to your load. The rest is consumed by the motor itself.
Simple question. What is the efficiency of a motor with no load? 0%. It's chewing through power, but it's not doing any work.
Here's a graph I found on google. Electricl motor efficiency curve.
It's actually a very common shape for a graph. Here is the efficiency of a computer's power supply vs load.
Now here's a graph showing how high compression engines are more efficient than low compression engines.
For gasoline engines, peak efficiency is often higher rpm than peak torque. For something like my Corolla, it's at max efficiency with the throttle open all the way and the engine spinning at about 4500rpm. Great for efficiency, not so easy on the car.
http://en.wikipedia.org/wiki/Engine_efficiency
wikipedia said:
One thing I've always wondered is why we don't put gasoline in diesel engines. You know that effect where your put regular gasoline in a high compression engine and it runs like dog shit because the gasoline is exploding before the thing even has a chance to spark? That's exactly how a diesel engine works. You compress diesel until it explodes; diesel engines do not have spark plugs. So...... let's make a diesel engine with a compression rate of 15:1 then just use regular unleaded gasoline in it. Why the hell not? We just saw a graph showing that higher compression is more efficient, and this is one of the reasons diesel engines are more efficient (the other reason being that the fuel itself has more energy than gasoline).
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Good point.
Anyway, the reason flooring it doesn't ruin mileage is because electric motors are most efficient when they are loaded. Efficiency drops like a rock once the thing is up to speed. You use 2W of energy and only 1W of that goes to your load. The rest is consumed by the motor itself.
Simple question. What is the efficiency of a motor with no load? 0%. It's chewing through power, but it's not doing any work.
Here's a graph I found on google. Electricl motor efficiency curve.
It's actually a very common shape for a graph. Here is the efficiency of a computer's power supply vs load.
[img]
Now here's a graph showing how high compression engines are more efficient than low compression engines.
[img]
For gasoline engines,[B] peak efficiency is often higher rpm than peak torque[/B]. For something like my Corolla, it's at max efficiency with the throttle open all the way and the engine spinning at about 4500rpm. Great for efficiency, not so easy on the car.
[url]http://en.wikipedia.org/wiki/Engine_efficiency[/url]
[/QUOTE]
Uh... peak engine efficiency is almost always at the same, or lower, engine speed as peak torque. You need to skip these other graphs and take a look at a brake specific fuel consumption (BSFC) plot from a real IC engine like this one:
[IMG]http://i490.photobucket.com/albums/rr266/TestDrive100/CombinedBSFCOverlay21.jpg
The lines are called "iso-efficiency" lines, points of load and speed where the engine is creating power at the same efficiency. Notice that the units are grams/kw.h, or grams of fuel burned to generate 1 kw.h of brake energy (this means that powertrain and aerodynamic losses associated with a particular vehicle are not considered). You can clearly see that the peak thermodynamic efficiency of 255 g/kw.h is achieved right around the peak torque, at 2500rpm.
Also, here is one for a Corolla, notice that the peak efficiency is well below 4500rpm. The "previous engine" is appaently the 3VZ-FE corolla engine, fwiw.
Essentially this type of engine has been in development for quite some time, they're called HCCI engines, homogeneous charge compression ignition.
One thing your missing about diesel engines is that the diesel cycle itself is less efficient than the otto cycle for the same compression ratio. The saving grace of a diesel engine is it's VERY high compression ratio, around 18-24:1, as compared to 8-11:1 for most gasoline engines. Dropping a diesel cycle engine's CR down, and then using a less energy dense fuel, probably won't gain you out much.
PricklyPete
Lifer
- Sep 17, 2002
- 14,582
- 162
- 106
I don't think anyone was arguing that. Most in this post agree that miles per $ is better...but the amount of gas you consume is still an important factor to people who are going to be buying this vehicle from an environmental standpoint (whether you agree with them being environmentally friendly or not).
Quick grade 10 physics problem. Which uses more energy: accelerating a car up to 60mph in 20 seconds or accelerating in just 10 seconds? The answer absolutely will not shock you, but I'm asking anyway
Depends. Usually 20 seconds, unless the 10 seconds is done in open loop at which point it might be a toss up.
Quick grade 10 physics problem. Which uses more energy: accelerating a car up to 60mph in 20 seconds or accelerating in just 10 seconds? The answer absolutely will not shock you, but I'm asking anyway
In an ideal world the answer is they use the same energy. You're trying to be :ninja: and confuse people with power. It takes twice the power to reach the same speed in half the time.
In the real world most engines (electric, diesel, or gasoline) do not have the same specific power output efficiency at, for example, 100% and 50% load. Using the Saturn 1.9L engine BSFC that could be the difference between 255 and 300 g/kw.h during acceleration, or ~15% decrease in efficiency.
sdifox
No Lifer
- Sep 30, 2005
- 100,692
- 18,029
- 126
http://images.thetruthaboutcars.com/2010/10/Picture-602.png
Explain the 126mpg in that table please.
This graph doesn't seem right. If efficiency were highest around 2000rpm, then wouldn't cars intended for good gas mileage operate in that range when traveling at highway speed? As it stands, most 4 cylinder cars operate closer to 3000rpm when going the speed limit. My Honda Civic didn't even function properly below 2000rpm; flooring it would make the car shake a little and it sounded terrible. I assume my Corolla is the same way, but I can't test that because it's an automatic and it won't let me bog the engine like that.Uh... peak engine efficiency is almost always at the same, or lower, engine speed as peak torque. You need to skip these other graphs and take a look at a brake specific fuel consumption (BSFC) plot from a real IC engine like this one:
In theory yes, but why is this not observable in real life? I was ballin' when I bought my Corolla, so I drove like a dick because it was fun. When I was running low on money, I tried driving a lot slower and more annoying, and it had no effect on gas mileage. No matter what I did, it just didn't burn less gas. Driving exactly the speed limit vs 10 over makes no difference. Accelerating up to 60mph with the pedal all the way down and hitting red line vs taking twenty mninutes to reach the speed limit also made no difference. There's just no way to make it more efficient without fundamentally changing the vehicle (ie smaller vehicle, using diesel instead of gas, hybrid instead of normal, etc).
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It is generally not practical to gear an engine for highway speeds at it's maximum efficiency for reasons like driveability, and gradeability.
Don't say that that something is not observable simply because you have not seen it before.
Go explain how driving style makes no difference to the thousands of people who successfully hypermile their cars to get higher fuel economy. There are likely some mitigating circumstances that explain why you didn't get better fuel economy rather than the thousands of available stories and technical papers being wrong. Read this wiki article as it explains basically everything that you seem to not believe. If you don't believe the wiki article, click on all of the cited links.
Are you sure you read that article? Most of these methods say exactly what I said - you need to modify the car itself to change the mileage. Most of the behavior things on the list are things caused by other people; things you have no control over.
http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors#Basic_techniques
Maintenance - modify the car
Minimizing mass and improving aerodynamics - modify the car
Efficient speeds - use your psychic powers to make other people drive at least the damn speed limit instead of going 20mph below the speed limit (it says a chevy impala works best at 42mph, which would actually be speeding on most city streets)
Choice of gear (manual transmissions) - I just said my car is an automatic, so that's out
Acceleration and deceleration (braking) - You can't control traffic lights and you can't tell people to stop abruptly slowing down in front of you for no reason at all, so that's out
Coasting or gliding - instead of stopping at red lights, just coast through them
Anticipation - use psychic powers to predict which people are going to slow down for no reason, then go around these people
Trip computer - horribly misleading because people then take a full minute to get up to the speed limit even though we've already established that your chevy impala should be speeding in order to get the best mileage (because it's in the highest gear at that speed)
Drafting (tailgating) - This actually does work. I once got about 60mpg in my Corolla because I tailgated a huge truck for about 2 hours while going 70mph. In the city, this does basically nothing because most of your losses are from braking and idling which you have no control over.... unless you turn the engine off at red lights.
Tailgating at high speeds is the one behavior on that list that makes any difference. The other ones are just silly. Try to resist using your brakes? Really? The only reason anyone uses the brakes is to avoid crashing into things such as other cars that are slowing down. You either use your brakes or you crash. There's no middle ground at all.
OK. If you look at the third to last row, it is labeled "Total miles driven," and is followed by the number 299. Then the row after that is labeled "Total gas used," followed by 2.359. In order to calculate mpg you have to divide total miles by total gas used. This would give you the equation 299 / 2.359. The result of that calculation would give you 126.7 miles/gallon.
What exactly is so hard to understand about elementary school arithmetic?
PricklyPete
Lifer
- Sep 17, 2002
- 14,582
- 162
- 106
sdifox
No Lifer
- Sep 30, 2005
- 100,692
- 18,029
- 126
The fact that most of it was powered through nightly plug in. A bit more than 2/3 in fact. Yet that mileage is incorporated in the mpg calc.
Gas miles =84.6
Gas used 2.359
MPG is really 35.86.
Here is my example again
a traditional gas powered car.
You drive 84.6mi, load it up in an electric train, have the train drives 214.4mi
then you claim you drove 299 mi with 2.359gallon of gas...
The article did not make the numbers clear, that is my problem with it.
Volt would have infinite mile per gallon by this calculation if they kept all mileage under EV limit.
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If they dropped the mileage down to 100mpg to reflect the true operating cost of the car, would that satisfy you?
Some quick googling says the enthalpy for burning gasoline is about 34kWh per gallon. Gasoline engines are only about 25% efficient, so it's really more like 8.5kWh per gallon output. Apparently the average gas price in Texas is about $2.65 per gallon. That means gasoline costs about 31-32 cents per kWh.
Electricity where I live is 10 cents per kWh. High efficiency motors are up in the 90% efficiency range, but this is a car so we can probably round that down to maybe 50% just to stay on the conservative side of things. That would make electricity effectively about 20 cents per kwH.
So it's better, but it's not that much better. It's not free driving. If driving in a 100% electric mode, you're still looking at 1/2 to 2/3 the fuel cost. Actually that sounds a lot like the Prius. A Corolla gets about 30mpg in the city and the Prius gets about 40 if you really really try to drive as retarded as possible (race up to red lights, drive with parking brake on, etc). So that's like 1/2 to 2/3 the fuel cost....
I did not say that those things don't help, they certainly do. I'm also not saying that every hypermiling method is practicable by you in your situation. What I am saying is that there is an overwhelming amount of evidence that suggests that traveling too fast will reduce your fuel economy. Clearly there are many, many factors involved that can change each situation. You, however, appear to simply fixate on what supports your ideas and ignore everything else.
Here is a chart that shows, for the average vehicle in the US, fuel economy vs speed. From the DoE, made by people who analyze this stuff for living. Believe what you will, but don't ignore the mountains of evidence.
sdifox
No Lifer
- Sep 30, 2005
- 100,692
- 18,029
- 126
All I wanted was realistic reporting and not magic math. My beef is with MT, not Volt.
Actually that graph shows that in 99.999999% of cases driving faster improves your gas mileage
The graph looks like it peaks at about 55mph. The unposted speed limit on residential streets is often 30-35mph depending on where you live. If you drive faster, your gas mileage goes up, according to that graph. Main roads have speed limits of 40-45mph. If you speed, your gas mileage goes up, according to the graph.
The unscientific psychological aspect of going faster is that cars traveling at high speeds tend to leave bigger gaps between the cars, and this allows for "burn and coast" driving as described by wikipedia. When the car infront of you appears to be slowing down a little, you can take your foot off the gas and coast a little, and you might not even need to use the brakes or downshift. In slower traffic, like a certain 50mph strip I go through every day on a highway that passes through my city, the cars are bumper to bumper. It feels like everyone is constantly riding their brakes in situations like that. You accelerate just a little bit, then suddenly the guy in front of you is braking for no reason and you need to brake too. If you leave a big gap, people in other lanes jump in front of you and close the gap so you're still bumper to bumper. Slow driving is what causes that type of stop and go driving or riding your brakes driving.
The other other part of going slow is that you can't tailgate when you drive slow. About half of my trip to work is dense city driving leading to a highway that goes to the outer part of the city, and the other half is going 70-80mph on a highway that runs along the outside of the city. On that highway, 18 wheelers drive about 70mph. If you feel like driving 70mph, you can tailgate them and save gas. If you drive slow as hell, you're the head of the pack and people are going around you. You're not able to tailgate anyone, so all of the wind resistance is slamming you and only you.
I'm so proud to say that I'm still getting 35-36mpg even though I red line my piece of shit car every day, half of my driving is stop and go, and the other half is driving 70-80mph. The US EPA says this car should only get about 30mpg in combined highway/city driving if you drive it the way the EPA drives it. I still think they run those tests with 4 flat tires and the parking brake engaged
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