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2008-2013 Cadillac CTS General Discussion Discussion, Trip computers are the most reliable way to track fuel economy in Cadillac CTS Second Generation Forum - 2008-2013; Originally Posted by Sailirish7 Then how do you account for the fluctuation of the energy content as the fuel expands/contracts ...
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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by Sailirish7 View Post
    Then how do you account for the fluctuation of the energy content as the fuel expands/contracts due to temperature? Not trying to be funny, but this must have an effect even if it is negligible.
    That's an interesting observation. That would make an accurate trip computer always underestimate the mileage in comparison with an accurate filling station pump reading, since it sees a larger volume at the warm engine with all the radiator and exhaust heat passing under the car. The difference should be about 1% for every 10C (18F) between the in ground tank and metering device at the engine.

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    Re: Trip computers are the most reliable way to track fuel economy

    well what I have done to ensure I won't get stranded...and I have done on the last three new cars that had a "'LOW FUEL" light was to look in the manual and verify the tanks capacity. Then Fill it up....set one of the trip odometers, just for S-N-G's. then just drive like normal, both city and highway, and as soon as the light comes on....pull into the nearest gas station and fill up.....do the math, and it will give you a very good estimate of how far you can go.....without getting stranded. personally....for the most part, I try to fill up once the tank gets down to 1/4.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by Sir Nuke View Post
    for the most part, I try to fill up once the tank gets down to 1/4.
    you should, because the electric pumps in the tanks of most modern cars are cooled by the fuel around the pump. Constantly running near empty may limit pump life.

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    Re: Trip computers are the most reliable way to track fuel economy

    THAT is a fact.....and a fact that a lot of people are not aware of.
    MoFex likes this.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by hauler View Post
    That's an interesting observation. That would make an accurate trip computer always underestimate the mileage in comparison with an accurate filling station pump reading, since it sees a larger volume at the warm engine with all the radiator and exhaust heat passing under the car. The difference should be about 1% for every 10C (18F) between the in ground tank and metering device at the engine.
    Yikes! That does not make me feel any better considering the wacky winter weather in Texas...lol The ground here tends not to freeze, so assuming I fill up on a cold day (recently 28F), the fuel would expand as it is being pumped (not sure by how much) and then futher expand once in my tank due to the cold. That can't have a positive effect on the energy content...

    Admittedly, we're talking small amounts of difference, but difference none the less.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by Sailirish7 View Post
    Yikes! That does not make me feel any better considering the wacky winter weather in Texas...lol The ground here tends not to freeze, so assuming I fill up on a cold day (recently 28F), the fuel would expand as it is being pumped (not sure by how much) and then futher expand once in my tank due to the cold. That can't have a positive effect on the energy content...

    Admittedly, we're talking small amounts of difference, but difference none the less.
    But you actually get more gas for less price as the gasoline is more dense due to being cold. Therefore, fill up in the morning as a rule.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by tinman View Post
    But you actually get more gas for less price as the gasoline is more dense due to being cold. Therefore, fill up in the morning as a rule.
    Not necessarily, which is what I found interesting. If the fuel expands, the same energy content is taking up more volume. Therefore, you are getting less total energy in the same volume. Make sense?

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    Re: Trip computers are the most reliable way to track fuel economy

    I basically agree...for our real life there is nothing to gain or lose by step differential. However, a gallon of gas at -20 C is "smaller" than a gallon at 100 C. As it passes thru the pump and is " measured" you should end up with more fuel pumped at -10 once it is at warm ambient temperature than gas pumped " hot" that cools to warm ambient temperature.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by Sailirish7 View Post
    Not necessarily, which is what I found interesting. If the fuel expands, the same energy contents is taking up more volume. Therefore, you are getting less total energy in the same volume. Make sense?
    Gasoline is sold by volume, not by the number of British Thermal Units. To get more BTUs, ya gotta buy high-test, by volume.

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    Re: Trip computers are the most reliable way to track fuel economy

    High test is less susceptible to pre-ignition detonation ( knock), not more powerful per se.

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    Re: Trip computers are the most reliable way to track fuel economy

    Wow, lots of myths here. First, cooler fuel isn't denser--liquids don't work that way, at least not in the range where humans live (and some, like water, actually get less dense as they get colder). Additionally, the fuel in the tanks in the ground at your local gas station doesn't heat up and cool off to any significant degree--more than three feet or so underground is always around 64 degrees except in extreme places like the desert or arctic, and even then the variation is only a few degrees. In fact, that consistency of temperature underground is what geothermal heating systems are based on. In short, you're not getting more gas per dollar by filling up before the sun comes up.

    Second, even if the gas in your tank is 150 degrees from sitting in the sun all day, it isn't notably less dense at that temperature. Maybe an infinitesimal amount, but not enough to affect power or mileage in a measurable way. Liquids are pretty stable at the temperatures at which humans are comfortable. Evaporation does happen, but that's different from density changes due to temperature. Racers who use "cool cans" to cool the fuel are not trying to make the fuel more dense, but are trying to keep it from evaporating early in the fuel lines and carburetor (vapor lock) and may also be thinking that they are cooling the air/fuel mixture a bit, since in the cylinder, the fuel is also the primary coolant. Starting with cooler fuel never hurts, but in terms of BTU/gallon sitting in your tank, it's a non-issue, especially in a fuel-injected car that pressurizes the fuel anyway. In a car with direct injection, the extreme fuel pressure (more than 2000 PSI) probably adds quite a bit of heat, not to mention running through fuel lines on top of a hot engine and next to a hot exhaust system, totally erasing any gains from using "cool" fuel. It's a non-issue.

    Also, I don't believe the fuel pump measures the fuel flow or even particularly cares. It's just a motor turning an impeller (or something like that depending on the pump) and doesn't care whether it's pumping fuel or water or air. It's not measuring anything, it's just spinning. The primary feedback is the O2 sensors, which will tell the ECM if the mixture is too rich or too lean and the ECM will adjust injector pulse width accordingly to bring it back in line. Again, the density of the fuel doesn't make any difference, the O2 sensors are looking for a specific level of oxygen in the exhaust and the ECM is changing the amount of fuel injected to achieve it. There's just no way for fuel to be enough denser (is that a word?) to make any kind of difference at such minute levels (we're talking microseconds of pulse width and nanoliters of fuel).

    And finally, high octane fuel doesn't have more potential energy. It just has higher octane, which is simply a measurement of the fuel's resistance to ignition. It doesn't make a hotter or bigger explosion and feeding it to a car that doesn't need high octane gas doesn't make it more powerful. It only allows the engine to make as much power as it was designed to make. Now, it's possible that running a car on cheap gas will reduce power, but putting the good stuff in your 12-year-old Cavalier won't make it run better or cleaner or make more power and in fact it may run worse due to the fuel's slower-traveling flame front. If you don't need it to prevent knocking, you're just throwing your money away.

    Don't fall for the old wives' tales!

    Now, as far as the trip computers being accurate, I don't believe it. I believe most calculations are related to pulse width of the injectors, which already takes into account temperature, speed and load. Unless the injectors are completely uniform and operating perfectly, the engine is in a perfect state of tune, and other internal factors match what they are "supposed" to read, there's no way this calculation can be 100% accurate. Most cars have trip computers that are adjustable to fine-tune their accuracy in the field, usually in response to customer complaints about bad fuel economy. If they were getting information from the car that was accurate enough to make an accurate calculation, why does it need such a thing? My old Audi allroad, for example, had as much as a 5% adjustment either way to fine-tune the MPG calculations. I didn't bother with it and found that the computer was always about 1.5 MPG optimistic (using an average over several fills), but at this point, who cares? 21.7 or 21.2 MPG, does it really matter in a car that costs as much as a CTS? And then there's the fact that when you're coasting, you're not using any fuel, yet it still says 70 MPG on the display. Which is it? 0 (or infinity, depending on how you look at it) or 70 MPG? There's obviously a lot of gray area in the calculations.
    99flhr and Sailirish7 like this.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by toolman8 View Post
    Wow, lots of myths here. First, cooler fuel isn't denser--liquids don't work that way, at least not in the range where humans live (and some, like water, actually get less dense as they get colder). Additionally, the fuel in the tanks in the ground at your local gas station doesn't heat up and cool off to any significant degree--more than three feet or so underground is always around 64 degrees except in extreme places like the desert or arctic, and even then the variation is only a few degrees. In fact, that consistency of temperature underground is what geothermal heating systems are based on. In short, you're not getting more gas per dollar by filling up before the sun comes up.

    Second, even if the gas in your tank is 150 degrees from sitting in the sun all day, it isn't notably less dense at that temperature. Maybe an infinitesimal amount, but not enough to affect power or mileage in a measurable way. Liquids are pretty stable at the temperatures at which humans are comfortable. Evaporation does happen, but that's different from density changes due to temperature. Racers who use "cool cans" to cool the fuel are not trying to make the fuel more dense, but are trying to keep it from evaporating early in the fuel lines and carburetor (vapor lock) and may also be thinking that they are cooling the air/fuel mixture a bit, since in the cylinder, the fuel is also the primary coolant. Starting with cooler fuel never hurts, but in terms of BTU/gallon sitting in your tank, it's a non-issue, especially in a fuel-injected car that pressurizes the fuel anyway. In a car with direct injection, the extreme fuel pressure (more than 2000 PSI) probably adds quite a bit of heat, not to mention running through fuel lines on top of a hot engine and next to a hot exhaust system, totally erasing any gains from using "cool" fuel. It's a non-issue.

    Also, I don't believe the fuel pump measures the fuel flow or even particularly cares. It's just a motor turning an impeller (or something like that depending on the pump) and doesn't care whether it's pumping fuel or water or air. It's not measuring anything, it's just spinning. The primary feedback is the O2 sensors, which will tell the ECM if the mixture is too rich or too lean and the ECM will adjust injector pulse width accordingly to bring it back in line. Again, the density of the fuel doesn't make any difference, the O2 sensors are looking for a specific level of oxygen in the exhaust and the ECM is changing the amount of fuel injected to achieve it. There's just no way for fuel to be enough denser (is that a word?) to make any kind of difference at such minute levels (we're talking microseconds of pulse width and nanoliters of fuel).

    And finally, high octane fuel doesn't have more potential energy. It just has higher octane, which is simply a measurement of the fuel's resistance to ignition. It doesn't make a hotter or bigger explosion and feeding it to a car that doesn't need high octane gas doesn't make it more powerful. It only allows the engine to make as much power as it was designed to make. Now, it's possible that running a car on cheap gas will reduce power, but putting the good stuff in your 12-year-old Cavalier won't make it run better or cleaner or make more power and in fact it may run worse due to the fuel's slower-traveling flame front. If you don't need it to prevent knocking, you're just throwing your money away.

    Don't fall for the old wives' tales!

    Now, as far as the trip computers being accurate, I don't believe it. I believe most calculations are related to pulse width of the injectors, which already takes into account temperature, speed and load. Unless the injectors are completely uniform and operating perfectly, the engine is in a perfect state of tune, and other internal factors match what they are "supposed" to read, there's no way this calculation can be 100% accurate. Most cars have trip computers that are adjustable to fine-tune their accuracy in the field, usually in response to customer complaints about bad fuel economy. If they were getting information from the car that was accurate enough to make an accurate calculation, why does it need such a thing? My old Audi allroad, for example, had as much as a 5% adjustment either way to fine-tune the MPG calculations. I didn't bother with it and found that the computer was always about 1.5 MPG optimistic (using an average over several fills), but at this point, who cares? 21.7 or 21.2 MPG, does it really matter in a car that costs as much as a CTS? And then there's the fact that when you're coasting, you're not using any fuel, yet it still says 70 MPG on the display. Which is it? 0 (or infinity, depending on how you look at it) or 70 MPG? There's obviously a lot of gray area in the calculations.

    Thank you for the in depth reply, that was great to read. My only point was that temperature could effect the energy content of the fuel due to expansion/contraction. That doesn't necessarily translate to amount great enough for performance gain/loss. It was mostly just something I found interesting on a strictly academic level. Thanks for clearing that up though.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by toolman8 View Post
    Wow, lots of myths here.
    Yes, and you just added a doozer, below.

    Quote Originally Posted by toolman8 View Post
    And then there's the fact that when you're coasting, you're not using any fuel, yet it still says 70 MPG on the display. Which is it? 0 (or infinity, depending on how you look at it) or 70 MPG? There's obviously a lot of gray area in the calculations.
    Really?

    I read every word of your reply and agreed with most of your thoughts until your totally illogical statement above. My engine DOES NOT shut off while coasting and does continue to use fuel. The calculations do not change. What would you expect the instantaneous fuel milage to be when the injector pulse width goes to that required to sustain idle and the car is moving at 70 MPH? Certainly not zero or infinity.

    Could be the reading should be higher then 70 MPG.

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    Re: Trip computers are the most reliable way to track fuel economy

    Quote Originally Posted by Mistercoffee2 View Post
    Yes, and you just added a doozer, below.



    Really?

    I read every word of your reply and agreed with most of your thoughts until your totally illogical statement above. My engine DOES NOT shut off while coasting and does continue to use fuel. The calculations do not change. What would you expect the instantaneous fuel milage to be when the injector pulse width goes to that required to sustain idle and the car is moving at 70 MPH? Certainly not zero or infinity.

    Could be the reading should be higher then 70 MPG.
    OK, well, I might be wrong about Cadillac DI engines, but when I was working with GM on the C5 Corvette doing powertrain development work, and when I was building performance fuel injected Fords before that, and when I was researching the fuel injection systems on my Audi allroad before I chipped it, the one thing they ALL had in common was that when throttle position was zero and vehicle speed was above a certain threshold (12-15 MPH for most), the fuel injectors were shut off. That means when you're coasting at speeds above, say, 15 MPH, there is no fuel being injected into the engine at all, it's 100% momentum. That's what causes the engine braking sensation when you lift off the pedal. Mind you, this is only with the car in gear--putting it in neutral simply reverts to the idle circuit same as if you were standing still, although in some cars you will see a slightly higher idle at higher vehicle speeds because of the algorithm designed to keep it from stalling as you coast down, but most OBD-II systems have eliminated this function thanks to more precise air metering.

    Now, when you get below a certain speed and at idle, of course the injectors are running to keep the engine from stalling. Having logged tens (maybe hundreds? I don't keep track) of thousands of miles in late-model Corvettes and EFI Mustangs, I can actually feel the moment when the injectors come back online as I slow down to a stop with the car in gear, and any laptop monitoring software that plugs into the OBD-II port will show injector duty cycle as 0 when you're coasting. Say you're going 60 MPH down a steep hill and you are not using the accelerator pedal at all, your foot is completely off it. At that point, the injectors are off and not injecting any fuel at all. Air is still pumping through the engine, all the accessories are still turning because the engine is being driven by the wheels, but there's no fuel involved. Zero. That's why the instant fuel economy indicator in my Audi, for example, goes to "---" whenever I'm coasting: there's no fuel being used, effectively rendering fuel economy as either 0 or infinity, depending on how you look at it. Hell, going down a particularly long hill near my house, I could watch the engine physically cooling off on the temperature gauge because there was no heat from combustion, but the cooling system was still going full tilt. Coasting down a hill in neutral will actually use MORE fuel than coasting down in gear at 4000 RPM because in one case the injectors are on the idle circuit and in the other they're completely off. It's negligible, but it's still true.

    As I said, I'm not 100% up-to-date on the function of a Direct Injection engine and it may need to use a bit of fuel for some reason I'm not aware of, even during coast-down, but I bet it doesn't. Perhaps there's someone here with some more specific information on the fuel mapping of the CTS that can chime in. I'm perfectly happy to be wrong about this, but almost since the advent of electronic multi-port fuel injection, and perhaps even before with TBI, most systems shut off the injectors during coast-down at throttle position 0.

    I know this is often a hard thing for guys to wrap their heads around, and there's no crime in that; it's counter-intuitive and this isn't the first time I've had this conversation, even with car guys who really know their stuff. After all, it sure feels like the engine is still running, right? It's still going the same RPM, just slowing down slowly, it's not stalling. The alternator is still making juice, the A/C is still cold, the power steering still works, and when you step on the gas, it goes instantly (the injectors come back on line in milliseconds, so you won't catch them sleeping). What you have to remember is that shutting off fuel doesn't lock the engine solidly, and it is NOT idling unless you clutch in or put it in neutral (at which point you ARE using fuel). It doesn't take a lot of effort to turn the engine over--you can do it by hand with a wrench, the starter can do it, and the momentum of the car does it easily by pushing back through the driveline, particularly with the HUGE leverage that the gears, especially overdrive gears, offer. Shutting off the fuel doesn't instantly lock it up and throw you through the windshield. You can actually feel it as driveline lash, that exact moment when you go from driving the wheels to the wheels driving the engine. All the tiny tolerances in all the gears, bearings, universal joints, CV joints, etc, all subtly shift in the other direction as the wheels start pushing back through the driveline and continue to turn the engine. That's what driveline lash is, and on my ancient Dodge 4x4 pickup with sloppy U-joints, there's an audible thunk as they all shift from one driving surface to the other. Try it next time you're out--just lift off the throttle completely at highway speeds. The moment you feel the car start to slow down, and it'll be something you can easily feel, is the moment that the injectors are off and the wheels are driving the engine. It's also the exact same moment when your instant fuel economy reading goes to 70 MPG, which is apparently some GM programming quirk and not a reflection of real MPG at that point (I know why they did it that way, but that's another discussion).

    Here are some links you can read from guys who aren't me (and who aren't just random guys on message boards) that all say the exact same thing:

    http://www.caranddriver.com/features...neutral-page-3

    http://www.popularmechanics.com/cars...l-fuel-economy

    http://www.kfz-tech.de/Engl/Schubabschaltung.htm

    I KNOW it's counter-intuitive and sounds fishy. It's still true.

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    Re: Trip computers are the most reliable way to track fuel economy

    I personally don't find it counter intuitive that our engines shut off. As I glide off a large bridge into work I have often wondered if my (various car) engine just reverts to idle or shuts down. I knew my Hemi had half the cylinders shut down at highway speed. The new Corvette does too.

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