cutlass1991 Posted March 30, 2005 Author Report Share Posted March 30, 2005 does anyone have answers to the chem test on thermodynamics? seriously, i drive around on a 1/4 tank all the time. you guys are really making me consider not doing that anymore. Quote Link to comment Share on other sites More sharing options...
Brian P Posted March 30, 2005 Report Share Posted March 30, 2005 does anyone have answers to the chem test on thermodynamics? seriously, i drive around on a 1/4 tank all the time. you guys are really making me consider not doing that anymore. I'm taking thermodynamics right now at my university. NOT easy at this level. I can also note regarding pump life vs amount of fuel in the tank: My old Cutlass Supreme ('90) I rarely ran below 1/4 tank (also when the gage read E, I still had about 1.5 gals)- The pump was still original at over 200,000 miles. My sister's '93 Cutlass fuel pump just died at 131k, yet she really never went below 1/4 either....BUT. At the time I was changing the pump, the gage read just below 1/2 tank. After dropping the tank, there was at most a gallon of gas in there. So all this time she thought she had a decent amount of gas, she didn't. So technically she ran the car below 1/4 tank most of the time (this would also explain why the car ran out of gas on me shortly after the "low fuel" light came on). Both cars had the same engine, fuel pump and tank, but the one that lasted longer was 3 years and over 70,000 miles older. Quote Link to comment Share on other sites More sharing options...
ursus Posted March 30, 2005 Report Share Posted March 30, 2005 Another data point, 1988 Regal on which I have kept records since new 201,000 miles and has original fuel pump which currently puts out 42psi, 50psi with vacumn 635 tanks of gas, average fillup size is 54.3 litres which leaves the 62 litre tank with 7.7 litres (2 gallons) 13% or about 1/8 full. Probably 1/2 the time it was 56-57 litres and 1/2 the time 51-52. Car is in Canada so I guess 1/2 the year the gas is friggen COLD! maybe somone from Death Valley could chime in Quote Link to comment Share on other sites More sharing options...
bartonmd Posted March 30, 2005 Report Share Posted March 30, 2005 Car is in Canada so I guess 1/2 the year the gas is friggen COLD! This is a valid point... you know the thing about -10deg F = twice the life... What part of Canada do you live in? Also, when the fuel pump was going out in my '87 bonne, it took me a couple months of "gathering data" to diagnose it because it was intermittant (and I was still in college, so I didn't just want to start playing the part game... On the plus side, I did replace every small vaccum line on the car because it was cheap...), and it wouldn't just quit, but it had stopped making pressure, so it would run flawlessly on the highway and for quite awhile in town with a full tank of gas, and would run just fine on the highway down to 1/4 (which is usually where I get gas), but would start to run like ass in town very quickly with less than a half tank... Found it didn't do it as much or as fast at night and on colder days as it did during the day on hot days... Finally bit the bullet and took it to somebody with a reader and paid the $25 diagnostic fee and found out that the fuel trim was set way too rich... It also would be really hard to start sometimes, so I pressed on the schraider valve and fuel just barely came out... It wasn't making enough pressure in the 2 seconds of on-time, so a couple times, I had to hot-wire the fuel pump (fuel pump test connector right next to the battery on that car) and it started up fine... Mike Quote Link to comment Share on other sites More sharing options...
pitzel Posted March 31, 2005 Report Share Posted March 31, 2005 If you're not breaking 20mpg at least, time for a new oxygen sensor and maybe plugs. my plugs are new, so do you think replacing the oxygen sensor will help my mileage? less than 200 miles per tank is sucking. Absolutely. I had terrible mileage when I first got my car, and if I left it idling on my driveway for 10 minutes, I could obviously smell the richness of the exhaust. Oxygen sensor replacement fixed the problem real fast. Quote Link to comment Share on other sites More sharing options...
pitzel Posted March 31, 2005 Report Share Posted March 31, 2005 In a closed system the amount of gas in the system has no effect on the equalibrium temperature In theory, in a perfect world (or in all stop and go city driving, never getting up to speed), this is correct... BUT... very rarely are you at equalibrium, which is what I'm talking about... What more gas in the tank I'll just chime in here. The equilibrium temperature of the tank can be determined essentially through this equation: T=To + dQ/Cv + k, whereas T is the tank temperature, Q is the change in heat, Cv is constant volume heat capacity of the tank, To is the initial temperature, and k represents the cooling/heating of the tank due to airflow/etc. The *only* meaningful variable involved will be the constant volume heat capacity of the tank, since we can probably fairly safely assume that k will remain constant for a full tank or an almost empty tank. Thus, the overall Cv of the tank will be the determining factor. The Cv of the tank will be determined by the percentage petrol and the percentage of air present in the tank. Since petrol stores heat far better than does metal, the fuller the tank, the cooler the tank will be overall. Quote Link to comment Share on other sites More sharing options...
cutlass1991 Posted March 31, 2005 Author Report Share Posted March 31, 2005 thanks pitzel. leave it to a fellow EE to help me out. on another note, i apologize to anyone who is pissed because i started this post. I know it's been done a bunch of times already. I hate people that don't use the search function. i swear i do 99% of the time, except in this case. :oops: Quote Link to comment Share on other sites More sharing options...
Garrett8 Posted March 31, 2005 Report Share Posted March 31, 2005 116k i get about 20 Quote Link to comment Share on other sites More sharing options...
Crimson Fury Z Posted March 31, 2005 Report Share Posted March 31, 2005 my mileage sucks, poppin gas in it every other day to 2 days, enough to get me between 1/2-3/4 tank, then i watch the needle drop........ Quote Link to comment Share on other sites More sharing options...
bartonmd Posted March 31, 2005 Report Share Posted March 31, 2005 Fury, Retarded timing will KILL your fuel mileage... why is your timing retarded anyway?? Mike Edit: Pitzel... correct... but the fuel does get warmer when you're in town... that's why my fuel pump would start turning to shit when in town and run perfectly on the highway... but you're right... the more fuel in the tank, the cooler it will be... Quote Link to comment Share on other sites More sharing options...
CuttySup Posted March 31, 2005 Report Share Posted March 31, 2005 250 miles to a tank. Quote Link to comment Share on other sites More sharing options...
Jcrow Posted March 31, 2005 Report Share Posted March 31, 2005 I just took a small road trip last night, kept it between 70-75 MPH and I averaged 32.5 miles per gallon (93 octane) Quote Link to comment Share on other sites More sharing options...
ursus Posted March 31, 2005 Report Share Posted March 31, 2005 I'll just chime in here. The equilibrium temperature of the tank can be determined essentially through this equation: T=To + dQ/Cv + k, whereas T is the tank temperature, Q is the change in heat, Cv is constant volume heat capacity of the tank, To is the initial temperature, and k represents the cooling/heating of the tank due to airflow/etc. The *only* meaningful variable involved will be the constant volume heat capacity of the tank, since we can probably fairly safely assume that k will remain constant for a full tank or an almost empty tank. Thus, the overall Cv of the tank will be the determining factor. The Cv of the tank will be determined by the percentage petrol and the percentage of air present in the tank. Since petrol stores heat far better than does metal, the fuller the tank, the cooler the tank will be overall. No you have misapplied that formula, If you take a sealed container with heat source inside the temperature rise above ambient inside the container will be dependant on: 1. the amount of energy being added to the system 2. the ability of the system to conduct energy to ambient 3. The ambient temperature The Temperature in the Tank will rise until the amount of heat per unit time we add to the system is equal to the heat transfer from the tank to ambient per unit time. When they are equal we will be at the equalibrium temperature. The amount of heat we add per unit time , Q/t, is simply dependent on the energy source, the hot fuel rail in our case The amount of heat we transfer to ambient per unit time is given by the equation: Q/t = k*A*( Ttank -Tair)/d solving for Ttank we get: Ttank = Tair + (Q * d) / ( t * k * A) where: Q/t is heat transfered from the hot part of the system to the cold part of the system k is the thermal conductivity of the barrier A is the surface area T are the temperatures inside of the tank and ambient air and d is the thickness of the barrier Cv is meaninless in our case as it is the Constant volume heat of a gas used to normalize the equation for preassure. The fluid within the tank does not affect final Temperature whether that fluid is air, water gas, gas fumes or any combination. Only the thermal conductivity of the material of the tank which is our thermal barrier, which is why the plastic tank would be warmer than the steel tank, and the surface area of the tank are relevant. In your formula dQ/Cv is the rate of heat transfer and this term becomes less and less relevant as time increases. The formula rather will give what the temperature of the tank will be after some specific time and shows as bartonmd correctly pointed ouy yesterday that the tank with more gas in it and hence more thermal mass or Cv will take longer to heat up/cool down, however,,, the thermal mass will only effect the time required to reach equalibrium not what that equalibrium temperature is. Think of it this way if I live in a house in the summer, if I fill up a 1000 gal container of water and put it in my living room will the house be cooler, the answer is no, it will slow the time it takes for the house to heat up to its final temperature so maybe it will be 3pm before we hit 90 degrees instead of 2 pm but it will not effect the final temperature of the house. If you were correct we wouldn't need air conditioners just a big bucket of water in a closet in the middle of the house to stay cool all summer and the temp of the house would just be dependant on the size of the bucket. Quote Link to comment Share on other sites More sharing options...
Bzdel Posted March 31, 2005 Report Share Posted March 31, 2005 Highway I was getting around 32 mpg. City its about 20 mpg or so. Quote Link to comment Share on other sites More sharing options...
3.1LUMITIC Posted April 1, 2005 Report Share Posted April 1, 2005 Well i took a trip to Louisville Ky, from Detroit MI witch is 400 miles in my 90 Lumina euro coupe. Made it there with alil more then e left in my tank with the rpms right above 2k like the second line up from 200 rpm. So i guess i got great gasmilage but not anymore . I need a fpr and the last time i went to KY i had to fill up 2 times thats bad.... Quote Link to comment Share on other sites More sharing options...
riceca Posted April 2, 2005 Report Share Posted April 2, 2005 Last tank I got a little over 21 mpg with most city driving with maybe a few short trips on the highway. lata CHRIS Quote Link to comment Share on other sites More sharing options...
pitzel Posted April 2, 2005 Report Share Posted April 2, 2005 IThe amount of heat we transfer to ambient per unit time is given by the equation: Q/t = k*A*( Ttank -Tair)/d solving for Ttank we get: Ttank = Tair + (Q * d) / ( t * k * A) I follow your logic. I used the variable 'k' to denote the per unit time heat loss, which, in my estimation of the materials and the physical location and geometry of the fuel tank, would be similar, if not identical for both the situation where the tank is full, as well as when the tank is empty. We come to the same conclusions in essence, that a full tank will heat more slowly than an almost empty tank, and the peak temperature will be lesser, and only reached when the heat input is equal to the heat loss due to conduction/radiation to the ambient air. Whether this situation ever occurs in real-life is an entirely different, yet legitimate matter for debate. Sorry, it was a quick and dirty expression of math, and I should have been more clear about things instead of leaving the arbitrary heat loss relationship, which I denoted as k. Quote Link to comment Share on other sites More sharing options...
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