is that PH. -Dave
You keep ignoring the FACT that it takes more energy to push a car along at 65 mph than at 45 mph due to aerodynamic drag. It is very possible (even likely) that your engine may be more efficient at 65 mph than at 45 mph if you are defining efficiency as Efficiency = Work Out Energy In. However, is less than at 45. You don't seem to understand this despite multiple people telling you this over and over again.
I've tried repeatedly myself. One more time.
When your car is moving down the road, the power output of the engine is "consumed" by several factors -
1) powering accessories like power steering, alternator, A-C, water pump, etc. This is not completely independent of engine speed, but it is close, so just pick a steady state value, say PA = 2 horsepower 2) overcoming rolling resistance. Again this is not completely independent of speed, but it is close enough so that we can treat it as such. In general it is treated as a coefficient times the weight of the vehicle. A 1995 Galant should weigh around 1300 kg with you aboard. I'll arbitrarily select the coefficient of rolling resistance as 0.01. The power to overcome rolling resistance = PR = u*m*g*v (u= coefficient = 0.1, m = mbutt, g = acel due to gravity, v = vehicle velocity) 3) overcoming aerodynamic drag. This is almost entirely based on three factors - vehicle coefficient of drag, vehicle frontal area, and vehicle speed. The power needed to overcome aerodynamic drag = PD = Af*Cd*(V**3)*Da-2 (Af = frontal area = 2.3 square meters for a 1995 Galant, Cd = 0.29 for a 1995 Galant, Da = density of air = 1.225 kg-cubic meter at sea level)
You must also include the driveline efficiency when calculating engine horsepower required. I'll buttume the Galant has a 90% efficient driveline
So if we use this information, we get the following power requirements (in horsepower) for the following speeds:
Required Spd PR PD PR+PD PR+PD*1.1 PA Engine Power 45 3.4 4.5 8.0 8.8 2.0 10.8 50 3.8 6.2 10.0 11.0 2.0 13.0 55 4.2 8.3 12.5 13.8 2.0 15.8 60 4.6 10.7 15.3 16.8 2.0 18.8 65 5.0 13.6 18.6 20.5 2.0 22.5 70 5.3 17.0 22.4 24.6 2.0 26.6 75 5.7 20.9 26.7 29.3 2.0 31.3
This table is a good approximation of how much engine power it takes to drive a car at these various speeds. The next thing to do is to figure out how much energy per mile this consumes.
Total energy required per mile = power * time to go one mile (1 HP = 33,000 ft-lb-min, 1 ft-lb = 0.0012851 Btu, 1 HP = 42.4 ft-lb-min) Min per BTUs per Spd Eng Pwr Mile Mile 45 10.8 1.3 595 50 13.0 1.2 662 55 15.8 1.1 731 60 18.8 1.0 797 65 22.5 0.92 881 70 26.6 0.86 967 75 31.3 0.80 1062
One gallon of gasoline can produce approximately 115,000 BTUs. So if your engine was 100% efficient, the following gallons of gasoline would be required to go 1 mile at each speed - I have also converted this to miles per gallon
Gallons Miles Spd per mile per Gallon 45 0.0052 193 50 0.0058 174 55 0.0064 157 60 0.0068 plus 1 144 65 0.0077 131 70 0.0084 119 75 0.0092 108
Remember that this is the number of gallons required for a 100% efficient engine, which of course does not exist. You have claimed that your Galant gets around 42 miles per gallon at 75 mph. This means your engine would have to be almost 40% efficient at that speed. This would be remarkable (actually incredible, unbelievable). The very best Otto cycle engines are only less than 30% efficient under ideal wide open throttle conditions,. Therefore I think it is ridiculous for you to claim you can get 42 mpg at 75 mph. Therefore I conclude that your claim that you get 42 miles per gallon at a steady 75 is bogus and all the other conclusion you reached based on the initial claim are bogus as well. Reality is giving you a huge dope slap.
Here are some references you should study:
****** In particular you should look at the following********
This includes a table showing fuel economy at various speeds for a selection of late 90's vehicles. There were no vehicles listed that got better mileage at 75 than at 55. There was one vehicle that got better mileage at 60 and 65 than at 55 (1997 Toyota Celica), but it got it's best mileage at 25! This study was conducted by people that could make careful measurements. If it doesn't blow enormous holes in your bogus 40% of red line theory, nothing should. But then I guess no FACTS will get in the way of your good story.
Regards,
Ed White
