Date: Tue, 8 Sep 1998 09:33:07 -0700 (PDT) From: Anthony Jackson Subject: Re: Engine Question > Fuel is consumed in gph (gallons per hour) by fuel burning engines. In > my car (a vauxhall cavalier) if i were to drive at 60mph i would > probably use about 1.5 gph. If i was to drive at 90mph this would > probably go up to 2 gph. Hm...I don't know what model of car you actually drive, but I'd guess at 60 mph you use between 2 and 3 gph (i.e. you get between 20 and 30 miles per gallon) and at 90 mph you use more than twice that. The model for fuel consumption in Vehicles is fairly inaccurate, but handling it correctly results in moderately annoying math. Following is an (unpolished) article I was playing with on real ground vehicle performance, I was originally going to polish it up a bit more and eventually put it in the archives, but if anyone's interested it's here. ------------------------------------------------------------------------ p128 Ground Speed Ground speed is fairly complicated, and the math used in Vehicles bears fairly little resemblance to the actual performance of any real ground vehicle. This is a bit more realistic of a treatment. These rules probably should not be applied to legs or flexibody drivetrains, the mechanical issues are much more complicated (particularly for legs -- performance will depend on such things as the weight and length of the leg subassembies, and most likely will be limited more by strength than by available power). For modern vehicles, I recommend using a gear system -- first gear, etc. TL 5 vehicles probably only have one gear, most TL 6-7 vehicles will have 3-5. If you want, figure that the weight of the drivetrain is 80% of list plus 5% per gear. From what I've heard, the electric motors used in recent electric cars (basically TL 8 designs) also functionally have gears, though their performance statistics are significantly different. For a vehicle with gears, choose 'motive force' available in each gear. A typical car will have about 20 x kW / (gear #). A two-wheel drive vehicle is limited to about 60% of its weight in motive force, a four-wheel (or tracked) drivetrain is limited to 100%. To compute ST, divide motive force by 25 (cars aren't really all that strong. However, they can apply that full force at 20-25 mph, which a human can't). A given gear can apply full force for a speed up to (kW * 500 / motive force), beyond that speed reduce available force by 10% per 10% excess speed, and going more than about 20% over max can damage the engine. For an electric drivetrain, available force is 500 * kW / speed, with a maximum value of 5 * kW at TL 6, 10 * kW at TL 7, doubling every added TL (this can be improved by adding a gearbox -- this is just an electric motor attached to an axle). Now, you need to figure out the amount of resistance you will need to overcome. These basically work out to ground friction (from tires, axles, etc), internal friction (energy requirements for sucking air in and out of your pistons, mostly), and air resistance. These have the following values: Ground friction: equal to 4 * weight / (speed factor * HT) Internal friction: equal to 10% of maximum motive force in gear. This may be very low or even zero for an electric drivetrain. Air resistance: equal to (aerodynamic drag)*(mph^2)/7500. There are some ground effects which limit the degree to which a car may be streamlined, assume that (a) wheels cannot be streamlined, and (b) maximum streamlining is 'good' (higher levels with chop MR and SR). Now we can work out performance. It is relatively difficult to compute max speed based on power, and this isn't all that useful in any case; I suggest just generating a table listing MPH, gears, resistance, power usage, and acceleration. Don't list any speeds for which resistance significantly exceeds motive force, or more than 20% over the normal maximum for the gear. Power usage is */500, acceleration is *20/. Unusual surfaces: in most cases, unusual surfaces cause fairly flat resistance, and thus can be treated as a constant deceleration per turn; offroad travel is about 1 mph/turn, extremely bad surfaces can be even more than that. Slope is 1 mph/turn per 5% slope. If you have a deceleration of 1 mph/turn, this works out to an additional power requirement of (wt in tons)*(speed)/5. Determining MPG: equal to 60 / (power requirement at 60 mph * fuel multiplier). You should multiply fuel usage by 2.5 from the Vehicles stats (round up to nearest 0.01) -- so for a TL 7 vehicle, it is 500 / power requirement at 60 MPH. Example: the TL 7 car in Vehicles -- fully loaded, total weight roughly 4600 lb, HT 12, total drag 300, 95 kW drivetrain. It has four gears, motive force is 1900 lb in first(max 25), 1000 in second (max 47), 650 in third (max 73), 500 in fourth (max 95); HT is 12. Resistance is 4*4600/(16*12) = 95 lb plus 0.04*speed^2 plus 10% of motive thrust (190 lb in first, 100 in second, 65 in third, 50 in fourth). speed gea res acc pow gea res acc pow gea res acc pow gea res acc pow 10 1 289 7 5.8 2 199 3 4 3 --- --- --- 4 --- --- --- 20 1 301 7 12 2 211 3 8.4 3 176 2 7 4 161 1 6.4 30 1 321 7 19 2 231 3 14 3 196 2 12 4 181 1 11 40 1 --- --- --- 2 259 3 21 3 224 2 18 4 209 1 17 50 1 --- --- --- 2 295 3 30 3 260 2 26 4 245 1 25 60 1 --- --- --- 2 --- --- --- 3 304 1 37 4 289 1 35 70 1 --- --- --- 2 --- --- --- 3 356 1 50 4 341 .5 48 80 1 --- --- --- 2 --- --- --- 3 416 1 67 4 401 .5 64 90 1 --- --- --- 2 --- --- --- 3 --- --- --- 4 474 .25 85 100 1 --- --- --- 2 --- --- --- 3 --- --- --- 4 550 -.5 110 MPG: 60 / (35 * .10) = 17. 0-60: 0-28 in first(4 sec), 28-49 in second (7 sec), 49-60 in third(5.5 sec) = 16.5 seconds. Modifications: a 1400 lb load is equivalent to a family of four on a trip, including 90 lb of gasoline, 600 lb of people, and 700 lb of cargo. If being used for commuting (or performance testing) this is probably about a thousand pounds too high. That subtracts around 20 lb of resistance at every speed and increases acceleration by about 25% at every speed, giving a 0-60 time of about 13 seconds and increasing fuel economy to about 18 mpg. Upgrading to 'fair' streamlining (typical of essentially all late TL 7 vehicles) reduces drag by about 40%, for power usage of 28 kW fully loaded (21 mpg), 24 mpg with a light load.