On Fri, May 3, 2013 at 3:46 AM, Ben Potter <[email protected]> wrote: > Only consider rolling resistance and air resistance. Also assume constant > velocity. Take a fairly typical car - Frontal area around 3 m2; Cd of 0.33; > mass of 1200 kg; Urr of 0.015 (possibly a bit high these days, pretend it's > constant). Since it's diesel let's call fuel energy 35.8 MJ/L. > > At 70 mph 200 mp(imp)g needs an engine efficiency of ~62%. If you prefer > mp(US)g then you need an engine efficiency of ~74% > At 55 mph you're talking ~44% and ~52%
Thanks for suggesting these interesting calculations, but I get slightly different results. Not a lot different, but we're arguing over fairly fine differences in already advanced technology so ten percent here and there and pretty soon we're talking a difference between workable and thermodynamically impossible. First, we're using 1/2 rho A Cd v^2 and m g Urr for air and rolling resistance, right? Then, diesel is usually quoted at 37-38 MJ/l while gas is 32-35 MJ/l---your 35.8 looks like an average?? I use STP air density at 1.17 kg/m3 and US gallons, and plug everything into my favorite 'units' program: ((1/2) (1.17 kg/m3) 3m2 .33 (70 mph)^2 + 1200 kgf .015) 200 mpg / (37.5 MJ/l) to get the ratio between required energy to maintain the speed, and energy from fuel. Apparently 200 mpg would require 169% efficiency. A 200mpg vehicle with your mechanical parameters can only do at most 48 mph even if it had a 100% efficient engine; given a realistic efficiency, we're talking 25mph max speed. Please look over those numbers and let me know if I'm off base here. Greetings Przemek "I'm for responsible gun ownership, with three requirements: training, license and insurance" ------------------------------------------------------------------------------ Get 100% visibility into Java/.NET code with AppDynamics Lite It's a free troubleshooting tool designed for production Get down to code-level detail for bottlenecks, with <2% overhead. Download for free and get started troubleshooting in minutes. http://p.sf.net/sfu/appdyn_d2d_ap2 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
