> From: Steve Blackmore [mailto:st...@pilotltd.net] >> On 1 May 2013 22:54, Steve Blackmore <st...@pilotltd.net> wrote: >> >>> Some years ago I worked for a large multinational company, who I can't >>> name for legal reasons. One of the many things they were working on >>> were more efficient fossil fuel engines. They had a 2litre Diesel >>> engine that was capable of 200 mpg plus
Entirely possible in a lab setting, but still a nonsensical measure of engine performance. About as odd as "My 90 frame ACIM spindle motor can achieve an MRR of 310 cc/min". >>> and they had it working fine >>> in a popular large saloon car as a demonstrator. Great! But not at 200 mpg under real-world conditions. The 200 mpg car is one of those topics of conversation that seems to come up once in a while which usually result in me banging my head against a brick wall. Using first approximations only it's pretty easy to see why the market hasn't been flooded by cars with that sort of fuel efficiency. 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% Theoretical max efficiency is around 75%, the highest efficiency diesel I'm aware of is a touch over 54%. That's a freaking huge low speed engine - and not suitable for a car. Typical diesel efficiency is 30-35% (I'm guessing here). So somewhere around 42 mph would give you 200mpg. Plugging the numbers in for my [petrol] car I get around 70 mpg at 70 mph. On a good day, under conditions as close to these as possible - I get between 35 and 40. Some of those losses come from all the inefficiencies that were ignored (Transmission losses, alternator, changes in road gradient, air con). Most of the rest comes from the irritating problem of having to slow down and accelerate to avoid other road users. Even at a 75% engine efficiency I'm not going to get a real world 200 mpg without completely redesigning the entire car. It may one day be possible, but I'm not going to hold my breath. >> >> Bollocks. > > Sort of reply I'd expect from someone who works for the motor industry. > I don't work for the motor industry. Can't help but agree with Andy there. ------------------------------------------------------------------------------ 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
