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RE: fuel efficiency How Bikez could be improved discussion group:
RE: fuel efficiency

RE: fuel efficiency 
tahrey said 2009-05-31 01:42:35 
 
Erm? Not a great deal. Allegedly running the engine at its peak torque speed gives you the best economy, but I\\\īve never actually found that to be true in practice if you test it. In most cases it seems you\\\īre best to run as slowly as you can stand to in top gear, without labouring the motor (e.g. for my current car - which makes it easy to test because of the built in economy meter - it\\\īs around 40-45km/h... for some old Honda 50s it was as low as 25-30k, which is fair enough if you\\\īre on some old dirt road but not good for much else).

Just at a guess the incorrect theory came about either because max torque used to come in not much above the revs where the engine would start rattling (mid 2000s rpm, instead of 3500-4000 for cars or a minimum of about 5000 for bikes these days), or because the \\\īspecific consumption\\\ī is generally lowest at max torque (as the engine gives the most twisting force - NOT power - because its burning the supplied fuel most efficiently), as can be seen on various torque/power vs consumption curves in the rare occasions these are supplied. Problem is, that\\\īs for running at full throttle... not exactly normal running conditions, unless you have a quite low powered, very high geared vehicle that you run at constant low speed on level ground. (typical vehicle should theoretically still be capable of 65-80% of its absolute max speed at max torque... even if it can only do 100km/h, 65-80km/h is only really sustainable on good out-of-town roads, and still won\\\īt be the most efficient running speed). You\\\īre more likely to be running with gearing far below that required for constant speed at max torque (in some cases you\\\īll rev out a bit above max power, even in top gear; quite what speed the torque peak is for any particular vehicle is anyone\\\īs guess - anything between 70 and 140km/h wouldn\\\īt be unusual... only down into the 50k-and-less zone for really weak scooters), at 50% throttle or less, and we don\\\īt tend to be given figures for such settings except in very rare cases. EG a map of grams of fuel used per horsepower at rpms & % throttle i once saw ... for a medium sized diesel truck. I think it was on a site promoting some eccentric new kind of transmission that meant you could say in the high-efficiency range almost continually. The map does change quite radically when you back off the power, as you\\\īre letting in a much lower quantity of fuel-air mix, at a lower pressure, altered ignition timing and possibly different mixture richness... the way it burns will necessarily change because of it.

So long story short, \\\īnot much\\\ī. Just ride slow and light and aim for as high a geared transmission as possible. It doesn\\\īt even seem to matter much torque the engine puts out or at what revs, or torque/litre rating. Bike experience isn\\\īt much, but first car was wimpy and low revving 1-litre --- about 40mpg. Second, 1.6, like an inflated version with longer gears, about 34mpg in the same driving conditions. Now a higher tuned 1.6, with in between gears ... 38mpg. Would probably make 40+ with decent gearing, despite its motor being worlds apart in terms of power (and torque) output from the first one and distinctly higher revving.
 
 
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