fuel consumption (long)

[Chris Krok]

> > I have always wanted to look at the KLE 500: it would seem to offer a small > sportbike motor in a dual sport envelope, which is an intriguing notion. I > couldnt live with 1/2 the fuel mileage, though. >

Never fear, folks. Fuel consumption depends primarily on engine power output. In short, fuels have a particular amount of stored chemical energy, measured in energy per unit mass (i.e., joules per kilogram). Of course, not all of this energy reaches the countershaft when you burn it in an engine. There are various inefficiencies. But, when you burn fuel at a certain rate (kilograms per second) in a given engine, you get a certain amount of energy out (joules per second). Joules per second are known as watts, which you know are usually associated with electrical power. However, they also hold for engine power output, and 746 watts = 1 horsepower. Fuel consumption rate thus depends on the amount of power you are demanding of the engine, and to some degree is independent of engine size (more on that below). When you are just cruising down the highway, you don't need a lot of engine power, so you get your best mileage. When a lot of accelerating is involved, you are demanding more from the engine, so you use more gas. In city driving, all the energy you use to bring the bike up to speed is wasted when you have to hit the brakes and stop for a light, inattentive cage driver, whatever. So, you use more gas. Offroad conditions are similar. THEORETICALLY, then, for a given power output, my GS1100E should burn fuel at the same rate as my KLR650. But, then you get into secondary effects, namely that for lower power outputs, the 1100 cc engine is oversized, and doesn't run as efficiently there, so you burn some extra gas. (Counterpoint: the 650 is designed for low-end torque, not horsepower, so that has an effect too.) In terms of mileage, other aspects of the bike become important. My KLR650 got 65 mpg on Colorado highways with stock jetting, and when I went to the Dynajet kit, it went down to 50-55 mpg (also with California gas... many additives). My GS1100E weighs 200 pounds more, is rated for 108 hp peak and has 4 carbs, but still gets 44 mpg on the highway. Unless I'm averaging 90 mph en route to Vegas, when it becomes considerably lower. But then, air drag is way up, so I'm demanding more power out of the engine. Sorry this got so long, but my thesis was based on combustion, so it's somewhat dear to my heart. Feel free to e-mail me with any questions or if I made a mistake somewhere. Thanks! Chris -- Dr. J. Christopher Krok Project Engineer, Adaptive Wall Wind Tunnel Graduate Aeronautical Laboratories, California Institute of Technology MS 205-45 Phone: 626.395.4794 Pasadena, CA 91125 Fax: 626.449.2677