Mpg Economics

MPG economics

TDC

Kevin Cameron

YEARS AGO, I ZOOMED AROUND THE city of Boston on a little Kawasaki 90 that I bought for insignificant money. Riding it was definitely cheaper than driving a car, and I could park it anywhere. I thought of that bike as I gazed into a Spanish magazine at Honda's CB125F, a very similar little runabout that seems to suit this moment of economic reverse. It has a simple, air-cooled, four-stroke Single with five speeds. It does not cost $8000 and it cannot reach 100 mph. It is not sold here.

Selling CB125Fs would be a letdown for dealers accustomed to moving 600 and 1000cc sportbikes, but it could have advantages in the long run. If many people stop buying high-ticket motorcycles during this recession, will they start right in buying them again once it ends? Can the industry afford to sit and wait? Small, genuinely economical motorbikes might be one way to hold onto market share-and even create a few new buyers in the meantime.

This opens the whole question of ef ficiency in road vehicles. As a crude rule of thumb, a four-stroke gasoline engine burns half a pound of fuel for every horsepower it makes for each hour of operation. To a first approximation, if my bike needs 15 hp to cruise on the freeway, it will use half that number of pounds of fuel per hour, or 7.5 pounds. At 60 mph that would work out to 48 miles per gallon.

In fact, fuel consumption is more complicated than that. As we close the throttle from full-open, we create a loss of engine efficiency that is associated with the power used in pulling a partial vacuum during the intake strokes. This is called "pumping loss," and at small throttle openings it becomes quite large, causing our engine to need much more than a ½-lb per hp per hour of fuel. It may in fact consume more like a whole pound of fuel per hp-hour, or even a bit more. So there we are on the four-lane glideway, throttled back, and perhaps getting more like 25 or 30 miles per gallon. There are five-passenger sedans that do significantly better.

As we speed up, friction losses in crease because bearing loads increase as the square of engine rpm. We are also leaving behind us a swirling, turbulent wake that is rich with energy-energy that our engine is having to generate to push the bike through the air. That, too, causes fuel mileage to worsen as speed rises. Airplanes and even cars do better with aero than motorcycles because they have the length to close the holes they make in the air.

Pushing air aside is just the beginning. For high aerodynamic efficiency, we also have to smoothly put our wake back together. Air pressure, acting on an airplane's long tapered tail, is like a hand squeezing a wet bar of soap, pushing it forward. This "push" on a tapered tail balances out some of the force air exerts on the front of the moving vehicle, reducing the energy that the engine must supply to maintain speed.

Craig Vetter (www.craigvetter.com) is trying to design a closed-wake motor cycle that can achieve 100 miles per gallon at 70 mph. His prototypes closely resemble that most streamlined of all nature's creations, the fish.

Superior aero is only one reason for the good fuel consumption of latemodel autos. Perhaps the major reason is that low fuel consumption has been a design goal for cars for many years, while motorcycle design-at least for the moment-is aimed at high performance. Auto engines typically turn a moderate 2500 rpm at highway speed, while many motorcycle engines are spinning at least twice as fast. All techniques that have been used to make auto engines highly efficient can be applied to motorcycle engines; it just hasn't happened yet because economy has not become a major motorcycle selling point.

Pumping loss is a major reason for the existence of hybrid autos. An engineer, charged with cutting fuel consumption, would immediately seek to avoid all opera tion on low throttle. Let's see, how could we do this? Well, we could take surplus energy from the engine during the times when it was running efficiently on closer to full throttle, and we could store this in a battery pack to power the vehicle at low speeds when the combustion engine is least efficient. Hmm, if we use both the engine and the stored energy 7 during acceleration, we can afford to make the combustion engine smaller, which will reduce the size, number and drag of its friction-producing parts such as piston rings, bearings, cams and so on. --

5" A major source of inefficiency during stop-and-go driving is the power required to re-accelerate the vehicle after braking. How about if we use the on-board generator to perform a lot of the routine braking, and save the conventional friction brakes for hard stopping? The power we generate during this "regenerative braking" can be added to the stored energy described above. In this way, maybe 30 percent of what would otherwise become waste heat in the brakes can now be stored to re accelerate the car when the lights turn green. That's another part of why hybrids have such good city-mileage figures.

At speed, the picture is not so green. Above 35 mph, the Toyota Prius can go about four miles on its battery aloneluckily, the evil ol' internal-combustion engine is there to provide forward motion and a recharge. This has prompted some owners to add batteries (at a cost of up to $12,000) to create "plug-in" hybrids that are recharged from mains power at home. Cheap electric power instead of expensive gasoline! Low carbon footprint, here I come.

But now one of your clever, Internet browsing children pipes up, saying, "Daddy, did you know that more than half of U.S. electricity is generated in coal-burning plants? By charging your car at home, you are turning it into a coal-burner!"

Maybe I should just walk.