The Mpg Papers

TDC

The MPG papers

Kevin Cameron

PASSENGER-VEHICLE FUEL ECONOMY has two faces. Little power is used while cruising at highway speeds, but for acceleration, eight or 10 times more may be required. In the past, this has been “solved" by giving every passenger vehicle an engine much bigger and heavier than needed for cruising-just so it could accelerate as necessary. We paid the fuel cost of carrying this bigger engine, and we paid to overcome its extra friction.

The initial, panic approach of industry to social demands for better mileage was simply to gear taller, so engines would turn more slowly. This cut friction slightly, and improved mileage, but acceleration suffered. This was no fun-when you pushed down the throttle, the large but overgeared engine seemed to look back at you and ask feebly, “Did you want something?”

The next move was to reduce vehicle weight, air drag and engine size. Look down on any parking lot from above and you can easily find the older cars; they are the wide ones. Narrow' cars push less air, and lighter cars require less power to accelerate. Smaller engines, sized closer to cruising power to waste less power in friction, had less surplus power available for acceleration, so they were even less fun to drive than the overgeared big engines of the previous era.

In the early 1980s came the turbocharger. By cramming more fuel-air mixture into an engine, a turbo could make a small engine act like a 50 percent bigger one, making a small car and small engine less dreary to drive. There were drawbacks; compression had to be lowered to prevent knock when onboost, but this increased fuel consumption at cruise. Turbos take time to generate boost, and this lag annoyed people accustomed to instant thrust.

Manufacturers, therefore, next brought out sportier, non-turbo engines, with motorcycle-engine-like features; four valves per cylinder, separate intake runners, overhead camshafts. The aim was to get more peak power from smaller engines, but without lag and without crippling their ability to pull efficiently at low revs. These engines brought back instant response and some degree of driving pleasure. This progress is widely applauded, but it’s hardly original. Narrow, light, small-engined vehicles that require minimal materials and fuel have been with us through this whole century: Motorcycles.

Soon another step in the rise of auto economy may begin. If a vehicle needs low power for cruise, and much more power for acceleration, why assume that both must come from the same source? This is the basis for the idea of hybrid drive systems. For cruise, power comes from a small sustainer engine, operating efficiently near full throttle. For acceleration, extra power comes from some form of short-term storagea battery, a flywheel or compressed air. The sustainer could be a conventional piston engine, a small, low-compression-ratio regenerative turbine or even a fuel cell. When operating on shortterm storage alone, the vehicle would emit no pollutants-useful in urban environments. When hybrid-drive autos appear, they will be light and extremely well streamlined. They may use only about half the fuel of the most economical present autos. Motorcycles as they are today cannot equal this performance. If small cars begin to get 60-80 miles per gallon, today’s 40-mpg motorcycles could draw criticism as a wasteful indulgence. They don’t need that kind of attention. Let’s hope that long-range planning for this contingency is under way in the R&D centers of the major makers.

Would hybrid drive work on motorcycles? Since bikes are quite heavy enough as they are, it’s hard to imag-

ine them further burdened with several auto-sized lead-acid storage batteries. A flywheel-storage system might package better, but would be complex. Compressed air would be bulky. In fact, each of these possibilities violates the basic nature of the motorcycle: simplicity in human scale. Therefore the most likely early response from the motorcycle industry would be-as it was from the auto industry-lighter weight, with smaller, harder-working engines.

One candidate for the engine is the direct-injection two-stroke, which is particularly efficient at part-throttle, where most riding is done. Such an engine is small and light, and would be able to deliver big-bike performance in a package substantially smaller than today’s lOOOcc-class machines. Unlike conventional twostrokes, the direct-injection engine has a perfectly regular idle, and its emissions are comparable with those of the best four-strokes. Whether the public could accept a new powerplant is an open question.

Lighter weight would be a smaller problem. Much of the weight of present designs is there just to carry the large, heavy engine, its fuel and driveline. That weight could be cut if lighter, smaller engines could be found. Motorcycle manufacturers already know how to build lightweight vehicles.

Motorcycles are not particularly well-streamlined now, but that could be improved and would do its part to reduce fuel consumption to more seemly levels. Bikes look swoopy with all their dashing graphics, but they achieve their speed through brute power, not through aerodynamic sophistication. The drag coefficients of bikes remain no better than those of big trucks.

If it becomes necessary to boost the fuel economy of motorcycles, it can be done with existing technologies. And a motorcycle has special virtues outside how much or how little fuel it uses to carry one or two people. An automobile carrying one or two people is a waste in the same sense as is a big V-Eight engine operated at a fraction of its peak power. A motorcycle contains less of everything than does a car-less rubber, less metal, plastic, and lubricant. It occupies little space at rest or in motion.

Simplicity in human scale. □