November 2002

Why Not a 40-MPG SUV?

Continued from page 3

By Mark Fischetti

Digital Engine

The most radical advance for improving gas mileage would come about by remaking the engine itself. This means rethinking the century-old mechanics that open and close the engine valves, which let a fuel-and-air mixture into combustion chambers and release exhaust gases. For decades, a camshaft has performed this job. A spinning shaft, it moves levers that open and close valves approximately 100 times per second in a fixed pattern.

The camshaft technology works well, but it wastes fuel. The traditional configuration provides no way to change patterns in order to deliver, for example, lots of power for accelerating onto a highway and to cut back on unneeded power-saving fuel-at highway cruising speeds. In recent years, though, engineers have added mechanical equipment to the camshaft, allowing some enhanced valve control. This control includes, for example, the ability to open valves only partway when little power is needed. Honda and BMW have developed and installed such "variable valve" systems in many production cars, improving fuel economy by 5 to 10 percent.

But the ultimate move toward optimization throws the camshaft away. Instead, electromechanical actuators would provide software-driven control for each valve (see "The Camless Engine," below). By providing full control over the timing, lift, and duration of each valve motion, such a camless engine optimizes power delivery with the least possible fuel at every engine-rotation speed. The payoff is huge: a camless engine could improve fuel economy by 10 to 18 percent while also increasing engine torque by 15 to 20 percent at low speeds for faster acceleration.

Illustration by John MacNeil

The problem is that to prevent excessive wear and minimize engine noise and vibration, valves must decelerate before landing. A camshaft, though relatively inefficient, does this quite well, thanks to its ovoid shape, which produces a corresponding acceleration and deceleration in the valve motion. Actuators are different; they slam up and down, on and off. The way to make actuators as gentle as camshafts involves a combination of hardware and software, and many companies are working on the problem. Anna Stefanopoulou, a mechanical engineer at the University of Michigan, has already designed several promising software schemes. Within the past year Stefanopoulou's team has optimized several algorithms and is now testing ways to use feedback from the valves to achieve high-speed motion with gentle landings. Meanwhile, to aid in this soft landing, Mohammad Haghgooie, a physicist with Ford Motor, is testing springs and pneumatic and hydraulic dampers to lessen the impact of valves without slowing them down. If successful, these improvements in electromechanical valve software and hardware could bring a camless engine to market in 2008, Haghgooie says.