Variable Valves
Variable valve operation seems to offer plenty of promise, and has become almost ubiquitous in the car world. Essentially, if you can alter the timing and/or the amount of lift of the valves, you can alter the engine's breathing characteristics to suit the rev range it's operating at. An engine needs different things from its valve setup at different engine speeds: you get better running at low revs if you have `softer' cams with less `overlap' time (when inlet and exhaust valves are both open). Conversely at high rpm, there's hardly any time to get the gasses into and out of the combustion chamber, so you get better performance with radical cams that have more overlap, advanced timing and higher lift.
Currently, there are two bikes with variable valves: Kawasaki's Concours 1400 and the Honda VFR Interceptor. On the Kawasaki, there's a clever hydraulic actuator on the inlet camshaft, which advances or retards the cam timing according to engine speed. The ECU opens an electronic valve, allowing high-pressure oil into the actuator, and moving the cams forward or backward in relation to the crankshaft rotation. So at low revs, the cams open the valves later, and at high revs, the reverse is true.
The hydraulic actuator alters...
The hydraulic actuator alters cam timing on kawasaki's concours 1400 to broaden the powerband.
On Honda's Interceptor, the VTEC system also uses an ECU and pressurized oil. But the rest of the system is very different. Rather than alter the timing of the valve opening, Honda's system actually engages and disengages two of the four valves in each cylinder. Two valves (one inlet, one exhaust) work better at low engine speeds, so below about 6,500 rpm, the cams only open two of the four valves in each cylinder. When revs get higher, the ECU uses oil pressure to push small pins into place on the non-opening valves, and the cam then opens all four valves, improving high-speed breathing and adding a stack of horsepower.
The ultimate valve setup?
Perhaps the ultimate in four-stroke valve operation was shown by BMW's car division in the late 1990s. In place of cams and springs, the system had electric solenoids - not unlike the technology that operates door bells or car door locks. A solenoid is simply a coil of wire around a movable steel rod - when you pass electricity through the coil of wire, it creates a magnetic field that moves the rod in a linear fashion through the coil. Put something on the end of the rod - like, say, an engine valve - and you can move it back and forth as the electricity is switched on and off. Get an ECU to control the electricity, and you can open your valves whenever you want, for as long as you want, and in varying amounts.
In fact, your bike probably already has such a system. Fuel injectors are basically just small solenoids connected to a valve with pressurized gas behind it. Your bike's fuel injection computer sends electricity to the injector solenoid when it wants to squirt gas into the engine, varying the amount and timing to suit the engine's needs.
Sizing a fuel injection system up to an engine valve operating system isn't a trivial task however - the forces are much greater, and operating conditions tougher. Your solenoids need much more power and are, necessarily, bigger and louder (on the prototype system, they made a horrid racket). BMW's system needed a 48 volt electricity supply and a huge alternator to run, neither of which are simple to put on a small motorcycle engine. And, again, standard camshaft and spring valve operation has done a very fine job up till now.