We’ve all just been reading about developments revealed at the first preseason MotoGP test at Sepang, Malaysia. This is the first public running of the prototype “Factory” Honda and Yamaha engines under the new rule reducing fuel quantity from 21 liters to 20 (“Open” entries get 24 liters). Although most accounts have the Hondas dealing with this better than the Yamahas, engine operation is harsher, and coming on-throttle is now less smooth.
A second interesting development is the new rear Bridgestone tire. It’s hard to interpret limited rider commentary this early, but it appears this tire’s design is more in the direction of abuse tolerance (better able to handle throttle-steering and wheelspin) and with a bit less in terms of edge grip (so essential to a corner-speed style). One might almost think this tire was developed in anticipation of rougher engine response.
Now the question: Why should engine response be rougher when fuel is reduced? Long ago, engineers found that normal ignition sparks could ignite fuel-air mixtures in the range 10:1 (rich) to 18:1 (lean). As the edges of that range are approached, misfiring appears. Beyond that range, special high-energy ignitions such as laser- or plasma-based types are required.
In real-world engines, mixtures are far from uniform. Fuel spraying from the injectors enters the engine in three forms: 1) fully evaporated vapor; 2) unevaporated vapor; and 3) as a wet film creeping along the walls of the port and being stripped off of edges by the airflow. Digital fuel injection must be based upon a model incorporating these modes.
Dani Pedrosa in fuel conservation mode.
As air and fuel enter the engine, vapor can turn corners with the air, but droplets and wall-sheet flows tend to go straight. Back in the ’50s, former TT rider-turned-engineer Jack Williams sought at AJS to learn about this by lining the engine cylinder on his home-made flow rig with blotting paper, then flowing a few cc of fuel through the carb’s needle jet. He knew fuel that hit the cylinder wall could mix with the oil and be swept down to the crankcase by the piston rings (Suzuki had this trouble with the early fuel injection on TL1000 V-twins). What he wanted was to direct droplets away from the wall, possibly onto the hot piston crown, which would evaporate them. Any fuel passing through the engine still in droplet form is wasted.
Even the vapor may not be thoroughly mixed into the air, resulting in rich and lean zones. The leaner the overall mixture is made (in trying to finish races on just 20 liters of fuel), the more often the engine will misfire when the spark hits such a lean zone.
This brings us to two-strokes and their irregular “ring-ding” idle. In an idling two-stroke, a small volume of fresh charge is blown into a cylinder filled with leftover exhaust gas, and the result is as above: in the turbulent cylinder charge are regions of mostly exhaust gas and few of fresh charge. When the spark passes through exhaust gas, a misfire occurs. The crankshaft’s inertia keeps the engine spinning, and a second shot of mixture is blown into the cylinder. This roughly doubles the chance of ignition, but even so, the abundance of exhaust gas may still result in a misfire. So it can take several revolutions before enough fresh mixture accumulates in the cylinder to assure ignition.
As the rider begins to open the throttle of a two-stroke, more fresh charge is admitted but at first, misfiring persists, so the engine eight-strokes. With more throttle angle, eight-stroking turns to four-stroking, and engine torque doubles. This is not smooth power; the doubling of torque is roughness that can knock a hard-working rear tire loose mid-corner. Two-stroke exhaust gates (Yamaha’s Power Valve, for example) were developed partly to smooth out this response.
Jorge Lorenzo spinning, sliding, sideways.
In four-strokes, as the mixture is leaned down, misfiring can sometimes be reduced by changing the fuel-injection timing, changing the aim of the injector or even by relocating the spark plug (next to impossible in a four-valve engine). The goal of these and other such changes is to put richer mixture around the spark plug to reduce or eliminate misfiring and throttle-up roughness like that of two-strokes. In auto engines intentionally designed for “lean burn,” the low-to-mid throttle region is often operated in a stratified-charge mode. Typically, the piston crown is shaped to hold injected fuel around the spark plug, allowing regular ignition to take place even when the mixture as a whole is too lean to be spark-ignited. This works because, once the ignitable mixture around the plug is burning, its rate of heat release is enough to burn the rest of the charge.
Because a lean mixture releases heat at a lower rate than does a chemically correct mixture, it burns more slowly. Therefore, the ignition timing must be advanced to bring the point of peak combustion pressure to the “sweet spot” at around 11 degrees ATDC. If peak pressure comes too early, heat is being lost rapidly to piston crown and combustion-chamber walls while the piston remains virtually motionless. If peak pressure comes too late, the piston moves significantly, expanding the combustion gas so that it can never even reach normal peak pressure.
If low fuel consumption were the only goal of MotoGP, the combustion chamber could be redesigned for stratified-charge operation. In a production car engine, with essentially equal bore and stroke, the piston shape needed for this can be combined with a high compression ratio. But MotoGP is still a form of racing, so its engines have high-rpm-style 81.0 x 48.5mm bore and stroke that make it tough to combine high compression and the piston shape necessary for lean-burn, stratified-charge operation. Make up your mind, MSMA! Is this still racing? Or has it become a Mobil Economy Run for motorcycles?
Meanwhile, just like a two-stroke, as this year’s MotoGP riders begin to throttle-up mid-corner, their lean-running engines will pass through zones in which the ignition spark hits mixture too lean to ignite and misfires will happen. Turn the throttle a bit more, the misfire stops and torque jumps very unsmoothly to a higher value—just like a two-stroke making the leaps from eight-stroking to four-stroking to two-stroking. Rough throttle response works the tire harder, for if you’re already near the edge of traction when the misfiring stops, the tire’s going to slide.
Stay tuned for the next pre-season test at the Sepang International Circuit on February 26–28.
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