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.