As the piston rises on compression and is nearing TDC, a small amount of high-reactivity diesel fuel is injected, mixing into the charge of gasoline and air. This small injection of diesel becomes hot enough to auto-ignite, adding an amount of heat to the overall mix that is enough to result in HCCI’s famous “thousand points of light” combustion. But because the amount of injected diesel is small and well-dispersed, no diesel-like zone of hot, chemically correct combustion takes place. Because this engine’s intake is unthrottled, at anything less than full throttle its overall mixture is lean—most of the time, too lean to be spark-ignited. But the presence of the extra heat added by diesel injection, plus the engine’s high compression ratio, results in widespread ignition. Over the engine’s range of operation, the ratio between gasoline and diesel fuel is varied to obtain efficient, low-emissions combustion.