HCCI engine is a hybrid of spark ignition (SI) and compression ignition (IC) diesel engines with a goal to theoretically harness advantages of both in a single setting. Similar to SI engines, a homogeneous fuel/air mixture is inducted into the engine. During the compression stroke the temperature of the mixture increases and reaches the point of autoignition; i.e. the combustion is initiated without the help of any ignition system. The early studies of this phenomenon were conducted on two-stroke engines with the goal to reduce the hydrocarbon (HC) emission at part load condition. The historical development in two-stroke engines by itself is educational. In the l970s, Eiji Toyoda, founder of Toyota Motor Co. In Tokyo, experienced the abnormal run-on behavior with his motorcycles and challenged his staff engineers to explain it. Masaaki Noguchi and a team of researchers from Toyota and Nippon Soken Inc. investigated this unusual effect, and determined that it was caused not by the assumed hot spots but by an interaction between fuel chemistry and the temperature and pressure conditions in the engines' cylinders. Specifically, the self-ignited combustion tends " to occur at relatively low cylinder pressures and temperature (compared to diesel combustion), presumably by virtue of intermediate reaction products [active radicals] ". Using partially-transparent optical engines with quartz observation ports, the researchers detected that the stable combustion behavior was characterized by an intense blue glow, which was later shown to be the fluorescence of active radicals. The group named the phenomenon Toyota-Soken combustion. Around the same time, Shigeru Onishi and his coworkers at Nippon Clean Engine Research Institute Co. Ltd. showed a small single-cylinder motor/generator set (called the nice engine) that ran with no spark. The team called the phenomenon they observed active thermo-atmosphere combustion. In both these research projects, the engineers could make their engines operate sparkless on radical-initiated combustion, but only at constant speed and load. They had, however, determined some empirical characteristics of this third type of burning. The autoignition occurred only at low loads, when there was a large amount of hot residual gas in the cylinders. As the temperature rose further during the compression, the initiating reactions in the fresh charge were beginning, and the hydrocarbons in the fuel were breaking down into simpler compounds and activated radicals.