Various Fuel Types Evaluated through Engine Tests for Measurements of Ignition Delay, Emissions, and Other Variables: PCI combustion can simultaneously reduce soot and NOx in a certain engine operating range where the temperature is not too low or too high. Technologically, it is still difficult to expand the operating range, and research continues for a way to overcome this difficulty. In the meantime, some manufacturers are developing technology to use PCI combustion by switching the combustion mode between PCI and conventional diesel combustion so that overall exhaust emissions and fuel economy can be improved. Although indexes such as cetane number and octane number are generally referenced to characterize ignition of a fuel type, they may not be suitable for evaluating ignition characteristics of PCI combustion. Therefore, researchers at Hokkaido University and Japan Energy Corporation suggested a systematic categorization of fuel properties that affect PCI combustion characteristics and conducted parametric tests with various fuel types. The information is expected to increase understanding of ignitability in PCI combustion. The chapter consists of 12 pages including 10 figures and 2 tables.