Measurements Provided by Engine Tests Demonstrate Reliability of Improved HIDECs: Hiro Technology Brain, Inc., continues development of the Hiroshima University Diesel Engine Combustion Simulation (HIDECS) program to improve its capability. By employing the statistical thermodynamics and n-dimensional topological space created in a fuel spray, HIDECS has been modified to increase its versatility in simulating various kinds of diesel combustion systems including combustion with unconventionally advanced fuel injection timing, combustion with multiple fuel injections, and premix compression ignition (PCI) combustion in addition to conventional diesel combustion. Yet the computational time is kept extremely short, e.g., 2 to 3 seconds on a personal computer. Because of the fast computational time to complete calculations of heat release rate, cylinder pressure, soot, and nitric oxide (NO), HIDECS is attractive not only to engine designers who wish to acquire quick results of diesel combustion analysis, but also possibly to engine control engineers working to design a model-based engine control system for both engine and aftertreatment devices. This chapter reports the improved HIDEC modeling approaches and governing equations, validation results, and a few demonstration results. HIDECS is available through iTEP (contract sariga@itepsa.com for more information). The chapter consists of 14 pages including 8 figures and 1 table.