Parametric Tests Reveal Relationship between Fuel Injection Parameters and Load: Natural gas engines with diesel pilot, so-called dual-fuel engines, can operate on lean mixture of natural gas with a relatively high compression ratio similar to that used for diesel engines. Thus, thermal efficiency is significantly higher than a natural gas engine that operates on premix natural gas with spark ignition. Under low to medium load, however, the dual-fuel engine emits an excess of unburned natural gas. Researchers at the National Institute of Advanced Industrial and Science and Technology and Denso Corporation developed a technique to improve both exhaust emissions and thermal efficiency of a dual-fuel engine through a relatively simple approach. This chapter reports the test results and observation of dual-fuel combustion in a single-cylinder engine.