In pilot-ignited HPDI, a small quantity of pilot diesel fuel is used to provide a very reliable and effective source of ignition for the directly injected natural gas. The pilot diesel fuel is injected just before the main gas injection event. The pilot fuel is only used for ignition and represents as little as 2% of the overall fuel energy. The pilot and the gas are injected through a unique unit injector that provides both a small quantity of pilot diesel fuel and a main quantity of gaseous fuel directly to the engine cylinders. This patented combined injector has been designed to fit into the engine cylinder head in the same location/hole as the diesel injector does. No modifications to the engine such as spark ignition, throttling or other performance-reducing compromises are required. As no engine combustion chamber design modifications are required, the cost and risk of introducing the technology to a particular engine are relatively minimal. As a result, the technology can be easily adapted for a wide range of heavy-duty diesel engines. Today, this technology is used on heavy-duty trucks and power generation units.<br>photo of fuel injection system