With some tradeoff in performance, diesel engines can be made to run on a mix- ture of diesel fuel and compressed natural gas. Benefits are reduced NOx and PM emissions, lower fuel costs, and for the five Caterpillar 3170B trucks operated by Pima Gro Systems in California, full warranty coverage. The gas, injected into the manifold at 125 psi, is the primary fuel, ignited by small amounts of No. 2 diesel. A dual-fuel ECU adjusts pulse width for both sets of injectors with respect to manifold pressure, charge-air and gas temperatures, and fuel mapping. At low rpm the engines “skip fire,” with three cylinders receiving diesel fuel and no fuel to the others. As loads and speeds increase, progressively greater amounts of natural gas are injected and the remaining three cylinders come on line. Skip firing reduces emissions and provides the surplus air to ease the transition from one fuel to the other. These trucks run about 350 miles a day without major problems.