Some haulers have been switching from trucks powered by diesel fuel to natural gas because of government mandates, environmental concerns and to reduce fuel costs. However, natural gas engines produce less power. One solution is to use dual-fuel engines that run simultaneously on approximately 85 percent natural gas and approximately 15 percent diesel fuel.

Dual fuel engines significantly lower tailpipe emissions. The converted engines reduce nitrogen oxide (NOx) and particulate emissions by up to 50 percent, which meets the EPA's Low Emission Vehicle (LEV) certification level requirements and California's Air Resources Board (CARB) optional low NOx levels. As an added bonus, these engines run on pure diesel when natural gas is not available.

In dual fuel engines - which are being offered through a partnership between Caterpillar, Peoria, Ill., Clean Air Partners, San Diego and Power Systems Associates, Los Angeles - diesel fuel acts as a pilot for combustion that ignites with compression heat. Working as a combustion aid, diesel fuel allows compression ratios of 16-to-1, which makes diesels powerful and efficient. In contrast, when an air and natural gas mixture are compressed to greater than 11.5- to-1, spark ignition is no longer reliable.

Diesel engines are relatively easily converted to run on compressed natural gas (CNG), liquid natural gas (LNG) or liquid petroleum gas (LPG/propane). The process entails fitting Caterpillar engines with a natural gas fuel system, complete with electronically controlled, multi-point, sequential port injection valves, plus electronics for communication between systems.

The most expensive variable is adding the on-board fuel storage tanks. On a dual fuel vehicle, CNG and LNG storage is complex. CNG is compressed to either 3,000 pounds per square inch (psi) or 3,600 psi and is stored in high-pressure tanks. LNG is natural gas cooled at cryogenic temperatures and stored in low-pressure, vacuum-insulated tanks. While CNG and LNG fuel deliver equal engine performance, CNG tanks are twice as big as LNG tanks for equal fuel capacity. When running on CNG or LNG, engines are approximately 95 percent to 100 percent as efficient as corresponding diesel engines.

In contrast, propane offers an existing fuel distribution infrastructure and less expensive, simpler storage because it requires either cryogenic or high-pressure storage. Also, with an octane rating of 120 vs. 140 for CNG and LNG, propane offers less performance.

Compensating for higher costs are that natural gas costs less than diesel fuel, engines last longer because of natural gas use and fewer oil changes are required. Also, the engines are compatible with compression braking systems, computer Fleet Management Tools and electronic engine diagnosis systems.

Dual fuel versions of the Caterpillar of the 3126, C-10 and C-12 now are available with larger engines and propane conversions in the works.