In Anaheim, Calif., solid waste trucks running on a blend of mostly liquefied natural gas and a small volume of diesel perform as well as trucks running on pure diesel, but with significantly lower exhaust emissions.

The trucks are being operated as part of a joint project intended to demonstrate that LNG is a practical fuel for use in highway diesels and to showcase dual-fuel technology as a practical alternative to dedicated natural gas engines.

The engines are production-model electronically controlled Caterpillar truck diesels with the combustion systems modified to combine the injection of natural gas with diesel. 
The generic dual-fuel system uses a mix of approximately 15 percent diesel and 85 percent natural gas, with the diesel injection serving as the pilot for combustion ignition. 

Taormina manages all solid waste collection and processing for six Orange County cities and recycles, processes and hauls waste from another 55 nearby communities, handling over 4000 tons of solid waste per day. It operates a fleet of 270 Peterbilt, International, White/GMC and Volvo/ White collection, roll-off trucks and transfer tractors. Two years ago, the mechanical diesels in 38 of the trucks were replaced with dual-fuel conversions of Caterpillar's 31767B, C-10 and C-12 electronic diesels. Power Systems Associates, the Los Angeles distributor for Caterpillar made the conversions and over-saw engine replacement. 

New versions of the ever-evolving electronic controls and the more efficient injection/combustion systems of the new diesels hastened dual-fuel moving from concept to a practical technology. Clean Air Partners reportedly had to wait for introduction of Caterpillar's new generation of electronically controlled diesels before it could fully prove and market its dual-fuel concept. 

The precise orchestration of the injection and combustion of separate diesel and LNG fuel streams is managed by the ECM in data link coordination with an electronic control unit on the LNG circuit. The discrete electronic controls are programmed to start the engine as a conventional diesel, then automatically switch to dual-fuel when the engine reaches its operating temperature and accelerates above idle speed. 

This is a seamless switchover - truck operators reported they could scarcely detect when it takes place. Nor can they determine when the engine switches back to full diesel as the engine drops back to idle speed. More than gee-whiz electronic coordination of two fuel flows is needed for this; there must also be a near-perfect rpm/ torque match. Fine-tuning of the volume and injection timing of natural gas and diesel fuels accomplishes this. The engines in Taormina's trucks have run effectively on diesel-to-gas ratios as low as 5/95 percent. Extensive on highway testing has shown that for Taormina a ratio of 15/85 percent, gives good engine performance on dual-fuel and an imperceptible torque change when switching between the two fuel modes.

As an example, Campbell points to the Cat C-10 diesel, whose high-torque setting is 1100 ft-lb, but which on Taormina's dual-fuel C-10s was lowered to 975 ft-lb, well below the gas/air detonation point.

Little is done to Cat's production-line engines in converting them to dual-fuel. The only noticeable modification is the addition of an air intake manifold that has been machined to accept six Servojet natural gas injector valves. Truck modifications are similarly modest. On Taormina's waste haulers, a single 97-gallon cryogenic tank is mounted on the truck frame rail near the conventional 75-gallon diesel tank (which for weight saving Taormina will soon replace with a smaller tank). The LNG is piped from the cryogenic tank to an in-line engine-coolant heat exchanger where it is vaporized into low-pressure CNG, then passes through a pressure regulator to the gas injector valves on the intake manifold.

Installed on the modified air intake manifold, along with the gas injectors, are gas pressure and temperature sensors. They provide data the ECU uses to calculate natural gas fuel density, which is essential to determining the volume of natural gas to be injected. A malfunction anywhere in the natural gas circuit will cause the ECM/ECU combination to trigger an in-line on-off solenoid, instantly stopping the natural gas flow simultaneously switching the engine to full diesel operation.

Ron Stewart, Taormina's director of fleet maintenance, reported other benefits from the dual-fuel engines. "Our oil sampling shows much less contamination in the lube oil of the dual-fuel engines," he observed. Because of this, Caterpillar allowed Taormina to extend oil change intervals without resorting to special lubrication oil. "We doubled time intervals on test trucks and see no problems," Stewart added. This reduced Taormina's lubricant and filter costs, lowered maintenance labor costs, and means more on-road and less in-shop time for the trucks. "If the engine wear rates stay as low as our oil samples indicate then we could see engine life extended significantly," he said.
 

By doubling a truck's LNG tankage (LNG weighs 30 percent less than diesel), the intra-city truck can become an inter-city low emissions hauler. With the expected increase in the number of LNG fueling facilities across the nation the home-at night truck can as easily go interstate, reducing exhaust emissions by some 50 percent wherever LNG is available.