LNG Dual-Fuel Conversions Hit the Road with Refuse Fleet
DIESEL PROGRESS North American edition July 1999 pp. 54-55
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. 
Liquefied natural gas instead of compressed natural gas is used because of its storage density.
The technology of combining natural gas and diesel to achieve diesel ignition is the result of a
development partnership between Clean Air Partners, San Diego, Calif., and Cat's Engine Division.

Because these are production model engines and modified by Cat dealers to run on dual-fuel, their installed costs are relatively low. At the end of an engine's useful life, removal of the dual-fuel conversion components returns the engine to normal electronic diesel operation. The trucks are owned by Taormina Industries, one of California's largest and most innovative solid waste management firms. 

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. 

The repowering project is part of a joint research program that includes Taormina Industries, 
California South Coast Air Quality Management District
Amoco Oil Company

Power Systems Associates
Clean Air Partners and 
Caterpillar Engine Division.
Taormina's interest in this technology was sparked by a desire to find a practical way to reduce exhaust emissions without sacrificing truck performance or skyrocketing capital costs. Air quality experts of the research consortium calculated that the 38 dual-fuel trucks would reduce exhaust emissions by more than 300,000-lb. cutting particulate volume in half over the service life of the trucks.
Natural gas in liquefied rather than compressed CNG form appealed to Taormina. LNG’s high density simplifies both on-site and truck storage and speeds truck fueling. A 90-gallon tank of LNG will run a Taormina truck for up to 12 hours, a third the tankage needed for CNG. On-site storage and truck fueling with LNG are also more efficient. At Taormina's Anaheim complex, a compact 6000 gal. Amoco LNG fueling facility allows drivers to safely fill their onboard tanks at 25 gpm.

Installed costs for dual-fuel engines are reported to be lower - usually much lower - than for typical alternative fuel engines. The like-diesel performance is important to Taormina's management because engine output impacts everything from truck scheduling to driver morale and turnover. Compression ignition of the blended natural gas and diesel is the reason for the strong engine performance, the company said.

In conventional natural gas engines, combustion is achieved by using spark ignition, restrictive air throttles, and relatively low compression ratios. The combination elevates engine-operating temperatures, reduces engine power and lowers fuel combustion efficiency, sometimes enough to cancel an often-attractive price spread between natural gas and diesel. 

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.

Power Systems Associates' Kevin Campbell noted that with certain high, torque-rise dual-fuel engines, there is a torque drop-off of less than 15 percent at the highest power levels. He explained that this results from fine-tuning the engines' power settings in order to eliminate a naturally occurring detonation of the compressed blend of natural gas and intake air. The point at which the detonation occurs is well defined and can be predetermined, permitting the power setting to be lowered to just below the detonation mark, thereby eliminating a noticeable power drop-off even at high power levels.

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.

Trucks that use alternative fuels are typically in home-at-night fleets: short haul truckers, transit buses and municipal service trucks like Taormina's refuse trucks that are tethered to one or two central natural gas fueling facilities. This structure may ease for truckers using dual-fuel engines. 

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. 

When it is time to trade or sell the truck its dual-fuel components can be easily removed, returning the truck to the configuration and value of a truck powered by an electronic diesel.

To fuel the Taormina LNG fleet Amoco Oil Co installed a 6000 gal, fully automated LNG fueling station that communicates with Taormina’s computerized management system for instant and accurate accounting of fuel use