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Carburetion & Turbo Systems, Inc
Dual-fuel Caterpillar C-12 engines draw raves from California's fleet for performance, meet CARB optional low NOX standards
By Paul Harder Paul Harder is an industrial journalist based in Port Townsend, Wash. DIESEL PROGRESS NORTH AMERICAN EDITION October 2000 p. 40

Natural gas, after a decade of fruitless promotion as fuel for heavy-duty trucks, is being helped by an improbable ally to become the fuel-of-choice among firms that formerly relied entirely on diesel for long haul service.

Surprisingly it is automotive diesel - the fuel that natural gas is calculated to replace - coupled with the latest in electronic engine control technology, that is transforming trucker attitudes about natural gas as an over-the-road truck fuel.

In central California's San Joaquin Valley, Harris Ranch, one of the largest feedlots west of the Mississippi, has joined the ranks of several other heavy-duty fleets in embracing natural gas, pointing to a dual-fuel combustion technology as the reason for its change of heart.  Like other firms that have modified Caterpillar electronic diesels to dual-fuel technology, Harris Ranch has found that performance of its converted engines matches that of the same engines running on straight diesel.  In addition, the dual-fuel engines have demonstrated lower fuel costs and achieve CARB's optional low NOx certification standard of 2.5 g/bhp-hr NOX, and 0.10 g PM.

The Harris Ranch trucks operating on natural gas are Freightliner Century heavy-duty tractors.  They are additions to a 95-truck fleet used for hauling livestock, cattle feed and beef between Coalinga and communities throughout the west and southwest.  Half of a 24-truck order had its 380/430 hp Caterpillar C- 12 electronic diesels converted to Caterpillar natural gas dual-fuel C-12 electronic engines by the local CAT dealer, Quinn Engine Systems.  The other 12 tractors entered service with the factory configuration and rating of their C-12 engines unchanged.

Converting the engines to dual-fuel natural gas was done as part of a licensing agreement between Caterpillar and its L.A. dealer, Power Systems Associates (PSA), under which CAT sells electronic on-highway diesels equipped with dual fuel equipment jointly developed by PSA and Clean Air Partners, San Diego, Calif.  Until recently dual-fuel modifications of selected CAT diesels were made only by CAT engine dealers, usually under the supervision of PSA. 

Recently, Caterpillar began delivering factory-assembled dual fuel engines to Freightliner and other truck manufacturers, allowing CAT dual fuel engines and their related alternative fuel fueling systems to be factory-installed.  Retrofit conversions of the selected CAT on-highway diesel models can also be made by CAT engine dealers.  Removal of the dual-fuel equipment returns the engine to its original electronic diesel configuration and performance.

At 12 L and 410 hp, the engines in the Harris Ranch Freightliner trucks are arguably the largest commercially available alternative fuel engines on the highway.  Though smaller displacement dual fuel CAT conversions have been performing successfully for several years (see May 1999 Diesel Regress), this is believed to be the first application where the performance of the C-12 engine in typical 80,000 lb. heavy-duty service has been matched to that of identical electronic 380-430 C- 1 2 diesels carrying the same loads over routes that include the Grapevine, California's Pacheco Pass and Donner Summit.

Drivers of the Harris Ranch trucks, who alternate between driving the diesel and dual-fuel versions of the C-12 engine, are vigorous enthusiasts for the dual-fuel trucks.  They report they perform as well as their diesel counterparts and run significantly smoother and quieter.  Clean Air Partners, in acknowledging this phenomenon, explained the improvements as the result of a change in the timing of combustion events programmed into ADEM 2000 electronic engine control, which assists in achieving low NOx levels.

Natural gas dual-fuel engines run on mostly natural gas with a small volume of injected diesel serving as the pilot for combustion ignition.  For natural gas to combust in the high-pressure diesel environment, a specific air/natural gas ratio must be achieved and its injection precisely coordinated with that of the pilot diesel.  Computerized electronic monitoring, control and interlacing of the two streams of fuel make this possible.  It was not until CAT's development of its ADEM and ADEM 2000 electronic control modules that Clean Air Partners had the level of combustion event control needed for its dual-fuel technology to work over the full operating range of the on-highway diesel.

By attaining compression combustion of natural gas at diesel compression levels, there is no need for spark plugs and restrictive air throttles and no increase in heat. In providing diesel-like performance at diesel compression ratios the dual fuel engine makes full use of the braking capability of Jacobs engine brakes essential safety factor for a fleet whose trucks regularly travel some of the west's steepest grades.
The typical dual-fuel combustion mix in highway service averages 15 percent diesel and 85 percent natural gas, with the diesel ratio lowering to 10 percent or less under steady, heavy load conditions.

CAT and Clean Air Partners representatives noted that pilot diesel concentration can drop as low as 1 percent in very large stationary engines - machines whose weight, complexity and cost can accommodate the requisite two discrete diesel injection systems.  It is understood that to approach this level of pilot fuel through addition of a microinjection circuit to a highway engine's design would greatly increase its weight, complexity and cost.
Most important, it would eliminate one of the major attractions of CAT's dual fuel technology: the ability to have off the-shelf electronic on-highway diesels run on natural gas, satisfy tough emissions goals, do it at an affordable cost and in the process have an engine that instantly reverts to its conventional diesel operation if the natural gas fuel system fads.

This automatic diesel fall-back provision was an important consideration of Harris Ranch in choosing the dual-fuel C-12 engine.  With few liquefied natural gas (LNG) facilities along its normal routes, fleet management wanted an alternative fuel system that would get its trucks to their destination, or a service facility, in case of a malfunction in the dual-fuel system.  The ADEM 2000 control provides that insurance.  It is proved to instantly convert the engine from dual-fuel natural gas to full diesel operation when signaled by the natural gas ECU of a loss of fuel or a failure within the natural gas system.
Also spurring Harris Ranch to purchase trucks with the dual fuel systems is the availability of grant funding that covers the incremental cost of converting the original electronic C-12 diesel to dual-fuel operation.  Funding is tied to the engine satisfying CAR-Bs optional low NOx standards, which the manufacturer of record, Power Systems Associates, claims it does with ease. Grant money comes from a consortium of public-private groups seeking reduction of air pollution along an Interstate Clean Air Transportation corridor linking San Diego, Los Angeles, the San Francisco Bay area, Sacramento, Reno, Salt Lake City, Las Vegas and soon Phoenix.  Grant money is also available for construction of LNG fueling stations along this mute. It was used in part to build the 16,000-gallon LNG station at the Harris Ranch complex in Coalinga.

Converting an engine to dual fuel requires relatively minor and mostly external changes to the original electronic diesel engine.  The most significant and noticeable alteration is the replacement of the original intake manifold by one fitted with natural gas injectors and sensors and associated wiring harnesses.  The ECU that monitors and manages the natural gas system is mounted on the cover of the CAT ADEM 2000 ECM.  Combined with the C-12 engine's standard diesel injection system, the new air/natural gas manifold supplies multi-point, sequential natural gas and diesel injection, with each fuel using its own delivery and injection system. The ECM and ECU are programmed to start the engine on straight diesel, then switch to dual-fuel natural gas when the engine reaches operating temperature and accelerates above idle rpm.  The switchover is so seamless that veteran drivers reported that the only way they know when it takes place is when they see the illumination of a dash mounted dual-fuel mode light.  They reported no hesitation or loss of power when the change in fuel takes place.

Though the dual-fuel engine can still run as a full power, high performance electronic diesel, the driver has no way to make it do so.  As part of converting the engine to dual fuel, the ECM is programmed for the engine to run in emergency conditions on straight diesel, though at a 20 percent power derating.  Appropriately called the "limp home" mode, this reduced power diesel operation is triggered when the air/natural gas ECU signals that it is out of natural gas or that the natural gas system is malfunctioning.  Derating is an effective means to discourage driving on straight diesel for anything more than short distance emergency driving.

To maintain low emissions and high fuel efficiency at low power demand an operating level when most diesels are at their worst - the dual-fuel engine switches to a "skip-fir-e" mode.  This kicks in when power demand drops below approximately 60 percent of rated engine power, progressively eliminating cylinders from the combustion sequence by cutting off each cylinder's supply of natural gas and pilot diesel as power demand decreases. As many as three cylinders will be dropped, at which point the ECM switches the engine to full diesel idle.  Depressing the throttle instantly restores optimum fuel to all cylinders and full power from the engine.

To obtain the minimum operating range of 425 miles - a distance dictated to Harris Ranch by the limited number of alternative fueling stations along its fleet routes - the trucks use LNG rather than compressed natural gas (CNG).  The standard two 120 gallon diesel fuel tanks were replaced by a single 60-gallon diesel tank and a single 119-gallon LNG tank.  The cryogenic LNG tank is piped to a heat exchanger that uses the engine cooling circuit to convert -260 degree LNG to vaporized 120-psi gaseous fuel.  A gas pressure regulator, monitored by the ECU, provides the pressure and volume of natural gas needed for the appropriate air/natural gas ratio. With a year of dual-fuel operating experience behind it, Harris Ranch's transportation management stated that it is "bullish on the dual-fuel technology.  The more dependent we are on natural gas the better off we'll be."

Along with reducing engine emissions, Harris Ranch found that natural gas as fuel made it easier to project operating costs because of its relative price stability.  It also sees more long haul dual fuel in its future as the LNG fueling infrastructure improves - something that is already taking place as western states struggle with the need to keep their heavy-duty trucks rolling and at the same time comply with ever-tighter emission restrictions. 

A Harris Ranch Freightliner Century 120 truck equipped with a dual-fuel Caterpillar C-12 engine pulls an 80,000-lb load on California's long, steep Pacheco Pass.  Harris Ranch has 12 tractors that it said match diesel performance while meeting CARB's 2.5-gr/bhp/hr optional low NOx standard.

An LNG fueling station at Harris Ranch was partially paid for by public grants.  To spur greater use of natural gas along a three state clean air corridor, 20 more subsidized LNG stations are planned.  On the Harris Ranch Freightliner trucks, the standard two, 120-gallon diesel fuel tanks were replaced by a single 60-gallon diesel tank and a single 119-gallon LNG tank.

The CAT C-12 diesels used in the Harris Ranch trucks utilize a dual-fuel intake manifold developed by Clean Air Partners.  The manifold incorporates pulse width natural gas injectors and a range of sensors for detecting such parameters as manifold pressure, charge-air temperature and gas pressure and temperature.