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| Illustration 1 | g02192355 |
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Fuel system schematic (typical example) | |
The fuel supply circuit is a conventional design for engines that use fuel injectors. The fuel supply circuit uses a fuel transfer pump to deliver fuel from the fuel tank to the electronic fuel injectors. The transfer pump is a fixed displacement gear pump.
The fuel flows then through a fuel filter before entering the fuel supply manifold. A fuel priming pump is on the fuel filter base to fill the system. The system must be primed after the filter changes. The system must be primed after draining the fuel supply and return manifolds, when the fuel injectors are replaced.
The fuel flows continuously from the fuel supply manifold through the fuel injectors. The fuel flows when either the supply or the fill port in the injector is not closed by the injector body assembly plunger. The fuel that is not injected into the cylinder is returned to the tank through the fuel return manifold.
A pressure regulating valve is at the end of the fuel return manifold. The pressure regulating valve controls the entire fuel system pressure. The regulation provides proper filling of the fuel injectors.
The electronically controlled mechanically actuated fuel injector system provides total electronic control of injection timing. The injection timing is varied to optimize the engines performance.
The timing ring is part of the rear gear group. A signal is generated by the engine speed/timing sensor. This information is for detection of crankshaft position and for engine speed. Other information and these data allow the ECM to send a signal to the injector solenoids. The fuel injector solenoid is energized to begin fuel injection. The fuel injector solenoid is de-energized to end fuel injection. Refer to Systems Operation, "Fuel Injector".
Cat® Large Mining Truck (LMT) products powered by diesel and liquid natural gas (LNG) fuels are equipped with fully integrated diesel and LNG fuel tanks located in the same space claim as the standard diesel tank. In the pump-less LNG system designed for the Cat LMT, the bulk temperature of the LNG fuel in the tank effectively determines the tank pressure driving natural gas to the engine. The system is designed to operate at
Clean LNG fuel that meets Caterpillar fuel recommendations provides outstanding engine service life and performance. As per fuel cleanliness standard ISO 4406, the LNG fuel needs to meet or exceed a standard of ISO 18/16/13 cleanliness. Fuels not meeting Caterpillar’s minimum specifications will adversely affect the perceived performance of the combustion system, fuel filters, and the service life of the gaseous fuel injection system, valves, pistons, liners, and bearings.
Refer to the LNG Powered Large Mining Truck Application and Installation Guide, LEBT0003, for more information on LNG fuel specifications and LNG fueling station specifications. The publication is available from the Electronic Media Center.
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| Illustration 2 | g06311772 |
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(1) Gas admission valve
(2) Gas line to inlet runner (3) Gas supply from vaporizer (4) Gas shutoff solenoid (5) Gas pressure regulater | |
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| Illustration 3 | g06311747 |
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(1) GAV
(2) Gas line to inlet runner (6) Rear right-hand fuel line to fuel rail (7) Fuel rail (8) GAV ECM (right-hand side) (9) Engine ECM | |
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| Illustration 4 | g06311748 |
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(1) GAV
(2) Gas line to inlet runner (7) Fuel rail (10) In Cylinder Pressure Sensor (ICPS) ECM (11) GAV ECM (left-hand side) (13) Rear left-hand fuel line to fuel rail | |
Dynamic Gas Blending (DGB)TMor DGBTM is Caterpillar’s brand of dual fuel engines and retrofit kits. The DGBTM system utilizes key technology building blocks like multiport gas admission, in-cylinder pressure sensing, and advanced controls to deliver diesel-like performance with significant fuel cost savings.
Natural gas flows from the LNG tank module to the engine at the rear gaseous fuel train. The rear gaseous fuel train components include a gaseous fuel shutoff valve (GSOV), pressure reducing regulator, and a manifold block with pressure and temperature sensors. Downstream of the manifold, natural gas is supplied to a gas rail mounted on each bank of the engine. The gas rail has solenoid-operated gas admission valves (SOGAV) at each cylinder that provides gaseous fuel to the air intake runner and into the cylinder head to the combustion chamber.
The DGBTM system requires a minimum natural gas pressure above boost pressure to overcome the intake manifold air pressure during gaseous fuel injection. Routed to the gas regulator is a boost reference line that is used to maintain approximately a
The DGBTM engine ECM optimizes and controls the ratio of diesel to LNG by monitoring operating conditions that include engine speed, engine load, ambient pressure and temperature, and other engine conditions. The In Cylinder Pressure Sensors (ICPS) monitor the combustion characteristics to provide feedback for optimization of the gas and diesel ratio. Diesel displacement is maximized at medium to high load factors due to the physics of the combustion process. An engine continuously producing full power is operating at a load factor of 100%. Earth moving machines may reach 100% load factor intermittently, but seldom operate at this level for extended periods of time. Periods spent idling, traveling unloaded, traveling in reverse, maneuvering at part throttle, and operating downhill are examples of conditions which reduce load factor.
The terms "LNG substitution" and "diesel displacement" are often used interchangeably to quantify the amount of diesel and LNG consumption on dual fuel engines. In a practical sense, diesel displacement is the volume of diesel displaced by LNG as follows:
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| Illustration 5 | g06310434 |
Diesel displacement can be measured in several ways, but is most easily quantified using the engine ECM, which calculates the volume of diesel consumed during dual fuel operation and the volume of diesel that would have been consumed if running in diesel only mode. Cycle displacement is influenced by several variables, like truck application and idling, engine load factors, haul gradient and profiles, and operator performance. All these factors need to be accounted for in estimating cycle diesel displacement
The figure below shows the expected average diesel displacement over an operating cycle load factor for the 785C DGBTM product.
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| Illustration 6 | g06310425 |
The DGBTM truck is designed and capable of full rated power while running on a 100% diesel without affecting machine performance. The dual fuel engine system is designed to deliver the same speed on grade as the existing diesel truck under the same operating conditions.
Caterpillar expects engine and truck life to be the same as a diesel only truck, with the same maintenance and planned component replacement (PCR) intervals. There may be an upside potential for engine maintenance intervals and life, as LNG burns cleaner then diesel. But this has not been practically verified.