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LNG Tractor-Trailers with HPDI Engines

DOE’s National Renewable Energy Laboratory (NREL) and Westport Innovations are working together to develop and to deploy LNG (liquefied natural gas) fueled engines with high-pressure direct-injection (HPDI) technology in tractor-trailers. This project, part of the Next Generation Natural Gas Vehicle (NGNGV) program, is targeted at delivering a heavy-duty engine suitable for tractor-trailers than can be certified to US EPA 2007 emission standards.

The NGNGV program is an activity of the Natural Gas Vehicle Technology Forum (NGVTF)—a group of government and industry stakeholders working to advance gaseous fuel vehicles and infrastructure. Gaseous fuels of interest include natural gas, hydrogen, and hydrogen-natural gas blends.

Current medium- and heavy-duty natural gas engines use lean-burn spark ignition technology to achieve low emissions with full-load efficiencies slightly lower than those of conventional diesel engines. Part-load efficiencies for the natural gas engines, however, are much lower than those of diesel engines.

The Westport HPDI system directly injects a small quantity of pilot diesel fuel into the engine cylinder to provide diesel-like compression ignition for natural gas. This gives the engine the efficiency and low-speed torque advantages of compression ignition while using natural gas as the primary fuel.

Westport and Cummins started development of the engine by combining a standard Cummins 15-liter ISX 450 diesel engine with Westport HPDI natural gas fueling and exhaust gas recirculation (EGR). The resulting engine—what would be called the 15L ISX G—was to meet the following specifications:

  • Peak torque of 1,650 lb-ft (2,236 Nm)

  • Rated power of 450 hp (335 kW)

  • Peak thermal efficiency of 40% with typical diesel part-load efficiency

  • NOx emissions of 0.5 g/bhp-hr

  • PM emissions of 0.1 g/bhp-hr

  • Substitution of diesel with natural gas greater than 90% over the certification

The engineers discovered that the otherwise unmodified Cummins ISX engine with EGR (Exhaust gas Recirculation) and HPDI fueling could not meet the program goals. The development team then fitted a turbocharger using a smaller trim compressor and a second EGR cooler to the engine along with an oxidation catalyst. It worked.

Cummins Westport HPDI Ultra-Low NOx Natural Gas Engine
 Program GoalEngine Result
Power450 hp450 hp
Torque1,650 lb-ft (2,236 Nm)1,650 lb-ft (2,236 Nm)
Peak thermal efficiency40%39.5%
Average thermal efficiency32–35%34.1%
NOx emissions 0.5 g/bhp-hr0.6 g/bhp-hr
PM emissions0.1 g/bhp-hr0.03 g/bhp-hr
Diesel % of total fuel consumption<10% 6%

The EPA 2007 emissions targets to meet are 1.2 g/bhp-hr NOx and 0.01 g/bhp-hr PM. The current prototype engine thus beats the NOx standard but still has further to go to meet that PM target.

(Meeting that PM target is going to prove challenging whatever the technology system. Truckmakers are hard at work on a variety off approaches, including modified HCCI (Homogeneous Charge Compression Ignition) systems with accompanying improved engine control software.)

After some further development, the engines will be installed in tractor-trailers with liquefied natural gas (LNG) fuel systems, and the trucks will be deployed in a California-based fleet. NREL and Westport will evaluate the fleet’s operating costs, fuel economy, reliability, and emissions.

NREL is cost-sharing $1.5 million of the 2-year, $1.9 million project, with the remainder cost-shared by Westport. NREL’s funding is provided by DOE’s FreedomCAR and Vehicle Technologies and Clean Cities Programs.




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