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The high-pressure direct injection (HPDI) of natural gas permits diesel engines to retain their performance and high fuel economy while reducing regulated emissions. In the work presented in this paper, a pilot diesel fuel ignites directly injected natural gas, and both fuels are injected through a single injector. Recently the HPDI engine achieved a combined NOx+nmHC emissions of 2.38 g/bhp-hr during official certification tests performed under the US EPA specified FTP cycle for heavy-duty diesel engines. NOx, nmHC and PM emissions were reduced by 45%, 85% and 71%, respectively, compared to the 1998 EPA emissions requirement. These results are consistent with previously reported results on a two-stroke engine. The present study clearly demonstrates that a combination of gas injection timing and pressure can significantly reduce NOx emissions while retaining the overall thermal efficiency.

A heavy-duty compression ignition engine using EGR and pilot-ignited directly injected natural gas fueling was calibrated for low NOx emissions. A Cummins ISX engine using cooled EGR was fitted with a Westport HPDI™ fuel system and an oxidation catalyst. The base engine hardware was modified to increase EGR rates (up to 40%). The engine, rated at 336 kW (450 hp) and 2236Nm (1650 ft-lbs), was calibrated and tested over steady state and transient test cycles. Steady state testing over the ESC 13-mode test cycle resulted in weighted composite NOx emissions of 0.36 g/bhp-hr and particulate matter emissions of 0.04 g/bhp-hr. Transient testing over the US EPA specified FTP cycle resulted in average NOx emissions of 0.6 g/bhp-hr and PM emissions of 0.03 g/bhp-hr.

A turbocharged lean burn natural gas engine was upgraded to operate on a blend of hydrogen and natural gas (HCNG). Tests were carried out to determine the most suitable H2/NG blend for H2 fractions between 20 and 32 vol%. A 20 vol% H2 content was found to provide the desired benefits when taking into consideration the engine and vehicle performance attributes. A full engine map was developed for the chosen mixture, and was verified over the steady-state AVL8 cycle. In general, the HCNG calibration included operation at higher air-fuel ratios and retarded spark timings. The results indicated that the NOx and NMHC emissions were reduced by 50% and 58% respectively, while the CO and CH4 emissions were slightly reduced. The HCNG engine torque, power and fuel consumption were maintained the same as for the natural gas fuel. The chassis dynamometer transient testing confirmed large NOx reduction of about 56% for HCNG operation.

A new fuel injector prototype for heavy-duty engines has been developed to use direct-injection natural gas with small amounts of entrained diesel as an ignition promoter. This “co-injection” is quite different from other dual-fuel engine systems, where diesel and gas are introduced separately. Reliable compression-ignition can be attained, but two injections per engine cycle are needed to minimize engine knock. In the present paper the interactions between diesel injection mass, combustion timing, engine load, and engine speed are investigated experimentally in a heavy-duty single-cylinder engine. For the tests with this injector, ignition delay ranged from 1.2–4.0 ms (of which injector delay accounts for ~0.9 ms). Shorter ignition delays occurred at higher diesel injection masses and advanced combustion timing. At ignition delays shorter than 2.0 ms, knock intensity decreased with increasing ignition delay.