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A diesel piloted natural gas engine's performance varies depending on operating conditions and has performed best under medium to high loads. It can often equal or better the fuel conversion efficiency of a diesel-only engine in this operating range. This paper presents a study performed on a multi-cylinder Cummins ISB 6.7L diesel engine converted to run stoichiometric natural gas/diesel micro-pilot combustion with a maximum diesel contribution of 10%. This study systematically quantifies and ranks the sensitivity of control factors on combustion and performance while operating at medium loads. The effects of combustion control parameters, including the pilot start of injection, pilot injection pressure, pilot injection quantity, exhaust gas recirculation, and global equivalence ratio, were tested using a design of experiments orthogonal matrix approach.

Natural gas offers the potential to reduce greenhouse gas emissions from heavy-duty on-road transportation. One of the challenges facing natural gas as a fuel is that its composition can vary significantly between different fuel suppliers and geographical regions. In this work, the impact of fuel composition variations on a heavy-duty, direct injection of natural gas engine with diesel pilot ignition is evaluated. This combustion process results in a predominantly non-premixed gaseous fuel combustion event; as a result, end-gas autoignition (knock) is not a concern. Changes in the fuel composition do still impact the combustion, both through the changes in the chemical kinetics of the reactions and due to changes in the density of the fuel. Increasing concentrations of heavier hydrocarbons, such as ethane or propane, in the fuel lead to higher fuel densities and hence greater fuel mass being injected for a given injection duration.

Retaining the diesel combustion process but burning primarily natural gas offers diesel-like efficiencies from a natural-gas fuelled heavy-duty engine. This combustion event is limited by the injection pressure of the fuel, as this dictates the rate of mixing and hence of combustion. Typical late-cycle direct injection applications are limited to approximately 300 bar fuel pressure. The current work reports on tests for the first time at natural gas injection pressures up to 600 bar. The results show that significant efficiency and particulate matter reductions can be achieved at high loads, especially at higher speeds where the combustion is injection rate limited at conventional pressures. Increases in combustion noise and harshness are a drawback of higher pressures, but these can be mitigated by reducing the diameter of the nozzle gas holes to control the fuel injection rate.

Natural gas high pressure direct injection (HPDI) engines represent a technology with the potential for lower engine-out emissions and reduced fuel costs over a diesel engine. This combustion process uses a direct injection of natural gas, into the combustion chamber of a high compression ratio engine, to maintain diesel engine performance. As natural gas will not auto-ignite at typical engine conditions, a small quantity of diesel pilot fuel is used to initiate the combustion event. One potential technique to improve engine performance is the optimization of the intake and exhaust valve timings. To experimentally investigate these effects, tests were performed on a single cylinder engine based on Westport Innovation's 15L HD engine. The intake valve closing time was varied both before and after the standard closing (EIVC and LIVC). Early closing of the exhaust valve was also tested (EEVC).

To maximize payback for operators, it is important that natural gas engines for heavy-duty on-road applications minimize fuel consumption. To directly replace a diesel engine for a given vehicle mass and duty cycle, the natural gas engine also needs to match the diesel's power and torque characteristics. This paper reports the results of a development project to increase the torque and power of Westport's 15L 356 kW pilot-ignited, late cycle direct injection of natural gas engine by 10%, while matching or improving efficiency and maintaining emissions compliance. The strategies evaluated to achieve these objectives were to recover some of the exhaust energy with a power turbine, to increase the injector flow area to avoid excessively long combustion durations and to reduce the compression ratio to keep peak cylinder pressure below its maximum limit.

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.

Development status and insight on a “research level” piezoelectric direct injection fuel injection system for prototype hydrogen Internal Combustion Engines (ICEs) is described. Practical experience accumulated from specialized material testing, bench testing and engine operation have helped steer research efforts on the fuel injection system. Recent results from a single cylinder engine are also presented, including demonstration of 45% peak brake thermal efficiency. Developing ICEs to utilize hydrogen can result in cost effective power plants that can potentially serve the needs of a long term hydrogen roadmap. Hydrogen direct injection provides many benefits including improved volumetric efficiency, robust combustion (avoidance of pre-ignition and backfire) and significant power density advantages relative to port-injected approaches with hydrogen ICEs.

The use of exhaust gas recirculation (EGR) to control NOx emissions from direct-injection engines results in the reintroduction of exhaust particulate matter (PM) into the intake manifold. The influence of this recirculated PM on emissions from a pilot-ignited direct injection of natural gas engine was studied by installing a filter in the EGR system. Comparison tests at fixed engine conditions were conducted to identify differences between filtered and unfiltered EGR. No significant variations in gaseous or PM mass emissions were detected. This indicates that the recirculated PM is not contributing substantially to the increases in PM mass emissions commonly observed with EGR. Reductions in black carbon and ultra-fine particle exhaust concentrations in the exhaust were observed at the highest EGR fractions with the filter installed.

A diesel pilot-ignited, high-pressure direct-injection of natural gas heavy-duty single-cylinder engine was fuelled with both natural gas and blends of 10% and 23% by volume hydrogen in methane. A single operating condition (6 bar GIMEP, 0.5 ϕ, 800 RPM, 40%EGR) was selected, and the combustion phasing was varied from advanced (mid-point of combustion at top-dead-center) to late (mid-point of combustion at 15°ATDC). Replacing the natural gas with hydrogen/methane blend fuels was found to have a significant influence on engine emissions and on combustion stability. The use of 10%hydrogen was found to slightly reduce PM, CO, and tHC emissions, while improving combustion stability. 23%hydrogen was found to substantially reduce CO and tHC emissions, while slightly increasing NOx. The greatest reductions in CO and tHC, along with a significant reduction in PM, were observed at the latest combustion timings, where combustion stability was lowest.

The use of pilot-ignited, direct-injected natural gas in a heavy-duty compression-ignition engine has been shown to reduce emissions while maintaining performance and efficiency. Adding recirculated exhaust gas (EGR) has been shown to further reduce emissions of nitric oxides (NOx), albeit at the cost of increased hydrocarbons (tHC), carbon monoxide (CO), and particulate matter (PM) emissions at high EGR fractions. Previous tests have suggested that reducing the delay between the diesel and natural gas injections, increasing the injection pressure, or adjusting the combustion timing have individually achieved substantial emissions benefits. To investigate the effectiveness of combining these techniques, and of using them over a wide range of operating conditions, a series of tests were carried out. The first set of tests investigated the interactions between these effects and the EGR fraction.

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.

Determining what exhaust gas recirculation (EGR) control parameters have the largest impact on engine performance and emissions is of critical importance when developing an EGR-equipped engine. These tests studied the effects of varying the net charge mass, the fresh air charge mass, the indicated power, and the oxygen equivalence ratio at various EGR fractions. The research was carried out on a direct-injection, natural gas fuelled, pilot-ignited four-stroke heavy-duty engine using Westport Innovations Inc.'s pilot-ignited, direct injection of natural gas technology. The testing was carried out using a prototype injector and the standard diesel-fuelled engine's combustion chamber. The results indicate that fuel efficiency, as well as emissions of Nitrogen Oxides (NOx) and Carbon Monoxide (CO) depend primarily on the EGR level, and not on the values of the EGR control parameters.

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.

Pilot-ignited direct injection of natural gas fuelling of a heavy-duty compression ignition engine while using recirculated exhaust gas (EGR) has been shown to significantly reduce NOx emissions. To further investigate emissions reductions, the combustion timing, injection pressure, and relative delay between the pilot and main fuel injections were varied over a range of EGR fractions while engine speed, net charge mass, and oxygen equivalence ratio were held constant. PM emissions were reduced by higher injection pressures without significantly affecting NOx at all EGR conditions. By delaying the combustion, NOx was reduced at the expense of increased PM for a given EGR fraction. By reducing the delay between the pilot and main fuel injections at high EGR, PM emissions were substantially reduced at the expense of increased total hydrocarbon (tHC) emissions. In this research, no attempt was made to optimize the injector or combustion chamber for natural gas fuelling with EGR.

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.