Injection strategies for low HC raw emissions in SI engines with CNG direct injection

Abstract

The energy source natural gas represents an attractive alternative fuel for SI engines due to its suitable chemical properties. But the lower performance yield and the insufficient infrastructure lead to a rejection by the customer. One approach to gain acceptance is the improvement of the driveability of the natural gas-powered vehicles. Direct injection of CNG is a technology with potential to achieve this objective. Nevertheless, the combustion concept has high requirements on injection system and mixture formation. Among other challenges the methane slip raises major issue. The compact molecular structure of the CNG's main component, methane, leads to an extremely slow reactivity, which expresses itself in a higher catalytic converter light-off temperature and thus aggravated exhaust aftertreatment. Especially, since methane's global warming potential is 21 times higher than CO 2 , the greenhouse gas footprint is getting worse again. Thus, for the future implementation of such a combustion system concept it will be necessary to compensate the limited conversion rates of the catalytic converter with low raw emissions. For this, the comprehension of the mixture formation is necessary. This paper deals with different injection strategies at part load and charged operation to achieve HC raw emissions as low as possible. The testbench experiments are executed with a two-cylinder SI engine under homogeneous stoichiometric conditions. The DI injector used for this study is a prototype model fabricated by Robert Bosch GmbH. In summary, the need of adaption of the injection strategy to the respective engine map area is becoming evident. Furthermore, various injection timings and injection splitting strategy turned out as an appropriate measure to make the most of CNG-DI's potential and to achieve low HC raw emissions.

A comparative study of directly injected, spark ignition engine performance and emissions with natural gas, gasoline and charge dilution

“…Though not depicted here, similar trends are observed at other tested engine speeds and loads up to 9 bar BMEP. Note that CO emissions increase when SOI advances from 240 to 280 deg bTDCFire (Figure 4e) despite the increased mixing time, which was also observed in previous studies [15,36]. These other studies reasoned that the early start of fuel injection likely disrupts the incoming air stream, leading to rich pockets that increase CO emissions.…”

“…DI CNG has the potential to overcome the limitations of PFI systems by mitigating the loss of volumetric efficiency. Furthermore, DI CNG allows for improved scavenging and in-cylinder, multiple injection capability as opposed to CNG PFI technology [2,9]. Husted et al [10] showed that up to two-thirds of the torque deficiency between GDI and PFI CNG operation can be recovered with DI CNG.…”

Experimental Design, Testing, and Evaluation of Methods to Improve the Efficiency and Reduce Emissions from a Small Two-stroke Natural Gas Engine

“…Injection pressure was found to have minimal effect on the emission levels of HC. Injection timing was paramount in forming the proper mixture formation to achieve a low HC emissions [52].…”

“…Basic conical injector flow schematic[64].Based on calculations using Equations 4-6[52], I found that the ideal injection pressure window to maintain choked (or maximum) injection flow occurred from 180 to 65 CAD BTDC for an injection pressure of 275 kPa gauge. For an injection pressure of 137 kPa gauge this pressure window reduced into 180 to 83.5 CAD BTDC.…”