Development of a High Performance Natural Gas Engine with Direct Gas Injection and Variable Valve Actuation

Abstract

Natural gas is a promising alternative fuel for internal combustion engine application due to its low carbon content and high knock resistance. Performance of natural gas engines is further improved if direct injection, high turbocharger boost level, and variable valve actuation (VVA) are adopted. Also, relevant efficiency benefits can be obtained through downsizing. However, mixture quality resulting from direct gas injection has proven to be problematic. This work aims at developing a mono-fuel small-displacement turbocharged compressed natural gas engine with side-mounted direct injector and advanced VVA system. An injector configuration was designed in order to enhance the overall engine tumble and thus overcome low penetration. Gas injection, interaction thereof with charge motion and geometrical bounding walls, and the resultant mixture formation process was investigated in detail by the combination of planar laserinduced fluorescence (LIF) in an optical engine and computational fluid dynamics (CFD) analysis with moving injector model to verify the design of the injector and combustion chamber. Then a prototype engine was tested to compare the rated torque against target performance. The planar LIF investigation underlined the influence of the Coandǎ effect whereby the gas jet was deflected to the adjacent injector niche and then to the combustion chamber roof. Such effect was inhibited at early injection timings due to strong intake air flow. CFD analysis confirmed this behavior and pointed out that the mixing process is dominated by the gas jet during injection and flow patterns promoted by it. It was concluded that the principal mixing mechanism is the jet-promoted tumble and elliptical swirl motion, and the mixing rate is thereby scaled with absolute time, rather than crank angle degree, and mainly determined by the strength of these two motion patterns. It was in addition found that the injection contributes to combustion-relevant turbulence mainly by intensifying the large-scale charge motion. Overall high mixing capacity was observed, and the injector and combustion chamber design deemed efficacious. The engine design has been successfully accomplished and the prototype multi-cylinder engine (MCE) is ready for extensive performance and emission analysis on the test rig.

Experimental Investigation of Methane Direct Injection with Stratified Charge Combustion in Optical SI Single Cylinder Engine

“…Also, to inject the fuel at a higher load and maximize the pressure difference between the fuel injection system and the cylinder compression pressure, the fuel was injected at the highest possible pressure -18 bar. As mentioned previously, injection pressures of 18 bar are sufficient for operation at full load [20,24].…”

“…In port fuel injection (PFI) systems, the ICE's volumetric efficiency is reduced by occupied air volume in the intake manifold, which reduces engine performance [23]. Despite this drawback, PFI is still currently the dominant technology for injecting gaseous fuel into engines for light duty vehicles [24].…”

“…Direct injection (DI) systems could be used to achieve a better torque and power output as well as higher volumetric efficiencies that could rival those of gasoline engines [21]. Several recent industrial and academic research projects have examined the use of natural gas (methane) DI systems in experimental single cylinder engines and standard vehicle research engines [20,24,25,26].…”

“…If the latter approach is chosen, the compression work will reduce the overall engine efficiency [22]. Several studies have shown that lower gas injection pressures of ~16-30 bar are sufficient to deliver the quantity of fuel required for stable engine operation, even under full load [20,21,24].…”

“…Consequently, the difference between the injection and compression pressures must be large enough to ensure that sufficient fuel is injected during the injection period [29]. The injector's positioning (top or side injection) can also influence the mixing of the injected gas and the air flow in the combustion chamber [24,26]. Some studies have evaluated the performance of CNG DI systems using combustion modes including homogeneous stoichiometric, homogeneous lean burn, and stratified lean burn.…”