Performance of Diesel-Compressed Natural Gas (CNG) Dual Fuel (DDF) Engine via CNG-Air Venturi Mixjector Application

The performance of the diesel engines can be improved by integrating the existing diesel fuel system with another type of fuel which is compressed natural gas (CNG) and this system is called as diesel-CNG dual fuel (DDF). In DDF system, the performance of the engines are greatly influenced by mixing quality of CNG and air before combustion chamber. There are many possible designs of CNG and air mixer available in the market; however, there are limited design using a combination of CNG injector and air mixer which is named as CNG-air venturi mixjector. Hence, this research is conducted to design new type of CNG-air venturi mixjector and to determine the effects of injector numbers in mixjector toward diesel engine performance. CNG-air venturi mixjector was installed at the entrance of the intake manifold, and only three different numbers of injectors were tested. The engine performance tests under full load condition were conducted over the engine speed range from 1,000 to 3,000 rpm for power output and for exhaust gas concentrations, the range was set from 1,000 to 2,800 rpm. Exhaust gas concentrations under various loading conditions were analyzed from light to heavy loading using rated power speed that can produce higher power output. The concentrations tested under both conditions included nitric oxides (NOx), carbon dioxide (CO2) and carbon monoxide (CO) concentrations. Meanwhile, unburned hydrocarbons concentration tests were only conducted at full load condition. DDF system with four injectors in CNG-air venturi mixjector shows higher power output at rated power speed of 2,800 rpm and less exhaust gas concentrations under maximum loading condition (100% load) compared to the conventional diesel engine, DDF system without CNG-air venturi mixjector and DDF system with two and six injectors in CNG-air venturi mixjector. CFD simulator was used to study the possible mechanism for an optimum numbers of injectors that resulted in better engine performance focused in flow behavior of CNG and air in the CNG-air venturi mixjector connected to the intake manifold. CNG-air venturi mixjector with four injectors results in homogeneous mixture and thus lead to the high combustion efficiency. Based on the findings, the new design of CNG-air venturi mixjector can be very valuable if opted to apply this technology on existing diesel engine.