Dual fuel engine combustion and emissions -- an experimental investigation coupled with computer simulation

Abstract

Alternative fuels have been getting more attention as concerns escalate over exhaust pollutant emissions produced by internal combustion engines, higher fuel costs, and the depletion of crude oil. Various solutions have been proposed, including utilizing alternative fuels as a dedicated fuel in spark ignited engines, diesel pilot ignition engines, gas turbines, and dual fuel and bi-fuel engines. Among these applications, one of the most promising options is the diesel derivative dual fuel engine with natural gas as the supplement fuel. This study aims to evaluate diesel and dual fuel combustion in a natural gas-diesel dual fuel engine. More dual fuel engines are being utilized due to stricter emission standards, increasing costs of diesel fuel and decreasing costs of natural gas. Originally sold as diesel engines, these units are converted to natural gas-diesel fuel engines using an aftermarket dual fuel kit. As natural gas is mixed with air intake, the amount of diesel used is reduced. The maximum natural gas substitution is limited by knock or emissions of carbon monoxide and total hydrocarbons. In this research a John Deere 6068H diesel engine is converted to dual fuel operation. The engine is a Tier II, 6 cylinder, 6.8 liter, 4-stroke compression ignition engine with a compression ratio of 17:1 and a power rating of 168 kW at 2200 rpm. A natural gas fuel system is installed to deliver fuel upstream of the turbocharger compressor. The engine operates at 1800 rpm through five different load points in diesel and dual fuel operating modes. Crank angle resolved high speed combustion pressure data is obtained and analyzed. The natural gas substitution values tested are representative of standard dual fuel tuning, with a maximum diesel displacement of 70%. Data for thermal efficiency, combustion stability, in-cylinder pressure and net heat release rate are also presented in this study. In addition, fuel consumption and pollutant emissions are measured. Elevated CO and HC emissions are observed at low loads for dual fuel operation. Overall, CO and unburned HC emissions increase for dual fuel operation. However, the average levels of PM and NOx substantially decreases. A series of natural gas and injection timing sweep are conducted to optimize the combustion and emission in dual fuel engine. To understand the location of emissions inside the cylinder, a model study of a natural gas-diesel dual fuel combustion and emission is performed using the commercial CONVERGE CFD code. A reduced chemical kinetic mechanism with 86 species and 393 reactions for n-heptane, methane, ethane and propane is used. A preliminary hypothesis for these emissions is formulated based on the values of experiment equivalence ratio. Findings indicate that a large amount of CO and HC emissions in dual fuel engines are mainly located on the cylinder wall and nozzle area. High temperatures are not able to propagate through the lean mixture of natural gas and air in dual fuel engine hence high unburned fuel trapped at wall. It is concluded that dual fuel engines are capable of reducing emissions and cost saving (through diesel displacement up to 70%) in diesel fuel engines. CO and unburned HC can be reduced with the application of a dual fuel optimization map. Further investigation using oxidation catalyst is recommended in order to meet with emission regulations.