Development of a Fuel Injection Strategy for Diesel LTC 2008-01-0057
A production V-8 engine was redesigned to run on low temperature combustion (LTC) with conventional Diesel fuel. Two fuel injection strategies were used to attain reduction in soot and NOx; a) early premixed injection strategy: fuel injected early during the compression stroke and b) late premixed injection strategy: fuel injected close to TDC with heavy EGR. The early premixed injection strategy yielded low NOx and soot but struggled to vaporize the fuel as noted in unburned hydrocarbons readings. The late premixed injection strategy introduced the fuel at higher in-cylinder temperatures and densities, improving the fuel's vaporization and limited the unburned hydrocarbon and carbon monoxide. The use of high EGR and high injection pressure for late premixed injection strategy provided sufficiently long ignition delay that resulted in partially premixed cylinder charge before combustion, and thereby prevented high soot, even in presence of high EGR. The engine operation was enhanced by a specially designed controller to balance all engine cylinders with the aide of in-cylinder pressure transducers. Under early premixed injection timings, where combustion phasing control was more challenging, the controller was responsible for reducing the soot in some cases by over 50%. The engine mapping was performed for a range of BMEP up to 7 bar.
The CHEMKIN simulation of the combustion process reproduced the experimental results, specially the start of cool flame reactions for the early premixed injection strategy. The simulations provided a useful development tool to frame the boundary conditions towards optimum ignition timing.