Cold Start on Diesel Engine: Is Low Compression Ratio Compatible with Cold Start Requirements? 2008-01-1310
Future emission standards for Diesel engine will require a drastic reduction of engine-out NOx emissions with very low level of particulate matter (PM), HC and CO, and keeping under control fuel consumption and combustion noise. One of the most promising way to reach this challenge is to reduce compression ratio (CR). A stringent limitation of reducing Diesel CR is currently cold start requirements. Indeed, reduction of ambient temperature leads to penalties in fuel vaporization and auto ignition capabilities, even more at very low temperature (-20°C and below).
In this paper, we present the work operated on an HSDI Common rail Diesel 4-cyl engine in three area: engine tests till very low temperature (down to -25°C); in cylinder imaging (videoscope) and CFD code development for cold start operation.
First, combustion chamber is adapted in order to reach low compression ratio (CR 13.7:1). Then, tests at low CR are compared with conventional CR on the same engine basis and results are discussed. After that, a complete recalibration of injection settings (fuel quantity and timing, injection pressure...) is operated. It allows to reduce significantly start delay with low CR, and reference start delay with conventional CR becomes reachable. Moreover, effects of combustion chamber design such as spray position vs glow plug are studied and show a great potential regarding behaviour in cold conditions notably in order to reduce again start delay in such conditions.
In addition to engine tests, CFD combustion calculations are performed during starting operation in ambient then cold conditions. Results are in accordance with optical observations and cylinder pressure data recorded on the engine. Correlations between tests and calculations give consistency explanations justifying cold start mechanisms and first order phenomena concerning vaporization and combustion.