Optimizing diesel engine operating conditions

  • 20-Apr-2009 02:13 EDT

Schematic of the four-cylinder common-rail injection premixed charge compression ignition diesel engine used by researchers in a design-of-experiments technique to optimize operating conditions.

More stringent emission standards have led to many innovative developments of diesel engine technologies in terms of engine performance and emissions improvements. Such developments generally fall into two categories: advanced combustion concepts and aftertreatment systems. Because aftertreatment systems tend to be costly, there is an increasing effort by some in the industry to reduce heavy dependency on the aftertreatment systems by improving the emissions performance of the engine itself.

Premixed charge compression ignition (PCCI) diesel engines have the potential to achieve homogeneous mixture in the cylinder, resulting in lower NOx and PM as well as performance improvements. However, it is necessary to optimize operating conditions such as injection timing, multiple injection strategy, cooled exhaust gas recirculation (EGR), intake charging, and swirl control valve (SCV) to achieve the uniform mixture formation. PCCI combustion is typically no simple task due to a trade-off relation between NOx and PM.

The large number of parameters that effect emissions and combustion characteristics requires a complex calibration process, which could generate a seemingly infinite number of experimental conditions. The use of design-of-experiments (DoE) technique can considerably reduce both the number of experimental sets and time requirement. In short, DoE is a statistical analysis technique capable of evaluating experimental responses of a physical system that is effected by numerous factors and interactions between the factors. The approach has been often used in diesel engine optimization studies for achieving low emissions and high combustion performance.

During Wednesday's HCCI (part 5 of 7) session, starting at 8:30 in room W2-65, researchers from Imagineering and Hanyang Universty will present the results a DoE analysis and optimization tool they used to simplify the calibration process of a four-cylinder PCCI engine system.

Studied were the effects of the engine's two-stage fuel-injection strategy along with engine operation conditions (e.g., injection timing, multiple injection strategy, cooled EGR, intake pressure, SCV) on the combustion and emissions, with the goal of achieving the optimum combustion performance with the lowest NOx and PM. However, considering all these parameters and their interaction increases the degrees of freedom available making the engine optimization and calibration processes extremely complicated.

PCCI combustion attained by a multiple-injection strategy can reduce NOx and PM emissions without sacrificing thermal efficiency, but a low combustion temperature resulting from early fuel injection, and ignition occurring prior to TDC, can cause higher total hydrocarbons  and CO emissions and fuel consumption.

In the light of this, the injection strategy has been applied to diesel engines along with many technologies such as cooled EGR, intake turbo-charging, and SCV control to achieve homogeneous mixture in the cylinder, which results in lower NOx and soot emissions as well as performance improvements.

Just two of the results of the DoE optimization to be discussed include, first, an increase in the EGR rate decreases the combustion temperature and the air-to-fuel ratio. As a result, BSNOx emissions decrease rapidly, but BSPM emissions and BSFC increase.  Second, an increase in injection pressure up to 1150 bar results in a slight decrease in PM and BSFC, but a slight increase in NOx emissions. This is attributed to the promotion of the atomization of fuel and combustion efficiency due to the increased injection pressure. However, the high injection pressure of 1300 bar along with the early injection timing of ATDC -60° results in wall wetting that causes high BSFC.

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