Laboratory tests were performed to measure cooling rates o f an impinging oil-jet on the underside o f an automotive piston as func tions o f oil nozzle-to-piston surface spacing, oil pressure, oil temperature, and piston temperature. Based on these results, area-average Nusselt number correlations were derived fo r a Reynolds number range o f 100^1500, a Prandtl number range o f 90-750, and a nozzle-to-piston surface spacing range over 73-160 mm, which are within the ranges expected fo r oil-jet cool ing o f automotive pistons.
<div class="section abstract"><div class="htmlview paragraph">Prior research studies have investigated a wide variety of gasoline compression ignition (GCI) injection strategies and the resulting fuel stratification levels to maintain control over the combustion phasing, duration, and heat release rate. Previous GCI research at the US Department of Energy’s Oak Ridge National Laboratory has shown that for a combustion mode with a low degree of fuel stratification, called “partial fuel stratification” (PFS), gasoline range fuels with anti-knock index values in the range of regular-grade gasoline (~87 anti-knock index or higher) provides very little controllability over the timing of combustion without significant boost pressures. On the contrary, heavy fuel stratification (HFS) provides control over combustion phasing but has challenges achieving low temperature combustion operation, which has the benefits of low NO<sub>X</sub> and soot emissions, because of the air handling burdens associated with the required high exhaust gas recirculation rates. This work investigates HFS and PFS combustion, efficiency, and emissions performance on a single-cylinder, medium-duty engine with a regular-grade gasoline (91 research octane number) at 1,200 rpm, 4.3 bar, and 3.0 nominal gross indicated mean effective pressure operating points with boost levels similar to those in a medium-duty diesel application. Authority of combustion phasing with main injection timing sweeps for HFS and second injection timing sweeps and fuel split sweeps for PFS are shown. In addition, this work is discussed in the context of previous findings with a light-duty diesel platform, and next steps and future direction for this work are presented<span class="xref"><sup>1</sup></span>. </div></div>
Gasoline particulate filters (GPF) are being utilized in certain markets on gasoline direct injection (GDI) vehicles to reduce tailpipe particulate emissions as required by particle number regulations. GPF filtration efficiency is dependent on soot build-up within the filter. Since soot oxidizes within the GPF during normal vehicle operation, an understanding of soot reactivity is important for optimizing aftertreatment architecture and engine calibration. Past work has indicated that gasoline soot reactivity may depend on levels of metallic ash species. In this work, carbonaceous particulate matter from a GDI engine are evaluated from engine operation at a consistent speed and load but with different levels of fuel injection pressures and timings to vary the relative ash to soot ratio. Soot reactivity is found to vary significantly with the ratio of ash to soot present. Interestingly, the more reactive soots possess a unique oxidation profile by which a conventional Arrhenius type expression cannot be used to quantify reactivity. To understand the mechanisms driving such distinct oxidation differences, soot samples are analyzed after being partially oxidized. Particulate characteristics are evaluated by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM þ EDS). A mechanism is proposed that may explain further why ash affects gasoline soot reactivity to the extent seen in this and other work.
Low temperature and dilute homogenous charge compression ignition (HCCI) and spark-assisted compression ignition (SACI) can improve fuel efficiency and reduce engine-out NOx emissions, especially during lean operation. However, under lean operation, these combustion modes are unable to achieve Environmental Protection Agency (EPA) Tier 3 emissions standards without the use of lean aftertreatment. The three way catalyst (TWC)-SCR lean aftertreatment concept investigated in this work uses periodic-rich operation to produce NH3 over a TWC to be stored on a selective catalytic reduction (SCR) catalyst for NOx conversion during subsequent lean operation. Experiments were performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation and between lean and rich spark-ignited (SI) combustion to evaluate NOx conversion and fuel efficiency benefits. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system calculations are also presented to allow for comparisons of system performance for different levels of stored NH3. With the configuration used in this study, lean/rich HCCI/SACI operation resulted in a maximum NOx conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NOx conversion efficiency of 60%. If the low conversion efficiency of HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, calculations indicate that TWC-SCR aftertreatment has the potential to provide attractive fuel efficiency benefits and near-zero tailpipe NOx. Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7–17% for lean/rich HCCI/SACI with zero tailpipe NOx and −1 to 5% for lean/rich SI with zero tailpipe NOx emissions. Although the previous work indicated that the use of HCCI/SACI increases the time for NH3 to start forming over the TWC during rich operation, reduces NH3 production over the TWC per fuel amount, and increases NH3 slip over the SCR catalyst, if NOx conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NOx standards.
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