Kazuhisa Inagaki

Low Emissions and High-Efficiency Diesel Combustion Using Highly Dispersed Spray with Restricted In-Cylinder Swirl and Squish Flows

A new clean diesel combustion concept has been proposed and its excellent performance with respect to gas emissions and fuel economy were demonstrated using a single cylinder diesel engine. It features the following three items: (1) low-penetrating and highly dispersed spray using a specially designed injector with very small and numerous orifices, (2) a lower compression ratio, and (3) drastically restricted in-cylinder flow by means of very low swirl ports and a lip-less shallow dish type piston cavity. Item (1) creates a more homogeneous air-fuel mixture with early fuel injection timings, while preventing wall wetting, i.e., impingement of the spray onto the wall. In other words, this spray is suitable for premixed charge compression ignition (PCCI) operation, and can decrease both nitrogen oxides (NOx) and soot considerably when the utilization range of PCCI is maximized. However, in diffusive combustion, especially at full load, a low-penetrating spray potentially causes higher soot emissions and results in lower maximum torque. In this case, item (2) is applied to recover full-load performance. The lower compression ratio enables diffusive combustion phasing to be advanced more with an earlier injection timing because of a larger margin between the compression-end pressure and the allowable maximum in-cylinder pressure. This results in lower soot emissions because enough time is created to oxidize soot before the end of the combustion period. A lower compression ratio often leads to worse cold-condition engine performance aspects, such as cold startability, unburned hydrocarbons, and white smoke. Item (3) is applied to compensate for such practical problems. Drastically weakened in-cylinder flow keeps the compression-end temperature to the same level as a conventional engine with an ordinary compression ratio by decreasing heat-flux escaping through the chamber wall (i.e., heat-loss). Although a weak in-cylinder gas motion might lead to higher soot emissions due to slower fuel-air mixing, it should be noted that the highly dispersed spray of item (1) enables PCCI-dominant combustion in which the fuel-air mixing process is less dependent on in-cylinder flow. In this way, these three items act mutually to compensate for each other’s drawbacks, while maximizing their advantages. Consequently, NOx emissions in the New European Driving Cycle (NEDC) can be reduced drastically to less than 1/4 of the level of a conventional engine, or less than half of the Euro 6 standard without deteriorating fuel consumption, full-load torque, or cold-condition performance.

In-cylinder stratification of external exhaust gas recirculation for controlling diesel combustion

Abstract A technique for achieving the in-cylinder stratification of external exhaust gas recirculation (EGR) gas in direct-injection (DI) diesel engines has been developed to reduce toxic exhaust emissions. The external EGR gas is supplied from one of the two intake ports which can create a swirl flow in either the upper or lower portion of the cylinder during the intake stroke. In the final stage of the compression stroke, a squish flow conveys the vertically stratified EGR gas into the piston cavity, generating a radially stratified EGR gas in the piston cavity at the end of the compression stroke. This strategy for achieving EGR gas stratification in the piston cavity was developed by using an unsteady computational fluid dynamics (CFD) code. Prior to the exhaust emission tests, the accuracy of the simulation was evaluated by planer laser-induced fluorescence (LIF) imaging. The exhaust emission tests showed that there was less smoke emission under medium load conditions when the EGR gas was delivered to the inner part of the piston cavity. The mechanism of this smoke reduction was investigated using CFD simulation, which is based on a series of calculations related to the internal flow of the injector nozzle, the in-cylinder fuel spray, and mixture formation and combustion. It has been shown that, at the beginning of the combustion, the higher concentration of EGR gas in the inner part of the cavity lowers the combustion temperature and reduces the soot formation rate. Air, which exists in the outer part of the cavity at the start of fuel injection, enhances the oxidation of the soot cloud in the piston cavity periphery in the latter half of the combustion period.

Effect of hydrocarbon molecular structure in diesel fuel on in-cylinder soot formation and exhaust emissions

Exhaust emissions and combustion characteristics from well-characterized diesel test fuels have been measured using two types of single-cylinder HSDI diesel engines. Data were collected at several fixed speed/load conditions representative of typical light-duty operating conditions and full-load performance (smoke-limited maximum torque) points. In addition, in-cylinder soot formation processes of these fuels were investigated via Laser Induced Incandescence (LII) using an optically accessible single-cylinder engine. The test fuels used in this study have been formulated with a sophisticated blending algorithm that systematically varies the hydrocarbon molecular structure in the fuels while maintaining the distillation characteristics of market diesel fuels. The following results have been obtained from this study. (1) The lowest PM emissions were observed with a fuel containing approximately 55% iso-paraffins and 39% n-paraffins with CN=52.5. Compared with the base fuel (corresponding to average market fuel in Japan), this fuel yields a 40 - 70% PM reduction and an increase in the maximum torque of approximately 8%. (2) A highly n-paraffinic fuel representative of a Fischer-Tropsch liquid did not yield PM reductions as high as expected. This is due to its very high cetane number (CN=80.5), resulting in a decreased ignition delay which initiates combustion before sufficient fuel-air mixing has occurred. This conclusion is corroborated by LII analyses of highly n-paraffinic fuels which show regions of high soot concentration in the burning fuel spray jet near the injector. (3) Under low and medium loads, cyclo-paraffins (naphthenes) have a higher PM formation tendency than iso- or n-paraffins. Under high load conditions, however, paraffin molecular structure has a very small effect on PM formation. (4) Aromatics have a higher soot/PM formation tendency than paraffins under all speed/load combinations investigated. A correlation of PM formation with fuel chemical composition has been developed from a statistical analysis of the data. Expressing the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.

Quantitative Analysis of Soot Formation and Oxidation Process using Laser-Induced Incandescence

A new technique using Laser-Induced Incandescence (LII) has been developed to quantify the soot concentration in a diesel engine. Characteristic problems in quantitative measurements, such as LII signal attenuation by soot clouds between the camera and the measurement plane, and incident laser attenuation due to soot clouds in the laser path, were corrected by the multi-layer correction method developed in this work. When this LII measurement method is applied to an optically accessible engine, the developing soot clouds in spray combustion can be visualized in detail. The changes in soot formation process caused by increasing fuel injection pressure with reduced hole size of injector, and by altering fuel chemical property, are both clarified quantitatively in this paper.

Computed tomography measurement of gaseous fuel concentration by infrared laser light absorption

A system to measure gaseous hydrocarbon distributions was devised, which is based on IR light absorption by C-H stretch mode of vibration and computed tomography method. It is called IR-CT method in the paper. Affection of laser light power fluctuation was diminished by monitoring source light intensity by the second IR light detector. Calibration test for methane fuel was carried out to convert spatial data of line absorption coefficient into quantitative methane concentration. This system was applied to three flow fields. The first is methane flow with lifted flame which is generated by a gourd-shaped fuel nozzle. Feasibility of the IR-CT method was confirmed through the measurement. The second application is combustion field with diffusion flame. Calibration to determine absorptivity was undertaken, and measured line absorption coefficient was converted spatial fuel concentration using corresponding temperature data. The last case is modeled in cylinder gas flow of internal combustion engine, where gaseous methane was led to the intake valve in steady flow state. The fuel gas flow simulates behavior of gaseous gasoline which is evaporated at intake valve tulip. Computed tomography measurement of inner flow is essentially difficult because of existence of surrounding wall. In this experiment, IR laser beam was led to planed portion by IR light fiber. It is found that fuel convection by airflow takes great part in air-fuel mixture formation and the developed IR-CT system to measure fuel concentration is useful to analyze air-fuel mixture formation process and to develop new combustors.