Experimental Study on the Characteristics of Dual-fuel Combustion Modes and Extension of Dual-fuel PCI Operating Range in a CI Engine

Abstract

The thermal efficiency and the durability of diesel engines which are based on compression ignition system are superior to those of gasoline engines with a spark ignition system. However, diesel engines are suffering from the high level of particulate matter (PM) emission which is caused by locally rich regions in-cylinder, related to the heterogeneous air-fuel mixture combustion. Also, since diesel engine is based on auto-ignition with high compression ratio, the pressure rise rate which causes engine noise is higher than that of gasoline engine. In addition, a diesel engine needs to separate after the treatment systems to oxidize the unburned components and deoxidize the nitrogen oxides (NOx) emission, while a gasoline engine with theoretical air-fuel ratio can reduce emissions using only a three-way catalyst (TWC) As a result, novel diesel combustion concepts, especially premixed compression ignition (PCI) which is a practical realization of homogeneous charge compression ignition (HCCI), have been studied. Most of the novel diesel combustion concepts are based on the higher exhaust gas recirculation (EGR) rate to suppress NOx emission by reducing oxygen concentration, combustion temperature and earlier diesel injection timing to implement the premixed air-fuel mixture condition to reduce PM emission. Engine-out NOx and PM emissions could be effectively reduced by novel diesel combustion, but a higher in-cylinder pressure rises rare from the increasing premixed combustion and incomplete combustion due to low combustion temperature are inevitable. In particular, almost all novel diesel engines have a trouble with the extension of the operating range. Since novel diesel combustion concepts need enough time for air-fuel mixing, higher speeds and load conditions could not be realized. In this circumstance, a reactivity controlled compression ignition (RCCI) which can be implemented by two different fuels (eg. gasoline and diesel) is one of good methods to extend the operating range with a clean combustion. However, in-cylinder pressure rise rate of RCCI is higher than that of conventional diesel combustion similar to HCCI combustion, because the major profit of RCCI combustion is higher thermal efficiency from rapid combustion. Especially, since the dual-fuel combustions including RCCI has unusual characteristics as the ratio between two fuels changes, the study of dual-fuel combustion characteristics must be conducted to suggest proper operating strategies under different load conditions. Thus, in this work, the characteristics of dual-fuel combustion under various modes were studied and the investigations of the higher thermal efficiency and the load of dual-fuel premixed compression ignition (PCI) were verified. Especially, from this research, the definition of dual-fuel PCI was introduced and the potential for more practical applications of dual-fuel combustion for commercial CI engines can be confirmed. The first experimental result was that dual-fuel combustion modes can be divided into three cases. The first one was dual-fuel combustion which was comprised of a premixed combustion of mainly diesel and some of gasoline which were entrained during the spray motion and the mixing controlled combustion of the residual air-fuel mixture. On the other hand, if the reactivity stratification was adjusted with a high portion of low reactivity fuels enough to combustible and the in-cylinder temperature and pressure reached a certain point, then split auto-ignition occurred, which can be verified by two peaks of HRR. The first peak of HRR came from the diesel and some of the gasoline fuel, but the second peak of HRR came from faster auto-ignition of residual gasoline and some of the diesel fuels. The third dual-fuel combustion mode was entirely premixed combustion from two fuels simultaneously. As reactivity stratification of in-cylinder became smooth and gradual, then stratified auto-ignition occurred from diesel and gasoline fuels. Therefore, in this work, the second and third dual-fuel combustion modes were selected to achieve a higher thermal efficiency with low NOx and PM emissions. Then these dual-fuel combustion modes based on the earlier diesel injection strategy whose ignition delay was longer than the diesel injection duration and a large amount of low reactivity fuel are called as dual-fuel PCI. The second objective was evaluating the relation between combustion index and dual-fuel combustion modes. As varying the total equivalence ratio, gasoline fraction, and diesel injection timings, LTHR (Low-Temperature Heat Release) region occurred from the third mode (single auto-ignition) and the tendency of MFB 50 location became opposite to the behavior of diesel injection timing. Therefore, although the second and third modes are based on the PCI region, the second mode was still related with diesel injection timing (spray motion) which means late injection PCI and the third mode was seemed like an early injection PCI which is based on the chemical reaction rather than combustion from spray. Additional finding was the source of THC emission under dual-fuel combustion. From the results, too much leaner equivalent ratio condition is not suitable for the dual-fuel combustion. Also, there might be criteria for the maximum fraction of gasoline and diesel injection timings. In addition, in this work, split diesel injection strategy can be suitable for the improvement of combustion efficiency in dual-fuel combustion. Using split diesel injection strategy, the distribution of diesel, which has a role of ignition source mainly, was improved and wall-wetting can be reduced by shortened the diesel pray length. However, under high load condition, split diesel injection strategy was not recommended because of higher PRRmax. The last experimental achievement of this work was the optimization of dual-fuel PCI under low and high loads respectively. Under a low load condition, since PRRmax was low enough to satisfy the criteria (especially, 10 bar/deg), the third dual-fuel combustion mode which means simultaneous premixed combustion of diesel and gasoline fuels was favorable. In this research, 48 % of gross indicated thermal efficiency was achieved under 1,500 rpm/gIMEP 6.5 bar condition with low NOx (13 ppm) and near-zero PM emissions (Below 0.05 FSN). On the other hand, under a higher load condition, PRRmax which is related with a knocking from gasoline fuel is the main challenge. Thus, the second dual-fuel combustion mode was applied to extend load. Using the split auto-ignition combustion strategy gIMEP 14 bar was successfully achieved without the knocking phenomenon under 2,000 rpm condition. This maximum value implied that there is a potential of dual-fuel PCI to cover the entire region of NEDC test mode. This work includes the study of the characteristics of dual-fuel combustion modes and the investigation into the improved operating strategies of dual-fuel PCI. From the results of combustion characteristics, appropriate combustion modes were suggested as load conditions to achieve low NOx and PM emissions, higher thermal efficiency and especially the highest operating loads. Thus, this research can contribute the practical application of dual-fuel combustion in passenger cars with light-duty diesel engines