Study of Antiknock Performance Under Various Octane Numbers and Compression Ratios in a DISI Engine

Abstract

This paper presents a study of antiknock performance under various octane numbers and compression ratios in a direct injection spark ignition (DISI) gasoline engine. The relationship between the octane number and engine performance in the DISI engine-the engine torque and the break specific fuel consumption (BSFC)-was investigated in comparison with a multipoint injection (MPI) engine. Due to the improvement in the charging efficiency and the advance of the ignition timing by cooled aspiration, the engine torque of the DISI engine was improved over that of the MPI engine. It was also found that the octane number requirement (ONR) was reduced. In addition, the possibility of engine performance enhancement at high compression ratios was studied. At high compression ratios, the engine torque is reduced due to the heavy knocking when low octane gasoline is used. However, an improvement in the engine torque has been observed with high octane gasoline. An increase in the ONR at a high compression ratio (15.0:1) was observed in both DISI and MPI engines, but the increase in the ONR in the DISI engine was smaller than in the MPI engine. The BSFC got worse under low-speed, high-load conditions at high compression ratios for retardation due to heavy knocking, while the BSFC was improved at low-speed, low-load and at middle speed for better thermal efficiency at high compression ratios. Finally, the benefits for fuel economy with high compression ratios and high octane gasolines were evaluated using J10-15, ECE-EUDC and LA-4 mode simulations.