Improvement in high load ethanol-diesel dual-fuel combustion by Miller cycle and charge air cooling

At high load, dual-fuel compression ignition engines often rely on exhaust gas recirculation (EGR) to avoid excessive in-cylinder pressure rise rates caused by the autoignition of the premixed fuel. This can adversely affect the net indicated efficiency depending on the resulting fuel/air equivalence ratio and pressure differential across the cylinder used to drive the requested amounts of EGR. In this work, advanced combustion control strategies have been experimentally investigated to improve ethanol-diesel dual-fuel operation at a high engine load of 1.8 MPa net indicated mean effective pressure. Miller cycle and charge air cooling have been explored to reduce the in-cylinder gas temperature and help control the ethanol autoignition process, potentially minimising the EGR requirements and increasing net indicated efficiency. Experiments were carried out on a single-cylinder heavy-duty engine equipped with a high pressure common rail diesel injection, an ethanol port fuel injection, and a variable valve actuation system on the intake camshaft. Exhaust emissions and net indicated efficiency were measured and discussed for different ethanol energy fractions. Early autoignition of the premixed ethanol fuel at the baseline intake valve lift profile resulted in high levels of pressure rise rate, which limited the ethanol energy fraction to 0.26. The application of a Miller cycle strategy via late intake valve closing events effectively delayed the ethanol autoignition process. The reduction of the intake manifold air temperature via an air-to-water charge air cooler also suppressed the early ignition of ethanol. Both approaches allowed for a substantial improvement in terms of the maximum ethanol energy fraction, which was increased to 0.79 without EGR. Moreover, high load dual-fuel operations with Miller cycle and charge air cooling attained higher net indicated efficiency and lower nitrogen oxides emissions than conventional diesel combustion. These improvements can help generate a viable business case of dual-fuel combustion as a technology for future compression ignition engines.