Effects of variable Miller cycle on combustion performance and fuel economy of highly intensified diesel engines

The Miller cycle has received increasing attention and application in the field of diesel engines. However, studies that have addressed the practical application of the variable Miller cycle in multi-cylinder diesel engines remain limited. Here, a six-cylinder highly intensified heavy-duty diesel engine equipped with a high geometric compression ratio and two-stage turbocharger was employed to evaluate a self-designed fully hydraulic variable valve system (FHVVS). This novel system enables early intake valve closure, allowing diesel engine operation in both Diesel and Miller cycle modes. Comparative experiments on a performance test bench were conducted to assess the effects of the variable Miller cycle on combustion and intake characteristics. Combustion matching was achieved by optimizing the injection advance angle. A fuel economy comparison between operating modes suggested that the Miller cycle effectively reduced pumping losses compared with the Diesel cycle, lowering pumping mean effective pressure (PMEP) by an average of 71.5 kPa (60.2%) at 1,400 rpm. The Miller cycle also decreased maximum combustion pressures, reducing the mechanical load as well as creating highly favorable conditions for optimal matching of the crank angle at 50% heat release, thus improving combustion performance in highly intensified diesel engines. By improving pumping losses and reasonably matching combustion parameters, the Miller cycle substantially extended the low fuel consumption range at medium speed and medium-to-high load. Compared with the Diesel cycle, the Miller cycle reduced the average fuel consumption rate by 8.5 g/(kW·h), a 4.0% decrease. By ensuring rapid and smooth conversion between operating modes (conversion time