Comparison of the combustion, performance and emissions of HVO/NG and diesel/NG RCCI engine based on energy balance and correlation analysis

Abstract

Reactivity controlled compression ignition (RCCI) is regarded as an advanced dual-fuel low temperature combustion concept for next generation engines, in which combustion phase and heat release rate can be controlled by regulating the in-cylinder mixture concentration and reactivity, thereby achieving high efficiency while breaking the NOx-soot trade-off. However, diesel/natural gas (NG) RCCI combustion still suffers from high unburned CH4 emission at low engine loads. In this paper, renewable hydrotreated vegetable oil (HVO) with higher reactivity than diesel is chosen to create the higher mixture reactivity gradient for further optimizing engine combustion and emissions. An experimental-data-driven energy balance method is proposed to quantify the energy distributions during the process of heat-to-work conversion, and Pearson correlation analysis and distance correlation are combined to reveal the correlations of controllable parameters of start of injection (SOI), manifold absolute pressure (MAP), and exhaust gas recirculation (EGR) ratio with combustion and performance of HVO/NG and diesel/NG. The results indicate that HVO/NG RCCI combustion can reduce the incomplete combustion by 2.0% and gain similar maximum indicated thermal efficiency (ITE) compared with diesel/NG RCCI combustion, while CH4 and NOx emissions are simultaneously reduced by 45.3% and 65.0%, respectively. However, under identical control conditions, the higher reactivity of HVO leads to earlier combustion phasing, which increases exhaust heat losses and wall heat transfer losses, thereby slightly reducing the ITE. Thanks to its high cetane number and zero aromatic content, HVO reduces incomplete combustion losses by >10% and increases net indicated work by 4.7–6.8% under extreme conditions (λ = 2.68, EGR = 21.6%). This is accompanied by substantial CH4 and CO reductions while meeting Euro VII NOx limits. The higher reactivity of HVO increases engine tolerance to leaner mixture and higher EGR, thereby showing the potential to expand the operable MAP and EGR range at an IMEP of approximately 0.8 MPa. However, the same SOIs lead to earlier combustion which increases wall heat transfer losses and compression negative work, resulting in slightly lower ITE under same conditions with diesel.