(i) Changing temperature history of mixture: (a) VVT and residual/exhaust gas trapping (1) Exhaust gas trapping [28, 29] (2) Modulating intake and exhaust flows [30, 31] (3) Combination of both [32] (b) Variable compression ratio (VCR) [33–36] (c) Variable EGR [31, 37, 38] (d) In-cylinder injection timing [39–41] (e) Modulating intake temperature [42–44] (f) Water injection [45] (g) Variable coolant temperature [46] (ii) Changing mixture reactivity: (a) modulating two or more fuels [21, 47–49] (b) fuel stratification [50–54] (c) fuel additives and reforming [55–57] (d) variable EGR [37, 38, 58]
Limited power output
(i) Boosting intake air flow: (a) supercharging [35, 59–61] (b) turbocharging [61–63] (ii) Dual-mode engines (HCCI at low load): (a) SI-HCCI [58, 64, 65] (b) diesel-HCCI [66, 67]
Homogenous mixture preparation
(i) Fuel injection in a highly turbulent port flow for gaseous and highly volatile fuels [68, 69] (ii) Early in-cylinder injection with sophisticated fuel injectors for diesel fuels [60, 70]
