# A Review of Early Injection Strategy in Premixed Combustion Engines
## 3 Injection Timing
### Two-Stage Early Injection
#### 3.2.3 First Injection Quantity
NOx emissions increased with the increment of first injection quantity. Soot emissions were slightly influenced by the first injection quantity. [PERSON] et al. [91] experimentally investigated the effects of first injection quantity on the emissions and combustion of a PCCI diesel engine. The first injection quantity was varied from 15% to 35% of the total fuel mass, and the first and second injection timings were fixed at 38\({}^{\circ}\) and 4\({}^{\circ}\) BTDC. Results showed that the combustion temperature increased and the start of combustion advanced as the first injection quantity increased, resulting in increased NOx emissions. CO emissions also increased because the portion of wall-wetting fuel increased, causing incomplete combustion. The increased incomplete combustion also increased the BSFC. [PERSON] et al. [92] investigated the effects of first injection quantity on the emissions of a Partially Premixed Compression Ignition (PPCI) engine. The first injection quantity was varied from 0 to 10 mg/cycle, with the first injection timing fixed at 50\({}^{\circ}\) BTDC. Results showed that soot emissions decreased with the increased first injection quantity due to the leaner homogeneous charge mixture, while HC emissions increased slightly. [PERSON] et al. [56] investigated the effects of first injection quantity on the emissions and performance of a diesel engine. The first injection quantity was swept from 20% to 60%, with the first and second injection timings fixed at 35\({}^{\circ}\) and \(-\)10\({}^{\circ}\) BTDC. Results showed that NOx and soot increased sharply after the first injection quantity reached 40%. [PERSON] and [PERSON] [93] investigated the effects of first injection quantity on the combustion and emissions of a PCCI engine. The first injection quantity was varied from 0 to 50%, and the first and second injection timings were fixed at 30\({}^{\circ}\) and 25\({}^{\circ}\) BTDC. Results showed that as the first injection quantity increased to 30%, the peak of heat release was raised and the combustion phase was advanced, resulting in increased NOx but decreased soot. However, the peak of heat release was reduced and the combustion phase was retarded again when the first injection quantity exceeded 40%, resulting in decreased NOx and increased soot emission. HC and CO emissions increased with the increased first injection quantity due to wall-wetting. [PERSON] et al. [94] studied the influence of pilot injection quantity on the performance and emission characteristics of an HCCI-DI combustion engine. The pilot injection quantity was from 0 to 10 mm\({}^{3}\). Results showed that when the pilot quantity was smaller than 8 mm\({}^{3}\), smoke emission increased with the rise of pilot quantity, and then dropped a little when it was bigger than that. NOx emission was opposite to soot due to the trade-off between the reduction of NOx during HCCI combustion and the increase of NOx during diffusive combustion. HC and CO emissions increased as the pilot quantity increased, which is a major problem of HCCI combustion due to incomplete combustion and fuel impingement. BSFC increased with increased pilot quantity. The reason was off-phasing of the combustion process and the negative work; meanwhile, HC and CO increased. [PERSON] et al. [95] investigated first injection proportions in the combustion and emission characteristics of a diesel engine. In that study, the first injection timing of the two-stage injection strategy was fixed at 30\({}^{\circ}\) BTDC, and the first injection proportion varied from 10% to 90%. The results showed that as the first injection proportion rose, indicated specific fuel consumption (ISFC) first decreased then increased, and the lowest value was obtained at 40%. At the same time, soot emission decreased sharply, but NOx emission was opposite to BSFC, and the turning point was 30%.