**3. Introduction to low temperature combustion techniques**

Conventional diesel engine running on petroleum and diesel fuel emits more oxides of nitrogen (NOx), oxides of carbon (COx) and particulate matter (PM) around the world. Low-temperature combustion (LTC) technology in engine development has dropped the environmental effects by providing better combustion efficiency, and increased the engine efficiency and fuel economy. Several low-temperature combustion strategies are available such as homogeneous charged compression ignition (HCCI), premixed charged compression ignition (PCCI), and reactive controlled compression ignition (RCCI). Before combustion, the entire air and fuel is premixed in the LTC combustion mode. The combustion is controlled by a predetermined equivalency ratio and cylinder temperature which leads to reduce the soot formation, PM, and NOx emissions. In LTC mode, the combustion temperature could be maintained between 1800 and 2200 K, which means no NOx emissions are produced in the rich mixing region and no soot is formed below 1800 K in the lean mixing by Hoekman and Robbins.

### **3.1 Homogeneous charge compression ignition (HCCI)**

The homogeneous charge compression ignition (HCCI) engine combines the combustion characteristics of both SI and CI engines in an IC engine. The fuel is

premixed in the HCCI engine in the same in SI engines, and the fuel is auto-ignited to start the combustion in the same way in CI engines. Before combustion begins, the fuel is vaporized and homogeneously premixed with air. Due to lean-burn combustion, the HCCI has the ability to reduce NOx emissions and increased the brake thermal efficiency. The in-cylinder temperature is reduced via lean-burn combustion, resulting in decreased NOx emissions as observed by Komninos and Rakopoulos [36]. In addition, due to the increased displacement capacity, HCCI combustion improves brake thermal efficiency by 50%, while emitting less smoke than conventional diesel combustion. The HCCI engine's compression ratio and premixed fuel combustion has improved the brake thermal efficiency of engine and lower the smoke emissions as noticed by Desantes et al. [37]. The multi-zone auto ignition and spontaneous combustion of the entire mixture is promoted by the homogenous mixture and uniform equivalence ratio in the cylinder. Furthermore, flame propagation has little effect on combustion in the HCCI mode [38].

The unanticipated pressure rise and cycle to cycle variation are exacerbated by multi-zone combustion and unexpected ignition location. Also, knocking is caused by high oscillation frequency and unanticipated pressure surge as noticed by Ganesh and Nagarajan [39]. Contino et al. [40] reported that some of the techniques such as early direct injection, early multiple injection, water injection, port fuel injection, external cold EGR, variable valve timing, variable compression ratio, air preheating, and alcohol injection are commonly employed in HCCI to control combustion and emission. The biofuel auto ignition temperature and viscosity are higher than diesel hence a higher compression ratio was used in HCCI engine. The compression ratio for the various loads can be adjusted to enhance the combustion efficiency as noticed by Zhang, et al. [41]. By modifying the spark timing and spark plug placement, the spark aided HCCI engine was able to achieve combustion phasing and emission reduction [42]. The key factors that have been employed to detect the combustion phenomena in the HCCI engine are the pressure increase rate, combustion noise, and ringing intensity. In a real-time combustion application, the ringing intensity is primarily employed to detect the combustion noise for the needed cylinder pressure [43].

Because of the increased stroke volume, the higher compression ratio HCCI engine improves brake thermal efficiency by achieving the auto-ignition temperature of the fuel. High to low octane fuels can be utilized as a port fuel to solve knocking and NOx formation. In HCCI engine, keeping the inlet charge temperature is critical. Similarly, the HCCI engine's compression ratio could be maintained effectively between 10:1 and 28:1. Compression ratios of 10:1 were favored for higher cetane fuels like n-heptane, and 28:1 were preferred for high octane fuels like iso-octane. For biodiesel, the intermediate compression ratio was favored [44]. Alternative method for achieving lower emission in HCCI engine includes use of alcoholic fuels such as ethanol, n-butonal, and methanol. Due to oxygen enrichment, alcohol fuel accelerated premixed burning and complete oxidization of fuel. Also, because of the latent heat of vaporization is higher, it lowers the combustion temperature, enhancing the quenching effect [41]. The HCCI combustion's power output is mostly determined by the equivalency ratio and fuel intake. For the higher power production, the equivalence ratio should remain at 1 as noticed by Vinod Babu et al. [45].
