# Overview of Clean Automotive Thermal Propulsion Options for India to 2030
## 2 The Current and Future Technological State-of-the-Art
### Advanced Internal Combustion Engine Solutions for Near Zero Emissions
#### Advanced Combustion  
When compared to the other engine types the Diesel engines are the most efficient due to their higher compression ratios and overall fuel-lean combustion, which provides low carbon monoxide and hydrocarbon emissions when compared to gasoline engines. The main focus is on reducing nitrous oxides and particulate matter emissions because of high temperatures and fuel-rich regions respectively, as discussed in [36]. Four general methods are extensively being developed to satisfy emission standards and to overcome the NOx and PM emissions i.e., improvement of the combustion process, usage of alternative fuels, introduction of advanced combustion concepts and exhaust after-treatment devices. From the diesel combustion perspective, to reduce NOx and PM emissions, high temperature stoichiometric and fuel-rich regions should be averted, concurrently. Consequently, one of the most effective approaches is Low Temperature Combustion (LTC) which shows improvement in fuel atomization, lower equivalence ratios and decreased combustion temperature which decreases NOx and PM emissions instantaneously. This maintains high thermal efficiency complying with the emission standards and increasing in fuel demands. Low Temperature Combustion: The concept of LTC reduces the flame temperature and permits adequate homogenous air-fuel mixture, leading to a simultaneous reduction of PM, NOx and smoke emissions [37]. As claimed by the [38], LTC was able to attain a very low NOx emission (<35 ppm) by using exhaust gas recirculation and PM (<0.05) by means of advanced fuel injection timing. Hence the technology has become a very popular research topic. LTC is an advanced combustion technology that is achieved by early fuel injection in the combustion chamber that improves the air-fuel mixing before the start of combustion inside the cylinder. By injecting the fuel nearer to the top dead centre (TDC) with EGR controlled combustion, dual fuel injection, the resultant air-fuel mixing avoids fuel-rich regions that lower the temperature below 2100K, which in turn reduces NOx and PM emissions. This type of combustion increases the engine efficiency and reduces emissions. LTC is achieved through a variety of methods such as Homogenous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI), Partially Premixed Combustion (PCC), Reactivity Controlled Compression Ignition (RCCI), Gasoline Compression Ignition (GCI), High Efficiency Clean Combustion (HECC) and Spark Assisted Compression Ignition (SACI). However, we will be briefly looking into HCCI and PCCI methods in this literature. Currently, numerous researchers are concentrating on reducing the use of conventional fuel, forming new alternative fuels and reformulating the fuels to reduce emissions such as a project on the effects of bio-diesel and binary blends in a turbocharged diesel engine [39]. Homogeneous Charge Compression Ignition (HCCI): The HCCI method was developed in 1979 using a gasoline engine to increase the combustion process stability for a 2-stroke engine. In HCCI a well premixed fuel-air mixture gets ignited at the end of the compression stroke devoid of any spark assistance. In this process, the combustion occurs at multiple places because of auto-ignition of the mixture which achieves its chemical initiation energy. Due to compression pressure and hot temperatures in the cylinder, the mixture ignites at different hot-spots inside the cylinder resulting in auto-ignition of the air-fuel mixture. This process reduces high flame fronts and maintains the in-cylinder temperatures at lower levels. In a study of HCCI and low temperature combustion with the aid of optics [40], it was demonstrated that near-zero NOx emissions were feasible due to low combustion temperature and high EGR attenuation. The overall homogenous lean air-fuel mixture also tends in avoiding the soot formation in emissions. Therefore, in HCCI engine technology the air-fuel mixture ignition is controlled by its composition and the in-cylinder temperatures.