**3.3 Partially premixed charge compression ignition (PPCI)**

Partially premixed charge compression ignition is related to the PCCI method, which is a hybrid of traditional diesel and HCCI combustion. However, for low cetane fuels, PPCI combustion is favored. Similar to the PCCI combustion method, a longer ignition delay period and improved air-fuel mixing can be accomplished. Few studies have shown that improved and delayed injection strategies can result in extended ignition delay in PPCI combustion. To achieve a longer ignition delay, low and moderate compression ratio were used, as well as moderate to high EGR dilution. The key benefit of PPCI mode over HCCI mode is that it releases less particulate matter and NOx while providing better combustion phasing. PPCI is divided into two categories: early injection PPCI and late injection PPCI. The fuel is injected at the middle of the compression stroke in early injection PPCI, and at the end of the compression stroke in late injection PPCI. The fuel-injection gases of the early injected PPCI variant are denser and cooler due to partial compression. Similarly, in the late injected PPCI model, the fuel-injected gases are colder and denser due to injection occurring on the expansion cycle, which lowers the temperature in the later stage [54]. Due to incomplete oxidation and nonoptimal combustion phasing, the PPCI combustion used slightly more fuel than standard diesel combustion [55].

At low load, a greater EGR rate and a delayed injection time reduce the power output of both low and higher power engines. In EGR assisted PPCI combustion, the advanced injection method was used to avoid a reduction in power output. Another disadvantage of PPCI combustion is that it produces more HC and CO because the amount of non-oxidized fuel in the piston bowl and high-pressure squish region increases [56]. The addition of gasoline to the PPCI is another way to achieve lower NOx and soot emissions without using EGR. The main benefits of adding gasoline to the PPCI are that it reduces HC and CO emissions by reducing residual products in the cylinder [57]. For longer ignition delay times, most of the premixed heat release phase was seen, resulting in higher peak cylinder pressure and noise levels. When the ignition delay periods shorten, the diffusion heat release phase occurs, resulting in a state similar to that of ordinary diesel combustion [58].

### **3.4 Reactive controlled compression ignition (RCCI)**

HCCI, PCCI, and RCCI are examples of sophisticated low-temperature combustion technology that have recently been created. RCCI, for example, increases research focus due to its versatility. By achieving low-temperature combustion, HCCI and PCCI improve engine efficiency and reduce pollutants, according to previous studies. These two technologies, however, have considerable limits, and they are not ideal for low and high load settings due to knocking, misfire, and a faster rate of pressure rise. Fuel alteration is required in the HCCI and PCCI combustion to overcome the difficulties [59, 60]. They also stated that combustion quality had improved across a broad range of engine operations Bessonette et al. [61] investigated the effect of a partially mixed gasoline/diesel charge in a CI engine from low to high load. Raw diesel is favored for the lowest load situation, while a higher percentage of gasoline blend is suited for the highest load condition, according to them. In a subsequent stage, this dual fuel PCCI operation is referred to as RCCI combustion [62]. Adjusting the low to high reactive fuel ratio and the injection pattern of the high reactive fuels to achieve the NOx to smoke trade-off and higher efficiency. Reactivity stratification in RCCI combustion can also be influenced by fuel qualities such as viscosity, volatility, and ignite ability.

### *Zero Emission Hydrogen Fuelled Fuel Cell Vehicle and Advanced Strategy on Internal… DOI: http://dx.doi.org/10.5772/intechopen.102057*

Biodiesel has been tested in a variety of engines and under a variety of operating circumstances all around the world. Due to the presence of oxygen in the biodiesel fuel, NOx emissions were higher for the engine [63, 64]. The RCCI engine driven by gasoline/biodiesel was mathematically analyzed by Li et al. [65]. When comparing raw biodiesel to gasoline/biodiesel, the study found decreased NOx emissions in the gasoline/biodiesel operation. As a result, using biodiesel under the RCCI method may be a better alternative for reducing NOx pollution than using biodiesel-powered diesel engines. Hanson et al. [58] study the RCCI combustion utilizing direct-injected diesel and biodiesel mixture (B20) as a direct-injected fuel and gasoline, E85 (85% ethanol and 15% diesel blend), and E20 as a port fuel. In the RCCI combustion, the findings of the E20/diesel mixture show that maximum pressure and HRR dropped, allowing the peak load to increase by 2 bar (from 8 bar to 10 bar BMEP). The usage of E20 improves combustion efficiency while lowering the heat release rate and exhaust leakage. The combustion efficiency of gasoline/B20 RCCI operation was also increased by lowering the UHC, albeit with a greater CO. Fuel efficiency also improved, resulting in a 1.68 percent increase in BTE. In comparison to the RCCI gasoline/diesel operation, E85/B20 allowed the RCCI operation to increase the BTE from 40 to 43%. The use of biodiesel as a pilot fuel has improved the stability of the cyclic operation of RCCI engine powered by natural gas/biodiesel, according to Gharehghani et al. [66]. This is due to the fact that biodiesel contains oxygen, which raises the cetane number. In comparison to natural gas/diesel, the mixture of natural gas/biodiesel produced 1.6% higher BTE as noticed by Gharehghani et al. [66].

#### **3.5 Low-temperature combustion advantages and challenges**

The combustion temperature in the LTC mode was always lower than the combustion temperature in a regular diesel engine. There are primarily two strategies to achieve low-temperature combustion: one is to operate the engine with higher EGR, and the other is to operate with an excess air ratio 0 greater than 1 [67]. Fuel combusted and oxidized at higher temperatures under stoichiometric operating conditions, resulting in more NOx production. Also, due to a reduction in oxygen availability in the fuel spray periphery, maximum soot emission was observed under the stoichiometric condition compared to normal diesel combustion [68]. Higher fuel injection pressure is usually a viable approach for overcoming the aforementioned concerns. Higher fuel injection pressure promotes atomization, mixing, and vaporization. However, the key duty to be remedied in modern injection technology in low-temperature combustion is the wall impingement of fuel caused by spray tip penetration at increased fuel injection pressure [69]. Furthermore, improved injection strategies such as high-pressure injection and CRDI approaches reduce the ignition delay period and boost premixed phase combustion, resulting in increased NOx emissions. The ignition delay and combustion phasing will be lengthened by using a higher level of cold EGR, lowering the compression ratio, and using variable valve timing control to advance the exhaust valve opening. Increased ignition delay enhances airfuel mixing, resulting in increased homogeneity in the air-fuel combination. Higher EGR rate and lower compression ratio reduce the cylinder peak pressure and temperature, which has a major impact on engine performance and higher fuel consumption.

Getting LTC mode to work in real-time settings with heavy engine load is difficult. It is impossible to manufacture engines with a larger amount of EGR. In addition, the engine's higher BTE should compensate for the increased EGR. In the LTC condition, an external charge booster is necessary to produce higher BTE [70]. When the engine

is running at a higher RPM, moderate EGR with an intake charge booster raises the cylinder peak pressure. The combustion process changes depending on the engine load, and it is influenced by the different equivalency ratio and fuel mixing zone, making the engine demanding and difficult to modify the operating state for each load [71]. The real-time modern diesel engine employs dual fuel technology, multiple injection method, and negative valve overlapping. However, these technologies are costly and difficult to implement across the board. By increasing the premixed charge quantity while lowering peak pressure and temperature, these innovations reduce the fuel-rich zone [72].
