## 3 Results and Discussion
For the diesel/LPG dual fuel engine, the maximum pressure is always higher than pure diesel fuel operation, due to the combustion and extra heat released from gaseous fuel [1]. The higher LPG ratio in dual fuel operation leads to two effects. First, the premixed combustion and the speed of flame propagation increases but the mixing-controlled combustion for the liquid fuel reduces. Second, the reduced amount of pilot injection causes the smaller size of the ignition sources, therefore increases the path that the flame needs to propagate to consume all the premixed mixture in the chamber [44]. This may be postulated to higher ignition delay of the liquid fuel and/or the lower self-ignition temperature of the gaseous fuel. The LPG fuel has lower cetane number and this can increase the ignition delay period of the fuel compared to pure diesel. In addition, the LPG has a high self-ignition temperature compared to diesel fuel. Therefore, it is expected that diesel/LPG blend would exhibit a longer delay for pure diesel fuel to ignite and the lower self-ignition temperature of LPG can increase the rate of pressure rise during the combustion. Thus, NOx will increase due to the excessive change in pressure per unit CA and the increase of maximum temperature of the cycle. Figure 8: Variation of NOx emission depending on engine loads. Figure 7: Variation of Brake Specific Fuel Consumption (BSFC) depending on engine loads. In the case of diesel/LPG dual fuel operation, the injected more diesel fuel ratio generates bigger initial flame and this produces smoother combustion of the gaseous fuel when the load is increased. The increasing of gaseous fuel ratio which is admitted into the cylinder causes the fuel to burn at higher rates. The oxygen in the excess air taken into the cylinder at the air intake stroke combines with nitrogen due to the high burnt gas temperature and occurs increased NOx emissions. In the experiments, NOx emission increased as both the engine load and the proportion of LPG were increased. This situation has occurred because LPG exhibited a better combustion reaction and higher temperature at the end of combustion than D-100. These results are in concordance with other studies [21; 26; 38; 41; 42]. Smoke emission values of the test engine are shown in Figure 5.0 LPG is a cleaner fuel than diesel fuel because LPG has lower carbon content and can be mixed with air homogeneously. In addition, increasing the LPG fuel ratio and the combustion temperature in the cylinder led to a decrease in smoke emission effectively. Additionally, because of LPG fuel has a lower C/H ratio than diesel fuel, it exhibits lower smoke emissions. For this reason, the diesel/LPG dual fuel operation reduces the smoke emission at all engine load conditions as compared to pure diesel operation. Diesel/LPG dual fuel keep the engine clean and smoke-free. Smoke emissions are in concordance with other studies [24; 30; 34; 41; 42; 43; 45; 46]. The measured HC emission values depend on engine load ratios are given in Figure 5.1 HC emissions using different LPG ratio decreased by means of getting better combustion reaction with diesel pilot fuel and performing more effective combustion reaction with the help of LPG. The essential factors of forming HC emissions are lower combustion temperature in the extremely poor mixture, insufficient oxygen and limited reaction time in the excessively rich mixture. The direct injection of LPG has better air charging performance and this phenomenon led to decrease unburned fuel as well. In the experimental setup, using of extremely poor and excessively rich mixtures were prevented by ECU while LPG in the liquid phase was injecting into the cylinder. Thus, HC emissions of diesel/LPG operation were reduced about 20% when it was compared with D-100 fuel. Similar results were obtained by other studies [24; 26; 36; 46]. The measured CO emission values as a function of engine load are shown in Figure 5.2 In general, CO2 emissions were produced because of full combustion of a large amount of fuel in the cylinder.