**3. Results and Discussion**

The effect of experiment fuels on the effective efficiency depends on the engine load is shown in Figure 4. The highest effective efficient was achieved about 28.34% using LPG-70 fuel at 1250 W engine load. The ignition delay, uncontrolled combustion, and post-combustion phases were occurred in a short time because of better atomization of the LPG fuel in the cylinder. The lower heating value and C/H ratio of LPG lead to higher flame temperature and effective combustion. In addition, LPG direct injection does not affect intake air and maximum air charging results in the air intake stroke. This phenomenon led to decreasing the fuel consumption and increasing the effective efficiency overall LPG fuel ratios. Effective efficiency results are in concordance with other studies [21,22,36,37].

**Figure 4.** Effect of engine loads change on effective efficiency.

The changes in EGT depends on the engine load are shown in Figure 5. In the experiments, the amount of consumed fuel and EGT increased continuously through the increase of engine load. In addition, EGT presented incremental behavior for all fuel types. However, as the engine load increased, the increase in the LPG ratio reflected more on the EGT. At the lower engine loads ignition delay period of diesel fuel increases and also, fine ignition and combustion of LPG do not occur due to low temperature and pressure inside the combustion chamber. However, at the higher engine

loads ignition delay period of diesel fuel decreases and also pressure and the temperature inside the cylinder become higher, an increase in EGT occurs due to this fine ignition and combustion of LPG. In addition, the low C/H ratio of LPG, high combustion rate, and better fuel atomization than diesel fuel have improved combustion process. These similar results were also observed by other researchers [21,34–38].

**Figure 5.** Effect of engine loads change on Exhaust Gas Temperature (EGT).

The effect of engine loads change on the amount of the fuel consumption is given in Figure 6. The fuel consumption was reduced by means of occurring better combustion because the lower heat value of LPG is higher than diesel fuel about 8%. This situation may be explained by increasing of the LPG flow rate increases the heat release because of the overall equivalence ratio and the combustion is inclined to be more complete, leading to high in-cylinder pressures and increased power output. The fuel consumption results are in concordance with those of other researchers [21,30,32,37,39].

**Figure 6.** Effect of engine loads change on fuel consumption.

The BSFC for experiment fuels is given as a function of engine loads in Figure 7. The lowest BSFC was achieved using LPG-70 fuel at 1000-Watt engine load. When the BSFC was compared in terms of D-100 and LPG-70, it was seen that BSFC demonstrated reducing behavior by about 6%. Thus, the BSFC decreased because of the lower heat value of LPG was higher than pure diesel. Similar results were reported in other studies [21,30,40–42].

**Figure 7.** Variation of Brake Specific Fuel Consumption (BSFC) depending on engine loads.

The NOx emission is given as a function of engine load in Figure 8. Although better combustion in the cylinder and increasing effective efficiency is desired in the automotive industry, they cause to rise NOx emission directly. It is well known that NOx emissions are the result of nitrogen reacts with oxygen at the high temperature in the cylinder. The cylinder peak pressure, the maximum heat release rate, the maximum cylinder mean gas temperature, the proportion of the premixed heat release, and NOx emission increase while increasing the propane proportion in the fuel blends [42,43]. As the load is increased, the richer mixture results in higher temperatures which in turn results in higher NOx emissions. Due to locally rich combustion, NOx emissions of the diesel engine are less sensitive to temperature increases resulting from increasing load [31–33]. 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.

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 9. 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 [24,30,34,41–43,45,46].

**Figure 9.** Variation of smoke emission depending on engine loads.

The measured HC emission values depend on engine load ratios are given in Figure 10. 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 11. In general, CO2 emissions were produced because of full combustion of a large amount of fuel in the cylinder. On the other hand, CO emissions occurred when the remained fuel from full combustion was burned inadequately. HC emissions were formed by the non-combustible portion of the fuel. The reasons of forming CO and HC emissions are very similar. The direct injection of LPG fuel into the cylinder under high pressure by an injector caused to reach better atomization level of the LPG compared to the diesel fuel. Thus, improvement of the combustion reaction using LPG fuel provided reducing CO emissions. The better combustion and higher calorific value of the LPG improve the flame propagation and oxidation reactions that reduce the HC and CO emissions slightly. Moreover, the lower C/H ratio

of LPG deceases HC and CO emissions. CO emission decreased using LPG-70 fuel about 30% than D-100 fuel. These results are in concordance with other studies [21,23,26,40].

**Figure 10.** Variation of HC emission depending on engine loads.

**Figure 11.** Variation of CO emission depending on engine loads.
