**4. Conclusions**

In this study, the effect of direct injection of LPG into the cylinder on performance and emissions has been investigated experimentally. The tests were performed using different fuel compositions and engine loads at a constant 3000 rpm engine speed. D-100, LPG-30, LPG-50, and LPG-70 fuels were used in the experiments. The test engine was loaded with 500, 750, 1000, 1250 and 1500 W loads through the loading unit. Depending on those parameters, fuel consumption and exhaust emissions were measured. As a result;

The best effective efficiency was reached using LPG-70 fuel. It was increased about 1% than D-100 fuel. The BSFC was reduced linearly depends on the lower heat value which increased with the LPG fuel ratios. The BSFC was decreased as 6% at 1000 W engine load when the LPG-70 fuel was compared with D-100 fuel.

In general, the EGT values for all LPG fuel ratios were higher than D-100 fuel. LPG fuels produced more temperature about 10% than D-100 fuel. NOx emissions increased about max. 4% when the engine operated with LPG fuels. CO and HC emissions were improved because of the low carbon content of LPG and increasing the in-cylinder temperature. The most emission improvement was gathered for smoke emission. The smoke emission reduced gradually by means of adding LPG content in the fuel mixture. In addition, the increasing pilot diesel fuel ratio caused the increasing smoke emission significantly. The smoke emission decreased as 70% for LPG-50 and 80% for LPG-70 when it was compared with D-100 results.

The dual fuel engine improves fuel economy and exhaust emissions, but it has some disadvantages like a second injector, separate fuel line, electronic and software modifications on fuel injection systems. Additionally, it can be reversed to a pure diesel engine easily when it is necessary.

**Author Contributions:** All authors designed the experimental setup, analyzed the data, discussed the results and implications and commented on the manuscript at all stages. M.A. and A.I. performed the experiments and wrote the paper. M.B.C. led the development of the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
