Motor diesel banyak digunakan baik untuk keperluan industri, transportasi maupun pembangkitan tenaga dan sebagainya. Meskipun demikian, motor diesel menghasilkan emisi gas buang yang cukup besar terutama NOx dan particulate matter. Ketergantungan yang sangat tinggi akan bahan bakar solar ini menimbulkan banyak masalah seiring dengan kenaikan harga bahan bakar minyak dari tahun ke tahun. Sebagai upaya mengurangi ketergantungan akan bahan bakar minyak dan mengurangi emisi maka perlu dilakukan penelitian mengenai penggunaan bahan bakar gas pada motor diesel dengan sistem dual fuel (dual fuel diesel engine). Tujuan dari penelitian ini adalah untuk mengetahui pengaruh saat injeksi dan pilot fuel quantity terhadap performa dan emisi gas buang motor diesel berbahan bakar LPG dan minyak solar. Penelitian ini dilakukan pada laboratorium Konversi Energi Universitas Gadjah Mada menggunakan motor diesel empat langkah satu silinder berpendingin air dengan volume langkah 353 cm3, panjang langkah 80 mm dan diameter silinder 75 mm, tekanan kompresi 22 dan 19: 1, dan tekanan injektor 125 kg/cm2. Mesin dioperasikan pada putaran konstan 1500 RPM dengan perubahan beban dan laju aliran LPG. Tekanan alir LPG dibuat konstan pada 0,1 kg/cm2 dengan variasi saat injeksi 12; 16,6 dan 20 derajat sebelum titik mati atas dan pilot fuel quantity sebesar 0,20; 0,24 dan 0,29 kg/jam. Hasil penelitian menunjukkan bahwa penurunan rasio kompresi dari 22:1 menjadi 19:1 dapat menunda terjadinya knocking pada motor diesel dengan sistem dual fuel. Efisiensi termal yang dihasilkan dengan sistem dual fuel lebih rendah dibandingkan motor diesel normal pada berbagai variasi saat injeksi dan jumlah solar yang disemprotkan. Konsentrasi CO berkurang dengan memajukan saat injeksi dan mengurangi jumlah solar yang diinjeksikan. Sebaliknya dengan kenaikan jumlah solar yang diinjeksikan akan menurunkan kadar HC. Kepekatan gas buang yang dihasilkan pada sistem dual fuel lebih rendah dibandingkan pada motor diesel normal.

Diesel engine is widely used for the need of industry, transport and generation of power, et cetera. However, diesel engine produces gas emissions that is quite high especially NOx and particulate matter. Very high dependence of diesel fuel causes many problems such the oil price increases from year to year. Hence, in order to reduce the dependence to fuel oil and to reduce emissions researches must be carried out to investigate the use of liquefied petroleum gas (LPG) to diesel engines with dual fuel system (dual fuel diesel engine). The aim of this research was to investigate the effect of injection timing and pilot fuel quantity on the performance and emissions of diesel engine fueled with LPG and diesel fuel. The research was carried out at the Conversion Energy Laboratory, at Mechanical Engineering Department of Gadjah Mada University using one cylinder four stroke diesel engine, water cooled, piston displacement of 353 cm3, with stroke length of 80 mm and the diameter of the cylinder 75 mm, the compression ratios were 22 and 19:1 and the injection pressure of diesel fuel was 125 kg/cm2. The engine was operated at the constant speed 1500 rpm with the change in the load and the mass flow rate of the LPG, the lpg pressure was made constant to 0.1 kg/cm2 with the variation of injection timing were 12; 16,6 and 20 degrees before top dead centre and pilot fuel quantity were 0,20; 0,24 and 0,29 kg/hour. The results of the research show that decreasing the compression ratio from 22:1 to 19:1 could postpone the occurrence of knocking to diesel engine with dual fuel system. Compared to normal diesel engine, thermal efficiency was lower in various injection timing and pilot fuel quantity. The CO concentration was decreased with advancing injection timing and reducing pilot fuel quantity, while increasing the amount of pilot fuel could decrease the HC level. The opacities of exhaust gas produced by dual fuel system was lower than those from normal diesel engines.

Kata Kunci : LPG,Dual fuel diesel engine,Saat injeksi,Pilot fuel quantity