Effects of 2-ethylhexyl nitrate (EHN) on combustion and emissions on a compression ignition engine fueling high-pressure direct-injection pure methanol fuel

Green methanol is a good carbon-neutral fuel, but the cetane number of methanol is so low that it is unsuitable for use in conventional compression ignition engines. Stable combustion of methanol is a great challenge at low load. Cetane improver such as 2-ethylhexyl nitrate (EHN), can improve the combustion stability of low cetane number fuel. In the current work, the effects of EHN on combustion and emissions were studied on a compression ignition engine at low load by using pure methanol fuel. Four methanol fuels with different EHN addition ratios were used, namely 0 % EHN (pure methanol), 0.1 % EHN, 1 % EHN and 3 % EHN, and diesel was used as the baseline. Results show that compared with diesel, the shortened combustion duration of pure methanol is mainly caused by the shortening of premixed combustion phase. The brake thermal efficiency (BTE) of pure methanol is relatively higher than that of diesel at 120 degrees C intake temperature. But the BTE of diesel at 50 degrees C intake temperature is slightly higher than that of pure methanol. As the ENH increases, the ignition delay decreases gradually, and the BTE decreases first and then increases. The NOx emissions of methanol are lower than those of diesel, but NOx emissions increase with the increase of EHN. NOx emissions of 3 % EHN are 31 % more than those of pure methanol and 77 % less than those of diesel. The soot emissions are close to zero for all test methanol fuels. The BTE of all fuels increases initially and decreases subsequently with the delay of start of injection. The higher EGR rate, lower intake temperature and injection pressure improve the BTE of all fuel. For 3 % EHN, the lowest intake temperature is not higher than 40 degrees C for stable operation, and the optimal operating strategy for the highest BTE (30.5 %) is single injection, 25 MPa injection pressure, 15 % EGR rate and 40 degrees C intake temperature. Methanol shows 0.6 % higher optimal BTE than diesel at the operating condition.