Experimental investigation on the effects of water blending on combustion and emission characteristics of diesel pilot high pressure direction injection methanol combustion

Water blending is a potential method to reduce NO x emissions for the diesel pilot high-pressure direct injection (HPDI) methanol combustion. In this study, experiments were conducted on a specially designed dual direct injection single - cylinder engine to explore the effects of water blending on the combustion and emission performance of diesel pilot HPDI methanol combustion under a high methanol substitution rate of over 90%. Results indicate that water blending increases the density, viscosity, and surface tension of methanol while significantly elevating specific heat capacity and latent heat. Combustion remains stable even under 30% water blending conditions, though water blending slightly delays ignition timing (CA10) and combustion phasing (CA50). As the water blending ratio increases, the peak in-cylinder pressure and heat release rates (HRR) decline, particularly at high loads, while combustion duration (CA90–CA10) shortens with 20–30% water ratio, effectively reducing exhaust losses. However, when the water blending ratio reaches 30%, it causes a significant increase in heat transfer loss, resulting in a decrease in thermal efficiency. Under all load conditions, increasing the water blending ratio can greatly reduce NO x emissions with slightly increased carbon monoxide and hydrocarbon emissions at low loads. When the water blending ratio reaches 30%, NO x emissions fully meet Tier III emission standards under all operating conditions, yet thermal efficiency is lower than that of pure methanol. By contrast, a 20% water blending ratio maintains thermal efficiency superior to pure methanol while still having the potential to meet Tier III NO x limits. Therefore, to balance emissions and efficiency in practical engine applications, a compromise ratio within the 20%–30% range is recommended.