Experimental Study of the Effect of Injection Mode on Knock Characteristics of High Compression Ratio Methanol Engines

To investigate the knock suppression methods for high compression ratio methanol engines, an experimental study is conducted on a single-cylinder naturally aspirated spark-ignition methanol engine with a compression ratio of 15:1, focusing on the impact of injection modes on knock characteristics under stoichiometric mixed gas conditions. First, the influence of load, ignition timing, and air-fuel ratio on knock characteristics is explored under the intake port injection mode. Second, the effect of injection timing on knock characteristics is examined under the direct injection mode. Finally, the suppression effect of multiple injection strategies on knock is analyzed under wide-open throttle conditions. The results indicate that in the intake port injection mode, the methanol engine experiences knock combustion at medium to high loads, with a strong knock phenomenon characterized by low frequency and high intensity, reaching a maximum knock intensity of 8. 42 MPa at an indicated mean effective pressure (p_i) of 0. 7 MPa. In contrast, under the direct injection mode, the knock phenomenon is significantly suppressed, with the maximum knock intensity dropping to 0. 43 MPa when p_i is 0. 91 MPa. A reasonable two-injection strategy further reduces knock intensity compared to a single injection strategy while maintaining a high p_i. This study confirms the effectiveness of combining direct injection with a two-injection strategy in suppressing knock phenomena in high compression ratio methanol engines, providing theoretical basis and data support for the development and application of methanol engines.