For the purpose of developing direct injection heavy-duty methanol engines which surpass diesel engines in purformace, this paper first clarifies the methanol concentration around the spark plug for achieving a high ignition stability by sampling the gas near the spark plug using a sampling valve. The combustion process of methanol is then observed by the method of high-speed Schlieren photography to clarify the mode of methanol combustion. A new methanol DISC combustion system having a protrusion in the combustion chamber is devised based on such results.This study clarifies that the methanol concentration at the point of ignition for high ignition stability is in the range of 6 to 22 vol%. The methanol mixture burns by flame propagation so far as the compression ratio is on the order of 16.5. The performance of the new methanol DISC combustion system can be characterized ➀ that the brake thermal efficiency is comparable to that of diesel engines at middle load but higher at high load due to early injection, ➁ that the brake mean effective pressure can be made higher than that of diesel engines, ➂ that the NOx emission is lower than that of diesel engines at high load where the emission is high, ➃ that the ignition stability is as high as that of diesel engines in all driving ranges, and ➄ that the maximum rate of pressure rise, (dP/dθ)max, is lower than that of diesel engines.