Combustion characterization of methanol in a lean burn Direct Injection Spark Ignition (DISI) engine

Lean operation is a promising approach to increase the engine efficiency. One of the main challenges for lean-burn technology is the combustion instability. Using a high laminar burning velocity fuel such as methanol might solve that problem. The potential of lean-burn limit extension with methanol was investigated through a comparison with conventional gasoline. In this work, a direct injection turbocharged SI engine was operated at wide open throttle (WOT), with the load controlled by a lean-burn strategy. The amount of fuel was decreased (or lambda increased) until the combustion became unstable. For methanol, the lambda limit was about 1.5, higher than the lambda limit for gasoline which was only about 1.2. The brake thermal efficiency for methanol increased as lambda increased and reached its peak at ~41% in a lambda range of 1.2-1.4. Then, the efficiency decreased as lambda increased. The increase of lambda also causes a change in the combustion process, e.g. prolonged flame development period (CA0-10). The relationship between the combustion characteristics and the combustion instability of methanol under lean-burn condition has not been reported previously. Values for the CA0-10 duration and the laminar burning velocity were searched for, able to define the combustion stability limit for the WOT operation with methanol. The laminar burning velocity was calculated using a previously developed correlation, with the unburned gas temperature and residual mass fraction derived from a three-pressure analysis simulation. A CA0-10 duration of 26.5 degree crank angle and a laminar burning velocity at ignition timing of ~0.4 m/s could be used to represent the combustion stability limit (5% coefficient of variance of indicated mean effective pressure). A longer CA0-10 and a smaller laminar burning velocity indicate an unstable combustion of methanol at WOT. At throttled conditions, those limits could not be employed to represent the instability limit for methanol combustion