Effects of piston preheating and friction reduction on cold-start charge preparation for DI methanol engine: an in-cylinder CFD study

Methanol is gaining interest as a renewable fuel for ICE application. A challenge for this fuel is its evaporation rate at low temperatures, making cold start at low environment temperature problematic. The first combustion cycles are characterized by a low combustion chamber temperature and high engine friction. In a previous work, a practical idea is presented to both pre-heat the pistons and pre-condition the gliding working surfaces, reducing friction. In this article, an in-cylinder CFD model is used to study the charge preparation of a DI methanol ICE until end of compression. The model is calibrated in-house against measurements of a warm methanol engine. The piston temperature is varied within the rage expected by the pre-heating – pre-lubricating device. The friction decrease is translated into the reduced amount of fuel needed to generate the IMEP required to idle the engine. Engine initial starting conditions at -20°C, 0°C and +20°C are considered. For these global conditions, different combination of piston pre-heating and friction decrease are studied. Warm engine (90°C) conditions are also modeled for comparison. From the results, it is clear that the piston is the main target of fuel the spray. As expected, for a warm engine, the fuel injected is completely evaporated. For an ordinary cold start, +20°C, the fuel distribution at end of compression is 79% evaporated, 15% is left as film and the rest as suspended droplets. In the cold start at -20°C, over 75% of the fuel is present as a film at end of compression. The rest is almost completely evaporated. By only pre-heating the piston, the fuel film is reduced to 60%. Alternatively, by reducing the friction load as per warm engine, the total fuel injected is drastically reduced, resulting in a fuel film of 5% relative to the cold engine reference.