Downsizing direct injection spark ignition engines: a timescale analysis

Downsizing direct injection spark ignition engines presents several challenges to the engine designer, but is a necessary requirement if significant savings in terms of fuel economy and CO(2) emissions are to be realised. This challenge becomes more acute if we wish to employ a flexible fuel supply, for instance a range of biological fuel blends. These typically require more mechanical and thermal effort to provide good fuel vapour-air mixture preparation at ignition.

In this chapter these issues are investigated by comparing engine timescales (a function of engine size, speed and injection timing) against droplet timescales (a function of drop diameter, liquid physical properties and local air thermodynamic conditions). The analysis is used to make predictions of the target drop diameter required for a given engine size, speed and injection timing (load). Finally, we briefly explore the possibility of employing electrostatic charging to reduce the mass transfer timescale, since this is the limiting timescale for small direct injection spark ignition operation.