04 S-11 Combustion in a Centimeter-Scale Four-Stroke Engine

Centimeter-scale liquid hydrocarbon fueled engines show promise for future use as high power density mobile power sources. Experimental results have shown that the peak power density of such engines is approximately 300 W/kg. For comparison, most electrochemical devices are limited to power densities of approximately 100 W/kg. While the power density of these devices is impressive, their operation is marked by a high percentage of unburned hydrocarbons and thus a low fuel conversion efficiency. The fuel-laden exhaust gives the impression of rich operation, however the system operates under overall near-stoichiometric conditions. We believe that two modes of combustion take place. An unburned wall film develops in the cylinder and acts as a cooling mechanism. Fuel evaporation from this film produces a relatively rich mixture near the walls that might sustain a diesel-like combustion process. Away from the walls, the mixture burns in a homogeneous charge compression ignition (HCCI) mode. Cylinder pressure measurements show that the compression process is far from ideal, mainly resulting from leakage past the piston ring.