The potential utility of spark discharges to assist low temperature, compression ignited engines has resulted in a number of experimental studies. These show mixed results: sometimes spark is effective at controlling heat release timing, sometimes it only stabilizes combustion, and sometimes it has no effect at all. In order to help understand the spark assisted process we propose a multi-mode combustion diagram to delineate the regimes of spark ignition, flame propagation, compression ignition, knocking combustion and spark-assisted combustion, in terms of unburned and burned gas temperatures near top dead center. An analysis of existing experimental data suggests that the effectiveness of spark assist is best at higher and middle loads and decreases as load is reduced. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.