Ion sensors have shown great potential as a low-cost diagnostic tool for detecting the start of combustion (SOC) in Homogeneous Charge Compression Ignition (HCCI) engines. Ion detection in an HCCI engine depends on both the combustion event as well as the type of probe used. Optimization of the shape and size of spark plug electrodes is experimentally examined in order to improve the overall quality of the ion signal for a wide range of engine operating conditions. Idling conditions for an HCCI engine are typically lean with equivalence ratios φ ≤ 0.25, which leads to reduced peak combustion temperatures. The ion signal in HCCI engines decreases exponentially with reduced combustion temperatures. Experiments are conducted on a 1.9L 4-cylinder VW TDI diesel engine (converted to run in HCCI mode) using four modified spark plugs to understand ion detection over various equivalence ratios and operating conditions. Increasing the surface area of the positive center electrode (using a positive bias voltage) provided a significant improvement in the peak ion signal.