Excess-Air Factor λ & Ignition Timing Effects on HC, NOx, CO Emissions - Manual

This manual page explains the relationship between the excess-air factor (lambda, λ) and ignition timing (αz) on engine exhaust emissions and fuel consumption. The text describes how the optimal ignition advance angle increases with higher excess-air ratios due to slower flame-front propagation in lean mixtures, and how operation near the lean misfire limit (LML) causes delayed combustion, misfiring, and rising specific fuel consumption. It details HC emissions, which bottom out around λ = 1.1 and rise again in extremely lean mixtures due to flame extinguishing on combustion chamber walls, and how advancing ignition timing can lower HC emissions in the lean range beyond λ = 1.25. It also explains the characteristic bell-shaped NOx emissions curve, which peaks near λ ≈ 1.05 due to increased oxygen concentration and peak combustion temperature, then drops sharply as the mixture becomes leaner. Figure 4 presents three graphs plotting HC, NOx, and CO emissions (in g/kWh) against excess-air factor λ (0.8–1.4) for four ignition advance angles (20°, 30°, 40°, 50°). This is theory/reference material on combustion and emissions behavior rather than vehicle-specific repair data, useful for understanding how air-fuel ratio and spark timing interact to affect emissions output.