Mechanisms that influence emissions of polynuclear aromatic hydrocarbons (PNA) from engines have been investigated with an on-line fluorescence PNA detector. PNA can accumulate at the surfaces and within the deposits of a combustion chamber, and some of these PNA are vaporized during engine operation. Increased heat input to established combustion chamber deposits (for example, from increased spark advance, knock, or high-speed operation) enhances PNA emissions, at least temporarily.At several steady speeds, PNA emissions were compared, using a rotary engine vehicle and a similar car with a reciprocating spark ignition engine. With the emission control system disabled, the rotary engine automobile exhibits very high PNA emissions relative to the piston engine. However, exhaust emission control devices, such as the thermal reactor on the rotary engine car, can be highly effective in destroying PNA.