<div class="htmlview paragraph">In order to simulate diesel exhaust of known composition, weighed amounts of various high-boiling hydrocarbons were evaporated into a stream of heated air. These mixtures were sampled continuously and the hydrocarbon contents measured with a heated flame ionization detector (FID). The evaporator unit and FID were operated at various temperatures and 375 F was optimum as regards percentage of input material accounted for (85–100%, for paraffins through C₁₆), fast response, and repeatability.</div> <div class="htmlview paragraph">The FID was then used at various temperatures to measure total hydrocarbons in exhaust from a 1-cyl diesel engine. Again, 375 F was optimum for obtaining maximum apparent hydrocarbon concentration, fast response, and repeatability.</div> <div class="htmlview paragraph">Finally, FID measurements were obtained at 375 F on exhaust from the engine at various operating conditions, to assess the effects of operating variables on hydrocarbons. Increasing compression ratio and temperatures of the inlet air and crankcase oil were effective ways of lowering hydrocarbons.</div> <div class="htmlview paragraph">Some principles for valid sampling and measurement of diesel exhaust hydrocarbons are discussed.</div>