Charged Species in the Exhaust of a Spark Ignition Engine as Studied With Langmuir Probes and a Mass Spectrometer

Abstract

The charged species present in the exhaust of a small single-cylinder spark ignition engine have been studied using spherical and cylindrical Langmuir probes and also by continuously sampling part of the exhaust gases into a mass spectrometer. Peak ion concentrations in the exhaust gases are of the order of 10^7-8 ionscm^-3 although ion densities vary markedly during an engine cycle. The positive ions arc usually hydrates of H30+, but some have large masses (> 10⁵a.m.u.) and are ultrafine charged soot particles (diam =. 10 nm) or their precursors. As for the negatively charged species, normally less than 1 % of them are free electrons. There are significant quantities of hydrates of the ions CO₃- and HCO₃-. In addition, negatively charged carbonaceous particles with masses up to and exceeding 105a.m,u. are present. One constant feature of the observations made here is that there are slightly more negatively charged large (mass > 10⁵a.m.u.) ions than positively charged ones. The origins of these ions are discussed; their abundances are largely determined by the time interval between the completion of combustion and the moment of measuring ion concentrations. However, the temperatures experienced by the exhaust gases arc also important, in that e.g. temperature determines the identity and mobility of the ions, as well us the mechanism for ion recombination. Preliminary computations have been made of the velocities and temperatures of the exhaust gases from such an engine, in order to calculate ion concentrations. The agreement with experimentally measured values is surprisingly good The dependence of ion concentrations on ignition timing, air-fuel ratio, engine load, octane rating of the fuel and on whether the gasoline is doped with potassium is investigated.