GASOLINE ENGINE EXHAUST ODORS - (A Study of Odors Produced Under Conditions of Deceleration 430114
THIS paper presents a study of the causes and cures of the pungent odor present in gasoline bus engines under certain conditions of operation - particularly during deceleration with the throttle closed and the clutch engaged.
To supplement the disagreeable task of smelling exhaust samples, the authors developed a chemical method for determining the odor intensity under the various operating conditions. This method was based on the discovery that formaldehyde was always present, among other aldehydes, in samples of gas that gave a characteristic odor and seemed to be its chief cause. Since it seemed that engine speed was one of the factors contributing to the objectionable odor, closed-throttle tests were made at various speeds. It was found that the engine was using approximately the same weight of charge per suction stroke at 1500 rpm as at 600 rpm. It seemed that the residual gas dilution, that is, the ratio of residual gas in the cylinder to the fuel mixture at the time the intake valve closes, was very high at the higher speeds. Analysis showed that, at the higher speeds, combustion was much more incomplete. This is a favorable condition for the formation of aldehydes, since they are intermediate products of combustion - and, in fact, a high concentration of aldehydes was found at the upper speeds.
It was found also that large amounts of fuel in the manifold at the time the throttle is closed (and deceleration begins) cause an increase in pungency.
Using a more volatile fuel, installing higher temperature manifolds or, if the bus has been in operation a long time, cleaning the deposits from the inside walls of the manifolds, are some of the remedies suggested by the author.