Exhaust Hydrocarbon Emissions from Gasoline Engines - Surface Phenomena

A detailed understanding of the surface phenomena associated with exhaust hydrocarbon (HC) emissions is required to be certain that all avenues of emission control, internal to the combustion chamber, have been exploited. This paper presents information obtained on a single-cylinder research engine, which supports the following hypotheses: there exists a critical piston top land-to-bore clearance above which HC emissions decrease sharply; the presence of an oxidatively active catalytic surface in the combustion chamber increases HC emissions; using a clean engine operating on a fuel containing 5% lubricating oil, HC emissions increase within minutes to a level comparable to that obtained with equilibrium deposits; the first parcel of gas to leave the exhaust valve contains an order of magnitude lower concentration of HCs than does the total mixed exhaust; and a cylinder bore tapered outward towards the top eliminates oil-droplet formation in a motored engine.

In order to predict exhaust HC emissions from the top land volume, an equation is derived which accounts for the effects of piston and bore expansion, blowby, and cylinder-pressure ratio. This equation is applied to literature data obtained on the decrease of HC emissions with an increase in engine speed.