Honda - Accord Prelude 1996 2000

Crankcase Emissions


Crankcase emissions are made up of water, acids, unburned fuel, oil fumes and particulates. These emissions are classified as hydrocarbons (HC) and are formed by the small amount of unburned, compressed air/fuel mixture entering the crankcase from the combustion area (between the cylinder walls and piston rings) during the compression and power strokes. The head of the compression and combustion help to form the remaining crankcase emissions.

Since the first engines, crankcase emissions were allowed into the atmosphere through a road draft tube, mounted on the lower side of the engine block. Fresh air came in through an open oil filler cap or breather. The air passed through the crankcase mixing with blow-by gases. The motion of the vehicle and the air blowing past the open end of the road draft tube caused a low pressure area (vacuum) at the end of the tube. Crankcase emissions were simply drawn out of the road draft tube into the air.

To control the crankcase emission, the crankcase breather road draft tube was deleted. A hose and/or tubing was routed from the crankcase to the intake manifold so the blow-by emission could be burned with the air/fuel mixture. However, it was found that intake manifold vacuum, used to draw the crankcase emissions into the manifold, would vary in strength at the wrong time and not allow the proper emission flow. A regulating valve was needed to control the flow of air through the crankcase.

Testing, showed the removal of the blow-by gases from the crankcase as quickly as possible, was most important to the longevity of the engine. Should large accumulations of blow-by gases remain and condense, dilution of the engine oil would occur to form water, soot, resins, acids and lead salts, resulting in the formation of sludge and varnishes. This condensation of the blow-by gases occurs during cold engine start up and most frequently to vehicles used in stop and go driving. These severe operating conditions include numerous engine starting and stopping conditions, excessive idling and not allowing the engine to attain normal operating temperature through short runs. Vehicles that accumulate mostly continuous speed highway miles tend to purge the condensation blow-by gases from the engine oil that occur during start up.

Because the engine oil can accumulate these blow-by gasses, dirty engine oil can contribute to increased emissions as well as increased engine wear. For this reason, oil change intervals for vehicles driven in severe conditions are more frequent than for vehicles driven primarily on the open highway.