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APPENDIX D GLOSSARY OF TECHNICAL TERMS AIR/FUEL RATIO: Ratio of air to fuel, by weight, in a combustion process. BRAKE-SPECIFIC QUANTITY: The prefix "brake-specific" denotes a measure of some quantity divided by the power delivered by the engine. The total power generated in the engine is called the indicated power. The net or brake power output of the engine equals the indicated power minus the power required to overcome engine friction. CATALYTIC CONVERTER: Device in the exhaust systems of vehicles with spark-ignition engines, containing a catalyst to aid in reducing emissions. (See oxidizing converter and three-way converter.) CETANE INDEX: Value dervied from an empirical method for finding the cetane number of a diesel fuel, based on fuel's density and mid-boiling point. CETANE NUMBER: Equivalent percentage by volume of cetane (= n-hexadecane, cetane number l00) in a mixture with alphamethylnapthyene (cetane number O); indicates the relative ignitibility of the fuel upon in- jection into the engine cylinder. COMPRESSION RATIO: Ratio between the volume above the piston when it is at bottom dead center to the volume above the piston when it is at top dead center. DYNAMOMETER: Device for testing engines. Assures power output from the engine and provides measurement of engine torque. EGR: Exhaust gas recirculation. Recirculation of a portion of the engine exhaust gases into the air intake, to control NOX emissions. ENGINE LOAD: General term referring to power or torque demand on the engine. ENGINE SPEED: Crankshaft revolutions per minute. FOUR-STROKE CYCLE: Engine cycle consisting of a downward intake stroke, an upward compression stroke, a downward power stroke, and an upward exhaust stroke. HEAVY-DUTY: Applications more severe than passenger car service. Usually applied to service in buses or trucks in excess of 8,500 pounds gross weight rating. ll0
HI HYDROCARBONS: Substances whose molecules are made solely from hydrogen and carbon atoms, with multiple carbon atoms linked to one another and with hydrogen atoms attached to the carbon atoms. MANIFOLD: Branched-pipe passage device that connects openings from each cylinder to a common opening. OXIDATION CATALYST: Catalyst, operating with oxygen in the exhaust gas, designed to oxidize exhaust carbon monoxide and hydrocarbons to carbon dioxide and water. PERFORMANCE: General descriptive term covering engine power output, durability, and driveability. QUENCH: Cool to temperatures at which the flame will go out. SAC VOLUME: Volume of the space in the top of a diesel fuel injector. Zero volume is the ideal design objective for low hydrocarbon emissions. SPARK-IGNITION ENGINE: An engine whose combustion is started by an arc across the spark plug electrodes. SPARK TIMING: Point in an engine's crankshaft rotation at which the spark plug is fired. STEADY STATE: Constant operating conditions with no variation in speed or load. STEADY-STATE TEST: Current test cycle for heavy-duty engines. TEST CYCLE: The specific sequence of speed and load conditions at which an engine is run for measurement of emissions. TEST PROCEDURES: Full set of procedures for testing engines to determine compliance with emission standards. THERMAL EFFICIENCY: Energy output (of the engine, for example) divided by the energy in the fuel used. THREE-WAY CATALYST: Exhaust treatment catalyst designed for simultaneous removal of carbon monoxide, hydrocarbons and oxides of nitrogen. TORQUE: Measure of the twisting moment on a shaft. TRANSIENT TEST CYCLE: The new test cycle for heavy-duty engines, effective in l984.