will be trapped and will dilute the incoming fuel-airmixture when the intake valves open. Since the pistonhas little downward movement while in the rockposition, the exhaust valve can remain open during thisperiod and thereby permit a more complete scavengingof the exhaust gases.Ignition timing refers to the timing of the sparks atthe spark plug gap with relation to the piston positionduring the compression and power strokes. The ignitionsystem is timed so that the sparks occurs before thepiston reaches top dead center on the compressionstroke. That gives the mixture enough time to ignite andstart burning. If this time were not provided, that is, ifthe spark occurred at or after the piston reached top deadcenter, then the pressure increase would not keep pacewith the piston movement.At higher speeds, there is still less time for the fuel-air mixture to ignite and bum. To make up for this lackof time and thereby avoid power loss, the ignitionsystem includes an advance mechanism that functionson speed.CLASSIFICATION OF ENGINESEngines for automotive and construction equipmentmay be classified in several ways: type of fuel used, typeof cooling employed, or valve and cylinder arrange-ment. They all operate on the internal combustionprinciple. The application of basic principles ofconstruction to particular needs or systems of manu-facture has caused certain designs to be recognized asconventional.The most common method of classification is basedon the type of fuel used; that is, whether the engine burnsgasoline or diesel fuel.GASOLINE ENGINESDIESEL ENGINESMechanically and inVERSUSoverall appearance, gasolineand diesel engines resemble one another. However,many parts of the diesel engine are designed to besomewhat heavier and stronger to withstand the highertemperatures and pressures the engine generates. Theengines differ also in the fuel used, in the method ofintroducing it into the cylinders, and in how the air-fuelmixture is ignited. In the gasoline engine, we first mixair and fuel in the carburetor. After this mixture iscompressed in the cylinders, it is ignited by an electricalspark from the spark plugs. The source of the energyproducing the electrical spark may be a storage batteryor a high-tension magneto.The diesel engine has no carburetor. Air alone entersits cylinders, where it is compressed and reaches a hightemperature because of compression. The heat ofcompression ignites the fuel injected into the cylinderand causes the fuel-air mixture to burn. The dieselengine needs no spark plugs; the very contact of thediesel fuel with the hot air in the cylinder causes ignition.In the gasoline engine the heat compression is notenough to ignite the air-fuel mixture; therefore, sparkplugs are necessary.ARRANGEMENT OF CYLINDERSEngines are also classified according to the arrange-ment of the cylinders. One classification is the in-line,in which all cylinders are cast in a straight line abovethe crankshaft, as in most trucks. Another is the V-type,in which two banks of cylinders are mounted in a “V”shape above the crankshaft, as in many passengervehicles. Another not-so-common arrangement is thehorizontally opposed engine whose cylinders mount intwo side rows, each opposite a central crankshaft. Busesoften have this type of engine.The cylinders are numbered. The cylinder nearestthe front of an in-line engine is numbered 1. The othersare numbered 2, 3,4, and so forth, from the front to rear.In V-type engines the numbering sequence varies withthe manufacturer.The firing order (which is different from thenumbering order) of the cylinders is usually stamped onthe cylinder block or on the manufacturer’s nameplate.VALVE ARRANGEMENTThe majority of internal combustion engines alsoare classified according to the position and arrangementof the intake and exhaust valves. This classificationdepends on whether the valves are in the cylinder blockor in the cylinder head. Various arrangements have beenused; the most common are the L-head, I-head, andF-head (fig. 12-8). The letter designation is used becausethe shape of the combustion chamber resembles theform of the letter identifying it.L-HeadIn the L-head engines, both valves are placed in theblock on the same side of the cylinder. The valve-operating mechanism is located directly below thevalves, and one camshaft actuates both the intake andexhaust valves.12-8
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