Quantcast Classification of Engines

will be trapped and will dilute the incoming fuel-air mixture when the intake valves open. Since the piston has  little  downward  movement  while  in  the  rock position, the exhaust valve can remain open during this period and thereby permit a more complete scavenging of the exhaust gases. Ignition timing refers to the timing of the sparks at the spark plug gap with relation to the piston position during the compression and power strokes. The ignition system is timed so that the sparks occurs before the piston  reaches  top  dead  center  on  the  compression stroke. That gives the mixture enough time to ignite and start burning. If this time were not provided, that is, if the spark occurred at or after the piston reached top dead center, then the pressure increase would not keep pace with  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 lack of  time  and  thereby  avoid  power  loss,  the  ignition system includes an advance mechanism that functions on  speed. CLASSIFICATION OF ENGINES Engines for automotive and construction equipment may be classified in several ways: type of fuel used, type of cooling employed, or valve and cylinder arrange- ment.  They  all  operate  on  the  internal  combustion principle.  The  application  of  basic  principles  of construction to particular needs or systems of manu- facture has caused certain designs to be recognized as conventional. The most common method of classification is based on the type of fuel used; that is, whether the engine burns gasoline or diesel fuel. GASOLINE ENGINES DIESEL ENGINES Mechanically  and  in VERSUS overall  appearance,  gasoline and diesel engines resemble one another. However, many parts of the diesel engine are designed to be somewhat heavier and stronger to withstand the higher temperatures and pressures the engine generates. The engines differ also in the fuel used, in the method of introducing it into the cylinders, and in how the air-fuel mixture is ignited. In the gasoline engine, we first mix air and fuel in the carburetor. After this mixture is compressed in the cylinders, it is ignited by an electrical spark  from  the  spark  plugs.  The  source  of  the  energy producing  the  electrical  spark  may  be  a  storage  battery or  a  high-tension  magneto. The diesel engine has no carburetor. Air alone enters its cylinders, where it is compressed and reaches a high temperature  because  of  compression.  The  heat  of compression  ignites  the  fuel  injected  into  the  cylinder and causes the fuel-air mixture to burn. The diesel engine needs no spark plugs; the very contact of the diesel fuel with the hot air in the cylinder causes ignition. In the gasoline engine the heat compression is not enough  to  ignite  the  air-fuel  mixture;  therefore,  spark plugs are necessary. ARRANGEMENT  OF  CYLINDERS Engines 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 above the 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  passenger vehicles.  Another  not-so-common  arrangement  is  the horizontally  opposed  engine  whose  cylinders  mount  in two side rows, each opposite a central crankshaft. Buses often  have  this  type  of  engine. The cylinders are numbered. The cylinder nearest the front of an in-line engine is numbered 1. The others are numbered 2, 3,4, and so forth, from the front to rear. In V-type engines the numbering sequence varies with the  manufacturer. The  firing  order  (which  is  different  from  the numbering order) of the cylinders is usually stamped on the  cylinder  block  or  on  the  manufacturer’s  nameplate. VALVE  ARRANGEMENT The  majority  of  internal  combustion  engines  also are classified according to the position and arrangement of the intake and exhaust valves. This classification depends on whether the valves are in the cylinder block or in the cylinder head. Various arrangements have been used; the most common are the L-head, I-head, and F-head (fig. 12-8). The letter designation is used because the shape of the combustion chamber resembles the form of the letter identifying it. L-Head In the L-head engines, both valves are placed in the block  on  the  same  side  of  the  cylinder.  The  valve- operating  mechanism  is  located  directly  below  the valves, and one camshaft actuates both the intake and exhaust  valves. 12-8


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