Document n96QoXpemqj0kmr9jJXaQJ9az

- m-' AND BLOWERS FOR SCAVENGING AND SUPERCHARGING valve stem meet it must be provided on push-tod tide. In many eases, ful crum bearing and push-rod and valvestem contact points are pressure lu bricated. Scavenging. There are three basic methods of supplying low-pressure air for scavenging. In crankcete scareng. ing, the crankcase and underside of the piston act as an air compressor. In a somewhat similar scheme, power-pis/on scavenging, a separate chamber ts con nected to the space under the piston and separated from the crankcase in such a way that piston downstroke compresses air drawn into the chamber on the upstroke. Scavenging air is, however, most commonly provided by a pump.or blower, either driven direct ly from the engine or by an independent source of power. Crankcase Scavenging. In crankcasescavenging engines, partitions between cylinders divide crankcase into sepa rate compartments. Aa shown in Pig. 6, the slight vacuum created by piston upstroke draws air in through filters, and valves in partition walla. Downstroke of piston compresses air slight ly. Near end of downstroke, piston un covers exhaust ports and then scaveng* Ing-alr ports. Air from crankcase flows into cylinder, displacing burned gas and charging cylinder. farts, Velvet. Engines scavenged with air from a pump or blower may be classified according to the way air en ters the cylinder and exhaust leaves. The commonest arrangement is port cavenging; air enters and gas leaves by ports alternately, covered and ex posed by the piston, in Pig. $, ports are et same level and uncovered simultan eously. Automatic air valves keep ex haust gas from flowing out through the air ports until cylinder pressure falls below air pressure. Then air valves open and scavenging air flows in to expel burned gas. Air valves remain open while piston moves to bottom center, reverses travel and move* up ward, until cylinder pressure again ex ceeds that in air manifold. Shape of ports directs flow of scavenging air in a loop, as shown. In a ra/v*.scavenged engine. Fig. 7, blower forces air into manifold and into cylinder when piston uncovers the ports, which are arranged around the entire cylinder periphery. Scavenging ' elr flows upward and out through the exhaust valves. This is known as a uniflow system. Scavenging Pumps. Reciprocating pumps form a common means of sup plying scavenging air. In one design. Pig. 8, pump cylinders are at right an gles to power cylinders, with pistons driven from the same crankshaft Pumps may be single or double acting; one scavenging cylinder usually sup plies two or more power cylinders. More frequently, a vertical pump, Pig. 9, is mounted at one end of the engine end driven from an extension of the crankshaft. These are usually double-acting. To save space, two pis tons may be used, one above the other on the same rod. Valves are usually of the automatic ring or feather type. Scavenging Hewers. The positivedisplacement blower finds wide use as a device (or supplying scavenging air. Such blowers consist of two closely fitted rotors carrying two or three lobes, Pig. 10, 11. Meshing action of the roton transports air to a region of higher pressure where it is compressed by back-flow effect. The two-lobed impel ler gives maximum capacity from mini mum blower site and weight and with lowest power consumption. More than two lobes, either of straight or spiral design, are used where a different pulsotion frequency is desired or where it is necessary to meet o noise limitation with little or no muffling. The familiar centrifugal blower also finds use In scavenging. Because such units operate best at' relatively high speeds (upwards of <000-3000 rpm) they are usually driven from a separate motor or turbine instead of from the engine, which is common practice with moderate-speed positive-displacement blowers. Supercharging. Supercharging should not be confused with scavenging, al though in 2 -cycle engines it may be hard to tell where one ends and the other begins. The purpose of super charging Is to cram more air into the cylinder so mote fuel can be burned and engine output boosted without in crease in cylinder sUe. This is accom plished by forcing air into the cylinder under pressure substantially above atmospheric (up to 10 psi or more in some cases). In this country, super charging has been applied only, to 4cycle engines; 2-cycle engines have been successfully supercharged in Eu rope (sec Power, June 1946, pp 68-70). Supercharged engines usually show somewhat better fuel economy than unsupercharged units. Increase in out put depends on a variety of factors; capacity boosts from 30 to 60% have been reported and <0% may be taken as a conservative figure. Superchargers may be added to existing engines and a number of users report that capacity gained in this way was substantially less expensive than Installation of ad ditional engines. Because air at high altitudes is less dense, a given cylinder full of air con tains fewer pounds et high altitude than low. This means lets fuel can be burned and engine capacity suffers. Superchargers may be Wed . ? come this loss of capacity 10 Applies -m pcrchargmg to an engine involve small changes. It is cutomiy f ample, to increase clearance U.fS thus reducing compression t|'^ keeping maximum pressure dow^' the unsupercharged condition** avoids unduly Increasing beorin. t and stresses in the engine. * . In the normal 4-cycte engine , ovtriap is about 30-40 deg. Thli Q that inlet valve opens before top- end exhaust valve closes after. |_ this period when both valves tre q air blows through the cylinder f inlet manifold to exhaust, scavet^, burned gas that would otherwise^ trapped in the clearance volume, vi pockets, etc. It alto provides some et Ing efTect. particularly on cxl valves. Longer valve overlap U g ally beneficial but requires more'll and at high supercharge pressures ti may mean excessive power consul tion. Designer thus must balance g against losses. Overlaps In the r borhood of 130-160 deg are supercharged engines. Because the basic purpose of charging is to increase the. air e so more fuel may be burned, retell fuel injection must be increased. Tbisl may mean larger fuel-pump plunges] Turbocharging. The energy la ti exhaust gas from a diesel can be r covered in a gas turbine to drive's centrifugal blower supplying air ot aa-! percharging pressure. Usually turbintj and blower rotors are mounted cn-i common shaft in a single casing,-! 13. Such a unit is called a turbocharger] In the Buchl turbocharging ayitc valve timing, exhaust manifolding * turbine design are coordinated to cren timed pressure pulsations. Engines c four cylinder* and over employ a tiple exhaust manifold arrangement] Fig. 14, to permit complete caven<jf!lfl of one cylinder before exhaust va'" opens on another. No controls. needed; speed, air quantity and chs ing pressure follow engine toed * speed changes. Cnglaa-Drivan Chargers. Superchsilj| ing blowers can be driven from the gine or, for that matter, from a power source. Poaitive-displae*01*^! blowers ere widely used; centrifuges moy be used olso. The posillve-d|**jj placement type. Fig. 12, finds f*vor applications where full torque ovef1 wide speed range is required. <_ tetercoollng. Insertion of an air coo 1-3 between supcrchorgcr and engine duces manifold temperature, ^hl* *?i creases air density and thus 1^5 greater weight of air per unit vrfu . increasing supercharging effect rowsa F*ililvi-dbplocimsi blower* find Ida w (n tupplylng **avnging oparal* with little pressure pul- d^r****"' of air It proportional to pwr Mid drop* o* paod drops, output b rvt effected by change In ond!tlon. Working affl iV^jImthr at tram an* ta Hire* time* engine Poililva-dlsplecamant blowers era gaor ar <hokt driven from angina Aoril 1948 Cotoway via* of tvrbechorgar *how* oxHnu.t-go.-turblno binding ert 13 right) centrifugal charging compressor el center; elr Intake, left Mrhe Buchl system of turbocharging employs n divided exhaust mani fold te permit template scavenging ef one cylinder before the ex haust valve open* on another. An engine firing t-2-4-3 Is connected as above so no two cylinder* exhaust consecutively Ime one manifold H_ Ihroo-lob* Impel* tors, straight ar spiral, (below) may be vied te give different fre quency ef pulsation or te meet noise limit with no muffler or little muffling 12g% Po*llo dlsplecontens blower, with 9-lobed, straight Impeller*, used a* su percharger. Driven from engine by chain, V-bel or gears, tapercharger of this type proportions otr supply to engine speed, mak ing It particularly suHed te auto motive end mobile application* with variable-speed operation (213) 77