Things Ybu Didn't Know About Engines
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Things You Didn't Know About Engines By Boh Whittier. EAA 1235 Box 543, S. Uu.xbury, Mass. HK HIGH-SPEED, fourcycle gaso- of piston speed, for this velocity di- gines often revolve at the "alarming" T line engine is one of tho major rectly affects intake manifold design speed of 20.000 rpm. or even more. triumphs of modern mechanical en and intake valve timing. But figure it out for yourself on the gineering. It represents a high degree In modern gasoline engine design basis of stroke and rpm. and you'll of refinement in mechanical design work, it is usual to base a new power find they all operate within the ac- technique and metallurgy. A readily plant's dimensions and rotational cepted limits of piston speed. When grasped example of this is the sue speed on the average maximum piston you find an exception to the 3.000 cess that advanced engineering en- speed of 3.000 fpm. This average pis- fpm rule, if it's very much below that joys in the matter of taking into ac- ton speed has been found by experi- figure the engine is either a very count the expansion under operating ence to mark the upper limit if ac- old one with cruder metallurgy and heat of the various kinds and shapes ceptable reliability and durability are machining, and if it's much above that of metal, in various heat zones of an to be achieved from today's metals figure the engine is probably one that engine, to produce smooth and reli- and lubricants. was designed for a short but merry able operation. Persons unfamiliar with engine de- life. Another example is the successful sign practices tend to assume that re- The pist'Mi speed of 3.00!) fpm used mingling of different kinds of fluids liability and durability are functions in design work refers to average pis- —air, gasoline, oil, and coolant in of rotational speed, but this is not so ton speed. The piston of a reciprocat- either gaseous or liquid form — to at all. To understand why piston speed ing engine comes to a stop at each achieve controlled and efficient com- is the most reasonable and accurrt'.1 end of its stroke, at which times it bustion by means of which latent en- figure on which t:> base an evaluation is momentarily moving at /ero fpm. ergy in the fuel is converted into of an engine's reliability and dura- Now if we're working on the basis usable power. bility, consider first an engine having of 3.000 fpm maximum average piston Few airplane pilots or mechanics a stroke of 6 in. The piston will travel speed, it can be found by compu- have much of an understanding of 1 ft. for each complete revolution of tation of crankpin travel that the the principles and problems involved the crankshaft. Thus, the engine could highest instantaneous speed reached in designing an engine to handle revolve at 3.000 rpm without exceed by the piston is on the order of 4.000 those different fluids satisfactorily. ing the maximum recommended piston to 5.000 fpm. occurring when the But such an appreciation is necessary speed of 3,000 fpm. It follows that an crankpin has reached the 90 cleg, for those who would undertake to ex- engine having a 3 in. stroke can re- point in its rotation. In gasoline en- periment with engines of various kinds volve at 6,000 rpm. and one having a gine textbooks it is common to find to be used for aircraft propulsion, so stroke of 1'z in. can revolve at 12,000 references to piston speed. Always that they will not unwittingly upset rpm without exceeding the recom- take into consideration the age of any the careful figuring and reasoning of mended maximum average piston surh book. One published 50 years ago the engineers who designed these en- speed. will mention piston speeds of 1.000 gines. Practical examples of this general fpm, the average speed rising through We will consider in this article the rule are seen everywhere. The huge the years from those days to the mod- matter of how one fluid — the air-and- diesel engines used in ships and the ern average of 3.000 fpm. And. always gasoline mixture produced by the large natural gas engines used as sta- remember that such references usu- carburetor — is handled so as to ob- tionary power plants operate at such ally mean average piston speed. When tain efficient results. To do this we seemingly low speeds as 300 and 400 considering the matter of valve timing must start by considering the matter rpm, while tiny model airplane en- t pogo) <£_ CRANKSHAFT ROD 90° CRANKPIN TRAVEL€><L ROD PISTON B.D.C. ROD PISTON T.D f-.655-j-.845——| FIG. 1 SPORT AVIATION 19 NTAKE MANIFOLD ^ fe^ -- o or o co 10 o 1^ o d 00 CD co o O 10 CO co < E r- O) UJ UJ UJ * O CO 10 00 H CO < Ul o < Ul Ul o < rOr rr U. UJ Z Ul < > FIG. 2 ENGINES . things in an engine that a mechanic (Continued from page 19) cannot see, but which are there and THEORETICAL and induction systems, it is necessary which the designer has taken into ac- VELOCITY CHART to take into account the fact that pis- count, and ignorance of which can ton speed varies from zero to some lead the mechanic into deep trouble LOC FTyMIN. MP.H. when he undertakes to modify engines figure in the middle thousands of A 26,600 503 feet per minute in the course of any with no understanding of engines as geometric and mathematical entities. ) of its strokes — including the intake 36,800 696 When the piston is at top dead E stroke. center, a single line can be drawn 10,950 208 Now an interesting and little known through the center of the piston pin, 18,900 358 fact will be revealed. One would down along the center line of the con- think, upon first considering the mat- AVERAG necting rod, through the center of the 0 ter, that when a crankpin had turned crankpin and then down through the mooo to the 90 deg. point, the piston would center of the crankshaft main journal. 4714 then be half way down its stroke. F As the crankpin begins to rotate to _ •anr»r» This is not so at all! When the crank- start a piston stroke, this line is brok- TELAVG. ^OOO pin has moved from top dead center en up into a triangle formed by the to the 90 deg. point of its rotation, the piston pin center, crankpin center and piston will have traveled a distance main journal center. Of course, you probably can't wave of about 60 percent of the stroke! Visualize the crankpin as having a wand and dissolve the crankshaft so Piston travel in the first 90 deg. of turned 90 deg., with the connecting that that would happen. What happens crankshaft rotation from top dead rod and piston having followed it as in practice is that the connecting rod center and in the last 90 deg. before they must. Wave a magic wand to dis- length remaining fixed, the above top dead center is greater than it is solve the crankshaft metal. The con- mentioned extra length has to be al- between 90 deg. and 180 deg., and be- necting rod will respond to gravity and lowed for by having the connecting tween 180 deg. and 270 deg. it will move from its slanted position rod pull the piston down the stroke This seems as utterly impossible to a vertical position, hanging from as much more beyond the 50 percent as it would be for a chain and sprocket the crankpin. When it swings to this point as is necessary to make allow- drive to slip like a belt drive will. But vertical position, obviously its center ances. it's true, and not too hard to under- will be below the center point of the The practical effect of the geometry stand. It's a perfect example of the crankshaft's main journal. involved is to foreshorten the con- 20 FEBRUARY 1969 necting rod when the crankpin has rotary piston drills turn at as much As its name implies, the four-cycle made one-quarter of a revolution. as 250,000 rpm. Comparatively modest engine has four strokes — intake, Therefore the piston responds by be- changes in atmospheric pressure can compression, power, exhaust. Thus ing pulled farther down the length generate hurricanes. So it is not hard the incoming mixture is actually go- of the stroke. Study Fig. 1. Turn the to see that high air speeds are pos- ing into the cylinder only once out of page so that you look at this drawing sible within intake manifolds. four piston strokes. In a single cylin- as though you were looking at a sin- Consider an engine in which the der engine it is thus at a standstill gle cylinder engine having a vertical piston moves at 4,700 fpm at its mo- during three strokes, and in a multi- cylinder. It will dawn on you after ment of highest velocity. If the intake cylinder engine it is diverted to one a while that when the crankpin is at manifold is one-third the diameter or more of the other cylinders. Obvi- the 90 deg. position, the piston will of the cylinder bore, incoming mixture ously, there is pulsating flow.