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)

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SPORT AVIATION 19 NTAKE MANIFOLD

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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 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 E 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. There is be about 60 percent down its stroke. will have to travel at three times the possibility that a column of air If you still don't believe your eyes, piston speed to fill the cylinder. In- stopped suddenly by a closing valve you can prove it to yourself with any take manifold diameter squared times can rebound and arrive at some point small engine. Find a lawn mower piston speed gives 42,906 fpm or 480 in the manifold at just the right time motor with a 2 in. stroke. Remove mph! to interfere with a column of air the cylinder head. Beginning with the When you turn off a water faucet being drawn in by another cylinder. piston at top dead center, rotate the suddenly, the house resounds to the So, an intake manifold is not just a flywheel 90 deg. and your pocket banging of water pipes caused by "wa- pipe leading conveniently from the ruler will prove to your satisfaction ter hammer." It is likewise possible carburetor to the intake ports. It is that the piston has moved more than to have "air hammer" in an intake a carefully considered duct that tries 1 in. down the 2 in. stroke! manifold when a fast-closing valve to have the same distance from the stops a column of air rushing at such carburetor to each of the intake ports This peculiarity of piston travel while at the same time taking into must obviously affect valve timing, high speed. Engineers call it by the more sophisticated name of "reson- account the pulsating air flow and the for the first half of the intake stroke need to avoid conflicting flow as first (as reckoned in terms of crankshaft ance." It is not readily determinable one and then another intake valve rotation) draws in around 60 percent opens. of the charge. As the valve mechan- what would be the actual ram pres- ism is operated by a gear on the crank- sure in a column of air moving in an Depending on such variables as in- shaft, the significance is clear! intake manifold, but it is known that let air temperature, fuel volatility, pressure is very great. That's under- carburetor metering jet design and There is great variation in the valve standable, considering the air speed temperature of the intake manifold timing of engines of different sizes, involved. It is also easy to understand walls, some of the fuel mixture being speeds and types. Timing is worked that the logical time to close the in- drawn through the manifold will be out to suit the job an engine is to do take valve is when the pressure being completely vaporized gasoline and and the performance wanted from it. built up in the cylinder by the pis- some will be unvaporized fuel drop- An automobile engine has to be "flex- ton on its compression stroke is equal lets. The latter can form puddles and ible", by which is meant the ability to the ram pressure of the mixture when the engine is put into some dif- to run reasonably well and efficiently still coming through the intake port. ferent position, such as when an air- at the various speeds required on ev- plane banks or noses down, it can go erything from side streets to express- That would draw in the greatest pos- sible volume of fresh charge. For this places and do things such as to ad- ways. Because valve timing is thus reason, some racing cams open the versely affect the engine's running. worked out to suit the needs of mod- intake valve as early as 30 deg. be- erate speed operation, hot rodders In order to keep unvaporized fuel fore top dead center, and close it as seeking greater speed resort to a va- droplets entrained in the moving air riety of reground camshafts to get late as 90 deg. after bottom dead so they will not settle out and form what they want. As an airplane en- center. That puts ram effect to work puddles, air velocity in the manifold gine will spend much of its life cruis- to obtain maximum power at full must be around at least 40 ft. per ing at a selected rpm figure, its valve speed, which wins races. second in an updraft manifold, while timing may be worked out to provide But naturally when such an engine in a downdraft manifold gravity will best results at that speed. is slowed down and ram effect is ap- help carry the droplets along and preciably reduced, a condition is ar- puddling may be less of a problem. Now let us learn something of a rived at in which cylinder pressure When an engine originally fitted with basic factor influencing valve timing. starts blowing back through intake a downdraft carburetor is converted Engine mechanics know the import- valves and leads to rough, erratic run- for flight and fitted with an up- ance of smooth inner surfaces and ning. Ideally, we should have cams draft carburetor, this fine point has to tight joints in intake manifolds, these that afford variable timing, just be taken into account. things being important to smooth mix- as we have variable pitch propellers. Velocity within an intake system ture flow and steady fuel-to-air pro- But one can easily imagine the me- depends primarily on piston velocity, portions. But few of them know any- chanical complexities of designing a but is controllable for practical pur- thing about velocities inside intake camshaft with several variable lobes poses by varying intake manifold di- manifolds. Designers are very much that is both cheap and reliable. The ameters. Diameter need not be great- aware of this — and so should be any- only practical answer is a compromise er than is necessary to supply the one who undertakes to experiment cam — tailored to fit the engine in- valve opening with as much mixture with engines. volved and the task it is to perform. as it can pass. Valve opening is gov- Gases move with great rapidity Anyone involved in adapting an auto- erned by cylinder bore diameter and when subject to pressure. A few motive engine to aircraft use should related mechanical considerations. pumps of the lever on an air rifle learn something about valve timing One reason why engines having short are enough to propel a bullet at a practices. It can pay off in various stroke and large cylinder bore are speed of several hundred feet per sec- ways, including engine performance popular among modern designers is ond. Compressed air is able to make and reliability. (Continued on page 23) SPORT AVIATION 21 for Baby Groat Lakes plans and will Amelia Reid's J-3 Cub is back in the Pitts Special will fly this spring . . . be getting under way as soon as he airport hangar, leaving room for Mar- Bob Burke is working like mad on his gains experience in making ribs for tin DeBella and Hal Abegg's Cubs. Pitts also. the chapter project. Sam Haley has the inside of the cock- Bub Corcoran of Bedford, Mass., has pit of his WACO RNF completed and the tubing fitted and welded by this • CHAPTER 316-1A is ready for painting. He is waiting time for the fuselage of his new Antique Aircraft EAA Chapter for new springs for his landing gear. project, the Scorpion helicopter. Art SAN JOSE, CALIFORNIA For sale are a 1941 Ercoupe, excel- Sessi, Chapter 106 member, has his Hal V. Abegg, President lent original condition by Chuck Flem- wing jigs up on the Midget Mustang 12845 Clayton Rd. ing and a Smith Miniplane owned by and is fitting the ribs to the spars San Jose. Calif. Ralph Cox. . . . seems to be going great guns, A dinner meeting was held recent- rivet gun, that is ... Bob MacDonald ly and the members had the pleasure NEW ENGLAND SPORT AVIATION is getting his geared-down VW engine of viewing some slides taken by Chuck NEWS ready for a test run . . . Walter Pratt Smith at Cole Palen's Aerodrome in of Brockton, Mass., is going all the Rhinebeck, N.V. During the course of (Excerpts taken from the New England way for the maiden flight of his Star- the evening, the chapter charter was Sport Aviation News printed by Chap- duster Too . . . Bob Sceppa has his signed by most of the chapter charter ter 136). wing ribs from Bob Bushby and is members in attendance. The chapter making progress on his Mustang II is chartered by EAA as a Special In- HOMEBUILDER'S NEWS FLASHES . . . Charlie Cotulli of Chapter 136 terest Group, and was our first to John Powell of Greater Boston is busy as a beaver with his Stits qualify under our rules for chapter re- Chapter 106 is going like a white tor- Playmate. quirements. They are a good group nado on his original parasol-winged George J. Chandler, Jr., of Barre, and we are pleased to have them with- Acey Ducey. The engine is fitted, fuel Vt., reports that as far as he knows, in our ranks. tank and nose cowl are now being there are only two homebuilts in Ver- In the project department. Bill molded from fiberglas. mont. They are the Miniplane which Ewertz' J-5 now has one coat of silver Russ Gage from down Canton, he is building (or I should say re- on the fuselage and tail, and the wings Mass, way reported at a recent meet- building) and a Baby Ace owned by still need fuel tanks. His Stearman ing of the Central Massachusetts chap- Cecil Wright of Newport. Is this is stored until the Cub is complete. te meeting that it is definite that his true? ®

It seems likely that the "Mustang II" being built by John This bare Emeraude 300-series fuselage and tail is a pride- Pambianco, EAA 33295, of 89 E. Pettibone St., Forty-Fort, ful accomplishment in itself. And, certainly good work- Pa., will be the second of its type to fly. The picture was manship does not appear to be lacking. Eight months taken one year after the start of the project. A zero time of work had been invested to this point by Jack De Jong, 160 hp Lycoming will be installed, and slightly larger EAA 23905, of Box 56, Courtenay, British Columbia, wheels will be used. Shown, from left, are friends Jack Canada. Kendra, Pambianco, Bob Campbell, and Hank Ward.

ENGINES . . . limit power output and secure very pulsating flow in a four-cycle engine's ^Continued from poge 21) long life and reliability. intake manifold it is not feasible to that the larger bore facilitates build- Figure 2 illustrates fairly typical in- take measurements that are reliable ing larger valves into the cylinder take system diameters. When pro- and meaningful. heads, for any given cylinder displace- nounced variation from these aver- This article is not a how-to-do-it ment. ages is encountered, it is for some text; it is meant only to call to the At the other end of the scale, reason having to do with governing attention of aviation experimenters minimum intake passenger diameter is mixture volume or controlling mix- some little-known facts about piston based on a combination of the need ture velocity, and with what you have travel and its effect on timing, the to maintain adequate velocity of the learned in previous paragraphs you surprisingly high velocities found in mixture, and to some extent on limit- can probably deduce the reason and intake manifolds, and the need to ing mixture flow for reasons of engine decide how best to handle the situa- guard against resonance and fuel durability. There are engines in use tion in your experimental work. The puddling when reworking the intake today which have intake passages de- mixture velocities given are of neces- systems of stock engines for aviation liberately kept on the small side to sity theoretical, because due to the use. ®

SPORT AVIATION 23