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FAQ's & Technical Articles => Techincal Section => General 2 Stroke => Topic started by: rsss396 on July 15, 2013, 02:47:24 PM

Title: Hot Engine Design
Post by: rsss396 on July 15, 2013, 02:47:24 PM

website link:http://modelenginenews.org/design/hot_design.html

[TABLE=align: center]
[TR]
[TD=width: 60%][h=1]Hot Engine Design[/h]   [/TD]
   [TD]    [TABLE=align: right]
     [TR]
[TD]    by Edward D Ingram
     [/TD]
[/TR]
    [/TABLE]
   [/TD]
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 [/TABLE]
   IN THE design of racing engines, where the chief  objective is maximum output, particular attention must be given to  proper portings, port timing, compression ratio, and reduction of  reciprocating weight. For high operating speeds, the cylinder intake and  exhaust ports should be large, but the amount of port area obtainable  is limited by the circumference of the cylinder, which places a limit on  port width, and by the port timing, which limits port height, for even  in a racing engine the ports cannot be opened too early.  
(http://modelenginenews.org/design/images/tn_ingram_1.jpg) (http://modelenginenews.org/design/images/ingram_1.jpg)
Consideration must be given to the disposition of available space  between intake and exhaust ports. Usually the area of the exhaust port  is made greater than that of the intake port. Intake port area should be  large, however. Manufacturers of racing engines have not always taken  full advantage of the available space for porting. Even in the Hornet,  which has an outstanding performance record, it is only in the converted  model that maximum intake port area is provided. An increase in intake  port area of approximately 75%, together with certain other changes  including a lighter piston, has resulted in the brake horsepower being  increased about 50%, according to figures given out by the manufacturer.  
By increasing intake port area 50%, the speed of the Bungay 600 (http://modelenginenews.org/ad/bungay.html)  has been increased 1,000 rpm, that is, to over 21,000 rpm. What is  probably the extreme limit of port area is found in the Dooling, where  the five intake and six exhaust openings completely ring the cylinder  with very narrow bridges between each opening.  
(http://modelenginenews.org/design/images/tn_ingram_2.jpg) (http://modelenginenews.org/design/images/ingram_2.jpg)
Among the curves presented here, is one which shows not only the  port timing of a typical crankshaft rotary valve racing engine-the  Bungay 600-but also the variation in port area with angular travel of  the crankshaft. It will be seen that the exhaust port opens 70° befor4i  bottom dead center. The rising curve is steep and convex, which shows  that the port opens rapidly, but the area becomes greater at a  decreasing rate because the piston is decelerating. The intake port  opens 63° before bottom dead center, that is, 7° after the exhaust port  opens. When the piston reaches the bottom of its stroke and the ports  are wide open (peaks of the curves), it will be seen that the intake  port area is .166 sq. in. and the exhaust port area .281 sq. in. As the  piston moves up, the ports close slowly at first and then more rapidly  as the speed of the piston increases. Both ports close at the same  number of degrees after bottom dead center that they opened before  bottom dead center.  
The graph shows that the shaft type rotary valve starts to open  at 69° after bottom dead center or just about as the exhaust port  closes. It opens at a uniform velocity for 60° when it is fully open. It  remains open for 50 ° and then starts to close at uniform velocity. It  is fully closed at 59° after top dead cen,ter, the total period of  opening being 170°.  
While it is comparatively easy to plot the rotary valve graph, in  order to plot the cylinder intake and exhaust port curves it is  necessary to calculate or determine graphically the distances the piston  moves for each 10° of crank rotation and then calculate the partial  port areas corresponding to each of these distances. The length of the  connecting rod must be known since this affects the piston motion.

Title: Hot Engine Design
Post by: rsss396 on July 15, 2013, 02:48:23 PM

of the connecting rod must be known since this affects the piston motion.  (http://modelenginenews.org/design/images/tn_ingram_3.jpg) (http://modelenginenews.org/design/images/ingram_3.jpg)
Because of the angularity of the connecting rod-which increases  as the length of the rod is decreased-on the down stroke the piston  accelerates to maximum speed in less than 90° of crankshaft rotation,  and therefore has correspondingly more than 90° of shaft rotation to  decelerate to zero speed, reached at end of the stroke (180° of shaft  rotation). Since at any given engine speed degrees of crankshaft  rotation correspond to units of time, this shows that the piston has  less time to accelerate to maximum speed than it has to decelerate to  zero speed. Going up, the piston has more time to accelerate than to  decelerate. These irregularities would be eliminated if the rod could be  made infinitely long.  
The inertial forces due to the varying speed of the reciprocating  parts are the chief cause of engine vibration and high bearing loads.  While the energy stored in the reciprocating parts during acceleration  is returned to the crankshaft during deceleration, if the parts are  heavy, the inertia forces and therefore the bearing loads will be  higher, and engine power will be reduced because of frictional losses.  Since the inertia forces increase as the square of the engine speed, the  importance of keeping the reciprocating weight of high-speed engines  low is obvious. Because the extreme upper end of the connecting rod is  entirely reciprocating weight and the extreme lower and entirely  rotating weight, it is usual to assume that one-half of the weight of  the rod is reciprocating weight.  
(http://modelenginenews.org/design/images/tn_ingram_4.jpg) (http://modelenginenews.org/design/images/ingram_4.jpg)
The primary unbalanced inertia forces are those due to variation  in piston speed (neglecting the irregularity caused by rod angularity)  while the secondary unbalanced inertia forces are those due to rod  angularity alone. In a single-cylinder engine both cause vibration,  although the former can be reduced by a crankshaft counterbalance  weight. Increasing the length of the connecting rod decreases rod  angularity but increases reciprocating weight, so a compromise must be  made. In model racing engines the ratio of connecting rod length to  stroke usually is around 1.66 to 1. In the Bungay the ratio is 1.68 to  1. The Hornet (http://modelenginenews.org/ad/hornet.html) has about the same ratio. The McCoy 60 has a somewhat longer rod, the ratio of rod-length to stroke being 1.84 to 1.  
Most manufacturers of racing engines, including McCoy, Hornet,  Dooling and Orr, prefer the backplate or disk rotary valve to the hollow  crankshaft type. With this design the end of the crankpin projects into  a hole in the disk so the disk revolves with the crankshaft. The rear  of the crankcase forms the seat of the valve. A long slot in the disk  registers for the proper interval with the port in the backplate to  which the carburetor is connected. Gas pressure combined with the  capillary attraction of the oil film hold the disk to its seat. The  rotary disk design permits a larger port area to be used because the  port is not limited by the diameter of the crankshaft. The path from  carburetor to crankcase is direct, so the gas mixture is not required to  turn any sharp bends.  
Also presented is a graph showing the variation in port area with  shaft rotation in the disk rotary valve used on the Dooling. The  figures are based on measurements rather than on data supplied by the  manufacturer and therefore are approximate. It will be seen that the  disk valve opens faster than the shaft type valve. The maximum opening  is more than twice as great, although this may not mean too much because  the passage to the carburetor has a cross-sectional area of less than  one-half the area of the disk valve opening. Curves showing the  variation in rotary valve and cylinder port area with shaft rotation in  the case of the McCoy 60 are shown at the start of this article.  
An objection to the disk rotary valve is that it is difficult to  supply the bearing supporting the disk with adequate lubrication and  sometimes this bearing overheats and fails. Some experienced modelers  install a special ball bearing at this point. A ball bearing is standard  equipment on the Orr 65 engine. It would seem that the use of an Oilite  bearing at this point would provide a simpler solution to the  difficulty. Oilite bearings are pressed from powdered bronze and  graphite and oil-impregnated. Disk valves must be carefully fitted with  the proper clearance between the disk and its seat (from .001 to .002  in.) to reduce drag and wear.  
The shaft type of rotary valve is easy to lubricate and involves  less frictional loss, but with this type it is difficult to provide a  free gas flow to the crankcase, the hollow shaft in particular tending  to be a bottleneck. Nevertheless, with careful design the shaft type'  rotary valve gives good results. I have had direct experience with the  operation of two racing engines using shaft type rotary valves-the Ball  and Bungay-and have found both to be fast and efficient.  
In most racing engines the bypass or transfer passage is run from  the cylinder intake port directly to the crankcase, with the addition  in some cases (as in Hornet, McCoy and Bungay) of small auxiliary ports  in the lower part of the cylinder wall and piston skirt. In the Dooling,  however, all the gas mixture passes through large ports in the piston  skirt and cylinder wall and then to the cylinder intake port by way of a  very large, curved bypass.  
The stroke of model racing engines is usually made a little less  than the bore. The Dooling is a very short-stroke engine, the  stroke-bore ratio being .74 to 1. The short-stroke design tends to  result in a compact engine of low weight.  
It may be pointed out that the stroke/bore ratio used does not  affect the amount of gas expansion during the power stroke. It is  obvious that with two engines of the same piston displacement but  different stroke-bore ratios, the initial- to - final volume of the  cylinder will be the same, assuming the same compression ratio is used.  Or, if we consider the bore to remain fixed and the stroke increased,  which will increase the displacement of the engine, then a greater  charge will be taken in during the intake period and so expansion will  be no greater, assuming the compression ratio is maintained constant. In  other words, gas expansion is not a function of the stroke-bore ratio.  And regardless of the stroke-bore ratio used, any desired compression  ratio may be chosen.  
When two engines with the same displacement and general design,  but different stroke-bore ratios are operating at the same number of  rpm, they will both be handling the same amount of gas per' minute and  should develop the same power, but the piston speed of the short stroke  engine will be lower. A low piston speed is favorable to reduced piston  wear!

Title: Hot Engine Design
Post by: rsss396 on July 15, 2013, 02:49:09 PM

The thermal efficiency of an engine is the percent of heat units of  the fuel that is transformed into work. The thermal efficiency increases  as the compression ratio is increased, but for ratios above 11 to 1,  the increase in efficiency is small. I know of two manufacturers who  experienced an actual falling-off in engine power, when the compression  ratio was raised above 11 to 1. The cause may have been detonation,  although methanol does I not usually have much tendency to detonate. The  cause of detonation is not fully understood despite the extensive  research work on this subject. One theory is that after the gas in the  vicinity of the spark plug is ignited, the rapidly moving flame front  compresses and heats the remaining portion of the charge so that it  reaches the self-ignition point and explodes violently.  
The shape of the combustion chamber has a marked effect on  detonation. In two-cycle engines the shape of the combustion chamber is  influenced by the shape of the piston head, which must be designed to  form a baffle to prevent the incoming charge from passing out of the  exhaust port. Manufacturers of racing engines are giving considerable  attention to improved piston head design. In the McCoy 60 the contour of  the piston head has been changed for better performance with hot fuels.  The piston is now made of a special aluminum alloy with higher heat  resisting properties and is webbed inside.  
Presented in the accompany tables are the cylinder port timing  for two Class D racing engines and a Class B engine; the rotary valve  timing for four Class D engines and a Class B engine; and the weights of  the reciprocating parts of four Class D engines.  
   
[TH][/TH]
[TH=colspan: 2]

Exhaust

[/TH]
[TH=colspan: 2]

Inlet

[/TH]
[/TR]
   [TR]
[TH]Engine[/TH]
[TH]Open BBCD[/TH]
[TH]Close ABDC[/TH]
[TH]Open BBDC[/TH]
[TH]Close ABDC[/TH]
[/TR]
   [TR=class: odd]
[TD]Bungay 60[/TD]
[TD=class: num]70.0°[/TD]
[TD=class: num]63.0°[/TD]
[TD=class: num]70.0°[/TD]
[TD=class: num]63.0°[/TD]
[/TR]
   [TR=class: evn]
[TD]McCoy 60[/TD]
[TD=class: num]68.5°[/TD]
[TD=class: num]55.5°[/TD]
[TD=class: num]68.5°[/TD]
[TD=class: num]55.5°[/TD]
[/TR]
   [TR=class: odd]
[TD]Forster 60[/TD]
[TD=class: num]65.0°[/TD]
[TD=class: num]55.0°[/TD]
[TD=class: num]65.0°[/TD]
[TD=class: num]55.0°[/TD]
[/TR]
   [TR]
[TD=colspan: 5]

Cylinder Port Timing

[/TD]
[/TR]
  [/TABLE]
     
[TH]Engine[/TH]
[TH]Valve Type[/TH]
[TH]Opens ABDC[/TH]
[TH]Closes ATDC[/TH]
[TH]Duration[/TH]
[/TR]
   [TR=class: odd]
[TD]Ball 60[/TD]
[TD]Shaft[/TD]
[TD=class: num]50°[/TD]
[TD=class: num]70°[/TD]
[TD=class: num]200°[/TD]
[/TR]
   [TR=class: evn]
[TD]Bungay 60[/TD]
[TD]Shaft[/TD]
[TD=class: num]69°[/TD]
[TD=class: num]59°[/TD]
[TD=class: num]170°[/TD]
[/TR]
   [TR=class: odd]
[TD]Dooling 60[/TD]
[TD]Disc[/TD]
[TD=class: num]55°[/TD]
[TD=class: num]55°[/TD]
[TD=class: num]180°[/TD]
[/TR]
   [TR=class: evn]
[TD]McCoy 60[/TD]
[TD]Disc[/TD]
[TD=class: num]56°[/TD]
[TD=class: num]56°[/TD]
[TD=class: num]180°[/TD]
[/TR]
   [TR=class: odd]
[TD]Forster 60[/TD]
[TD]Disc[/TD]
[TD=class: num]25°[/TD]
[TD=class: num]40°[/TD]
[TD=class: num]195°[/TD]
[/TR]
   [TR]
[TD=colspan: 5]

Rotary Valve Timing

[/TD]
[/TR]
  [/TABLE]
     
[TH]Engine[/TH]
[TH]Piston+rings[/TH]
[TH]Wristpin[/TH]
[TH]conrod[/TH]
[TH]piston+rod assy[/TH]
[/TR]
   [TR=class: odd]
[TD]Ball 60  [/TD]
[TD=class: num].418[/TD]
[TD=class: num].098[/TD]
[TD=class: num].114[/TD]
[TD=class: num].631[/TD]
[/TR]
   [TR=class: evn]
[TD]Bungay 60[/TD]
[TD=class: num].256[/TD]
[TD=class: num].062[/TD]
[TD=class: num].150[/TD]
[TD=class: num].468[/TD]
[/TR]
   [TR=class: odd]
[TD]Hornet 60[/TD]
[TD=class: num].302[/TD]
[TD=class: num].117[/TD]
[TD=class: num].187[/TD]
[TD=class: num].606[/TD]
[/TR]
   [TR=class: evn]
[TD]McCoy 60 [/TD]
[TD=class: num].382[/TD]
[TD=class: num].096[/TD]
[TD=class: num].169[/TD]
[TD=class: num].647[/TD]
[/TR]
   [TR]
[TD=colspan: 5]

Weight of Reciprocating Parts (oz)

[/TD]
[/TR]
  [/TABLE]
 
   

Ref: Model Airplane News, November 1949, p26

Title: Hot Engine Design
Post by: 86honda250 on July 15, 2013, 09:44:20 PM

Thanks rsss396 for all the tek info but I am having a hard time keeping up reading all your wealth of knoledge. Keep the info coming

Title: Hot Engine Design
Post by: rsss396 on July 15, 2013, 09:58:11 PM

Oh its not all my knowledge, but I have saved allot and still dig for tech stuff all the time, its kind of nice to post it all in one place.
I have so many dead links I have saved over the years, I kick myself for not coping and saving them :(