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336 obtained. This is possible in the two-cycle only by making the normal pressure the low pressure. In other words in the two-cycle the pressure of compression can be varied only by increasing the pressure of the scav- enging air. Any regulation of the pressure by a reduction of the scav- enging air below the normal would seem to be very difficult to obtain, that is, the normal compression should be that pressure which would result from having a full cylinder of air at 'atmospheric pressure. It is diffi- cult to see how any pressure lower than this: is pfactical. in the two- cycle engine. A higher pressure than this can be obtained by increasing the pressure of the scavenging air. In the condition of the engine today there are many points which will re- main a matter of opinion till settled by exhaustive tests. 3) THE MARINE REVIEW very carefully to see if a lower pres- sure cannot be used with better re- sults. We know that a pressure of com- pression of about 500 pounds will give a temperature sufficient to ignite the fuel spray. If this pressure can be reduced, the result will be lower- bearing pressures and reduced strains in the crank shafts. Since this means a lighter and more durable engine, it becomes of vital interest to know just how low a pressure of compres- sion canbe used. It has been fixed that from 450 to 500 pounds will be an average figure to use for the con- ventional type of engine. This. pres- sure can be very greatly reduced by warming the cylinders, by steam for instance, before attempting to start the engine. The estimated pressure of compression will not vary from August, 1918 dling on the part of irresponsible parties, and it is suggested that the connecting rods may be lengthened or shortened to make the needed variation. In addition to this method, it is possible to get the same result by taking a chip from either the head or the cylinder, or a slightly thicker gasket might be used in place of the original gasket. Even this change in the gasket might be objected to, while absolutely no objection can be found to taking a chip from the head or the cylinder in case the engine is found to have too great a clearance and in case the clearance be not large enough to take a chip from the connecting rod. In fact, with a ma- rine type connecting rod, all the varia- tion which is necessary can be made by either lengthening or shortening the rod. This matter of adjustment will not be a seri- With a four-cycle ous one, and when engine these points c Table I the designer under- in some instances stands that his can be settled by Comparison of Diesel and Low Pressure Types estimated clearance the use of such a Description. Joi can easily be type Jot. walve Breen sizes, rae ga bs Gyan oes ae A Ce aie a eee se 12 eee altered, in case of : istone area, "Sqllate ANCMeS* 1... .i ccc cc os ce amie coiaet rier Slee : . action : that the Piston circumference, INCHES eGo ccs coe ie ee ee ee STE "188.5 need, without the same engine can be Piston «displacement, cubic inches: ...50.00.0.. 00. 0s cae ses 1,357 172,500 waste of any ma- ait t Cylinder contents diesels cubic inches? 440. 2s Ae ee 1,460 185,500 bined run on iireren al on, low. pressure; cubic inches... 0s. ...00 Cees 1,697 215,500 chined parts, great- cycles or types of eueta Of eviinder diesel, inches... 06 oy cvs foe es 12.91 64.60 oe ' yP Length of cylinder, low pressure, INCHES rio es ek et 15 279 oak confidence indicator cards. use ae diesel, cubic a ROE eae 103 13,000 should exist in his earanee volume low pressure, cubic inches................. 340 43,000 : : From such _ tests Clearance surface diesel, square TWVCNES ce Ae spas ce GA oe sani 260 6,622 mind in regard to much valuable data Clearance surface low pressure, square inches................ 339 8,582 the final product. id he obtained Wits Ol air per 'stroke, diesel; pounds. ..2:3.....-5...0.005-. 0.0508 6.47 ' cou e€ ' ae a per soe oe MOUNdS sa5 he a ns 0.0592 7.53 Having shown how do so. it for no Hilo. Cleanatice sutiace; (COME) i. i. eet se occ c as ans 0.788 0.772 sre d it Ratio. clearance. vol: (diesel low pressure) 23 tha wg ce ws 0.303 0.303 the clearance can other enon. co easily be altered to seems wise to ee air = beginning of camper aion is 200 degrees Fahr. compression suit the needs of u es n : adopt the four- pressure diese pounds; compression pressure low pressure type 135 pounds. the engine, the cycle type. Also necessity 'of wide the matter of sim- plicity must be considered. Is it simpler to have a given number of cylinders with a more elaborate system of valves in the head or is it better to increase the number of cylinders by adding scavenging cylinders? The author holds that the four-cycle _type is. the more simple. Notwithstanding this opinion, con- siderable attention has been given to the two-cycle type in order to make as many of. the .author's. theories applicable to the two-cycle as is pos- sible. it is hard to see how. the weight of the four-cycle type can be reduced by making a two-cycle, and it seems that the simplicity of valve gear is more than offset by compli- 'cations in the engine as a whole, and also that a great amount of flexi- bility is sacrificed by adopting me two- cycle type. Notwithstanding Diesel's theory, the compression pressure in the oil en- gine is primarily carried to the high point for the purpose of producing a temperature of ignition. The de- signer should consider this feature the actual pressure by any consider- able amount. We. know that a small four-cycle oil engine, when warm, will fire on a compression of 125 pounds. It is certain that under the same conditions--namely, after the engine has been warmed up by opera- tion--the larger sizes will be able to run on pressures as low as 150 to 175 pounds. This latitude may be objected to as being altogether too great to be tolerated in a design which is to be built. Just as the main bearings have to be scraped to fit the shaft, so it is possible to fit, the cylinders with a proper clearance. This is not a very difficult matter; that is, in case of an error in estimate, no parts need be scrapped. There are several ways in which considerable adjustment can be made in the cylinder clearances. In the original engines put out by the American & British Mig. Co., the shaft was raised or lowered by means of wedges. upon which the bearings rested. This seems to offer a little too much chance for med- 'run on a lower compression since the experience is done away with. The clearance will vary for the different types to be built. For in- stance, a small high-speed engine will length of time is a minimum during which the compressed air at its maxi- mum temperature is exposed to radia- tion losses under the worst condition. On the other hand, in a large and slow-speed engine, while the air at the end of the stroke has a longer time in which to lose sufficient heat to prevent ignition, the space in which the air is compressed is much greater and hence the facilities for heat losses are less. It is very probable that there will not be a very great differ- ence in the clearance which can be used. The time element in the high- speed engine is to a very great degree counteracted by the increased clear- ance of the larger engine. The data presented in "The Thermo- dynamics of the Marine Oil Engine" prove conclusively that the full pres- sure of compression is not needed after the engine has been run long enough to get warmed up. Since