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temperature necessary for the first working cycle when starting from cold, the higher the average mean effective pressure reached in the working cylinder, and consequently the smaller the main piston-swept volume for å given indicated horsepower. On the other hand, the lower the compression the more even the turning moment, generally the higher the mechanical efficiency, and the less the effort required to start the engine from rest by way of overcoming the negative work of the first compression stroke.
The semi-Diesel engine in type is a variable compression oil engine, and can be designed to work between, but excluding, the two extreme limits the higher, that at which the heat of compression alone suffices to ignite and vaporize the injected charge (the Diesel cycle), and the lower, that at
Fud Los.per BHPH.
Gylinder Diam. in Inchos.
which the size of the hot bulb becomes inconveniently large for reasons of strength, when the loss from the hot bulb, by radiation, would be a serious factor, and together with the small power output obtainable with low compression, would tend towards an excessively high fuel consumption and a large engine.
The Disadvantage of High Compression. There are disadvantages attendant upon high compressions, and a compromise between the theory that the higher the compression the greater the economy and practical considerations, must be struck. In comparison with steam prime movers, the mechanical efficiency of internal-combustion engines is low, due primarily to the friction of the piston rings. (See Appendix I.) The higher the
compression and consequently the maximum pressures the greater this loss on account of the larger number of rings required to ensure satisfactory gas tightness, and the greater the pressure exerted by these rings when forced against the piston walls by the cylinder pressure operating behind the rings.
A further outcome of high pressures is increased piston-ring leakage, and the effects of piston-ring leakage upon economy are very considerable. The higher the compression pressure the greater the heat transfer from the charge in the cylinder to the jacket cooling water.
The Effect of Compression.-From these considerations it is clear that there is a compression pressure beyond which practical considerations will cause a diminution rather than an increase in overall efficiency.
With present-day designs of semi-Diesel engines little further economy of fuel consumption is to be sought in this direction. Designers adopt various compressions according to the means foreseen for attaining flexibility and depending on whether the water drip is retained or not. The effect of increase of compression on economy between the limit of 180 lbs. per square inch and 450 lbs. or 500 lbs. per square inch is not, per se, considerable, due to the practical considerations outlined. The principal effect is the pos
Poundo per sq.Inch (Gauges
Volume Fig. 5.-Full-Power Indicator Card of 2-Cycle Semi-Diesel Engine.
sibility of sustaining higher mean effective pressures with smaller cylinders for a given output, and so attaining somewhat better fuel economy.
Fuel Economy.—The fuel economy of semi-Diesel engines is surprisingly good (see Fig. 4), and is accounted for by the cycle of operation being nearer to the more economical explosion cycle than to the constant-pressure burning or Diesel cycle (see Fig.5).
Cycle of Operation.-Actual indicator diagrams do not quite so rigidly follow the theoretical cycle as the gas engine, on account of the difficulty with semi-Diesel engines of regulating the injection and ignition for all loads.
Flexibility. Experience of the operation of internal-combustion engines teaches that this prime mover is primarily a constant speed, and to a somewhat lesser extent a constant load engine. Innumerable designs of details and countless patents have been concerned with the problem of flexibility and compromise is generally the outcome. Flexibility can be considered under three headings:
(a) Constant mean effective pressure, with varying revolutions and consequently power.
(b) Constant speed of revolution and varying mean effective pressures and power.
(c) Varying speeds and mean effective pressures.
Condition (a) is not required in practice, and cannot normally be met, with maximum or even full load m. e. p. since with a reduction in speed of revolution, conditions affecting scavenging efficiency and compression, heat loss, etc., also change, and a small drop in speed of revolution is accompanied by a reduction in m. e. p., and so by a cumulative falling-off in power developed. A low m. e. p. can, of course, be maintained as a constant over a certain range of speed of revolution.
Condition (b) constant speed of revolution and varying power as affecting generator engines, etc., requires most frequently to be met, and may be considered in detail. At reduced power and m. e. p.'s-
(1) The charge drawn into the crank chamber remains relatively constant in volume or may even be slightly augmented, due to the engine running cooler as the mean effective pressure falls, unless means are provided to throttle the water cooling supply.
(2). The volume of the scavenging charge is approximately the same as at full power, but may be at a lower temperature and pressure.
(3) The compression pressure will reduced on account of: (a) Lower scavenging pressure (see 2); (b) less heat abstracted from the cylinder walls, which in turn is due to the less fuel burnt per stroke and so the lower temperature of these walls. . Condition (c) requires to be met with various types of machinery, and no difficulty is experienced provided the power of the engine is suitable for its work and a higher m. e. p. is not demanded than can be sustained for the speed of revolution under consideration.
Even where means are provided to throttle the cooling water and the scavenging air at low power, the point is quickly reached where the heat of the bulb is insufficient to vaporize and ignite the charge of injected oil, and the engine will “miss” and stop, unless heat be externally applied to the bulb as, for instance, by the blow lamp.
Range of Working.–The range of working must be extended to cover from full load or overload to a small load without having recourse to the blow lamp, and for this purpose the water drip has been retained on some designs. At full load water is allowed to enter the working cylinder with the scavenging air and serves by evaporation to take heat from the bulb, so that with a relatively large bulb and a low, compression engine, from three-quarters to full power can be satisfactorily developed without overheating of the bulb, and with the water drip cut off the engine will run satisfactorily down to low loads. An overheated bulb will give bad combustion and “coking” of the fuel, and is, besides, a source of danger due to weakening of the metal of the bulb (see annexed table). TABLE SHOWING TENSILE STRENGTH OF COMBUSTION CHAMBER MATERIALS AT VARIOUS TEMPERATURES
Tensile Tensile strength strength
of cast iron of mild steel
Temperature tons per tons per Load on engine
deg. F. Light load....
ŞJust showing color in the
24 Normal load...
Between dull and cherry
7.5 Over-load ..Bright cherry red.. 1,400
2.5. Consumption of Water.-The consumption of water through the water drip is very considerable, and varies according to the quality of attention given to the running of the engine, but may reach a value at high powers much in excess of the quantity of fuel burnt. The water should be as pure as possible to cause the minimum harm from deposits on the working surfaces. 'Water has a deleterious influence on the lubrication of the internal parts, although it is credited with preventing carbonizing of the main
piston rings. The water drip, however, is a crude solution of the problem of flexibility, requiring a large supply of fresh water, and with varying loads, regular attendance to the engine, since it is somewhat difficult and calls for complicated gear to connect the water supply with the governor in the same way as is necessary with the fuel supply.
The better solution is to take advantage of another law which is not yet completely explained, viz., that the temperature generated within the cylinder of an internal combustion engine depends on the load, the compression temperature, and upon the nature of the ignition, whether early, normal or late. Normal ignition may be said to be that ignition which is correct for maximum economy and will give the highest power without trouble, the cleanest exhaust, the sweetest running, etc. Late ignition makes for excessive heat losses to the exhaust and high fuel consumption. Early ignition gives rise to abnormally high temperatures. This last fact is utilized with semi-Diesel engines to counteract the cooling of the bulb with reduced
Fig. 6.—Diagram Illustrating the Control of the Point of Commencement and Period of Injection for Varying Quantities of Fuel and Load Giving Corresponding Indicator Diagrams.
power. By advancing the point of ignition of the fuel charge, as the quantity of fuel is reduced to correspond with the load, the semi-Diesel engine can be made to give satisfactorily running at all loads from full load to no load with the minimum of attention and without requiring external heating of the bulb (see Fig. 6). The governor controls the quantity of fuel to correspond with the load by varying the stroke of the fuel pump, and gears have been designed whereby with reduced quantity of fuel the injection point is advanced according either to B or C in Fig. 6. Scheme C is most necessary for engines requiring to run for long periods at light loads, whilst B suffices generally; C is less easy of attainment by a simple gear.
Scavenging.-The next point of importance is the question of scavenging, which, so far as published data or the results of experimental work are concerned, is almost an unexploited field, in connection with either the twocycle Diesel or semi-Diesel engine. With two-cycle engines the efficiency of
scavenging is lower than with four-cycle engines, which has proved one of the most important deterrents in all spheres of application to that success so often predicted in the past for the two-cycle principle. With two-cycle semi-Diesel engines the amount of air available per working cycle or per revolution for scavenging is limited to the volume swept by the working piston. More air than this cannot be drawn into the crank chamber (unless an induction system to the crank chamber were so designed and fitted, as to give a momentum effect with a slight gain, which subject has not yet been studied for other than high-speed four-cycle engines where the maximum output per unit volume is essential). The air, after being drawn into the crank chamber, is impregnated with a certain amount of lubricating oil, as
Full lines show atheoretical disgrem. Chain
dotted lines show effect of incieased exhaust
centre to the opening point of the
scavenging air poit.
to the cylinder with corresponding
drop in pressure.
to the point where the crankcase
air inlet valve (pens.
Fig. 7.-Crank Case Indicator Diagram. Suction loss due to attenuation of the charge is shown by the air admission line of the actual diagram falling below the atmospheric pressure line. The actual compression line is below the theoretical A B, because compression commences at a lower pressure and on account of leakages. Volumetric efficiency of the scavenging pump is greatly affected by the exhaust back pressure, as the position of the point i controls the position of the point D, as shown. The chain dotted line shows the effect of increasing the exhaust back pressure from Ci to C2. The further D is from A the greater is the volume of air represented and dealt with in the crank chamber. Exhaust back pressure affects the quantity of air transferred to the working cylinder, but has no influence on the scavenging air pressure.