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compression pressure will probably be substantially retained. The low mean effective pressure in reducing heat stresses and temperature conditions within the cylinder makes for reliability in operation.

The vexed question of “solid” injection of the fuel becomes a simpler issue when associated with surface ignition. All considerations in the design of the fuel pump operating and controlling gear, and the injection means for semi-Diesel engines, have in the past been subservient to that of simplicity. With a demand for the same degree of flexibility, and a capacity to burn as wide a range of fuels, without recourse to the water drip, as obtains with the Diesel engine, considerable improvements after the war can confidently be anticipated.

Appendix I.-Mechanical Efficiency of Internal-Combustion Engines.The following notes have special reference to internal-combustion engines of the trunk piston type; but apply equally, with slight modifications, for crosshead engines.

Mechanical efficiency (the ratio between brake horsepower and indicated horsepower) is affected by the number of auxiliaries which are driven by the main engine.

1. Except in so far as auxiliaries are concerned, the difference between the indicated horsepower and the brake horsepower can be apportioned as follows:

(a) 50 per cent is due to piston and piston-ring friction.

(b) 28 per cent can be attributed to main cylinder pumping losses, suction, exhaust and scavenging.

(c) 22 per cent is allocated to valve gear and bearing friction, etc., in which are included windage losses and other factors of little importance.

2. Piston friction depends primarily on the following factors : (a) The quality of the metal of the liner, the piston and the piston rings.

(b) The quality of the lubrication. (Certain tests which have been carried out go to prove that a diminution in viscosity of oil increases the mechanical efficiency. In one case the mechanical efficiency was increased by water injection into the combustion chamber.)

(c) The clearance between the piston and the cylinder walls, has an influence on efficiency.

(d) The m. e. p. the compression pressure, and the pressure between the liner and the piston, and the liner and the piston rings, can probably have a most suitable value for the reduction of friction loss to a minimum.

(e) The fit and the condition of the piston rings.

(f) The temperature at which the engine runs will have an effect on the lubrication and on the clearance; and it has been substantially proved that there is a temperature of maximum mechanical efficiency.

3. The suction loss, 28 per cent of the total, is primarily a function of design of ports, valve setting, piston speed and gas speeds.

4. The valve gear and the bearings, 22 per cent of the total loss, will depend on the design of the engine, the alignment, the efficiency of the lubrication.

In addition to the foregoing, there are records of mechanical efficiency being reduced by increased weight of flywheel.

Generally, mechanical efficiency is adversely affected by increased speed and reduced m. e. p.; and decreased by mal-alignment, etc.

The mechanical efficiency may be affected by the form of the combustion chamber which may produce undue distortion of the piston working conditions, although this is probably extremely slight; distortion of the piston being more due rather to the condition of the gudgeon pin bearing than to any other cause.

The mechanical efficiency, assuming a constant m. e. p., is practically unaffected by the size of the engine.

In connection with the above, a large number of records of tests of engines have been investigated from Guldner, Supino, D. Clerk, etc.

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Appendix 11.-Reversibility.-Reversibility need only be discussed in relation to two-cycle semi-Diesel engines, since those working on the four-cycle principle have not, as yet, been made directly reversible. The compression of the scavenging air in the crank chamber fitted with automatic valves, and the operations of scavenging and exhaust within the working cylinder are directly reversible (see Fig. 2).

The only problem remaining is that of the timing of the injection of the fuel for the reverse direction of rotation. If the end of the delivery stroke of the fuel pump coincides with the top dead center of the main piston, no alteration in timing for fuel injection is required for astern running. This condition is satisfied in most of the multi-cylinder reversible semi-Diesel engines. However, should the timing of the fuel pump be such that the end of the delivery stroke does not coincide with the top dead center, then operating gears are necessary for the fuel pumps-one for ahead and one for astern-in an exactly similar manner to the arrangements adopted for Diesel engine cylinder head fuel injection valves with reversible marine engines.

The method adopted for changing the engine from the ahead direction of rotation to astern, may be:

(a) By means of a pre-injection and ignition of fuel. (b) Pre-injection of starting air.

(c) Stopping the engine, reversing the driving mechanism of the starting air valves and possibly of the fuel pumps as well, and starting up the engine in the astern direction.

With (a) and (b) the fuel pump is timed so that the point of delivery coincides approximately with the top dead center of the main piston, and arrangements are made, that when the fuel is cut off, the engine is declutched, the speed falls to a predetermined minimum and an injection of fuel or starting air is effected by the governor on the up stroke of the piston, so driving it in the reversed direction. Method (c) is only applicable to four-cylinder engines, since engines with a lesser number of cylinders cannot be started from any position of the cranks at which the engine may have stopped. Although a clutch is generally fitted, it is not necessary for method (c). For a full description of method (c), see Engineering for August 24, 1917.Engineering, 25/10.

AERONAUTICS THE LIBERTY MOTOR.Its Checkered Career and Details of Its Construction.-At last the restrictions of the censor have been lifted and we are able, without in any way giving aid or comfort to the enemy, to disclose to our readers pictures and drawings of that engine of mystery, the Liberty motor.

The career of this motor has been a checkered one. Announced first as a five-day creation, a masterpiece of ingenuity which could be turned out immediately in tens of thousands by typically American quantity production methods, it was loudly acclaimed as one of the greatest inventions of the war that would give America the mastery of the air. Then the pendulum swung to the opposite extreme. In the sharp reaction from this grossly exaggerated opinion of the Liberty motor, the machine was pronounced a pitiful failure, an immensely heavy brute with a voracious appetite for fuel, a mere automobile engine, absolutely unfitted to take on wings and soar among the clouds. It was now held up as a glaring example of inefficiency and incompetency. Ugly stories of graft were whispered about—of hundreds of millions of dollars absolutely thrown away. However, by the time the pendulum had reached that extreme the Liberty motor had passed through the long and tedious period of experimentation and preparation for quantity manufacture and it had undergone a thousand and one changes in minute details, all of which consumed a great deal of time, and when the public criticism had reached its highest tide this engine was already being turned out by at least one

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 be 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).

Temperature

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

Color
deg. F. sq. in.

sq.in. Light load.... {u load Ş Just showing color in the

"; dark .................. 750 - 12 Normal load s Between dull and cherry

red ................... 1,100 7.5 12 Over-load ......Bright cherry red........ 1,400

3.5

2.5

24

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

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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

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Cearance Volume

Full lines show theoretical diegrem. Chain

dotted lines show effect of incieased exlu uzt
b ck persure. Plain dotted lines show
actual dlagram.
A-B - Compression from the Inner dead

centre to the opening point of the

scavenging air poit.
B-CI Transfer of air ficm the crankcase

to the cylinder with corresponding

drop in pressure.
C1-D = Re-expansion of clearance air down

to the point where the crankcase

air inlet valve (peps.
D-A - Alr admission to crank-chamber at

atniospheric pressure.

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 Ci 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.

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