Motors Part 7
The next most serious loss is in the escape of heat through the burnt gases, which amounts to seventeen per cent. If the expansive force of the burnt gases at the time of ignition is 250 pounds per square inch, and at the time of the discharge it is fifty pounds, only four-fifths of its power is effectively used.
As, however, the discharge is against the air pressure of nearly fifteen pounds per square inch, it is obvious that thirty-five pounds per inch is driven away and lost.
The third loss is by conduction and radiation, which amounts to fifteen per cent. or more, so that the total loss from all sources is about eighty-four per cent., leaving not more than sixteen per cent. of the value of the fuel which is converted into power.
Engine Construction.--In the construction of engines the utmost care should be exercised in making the various parts. The particular features which require special care are the valves, which should be ground to fit tightly, the proper fitting of the piston rings, crank shaft and connecting rod bearings as well as the accurate relining of these bearings.
[Ill.u.s.tration: Fig. 30. Valve Grinding.]
Valve Grinding.--Fig. 30 shows a valve and valve seat. The valve has usually a cross groove so that a screw driver in a drill stock may be used to turn it and to exert the proper pressure. The finest emery powder and a first cla.s.s quality of oil should be used. The valve is seated and after the oil and emery powder are applied the drill stock is used to turn the valve.
After twenty or thirty turns, wipe off the parts and examine the contact edges, to see whether the entire surfaces are bright, which will indicate that the valve fits true on its seat. Never overgrind. This is entirely unnecessary. It is better also to rock the crank of the drill stock back and forth, instead of turning it in one direction only.
The Crank Shaft.--The crank shaft is the most difficult part of the engine to build. It is usually made of a single forging of special steel and the cranks and bearings are turned out of this, requiring the utmost care. Formerly these were subject to breakage, but improved methods have eliminated all danger in this direction.
The Cams.--Notwithstanding the ends of the push rods are provided with rollers to make the contact with the cams, the latter will wear, and in doing so they will open the valves too late. The slightest wear will make considerable difference in the inlet valve, and it requires care and attention for this reason, in properly designing the cams, so that wear will be brought to a minimum.
CHAPTER VI
CARBURETERS
A carbureter is a device which receives and mixes gasoline and air in proper proportions, and in which a vapor is formed for gasoline engines.
The product of the carbureter is a mixture of gasoline vapor and air, not a gas. A gas, as explained, is of such a character that it remains fixed and will not stratify or condense.
Functions of a Carbureter.--The function of a carbureter is to supply air and gasoline by means of its adjustable features so as to make the best mixture. The proportions of air and gasoline will vary, but generally the average is fifteen parts of air to one of gasoline vapor.
If there is too much gasoline, proportionately, a waste of fuel results, as a great amount of soot is formed under those conditions. If there is an excess of air the mixture, when ignited, will not have such a high temperature, hence the expansive force is less, and the result is a decrease of power.
While it is possible to get a rapid evaporation from gasoline by heating it, experience has shown that it is more economical to keep the gasoline cool, or at ordinary temperatures, provided the carbureter is properly constructed, because the vapor, if heated, when drawn into the engine, will be unduly expanded, and less fuel in that case is drawn in at each charge, and less power results.
Rich Mixtures.--There are conditions under which rich mixtures are advantageous. This is a mixture in which there is a larger percentage of gasoline than is necessary for instantaneous combustion. For ordinary uses such a mixture would not be economical.
At low speeds, however, or when carrying heavy loads, it is desirable, for the reasons that at a slow speed the combustion is slower.
Rich mixtures are objectionable at high speeds because, as the combustion is slow, incomplete combustion within the power stroke results, the temperature of the gas at the end of the stroke is very high, and this will seriously affect the exhaust valves. Furthermore, there is likelihood of the gas continuing to burn after it is discharged from the cylinder.
Lean Mixtures.--Such a mixture is one which has a less amount of gasoline than is necessary to make a perfectly explosive compound. For high speeds a lean mixture is desirable, princ.i.p.ally because it burns more rapidly than a rich mixture.
Types of Carbureters.--There are two distinct types of carbureters, one which sprays the gasoline into a conduit through which air is pa.s.sing, and the other in which a large surface of gasoline is placed in the path of the moving air column, which was originally used, but has been absolutely replaced by the jet carbureters on account of their better control features.
It will be remembered that reference was made to the manner in which vaporization takes place, this term being used to designate that tendency of all liquids to change into a gaseous state. All carbureters are designed with the object of mechanically presenting the largest possible area of oil to the air, so that the latter will become impregnated with the vapor.
The Sprayer.--The best known type depends on dividing up the gasoline into fine globules, by ejecting it from a small pipe or jet. The spray thus formed is caught by the air column produced by the suction of the engine pistons, and during its pa.s.sage through the throttle and the manifold, is in condition where a fair mixture of air and vapor is formed, which will readily ignite.
The Surface Type.--This form of carbureter provides a pool of gasoline with a large surface, within the sh.e.l.l, so arranged that as the air is drawn past the pool it must come into contact with the oil, and thus take up the necessary quant.i.ty of evaporated gasoline for charging the air.
The _surface_ type has not been used to a large extent, but the _sprayer_ is universally used, and of this kind there are many examples of construction, each having some particular merit.
Governing a Carbureter.--It is a curious thing that one carbureter will work admirably with one engine, and be entirely useless in another. This is due to several factors, both in the engine design and in the carbureter itself. The quality of mixture that an engine will take depends on its speed. The suction of the pistons depends on the speed of the engine.
If, at ordinary speed the carbureter gives a proper mixture, the throats and pa.s.sages through the pipes and manifold, as well as the valve which discharges the gasoline, may be in a prime condition to do good work; but when the pistons work at double speed the inrush of air may not carry with it the proper amount of fuel; or, under those conditions, the air may receive too great an amount of gasoline, proportionally.
The latter is usually the case, hence provision must be made for such a contingency, and we shall therefore take up the various features essential in the construction of the carbureter, so as to show what steps have been taken to meet the problems arising from varying speeds, differences in the character of the fuel, regulating the inflow and mixture of gasoline and air, and adjustments.
[Ill.u.s.tration: _Fig. 31. Carbureter._]
So many different types of carbureters have been devised, that it is difficult to select one which typifies all the best elements of construction.
In Fig. 31 we have shown a well known construction, and which will ill.u.s.trate the features of the sprayer type to good advantage. The body of the device, represented by A, has a f.l.a.n.g.e by means of which it is secured to the pipe which carries the carbureted air to the engine. The lower end of this tubular body is contracted, as shown at B, so as to form what is called a venturi tube.
Exteriorly this contracted tube is threaded, as shown at C, so as to receive thereon a threaded body D, the lower end of the body having an enlarged disk-head E, integral therewith, and an upwardly-projecting annular f.l.a.n.g.e F is formed around this disk to receive and hold a cylinder G, which const.i.tutes the float and fuel chamber.
The upper end of this cylinder rests against a seat cast with the body A, and packing rings are placed at the ends of the cylinder to prevent the oil from leaking out. Within the tubular body D is a vertical tube H, integral with the disk head E, and oil is supplied to this tube through ducts I, which communicate with the chamber within the reservoir G.
A drain c.o.c.k is at the lower end of this tube, and an adjustable cap K screws on the tubular stem of the drain tube, around which air is admitted, the air pa.s.sing upwardly through vertical ducts L, as shown, and thus mixes with air at the contracted part of the venturi tube.
A ring-like float N is placed within the gla.s.s chamber, and this is adapted to engage with the inner end of a lever N', this lever being pivoted at O, within a side extension P of the carbureter sh.e.l.l. The inner end of this lever has a link hinged thereto, the lower end of which serves as a needle valve to close the ejecting orifice of the tube L.
The outer end of the lever N' engages a shoulder on a vertically-disposed needle valve Q, which has its point in the inlet opening of the pipe R, through which gasoline is supplied to the gla.s.s chamber. A spring T serves to keep the valve stem normally on its seat.
Directly opposite this chambered extension P is another extension U, also cast with the sh.e.l.l, through which is a vertical stem V. This stem carries a downwardly-opening valve W, that seats against a plug, and a spring X below the valve, serves to keep it against its seat, unless there should be an extraordinarily heavy pull or suction.
This is the auxiliary air inlet, and the lower spring is actuated only when the engine is running at moderate speeds, but when running at high speed and an additional quant.i.ty of air is required the upper spring Y is compressed, and thus a much greater quant.i.ty of air is allowed to pa.s.s in and mingle with the spray at the throttle valve Z.
The throttle valve is mounted in the discharge opening, and is controlled by a lever on the outside of the carbureter.
The device operates as follows: Primary air enters the opening between the cup K and the disk-head E, pa.s.sing up into the s.p.a.ce around the oil tube H. As the spring T, around the needle valve Q, draws up the valve from its seat, oil is permitted to flow in through the duct R and fill the chamber, until the float engages with the inner end of the lever N, and raises it, thus uncovering the ejecting end of the tube H, and at the same time closing the inlet tube R.
The suction from the engine then draws air through the primary duct, as stated, and also an additional quant.i.ty through the secondary source, by way of the valve W, this valve being so regulated as to supply the requisite quant.i.ty.
The auxiliary air source serves the purpose that means should be provided to supply more than the ordinary amount of air, when running at high speeds.
From the foregoing it will be observed that a carbureter must be so constructed that it will perform a variety of work. These are: First, Automatic means for filling the float chamber when the gasoline goes below a certain level. Second, Cutting off the supply of gasoline.
Third, Providing a primary supply of gasoline for spraying purposes.
Fourth, Furnishing an auxiliary air supply. Fifth, Throttling means in the discharge opening.
It is thus a most wonderful contrivance, and considering that all the elements necessary to make it work satisfactorily are provided with adjustable devices, it may be seen that to make it perform correctly requires a perfect understanding of its various features.
Requirements in a Carbureter.--In view of the foregoing it might be well to know how to select a carbureter that is ideal in its operation.
First. The adjustment of the auxiliary valve should be of such a character that at the slowest speed the valve should not be lifted from its seat.
Second. It must be so arranged that it is not difficult to change the relative amount of air and gasoline.