further description here. The jet B is surrounded by the inducing tube C, which is in one piece with the throttle valve D. This throttle valve is connected to the engine governor, so that as the speed of the motor falls the piece D will be raised. At the same time that the throttle valve is opened the effective area of the inducing tube will be

B

A

N

FIG. 17.

increased, owing to its form, and thus the air will not greatly increase its velocity. If carefully proportioned, a carburettor on these lines could be constructed to maintain the mixture constant in quality. To compensate for variation in the specific gravity of the petrol, or the state of the atmosphere, a needle valve is provided at N to control the size of the jet aperture. Other things being

equal, the efficiency of a carburettor is the measure of the freedom with which the mixture passes to the engine. Were it not for the unequal vapourization of the petrol, and the constant variation in the proportions of the mixture, surface carburettors would be preferable to the jet type if only on account of the free passage of the air and gas through them. One other defect of the surface type is the liability of explosion should a flame find its way into it, as even at temperatures considerably below 32° Fahr. there will be an explosive mixture formed.

Owing to the rapid evaporation of the petrol, heat is quickly absorbed from the metal forming the walls of the vapourizing chamber; hence provision must be made for supplying the heat necessary for the evaporation of the spirit. The usual plan is to provide the vapourizing chamber with a jacket, through which the heated water from the cylinder jackets, or a part of the exhaust gases, is allowed to circulate. The use of the water circulation is preferable to that of the exhaust gases, as the temperature is more likely to be kept even.

Governing. There are two principal methods of governing the speed of a petrol motor. Either the force of each impulse may be varied, or the number of impulses in a given time may be changed. All motors that are provided with a governor are controlled by one of these two methods, or some modification thereof. By the first method the force of the explosion is diminished by admitting a smaller charge into the cylinder. The efficiency of an engine governed on this system will not be so high as when the governing is effected by cutting out the impulses entirely, when it becomes necessary to reduce the speed. In the first method, by reducing the quantity of air and gas taken into the cylinder, the compression pressure is lowered, and this does not tend to make the engine economical, as explained on p. 8. But the turning

moment of the crank shaft is more even than when the charges are cut out altogether. The diminution of the amount of the charge can be effected in a variety of ways. A throttle valve in the pipe leading from the carburettor is the most usual device, but the same end may be accomplished by altering the lift of the inlet valve, or the time it remains open. The Crossley motor employs an auxiliary cut-off valve, through which the mixture has to pass on its way to the inlet valves, and which is acted upon by the governor to cut off the supply of air and gas before the suction stroke is completed, when the speed of the engine increases to such a point as to render this desirable.

Governing by cutting out the impulses may be effected in two ways when automatic inlet valves are used-either by retaining the products of combustion in the cylinder by causing the exhaust valve to remain inoperative during one or more cycles, or by allowing the exhaust valve to remain open during one or more suction strokes, either way preventing the formation of a sufficient vacuum to open the inlet valve. The first of these methods, i.e. keeping the burnt gases in the cylinder, was the system adopted in the original Daimler motors, and was economical of fuel. By suitably designing the cams which operate mechanically opened inlet valves, the governor may be made to render these cams inoperative for as long as may be required to bring the engine speed down to the normal, or the valve rods may be acted upon with similar effect.

It is possible to vary the speed of the engine by altering the timing of the ignition; but this is not to be recommended, as it is wasteful of fuel. By governing in this manner, the amount of fuel consumed will remain constant at all loads. The only use for this way of altering the engine speed is for temporary occasions, when other means are not so convenient, or in emergencies.

For stationary motors, such as those used for dynamo driving, the best system of governing is that in which the impulses are cut out entirely, as the utmost fuel economy is obtained, and the slight irregularities in speed can be compensated for by having a heavy flywheel. This system would also find acceptance for marine work. In the case of an automobile it is necessary to have some means of regulating the speed of the motor, within fairly wide limits, from the driver's seat, and in this connection there is nothing better than the throttle valve. The hand-operated throttle valve gives the driver the power of adjusting the speed of the vehicle without constant recourse to the speed gearing, or rather it gives a means of control supplemental to that of the gearing.

Upon whatever system the governor works, it should be so designed that the driver of the car can nullify its action at will, when the greatest speed is required from the motor. It is good practice to arrange for an auxiliary throttle valve connected to the brake gear, so that when the vehicle is stopped the motor will be automatically slowed down, thus avoiding waste of fuel, and preventing undue vibration.

The actual design of the governor may be left to individual judgment. Formulæ are of little value in connection with such small governors as are required in automobile work, and, moreover, there is generally a good deal of latitude allowed in the adjustment of the springs. The governor weights need only be small, even for highpowered engines, as the work imposed upon the governor is usually very slight.

Ignition. At the present time the ignition of the charge by an electric spark may be said to be universal, the hot-tube method having been quite abandoned for automobile engines. The only advantage to be claimed for the hot-tube method of ignition is its reliability and

simplicity. It is, however, not adjustable in regard to timing the moment of igniting the charge, and thus motors equipped with it can only be run at one speed economically. It is obvious that there is a risk of fire should anything cause an upset of the petrol, and this was by no means an unknown danger in the days when tube ignition was universal, or practically so. For stationary work, such as pumping or dynamo driving, tube ignition still has its uses, and is in many cases to be preferred to electric ignition for such purposes.

There are two systems of electric ignition in use, either of which is capable of giving satisfactory results, provided it be properly installed and maintained in working order. These two systems are known as the lowtension and the high-tension. The low-tension system requires a make-and-break device inside the combustion chamber, operated from the outside, usually by a cam on the valve-gear shaft. The system has the advantage that all the wires are easily insulated, owing to the low voltage of the circuit. The necessary fittings, with the exception of the sparking device in the combustion chamber, are cheap, and require few repairs.

In the high-tension system there are no moving parts within the cylinder, but there is greater difficulty in insulating the conductors of the secondary circuit, owing to the high tension, which may be as great as 30,000 volts. In the matter of cost of installing, it is believed that there is very little to choose between the low and high tension systems. An efficient low-tension coil may be constructed as follows. The core should consist of a bundle of soft iron wire of about 20 B.W.G., and should be about 9 inches long by 1 inch diameter. A thin tube of insulating material is placed over the core, and upon it is wound double cotton-covered wire of 14 B.W.G. till there are three layers. The connections for such a coil