When the fuel-air charge is compressed too much or becomes too hot before ignition takes place, a sharp metallic noise is often heard in the engine. This is called detonation or fuel knock. Fuels vary in their resistance to detonation and are rated in this respect by octane number. The higher the octane number of a fuel, the less susceptible it will be to detonation or knock. High compression engines, or those in which carbon has accumulated, require higher octane number fuels than do low compression en gines in which the carbon has recently been removed. The octane number of a fuel is determined by comparing it with a mixture of iso-octane (a non-knocking compound) and normal heptane (a hard-knocking compound) in a special single cylinder test engine. An arbitrary value of roo has been assigned to iso-octane and o to normal heptane. When a mixture of iso-octane and heptane has been found which produces the same degree of knock as the fuel under test, the percentage of iso-octane in the mixture will equal the octane number of the fuel ; that is, if the percentage of iso-octane in the mixture is 70, the fuel would be said to have an octane number of 70.
Starting.—Occasionally, owing to the improper functioning or failure of a particular part of the mechanism, the engine will not start. There are then three main lines of investigation to follow:—(f) Is there petrol (gasoline) in the tank and is it being supplied to the carburettor ? (2) Is the ignition system in order and producing sparks at the sparking plug? (3) Is the compression poor because of pitted nr leaking valves, blown-out gaskets or imperfectly fitting pistons and piston rings ? If there is fuel at the carburettor, a good spark and good compression, and yet the engine does not start, it may be that the carburettor is delivering to the engine an improper air-fuel mixture. If the trouble is due to too rich a mixture and the manifold has been flooded with fuel, the engine may often be started by opening the throttle wide while using the starter. If. however, the engine fires back through the carburettor the mixture is probably too lean. This may be corrected by means of the choke if manually operated. In the case of automatic chokes, it will be necessary to readjust the mechanism. Over-choking results in diluting the lubricating oil with fuel and should be avoided.
the starter turns the engine over slowly and with much difficulty, it is wise to start the motor with the hand crank, by pushing the car with another vehicle, or coasting downhill if possible ; this relieves the elec trical system of strain due to the starting motor drawing a heavy current. In American cars in which coil ignition is employed, when the battery is badly run down, the voltage drop is so great that the bat tery cannot at the same time actuate the ignition system ; when a mag neto is fitted, as in European cars, the rate of revolution may be in sufficient to produce a spark. The battery should be kept clean and the terminals covered with vaseline to prevent corrosion. The cells should be tested regularly and enough distilled water put in to cover the in sulators slightly. If the car is laid up, the battery should be removed and kept charged. [See Accum ULATOR (STORAGE CELL) .] The ammeter, usually fitted on the dash-board, indicates whether the battery is being charged or discharged. When the lighting and ignition switches are turned off, the ammeter should register zero.
Poor performance, excessive consumption of fuel, and misfiring are often caused by dirty, damaged or unevenly adjusted sparking plugs.
The sparking plug insulators should be well cleaned. It is well to use plugs recommended by the manufacturer of the car, and to change them at stated mileage intervals, since deterioration of porcelain in sulators may cause leakage of current, resulting in a weak spark. To discover a defective plug, short-circuit each plug one after another by holding a screw-driver or hammer so as to touch the metal or ter minal of the spark plug and the metal of the cylinder at the same time. If, when any one plug is short-circuited, no difference is noted in the running of the engine, that particular spark plug may be at fault. Other defects in ignition causing engine trouble are the following:— switch not on; fuse burned out ; disconnected wire on switch, ammeter or battery; and spark retarded or advanced too far. In the case of coil ignition, the coil or condenser may be defective or the resistance burned out, or the contact breaker points may be dirty, pitted or burned. Where a magneto is used the following may be the source of the trouble:—dirty magneto contact breaker points, contact breaker sticking, gap incorrect, or condenser short-circuiting. (See MAGNETO, HIGH-TENSION.) Cooling.-Two systems of cooling motor car engines are used: (f ) by air and (2) by water. The cooling of an engine by air depends on the surface exposed, on the temperature of the air and on its volume.