Knocking.—Freedom from the known tendency to detonate or knock in engines, the third essential property of an auto mobile fuel, is important for two reasons. The first reason is that the knock, which is the metallic "ping" that smites the ear of the driver sometimes when he is "crowding" his engine in traffic, or when his car is labouring up a hill trying to make it "in high," or top gear, is very unpleasant. The second reason is that a knocking fuel places a definite limitation upon the power and the economy of an engine. If it is to be powerful and effi cient, the automobile engine must tightly compress the mix ture of air and gasoline in its cylinders, for it is only in that way that maximum expansion can be obtained, and it is the ex pansion of the burned gases in its cylinders that imparts power to the piston. So the higher the compression ratio of an engine can be made, the greater is its power and the lower its fuel con sumption. The actual compression that can be employed in practice however is limited by the tendency of the available fuel to knock, for the knock gets worse and worse the higher the compression is made, and soon a point is reached at which the knock becomes violent enough to cut down power, so that any further increases in compression results in a loss instead of in a gain.
The knocking properties of the average automobile gasoline as sold in the United States in 194o were such that the com pression ratios of automobile engines varied from 5.5:1 up to over 7:1, depending upon the size and the design of the engine. As a result of much research upon detonation in automobile engines, compression ratios are now being increased (I) because automobile fuels are treated to improve their anti-knocking characteristics, and (2) because improvements in engines them selves make them freer from detonation on any given fuel. Four ways have been found of decreasing the knocking tendencies of automobile fuels ; viz., (I) adding to the fuel very small amounts of extremely effective materials for preventing knock, such as lead tetraethyl and iron carbonyl; (2) adding to the basic fuel a percentage of another fuel which is comparatively free from tendency to knock. Blends of benzol and gasoline and of alcohol and gasoline are the most familiar examples of automobile fuels of this second class. Gasolines produced in this way usually have good anti-knock qualities; (3) "cracking" heavy petroleum oils into gasoline by means of heat, i.e., breaking the large molecules of a heavy oil into smaller and lighter ones (see PETROLEUM) (4) selecting crude oils which by nature yield gasoline of good anti-knock properties. Gasoline obtained from some of the Cali
fornia petroleums, for instance, is much freer from knock than that produced from Pennsylvania petroleum.
Conversion of Heat into Energy Driving the Wheels.— The manner in which the automobile engine converts into mo tion the heat produced by the burning of fuel in its cylinders is briefly as follows : The gas in the cylinders of the automobile engine, just as everything else in the world, is composed of multi tudes of minute particles or molecules. Every one of these mole cules is in rapid motion. The hotter a molecule is the faster it moves, and the colder it is, the slower it travels. All of this is the basis of the kinetic theory of heat (see KINETIC THEORY OF MATTER), which states that the motion of the tiny particles of which matter is made is dependent on their temperature. The automobile engine first draws into its cylinders a mixture of fuel and air, which is done simply by a downward stroke of the piston away from an open intake valve. That breathed-in vapour consists of billions upon billions of tiny molecules racing about. Next, the piston is pushed up, thereby crowding or compressing the swarming molecules of fuel and air ahead of it into a much smaller space. The result is that several minute particles now batter against the piston top, as they speed about, for each one that did so before ; and so the piston feels a much greater push or pressure against it than at the beginning of the compression stroke. At this point a hot electric spark sets fire to the com pressed and highly inflammable mixture of fuel and air, and the whole burns very quickly, releasing an extremely large amount of energy in the form of heat. This increases the heat of the mixture, and the motion of those crowded and swarming mole cules in the cylinder is speeded up to a terrific velocity. In their violent agitation they beat against the piston top until they thrust it down. The piston, by pushing downward, turns the crankshaft, the crankshaft operates through the clutch and the transmission to turn the driveshaft ; and the driveshaft rotates the gears in the differential, the differential gears turn the rear axle, and the axle drives the wheels that roll the car along the road. It is thus that motion of molecules is converted into move ment of the automobile. The function of the fuel is to speed up the molecular motion.