These instances of correlation among merits and defects—out of many that could be mentioned— are characteristic of the auto mobile engine as an organism in which success or failure comes from harmonious interaction among all mechanical features as much as from the design and peculiarities of any of them. Carburation, fuel induction, ignition, compres sion, combustion and expansion and even lubri cation — all important and complicated func tions — must in the highest type of engine often be performed several thousand times each min ute and each time in a preconceived manner and in minutely regulated time and space rela tions of one to another, these relations varying moreover according to the engine speed.
Evolution of the automobile engine has for the reasons implied in these facts been a rapid succession of compromises among conflicting factors. Many advance steps have at first been too boldly conceived and had to be retraced to wait for rel: d not yet ripe, at the time, for mechanical realization.
The modern single-cylinder motorcycle en gine (see MOTORCYCLE) cooled directly by the atmosphere represents the simplest engine unit in which every unsuitable feature of the sta tionary gasoline engine prototype has been changed in kind or degree to gain reliability and meet the requirements for vehicle work on essentially the same plan that has been followed for automobile engines, although the urgent need of lightness has here caused the adoption of cooling by air instead of by water and has reduced the number of cylinders, in most cases, to one or two.
For automobile engines, on the other hand, cooling by water was originally found so much easier to manage that its cost, weight and encumbrance were almost universally accepted as a lesser evil than the unreliability thought to be inseparable from cooling by air alone, and only one American manufacturer of motor cars now employs the latter system, using forced air draft for each cylinder. As a small cylinder is more easily cooled than a large one (having a larger surface to which the cooling medium can be applied in proportion to its volume and capacity for beat generation), and as 6, 8 and 12-cylinder motors with small cylinders are gain ing in favor, while aluminum is also coming into use as an engine material for pistons and cylinders and is more readily cooled than iron, having much greater conductivity (see HEAT), the possibility of a much wider adop tion of air cooling is not excluded, especially to conjunction with a moderation of engine speed; for example, for light vehicles for which the smallest practicable dimensions of a four or six cylinder engine give an abundance of power without pushing the engine speed to the attain able maximum.
Among the features in the original engine, of which no trace is left in the modern designs shown in the accompanying illus trations, the surface-evaporation carburetor, the automatic inlet valve, the hit-and-miss speed control acting by blocking the ex haust, and the hot-tube ignition were those which were mostly responsible for the immense difficulties in utilizing the engine with equal facility for different vehicle speeds, different loads and different road grades. To change the constant-speed engine into a source of variable Dower, with the amount of nower and the rate of its delivery — the torque and angular speed of the crankshaft — under the required instantaneous control, it was first of all necessary to revolutionize the carburetor. As the engine is the heart of the motor car, so the carburetor is the soul of the engine. Theories on the necessity for complete evapora tion delayed the advent of the float chamber and the jet. Years followed, during which the Krebs diaphragm or a sliding sleeve was em ployed to secure approximately the same com position of the explosive mixture at one engine speed as at another, the spray from the jet in conjunction with a butterfly valve in the induction pipe having already made it possible to vary the engine speed considerably. But
moving parts wore at their bearings, so that adjustments did not stay true, and much finer control was wanted. The whole subsequent de velopment of carburetor mechanics has been• divided between the two objects of securing this fine control for all desired degrees of engine power and speed (high engine speed with low torque being as frequently wanted, on hard and level roads as maximum torque at low speed to turn the crest of a hill without gear change) and of reconciling it with the constantly de creasing volatility of the fuel obtainable in the market. Increasing number of cylinders in the engines, all to be supplied from the same carburetor, and rapidly increasing maximum speed of the engines due to other advancement in the construction, made the task an unceasing one. Hereto came a demand for rapid accelera tion or gpick-up,)) also to be complied with. The expedients adopted may be classified in groups mechanically, but are so variously modified and combined in the several hundreds of carburet ors manufactured for the market that no single scientific principle has been demonstrated as governing the control of carburation when it is undertaken on the basis of a spray-jet from which a variable air current must siphon the fuel. As the volume of the liquid gasoline should be only about one eight-thousandth part of the volume of air with which it is mixed in atomized or vaporized form, the vari able features in the control cannot suc cessfully be applied directly to the flow of the liquid, such as by having the throt tle reduce or enlarge the apertures of the jet, but must be applied to the air, its course and its velocity, and the air current, so varied on one plan or another which has been found suitable, must cause the discharge of liquid to be varied in proportion to the volume of air used. The greatest difficulty arises from the fact that an air current of doubled volume siphons much more than double the amount of fuel from an ordinary tubular jet, and com pensation for the excessive siphoning action must be made. Also, when the air current is strong, with the throttle open, and the engine cylinders are filled up completely by the suction stroke, the compression of the charge becomes greater than if the cylinder is only partly filled, and to realize the best fuel economy, the highly compressed fuel mixture should be leaner than one less compressed. To compensate for this factor special devices are sometimes employed which admit air above the throttle to thin the gas mixture at the engine speeds giving the highest torque. The expedients which are nor mal features in the carburetors are exemplified on the constructions shown in Figs. 1, 2, 3 and 4, and are briefly explained in the captions for each of them. In order to produce a notice ably rapid acceleration it has recently become a somewhat general practice to have a plunger connected with the throttle or accelerator lever with poppet valves are of the L-head, the T-head or the top-valve type. The questions involved in the designer's choice among these, aside from cost of construction and commercial expediency, have changed in the course of years. Sound cylinder castings were hard to obtain in such manner that for a moment after the throttle has been opened to cause acceleration an additional air current is sent from the plunger pot through or past the jet, to produce an action which is at once as strong as that which the same throttle opening otherwise gradually causes by increasing the engine speed.