for many years, with intricate coring for water jackets, except in symmetrical shapes, and this factor gave the preference to the T-head type before the top-valve type was known. It is now seldom used, the art of casting having advanced, bringing the functional factors of the engine into relatively greater importance.
The arrangement of valves and valve-actuat ing mechanicism largely determines the dimen sions of valves that can be used for a cylinder of a given bore, and thereby the maximum engine speed under load. According to what arrangement is adopted, automobile engines These functional requirements are (1) compac combustion chamber with a minimum of oval area to get rapid ignition and inflammatioi from a centrally located spark plug and ful benefit of as expansion against the pistol before cooling can take place in appreciably degree; (2) large valves and direct passages for fuel charges and exhaust gases, these being the main factors in (volumetric efficiency( which is the ability of the engine to take in and discharge gas with a minimum of skin friction and vortex action; (3) minimum weight of valve mechanism, to reduce vibration and In the Quest for a small engine with a high maximum power and a wide range of speed these factors are all of importance, and it is usually admitted that the top-valve type, with the camshaft located on top of the cylinders, admits of the highest power development, closely followed however by the L-type ar ranged with the intake valve in the wing and the exhaust valve in the top and operated by a rocker arm. In all cases the possibility exists of obtaining equal maximum power and greater (flexibility° in operation from a slightly larger 'and heavier engine with relatively smaller valves. The valve timing can be made nearly correct throughout the smaller range of engine speeds. Thus the L-type with both intake and exhaust valves in the wing (and each of them noise; (4) accessibility of valves and valve seats, to facilitate inspection and repair; (5) minimum formation and ready removal of carbon deposits, the latter being due mostly to residues from incomplete combustion of lubri cating oil on the cylinder walls; and (6) re liable lubrication of the valve actuating mechanism.
therefore rather small) is commonly employed. A modern instance of the T-type, with two cam shafts, for a 6-cylinder engine of high class and cost is shown in Fig. 5 the valve tappet rods being encased by means of aluminum covers on both sides. Though little used now, this type can have large valves and high speed.
Fig. 6 shows a sectional view of a 4-cylinder L-type engine, all cylinders cast in one block, as now usually done. Fig. 7 shows a 4-cylinder top-valve engine with details of mechanism in Fig. 8. Fig. 9 an 8-cylinder engine. To make the power impulses follow one another at equal intervals, one row of cylinders in this engine forms a right angle with the other. In 12 cylinder engines of the same type, which are also used, this angle is reduced to 60 degrees, whereby the engine becomes narrower and more easily disposed of under the hood, the parts are more accessible, while the great ad vancement in the utilization of automatic machinery in the production of engine parts makes the difference in cost less than it could have been a few years ago. A tendency is thereby established toward the use of many small cylinders rather than fewer and larger ones. The crankshaft of a multicylinder engine fully balanced by counterweights at every point to obviate vibration in the last degree is, how ever, a complicated forging. The ability to produce such shafts by drop-forging from one piece of steel has been developed mainly in the United States, while on the other hand the art of producing complicated and yet sound cylin der castings was first developed in Ger many, France and Belgium. In the beginning of the industry, cylinder heads were cast sepa rately from the cylinders and were bolted on.
The gasket used at the joint frequently leaked after much exposure to heat, and the practice of casting all in one block was universally adopted. Lately, however, a return to separate casting of cylinder heads is noticed, this being due to considerations of cost, of new valve arrangements, better gasket materials and cheaper and more convenient repairs.