and Fig. 53 A. When both crank pins are on the same plane in a two cylinder vertical engine it is possible to have the explosions follow in regular sequence, but the engine is not in correct mechanical balance. If the crank pins are spaced as outlined at Fig. 53 A, the engine is in better mechanical balance, but the explosions are not separated by regular intervals. The best method of utilizing two cylinders is to use a double opposed motor. In this case the crank pin arrangement is as shown at Fig. 52 C. Counter weights are necessary when both connecting rods act on the same crank pin, just as in a one cylinder engine.

Fig. 53.-Unconventional Two-Cylinder Crankshaft. A-For TwoCylinder Vertical Engine. B-For Two-Cylinder V Engine.

Q. How are the crankpins spaced in three cylinder motor? A. Crank pins of a three cylinder motor are arranged on thirds of a circle or 120 degrees apart. (Fig. 54 A.)

Q. How are crankpins arranged on four cylinder crankshafts? A The conventional arrangement of crank pins on a four cylinder crankshaft is outlined at Fig. 54 B. Four separate crank throws are provided on two planes, spaced 180 degrees apart. The crank pins for connecting rods 1 and 4 are on a line and the pistons move up and down together. The crank throws of pistons 2 and 3 are on the same line.

Q. How are crankpins spaced on six cylinder engines?

A. The crank pins of six cylinder crankshafts may be arranged in two ways. The simplest arrangement is outlined at Fig. 54 C. In this three crank pins are provided, spaced 120 degrees apart, just the same as in a three cylinder crankshaft, each crank pin serving for two cylinders. The other method outlined at Fig. 54 D may be said to be composed of two three cylinder crank shafts joined together. The crank pins for cylinders 1 and 6 are on the same plane, the pistons of cylinders 2 and 5 move together, as is also true of the pistons of cylinders 3 and 4.

Fig. 54.-Outlining Construction and Arrangement of Throws of Crankshaft for Three, Four and Six-Cylinder Motors.

Q. How many main bearings on crankshaft?

A. The crankshafts of one, two, and some four cylinder motors have but two main bearings, one at each end. A three cylinder crankshaft usually has four main bearings. Four cylinder crankshafts used when motor cylinders are cast in pairs usually have three bearings, and when individual cylinder castings are used a four cylinder crankshaft will have five main bearings. Six cylinder crankshafts have either five or seven main bearings as a rule, though

on some very compact six cylinder engines a three main bearing crankshaft may be used.

Q. What is the influence of cylinder design on crankshaft length?

A. Motors employing block casings can have shorter crankshafts than those in which cylinders are cast in pairs or individually. Q. What are crankshaft main bearings made of?

A. The crankshaft main bearings are generally made of the same material as employed for the bushings for the connecting rod big ends. Ball bearings have been used as main bearings and their use is more general in this application than it is in connecting rods. (Fig. 50.)

Q. What is the function of the crankcase?

A. The crank case is utilized to support the crankshaft and to act as a bed for the engine cylinders. It keeps the working part of the cylinder in perfect alinement with the crankshaft and camshaft carried and protected by the crank case and at the same time it serves as a carrying or supporting member by which the power plant is attached to the chassis.

Q. What are crankcases made of?

A. Automobile engine crankcases may be made of cast aluminum, cast iron, or bronze castings. The first named material is most generally used on account of its lightness. It has about the same strength as cast iron and weighs but one-third as much. On engines that are manufactured in large quantities, stamped sheet metal, such as steel and aluminum, have been utilized as the lower portion of the crank case.

Q. Name three common types of crankcase.

A. Crank cases may be divided into three types, the barrel form, the horizontally divided, and the vertically divided.

Q. What is the barrel type crankcase?

A. The barrel type is a form in which practically the entire crankcase is cast in one piece, end plates being provided for holding the main journals and also to cover the large openings in the ends of the crank case through which the crank shaft assembly is introduced.

Q.

What are the advantages of the two piece crankcases? A. When a two piece crank case is employed the lower portion serves merely as an oil container or cover and the crankshaft is supported by the upper half. When it is desired to adjust the main bearings or the connecting rod big ends the lower portion or oil pan. of the crank case may be removed and the entire interior of the crank case is readily accessible. (Fig. 55.)

Q. How can interior parts of barrel type crankcases be reached?

A. Most forms of barrel type crankcases are provided with large openings or hand holes at the side which are closed by easily removable plates to permit of ready access to the crankcase interior.

Q. What is the function of the flywheel?

A. The flywheel is a heavy cast iron member attached to the crank shaft and is utilized to keep the crank shaft turning during the idle strokes by virtue of power stored in its rim during the working strokes of the motor. The one and two cylinder forms of engines require much heavier flywheels than four and six cylinder forms need. (Fig. 57.)

Q. What determines flywheel diameter?

The flywheel diameter is determined by the materials of which it is composed and by the speed of the crankshaft. A certain peripheral speed limit has been established for cast iron, which it is not desirable to exceed.

Q. Define peripheral speed.

A. The peripheral speed of the flywheel is the distance that any one point on its circumference covers in a given unit of time. Peripheral speed depends directly on the size of the flywheel and the number of revolutions at which it turns. For a given diameter the flywheel that turns the fastest has the greatest peripheral speed.

Q. What is safe peripheral speed for cast iron flywheels?

A. The safe peripheral speed for cast iron flywheels of ordinary construction is approximately 5000 feet per minute. A flywheel 5 feet in circumference could not turn more than 1000 R. P. M. without exceeding this limit.