Learn more about crankcases



Where the cylinder block and crankcase are cast separately, the usual practice is to screw studs into the case and anchor the cylinder block to it with a gasket between block and crankcase. This allows the cylinder block to be handled more readily, and is an economy if either unit suffers serious damage, since they may be replaced independently of each other. Aluminium, because of its light weight, is an ideal crankcase metal for use when the cylinder block and crankcase are cast separately. It is light in weight and has good strength. It works readily, and has a fine appearance. It lends itself nicely to cleaning, and when clean adds much to the attractiveness of the power plant. It requires no finish in the way of paints or enamels, and will not rust or tarnish.

Aside from the mechanical duties which the crankcase must perform, such as carrying the crankshaft and bearings, the camshaft and bearings, the cylinder block, the water pump, the oil pump, the oil lines, the starting motor, the generator, and other units, it must act as a retainer for the lubricating oil. While it may be designed in a rather irregular form, it must be so designed that it can be machined to fine limits, so that all parts will fit and so that all gear covers and oil pans may be drawn down to a close, accurate fit on the gaskets laid on its machined surfaces, thus making it oil-tight.

The crankcase carries the crankshaft, and as a rule, the camshaft. One bearing is provided for the crankshaft at each end of the case. Wherever additional bearings are to be used, the case must have webs cast across to receive them. Some engineers provide main-shaft bearings to a number one in excess of the number of cylinders. Thus a four-cylinder car will have five main bearings and a six-cylinder car will have seven. A straight-eight would have nine bearings, but a V-8 would not have in excess of five. A web in the crankcase is necessary to carry each main except the end ones. The more webs provided the more rigid the crankcase, and consequently, other elements of design being equal, the greater the freedom from distortion and vibration of the crankshaft and case.

Crankshafts and firing orders

Figure shows several styles of crankcases which might be used for six-cylinder engines, one with three, one with four, and the other with seven main bearings.

Crankshafts and firing orders

The design of the crankshaft has a determining influence on the design of the crankcase. The style of shaft used also has a determining influence on the camshaft design. When the firing order of the engine has been determined by the engine builder, then the crankshaft and the camshaft must be manufactured according to a design which will give the desired results.

The early part of this page gives an explanation of the four-stroke cycle with reference to a single cylinder. For instance, figure 2 left side illustrates the first throw of a crankshaft and the two cams for the valve action for the first cylinder, these two being connected together and driven in time by means of a silent chain and sprockets. The ratio is two turns of the crank to one of the camshaft. If the student will remember that each cylinder of any engine must have these working parts working in conjunction with it, he will be able to gain an understanding of the more complicated construction of multicylinder engines.

In figure above here, the simplest type of crankshaft is shown at A. In principle of operation this is similar to the crank on the bicycle. In the case of the bicycle the force comes on the pedal which is like the crank pin of the shaft A. The crank pin carries the connecting rod and this is like the cyclist's leg in transmitting force from the pedal to the crank pin.

In order to balance the weight of the piston-and-rod assembly, counterbalances such as are shown at B, may be used. The two-throw shaft at C is an example of how a counterbalanced shaft for a two-cylinder engine may be built. This shaft has two throws, two crank pins, and two mains. A shaft for a single-cylinder engine or one for a two-cylinder engine presents no problem when it comes to an order of firing the cylinders.

Firing orders When a four-cylinder engine is designed the firing order does need consideration. The shaft shown in figure left has a firing order of 1-2-4-3 as may be traced out by the lines and arrows. The firing order might be 1-3-4-2. This same shaft might be used for a V-8 design if the rods were set side by side, two rods for each crank pin.


Crankcase basics The sequence of operations in a six-cylinder engine is illustrated in figure left. Cylinder Number 1 shows at the end of the firing stroke, direction of rotation being indicated by the curved arrow. Five is just firing so we have the beginning of the firing order 1-5. Three is on compression and will be fired when it reaches top dead centre so we have 1-5-3. Six is shown on intake (suction) so that it will follow next, and since 2 has just started intaking and four is on exhaust they will follow in order, that is, 1-5-3-6-2-4. The valve and valve ports in the cylinder bead above are indicated. Cam action must be such as to open these valves in time and proper order for the firing order.