Reading Materials

Passage 1

Pistons, Connecting Rods, Crankshafts

Pistons

The engine piston is the workhorse of the engine.It is round and made in a variety of sizes to fit the cylinders in the block.Pistons serve several purposes:

(1)Transmit force of expansion to crankshaft through connecting rod.

(2)Act as a guide for upper end of connecting rod.

(3)Serve as a carrier for piston rings used to seal piston in cylinder.

(4)Aid in the burning of the fuel mixture by introducing a swirling action to the air-fuel mixture, which is particularly true in the case of diesel engine pistons.The swirling is accomplished by altering the contour of the piston head.

Pistons operate under exceedingly difficult mechanical and thermal conditions so they must be made and installed with the utmost care.Pistons must be strong enough to stand the force of the expansion, yet light enough to avoid excessive inertia forces when their direction of travel is reversed twice each revolution.

Connecting Rods

The connecting rods link the pistons with the crankshaft.In operation, the connecting rod is subjected to both gas pressure and inertia loads, and therefore, it must be adequately strong and rigid and light in weight as well.Connecting rods are generally fabricated from a high-quality steel in the form of a bar with ring shaped heads at its ends.The heads are being known as the connecting rod big end and small end, and serving to attach the rod to the crank pin and the gudgeon pin of the piston respectively.

Crankshafts

The crankshaft is regarded as the“backbone”of the engine.Connecting rods are attached to the crankshaft.This is where reciprocating engine is converted to rotating energy.The area where the rod connected to the crankshaft is called a journal.They are precisely surfaced to provide smooth operation.For every cylinder in the engine, there will be a journal where the connecting rod connects.

The crankshaft consists of main bearing journals, crank pins, front end, and a rear end.There are usually a series of offset weights to counterbalance the rotation of the crankshaft.The main bearing journals and crank pins are induction hardened to improve their wear resistance.The front of the crankshaft carries a gear or sprocket that drives the camshaft.The fan-belt pulley is also located at the front of the crankshaft along with a heavy balanced disc called a vibration damper.A flywheel is bolted to the rear of the crankshaft.This is a large gear about 20 inches in diameter which the starter will engage to crank the engine for starting.

The crankshaft is located at the bottom side of the cylinder block and is covered by what is called the pan.The area within the pan is called the crankcase.A removable plug is located at the back end of the pan to simplify the removal of oil during an oil change.