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Engine Building Fundamentals: Rotating Assembly

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Rotating Assembly


00:00 - Now we can discuss the internal components that will be fitted into the engine block.
00:05 When I refer to the rotating assembly, I'm talking here about the crankshaft, the connecting rods, or conrods for short, and the pistons.
00:14 While you could consider the camshaft to be a rotating component, this is normally separated and classed as a valve train component instead.
00:24 Let's start by discussing the crank shaft.
00:27 As we discussed in the last module, the crankshaft is supported in the engine block by bearings which run on the main bearing journals.
00:36 It's common to refer to a crankshaft by the number of main bearing journals it has.
00:40 For example, with a modern in-line four cylinder engine, the crankshaft will normally have a front and rear main bearing journal, and then a journal between each of the cylinders.
00:53 This means that the crankshaft has a total of five main bearing journals, and would be referred to as the five journal crankshaft.
01:01 The crankshaft will also include a pair of thrust flanges that positively locate the crankshaft fore and aft in the engine block.
01:11 This prevents, for example, the crankshaft from being forced forwards when the driver puts his foot on the clutch, and the clutch is disengaged.
01:19 At the front of the crankshaft, we have the snout, or nose, and this will often incorporate a machined keyway that's used to locate and drive a harmonic dampener.
01:29 Depending on the specific engine design, there may also be keyways for camdrive sprockets, and, perhaps, machined flats on the part of the crankshaft to locate and drive an oil pump.
01:41 There will usually be a machined surface for the front crank seal to run on as well.
01:46 At the rear of the crankshaft, we will have a large flange that the flywheel or flexplate bolts to.
01:53 This will typically be drilled and tapped for the bolts responsible for securing the flywheel, as well as, potentially, a dowel to help positively locate the flywheel or flexplate.
02:04 If we view the back of the rear flange, there will often be a location for a spigot bearing, or pilot bearing, to be installed, that's job is to support the front of the input shaft from the gearbox.
02:17 The rear flange will also double, often, as the sealing surface for the rear main seal.
02:24 Offset from the main bearing journals on the crankshaft are the conrod journals, which locate the connecting rods.
02:29 These are often referred to as crankpins.
02:33 Depending on the engine configuration, each conrod may have a separate journal, such as the case of an in-line four or six cylinder engine, while in some cases, such as a V8, the conrod journals will be shared by two conrods, one on each opposing bank of the engine.
02:52 Between the main journals, and the conrod journals are the counterweights used to balance the crankshaft.
02:59 In order to reduce stress in the crankshaft, and prevent cracking, there will be a smooth and gentle radius, machined with the journal surfaces transitioned into the crankshaft counterweights.
03:11 It's also common to see the counterweights on the crankshaft showing signs of drilling, which is part of the factory balancing process.
03:20 Since the crankshaft will be supplied with oil from the main bearing supply, it also includes a network of oil galleries internally, that take the oil from the main journals, and supplies it to the connecting rod journals.
03:34 These galleries are drilled through the crankshaft during the manufacturing process, and the ends of the gallery may be blanked off, either by a ball bearing, in the case of most factory crankshafts, or, in the case of some aftermarket crankshafts, these galleries may be drilled and tapped so that the plugs may be removed for cleaning.
03:55 Moving on to the connecting rods, or conrods, these are used to support the pistons on the crankshaft.
04:03 The conrod is one of the most highly stressed components in the engine, constantly being exposed to forces in both compression, and tension, as the engine moves through the engine cycle.
04:15 If we start where the conrod attaches to the crankshaft, this is referred to as the big end of the conrod.
04:22 The conrod is two-piece, and the cap, or conrod cap, as it's known, can be removed to facilitate installation on the crankshaft.
04:32 The cap is fastened to the body of the conrod, either by bolts and nuts, in the case of most factory components, or a bolt in the case of aftermarket components.
04:44 It's also typically located positively to the body of the conrod via a pair of dowels.
04:51 The big end of the conrod houses the bearings that run on the crankshaft journal, and there will be recesses machined into both the conrod and the cap that align the locating tang that you'll find on the bearing shells.
05:07 The opposite end of the conrod attaches to the piston, and is referred to as the small end, or sometimes the pin end of the conrod.
05:16 Where the wristpin is a floating fit in the small end of the conrod, the conrod will be fitted with a pressed-in bush, rather than a bearing, and this is what the wristpin will run in.
05:28 Lubrication of this bush is important, and this can be achieved via an oil supply hole on the top surface of the small end of the conrod, that relies on the oil most present under the crown of the piston, or sometimes via a forced oil supply from the big end bearing.
05:45 Conrods are typically supplied in two main forms, I-beam, or H-beam, and this refers to the shape of the beam of the conrod, joining the big end and the small end together.
05:56 If we cut the conrod in half through the centre, and looked end on at the cut that we've just made, this is where the terms H-beam and I-beam come from.
06:06 An I-beam rod looks like a capital I, while an H-beam rod looks like a capital H.
06:14 The shape of the conrod beam has implications for both the weight of the conrod, as well as its strength.
06:20 Lastly, we can move on and discuss the piston.
06:22 Pistons for automotive use are typically either cast or forged from an aluminium alloy, and they're a surprisingly complex component.
06:32 Starting at the top of the piston, we have the piston deck, or crown.
06:36 Depending on the desired compression ratio, the piston deck may have a dome formed on it to raise the compression, or, alternatively, a dish to lower it.
06:49 The crown of the piston may also have valve pockets, or valve reliefs machined into it to provide clearance to the valves as the piston moves through its stroke.
06:59 As we move down from the piston deck, we have the piston ring grooves, as well as the ring lands.
07:05 It's normal in most applications to fit three rings to the piston, comprising of two compression rings, and an oil control ring.
07:14 The rings are located in the machined ring grooves on the piston, and the area above and between the rings is known as the ring land.
07:22 On many aftermarket pistons, you'll find grooves machined into these ring lands.
07:28 On the top ring land, these are referred to as contact reduction grooves, or anti-detonation grooves, which are designed to reduce contact between the piston, and the bore wall, when the piston rocks over.
07:40 They also have the effect of disrupting flame travel, which can help prevent detonation, hence the name.
07:48 On the second ring land, we may see a more significant recess machined into the piston, which is known as an accumulator groove.
07:57 This groove helps to prevent pressure buildup between the top and second compression rings, which can destabilise the ring seal.
08:05 The oil control ring groove will include a number of oil drain holes, which are drilled right through the piston, and allow the oil scraped off the bores by the oil control ring to return to the crank case through the inside of the piston.
08:22 Often, these holes will also be used to force feed oil to lubricate the wristpin.
08:27 The wristpin, or gudgeon pin, as it's also known, is what attaches the piston to the conrod.
08:33 The wristpin locates through the wristpin hole in the piston and can be located in a number of ways.
08:40 In some instances, the wristpin is an interference fit, in either the small end of the conrod, or, alternatively, in the wristpin holes in the piston itself, while in the majority of cases in performance applications, the wristpin is known as fully floating, and is located via wire locks or spiral locks in machine grooves in the piston.
09:03 One of the key dimensions of the piston is the distance from the centre of the wristpin bore to the deck of the piston.
09:12 This is known as the compression height, or compression distance.
09:16 Below the oil control ring, we have the piston skirt which locates and stabilises the piston in the cylinder.
09:24 In modern racing pistons, it's common to see the skirt cut away, or reduced, around the sides where the wristpin bosses are, in order to reduce the frictional losses that come about due to the contact between the skirt and the cylinder wall.
09:40 The piston skirt helps to stabilise the piston in the bore, as it moves through the engine cycle.
09:47 One side of the piston, however, will be more heavily loaded against the cylinder wall during the power stroke, and this is known as the major thrust face.
09:56 The major thrust face will depend on the direction of rotation of the engine, and for most engines, which rotate clockwise when viewed from the front of the crankshaft, the major thrust face is the left side of the piston.
10:11 The opposite side is known as the minor thrust face.
10:15 This is important to know, as some piston manufacturers produce asymmetrical piston designs where the minor thrust face of the piston skirt is purposely reduced in size, in order to further reduce frictional losses.