B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 1

Transcription

1 Firing Order Every engine cylinder must fire once in every cycle. This requires that for a four-stroke fourcylinder engine the ignition system must fire for every 180 degrees of crank rotation. For a sixcylinder engine the time available is only 120 degrees of crank rotation. The order in which various cylinders of a multi cylinder engine fire is called the firing order. The number of possibilities of firing order depends upon the number of cylinders and throws of the crankshaft. It is desirable to have the power impulses equally spaced and from the point of view of balancing this has led to certain conventional arrangements of crankshaft throws. Further, there are three factors which must be considered before deciding the optimum firing order of an engine. These are: (i) Engine vibrations (ii) Engine cooling and (iii) Development of back pressure Consider that the cylinder number 1 of the four-cylinder engine, shown in Fig., is fired first. A pressure p, generated in the cylinder number 1 will give rise to a force equal to {pa [b/(a + b)]} and {pa [a/(a + b)]} on the two bearings A and B respectively. The load on bearing A is much more than load on bearing B. If the next cylinder fired is cylinder number 2, this imbalance in load on the two bearings would further aggravate the problem of balancing of the crankshaft vibrations & would result in severe engine vibrations. If we fire cylinder number 3 after cylinder number 1, the load may be more or less evenly distributed. Further, consider the effect of firing sequence on engine cooling. When the first cylinder is fired its temperature increases. If the next cylinder that fires is number 2, the portion of the engine between the cylinder number 1 and 2 gets overheated. If then the third cylinder is fired, overheating is shifted to the portion between the cylinders 2 and 4. Thus we see that the task of the cooling system becomes very difficult because it is then, required to cool more at one place than at other places and this imposes great strain on the cooling system. If the third cylinder is fired after the first the overheating problem can be controlled to a greater extent. B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 1

2 Next, consider the flow of exhaust gases in the exhaust pipe. After firing the first cylinder, exhaust gases flow out to the exhaust pipe. If the next cylinder fired is the cylinder number 2, we find that before the gases exhausted by the first cylinder go out of the exhaust pipe the gases exhausted from the second cylinder try to overtake them. This would require that the exhaust pipe be made bigger. Otherwise the back pressure in it would increase and the possibility of back flow would arise. If instead of firing cylinder number 2, cylinder number 3 is fired. then by the time the gases exhausted by the cylinder 3 come into the exhaust pipe, the gases from cylinder 1 would have sufficient time to travel the distance between cylinder 1 and cylinder 3 and thus, the development of a high back pressure is avoided It should be noted that to some extent all the above three requirements are conflicting and therefore a trade-off is necessary. For 4-Cylinder engines the possible firing orders are: or The former is more commonly used in the vertical configuration of cylinders. For a 6-Cylinder engine firing orders can be: or or or The first one is more commonly used. Other Firing Orders For 3 Cylinder engine Cylinder in-line engine Cylinder V shape engine , , , , Cylinder No. 1 is taken from front of the in-line engines whereas in V shape front cylinder on right side-bank is considered cylinder No.1 for fixing H.T. leads according to engine firing order. B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 2

3 Vibration Damper The power impulses tend to set up a twisting vibration in the crankshaft. When a piston moves down on its power stroke, it thrusts through the connecting rod, against a crankpin with a force that may exceed 2 tons. This force tends to twist the crankpin ahead of the rest of the crankshaft. Then, as the force against the crankpin recedes, it tends to untwist, or move back into its original relationship with the rest of the crankshaft. This twist-untwist action, repeated with every power impulse, tends to set up an oscillating motion in the crankshaft. This is called Torsional vibration. If it were not controlled, it could cause the crankshaft to break at certain speeds. To control torsional vibration, devices which are called vibration dampers, or harmonic balancers, are used. These dampers are usually mounted on the front end of the crankshaft and the drive-belt pulleys are incorporated into them. A typical damper is made in two parts, a small inertia ring or damper flywheel and the pulley. They are bonded to each other by a rubber insert about 4-inch [6-mm] thick. The damper is mounted to the front end of the crankshaft. As the crankshaft speeds up or slows down, the damper flywheel has a dragging effect. This effect, which slightly, flexes the rubber insert, tends to hold the pulley and crankshaft to a constant speed. This tends to check the twist-untwist action, or torsional vibration, of the crankshaft. Rubber Ring Fig. A torsional-vibration damper B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 3

4 Engine Bearings Bearings are placed in the engine wherever there is rotary motion between engine parts. These engine bearings are called sleeve bearings because they are shaped like sleeves that fit around the rotating shaft. The part of the shaft that rotates in the bearing is called a journal. Connecting-rod and crankshaft (also called main) bearings are of the split, or half, type. The upper half of a main bearing is installed in the counter bore in the cylinder block. The lower half is held in place by the bearing cap. The upper half of a connecting rod big end (or crankpin) bearing is installed in the rod. The lower half is placed in the rod cap. The typical bearing half is made up of a steel or bronze back, with up to five linings of bearing material. The bearing material is soft therefore, the bearing wears, and not the more expensive engine part. Then, the bearing, and not the engine part, can be replaced when it has worn too much. Fig. A typical thrust bearing half Fig A- Thrust-type main bearing and a connecting rod bearing, showing their positions on the crankshaft Fig. Typical sleeve-type bearing half with its parts named B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 4

5 Thrust bearing The crankshaft has to be kept from moving back and forth in the block. To prevent back-andforth movement, one of the main bearings is a thrust, or end-thrust, bearing. This bearing has flanges on its two sides. Flanges on the crankshaft fit close to the flanges on the thrust bearing. If the crankshaft tends to shift forward or backward, the crankshaft flange comes up against the thrustbearing flange. This prevents endwise movement. Bearing Lubrication Oil from the engine oil pump flows onto the bearing surfaces. The rotating shaft journals are supported on layers of oil. The journal must be smaller than the bearing so that there is a clearance (called oil clearance) between the two. In the engine oil moves through this clearance. The lubricating system feeds oil to the main bearings. It enters through the oil holes and the rotating journals carry it around to all parts of the bearings. The oil works its way to the outer edges of the main bearings. From there, it is thrown off-and drops back into the oil pan. The oil thrown off helps lubricate other engine parts, such as the cylinder walls, pistons, and piston rings. The connecting-rod bearings are lubricated through the oil holes drilled in the crankshaft. As the oil moves across the faces of the bearings, it also helps to cool them. The oil is relatively cool as it leaves the oil pan. It picks up heat in its passage through the bearings. This heat is carried down to the oil pan and released to the air around the oil pan. The oil also flushes and cleans the bearings. It flushes out particles of grit and dirt from the bearings. The particles are carried back to the oil pan by the oil. They then settle to the bottom of the oil pan, or are removed from the oil by the oil screen or filter. B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 5

6 Bearing Oil Clearances The greater the oil clearance, the faster oil flows through the bearing. Proper clearance varies with different engines, but inch [0.037mm] is a typical clearance. As the clearance becomes greater (owing to bearing wear, for example), the amount of oil flowing through and being thrown off increases. With a 0.003inch [0.076-mm] clearance only twice inch [0.037 mm], the oil throw off increases as much as five times. A 0.006inch [0.152-mm] clearance allows25 times as much oil to flow through and be thrown off. As bearings wear, more and more oil is thrown onto the cylinder walls. The piston rings cannot handle so much oil. Part of it works up into the combustion Fig. Oil clearance (exaggerated) between the chambers, where it burns and forms carbon. main bearing and the crankshaft journal Carbon deposits in the combustion chambers reduce engine power and cause other engine troubles. Excessive oil clearances can also cause some bearings to fail from oil starvation. An oil pump can deliver only a certain amount of oil. If the oil clearances are excessive most of the oil will pass through the nearest bearings. There won't be enough for the more distant bearings. Then these will probably fail from lack of oil. An engine with excessive bearing oil clearances usually has low oil pressure: The oil pump cannot build up normal pressure because of the large oil.clearances in the bearings. If bearing oil clearances are too small, there will be metal to- metal contact between the bearing and the shaft journal. Very rapid wear and quick failure will result. Also, there will not be enough oil throw off to lubricate cylinder walls, pistons, and rings. B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 6

7 Bearing Requirements Bearings must be able to do other things besides carry loads. Some of these are listed below. 1. Load Carrying Capacity-Modern engines are lighter and more powerful. They have higher compression ratios which impose greater bearing loads. Only a few years ago, bearing loads were around 1600 to 1800 psi [11,032 to 12,411kPa]. Today, connecting-rod bearings carry loads of up to 6000 psi [41,369 kpa]. 2. Fatigue Resistance-When a piece of metal is bent back and forth, over and over, it hardens and finally breaks. This is called fatigue failure. You have probably done this with a piece of wire or sheet metal. Bearings are subject to such loads and must withstand them without failing from fatigue. 3. Embedability This term refers to the ability of a bearing to permit foreign particles to embed in it. Dirt and dust particles enter the engine despite the air cleaner and oil filter. Some of them work onto the bearings and are not flushed away by the oil. A bearing protects itself by letting such particles sink into, or embed in, the bearing lining material. If the bearing were too hard to allow this, the particles would lie on the surface. They would scratch the shaft journal and probably gouge out the bearing. This would cause overheating and rapid bearing failure. Therefore, the bearing material must be soft enough for adequate embedability. 4. Conformability This is associated with embedability. It is the ability of the bearing material to conform to variations in shaft alignment and journal shape. For example, suppose that a shaft journal is slightly tapered. The bearing under the larger diameter will be more heavily loaded. If the bearing material has high conformability, it will "flow" slightly, from the heavily loaded areas to the lightly loaded areas. This slight flow evens the load on the bearing. A similar action takes place when foreign particles embed in the bearing. As they embed, they displace bearing material, producing local high spots. However with high conformability, the material flows away from the high spots. This prevents local heavy loading that could cause bearing failure. 5. Corrosion resistance - the by-products of combustion may form corrosive substances harmful to some metals. Bearing materials must be resistant to corrosion. Unleaded gasoline, required on cars using catalytic converters, changes the chemistry of the engine oil. Catalytic converters, are installed in the exhaust systems to reduce the pollutants coming out the tail pipe. The unleaded gasoline, in changing the chemistry of the oil, tends to increase bearing corrosion. Therefore, the composition of engine bearings has been changed. For example, instead of the copper-lead bearings used for years, some engines now have aluminum-lead bearings. These appear to withstand corrosion better. 6. Wear Rate The bearing material must be so hard and tough that it will not wear too fast. At the same time, it must be soft enough to permit good embedability and conformability. B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 7

8 Fig. Effect of a metallic particle that is embedded in the bearing material Fig. The crankshaft has oil holes drilled through it to carry lubricating oil from the main bearings to the connecting-rod bearings Fig. Standard crankshaft for a V-6engine (left) compared with a V -6 crankshaft with splayed crankpins (right) B Dinesh Prabhu, Asst. Professor, P E S College Engg., Mandya, KARNATAKA 8