Marine Internal Combustion Engines Lecture 1 PDF

Rudolf Diesel cooperated with Sulzer to construct the first Sulzer diesel engine in 1898 Marine Internal Combustion Engines Lecture 1: Introduction to Marine ICE Selandia (1912): The Worl...

Rudolf Diesel cooperated with Sulzer to construct the first Sulzer diesel engine in 1898 Marine Internal Combustion Engines Lecture 1: Introduction to Marine ICE Selandia (1912): The World's First Oceangoing Diesel Vessel In 2000’s the diesel engine dominated the merchant ship propulsion markets. Tonnage 6,800 dwt; 4,964 GRT Length 370 ft (112.8 m) Beam 53 ft (16.2 m) Speed 12 knots  DM8150X engine (MAN B&W Diesel) commissioned (1912) to power the first Selandia Installed power = 2Xengines(~10X2X8 meters) 2 x eight-cylinder, four-cycle, 1,250 hp diesel engines After 100 years, the engine volume is 7 Selandia’s, while the transported cargo is 13 times DWT: 88700 TEU: 6802 Reefer: 710 Loa: 299.99 (BB) Bmd: 42.80 Draft: 14.04 Engine: 1x oil Sulzer 12RTA96C 23X4X13.5 meters Power: 65880 kW Speed: 24.5 kn Say bye-bye to steam engines Steam turbines were the prevailing prim movers before diesel engines Examples include low and medium speed engines from large containerships, bulk carriers, VLCCs and cruise liners. Recently, even the remaining steam engines(seen on board LNGs) have been replaced by new dual-fuel diesel engine designs arranged to burn the cargo boil- off gas. The remorseless rise of the diesel engine at the expense of steam reciprocating and turbine installations was symbolized in 1987 by the steam-to-diesel conversion of Cunard’s prestigious cruise liner Queen Elizabeth 2. Cunard’s cruise liner Queen Elizabeth 2 (1987) a major refit at the Lloyd Werft yard at Bremerhaven in Germany transforming her from an ageing steamship to a state of the art diesel-electric motor ship. This refit, costing $162m, was hugely successful and allowed the ship to sail on for another 20 years as the Cunard flagship, the only transatlantic liner and the fastest and most powerful merchant ship in the world.  Engine mechanism Piston-cylinder(combustion chamber)-connecting rod-Crank shaft  Engine mechanism or wrist pin Piston-cylinder(combustion chamber)-connecting rod-Crank shaft  Engine mechanism Connecting rod and its Bearing  Engine mechanism Crank shaft or offset  Engine mechanism Crank shaft  Crosshead versus trunk piston engines Crosshead pistons: for low speed ( high power and torque engines ) The crosshead is a forged steel block secured to the foot of piston rod low lateral pressure so shorter skirt The name "crosshead" is derived from its shape. It typically has a cross-like or T-shaped configuration. The lower end of the piston rod is connected to a cross head, while the cross head is connected to the connecting rod by a crosshead pin. The crosshead slides up and down along the crosshead guides. This type of engines are mainly used in large 2 stroke engines and, particularly, in double acting engines  Crosshead versus trunk piston engines Crosshead pistons:  Crosshead versus trunk piston engines Crosshead pistons:  Crosshead versus trunk piston engines Trunk pistons:  Crosshead versus trunk piston engines Trunk pistons:  Crosshead versus trunk piston engines Trunk pistons: Usually, have lengths that are greater than their diameter. Top end of the trunk type piston is closed; this end is called the crown. The opposite or skirt end of the piston is open. The connecting rod is attached to the piston by means of the piston pin (wrist / Gudgeon pin).  Crosshead versus trunk piston engines Trunk pistons: The power of combustion is lower than that of a crosshead installations and thus it is only the piston who absorbs the lateral forces and transfer them to the cylinder liner and consequently to the engine frame This type of pistons is installed for medium and high speed main engines and generators that we have on board ships Trunk Pistons are usually installed for 4-stroke engines  Crosshead versus trunk piston engines Trunk pistons: Trunk piston example: Four stroke medium speed trunk piston  Crosshead versus trunk piston engines Trunk pistons: follower  Crosshead versus trunk piston engines Trunk pistons:  Crosshead versus trunk piston engines Trunk pistons:  Advantages and disadvantages of Crosshead versus trunk piston engines Crosshead pistons are easily lubricated, reduce liner wear and ensures uniformly distributed clearance around piston Crosshead pistons are simply constructed as it eliminates the use of gudgeon pin and its bearings that are inevitably installed to trunk pistons.  Engine operation= Sequence of events (cycle) 2-stroke versus 4-stroke cycle What do we mean by Stroke??? Stroke(=piston stroke): the travel of the piston between its extreme points (TDC and BDC). Each stroke is accomplished in half a revolution of the crankshaft. 4-stroke cycle The “four-stroke cycle” is completed in four strokes of the piston (or two revolutions of the crankshaft) This principle is generally applied to medium to high speed diesel engines employed in ship propulsion 4-stroke cycle The “four-stroke cycle” is completed in four strokes of the piston (or two revolutions of the crankshaft) Intake/suction stroke Power/expansion Exhaust stroke Compression stroke /working stroke TDC TDC TDC TDC BDC BDC BDC BDC Description of the 4-stroke cycle Consider the piston at a position known as top dead center (TDC). The inlet valve opens and fresh air is drawn in as the piston moves down At the bottom of the stroke (BDC) the inlet valve closes and the air in the cylinder is compressed (and consequently raised in temperature) as the piston rises. Fuel is injected as the piston reaches (TDC) and combustion takes place, producing very high pressure in the gases. The piston is now forced down by these gases and at (BDC) the exhaust valve opens. The final stroke is the exhausting of the burnt gases as the piston rises to (TDC) to complete the cycle Sequence of events: 4-stroke 4-stroke cycle timing diagram Sequence of events: 4-stroke 4-stroke cycle timing diagram  Engine operation= Sequence of events (cycle) 2-stroke versus 4-stroke cycle 2-Stroke cycle The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. air pump or blower 2-stroke cycle The difference between the two-stroke and four-stroke engine is in the method of removing the burned gases (gas exchange) and filling the cylinder with a fresh charge of air. In a two-stroke engine, gas exchange occur when the piston is near the BDC by-means of a separate air pump or blower In 2-stroke cycle, gas exchange is called scavenging This principle is generally applied to slow speed diesel engines employed in ship propulsion 2-stroke cycle: explained when the piston has traveled 80 to 85 percent of its expansion stroke, exhaust valves (e, e) are opened and the exhaust gases escape the cylinder. The piston continues to move toward BDC and soon uncovers ports (s, s) through which lightly compressed air enters the cylinder. The air, having a slightly higher pressure than the hot gases in the cylinder, pumps out the hot gases through valves (e, e). This operation is called scavenging. The air admitted is called scavenge air; the air admittance ports are called scavenge ports. About the time when the piston, on its upward stroke, closes ports (s, s), the exhaust valves (e, e), are also closed, and the compression stroke begins. Sequence of events: 2-stroke 2-stroke cycle timing diagram Advantages and disadvantages of 2-stroke versus 4- stroke cycle 1. Elimination of one scavenging (exhaust) and one charging (intake) stroke required in a 4- stroke cycle operation 2. The 2-stroke engines, theoretically, develop twice the power of a 4-stroke engine of the same swept volume and crank RPM. Notice the parameter “i” in the following equation: Z∙n∙VS ∙pE PE = i Where, pE = Effective/Shaft mean pressure measured at the crank shaft end/flywheel PE = Effective/Shaft engine power measured in the cylinder shaft end/flywheel Vs = π/4 x D2 x S = stroke or swept volume i = 1 for 2-stroke n = engine/crank shaft speed [rps] i = 2 for 4-stroke Z= number of cylinders D and S are piston diameter and stroke Notice: capital P is power and small p is pressure Advantages and disadvantages of 2-stroke versus 4- stroke cycle 3. The 2-stroke engines, practically, develop 1.6-1.7 the power of a four-stroke engine of the same swept volume. This is due to inefficient scavenging and other losses. 4. For a particular engine power the 2-stroke engine will be considerably lighter, which is an important consideration for ships 5. The 2-stroke engine does not require the complicated valve operating mechanism that is usually fitted to the 4-stroke engines. Advantages and disadvantages of 2-stroke versus 4- stroke cycle 6. In order for scavenging in 2-stroke engine to be successful the intake air pressure should be higher than that of exhaust gases inside the cylinder. 7. The film of lubricating oil on the piston and the cylinder liner is better maintained in 4-stroke engines 8. The 4-stroke engine however can operate efficiently at high (shaft and piston) speeds which offsets its power disadvantage; it also consumes less lubricating oil. 9. The thermal load of 2-stroke engines is higher than that of the 4-stroke ones due to the shorter time between consecutive high temperature power strokes. Engine speeds: Low, medium and high speed marine Diesel engines Low speed marine diesel engines: Max RPM=~ 240 or 4 RPS Cylinder bores=~ 260:1080 mm Requires no gearbox or no reduction of the engine RPM to match the propeller RPM (RPMeng=RPMprop) Example for one of the largest low Large weight and volume speed diesel engine Shaft power 97,300 kw Operated according to the 2-stroke principle bore 1080 mm No. of cylinders 14 Installed with crosshead pistons stroke 2660 mm Dry weight 2300 ton Propel large oil tankers, ore tankers and container ships Length X height 28X14 m Engine speeds: Low, medium and high speed marine Diesel engines Medium speed marine diesel engines: RPM=~240:960 or 4:16 RPS Cylinder bores 200:640 mm Shaft powers up to 30,000 Kw A gear box is necessary to reduce the engine RPM such that it matches the propeller RPM (RPMeng>RPMprop) Operated according to the 4-stroke principle Installed with trunk pistons Few old 2-stroke and crosshead medium speed engines still exist Engine speeds: Low, medium and high speed marine Diesel engines High speed marine diesel engines: RPM≥~ 960 or ≥~ 16 RPS. Cylinder bores=~ 40:200~300 mm Shaft powers up to 5,000 Kw A gear box is necessary to reduce the engine RPM such that it matches the propeller RPM (RPMeng>RPMprop) Operated according to the 4-stroke principle Installed with trunk pistons  Scavenging (gas exchange) in 2 stroke diesel engines Scavenging: (cleaning the exhaust from the cylinder by a pressurized intake air charge) Each cylinder should be adequately scavenged of gas before a fresh charge of air is compressed, otherwise the fresh charge is contaminated by residual exhaust gases from the previous cycle  Scavenging (gas exchange) in 2 stroke diesel engines Cross-flow scavenging Loop scavenging Uniflow scavenging Scavenging (gas exchange) in 2 stroke diesel engines Gas Exchange Control Elements Piston Ports Valves (control slide)  Scavenging (gas exchange) in 2 stroke diesel engines Scavenging: (cleaning/purging the exhaust from the cylinder by a pressurized intake air charge) Gas exchange has to occur while the piston is near BDC. consequently: A portion of the expansion and compression stroke is unusable. Piston velocity is low during the entire gas exchange phase Gas exchange can only occur when the intake pressure is sufficiently higher than the exhaust pressure to allow the incoming fresh charge to displace the burned gas in the time available.  Scavenging (gas exchange) in 2 stroke diesel engines Cross-flow scavenging The exhaust ports are closed after the scavenge ports Some of the air charge escape from the cylinder Simplicity of construction and of maintenance (does not use Cross-flow scavenging valves which must be kept tight)  Scavenging (gas exchange) in 2 stroke diesel engines Return-flow/Loop scavenging The scavenge-air and exhaust-gas ports are located on one side of the cylinder Some of the air charge escape from the cylinder Simplicity of construction and of maintenance (does not use Return-flow / Loop scavenging valves which must be kept tight)  Scavenging (gas exchange) in 2 stroke diesel engines Uniflow scavenging The exhaust gases and scavenging air are flowing in the same direction with less chance for formation of turbulences which are unavoidable with cross- and return-flow scavenging. Best scavenge efficiency (in terms of pressure and time) Higher maintenance and complexities Uniflow scavenging due to the valves Opposed piston Engine description Two pistons in each cylinder. The pistons are arranged in opposed positions in the cylinder Piston action is so timed that at one point of travel the two pistons come into close proximity to each other near the center of the cylinder As the pistons travel against each other they compress air between them. Opposed piston Opposed piston Engine description The scavenging air ports are located in the cylinder walls at the top of the cylinder and are opened and closed by the movement of the upper piston. The exhaust ports are located near the bottom of the cylinder and are opened and closed by the movement of the lower piston. All the upper pistons are connected by connecting rods to the upper crankshaft. All the lower pistons are connected by connecting rods to the lower crankshaft. Opposed piston Opposed piston Engine description The space between the two pistons thus becomes the combustion chamber. The point at which the two pistons come into closest proximity is called combustion dead center. Just prior to combustion dead center, fuel is injected and the resultant expansion caused by combustion drives the pistons apart. Opposed piston Opposed piston Engine cycle Opposed piston cycle Opposed piston Engine cycle 1. Both pistons are on the return travel from outer dead center, the upper piston has covered the scavenging air ports, the lower piston has covered the exhaust ports, and compression has begun. 2. Just as both pistons approach combustion dead center, fuel is injected. 3. Injection has been completed, expansion has begun, and both pistons are moving toward outer dead center. Opposed piston cycle Opposed piston Engine cycle 4. Expansion of gases from combustion drives the pistons apart, causing the crankshafts to turn. This is the power stroke of the cycle. 5. As the pistons approach outer dead center, the lower piston uncovers the exhaust ports and most of the expanded gases escape. Just before reaching outer dead center, the upper piston uncovers the scavenging air ports and scavenging air rushes into the cylinder, cleaning out the remaining exhaust gases. 6. The lower piston has covered the exhaust ports and scavenging air supercharges the Opposed piston cycle cylinder until the upper piston covers the scavenging air ports. Opposed piston Engine cycle 4. Expansion of gases from combustion drives the pistons apart, causing the crankshafts to turn. This is the power stroke of the cycle. 5. As the pistons approach outer dead center, the lower piston uncovers the exhaust ports and most of the expanded gases escape. Just before reaching outer dead center, the upper piston uncovers the scavenging air ports and scavenging air rushes into the cylinder, cleaning out the remaining exhaust gases. 6. The lower piston has covered the exhaust ports and scavenging air supercharges the Opposed piston cycle cylinder until the upper piston covers the scavenging air ports. Advantages of Opposed piston Engine 1. For a brief interval, the exhaust ports are closed while the intake ports are open. Thus, a supercharging effect is achieved in this engine, by the lower crankshaft lead and scavenging action. 2. It eliminates the necessity of cylinder heads and intricate(=complex) valve mechanisms with their cooling and lubricating problems. 3. There are fewer moving parts. 4. Good accessibility for the inspection and repair of most parts, except of the lower crankshaft. Double-acting engines description Use both ends of the cylinder and both faces of the piston to develop power. Double-acting engines description The piston is of the crosshead-type and is usually shorter than that of single-acting engines. The piston is closed at both ends (=has upper and lower crowns) and has a rigid piston rod extending from the lower end. Both ends of the cylinder are closed to form a combustion chamber at each end of the piston. Double-acting engines description The piston rod passes through a stuffing box in the cylinder head of the lower combustion chamber. The stuffing box prevents leakage of pressure. Double-acting engine cycle Combustion occurs in the upper combustion chamber, and the pressure of the gases of combustion is applied to the top end of the piston during the downward stroke. At the completion of this stroke, combustion occurs in the bottom combustion chamber and expansion pressure is applied to the bottom end of the piston during the upward stroke. The downward power stroke serves as the compression stroke for the lower combustion chamber and the upward power stroke serves as the compression stroke for the top combustion chamber. Thus the power strokes are double that of a single acting engine and the engine is referred to as a double-acting type. Advantages and disadvantages of Double-acting engines versus single-acting With twice as many power strokes as a comparable single acting engine and, with other conditions being equal, it develops practically twice as much power per cylinder. Engine operation is smoother due to the fact that the expansion stroke in one combustion chamber of the cylinder is balanced or cushioned by the compression stroke in the opposite combustion chamber. The double-acting crosshead type of piston construction requires considerably more cylinder length than that of single-acting engine types. Consequently, It is difficult to include a high and bulky double-acting engine in confined machinery spaces. Advantages and disadvantages of Double- acting engines versus single-acting Many difficulties are encountered in effecting a tight seal where the piston rod passes through the stuffing box. Naturally aspirated versus turbo charged engines Natural aspiration The engine draws air by means of the piston movement. Naturally aspirated engines admits air at or below atmospheric pressure Naturally aspirated versus turbo charged engines Supercharging or turbocharging Intake air is pressurized and subsequently transported to the engine by means of a turbo charger/turbo blower Most of the diesel engines, nowadays, are turbo charged Naturally aspirated versus turbo charged engines Supercharging or turbocharging The turbo blower draws in ambient air by a centrifugal pump/compressor The pressurized air passes through air inlet manifold Finally, the pressurized air enters the cylinders once the air intake valves open Naturally aspirated versus turbo charged engines Supercharging or turbocharging Naturally aspirated versus turbo charged engines Supercharging or turbocharging Naturally aspirated versus turbo charged engines Supercharging or turbocharging