This range of engines covers a broad spectrum of applications and fuels, for marine and land use, stationary and propulsion. There is a wide range of 2- and 4-stroke engines available covering power requirements from around 1 MW to 100 MW. Many of these engines are used for marine propulsion. Not only may there be problems due to variable quality of the fuel in this application, but the environment will not be kind to machinery.
5.2.1 Low Speed 2-Stroke Marine Diesel
Large tankers and container ships require powerful engines that are very reliable, with several years between overhauls. Their engine speeds range from 120 rpm down to 60 rpm for the larger cylinders, where the engine stroke is around 3 m and the bore around 0.6 m. There may be 12 or more cylinders, each producing in up to 4 MW. Figure 5.2 [2] shows an section through an example of a 2-stroke marine diesel engine.
Note the extreme engine height in comparison with the bore diameter.
FIGURE 5.2 Section through a crosshead-engine.
FIGURE 5.3 Crosshead-engine connecting rods.
These engines have a long piston stroke and a crosshead is placed between the piston and the crankshaft, effectively creating a taller piston. Both the crankshaft and crosshead journal bearings will be traditional white-metal bearings as used on smaller engines. The crankshaft thrust bearing will be of the tilting-pad type. Typical crosshead connecting rods are shown in Figure 5.3 [3]. The crosshead bearing may be hydrostatically lubricated. The taller cylinder will need lubricant fed via quills through the cylinder wall as splash lubrication would not effectively reach the upper cylinder walls. The cylinder head valves may be operated hydraulically or by solenoids, removing the need for a traditional camshaft.
Engine designs are aimed at economical operation, but the control of exhaust emissions is increas- ingly important. Marine diesel nitrogen oxides (NOx ) emissions need to comply with Annex VI of the International Convention for the Prevention of Pollution from Ships 1973/78, commonly known as the MARPOL Convention (www.imo.org) [4]. NOx levels must not exceed 9.8 g/kWh above 2000 rpm, rising to 17 g/kWh below 130 rpm. MARPOL was developed by the International Maritime Organisation (IMO).
This requirement may be met by control of compression ratio, timing of the fuel injection and valves, plus injector nozzle optimization. Rail fuel-injection is commonly used as it allows greater control of fuel volume and timing, for improved efficiency and reliability. Nonmarine standards are tougher and all applications will probably facer lower limits in the future.
Service intervals are largely determined by piston ring and bore wear. Liner material choice and ceramic coatings on piston rings are assisted by good design. A modern piston design is shown in Figure 5.4 [1].
Although the piston rings have similar designs to automotive rings, the piston is made from steel and the ring grooves may be chrome-plated. Careful liner honing and upper cylinder lubricant injection improve the extent of a good lubricant film. Reducing the piston-bore clearance above the top ring, which can be achieved by the use of a separate liner insert, minimizes the accumulation of carbonaceous deposits that can encourage bore polishing. Cooling of the piston crown is assisted by the delivery of lubricant
Diesel, Dual-Fuel, and Gas Engines 5-5
FIGURE 5.4 Sulzer Tribopack.
FIGURE 5.5 Cooling bores below the piston crown.
through the connecting rod to bores within the underside of the crown. The inverted piston crown in Figure 5.5 [2], shows such cooling bores. Piston crowns may also be water cooled by the cocktail-shaker method, using water delivered to a partially air-filled space where it can freely splash.
Good engine rigidity is important to ensure good alignment of all bearing surfaces. The cylinder jacket may be formed from a single cast iron unit or assembled from separate blocks. Access hatches must be provided for engine servicing.
FIGURE 5.6 Composite piston with steel crown and cast iron skirt.
The combustion chamber design is critical to achieve low surface temperatures and good lubrication.
With large volumes of injected fuel, impingement is to be avoided and efficient combustion requires good injector nozzle design. Controlled cooling of the liner is accompanied by the aforementioned oil-cooling of the underside of the piston crown.
5.2.2 Medium Speed 4-Stroke Marine Diesel
To achieve a more compact engine than the low-speed marine diesel, it is wise to demand a shorter piston stroke, which also allows for higher engine speeds for a given maximum piston velocity. Using an almost square configuration, with bore and stroke around 0.5 m and an engine speed of 500 rpm, a cylinder output of 1 MW is possible using a 4-stroke design. Trunk-type pistons are used, which are much more compact than crosshead pistons. They are basically similar to smaller automotive diesel engine pistons.
The piston traditionally has a steel crown and nodular cast iron piston skirt, as shown in Figure 5.6, with two compression rings and one oil scraper ring. Additional pressure lubrication of the skirt may be used in cases where severe running conditions are expected.
Raising engine speed, rather than cylinder pressure, means that turbochargers can be used to maximize the gains of the Miller cycle, with which there can be gains in engine efficiency by leaving the intake valve open for part of the compression stroke, reducing the work done by the piston. This results in lower compression end temperatures, leading to reduced NOx emissions [5]. Higher engine speeds demand superior balancing of the crankshaft and adjustable balance weights are provided. The crankshaft bearings may have temperature sensors fitted. Three-piece connecting rods (see Figure 5.7 [4]) are used to facilitate overhaul of the piston without disturbing the big-end bearing, and vice versa.
5.2.3 Medium Speed 4-Stroke Multi-Fuel/Diesel
Multi-fuel engines can run off gas oil, marine diesel fuel, intermediate fuel, heavy fuel, crude oil, and gas, allowing the most economical available fuel to be used. These fuels, however, may contain chemically
Diesel, Dual-Fuel, and Gas Engines 5-7
FIGURE 5.7 Three-piece connecting rod.
aggressive components, such as vanadium, sodium, and sulfur. Low temperature corrosion may occur due to the condensation of sulfuric acid. High temperatures, exceeding 430◦C, encourage the reactions of vanadium and sodium. The exhaust valves are particularly vulnerable to such temperatures.
Piston ring and cylinder wear is a major issue and good lubrication is an obvious requirement, but also bore polishing must be avoided as this increases lubricant consumption. A cylinder insert to reduce piston–bore clearance at the top land can reduce this problem by limiting the formation of carbonaceous deposits.
Wear in the cylinder head can be reduced by fitting rotators to both the inlet and exhaust valves, to positively encourage normal rotation and to distribute thermal and mechanical loading. The lobes on the camshaft will be designed to minimize vibration of the tappet rollers.
Exhaust emissions have to be controlled and the reduction of NOx can be achieved by direct water injection, which uses latent heat to lower the gas temperature, and by controlling the air–fuel properties to optimize combustion timing. Modern engines increasingly uses common rail technology, and control and monitoring is heavily automated.
5.2.4 4-Stroke Dual-Fuel
Dual-fuel engines run on natural gas or light fuel oil and may change from one fuel to the other during operation. These engines are used for propulsion as well as electrical power generation at 50 or 60 Hz.
Lean-burn technology is available for these fuels, with more air made available than is necessary for complete combustion. This lowers peak temperature and so reduces the production of NOx. The combustion begins with normal ignition of pilot fuel (light fuel oil [LFO]) that can be followed by either fuel as the main charge. Individual cylinder control is important to optimize the combustion. When full gas running is in operation, the LFO pilot fuel contributes less than 1% of the fuel consumed. Air–fuel ratios around 2.0 to 2.2 allow high thermal efficiency without the risk of knocking.
Pistons, connecting rods, and crankshafts are typically built to the same design as the diesel engines described above. However, the cleanliness of the gas combustion, when used, in comparison to diesel means that the operating conditions may be less detrimental to engine components and the demands on the lubricants less taxing.
5.2.5 4-Stroke Medium Speed Gas
Electrical power generators can make use of lean-burn, spark-ignited gas engines using the Otto cycle.
With bores and strokes in the range of 300 to 400 mm and up to 20 cylinders in a V-configuration, electrical power generation can approach 10 MW. The generating efficiency lies around the 50% region, but the heat output is used in cogeneration to produce an overall efficiency exceeding 90%. This efficiency gain can impose greater load on the engine and thus raise the specification requirements of the lubricant.