Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully. Sluggish valve action can be caused by dirty pistons or valve spindle guides and the like. In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances.
The ratio between the total surface area of the cylinder space and the volume of the space is such that as the cylinder dimensions increase the ratio between the values decrease.
In a small engine this means more heat is lost to the cylinder space surface during compression than in a larger engine. For this reason smaller engines require a higher compression ratio than larger engines.
An engine started in low ambient temperatures without preheating requires a higher compression ratio than an engine started in higher ambient temperatures.
The factors considered by the designer are therefore the cylinder dimensions and the ambient starting temperature of the engine’s operating environment. Common values of diesel engine compression ratios are as follows.
Slow-speed two-stroke cycle engine used for ship propulsion 12:1. Medium-speed turbocharged four-stroke cycle engine used for propulsion purposes 12:1.
Emergency electrical generator set 14:1 to 16:1.
Small, high-speed, naturally aspirated four-stroke cycle automotive engine fitted with glow plugs up to 23:1.
Note Large engines are usually preheated by raising the temperature of the cooling water. This aids starting and reduces cylinder liner wear. The lubricating oil is also preheated to reduce its viscosity and to assist starting by reducing the friction in bearings.
For cylinders with identical proportions, the total area of the cylinder surfaces varies as the square of the linear dimensions, and the volumes vary as the cube of the linear dimensions .
In normal engines the combustion chamber is formed in the space between the cylinder cover and the piston crown. The upper part of the cylinder liner usually forms the periphery to the space.
The shape of a combustion chamber may vary between that of a spheroid which will be formed from a concave piston crown and cylinder cover, to that of an inverted saucer, formed from a concave cylinder cover and a slightly convex piston crown. In opposed-piston engines the combustion chamber will be spheroidal. The piston crowns on the upper and lower pistons are usually identical in form. Combustion chambers of the shapes mentioned are referred to as open types.
The shape of a combustion chamber must be such that all parts of the space are accessible to the fuel sprays. If any part is not accessible, the space is wasted and combustion has to take place in a reduced space, which causes further difficulties due to less air being available in the region of the fuel spray. The wasted space is sometimes referred to as parasitic volume. The shape of the various parts must also be satisfactory in respect of their strength as they must be able to withstand the pressures in the cylinder without flexing.
With high-speed engines, open combustion chambers can create problems with very high rates of pressure rise due to the shortness of time available for injection and combustion. To overcome this problem the fuel is injected into a separate chamber which is connected to the main combustion chamber by a restricted passage. The restricted passage is at a high temperature, the fuel spray is long and narrow. Following injection the fuel commences to burn in the separate chamber and issues from the restricted passage at a high velocity due to the pressure rise in the chamber. The fuel enters the main combustion chamber as burning vaporized particles and combustion is then completed. The small chamber is about one-third of the clearance volume and is called a pre¬combustion chamber or antechamber. Its use allows high-speed engines to operate over wide speed ranges without combustion difficulties, and is a necessity in automotive engines. It is met in the marine field when automotive engines are used for electrical generation or other auxiliary purposes.
Diesel engine bearings are lubricated by fluid Aims. The journal is always smaller than its surrounding bearing. When the shaft is static it will make contact with the bearing and this contact will be a line. On each side of this line the normal distance between the shaft and the bearing will increase gradually and will in effect be a curved wedge. When the shaft revolves in the presence of an adequate liquid supply (lubricating oil), the oil is pulled into the wedge and pressure is set up. If these liquid pressures were to be plotted on lines drawn -radially from the centre of the bearing.it would be seen that the plot of these pressures would form a bulge something like a cam profile. The pressure of liquid in the wedge-shaped space sets the shaft over to one side and lifts the shaft away from the bearing so that it is supported on an oil film. The position where the oil film thickness is least will be a small distance away from the static contact line in the direction of shaft rotation. For pressures to be built up to a value high enough to separate the shaft from the bearing, the oil must have sufficient viscosity and the speed of the shaft must be above a certain value. This form of lubrication is referred to as fluid film or hydrodynamic.
Boundary lubrication occurs when the rotational shaft speed falls and the oil wedge is lost. Metal to metal contact then occurs. To prevent metallic contact under boundary conditions greases may be used or additives may be added to the oils. The bearings of a diesel engine do not work under boundary conditions. Very highly loaded crosshead bearings in two-stroke engines may approach boundary conditions.
Diesel engine bearings are lubricated by oil films built up under the conditions described. The bearings are supplied with large amounts of oil which are used to maintain the oil Aim and remove the heat generated. Removal of the heat generated keeps the working parts at temperatures that will not reduce the oil viscosity to values low enough to allow breakdown of the oil Aim.
The value of the maximum load on a cylinder cover and piston will be approximately the same, and will be the product of the area of the piston and the maximum gas pressure. In the case of the cover area it will be the projected area of the cylinder cover measured to the outer edge of the joint spigot. Ill some engines this may be considerably more than the piston area.
Cylinder diameter = 980mm Maximum pressure = 78.5 bars (80 kg/cm2)
Maximum load * 0.7854 x 982 x 80/1000 tonnes = 600 tonnes approx
The load on the cylinder cover is transmitted into the cylinder beam through the cover studs. The load on the cylinder beam is passed down to the bedplate through the tie-bolts and transverses supporting the crankshaft main bearings. The upward direction of the forces on the cylinder cover is balanced by the downward forces on the piston, which are transmitted through the piston-rod, crosshead block, bearings, connecting-rod, crankshaft and crankshaft main bearings.
The system of parts may be likened to a square flat plate tied to a square frame below it by four long tie-bolts at the corners. If a round shaft is placed across the frame, bearing on the two sides, and a jack is inserted between the shaft and the plate, and loaded, we may say that the system of engine parts is
simulated. The load on the jack, which is simulating the firing load on the piston and transmission of the load to the shaft, produces tension in the tie- bolts, a bending moment on the shaft and a bending moment on the two sides of the frame supporting the shaft. The fiat plate at the top will also be subjected to a bending moment. The actual parts of an engine will be subjected to the same loads as the simulation rig. The engine tie-rods will be in tension and the trans- verses supporting the main bearings will be subjected to a bending moment. The cylinder beam will also be subjected to bending moments.
Note The gas load coming on to the cylinder cover studs will be calculated on the area to the outer edge of the spigot. The tension on the tie-bolts from gas load will be calculated on the area of the piston.
The stresses coming about from the bending moment on the shaft in the simulated rig will be additive to the other stresses set up during engine operation.