What do you understand by the following terms: swept volume, clearance volume, compression ratio, volumetric efficiency, scavenge efficiency, air charge ratio, natural aspiration, supercharging? What other names are used for the supercharging process?

Swept volume. This term refers to the volume swept by the piston during one stroke and is the product of the piston area and stroke.
Clearance volume is the volume remaining in the cylinder when the piston is in the top-centre position. The difference between the total cylinder volume and the Swept volume is equal to the clearance volume. The clearance volume space forms the combustion chamber.
Compression ratio. This is the value obtained from dividing the total cylinder volume by the clearance volume and will be from 12 to 18, depending on the engine design. If the compression ratio is below 12 the engine may be difficult to start. High speed engines with small cylinders usually have high compression ratios. Slow speed direct-propulsion engines have compression ratios of around 14.
Volumetric efficiency. This is the ratio of the volume of air drawn into the cylinder (at normal temperature and pressure) to the swept volume. In naturally aspirated four-stroke engines the volumetric efficiency will be from 0.8S to 0.95.
Scavenge efficiency. This is the ratio of the volume of air (at normal temperature and pressure) contained in the cylinder at the start of compression to the volume swept by the piston from the top edge of the ports to the top of its stroke.
Air charge ratio. This is the ratio of the volume of air (at normal temperature and pressure) contained in the cylinder at the start of compression to the swept volume of the piston. This term has now more or less replaced the previous two terms. It is spmetimes referred to as air mass ratio or air supply ratio. In four- strolfe engines the value will vary from 0.85 for naturally aspirated types up to 4 or more in highly supercharged engines. In two-stroke engines the value will.be from 0.85 for simple engines with ported scavenge and exhaust, up to 2.5 for supercharged engines. •
Natural aspiration is a term applied to four-stroke engines where the air charge is brought into the cylinder only by the downward movement of the piston without other aids.
Supercharging is a term used to indicate that the weight of air supplied to the engine has been considerably increased. This allows more fuel to be used per stroke with a consequent increase in engine output power. More power is developed by a supercharged engine than by a non-supercharged engine of the same bore, stroke and speed. Supercharging has had the effect of lowering the specific weight of diesel engines, i.e. more horsepower is obtained per ton of engine weight. The term pressure-charging is now used generally instead of supercharging. Where use is made of an exhaust-gas turbo-driven compressor, the term turbocharging is often used.

What is the flash-point of an oil and what dies it indicate?

The flash-point is the lowest temperature at which an oil will give off suffi¬cient inflammable vapour to produce a flash when a small flame is brought to the surface of the oil. The flash-point may be measured as an open or closed
flash-point figure. Fuels for use aboard ships are tested in a Pensky-Martens instrument which measures the closed flash-point. The Department of Trade & Industry sets the lower limit of 65 °C for the flash-point of fuels used aboard merchant vessels and also stipulates that fuel in storage tanks must be kept at temperatures at least 14eC lower than its flash-point. The flash-point of an oil gives no indication of its suitability for use in a diesel engine. It only serves as a guide to the temperature below which it can be stored and handled with reasonable safety. A knowledge of the flash-point of the lubricating oil used in the crankcase of a diesel engine is useful, since lowering of the flash-point inducates that the lubricant may be contaminated with fuel.