Because of its compact form, the open combustion chamber system presents only a small surface area to the compressed air, and the amount of heat lost by conduction to the combustion chamber is relatively small. As a result, the temperature of the compressed air at the moment of injection is always considerably higher than the fuel’s self-ignition temperature, and the fuel ignites readily.
Unfortunately, the other combustion chamber systems present a large surface area to the compressed air, resulting in a large loss of heat. When the engine (and combustion chamber) is cold, the large difference in temperature between the compressed air and the combustion chamber causes a rapid transfer of heat from the air to the chamber. This heat loss, coupled with the fact that compression only raises the air temperature a certain amount above its initial temperature, makes starting a marine engine from cold a problem.
Many systems of overcoming this have been developed, the most common being:
• heater plugs or glow plugs
• thermostart devices
• the Ki-gass system, and
• the use of a volatile fuel.
Heater plugs or glow plugs for your marine diesel engine.
These units screw into the combustion chamber and supply additional heat to the air during the compression stroke. A heater element on the plug lies flush with the combustion chamber wall and, when prior to starting, a current of 20 to 35 amperes is supplied from the battery for from 30 to 40 seconds, this element glows bright red. If the engine is then cranked, some of the heat from the element will be transferred to the compressed air, giving a final air temperature that is high enough to ensure efficient ignition and combustion.
Heater plugs may be divided into two types – single-pole glow plugs and double-pole glow plugs. The difference lies in the terminal posts on the plugs – single-pole plugs have only one electrical terminal or connection, and double-pole plugs have two.
Single-pole glow plugs
Single-pole glow plugs are said to be connected in parallel. This means that the current is supplied from the battery to the single terminal of each plug, and the circuit is completed through the cylinder head to earth. In this system, full battery voltage is applied to each plug, and one plug can fail without affecting the others. Thus for a vehicle using a 12 volt electrical system, 12 volt single-pole glow plugs can be employed.
Double-pole glow plugs
Series connection is used with double-pole glow plugs, the current from the battery passing through one plug and on to the next. Thus the battery is connected to one terminal of the first plug, whose other terminal is connected to one terminal of the second plug. This system connects all the plugs, the free terminal of the last plug being connected either to earth on the engine, or earth at the battery.
Discover how to use glow plugs and make marine diesel engine repairs here.
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