2 - Background

2.2 - Specific examples

2.2.2 - Mechanical design of heat engines

2.2.2.4 - Lubrification

The most widely used engine has the most complex lubrication system. This is because the reciprocating engine has many lubrication points, in particular the piston-compression rings-cylinder assembly or cylinder sleeve which is the most difficult to lubricate for two reasons:

- firstly, combustion must take place at temperatures over 2,000 K to maximise combustion efficiency;

- Secondly, to prevent the pistons from seizing up, the operating temperature for the oil used to lubricate the piston guides in the cylinders must not exceed 125ºC.

In this respect, the gas turbine is more practical since the blades do not come into contact with their respective housing.

2.2.2.5 - Cooling

Given the fact that burning oils lose their power to lubricate, and the configuration of the the reciprocating engine, then the parts that are exposed to the heat generated by combustion must undergo intensive cooling to prevent the oil from losing its lubricating capacity.

This cooling causes the forced evacuation of over 30% of the thermal energy in the fuel, thereby removing part of the input heat..

2.2.2.6 - Exhaust fumes

In reciprocating engines over 30% of the energy in the fuel is dissipated through the exhaust:

- in the form of heat, because as the expanded volume is equal to the compression volume, the expanded volume is not greater than the compression volume in a ratio proportional to the amount of heat added during combustion;

- in the form of partially or totally unburnt hydrocarbons, because the time lag for the fuel to change state completely was too short, in effect, 0.002 to 0.01 s. are not enough, and the air-fuel ratio is not always favourable to satisfactory oxygenation

In gas turbines, the time lags for the combustion gases to pass through are too short to allow expansion and cooling to occur which would limit exhaust losses. It follows that 70 to 80% of the thermal energy in the fuel is dissipated in the form of heat, except where this heat is partially transmitted to the compressed air through a heat exchanger.

******