A possible method for reducing the combustion temperature is the dilution of the fuel/air mixture with relatively inert or non-combustible substances. Water can be defined as thermodynamically suitable for the dilution of fuel/air mixture since it has high latent heat of vaporisation, relatively low vapour pressure and low boiling points compared to maximum combustion temperature [15]. Water added to combustion presents a reactive substance in chemical reaction of hydrocarbon compounds with air upon which flame temperature and nitric oxide concentration are based.
The addition of water has both physical and chemical effects on combustion process, however the magnitude of these effects depends on how water is introduced into the combustion chamber. The physical effect can be presented as the temperature reduction and the micro-explosion effect, which occur in emulsified fuel. The chemical effects are the dilution of the reaction zone, which result in a longer ignition delay and the dissociation of water vapour to form OH radicals.
With injection of water into the intake air manifold, the water is added into the lean side of combustion process and is homogeneously distributed inside the combustion chamber (Fig.1)[17]. The gas temperature at the end of the compression stroke and consequently the combustion temperature decreases due to the combined effects of vaporisation (absorbing heat) and the relatively high specific heat of water[16]. On the other hand, by the dilution of the charged air, local concentration of oxygen will be reduced due to the low partial pressure of oxygen favouring further reduction in NOx emissions (Zeldovich mechamsm)[13][17]. The influence of these parameters on the temperature cannot be easily quantified, but seems to be disproportionate.
In summary, the effects of water addition on NOx reduction can be explained as:
・The added water vaporises by absorbing heat from the port wall, retake valve and cylinder wall. A small amount of heat can also be removed from the charge air. It is concluded that the water is more likely to cool hot surfaces rather than to cool the already relatively cool air.
・During the dilution of charge molar and energy densities by water vapour, concentration of O2 is reduced leading to lower flame temperatures. The importance of the reduced oxygen concentration on the NOx emission rate seems to be less significant than the effect of temperature reduction.
・The increase in the amount of water vapour inside the cylinder, which has higher specific heat than that of dry air leads to an increase of the overall specific heat of the intake charge. Hence peak compression and combustion temperatures are reduced.
4. EXPERIMENTAL SET UP
The expemnents are carried out on a naturally aspirated, three cylinder, four stroke, direct injection and high speed diesel engine. The engine specifications are given in table 1. A schematic layout of the engine and the associated equipment is illustrated in Fie. 2.
The water injection system used i nthe tests comprises a water supply pump and low pressure water nozzles. The required quantity of water is supplied in the form of fate atomised water spray penetrating into the intake air. The amount of water injected is controlled by varying the pump speed from 0 to 250 rpm (0-120 ml/min). Engine speed (fuel rack) is controlled by a linear actuator operated with an electronic servomechanism.
The engine is equipped with a data acquisition system for cylinder pressure measurement. Exhaust gas species, i.e. NOx, CO2, CO and O2, are measured by a Lancom 6500 gas analyser. Fuel consumption rate is recorded by using a volumetric fuel gauge. The variation in atmospheric conditions is considered negligible during the period of testing. Fresh water is used for all the tests.
