THE EFFECT OF WATER INJECTION TO INLET AIR ON ENGINE EMISSIONS AND PERFORMANCE
Chrysanthos Chrysanthou*, Ehsan Mesbahi**, Peilin Zhou**
Environmental concern has promoted marine industry to undertake research and develop engines with low emissions and high efficiency. This study aims to investigate the effect of water on exhaust emissions, engine performance, fuel efficiency and combustion characteristics. The experiment was conducted with induction of water into the intake air manifold. Water addition is found very effective in reducing NOx emissions without deteriorating engine efficiency. As a result, the compromising relationship between NOx and SFOC is slightly improved at moderate W/F ratios. NOx is reduced by up to 40% with water addition while CO emission seems to decrease only at high power. Furthermore water addition tends to increase the concentration of CO2 in the exhaust gas and to decrease the percentage of excess oxygen.
Apart from emissions, engine performance was closely monitored during the tests. The results suggest that the use of water is a viable method, which could lead to practical development in the future.
Key words: Water Injection, diesel, engine emission
On the evolution of diesel engines over the past decades, all design measures applied aimed to improve thermal efficiency, engine reliability and overall efficiency. At present, diesel engines face another major challenge, i.e. control of harmful emissions. This study aims to investigate the potential of NOx emission reduction and to assess engine combustion efficiency and performance when water is sprayed onto the engine intake air.
Due to the high thermal efficiency of diesel engines, the emissions of carbon monoxide and unburned HC are limited. Although the high combustion temperature results in high thermal efficiency, it is highly conducive to NOx formation. Research results [1-4] showed that shipping is responsible for 7-20% of global NOx emissions; the proportion at busy shipping areas can be as high as 40%.
There have been a nmnber of legislative measures for marine emission control. On global basis, IMO has developed a new Annex (Annex VI) added to the MARPOL 73/78 convection. However, this regulation could become more strict in the near future according to the improvement and development in emission control strategies.
In order to meet the current and future engine emission regulations many methods have been developed and tested under laboratory conditions and on board ships . Unfortunately, it has been proven that it is difficult to reduce NOx emissions from a diesel engine without inversely affecting fuel efficiency, PM and HC emissions . At the moment, by combining different m-cylinder control (primary) measures, most of new "buildings" are able to meet the IMO NOx limits. The challenge is how far we can continue to improve the diesel combustion concept and still achieve reduction in NOx emissions.
The addition of water into engine combustioninthe form of emulsified fuels, direct water injection and air fumigation's is currently being re-examined and a remarkable effect has been shown experimentally inreduction of NOx [7-13]. This paper is a further contribution to the discussion of pros and cons of water addition into diesel combustion for emission control.
Introduction of water to the combustion of IC engines can be retrieved back to a century ago. In 1913, Hopkinson reported the successful use of water to internally cool various power plants . Later, after the World War I, water was used to increase the operating efficiency of the Otto engines. Following these attempts, Ricardo (1935) used water injection to reduce engine knocking behaviour . The work of Zeldovich discovered the relation between NOx formation and peak combustion temperature, that has prompted many investigators to examine the influence of water introduction on combustion generated NOx emissions. In fact, it was Kopa et al (1964) who first suggested that water could be suitable for the reduction of noxious emissions . A growing interest in the potential of utilising water to mainly reduce NOx emissions has been evident throughout the last three decades.
3. THE EFFECT OF WATER ON NOx FORMATION
NOx is formed in the presence of O2 and N2 at high temperatures, therefore by lowering the combustion temperature, a reduction in NOx emissions can be achieved.
* Louis Cruis Lines, 5-7 Kanari Street, 185 38, Piraeus, Greece
** Dept. of Marine Technology, University of Newcastle upon Tyne, UK, NE1 7RU