Q1. With reference to diesel engine NOx emissions and control:
(a) Explain how exhaust gas recirculation and direct water injection reduce engine NOx emissions but result in increased specific CO2 emissions. (6)
(b) Describe, with the aid of a sketch, a Selective Catalytic Reduction system, stating, with reasons, the monitoring and control systems required.
2018/MAR >> (a) Most candidates linked the formation of NOx with high maximum cylinder temperature during combustion but many did not seem to understand that Exhaust Gas Recirculation (EGR) and Direct Water Injection (DWI) served the purpose of increasing the specific heat of the combustion gases thus reducing the peak temperature and so reducing NOx formation.
>> A number of candidates confused specific heat with latent heat, and although that is part of DWI it is specific heat which is the main factor. Most candidates failed to mention the increase in CO2 formation which is due to the increase in fuel consumption because the reduction in peak temperature reduces cycle thermal efficiency. Some candidates mentioned that there was an increase in CO2 emissions because exhaust gas containing CO2 was returned to the cylinder. Common sense would have indicated that if exhaust gas containing CO2 was returned to the cylinder it would not pass out of the engine as emissions; such CO2 cannot be counted twice.
(b) Most candidates were able to sketch a Selective Catalytic Reduction system and explain how it functioned, but most descriptions were rather basic. However, some candidates described an SOx scrubber system; lack of knowledge or unable to read an exam paper correctly?
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