Formaldehyde Emission Characteristic From Diesel/Methanol Compound Combustion Engine

Methanol as alternative fuel for vehicles in China has a broad prospect. Especially, the proposing of diesel / methanol compound combustion (DMCC) provided new and more effective approach for the development of methanol engine. Formaldehyde as the main pollutant from methanol fuel engine will do great harm to human health. To protect the environment, achieve no or low formaldehyde emission and promote DMCC, formaldehyde emission must be controlled. In order to control formaldehyde emissions, the concentration of formaldehyde from the exhaust must be detected accurately.There are various ways to detect the formaldehyde, such as spectroscopy, spectrophotometry, electrochemical sensor and chromatography. Concerning of the advantages and disadvantages of these ways, the complexity of the exhaust and the instability of the formaldehyde, gas chromatography (GC) is ultimately used to analyze formaldehyde from the engine exhaust. Namely, formaldehyde from the engine exhaust is absorbed by the acidic saturated solution of 2,4-dinitrophenyl hydrazine (DNPH) and converted to its hydrazone, the hydrazone is then extracted with CS2 and determinated using gas chromatography.According to this detection method, we designed a collection engine exhaust formaldehyde sampling devices, and finally determined this sampling system through repeated experiments. This sampling system and detection methods can be used together to detect formaldehyde content from the engine exhaust accurately.This system was used to study the characteristics of formaldehyde emission from the exhaust of DMCC engine. Experimental engine was reconstructed from a diesel engine to DMCC engine. This engine can fully achieve DMCC. On the experimental process, formaldehyde from the exhaust was collected by the sampling system when the engine was in different working conditions and different methanol/diesel blending ratio. We sampled the formaldehyde both before and after the catalytic converter. And we did the experiment with market diesel oil and testing diesel oil respectively. After the collected formaldehyde was placed stilly for 2 hours, it was extracted from the absorbing liquor, and then analyzed by gas chromatograph.The result shows: The concentration of formaldehyde from the exhaust at low load is higher than those at medium and high load which are approximately equivalent. So there should be no methanol injection in low load for DMCC. The concentration of formaldehyde at low substituting rate is lower than those at medium and high substituting rate which are approximately equivalent at medium and high load. The formaldehyde emission from the exhaust depends on the exhaust temperature after the catalyst. The formaldehyde will increase when the exhaust temperature is between 300℃and 410℃, while it will decrease when the exhaust is at other temperature. Because there should be no methanol injection at low load, the exhaust temperature should be increased to decrease the formaldehyde emission.