| Diesel engines are broadly used in various sorts of power-machines, boats and vehicles after being invented in 1892, owing to good power performance, fuel efficiency and durability. However, because of vehicle increase, oil resource shortage and strict exhaust emission laws, diesel engine development are facing great challenge and difficulties. Therefore, seraching clean alternative fuel and reducing harmful emissions of diesel engines are main approach.Aim at the above problems, this research introduces a new combustion method, combination combustion, which is launching in same set on board adopt the superior burnable way according to the request of the function and the work condition. Compound combustion theory is composed of two stages of the combustion occurring in diesel engine, the one stage adopts conventional diffusion combustion mode using diesel fuel only at starting, warming up as well as low load periods, avoiding the formaldehyde emissions of the engine. The other one utilizes Quasi-HCCI (homogenous charge compression ignition) mode, which is completed by pre-mixed alcohol formed in intake manifold ignited by diesel fuel at other operating conditions. Fully using the characteristic of alcohol, compound combustion used on diesel engines can not only simultaneously reduce the soot (PM)and mitride oxidation(NOX)emissions from the engines.Early research showed that the power performance of DMCC was enhanced and the fuel efficiency was improved compared to diesel engine. NOx and PM emissions of DMCC mode are obviously reduced compared to diesel engine. With an oxidation catalytic converter, HC, CO and PM from DMCC engine were remarkably decreased. However, aldehydes and ketones are produced during the combustion of Methanol, and formaldehyde is a kind of carcinogen. But there is not yet a standard method for detecting aldehydes and ketones in exhaust of engines. In this paper, a method is put forward to measure formaldehyde in exhaust of DMCC engine by using gas chromatograph.In this paper, a sampling system is designed through comparison and analysis among different sampling methods and conditions, and a method is provided to determinate formaldehyde in the engine exhaust. Formaldehyde is absorbed by the acidic saturated solution of 2,4-dinitrophenyl hydrazine (DNPH) and converted to its hydrazones, the hydrazones are then extracted with CS2 and determinated using Gas Chromatography (GC). Chromatography conditions: Gas Chromatography(SP-3420); FID; chromatographic column (1.5%OV-101/CW..D, 2m×6mm); N2 30ml·min-1, H2 30ml·min-1, air 300ml·min-1; chromatographic column temperature 180℃; inject temperature 270℃; detector temperature 270℃. The test method is applied to emissions of a DMCC engine, the result of determination of formaldehyde in exhaust is satisfied.The results show that formaldehyde is a main kind of aldehydes in exhaust of DMCC mode, and a little acetaldehyde also exists. The method is applied to not only DMCC mode but also diesel engine. The test method is applied to emissions of a DMCC mode engine,the results show that with the increasing of methanol the formaldehyde in exhaust gas is increasing at same rotate speed and same load. And formaldehyde in exhaust gas is higher in 50% load than in 75% load when the rotate speed is consistently.
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