Detection and analysis of formaldehyde and unburned methanol emissions from a direct-injection spark-ignition methanol engine

•A novel measurement method to formaldehyde and unburned emissions by spectrum is proposed.•Combination sampling method of bag sampling and absorbent sampling is used.•Measurement method of formaldehyde and unburned methanol for GCLS and LCLS is better than GCLC.•Formaldehyde and unburned methanol emissions from DISI methanol engine are detected and analyzed. A sampling system to collect exhaust emissions of a methanol engine by means of a sampling bag and an absorber was designed to absorb formaldehyde and unburned methanol emissions with deionized pure water. Three novel measurement methods that combine gas chromatography and liquid chromatography (GCLC), gas chromatography and the light spectrum (GCLS), and liquid chromatography and light spectrum (LCLS) were used to separate and measure formaldehyde and unburned methanol emissions. The three measurement methods of GCLC, GCLS and LCLS proved suitable and reliable to separate and measure the formaldehyde and unburned methanol emissions from a direct-injection spark-ignition methanol engine. The effects of methanol injection timing, spark timing, and excess air ratio on formaldehyde and unburned methanol emissions were analyzed experimentally by measurement method of GCLS. The formaldehyde and methanol detection limits are 0.16 μg/mL and 0.18 μg/mL for gas chromatography (GC), respectively, the detection limit for methanol is 0.15 μg/mL for liquid chromatography (LC), and the detection limit for formaldehyde is 0.21 μg/mL for light spectrum (LS). Retarding the spark timing increases the formaldehyde emission and decreases the unburned methanol emissions. The formaldehyde emission decreases rapidly and the unburned methanol emission increases significantly with an increasing excess air ratio.