Effect of lubricating oil characteristics on solid particle number and CO2 emissions of a Euro 6 light-duty compressed natural gas fuelled vehicle

Abstract

Lubricant oil affects the fuel consumption and exhaust emissions of passenger cars. Oil reduces the friction in combustion engines, yet simultaneously some lubricating oil is exhausted from the engine. The fugitive lubricant oil results in < 30 nm solid particle number (SPN) emissions in the exhaust gas. The characteristics and formation path of the small particles is not well known, but as the SPN regulation is extending down to 10 nm size range, they cannot be neglected. Vehicles using compressed natural gas (CNG) fuel are known to emit solid particles in the < 23 nm size range. Here we studied the emissions of a CNG-fuelled Euro 6 light-duty vehicle using different lubricants. The lubricant characteristics had a strong effect on < 23 nm SPN emissions. A high ash, high volatility lubricant was shown, to alter the SPN population also in > 10 nm and even > 23 nm size range, thus, underlining the importance of the lubricant quality. The difference between the high volatility lubricant and the two tested modern lubricants was most evident at high temperature, high speed driving, while the emissions at the start of the cycle were of the same order of magnitude for all oils. The lubricant formulation was also shown to affect the vehicle CO2 emissions. The vehicle CO2 emissions in the tests driven with the high volatility lubricant oil exceeded the emissions with optimized lubricant oil by as much as 2%. Thus, the right lubricant choice can lead to fuel savings in CNG-fuelled vehicles. The presented results demonstrate the origin of the < 23 nm SPN in CNG-fuelled vehicles.