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The reduction of friction losses is a subject of central importance in a diesel engine. The piston frictions of low viscosity engine oil and molybdenum dialkyl dithiocarbamate (MoDTC) have been measured by floating liner method. It was found that the low viscosity engine oil lower than 5W-30 is not effective against the reduction of friction mean effective pressure (FMEP) related to the fuel consumption. MoDTC showed a good performance against the reduction of FMEP. In the friction measurement points, the reduction ratio of 10W-30 with MoDTC to 10W-30 was greater than that of 5W-30 to 10W-30.

Applying friction modifier (FM) in low viscosity engine oil is one well known cost effective approach for improving a fuel economy of vehicles. At first, the characteristics and mechanisms of FMs on tribological phenomena were studied with surface analysis technics. The performance of FMs was also evaluated with engine component test and motored engine test to understand the friction property of FMs in engine application. Then the effect of driving cycle, lubricant viscosity and FMs in fuel economy performance under chassis dynamo were studied. Among tested FMs, molybdenum dialkyl dithiocarbamate (MoDTC) was the most effective at boundary lubrication, which is considered significantly important friction area for WLTP, latest procedure for fuel economy test, with low and ultra-low viscosity engine oil.

In order to confirm friction and fuel economy performance of engine oils, laboratory bench tests, motored engine tests and chassis dynamo tests with HEV under WLTP were conducted. The fuel economy improvement effect of reducing viscosity and MoDTC were confirmed under these tests. Moreover, MoDTC (std.) exhibited excellent fuel economy improvement effect compared to MoDTC (L) under low temperature condition particularly. Low viscosity oils formulated with MoDTC (std.) showed superior fuel economy performance even at HEV with relatively lower oil temperatures in this study.