The effect of engine and transmission oil viscometrics on vehicle fuel consumption

An extensive programme of work has been undertaken to assess the potential benefits of modulating the properties of both the engine and the transmission lubricating oils to achieve lower fuel consumption. The performance of the engine lubricants was evaluated on a production diesel engine on a transient test bed. The main engine lubricating-oil viscometric properties investigated were the cold cranking shear, the kinematic viscosity at 100 degrees C, and the high-temperature high-shear value. Up to 3.5 per cent fuel economy improvement was observed over the New European Drive Cycle (NEDC), relative to current production lubricants. A model relating the fuel consumption to the oil properties was developed and verified using an experimental programme conducted on a chassis dynamometer. In a related study, the effects of changes in the transmission lubricant properties were evaluated using a standard five-speed manual transmission fitted to a light-goods vehicle and tested on a chassis dynamometer. The lubricant was heated using an external energy source to simulate the effect of a more rapid warm-up; this reduced the viscosity of the lubricant and a fuel consumption improvement of 0.7 per cent was demonstrated over the NEDC from a 25 degrees C start. In addition, a lower-viscosity lubricant blend was evaluated, which delivered a 1 per cent improvement in the fuel economy over the standard blend from a cold start, and a further 0.4 per cent improvement if heated.