Investigating the Effect of Engine Lubricant Viscosity on Engine Friction and Fuel Economy of a Diesel Engine

Fuel economy is affected, both by fuel and engine lubricant quality. Engine lubricant quality plays a vital role in reduction of fuel consumption by effective reduction of friction between the contact surfaces of engine parts (piston ring assembly, bearings and valve train). Engine components are exposed to various lubrication regimes such as hydrodynamic, elasto-hydrodynamic, boundary and mixed lubrication during engine operation. In each of these regimes, the factors which influence engine friction are different. Hydrodynamic friction is influenced by lubricant rheology, film thickness and sliding speed of interacting surfaces, whereas boundary and elasto-hydrodynamic friction is a function of surface properties like roughness and hardness and the type of friction modifier used in engine lubricant. So the principal factors which influence engine friction power are speed, load, and surface topography of engine components, oil viscosity, oil temperature and type of friction modifiers used.

It is generally accepted that both the piston assembly and bearings are predominantly in the hydrodynamic lubrication regime, whereas the valve train is in the mixed/boundary lubrication regime. Hydrodynamic friction is proportional to sliding velocity of a pair, oil film thickness, operating temperature, lubricant viscosity and many other physical parameters.

To investigate the effect of engine lubricant viscosity on friction characteristics and fuel consumption of a heavy duty and light duty diesel engine, an experimental study was carried out on a 4-cylinder, Direct Injection off-highway, heavy-duty, diesel engine and 4- cylinder indirect injection, light duty diesel engine coupled with the appropriate eddy current dynamometers and instrumented with fuel consumption measurement unit, pressure sensor, angle encoder, speed sensor, temperature indicators, data acquisition system etc, to measure the fuel consumption, power/torque etc. Two engine lubricants were selected for both types of engine in such a way that both lubricants were of same performance category but having different viscosity grade. For DI diesel engine SAE 20W-50 and SAE 10W-30 engine lubricant complying with API CG-4 were chosen, whereas for IDI diesel engine SAE 15W-40 and SAE 5W-30 engine lubricants complying with API CF-4 were selected. It is to be noted that recommended engine oil was taken as baseline lubricant for the friction and fuel consumption study. Test results in terms of friction mean effective pressure (FMEP), friction power, fuel consumption (g/kWh) were analyzed for DI heavy duty diesel engine for both engine lubricants. Whereas test results in terms of fuel consumption and Fuel Efficiency (%FE) for the light duty IDI diesel engine were analyzed for both engine lubricants.

In order to determine the most dominant factor among the engine operating conditions such as speed, load and engine lubricant viscosity, which affect engine friction power significantly, a full factorial design of experiments (DOE) was formulated to analyze some of the important parameters by which engine friction power influenced significantly. Three factors; speed, load and oil viscosity were chosen as variables with each factor having two levels.

Statistical analysis for determining the dominant factor, affecting the friction power of an engine revealed that the engine speed and speed-load combination are the most significant factors on which engine friction is strongly influenced. An empirical model was developed based on the selected parameters i.e. speed, load and engine lubricant viscosity for predicting the distribution of possible outcomes (friction power) for the Off-highway, DI diesel engine. It may be seen with this investigation that there is consistent reduction in engine friction power at high speed when lower viscosity grade engine oil was used instead of the recommended viscosity grade engine oil. Hence it may be concluded from the experimental engine study that lower viscosity engine lubricant with the same API performance category levels as of OEMs recommended engine lubricant, used for both DI heavy duty and IDI light duty diesel engine, results in reduction in friction power, fuel consumption and yield better fuel efficiency than the recommended engine lubricant.