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Internal combustion engines (ICE) for the use in heavy-duty trucks and buses have to fulfil demanding requirements for both vehicle efficiency as well as for emission of greenhouse gases. Beside the piston assembly the journal bearings are among the largest contributors to friction in the ICE. Through a combination of measurements and validated simulation methods the journal bearing friction losses of a state-of-the-art heavy-duty Diesel engine are investigated for a large range of real world operating conditions. To this task recently developed and extensively validated simulation methods are used together with realistic lubricant models that consider the Non-Newtonian behaviour as well as the piezoviscous effect. In addition, the potential for further friction reduction with the use of ultra-low viscosity lubricants is explored. The results reveal a potential of about 8% friction reduction in the journal bearings using a 0W20 ultra-low viscosity oil with an HTHS-viscosity (The HTHS-viscosity is defined as the dynamic viscosity of the lubricant measured at 150• C and at a shear rate of 10 6 s −1 ) of 3.6 mPa s. For the investigated engine, HTHS-viscosity limitations are determined which indicate that the use of lubricants with further reduced HTHS-viscosity would require engine and/or journal bearing modifications to be able to maintain the high service life of the engine.

The friction power losses of a turbo-charged heavy-duty diesel engine of the 13 litre class are investigated both by fired engine tests as well as by pressurized motoring tests. During pressurized motoring compressed air is applied to the engine intake which creates loads comparable with fired operation but without the strong and changing thermal influence of combustion. By using this combined approach the influence of the load and the thermal influence of the combustion can be studied separately for the first time. It is found that pressurized motoring yields comparable but generally a bit higher friction power losses as in fired operation. In particular, for full load operation, the agreement between the two methods is very good which supports the reasoning that for full load operation the mechanical load is the dominant factor for the friction power losses. However, for part load operation significant differences arise. Without the thermal influence from combustion, increasing the load on the engine leads to a rather linear increase in the friction power losses as is seen from pressurized motoring. This is in contrast to the fired engine tests, where the friction power losses stay almost constant over a rather large range of part loads and only increase for full load operation. It is argued that the reason for this different behaviour is the thermal impact from combustion.

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