Friction and Ancillary Losses

Hannu Jääskeläinen

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Abstract: Engine mechanical losses result from friction in the power cylinder unit, crankshaft, and valvetrain, as well as from ancillary loads such as pumps and fans. Depending on engine operating conditions, mechanical losses can consume up to 100% of engine power and are thus a critical focus of efforts to improve engine efficiency.


Depending on engine operating conditions, mechanical losses can consume up to 100% of engine power and are thus a critical focus of efforts to improve engine efficiency. Engine mechanical losses result from two main sources:

Prior to significant modern efforts to reduce engine friction, mechanical friction could account for about 4% to 15% of the total fuel energy in diesel engines [4737]. Accordingly it would consume 10% to 30% of engine power output under load, 10% being typical for high loads and 30% for part loads. At idle, friction can consume 100% of engine power [4980].

Ancillary losses can represent about 20-30% of mechanical losses and include the work required to operate critical engine components such as the lubricating oil pump, coolant pump and cooling fans [4980]. While these components must ensure adequate flow and/or pressure for the worst case conditions, the conventional design approach makes little effort to reduce energy consumption at off-design conditions.

Electrification of ancillary loads could allow these devices to be driven by energy recovered via regenerative braking and provide a significant efficiency benefit for some applications.



The distribution of engine friction contributions from different component groups is illustrated in Figure 1. Clearly, friction from the power cylinder unit or power cell unit (PCU) consisting of piston skirt, piston ring and wrist pin friction dominates and is about equal to the sum of that from valvetrain and crankshaft. Of the various sources of friction within the piston assembly, the piston skirt/liner and the piston ring pack/liner interactions are most significant [4980].

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Figure 1. Distribution of engine friction losses

While any efforts to reduce engine friction necessitates addressing friction from the PCU, attention to numerous other friction sources can also deliver cost-effective options for engine efficiency improvement.

Low viscosity lubricating oils can be a very cost effective means to reduce engine friction in a number of key areas of the engine. For components experiencing full hydrodynamic lubrication, lower viscosity reduces friction so long as hydrodynamic conditions continue to be met and boundary lubrication does increase to offset any friction reductions. However, less viscous lubricants also mean the oil film thickness can decrease and for components experiencing mixed lubrication, the risk of metal-to-metal contact increases [4980]. Thus, to realize friction reductions from low viscosity lubricants, design details, surface finishes, coatings and other materials used in bearings, the piston assembly and the valve train may need to be revised to ensure friction reductions and that engine reliability is not compromised.

Increasing oil temperature is another option that will deliver a lower viscosity and provide benefits similar to using a low viscosity lubricant [4792]. Figure 2 illustrates the friction reduction potential of various changes in steady-state oil temperature ranging from 70-110°C [4988]. Obviously, raising oil temperature to the maximum value possible would provide the lowest friction. However, friction in different component groups responds differently to higher oil temperature, Figure 3 [4988][4995][4994]. In the crankshaft where hydrodynamic lubrication friction is significant, the friction reduction is greatest. In the PCU, friction can decrease at low loads and at high speeds but at high loads combined with low speed, friction increases can be significant. In the valvetrain where mixed and boundary lubrication dominate the friction losses, higher oil temperature typically increases friction.

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Figure 2. Effect of changing steady-state oil temperature in a 2.0 L 4 cylinder passenger car diesel engine
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Figure 3. Effect of steady-state oil temperature on motored friction from different component groups in a 2.0 L 4 cylinder passenger car diesel engine

While Figure 2 demonstrates that raising steady-state oil temperature will reduce friction, a number of other measures can affect local oil temperatures or oil temperature during engine warm-up without affecting the maximum steady-state oil temperature in the engine sump. Reducing oil pump flow will not only lower ancillary losses associated with pumping less oil (see discussion below), the lower oil flow can result in changes in local oil temperature on some lubricated surfaces that can either increase or decrease the local oil viscosity depending on engine design details [4989][4996]. Also, engine thermal management aims to manage engine coolant flow to more quickly raise engine temperature and thus oil temperature to lower viscous friction during engine warm-up. Often this entails lowering or even stopping coolant flow rates during engine warm-up that can also lower ancillary losses associated with coolant pumping.

Another general option that can reduce friction is engine downspeeding. Friction power losses tend change in proportion to mean piston speed and producing a given amount of brake power at a lower engine speed can with lower friction losses. Downspeeding also reduces other losses including those associated with intake and exhaust pumping and heat transfer.