Lube-Tech


It is recommended that equation (6) should be used to calculate mixed/boundary friction, since (1) it is based on experimental data, (2) the equation takes the form of a “reverse S-curve”, and this would be expected since the real contact area should increase (grow) with load (i.e. as 1/λ), and S-curves arise naturally in many growth processes [21].


Economic Impact of Mixed and Boundary Friction For passenger car engines, experimental data is available on the amount of mixed/boundary friction in valve trains, piston assemblies and journal bearings. The amount of mixed/boundary friction depends strongly on engine speed, and an analysis of reported data [6] has found that up to 10% of fuel consumed on a fuel economy test cycle, which used a 1990’s 2.0 litre gasoline engine, is used to overcome mixed/ boundary friction. For more modern engines, fitted with stop-start systems, and with more advanced valve train designs using lower weight valves and softer springs, the figure will most likely be closer to 5%.


Therefore, it can be assumed that, for passenger cars, approximately 5-10% of total fuel consumption is used to overcome mixed and boundary friction. In the UK, assuming a typical vehicle mileage of 16,000 km, and an average fuel consumption of 6 litres/100 km, the annual fuel consumption of an average car will be around 1000 litres. Therefore, each passenger car in the UK will use approximately 50-100 litres of fuel per year, simply to overcome mixed/boundary friction.


Given that there are around 30 million passenger cars in the UK, this equates to 1.5 to 3 billion litres of fuel per year, with an associated cost of about £2.25 to £4.5 billion per year.


If these figures can be extrapolated worldwide, with 1 billion passenger cars, mixed/boundary friction results in approximately 50 to 100 billion litres of fuel being consumed per year, at a total cost of about $100 to $200 billion.


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.152 page 5


Note that these economic estimates do not include any costs for wear, breakdowns, repairs etc, that may occur due to mixed/boundary friction. Further work is needed to provide realistic estimates for these additional costs.


Environmental Impact It is well known that CO2


emissions result from the


burning of fossil fuels. For gasoline, approximately 3 kg of CO2


are emitted for each litre burnt, of which


2.4 kg is emitted directly through combustion whilst another 0.6-0.7 kg is emitted during the manufacture of gasoline from crude oil, and subsequent transportation of gasoline to fuel stations [22].


The previous section has suggested that annually, approximately 50 to 100 billion litres of fuel are consumed simply to overcome mixed/boundary friction in passenger cars. Therefore, this would result in annual CO2


emissions of between 150 million to 300 million tonnes.


Some countries, or regions, set a “carbon tax”, i.e. a price on each tonne of CO2


producers to reduce their CO2 of CO2


such a price is set, at the time of writing, a typical figure is $50 per tonne of CO2


. Therefore, the cost emissions due to mixed/boundary friction, if


carbon taxes were to be widely used, is potentially in the region of $7.5 to $15 billion, which, whilst far less than the direct fuel costs associated with mixed/ boundary friction, is still a significant sum.


Mitigation Efforts to Reduce Impact of Mixed/ Boundary Lubrication Given the large costs and CO2


emitted, to encourage emissions. Where


emissions associated


with mixed/boundary lubrication, much research is ongoing to attempt to reduce mixed/boundary friction in machines. The following measures, for example, are now widely used:


LUBE MAGAZINE NO.181 JUNE 2024


35


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