Abstract
The oil distribution system of an automotive light duty engine typically has an oil pump mechanically driven through the front-endancillaries-drive or directly off the crankshaft. Delivery pressure is regulated by a relief valve to provide an oil gallery pressure of typically 3 to 4 bar absolute at fully-warm engine running conditions. Electrification of the oil pump drive is one way to decouple pump delivery from engine speed, but this does not alter the flow distribution between parts of the engine requiring lubrication. Here, the behaviour and benefits of a system with an electrically driven, fixed displacement pump and a distributor providing control over flow to crankshaft main bearings and big end bearings is examined. The aim has been to demonstrate that by controlling flow to these bearings, without changing flow to other parts of the engine, significant reductions in engine friction can be achieved. The study has been conducted on a 1.5litre, 4 cylinder turbocharged diesel engine. By reducing the feed pressure to the bearings from a baseline pressure of 3bar absolute to 1.5 bar absolute, reductions in engine rubbing friction mean effective pressure of up to 14% has been achieved at light load. Similar reductions in friction were recorded across a speed range of 1000-2000 rev/min and net indicated mean effective pressures up to 3.5 bar. The ranges were conservatively limited to protect against bearing damage. The paper reports details of the oil system modifications and the test results. The fuel economy benefit due solely to the friction reduction, not including any benefit from a reduction in oil pump work, is around 1½ % over the New European Drive Cycle (NEDC). The reduction in friction is demonstrably significant and represents an area with great potential to improve engine efficiency.
Highlights
The net indicated work done by the cylinder gases on the pistons of an internal combustion engine is greater than the brake work output
We have distinguished between rubbing friction mean effective pressure, FMEP, and FMEP* which includes AMEP and is determined from the difference IMEPn - BMEP, the ancillary work term does not include work done by the oil pump
Reducing the oil feed pressure to the five crankshaft main bearings and the four big end bearings from 3 bar absolute (barA) to 1.5 barA whilst maintaining the feed to the valve train and turbocharger at 3barA reduced engine FMEP by up to 14% at light loads, with diminishing benefit as load was raised
Summary
The net indicated work done by the cylinder gases on the pistons of an internal combustion engine is greater than the brake work output. A fraction of the indicated work is consumed driving ancillaries and the valve train and a further fraction is dissipated by rubbing friction at the various interfaces between surfaces moving relative to each other This can entail sliding motion such as between the piston and the cylinder liner, rolling-sliding motion between cams and roller followers, and rotational motion in the journal bearings. The higher friction is largely attributable to higher values of oil viscosity at low temperatures, and various routes to accelerate the rate of rise of bulk oil temperature after cold start-up have been explored These include reducing effective sump capacity [8] or the introduction of baffles to direct hottest available oil to the oil pump inlet [9], external heating of the oil [10] and the use of thermal insulation [11]. Introducing a high contact resistance at the outside surface of the bearing shells reduces heat transfer in this direction [13] but not into the journal, which limits the benefit
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