Automotive Engine Lubrication System Modelling and Analysis

The lubrication system is one of the most important systems of an engine.

This system should ensure:

  1. Delivery of the required oil amount to the moving parts (e.g.-Bearings);
  2. Dissipation of the heat generated due to friction by circulation of lubricant throughout the system; and
  3. Cleaning of the oil from contamination and impurities introduced during engine operation.

 

To meet the above requirements, the lubricant circulation (lubricant reaching each component) should happen at appropriate pressure and mass flow rate throughout the system.  This is also required in order to avoid cavitation caused by adverse pressure, and excessive heat generation due to less mass flow rate, at any place or particularly at any component. However, sometimes lubricant does not circulate properly to each corner of the system or to the rotating components. In some cases, the rotation of the crankshaft can actually starve the bearings and increase the internal heat due to insufficient supply of lubrication.

To avoid such problems, simulation engineers must model the whole system at all operating modes. They can predict the best system by varying flow rates (volumetric or mass flow rates), system pressures, temperatures, heat flows, as well as by changing the system geometry itself. Such modelling can be performed easily and with sufficient accuracy in a 1D Thermal Fluid analysis tool, such as AxSTREAM NET™ developed by SoftInWay.

It is worthwhile to use a 1D-Analysis tool in this case, because it can be used at any stage of the system design process to explore more options for improving the final design and to reduce development cycle time. The simulation engineer can easily create a model of automotive engine lubrication system, using different elements (components) which are available in the element database of AxSTREAM NET™. The system configuration can also be easily changed at any stage in the design process without rebuilding the complex 3D models.

Let us try to understand how to build a 1D scheme for an automotive engine lubrication system in a 1D tool (AxSTREAM NET™). First, we need to identify the major elements (components) which are part of the automotive engine lubrication system as per their order or sequence in the scheme. A typical engine lubrication system involves components like Oil – sump, strainer, pump and filter, all of which are parts of the initial oil suction line. In addition, the main gallery involves components like flow passages within the connecting rods, crankshaft, and bearings. The typical connections among these elements are shown in Figure 1.

Figure 1 Main elements of a lubrication system
Figure 1 Main elements of a lubrication system

Now let’s see the arrangement of a few components with their specific purposes towards the construction of the whole model.

The first component is the oil sump. It provides storage and cooling for the oil and collects friction wear products like debris, dust etc. Oil taken from the sump is circulated throughout the system by the oil pump. Before that, the oil passes through the suction strainer, which removes larger contaminants. After the pump, it passes through the oil filter for the second level of filtration, i.e. removal of smaller contaminants. The suction part (Oil pan, Suction strainer, Oil pump, and Oil filter) of the system is highlighted with a yellow box in Figure 2 below.

Figure 2 Automotive Engine (4-Cylinder Engine) Lubrication System designed in AxSTREAM NET
Figure 2 Automotive Engine (4-Cylinder Engine) Lubrication System designed in AxSTREAM NET™

After the filter, the oil is passed to the main collector. From there, it gets distributed to the main consumers of oil such as:

  • – Main bearings;
  • – Connecting rod bearings;
  • – Big-end bearings; and
  • – Piston cooling nozzles.

 

Now, let’s consider some key issues that the simulation engineer should pay attention to, when modelling an engine lubrication system.

From a designer’s point of view, the design of an oil pump is very important because the oil pump is responsible for delivering the oil to all the other parts in the system where it is required. The design engineers need to determine the oil pump capacity required to supply lubricant at specific components, under varying load conditions and temperature variations (because oil viscosity changes when the temperature changes). In addition, they need to analyse the failure scenarios of Oil-Pump in critical situations.

Just like the oil pump, there are several other critical design aspects associated with other components such as oil filter. If the oil filter gets blocked, the bearings will experience variation in oil flow and ultimately there will be excessive heat generation because of insufficient lubricant supply. The design or simulation engineer can observe the calculated fluid property values and identify problems (i.e. oil filter related issues) through 1D-analysis, and find alternatives by doing more simulations with different types of oil filters, which could extend the service interval.

Engine bearings are one of the key parts for engine’s long-term reliable operation. In this example (Figure 3), hydrodynamic bearings (Journal Bearings with bore and Journal Bearings with groove) are employed to take the lateral loads imposed on the crankshaft by the connecting rod.

Figure 3 Hydrodynamic Journal Bearing (AxSTREAM Bearing)
Figure 3 Hydrodynamic Journal Bearing (AxSTREAM Bearing™)

For optimization purposes, various types of bearings can be employed.  One can characterize the pressure losses, load carrying capacity, etc., for each of the employed bearings. One can also perform a number of simulations easily and quickly with different types of bearings and with different input parameters (RPM, bearing dimensions, etc.) with 1D Thermal-Fluid analysis tool. For a larger number of simulations with a number of variants, the 1D (Thermal-Fluid analysis) tool can be integrated with an optimization tool like AxSTREAM ION™ developed by SoftInWay. Such an optimization tool allows plotting a number of graphs for the specific objectives against different variables (Example: rpm) to find the most optimized solution. This approach is useful to find the most appropriate bearing type for a particular lubrication system.

The simulation engineer needs to test the whole lubrication network with varying operating conditions using a 1D CFD, i.e. 1D thermal-Fluid analysis tool, because during the engine design process, all parts of the lubrication system are designed and selected with experience. With this 1D lubrication network simulation, one can ensure reliability of the design and validate the selection of each element. If required, appropriate changes can be made at this stage, which effectively saves the development cost, greatly shortens the development cycle and allows developing a lubrication system that is effective in terms of size of system and safety.

Thus, this 1D modelling and analysis allows an engineer to obtain important information about the system performance, in order to reveal the necessary design changes and give a constructive outcome upon the new design process.

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