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Improved prediction of heat rejection in combustion engines
A new method to predict heat rejection in engine design using software-based virtual testing, backed up by data from physical tests, has been developed by Integrated Design Analysis
Gerald Seider, InDesA
Based north of Munich, Germany, InDesA is a consulting and engineering services firm specializing in simulation and analysis of complex fluid flow and heat transfer systems.
InDesA’s approach is built around a detailed STAR-CCM+ engine model embedded in a virtual underhood environment. Combustion and exhaust temperatures are derived from 1D engine process simulation using Gamma Technologies’ GT-Suite software, while friction heat is measured in physical testing.
Shortcomings of physical prototype testing for heat rejection
Using an early-development-stage engine prototype is a conventional approach for heat rejection measurement. To begin, the engine is instrumented with pressure indicators in the cylinder head. This gives a measure called indicated mean effective pressure (IMEP) and another called brake mean effective pressure (BMEP), representing the torque at the flywheel. The two measures are used to derive the friction for the complete engine, known as friction mean effective pressure (FMEP). To obtain these measurements, a specially cast and equipped cylinder head must be fitted to the engine, along with temperature sensors (thermocouples).This adds to the complexity and cost.
Early-stage prototype engines typically have built-in performance limits to safeguard them during testing. Heat rejection needs to be understood early in development, but the engine’s combustion and exhaust characteristics are often not mature enough at this stage for accurate evaluation based on physical testing.
Supplementing physical with virtual testing
To remedy these shortcomings, InDesA developed an approach that uses standard physical test procedures to calibrate simulation models based on 1D and 3D representation of fluid flow and heat transfer (Figure 1). Physical testing provides comprehensive information about the engine and fuel consumption. These measurements are used to populate and calibrate various 1D engine models in GT-Suite. The resulting 1D simulations yield predictions for fuel consumption, basic engine operating parameters, mass flow rates, pressure and temperature in the air induction and exhaust systems and in the coolant and lubrication circuit. This 1D simulation output then provides the boundary conditions for a STAR-CCM+ model of the engine as well as the underhood and full-vehicle environment.
Replacing physical test cell with virtual underhood environment
Engine heat rejection is controlled through thermal management technologies embedded in the engine thermal design, exhaust, cooling and lubrication system design, and underhood environment. To virtualize all of these, InDesA used STAR-CCM+ to model a virtual engine to demonstrate thermal simulation techniques with options for different thermal management technologies: split cooling, water-cooled exhaust manifold, engine oil cooler, and thermal encapsulation. The virtual engine is ‘brought to life’ with 1D simulation models from GT-Suite for engine performance, combustion with air intake and exhaust, heat transfer to the engine structure, lubrication and coolant circuit (Figure 2). Then, to improve on the traditional physical test cell, InDesA developed a virtual car named Pandora, with underhood and full-vehicle environment using STAR-CCM+, that brings together CFD and conjugate heat transfer (CHT) models.
Pandora is used to simulate engine thermal performance with heat transfer to the engine compartment and ambient environment (Figure 3). The model includes the engine and a simplified engine compartment, with air induction, exhaust and coolant systems and front-end heat exchanger module.
The goal is to provide greater fidelity and a wider range of operating conditions than a physical test cell is capable of (Figure 4). Air flow through the engine compartment is modeled in accordance with vehicle speed and cooling fan performance. Air temperature is also modeled in accordance with heat release from the front-mounted radiator. The 1D vehicle model is used for transient simulation in GT-Suite. Hence boundary conditions for any driving cycle from warm-up to race-track operation can be provided.
The engine model is detailed down to a level suitable for thermal stress analysis (Figure 5), with heat flux from combustion into the liner, piston, flame deck and exhaust ports. In addition, dissipated frictional heat is added to the engine liner to calculate internal heat flux, the heat exchange between the engine structure, coolant and engine oil. A unified vehicle model of between 100 and 150 million cells is created in STAR-CCM+, which is used to calculate the heat flux from the engine to the coolant and lubrication system from where it is transported to the heat exchanger pack in the front-end (Figures 7-8). There the heat is released to the cooling air passing through the heat exchanger and cooling fan (Figure 9). The engine experiences the correct underhood air temperature and flow conditions, a significant difference to conventional test cell testing, allowing various heat sources to be quantified precisely.
The model also reveals heat release to the coolant (79.6%), to the engine oil (14%) and to the ambient environment through the engine surface (5.8%). InDesA notes that due to internal heat fluxes and redistribution, the values revealed by simulation differ from those expected with engineering intuition.
Used in combination with bench testing of physical prototypes, InDesA’s virtual approach can predict heat rejection early in engine development with higher fidelity and confidence than bench testing alone. With further development, InDesA believes this methodology has the potential to replace physical prototype heat rejection testing altogether.
Figure 1: InDesA’s approach to engine and vehicle underhood thermal simulation
Figure 2: The virtual engine is ‘brought to life’ with data from GT-Suite
Figure 3: Pandora is a virtual concept car with underhood and full vehicle environment modeled with STAR-CCM+ to simulate engine thermal performance with heat transfer to the ambient environment
Figure 4: Engine installation of InDesA’s virtual concept car Pandora
Figure 5: Heat flux vectors in engine structure
Figure 6: Heat flux from engine surface to ambient environment at 240kmh and 135kW
Figure 7: Heat rejection of the engine combined with the cooling system at 240kmh and 135kW
Figure 8: Streamlines through engine compartment for 240kmh and 135kW