Handbook of Thermal Management of Engines

The term ‘thermal management’ refers to optimizing the thermal balance in an engine and vehicle subsystems, such that their operating temperatures are maintained at an optimum level. Thermal management has become a very crucial part of vehicle development, because it directly or indirectly affects engine performance, fuel economy, safety, reliability, engine component life, driver and passenger comfort, materials selection, emissions, maintenance, aerodynamics, etc. Thermal management of the engine has become critical in optimization for energy efficiency and emissions reduction.

Thermal management is a critical part of today’s advanced diesel aftertreatment systems due to considerations including stringent exhaust emissions regulations, exhaust temperature-induced catalyst performance limitations, aftertreatment recovery, implications of diesel exhaust fluid dosing, and removal of solid deposits in selective catalytic reduction systems. This chapter summarizes developments in thermal management of diesel exhaust aftertreatment with special emphasis on the effects of the technologies on exhaust heat, emissions, and fuel efficiency. The technologies are divided into the following major categories:

As regulations continue to become more stringent with ultralow NO_x and particulate matter mass and number standards, thermal management of diesel exhaust aftertreatment systems will remain an important topic for the foreseeable future. Also, with the advent of hybrids and alternative propulsion systems, thermal management now must be integrated into system-level strategies, increasing complexity and requiring advanced integration and controls, bringing in rich possibilities for further research and development.

The performance of the after-treatment devices depends on their working temperature and in turn on the turbine-out temperature. The target conversion efficiency and regeneration can be achieved by choosing an optimum strategy to increase the temperature at the inlet of the devices, at the same time addressing concerns on the engine fuel consumption. Diameters of exhaust and intake valves, valve timings as well as the use of multi-step openings were studied to predict the temperature at the turbine outlet, coupled to external models for heat transfer and friction losses in steady and transient conditions. The potential of every proposal is deliberated as a function of the engine operating range. The engine layout is guided by the trade-off between the turbine outlet temperature and fuel consumption.

Turbochargers are widely used in internal combustion engines for achieving higher power output, improvement in fuel economy, and reduction in exhaust emissions. With engine downsizing, the increase in low-end torque and rated power promotes further new engine development. Refer to Leduc et al. (Oil Gas Sci Technol Rev IFP 58(1):115–127, 2003 [3]) and Kirwan et al. (SAE Int J Engines 3(1):355–371, 2010 [4]), for similar power with smaller-sized engines, the reduction in carbon dioxide (CO₂) emission is around 20%. The turbocharger with fixed turbine geometry is widely used in medium engine-power intensity applications, and the turbocharger with variable turbine geometry copes with the high engine-power intensity requirements. Also, a turbocharger with variable turbine geometry is used for optimizing the exhaust gas heat energy for the catalyst operation during engine cold starting conditions and emission management. The benefits of using different turbocharger configurations for engine performance enhancement, thermal management, and emission reduction are discussed in this chapter.

In a passenger car design, thermal safety and performance are the key attributes that decide the successful launch and customer acceptance of a product platform. Thermal management is becoming more and more challenging with the stringent regulations about emissions and safety. The BSVI—stage two regulations and CAFE norms require the powertrain and the vehicle systems to be highly efficient in terms of fuel economy (FE) and at the same time meeting the regulatory requirements. It is very important that the vehicle packaging and insulation strategies should lead to the safe functioning of the vehicle system throughout the real world and the extreme conditions. Also, for better FE performance, the air intake system should be sufficiently masked from the high underhood temperatures resulting from engine operation. This chapter includes concepts and validation aspects of thermal insulation design. The topics covered include the basics of insulation, thermal design considerations, material selection, vehicle level thermal assessment, and manufacturing.

Part 1 The temperature for lighting off the combustion of soot stored in particulate traps is lowered from 600 to 300 °C by using catalysts, either coatings or fuel-borne, In CRT, it drops to 250 °C. Nevertheless, in a city bus in dense traffic only 150–200 °C is attained and even the catalysts fail to trigger the reaction. Low load duty cycles, similarly exist in many other applications also. Here, passive regeneration becomes ineffective and active support is desired. This chapter describes one active method for any diesel engine to increase the temperature in the exhaust whenever necessary. Since the load control of a diesel engine is only by the flow rate of the fuel, with the constant airflow for a given speed, the air–fuel ratio increases from when the load drops from λ = 1.5 to λ = 8; and it is the main reason for the low temperature at light loads. If the airflow is simultaneous with the reduction in the fuel flow, the air–fuel ratio can be reduced and the exhaust temperature can be increased up to 300 °C. Both computer simulation and experiments show intake throttling is an active tool to increase the temperature needed for the regeneration of a particulate filter. It is concluded that throttling downstream of the turbocharger compressor is preferred with negligible influence on fuel economy because the operation is only for a short time. To take care of fast changes in load the actuators must respond quickly according to the strategy based on the engine map. Furthermore, simpler solutions are possible for retrofitting DPF in older engines fitted with mechanical fuel injection equipment. It is concluded that air intake throttling in conjunction with a catalyzed particle filters or fuel-borne catalysts covers a wide range of applications and running conditions. Part 2 High PM is emitted by the Transport Refrigeration Units (TRU) powered by small diesel engines. The attendant health risks demand efforts to devise a cost-effective treatment to abate the emissions. Diesel particulate filters (DPF) of ceramic honeycomb construction efficiently trap even ultrafine particles below 300 nm that penetrate the lungs. Fuel-borne catalysts (FBC) facilitate trap regeneration at lower exhaust temperatures but do not assure reliable regeneration in all conditions. A Swiss development team with industrial partners developed a fully automatic active regeneration system for the California Air Resources Board using the FBC strategy in conjunction with a fast-acting intake air throttle valve that increases the exhaust temperature by 250 °C when closed and again provides high oxygen content to the heated filter when opened, thus decoupling the availability of temperature and oxygen for a controlled regeneration of the DPF. The electronic control unit (ECU) monitors exhaust backpressure, temperature, oxygen, and regeneration time and includes self-adapting elements. Here, the development and testing of a prototype unit in a TRU powered by a 26 kW diesel engine. The particle number of solid soot particles below 300 nm is reduced by 99%, EC-mass by 97%, PM by 86%, HC, and NO₂ on average by about 60%. This system is cost-effective for retrofitting small engines and off-road vehicles. Part 3 Recently, PM in winter and ozone in summer have increased in the cities. Diesel engines in heavy-duty vehicles such as garbage trucks, city buses, and construction vehicles are partly responsible for the emissions. The possibility of equipping vehicles older than Euro 3 with a universal retrofit kit to reduce the NOx emissions by 50% and the PM emissions by 99% without creating any secondary emissions are discussed. The soot on the DPF is burned with the oxygen in the exhaust relying less on NO₂. The optimal arrangement for EGR and DPF regeneration is evaluated on a car engine at the test bench. The research indicated that low-pressure EGR (NOx reduction) and intake throttling (DPF regeneration) are the best answers for a retrofit. To increase the EGR at low loads, a second throttle is introduced on the low-pressure side ahead of the compressor. Bench results revealed that with EGR, it is conceivable to decrease the NOx by 50% and the efficiency of the DPF by 99%. A control algorithm for the EGR and the regeneration systems is developed. Part 4 The application of intake or exhaust throttling assumes importance in vehicles applied to slow city traffic. DPF regeneration or the SCR light-off can be augmented by late injection of fuel in the cylinder or HC dosing ahead of a DOC. Early breakthroughs and the current status of the technology are described here.

The regeneration of a loaded diesel particulate filter (DPF), i.e. the burn-up of the stored soot to CO₂, depends on numerous factors, but the most important ones are the temperature and oxygen content of the exhaust gas. Temperature and oxygen, however, have opposite trends. At operation conditions where the temperature is high in a diesel engine, oxygen is always low and vice versa. The challenge to have high oxygen available at conditions where the temperature is high, however, can be solved in a dynamic process using heat inertia effects and can be used for perfect regeneration of the soot-loaded DPF as this proposal demonstrates.

The after-treatment systems function well when there is a combination of city, rural, and highway operations with the latter two helping in maintaining high operating temperatures above light off. However, when the vehicles operate only under urban conditions the engine-out temperatures in the exhaust rarely reach values above 150 °C for sufficient time. Injecting fuel late in the cycle to produce heat is wasteful since the problem prolongs throughout the city drive. The apparent difficulty to reach the temperature without heating is resolved by various schemes to recover the exhaust heat from the gases after the exothermic reactions in DOC and DPF. Ingenious schemes like the STARFILTER that integrate the catalysts in a compact enclosure, achieve both energy recuperation and reduction in heat losses to the surroundings. The endothermic reactions in an SCR require a small intermediary heat source within the recuperation loop to keep the temperature level sufficiently high.

Even though the current NO_x emission limits of heavy-duty engines are already very stringent, it is expected that these NO_x limits would be further tightened in near future. Accordingly, there is a consensus that the present SCR technology should be also further improved to meet next-step NO_x standards, in which a strong focus will be on reducing NO_x emission in low-temperature operations including real-road conditions. The aim of this article is twofold. The first one is to provide an insight into the answers to the questions: “What is the best next-generation SCR concept to meet future heavy-duty NO_x limits?” and “What is a proper thermal management strategy for next-generation SCR systems?” The second purpose of this article is to show that the catalytic simulation technique is a quite useful tool in the concept design of SCR systems. For this, transient SCR simulation was carried out to assess the potential of the thermal management strategies considered for next-generation heavy-duty SCR systems.

Methane is a greenhouse gas, and it is the second major contributor to climate change after CO₂. It is the most stable of all the hydrocarbons due to its strong C-H bond (434 kJ/mol) and thus the activation of its oxidation needs a high temperature. Hence, the light-off temperature for methane at a catalyst is higher than for other hydrocarbons. Engine-out emissions of the natural-gas-powered engines have a major component of methane in the total hydrocarbons. In the cold phase, if not treated or controlled properly, methane in the tailpipe can defeat the new standards. In this chapter, the reasons for high methane emission particularly during the cold phase, and the strategies to reduce it are discussed.

The cooling system in an automobile maintains its engine and related components at an optimum range of temperature for proper functioning and life of the engine. The first step in designing a heat exchanger for an engine is estimating the heat released by the engine. Analytical methods to calculate the heat release of an engine are presented. A brief explanation of the functioning of the cooling system and the components involved is provided. The factors to be considered in the design of a cooling system are listed out. Computational Fluid Dynamics (CFD) provides a method of simulating heat transfer to predict the temperatures achieved in the engine and related components by incorporating all modes of heat transfer—conduction, convection, and radiation. The methodology and equations adopted in CFD software to model the physics of flow and heat transfer are explained. Simulation of convective heat transfer requires accurate prediction of the flow of the fluid. An important area in CFD contributing to this is turbulence modelling. Various turbulence models and the kind of flows for which they work well are presented. The boiling phenomenon is also expected in engine coolant flows and requires special models in CFD to simulate it. The important factors that affect flow with boiling are explained. Conjugate Heat Transfer (CHT) is the method for solving conduction in solid regions along with convection. The numerics behind these calculations are presented. Radiation models in CFD provide an effective way of predicting radiative heat transfer which is very important in components that reach very high temperatures. One of the most common CFD analyses carried out for the cooling system, particularly at the vehicle level, is the under-hood thermal analysis, which involves simulating airflow through the heat exchangers. When analyses are done at a vehicle level, it is not practicable to capture every flow phenomenon happening inside all the components of the cooling system. Instead, the effect of the component is modelled using experimental data. An explanation of such models is presented. Virtual simulation methods such as CFD provide a method to evaluate the performance of the cooling system before prototyping. The increased use of CFD in industrial problems has been made possible only due to parallel processing. The considerations and some of the solutions for parallel processing in CFD are explained. The increasing computing power also provides the opportunity for more analyses to be carried out. This allows improving the design for more optimal performance. Optimization methods are described in the last section.

Theoretical and experimental studies of flow and heat transfer in exhaust systems, internal as well as external, are important to understand the underbody heat transfer, cold start warm-up, and aging of the catalytic converters and to provide sufficient thermal protection for the underbody components. The internal flow in a typical automobile exhaust system can be simplified using a 1-D model considering friction and heat transfer to the inner surfaces of the exhaust system; the heat transfer from the external surfaces due to the external flow and underbody of a vehicle is highly complex as treated in the next chapter by the use of a full-scale 3-D model of a vehicle. The predicted external heat transfer coefficients from the 3-D model are then used as a boundary condition for the 1-D model for heat transfer iteratively until convergence ensures an accurate prediction of the skin temperatures.

Automotive design is an extremely challenging process as it needs to take into account multiple competing design objectives. An increased number of automotive corporations has severely increased the competition and significantly decreased the time for marketing and release both consumers and competitors act as catalysts to escalate performance with minimal increases in cost. To meet these challenges, automotive manufacturers must find new ways to develop vehicles that can be predictable and less reliant on third parties. One factor that affects the cost and schedule of vehicle development is the use of physical models and testing facilities to design and evaluate the models. This process is heavily dependent on both suppliers and the limited number of test facilities that are located in remote geographic locations. Despite all the efforts that vehicle manufacturers take, physical tests have their limitations as they can only test a handful of model variations and driving conditions. Some of the driving conditions that are critical to ensure vehicle safety may not be tested in a test facility reliably. Traditionally, CFD methodologies were not used in vehicle development processes heavily due to the limitations of the solvers in handling production-level vehicle models and a lack of required predictive accuracy to make bold design decisions. There has been significant progress in this arena in the past decade that has made CFD a reliable tool to replace testing in many vehicle development areas. The computer hardware industry has made significant leaps with computational speed and a reduction in computational costs. All these factors are crucial for the adoption of CFD methodologies in the vehicle development process. In this article, we describe simulation workflows used for thermal design. The article is organized as follows. We begin with a discussion of design challenges and a review of the simulation methods for thermal design. In the next section, cooling airflow simulations are discussed, where simulations of the heat exchangers and fans are detailed using the CFD solver PowerFLOW™. Following that, we discuss how the surface temperature map of the entire vehicle is enabled and how this is relevant to the vehicle design process. This workflow known as thermal protection takes into account convection, conduction, and radiation contributions. In the section that follows, we describe the simulation process for a more generalized testing environment such as thermal duty cycle and key-off and soak. The discussion then moves onto automation of the thermal workflows, optimization of design, and post-processing. Finally, we discuss the 1D and system tools used for modelling vehicle components used in thermal workflows.

Engine performance deteriorates as the ambient temperature drops below 10 ºC. Fuel, engine oil, and battery are carefully selected for the efficient functioning of the engine in cold climates. The winter-grade fuels with better flowability and the ability to finely atomize when injected in order to burn the fuel efficiently are described. Then lubricating oils with flow improvers to reduce the oil viscosity and thereby lower the frictional losses and pumping loss are specified for easier cranking to start the engine.

EU Emission regulation at a cold ambient temperature of − 7 °C only speaks about hydrocarbons and CO emissions from all engines and particle numbers only from GDI and diesel engines. Tests on Euro-6 SI and CI engines as per World harmonized Light-duty Test Cycle, indicate that all emissions including particle numbers increase disproportionately at low temperatures. Such an increase is reported from Euro 5 onwards.