Intelligent Thermal Management for Hybrid Powertrains
Figure 1 High heat inputs and low heat capacity will characterize the engine cooling circuit of the future
These requirements make it necessary to respond much more quickly to changes in the operating mode. It must be possible to activate heat sinks much faster, i.e. in less than a second. Classic thermostat control is too sluggish for this. Instead, the engine and vehicle operating modes are taken as the basis for calculating the energy quantities that are initially present and proactively setting the temperature accordingly. It is already possible to implement a corresponding control strategy in the first generation of the thermal management module. This results in CO₂ savings of approx. 3.5 percent in the NEDC. In addition to directly reducing the CO₂ emissions, the thermal management module also enables the implementation of functions such as “active engine heating” or “active transmission heating” for which additional credits are issued according to the US CAFE standards. For each of these functions, 1.5 g of CO₂/m are credited to a passenger car certified in the US.
Second Generation of the Thermal Management Module
Figure 2 Cross-section of the second generation of the thermal management module
(Figures 3 and 4),
Figure 3 Main valve
Figure 4 Auxiliary valve
Decentralized Thermal Management in the Electrified Powertrain
Figure 5 Complexity of the different drive cooling systems
Figure 6 Smart single valve in near-production design
Figure 7 Modular design in intelligent valves for coolant control
Figure 8 Simulation model for a plug-in hybrid vehicle
Figure 9 Model calculations for the heat entry through the electric motor in the NEDC (left) and during full-load acceleration
To validate the simulation model, extensive measurements were taken on a standard plug-in hybrid vehicle and compared with the simulation results. The measurements included the following parameters:
• Volume flows in all cooling circuits
• Coolant temperatures
• Temperatures of the engine and transmission oil
• Component temperatures in the electric motor and battery
• Charge status, voltage, and currents in the battery
• Heat inputs of the relevant components
• Heat outputs of the heat exchangers.
The system assessment answers the question regarding at what points it makes sense to use controllable actuators for optimizing the efficiency of the overall system. This gain in efficiency is to be demonstrated and quantified soon on the basis of a standard plug-in hybrid vehicle modified by Schaeffler.
Figure 10 Measured and simulated heat exchange in the low-temperature heat exchanger of a plug-in hybrid vehicle
Models of the Cooling Circuit and the Thermal Management Module
The basis of the physical model is a mechatronic model for the actuator transmission and electric motor that factors in both the electric loads that occur as well as the torques and heat flows down to the level of individual rotary valve positions. It interacts with a hydromechanical model of the thermal management module or the individual actuators, which takes into account the friction caused by the valve movement. This submodel is in turn linked to the thermohydraulic model of the entire cooling circuit.
The purpose of this kind of a “digital twin” of the actual components of a thermal management system is not only for achieving greater accuracy during the concept phase, but it can also shorten the development time considerably, particularly for design variants, since the effect of individual parameter variations on the cooling circuit can be calculated very quickly.
Figure 11 Structure of a physical model for designing thermal management
In order to validate the above-mentioned simulation models and test modules and components for thermal management during the development phase, Schaeffler has been setting up its own test centers for thermal management in Germany, China, and the US over the past few years. Other test equipment is located in Korea and Japan. Various test stands have been built with the assistance of the in-house Special Machinery Department. The most important test equipment includes the following:
• A test bench called “Typhoon,” which is used to test complete modules under extreme conditions – such as greatly fluctuating coolant temperatures of between -40 and +125 °C. Valve movement, temperature gradients, and volume flow of the pump are recorded on this test bench. Accordingly, the test bench is suitable for durability tests, such as a test with a duration of more than 1,200 hours, corresponding to around 6,000,000 valve movements in a thermal management module.
• Equipment for electric function tests, mainly used for validating actuators under dynamic operating conditions. In addition to the motor function, the signal quality of the position sensor can also be checked.
• A special test bench for testing sensors with respect to accuracy, temperature behavior, and hysteresis.
Along with long-term durability and robust temperature behavior, especially for plastic components, electromagnetic compatibility is also an important goal of validation. Schaeffler is making every effort to develop future product generations that will satisfy EMC protection class 5.
Schaeffler was entering virgin territory with the first series launch of the thermal management module in 2011. By integrating the control system in smart actuators, the second generation of the thermal management system is forging a path to become a mechatronic module.
In Schaeffler’s opinion, future electrified powertrains will lead to central thermal management, combined in a single unit, being gradually replaced by decentralized actuators. The smart single valves (SSV) already represent a suitable technology to this end.
The development of thermal management for the drives of the future will be closely coupled to the overall heat balance of the vehicle. For example, the precise knowledge of the heat inputs and flows in hybrid vehicles can be utilized for predictive regulation, such as for preventing the vehicle interior or exhaust gas cleaning system from cooling off. Schaeffler is currently working on the development tools needed for complex system designs. At lower levels, they already have a high degree of maturity, making it possible for the heat balance to be designed for maximum efficiency.