Abstract
Research and/or Engineering Questions/Objective:
A major topic in the development of gasoline engines is the improvement of efficiency over the entire operation range. Besides improved part-load fuel consumption this particularly affects the reduction of the fuel enrichment requirement at high engine loads. The recirculation of cooled exhaust gas is a promising approach for gasoline engines to meet these challenges.
The paper analyses the potentials of different external exhaust gas recirculation (EGR) systems and evaluates their impact on the combustion, the engine cooling system and the engine durability as well as the influences on drivability and engine controls.
Methodology:
For the evaluation two different EGR layouts are generated for high-pressure and lowpressure recirculation. Both layouts are developed by means of computer aided engineering (CAE) methodologies for dimensioning and matching as well as for design and packaging. Based on these investigations two prototype systems are built in the following.
The basic control functions development and calibration to operate the overall engine system is done on the engine dyno. Then, the fundamental assessment of both EGR systems is done on the engine dyno and in the vehicle. This contains the steady-state mapping and the assessment of the transient behavior.
Results:
Besides the system dimensioning, layout and packaging for both systems, the study shows in detail the impacts of cooled recirculated exhaust gas on efficiency, heat release, transient engine response and durability.
The comparison of both layouts reveals the advantages of each system under different driving conditions and creates recommendations for the use of low- and high-pressure EGR. The main focus thereby is on efficiency and enrichment reduction. But these aspect are also related to the increased thermal load in the engine cooling circuit and the drivability-related aspects like low-end-torque and time-to-torque. The analysis of impacts on the durability of engine components caused by the introduction of an EGR system completes the system assessment.
Limitations of this study:
The application of the EGR systems in this study is limited to a singlestage turbo gasoline engine. Spark ignition and direct fuel injection are also boundary conditions that affect the significance of the results. The homogeneous combustion system which is operated with 95RON fuel (DIN EN228) is additionally limiting the generalization of outcomes.
What does the paper offer that is new in the field:
The paper shows a fundamental comparison of all available cooled external EGR systems (high-pressure and low-pressure) on one engine. The dedicated focus of this study is on the system requirements regarding vehicle integration (cooling system), the overall system economy (system costs and fuel efficiency for different use-cases) and impact on the durability of affected engine components.
Conclusions:
The study shows the individual potentials of cooled external high- and low-pressure EGR as enabler for combustion efficiency improvement in the overall engine operating range to fulfill future CO2 requirements. Besides efficiency both systems are also assessed regarding impacts on drivability (low-end-torque and transient response) and their increased heat transfer in the engine coolant system taking into account different design solutions. Additionally the presentation of required material and design changes to engine components due to durability issues completes the overall evaluation of EGR systems.
Key words : Engine System, New SI Engines and Components