Christian Brace

Stochastic Dynamic Programming in the Real-World Control of Hybrid Electric Vehicles

Stochastic dynamic programming (SDP) is applied to the optimal control of a hybrid electric vehicle in a concerted attempt to deploy and evaluate such a controller in the real world. Practical considerations for robust implementation of the SDP algorithm are addressed, such as the choice of discount factor used and how charge sustaining characteristics of the SDP controller can be examined and adjusted. A novel cost function is used incorporating the square of battery charge (C-rate) as an indicator of electrical powertrain stress, with the aim of lessening the affliction of real-world concerns such as battery health and motor temperature, while allowing short spells of operation toward the system peak power limits where advantageous. This paper presents the simulation and chassis dynamometer results over the LA92 drive cycle, as well as the results of testing on open roads. The hybrid system is operated at several levels of aggressivity, allowing the tradeoff between fuel savings and electrical powertrain stress to be evaluated. In dynamometer testing, this approach yielded a 13% reduction in electrical powertrain stress without sacrificing any fuel savings, compared with a controller that does not consider aggressivity in its optimization.

Development of a new method to assess fuel saving using gear shift indicators

European regulations set the emissions requirements for new vehicles at 130 g CO2/km, with an additional 10 g CO2/km to be achieved by additional complementary measures, including gear shift indicators. However, there is presently little knowledge of how much fuel or CO2 could actually be saved by the introduction of gear shift indicators, and there is no consensus on how these savings should be quantified. This study presents a procedure which allows these savings to be quantified over a New European Driving Cycle, and explores the trade-off between fuel savings and drivability. A vehicle model was established and calibrated using data obtained from pedal ramp tests conducted at steady speed using a chassis dynamometer, significantly reducing the time required to generate a calibration data set when compared with a steady-state mapping approach. This model was used for the optimisation of gear shift points on the New European Driving Cycle for reduced fuel consumption subject to drivability constraints. During model validation the greatest fuel saving achieved experimentally for a warm engine was 3.6% over the New European Driving Cycle, within the constraints imposed using subjective driver appraisal of vehicle drivability. The same shift strategy for a cold start driving cycle showed a fuel saving of 4.3% over the baseline, with corresponding savings in CO2 of 4.5% or 6.4 g CO2/km. For both hot and cold tests the savings were made entirely in the urban phase of the New European Driving Cycle; there were no significant differences in fuel consumption in the extra-urban phase. These results suggest that the introduction of gear shift indicators could have a substantial impact, contributing significantly towards the 10 g CO2/km to be achieved by additional complementary measures when assessed in this way. It is not clear whether these savings would translate into real world driving conditions, but for legislative purposes an assessment procedure based on the New European Driving Cycle remains a logical choice for simplicity and continuity.

Novel approaches to improve the gas exchange process of downsized turbocharged spark-ignition engines: A review

Engine downsizing, which is the use of a smaller engine that provides the power of a larger engine, is now considered a mega-trend for the internal combustion engine market. It is usually achieved using one or more boosting devices including a supercharger or a turbocharger. Although supercharging is beneficial for engine’s transient response, turbocharging technology is more widely adopted considering its advantages in fuel efficiency. Compared to turbocharged compression ignition engines, turbocharged spark-ignition engines tend to be more challenging with respect to the gas exchange process mainly due to their higher pumping loss, the need for throttling and the fact that spark-ignition engines demand more controllability due to the mitigation of knock, particularly with regard to minimizing trapped residuals. These challenges encourage the entire gas exchange process of turbocharged spark-ignition engines to be regarded as a complete air management system instead of just looking at the boosting system in isolation. In addition, more research emphasis should be focused on novel approaches to improve the gas exchange process of downsized turbocharged spark-ignition engines because the refinement of the conventional technologies cannot provide continuous gains indefinitely and only innovative concept may improve the engine performance to meet the fuel efficiency target and the more stringent emission regulation in the future. This article will first briefly review knowledge of the current state of the art technologies that are in production as opposed to approaches that are currently only being investigated at a research level. Next, more novel methods of the gas exchange process are introduced to identify the improved synergies between the engine and the boosting machine. The major findings to improve the gas exchange process that emerge from this review comprise four aspects (depending on the location where the novel technologies are implemented) which are as follows: charge air pressurization/de-pressurization improvement, combustion efficiency enhancement within the chamber, valve event–associated development and exhaust system optimization. Although the interaction between these technologies on different aspects of the gas exchange process was found to be highly complex, the optimization or the combination of these technologies is anticipated to further improve a downsized turbocharged spark-ignition engine’s performance.

Analysis of a Driver Behaviour Improvement Tool to Reduce Fuel Consumption

A number of technologies have been introduced into new automobiles with the aim of reducing CO2 emissions. One method of improving fuel consumption is to improve driver behaviour, since eco-driving techniques can help save 10-15% of fuel. A retro-fittable driver behaviour improvement device has been developed and tested in real world conditions. The device provides real-time audio and visual feedback to the driver to improve his/her driving style. It was tested on 15 vans belonging to various companies in the UK and over 39,000km of data was collected. It was observed that use of the device saved an average 7.6% of fuel. Further analysis showed that the savings were obtained as a result of improvement in driving behaviour through reduction in harsh accelerations and early gear shifting into higher gears. There was also a reduction in the pedal busyness of drivers with the system fitted. A model was created using the data obtained to predict the fuel savings that can be achieved if the device is fitted onto a new vehicle.

Review of Turbocharger Mapping and 1D Modelling Inaccuracies with Specific Focus on Two-Stag Systems

The adoption of two stage serial turbochargers in combination with internal combustion engines can improve the overall efficiency of powertrain systems. In conjunction with the increase of engine volumetric efficiency, two stage boosting technologies are capable of increasing torque and pedal response of small displacement engines. In two stage serial turbocharges, a high pressure (HP) and a low pressure (LP) turbocharger are connected by a series of ducts. The former can increase charge pressure for low air mass flow typical of low engine speed. The latter has a bigger size and can cooperate with higher mass flows. In serial configuration, turbochargers are packaged in a way that the exhaust gases access the LP turbine after exiting the HP turbine. On the induction side, fresh air is compressed sequentially by LP and HP compressors. By-pass valves and waste-gated turbines are often included in two stage boosting systems in order to regulate turbochargers operations. One-dimensional modelling approaches are considered for investigating the integration of boosting systems with internal combustion engines. In this scenario, turbocharger behaviour are input in the powertrain models through previously measured compressor and turbine maps in turbocharger gas stands. However, this procedure does not capture all the effects that occur on engine application such as heat transfer, friction and flow motion that influence the turbochargers operations. This is of particular importance for two stage serial turbochargers where the LP compressor may induce a swirling motion to the flow at the entry of the HP compressor. In addition, flow non-uniformities caused by bends between the two compressors can make the HP compressor perform differently. In this paper, a review of the available literature containing approaches to quantify the effects of heat transfer on turbocharger efficiency and the flow influence in the prediction of two stage serial turbochargers performance is explored.

Analysis of energy flows in engine coolant, structure and lubricant during warm-up

Improving engine warm up time is of great interest because cold start engines suffer from increased fuel consumption. This study comprises of two phases: the first looking at global benefits from a novel engine cooling system incorporating coolant flow control valves and dual EGR setup. The new cooling system showed promising signs of influencing energy flows during warm up, however only limited analysis was possible. The second phase used an instrumented engine to show new ways of quantifying performance enhancements through combustion chamber heat transfer, local bearing oil film temperatures and bearing heat transfer. 1 dimensional heat transfer analysis demonstrated the significant energy required to warm up the engine structure and 25~50kJ heat loss per main crank shaft bearing.

Experimental Characterisation of Heat Transfer in Exhaust Pipe Sections

This paper describes the characterization of heat transfer in a series of 11 test sections designed to represent a range of configurations seen in production exhaust systems, which is part of a larger activity aimed at the accurate modelling of heat transfer and subsequent catalyst light-off in production exhaust systems comprised of similar geometries. These sections include variations in wall thickness, diameter, bend angle and radius. For each section a range of transient and steady state tests were performed on a dynamic test cell using a port-injected gasoline engine. In each case a correlation between observed Reynolds number (Re) and Nusselt number (Nu) was developed. A model of the system was implemented in Matlab/Simulink in which each pipe element was split into 25 sub-elements by dividing the pipe into five both axially and radially. The modelling approach was validated using the experimental data. The steady state relationship between Re and Nu allow heat transfer in the test section to be predicted with acceptable accuracy over transient test cycles and demonstrates good agreement with relationships in the literature. The model accuracy was enhanced by developing empirical models of heat transfer during the warm-up stage of the transient test.

The control of chassis dynamometer fuel consumption testing noise factors and the use of response modelling for validation of test repeatability

The research described in this paper was aimed at demonstrating the implementation of statistically derived tolerances to so-called ‘noise’ factors that cause imprecision in the vehicle fuel consumption during chassis dynamometer testing. These tolerances were derived from previous work carried out by one of the authors co-workers and were set to achieve a repeatability target of 0.5% coefficient of variation in the vehicle fuel consumption. This target was successfully achieved during a test programme to determine the fuel consumption benefit of two candidate engine oils over production engine oil using a 1.0 l gasoline passenger test vehicle. Regression response modelling was used to determine whether the recorded variability was correlated with the variability in the vehicle fuel consumption and it was found that all the measured test noise factors were adequately controlled. A universal methodology is proposed for the use of the response modelling technique to verify adequate control of known noise factors and to allow for corrections to the vehicle fuel consumption to be performed where factors have not been adequately controlled, without the need to complete additional testing.

An empirical approach to predicting heat transfer within single- and twin-skin automotive exhaust systems:

This paper describes the further development of an exhaust system model based on the experimental characterization of heat transfer in a series of different pipe sections. Building on previous work published in this journal by the present authors, this study was undertaken to improve the operating range, accuracy, and usability of the original model as well as to introduce the ability to model twin-skin exhaust sections with an air gap. Convective heat transfer relationships for nine stainless steel exhaust bend sections of various wall thicknesses and radii were experimentally characterized over a range of steady state conditions. In each case a correlation between the observed Reynolds number Re and the Nusselt number Nu was developed. Based on measured experimental data, a generic model was built using MATLAB/Simulink; this model is capable of predicting the relationship between the Nusselt number and the Reynolds number for previously unseen pipe geometries falling within the experimental design range. To develop the usefulness of the model further, 15 twin-skin test sections, intended to represent a range of geometries applicable to production automotive gasoline exhaust systems, were also fabricated and characterized. Within the model, both skins of each pipe section were split into five axial elements and five radial elements with the inner and outer skins linked via the modelling of free convection and radiation between them. The predicted Reynolds–Nusselt relationships for each bend section and twin-skin configuration were validated using transient experimental data over a portion of the US06 drive cycle. The final model demonstrated an improved accuracy of exhaust gas temperature predictions, compared with the previous model iterations, with typical errors of less than ±1 per cent and a mean error over the US06 cycle of +0.2 per cent.

The use of multi-variate models for the prediction of heat transfer in vehicle exhaust systems

Abstract This paper describes the development of an exhaust system model based on the characterization of heat transfer in a series of 11 test sections designed to represent a range of configurations seen in production exhaust systems. These sections include variations in the wall thickness, diameter, bend angle, and radius. This work is part of a larger activity aimed at the accurate modelling of heat transfer and subsequent catalyst light-off in production exhaust systems consisting of similar geometries. For each section a range of steady state tests was performed on a dynamic test cell using a port injected gasoline engine. In each case a correlation between the observed Reynolds number Re and the Nusselt number Nu was developed. A model of the system was implemented in MATLAB—Simulink in which each pipe element was split into 25 subelements by dividing the pipe into five, both axially and radially. After each individual section had been characterized, a generic model was built that was capable of determining the relationship between the Nusselt number and the Reynolds number for previously unseen pipe geometries. Complex exhaust systems can be constructed from smaller elements of defined geometry. The modelling approach was validated using experimental data gathered from vehicle testing on a chassis dynamometer. The steady state relationship between Re and Nu allows heat transfer in the test section to be predicted over transient test cycles and demonstrates good agreement with relationships in the literature.