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Title: Matching of internal combustion engine characteristics for continuously variable transmissions
Author: Bonnet, Baptiste
ISNI:       0000 0001 3469 9356
Awarding Body: Cranfield University
Current Institution: Cranfield University
Date of Award: 2007
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This work proposes to match the engine characteristics to the requirements of the Continuously Variable Transmission [CVT] powertrain. The normal process is to pair the transmission to the engine and modify its calibration without considering the full potential to modify the engine. On the one hand continuously variable transmissions offer the possibility to operate the engine closer to its best efficiency. They benefit from the high versatility of the effective speed ratio between the wheel and the engine to match a driver requested power. On the other hand, this concept demands slightly different qualities from the gasoline or diesel engine. For instance, a torque margin is necessary in most cases to allow for engine speed controllability and transients often involve speed and torque together. The necessity for an appropriate engine matching approach to the CVT powertrain is justified in this thesis and supported by a survey of the current engineering trends with particular emphasis on CVT prospects. The trends towards a more integrated powertrain control system are highlighted, as well as the requirements on the engine behaviour itself. Two separate research axes are taken to investigate low Brake Specific Fuel Consumption [BSFC] in the low speed region and torque transient respectively for a large V8 gasoline engine and a turbocharged diesel V6 engine. This work is based on suitable simulation environments established for both engines in the powertrain. The modelling exercises are aimed at supplying appropriate models that can be validated against experimental data. The simulation platforms developed then allow the investigation of CVT powertrain biased engine characteristics. The V8 engine model in particular benefited from engine and vehicle dynamometer data to validate the model behaviour and the accuracy of the prediction. It benefited from the parallel work conducted on the Electrically Assisted Infinitely Variable Transmission [EASIVT] project in Cranfield University. The EASIVT vehicle is a parallel mild hybrid aimed at demonstrating the combined fuel economy benefits of a CVT technology and hybridisation. From the CVT powertrain requirements for fuel economy, BSFC operation can be further promoted in the low speed region if Noise Vibration and Harshness [NVH] counter-measures are developed. A study of the combustion torque oscillations at the crankshaft led to the elaboration of an Active Vibration Control [AVC] strategy for the hybrid Integrated Motor Generator [IMG]. Successful implementation of the strategy in both simulation and in-vehicle helped quantify the benefits and short comings of engine operation for best fuel economy. The development in parallel of the hybrid control functions for torque assist and regenerative braking made it possible to implement the low speed AVC in the vehicle without a driveability penalty. The V6 TDI model yielded a realistic and representative simulation for the transient torque response improvement research to be undertaken. For that purpose, the model was tuned against full-load data and the air path control sub-systems were designed and calibrated similarly to a real application. The model was able to highlight the turbocharger lag issue associated with a large combined speed and torque transient inevitable in the fuel economy biased CVT powertrain. This study proposes a Manifold Air Injection [MAI] system in the intake of the engine to help breathing when the VGT operating conditions cannot be shifted rapidly enough for a manoeuvre. The system design constraints were analysed and a suitable strategy was elaborated and calibrated. A sensitivity analysis was also conducted to demonstrate the influence of the MAI design and control variables on the engine performance in the CVT powertrain In conclusion, the benefits of the engine characteristic matching were highlighted in both cases. A review of the work achieved is available in the last chapter, including prospects for further improvements and investigations. The ideal engine characteristics for gasoline and diesel engine technologies integrated in a CVT powertrain are derived from the experience gathered in the research and the results obtained from the tests in low speed operation and transient torque control respectively for the gasoline and the diesel engines. The engine characteristics can be altered toward a better match with a CVT by the use of specific hardware and control strategy. This work recommends that a direct injected, variable valve actuated gasoline engine provides the ideal starting point for low fuel consumption powertrain. When integrated within a mild hybrid CVT powertrain, the full benefits are obtained with the use of low speed operation and AVC. If no electrical machine is available to torque assist the engine, then existing supercharging concepts for a downsized engine can be applied. Diesel engines can also be downsized because of their high torque density. Increased turbocharging boost levels allow steady state torque levels to be maintained in the downsizing process. The CVT powertrain can optimise the fuel consumption and emission levels by appropriate selection of the engine steady state operating points. The torque response lag then becomes critical for the CVT to control the engine speed. This can be improved by the use of Manifold air Injection to assist the turbocharger.
Supervisor: Vaughan, N. D. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available