Alessandro Romagnoli

Ultra Boost for Economy: Extending the Limits of Extreme Engine Downsizing

The paper discusses the concept, design and final results from the ‘Ultra Boost for Economy’ collaborative project, which was part-funded by the Technology Strategy Board, the UKs innovation agency. The project comprised industry- and academia-wide expertise to demonstrate that it is possible to reduce engine capacity by 60% and still achieve the torque curve of a modern, large-capacity naturally-aspirated engine, while encompassing the attributes necessary to employ such a concept in premium vehicles. In addition to achieving the torque curve of the Jaguar Land Rover naturally-aspirated 5.0 litre V8 engine (which included generating 25 bar BMEP at 1000 rpm), the main project target was to show that such a downsized engine could, in itself, provide a major proportion of a route towards a 35% reduction in vehicle tailpipe CO2 on the New European Drive Cycle, together with some vehicle-based modifications and the assumption of stop-start technology being used instead of hybridization. In order to do this vehicle modelling was employed to set part-load operating points representative of a target vehicle and to provide weighting factors for those points. The engine was sized by using the fuel consumption improvement targets and a series of specification steps designed to ensure that the required full-load performance and driveability could be achieved. The engine was designed in parallel with 1-D modelling which helped to combine the various technology packages of the project, including the specification of an advanced charging system and the provision of the necessary variability in the valvetrain system. An advanced intake port was designed in order to ensure the necessary flow rate and the charge motion to provide fuel mixing and help suppress knock, and was subjected to a full transient CFD analysis. A new engine management system was provided which necessarily had to be capable of controlling many functions, including a supercharger engagement clutch and full bypass system, direct injection system, port-fuel injection system, separately-switchable cam profiles for the intake and exhaust valves and wide-range fast-acting camshaft phasing devices. Testing of the engine was split into two phases. The first usied a test bed Combustion Air Handling Unit to enable development of the combustion system without the complication of a new charging system being fitted to the engine. To set boundary conditions during this part of the programme, heavy reliance was placed on the 1-D simulation. The second phase tested the full engine. The ramifications of realizing the engine design from a V8 basis in terms of residual friction versus the fuel consumption results achieved are also discussed. The final improvement in vehicle fuel economy is demonstrated using a proprietary fuel consumption code, and is presented for the New European Drive Cycle, the FTP-75 cycle and a 120 km/h (75 mph) cruise condition.

Mild Hybridization via Electrification of the Air System: Electrically Assisted and Variable Geometry Turbocharging Impact on an Off-Road Diesel Engine

Electric turbocharger assistance consists in incorporating an electric motor/generator within the turbocharger bearing housing to form a mild-hybrid system, without altering other mechanical parts of the engine. This makes it an ideal and economical short to medium term solution for the reduction of CO2 emissions.The scope of the paper is to assess the improvements in engine energy efficiency and transient response correlated to the hybridization of the air system. To achieve this, an electrically assisted turbocharger with a variable geometry turbine has been compared to a similar, not hybridized, system over step changes of engine load. The variable geometry turbine has been controlled to provide different levels of initial boost, including one optimized for efficiency, and to change its flow capacity during the transient. The engine modeled is a 7-litre, 6-cylindres diesel engine with a power output of over 200 kW and a sub-10 kW turbocharger electric assistance power. To improve the accuracy of the model, the turbocharger turbine has been experimentally characterized by means of a unique testing facility available at Imperial College and the data has been extrapolated by means of a turbine meanline model.Optimization of the engine boost to minimize pumping losses has shown a reduction in brake specific fuel consumption up to 4.2%. By applying electric turbocharger assistance, it has been possible to recover the loss in engine transient response of the efficiency optimized system, as it causes a reduction in engine speed drop of 71% to 86% and of 79% to 94% in engine speed recovery time. When electric assistance is present in the turbocharger, actuating the turbine vanes to assist transient response has not produced the desired result but only a decrement in energy efficiency. If the variable geometry turbine is opened during transients, an improvement in specific energy efficiency with negligible decrement in engine transient performances has been achieved.Copyright

Comparison between the steady performance of double-entry and twin-entry turbocharger turbines

Most boosting systems in internal combustion engines utilize ‘pulse turbocharging’ to maximize the energy extraction by the turbine. An internal combustion engine with more than four cylinders has a significant overlap between the exhaust pulses which, unless isolated, can decrease the overall pulse energy and increase the engine pumping loss. Thus, it is advantageous to isolate a set of cylinders and introduce the exhaust gases into two or more turbine entries separately. There are two main types of multiple entry turbines depending on the method of flow division: the twin-entry and the double-entry turbine. In the twin-entry design, each inlet feeds the entire circumference of the rotor leading edge regardless of inlet conditions. In contrast, the double-entry design introduces the flow from each gas inlet into the rotor leading edge through two distinct sectors of the nozzle. This paper compares the performance of a twin and double-entry mixed flow turbine. The turbines were tested at Imperial College for a range of steady-state flow conditions under equal and unequal admission conditions. The performance of the turbines was then evaluated and compared to one another. Based on experimental data, a method to calculate the mass flow under unequal admission from the full admission maps was also developed and validated against the test results.Copyright

Heat Transfer on a Turbocharger Under Constant Load Points

The processes occurring in turbo machinery applications are frequently treated as adiabatic. However, in a turbocharger significant heat transfer occurs, leading to a deficit of turbocharger performance. The overall objective of this experimental work is to improve the understanding of the heat transfer process taking place in a turbocharger when installed on an internal combustion engine. In order to do this, beyond the standard set of measurements needed to define the turbo operating point, a large number of thermocouples were installed on the turbocharger. The tests results allow the quantification of the temperatures within the turbocharger and revealed that a nonuniform temperature distribution exists on the compressor and turbine casings. This is partly attributed to the proximity of the turbocharger to the engine. This process plays a role on the deterioration of the compressor efficiency when compared to the corresponding adiabatic efficiency. A correlation that allows the calculation of the compressor exit temperature is proposed. The method uses the surface temperature of the bearing housing; it was validated against experimental data with deviations no larger than 3%. A simplified 1-dimensional heat transfer model was also developed and compared with experimental measurements. The algorithms calculate the heat transferred through the turbocharger, from the hot end to the cold end by means of lump masses. The compressor performance deterioration from the adiabatic map is predicted.Copyright

A New Turboexpansion Concept in a Twin-Charged Engine System

Engines equipped with pressure charging systems are more prone to knock partly due the increased intake temperature. Meanwhile, turbocharged engines when operating at high engine speeds and loads cannot fully utilize the exhaust energy as the wastegate is opened to prevent overboost. The turboexpansion concept thus is conceived to reduce the intake temperature by utilizing some otherwise unexploited exhaust energy. This concept can be applied to any turbocharged engines equipped with both a compressor and a turbine-like expander on the intake loop. The turbocharging system is designed to achieve maximum utilization of the exhaust energy, from which the intake charge is over-boosted. After the intercooler, the turbine-like expander expands the over-compressed intake charge to the required plenum pressure and reduces its temperature whilst recovering some energy through the connection to the crankshaft. It is anticipated that such a concept has benefits for knock resistance and energy recovery despite suffering higher pumping losses. This paper, for the first time, will investigate the net fuel efficiency benefit from this concept using a super-turbo twin-charger 1-D simulation model. By the operation of a switch between compressor and expander mode, the supercharger could provide boost at low engine speed whilst behaving like a turbine presenting an indirect means to recover exhaust gas energy at high engine speed and meanwhile reducing the intake temperature. The results showed that the BSFC improvement depended on the efficiency of the supercharger as an expander.

A high performance low pressure ratio turbine for engine electric turbocompounding

In order to enhance energy extraction from the exhaust gases of a highly boosted downsized engine, an electric turbo-compounding unit can be fitted downstream of the main turbocharger. The extra energy made available to the vehicle can be used to feed batteries which can supply energy to electric units like superchargers, start and stop systems or other electric units. The current research focuses on the design of a turbine for a 1.0 litre gasoline engine which aims to reduce the CO2 emissions of a “cost-effective, ultra-efficient gasoline engine in small and large family car segment”. A 1-D engine simulation showed that a 3% improvement in brake specific fuel consumption (BSFC) can be expected with the use of an electric turbocompounding. However, the low pressure available to the exhaust gases expanded in the main turbocharger and the constant rotational speed required by the electric motor, motivated to design a new turbine which gives a high performance at lower pressures. Accordingly, a new turbine design was developed to recover energy of discharged exhaust gases at low pressure ratios (1.05–1.3) and to drive a small electric generator with a maximum power output of 1.0 kW. The design operating conditions were fixed at 50,000 rpm with a pressure ratio of 1.1. Commercially available turbines are not suitable for this purpose due to the very low efficiencies experienced when operating in these pressure ranges. The low pressure turbine design was carried out through a conventional non-dimensional mixed-flow turbine design method. The design procedure started with the establishment of 2-D configurations and was followed by the 3-D radial fibre blade design. A vane-less turbine volute was designed based on the knowledge of the rotor inlet flow direction and the magnitude of the absolute speed. The overall dimensions of the volute design were defined by the area-to-radius ratios at each respective volute circumferential azimuth angle. Subsequently, a comprehensive steady-state turbine performance analysis was performed by mean of Computational Fluid Dynamics (CFD) and it was found that a maximum of 76% of total-static efficiency ηt-s can be achieved at design speed.Copyright

A New De-throttling Concept in a Twin-Charged Gasoline Engine System

Throttling loss of downsized gasoline engines is significantly smaller than that of naturally aspirated counterparts. However, even under extremely downsizing condition, the downsized engine could still suffer a relatively large throttling loss when operating under part load. Various de-throttling concepts have been proposed recently, such as using a conventional turbine on the intake as a de-throttling mechanism or applying variable valve timing to control the charge airflow. Although they all can adjust the mass air flow without a throttle in regular use, an extra component or complicated control strategies have to be adopted. This paper will, for the first time, propose a de-throttling concept in a twin-charged gasoline engine with minimum modification of the existing system. The research engine model which this paper is based on is a 60% downsized 2.0L four cylinder gasoline demonstrator engine with both supercharger and turbocharger on the intake. The idea is to use a CVT controlled supercharger to ‘throttle’ the intake mass flow. By the adoption of a CVT, the supercharger outlet pressure could be controllable. Depending on whether the outlet pressure is larger than the inlet, the supercharger could supply boost at high load consuming engine power or behave like an expander under part load presenting a means to recover the throttling loss to provide all the necessary need. A 1-D simulation model was used for this research with the experimental data on the supercharger as an expander from the test rig. The results showed that at part load, by recovering some throttling loss through the supercharger, up to 5% BSFC improvement could be achieved compared to the throttled counterpart depending on the engine operating points. The effect of the reduced supercharger outlet temperature on the combustion efficiency was also discussed. It shows in the end that by the speed control, extended working range of the supercharger can be achievable which could push the fuel efficiency of the downsized engine further.

Heavy-duty engine electric turbocompounding

This paper analyses the performance of a heavy-duty engine when integrating a turbocompound system including a low-pressure-ratio turbine. The main feature of the low-pressure-ratio turbine is that of achieving efficiencies greater than 75% at very low pressure ratios (of approximately 1.1). The engine performance is predicted by a one-dimensionanl simulation model of a 10 l heavy-duty engine. An electric turbocompound system with a low-pressure-ratio turbine is fitted downstream of the main turbocharger to extract the exhaust gas energy. This additional energy supplies the electronic units and batteries. A method to scale an existing low-pressure-ratio turbine, designed for a 1.0 l gasoline engine, is defined and the changes in the turbine losses and the blade stresses are discussed. Low-pressure-ratio turbines of different sizes are analysed and compared in order to obtain the greatest benefit. In all cases the sum of the engine power and the turbocompound power is matched to the baseline engine power output. The full-load results show a power output from the low-pressure-ratio turbine of up to 15.8 kW, an improvement in the brake specific fuel consumption of the engine of up to 4% and a reduction in the brake mean effective pressure of up to 5%. The recovered exhaust gas energy is higher than the engine power required to balance the increased back pressure due to the additional component within the exhaust system. The electrical power produced is sufficiently high to replace the alternator.

Unsteady Performance Prediction of a Single Entry Mixed Flow Turbine Using 1-D Gas Dynamic Code Extended With Meanline Model

Turbochargers are widely regarded as one of the most promising enabling technology for engine downsizing, in the aim to achieve better specific fuel consumption, thermal efficiency and most importantly carbon reduction. The increasing demand for higher quality engine-turbocharger matching, leads to the development of computational models capable of predicting the unsteady behaviour of a turbocharger turbine when subjected to pulsating inlet flow. Due to the wide range of engine loads and speed variations, an automotive turbocharger turbine model must be able to render all the frequency range of a typical exhaust pulse flow. A purely one-dimensional (1-D) turbine model is capable of good unsteady swallowing capacity prediction, provided it is accurately validated. However, the unsteady turbine power evaluation still heavily relies on the quasi-steady assumption. On the other hand, meanline model is capable of resolving the turbine work output but it is limited to steady state flow due to its zero dimensional nature.This paper explores an alternative methodology to realize turbine unsteady power prediction in 1-D by integrating these two independent modelling methods. A single entry mixed-flow turbine is first modelled using 1-D gas dynamic method to solve the unsteady flow propagation in turbine volute while the instantaneous turbine power is subsequently evaluated using a mean-line model. The key in the effectiveness of this methodology relies on the synchronization of the flow information with different time-scales. In addition to the turbine performance parameters, the common level of unsteadiness was also compared based on the Strouhal number evaluations. Comparison of the quasi-steady assumption using the experiment results was made in order to further understand the strength and weaknesses of corresponding method in unsteady turbine performance prediction. The outcomes of the simulation showed a good agreement in the shape and trend profile for the instantaneous turbine power. Meanwhile the predicted cycle-averaged value indicates a positive potential of the current turbine model to be expanded to a whole engine simulation after few minor improvements.Copyright

Nozzle Steam Piston Expander for Engine Exhaust Energy Recovery

This paper presents a concept for new piston expander utilizing nozzle as part of a secondary steam cycle to recover exhaust energy. A commercial 1D simulation tool, AVL BOOST, was used to model the system, and comparison study was carried out between the conventional and nozzle piston expanders. It was found the nozzle piston expander could increase output power from a minimum of 0.73kW up to a maximum of 4.75kW. The simulation study has shown that the concept of using nozzle to admit steam into the piston expander has potential to improve engine system level efficiency.