Pengfei Lu

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.

Analysis and Comparison of the Performance of an Inverted Brayton Cycle and Turbocompounding With Decoupled Turbine and Continuous Variable Transmission Driven Compressor for Small Automotive Engines

For an internal combustion engine, a large quantity of fuel energy (accounting for approximately 30% of the total combustion energy) is expelled through the exhaust without being converted into useful work. Various technologies including turbocompounding and the pressurized Brayton bottoming cycle have been developed to recover the exhaust heat and thus reduce the fuel consumption and CO2 emission. However, the application of these approaches in small automotive power plants has been relatively less explored because of the inherent difficulties, such as the detrimental backpressure and higher complexity imposed by the additional devices. Therefore, research has been conducted, in which modifications were made to the traditional arrangement aiming to minimize the weaknesses. The turbocharger of the baseline series turbocompounding was eliminated from the system so that the power turbine became the only heat recovery device on the exhaust side of the engine, and operated at a higher expansion ratio. The compressor was separated from the turbine shaft and mechanically connected to the engine via continuous variable transmission (CVT). According to the results, the backpressure of the novel system is significantly reduced comparing with the series turbocompounding model. The power output at lower engine speed was also promoted. For the pressurized Brayton bottoming cycle, rather than transferring the thermal energy from the exhaust to the working fluid, the exhaust gas was directly utilized as the working medium and was simply cooled by ambient coolant before the compressor. This arrangement, which is known as the inverted Brayton cycle (IBC) was simpler to implement. Besides, it allowed the exhaust gasses to be expanded below the ambient pressure. Thereby, the primary cycle was less compromised by the bottoming cycle. The potential of recovering energy from the exhaust was increased as well. This paper analyzed and optimized the parameters (including CVT ratio, turbine and compressor speed and the inlet pressure to the bottoming cycle) that are sensitive to the performance of the small vehicle engine equipped with inverted Brayton cycle and novel turbocompounding system, respectively. The performance evaluation was given in terms of brake power output and specific fuel consumption. Two working conditions, full and partial load (10 and 2 bar brake mean effective pressure (BMEP)) were investigated. Evaluation of the transient performance was also carried out. Simulated results of these two designs were compared with each other as well as the performance from the corresponding baseline models. The system models in this paper were built in GT-Power which is a one dimension (1D) engine simulation code. All the waste heat recovery systems were combined with a 2.0 L gasoline engine.

Fuel Efficiency Optimization for a Divided Exhaust Period Regulated Two-Stage Downsized SI Engine

In our previous paper, a new gas exchange concept termed Divided Exhaust Period Regulated 2-stage (DEP R2S) system has been proposed. In this system, two exhaust valves in each cylinder are separately functioned with one valve feeding the exhaust mass flow into the high pressure (HP) manifold whilst the other valve evacuating the remaining mass flow directly into the low pressure (LP) manifold. By adjusting the timing of the exhaust valves, the target boost can be controllable whilst improving the engine’s pumping work and scavenging is attainable which results in better fuel efficiency from the gas exchange perspective.This paper will continue this study by adding an appropriate knock model to examine the benefits this concept could bring to the combustion phasing. The results at full load showed that under knock limited spark advance (KLSA) and fully optimized exhaust valve timing condition, the DEP R2S system benefited from lower pumping loss and better scavenging due to the reduced backpressure and improved pulsation interference despite suffering from reduced expansion ratio and expansion work. The combustion phasing was advanced across the engine speed which is mainly attributed to the reduced residual and the reduced requirement of gross IMEP. The net BSFC was observed to improve by up to 3% depending on the engine operating points. At part load, the DEP R2S system could be used as a mechanism to extend the ‘duration’ of the exhaust valve. This will reduce the recompression effect of the exhaust residuals during the beginning and the end of the exhaust stroke compared to the original R2S model with late exhaust valve opening and early exhaust valve opening. In addition, increased internal EGR due to the increased overlap between the LP and the intake valve is also beneficial for the improved PMEP as the throttle can be further opened to reduce the corresponding throttling loss. The average net BSFC improvement is expected to be approximately 6–7%.Copyright

Explore and Extend the Effectiveness of Turbo-compounding in a 2.0 litres Gasoline Engine

After years of study and improvement, turbochargers in passenger cars now generally have very high efficiency. This is advantageous, but on the other hand, due to their high efficiency, only a small portion of the exhaust energy is needed for compressing the intake air, which means further utilization of waste heat is restricted. From this point of view, a turbo-compounding arrangement has significant advantage over a turbocharger in converting exhaust energy as it is immune to the upper power demand limit of the compressor. However, with the power turbine being located in series with the main turbine, power losses are incurred due to the higher back pressure which increases the pumping losses. This paper evaluates the effectiveness that the turbo-compounding arrangement has on a 2.0 litres gasoline engine and seeks to draw a conclusion on whether the produced power is sufficient to offset the increased pumping work. Furthermore, as mentioned above, the baseline engine model in this paper is not a heavy-duty diesel engine which is more appropriate to the turbo compounding mechanism for automotive application, but a small size gasoline engine. This paper also aims to explore a potential methodology for extending the operating range of the turbo compounding in light-duty petrol engine over its entire speed range under full load condition. The system models in this paper were built in GT-Power which is a one dimension (1-D) engine simulation code. Simulation results show that with the assistance of a variably driven supercharger, the output torque of the engine system is much larger (up to around 24%) at lower engine speeds. The fuel economy is also improved by up to about 8%.

Fuel efficiency optimization for a divided exhaust period regulated two-stage downsized spark ignition engine

In our previous paper, a new gas exchange concept termed divided exhaust period regulated two-stage (DEP R2S) system has been proposed. In this system, two exhaust valves in each cylinder are separately functioned with one valve feeding the exhaust mass flow into the high-pressure (HP) manifold, while the other valve evacuating the remaining mass flow directly into the low-pressure (LP) manifold. By adjusting the timing of the exhaust valves, the target boost can be controllable while improving the engines pumping work and scavenging is attainable which results in better fuel efficiency from the gas exchange perspective. This paper will continue this study by adding an appropriate knock model to examine the benefits this concept could bring to the combustion phasing. The results at full load showed that under knock limited spark advance (KLSA) and fully optimized exhaust valve timing condition, the DEP R2S system benefited from lower pumping loss and better scavenging due to the reduced backpressure and improved pulsation interference despite suffering from reduced expansion ratio and expansion work. The combustion phasing was advanced across the engine speed which is mainly attributed to the reduced residual and the reduced requirement of gross indicated mean effective pressure (IMEP). The net brake-specific fuel consumption (BSFC) was observed to improve by up to 3% depending on the engine operating points. At part load, the DEP R2S system could be used as a mechanism to extend the “duration” of the exhaust valve. This will reduce the recompression effect of the exhaust residuals during the beginning and the end of the exhaust stroke compared to the original R2S model with late exhaust valve opening and early exhaust valve opening. In addition, increased internal exhaust gas recirculation (EGR) due to the increased overlap between the LP and the intake valve is also beneficial for the improved pumping mean effective pressure (PMEP) as the throttle can be further opened to reduce the corresponding throttling loss. The average net BSFC improvement is expected to be approximately 6–7%.

Experimental analysis of the V-Charge variable drive supercharger system on a 1.0 L GTDI engine

A compound charging system that pairs a turbocharger with a supercharger seems to be a potential trend for future passenger car gasoline engines, as the strength of both could be enhanced and the deficiencies of each could be offset. The use of a fixed-ratio positive-displacement supercharger system on a downsized turbocharged gasoline engine has already appeared on the market. Although such systems can achieve enhanced low-end torque and improved transient response, several challenges still exist. An alternative solution to the fixed-ratio positive-displacement supercharger is the V-Charge variable ratio centrifugal supercharger. This technology utilizes a Torotrak continuously variable transmission (CVT) coupled to a centrifugal compressor for near silent boosting. With a wide ratio spread of 10:1 and rapid rate of ratio change, the compressor speed can be set independently of the engine speed to provide an exact boost pressure for the required operating points, without the need to recirculate the air through a bypass valve. A clutch and an active bypass valve can also be eliminated, due to the CVT capability to down-speed, thus improving the noise vibration and harshness performance. This paper will, for the first time, present and discuss the V-Charge technology optimization and experimental validation on a 1.0 L GTDI engine to achieve a better brake specific fuel consumption and transient response over the turbo-only and the fixed-ratio positive-displacement supercharger solution. The potential for the V-Charge system to increase the low-end torque and enable a down-speeding strategy is also discussed.

A Throttle Loss Recovery Concept Using an Expander for a 2.0 Litre Turbocharged Gasoline Engine

In the internal combustion engine, a large amount of energy is rejected in the form of exhaust heat without being converted into brake work.Additionally, in gasoline engines, throttle losses are also a considerable disadvantage limiting the capability to achieve higher thermal efficiency. Under part load conditions, both the power demand and engine speed are much lower than the maximum achievable. The throttle is partially closed to restrict inlet air mass flow to regulate the brake power production. To overcome the friction and turbulence losses at the small throttle opening, negative pressure is produced in the manifold at the cost of engine power.This paper explores the effectiveness of an expander installed in the inlet duct of the engine to lessen, even eliminate, the throttle losses by allowing power to be reclaimed from the pressure drop across the expander, which will otherwise be wastefully dissipated across the throttle. In this way the pumping losses are reduced.The engine system was modelled in GT-Power which is a 1-dimensional engine simulation code. The limits in decreasing in pressure drop through the throttle and the power generation from the expander were explored. Together with a turbo machine recovering energy from the exhaust flow, this system was able to enhance the fuel economy by about 5% when operating at 1.75 bar BMEP from 500–3000rpm compared with a conventional turbocharged engine. The influence of the expander machinery on the combustion and the turbocharger performance was also discussed. To achieve the highest performance level, careful optimization of the expander size and control strategy and proper matching with engine system are critical.© 2015 ASME