Analysis and control of a spark ignition free-piston engine generator

PhD ThesisIn this research, the performance analysis and control strategy of a spark-ignited free-piston engine generator were presented. A literature review of the free-piston engine fundamental information and the recent research development on the free-piston engine generator (FPEG) was provided, mainly focussing on previous work on numerical modelling, prototype design as well as the control strategy. The design and simulation of a dual-piston spark-ignited FPEG suitable for operation using either a two-stroke or four-stroke thermodynamic cycle were presented. Model validation and the general engine performance of the system were discussed. For the first time, this research demonstrated the potential advantages and disadvantages of the FPEG on using different thermodynamic gas-exchange cycles. A fast response real time model of the FPEG was designed and validated. The simplicity and flexibility of the proposed model make it feasible to be implemented and coupled with real-time hardware in the loop control system development. In addition, since it revealed how an FPEG operates according to a resonant principle, the model is useful for parameter selection in the design process. For the first time, cascade control was proposed and investigated for the piston stable operation control, using both the measured piston top dead centre of the previous stroke and the measured piston velocity at the current stroke as feedbacks, with the injected fuel mass as the control variable. The system performance was improved by implementing the cascade control compared with single loop control in terms of the controller response time, peak error and settling time

The impact of disruptive powertrain technologies on energy consumption and carbon dioxide emissions from heavy-duty vehicles

Minimising tailpipe emissions and the decarbonisation of transport in a cost effective way remains a major objective for policymakers and vehicle manufacturers. Current trends are rapidly evolving but appear to be moving towards solutions in which vehicles which are increasingly electrified. As a result we will see a greater linkage between the wider energy system and the transportation sector resulting in a more complex and mutual dependency. At the same time, major investments into technological innovation across both sectors are yielding rapid advancements into on-board energy storage and more compact/lightweight on-board electricity generators. In the absence of sufficient technical data on such technology, holistic evaluations of the future transportation sector and its energy sources have not considered the impact of a new generation of innovation in propulsion technologies. In this paper, the potential impact of a number of novel powertrain technologies are evaluated and presented. The analysis considers heavy duty vehicles with conventional reciprocating engines powered by diesel and hydrogen, hybrid and battery electric vehicles and vehicles powered by hydrogen fuel cells, and free-piston engine generators (FPEGs). The benefits are compared for each technology to meet the expectations of representative medium and heavy-duty vehicle drivers. Analysis is presented in terms of vehicle type, vehicle duty cycle, fuel economy, greenhouse gas (GHG) emissions, impact on the vehicle etc.. The work shows that the underpinning energy vector and its primary energy source are the most significant factor for reducing primary energy consumption and net CO2 emissions. Indeed, while an HGV with a BEV powertrain offers no direct tailpipe emissions, it produces significantly worse lifecycle CO2 emissions than a conventional diesel powertrain. Even with a de-carbonised electricity system (100g CO2/kWh), CO2 emissions are similar to a conventional Diesel fuelled HGV. For the HGV sector, range is key to operator acceptability of new powertrain technologies. This analysis has shown that cumulative benefits of improved electrical powertrains, on-board storage, efficiency improvements and vehicle design in 2025 and 2035 mean that hydrogen and electric fuelled vehicles can be competitive on gravimetric and volumetric density. Overall, the work demonstrates that presently there is no common powertrain solution appropriate for all vehicle types but how subtle improvements at a vehicle component level can have significant impact on the design choices for the wider energy system

The development and testing of a free-piston engine generator for hybrid electric vehicle applications

In this work, we present some of the first experimental results along with simulation results obtained in developing and testing a novel dual-piston free-piston engine generator (FPEG), designed for electric- vehicle (EV) range-extender or hybrid powertrain applications. The benefits of a high-efficiency, compact and lightweight design of the proposed range-extender are presented. The technical details and experimental set-up of a two-cylinder prototype and its instrumentation are also outlined. Results are presented for simulation and recent test programmes carried out across both 2-stroke and 4-stroke operational modes. The methods associated with engine control are detailed alongside key post-processed engine characteristics

Effect of the stroke-to-bore ratio on the performance of a dual-piston free piston engine generator

The free piston engine generator (FPEG) is considered as one of the next generation efficient energy conversion device because of its compact structure, high geometric power ratio and low pollution. This paper investigated the effect of stroke-to-bore (S/B) ratio on the system operation characteristics and engine performance, constructed a detailed numerical model in MATLAB/Simulink and verified the experimental data whose difference value could be controlled within 5%. The effect of five S/B ratios (0.84, 0.91, 0.99, 1.07 and 1.14) and three compression ratios (8, 9 and 10) was analysed at a constant bore diameter. The simulation results indicated that the operation frequency increased from 28.2 Hz to 48.3 Hz when the S/B ratio decreased from 1.14 to 0.84. The highest indicated power is 4.1 kW when the S/B ratio is 0.84 and the compression ratio (CR) is 10. While for high thermal efficiency and fuel economy design, larger S/B ratio and higher operating compression ratio should be selected while keeping the periodic energy input unchanged. The heat transfer loss decreased from 29.0% to 20.4% when the S/B ratio increased from 0.84 to 1.14. And in the long stroke, ignition position needs to lean back (from 6.8 mm to 24.8 when S/B increased from 0.84 to 1.14) so as to keep the compression ratio unchanged under different S/B ratios

Numerical Investigation on the Indicated Mean Effective Pressure and Integral Heat Release Rate Variations under Different Key Operating Parameters of a Spark‐Ignited Free Piston Engine Generator

Free-piston engine generator is a new type of hybrid power device and is regarded as the next-generation energy conversion device which can replace the traditional internal combustion engine. This paper focused on the combustion stability and combines experimental results to study the key factors affecting the stability of the free-piston engine, such as ignition time, intake pressure, equivalence ratio, and operating frequency. The simulation results showed that as the ignition advance angle increased, the indicated mean effective pressure increased significantly and the coefficient of variation of the indicated mean effective pressure was effectively reduced from 15.55% to 1.02% as the advance in ignition time from −15 to −30°ECA. When the intake pressure was increased to 1.2 bar, the average value of indicated mean effective pressure reached about 5.15 bar. When the equivalence ratio was in the range of 1.0–1.4, the coefficient of variation of the indicated mean effective pressure can be kept below 10%. The indicated mean effective pressure decreased monotonically from 4.79 to 3.62 bar, and the coefficient of variation increased by five times as the engine speed increased as the engine speed increased from 1,000 to 2,500 RPM

Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports or Valves

The free-piston engine generator (FPEG) is a linear energy conversion device with the objective of utilisation within a hybrid-electric automotive vehicle power system. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine (CE) of the same size, and the difference in the valve timing is compared for both port scavenging type and valve scavenging type, with the exhaust valve closing timing is selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software (2016.1), with the piston dynamics obtained from the simulation model in Matlab/SIMULINK (R2017a). For the CE and FEPG using scavenging ports, in order to improve its power output to the same level as that of a CE, the inlet gas pressure is suggested to be improved to above 1.2 bar, approximately 0.2 bar higher than that used for a CE. If a CE cylinder with exhaust valves is adopted or referred to during the development of an FPEG prototype, the exhaust valve is suggested to be closed earlier to improve its power output, and a higher intake pressure is also suggested if its output power is expected to be the same or higher than that of a CE

Electromagnetic–Thermal Characteristics Analysis of a Tubular Permanent Magnet Linear Generator for Free-Piston Engines

Temperature rise of the tubular permanent magnet linear generator (TPMLG) might lead to insulation failure and demagnetization of permanent magnets, affecting the safe and stable operation of other equipment and the entire system. Herein, a bidirectional electromagnetic–thermal coupling method for analyzing the electromagnetic loss and thermal characteristics of a TPMLG considering the effect of increased temperature on the permanent magnet was proposed. To study the electromagnetic–thermal characteristics of the TPMLG under stable power generation, a two-dimensional electromagnetic field model and a three-dimensional temperature field model were established and coupled. The temperature field of the TPMLG was numerically calculated using computational fluid dynamics over finite volume method under natural air cooling and forced air cooling conditions. Effects of loss and air flow velocity on the steady temperature field were investigated. Results indicated that copper loss increased by 24.5% considering the influence of temperature rise. The windings’ top central position in the TPMLG was the spot with the highest temperature of 127.8 °C and there was a potential demagnetization risk for the permanent magnets. Some reference for future research of clarifying thermal characteristics and cooling design was provided

Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports or Valves

The free-piston engine generator (FPEG) is a linear energy conversion device with the objective of utilisation within a hybrid-electric automotive vehicle power system. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine (CE) of the same size, and the difference in the valve timing is compared for both port scavenging type and valve scavenging type, with the exhaust valve closing timing is selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software (2016.1), with the piston dynamics obtained from the simulation model in Matlab/SIMULINK (R2017a). For the CE and FEPG using scavenging ports, in order to improve its power output to the same level as that of a CE, the inlet gas pressure is suggested to be improved to above 1.2 bar, approximately 0.2 bar higher than that used for a CE. If a CE cylinder with exhaust valves is adopted or referred to during the development of an FPEG prototype, the exhaust valve is suggested to be closed earlier to improve its power output, and a higher intake pressure is also suggested if its output power is expected to be the same or higher than that of a CE