Miller Cycle Research Articles

Effects of the continuous variable valve lift system and Miller cycle strategy on the performance behavior of the lean-burn natural gas spark ignition engine

In order to further improve thermal efficiency and fuel economy, the engine community pays more attention to the advanced controlling strategy. In this paper, the impacts of the continuous variable valve lift (CVVL) system and Miller cycle strategy on the performance behavior of the natural gas spark ignition (SI) engine were comprehensively investigated. The results indicated that pumping loss was decreased, and volumetric efficiency slightly increased with using CVVL. In addition, the intake air mass flow at the intake ports became more smooth, and turbulence flow and its intensity were strengthened with using CVVL, which were beneficial to accelerate combustion rate and improve combustion efficiency. Under the partial load of the 1200 rpm 5 bar, the ratio of the pumping mean effective pressure (PMEP) in indicated mean effective pressure (IMEP) was reduced by 2.74%. Furthermore, an extra 5.43% improvement of indicated thermal efficiency was obtained. On the other hand, peak in-cylinder pressure and peak combustion temperature of the natural gas SI engine operated on Miller cycle were decreased with late intake valve close (LIVC), particularly the pressure and temperature at the end of the compression stroke, which resulted in reducing NOx emissions formation and mitigating knocking combustion. However, the intake mass flow at the intake ports became fluctuation and the volumetric efficiency was decreased with using LIVC. In addition, brake specific fuel consumption was increased and indicated thermal efficiency decreased with employing LIVC.

Controls and Hardware Development of Multi-Level Miller Cycle Dynamic Skip Fire (mDSF) Technology

<div class="section abstract"><div class="htmlview paragraph">mDSF is a novel cylinder deactivation technology developed at Tula Technology, which combines the torque control of Dynamic Skip Fire (DSF) with Miller cycle engines to optimize fuel efficiency at minimal cost. mDSF employs a valvetrain with variable valve lift plus deactivation and novel control algorithms founded on Tula’s proven DSF technology. This allows cylinders to dynamically alternate among 3 potential states designated as: High Fire, Low Fire, and Skip (deactivation). The Low Fire state is achieved through an aggressive Miller cycle with Early Intake Valve Closing (EIVC). The three operating states in mDSF can be used to simultaneously optimize engine efficiency and driveline vibrations. Acceleration performance is retained using the all-cylinder, High Fire mode. mDSF can be implemented cost-effectively using an asymmetric intake valve lift strategy, with one high-flow power charging port and one high-efficiency Miller port.</div><div class="htmlview paragraph">Prototype mDSF cylinder heads were based on the EA888 Gen 3B engine by retrofitting the valvetrain with asymmetric intake cams, deactivatable roller finger followers and two oil control valves per cylinder. Event-based engine controls were developed to enable for each cylinder dynamic selection of the three mDSF operating modes: High Fire, Low Fire and Skip. Appropriate air estimation, fuel control and ignition control techniques were employed to ensure acceptable torque delivery and tailpipe emissions.</div><div class="htmlview paragraph">Engine dynamometer tests showed a 23% reduction in engine fuel consumption at 1500 rpm, 2 bar NMEP. Maximum torque and power from the baseline production engine up to 5000 rpm were also achieved. mDSF vehicle tests on the WLTC demonstrated a 6% reduction in CO<sub>2</sub> from Miller 2-step. Euro 6d compliant emissions were also reported. Further improvements in fuel economy, drivability and NVH may be possible by leveraging mixed firing densities more extensively.</div></div>

Lean-Stratified Combustion System with Miller Cycle for Downsized Boosted Application - Part 2

<div class="section abstract"><div class="htmlview paragraph">Automotive manufacturers relentlessly explore engine technology combinations to achieve reduced fuel consumption under continued regulatory, societal and economic pressures. For example, technologies enabling advanced combustion modes, increased expansion to effective compression ratio and reduced parasitics continue to be developed and integrated within conventional and hybrid propulsion strategies across the industry. A high-efficiency gasoline engine capable for use in conventional or hybrid electric vehicle platforms is highly desirable. This paper is the second of two papers describing the multi-cylinder integration of a technology package combining lean-stratified combustion with Miller cycle for downsized boosted applications. The first paper describes the design, analysis and single-cylinder testing conducted to down-select the combustion system deployed to the multi-cylinder engine. This paper defines a light-duty engine package capable of achieving a 35% fuel economy improvement at US Tier 3 emission standards over a naturally aspirated stoichiometric baseline vehicle. A multi-mode combustion system was developed enabling highly efficient lean-stratified operation at light-load and stoichiometric Miller-cycle operation at mid- to high-loads. A central direct-injection four-valve layout was designed with high-tumble ports and a bowl-in-piston capable of stable operation under highly dilute mixtures when properly matched to fuel spray characteristics, injection strategy, and a high-energy ignition system. Boost system layout and base engine architecture were selected to minimize parasitics and maximize aftertreatment effectiveness. A multi-cylinder engine was developed and calibrated over a steady-state map. Results fed vehicle simulations over the EPA city and highway test cycles to predict the fuel economy improvement and tailpipe emissions potential. Fuel economy improvements over the baseline vehicle were projected at 42% and 39% for active-urea and passive-ammonia SCR aftertreatment systems respectively. Controls complexity to achieve regulatory compliance and the cost of the lean aftertreatment system were identified as the primary challenges to commercial viability.</div></div>

Lean-Stratified Combustion System with Miller Cycle for Downsized Boosted Application - Part I

<div class="section abstract"><div class="htmlview paragraph">Automotive manufacturers relentlessly explore engine technology combinations to achieve reduced fuel consumption under continued regulatory, societal and economic pressures. For example, technologies enabling advanced combustion modes, increased expansion to effective compression ratio, and reduced parasitics continue to be developed and integrated within conventional and hybrid propulsion strategies across the industry. A high-efficiency gasoline engine capable for use in conventional or hybrid electric vehicle platforms is highly desirable. This paper is the first to two papers describing the development of a combustion system combining lean-stratified combustion with Miller cycle for downsized boosted applications. The work was completed under a multi-year US DOE project. The goal was to define a light-duty engine package capable of achieving a 35% fuel economy improvement at US Tier 3 emission standards over a naturally aspirated stoichiometric baseline vehicle. A multi-mode combustion system was developed enabling highly efficient lean-stratified operation at light-load and stoichiometric Miller-cycle operation at mid- to high-loads. Some of the unique challenges in synergistically combining the two concepts are highlighted in this paper. A central direct-injection four-valve layout was designed with high-tumble ports and a bowl-in-piston capable of stable operation under highly dilute (lean plus EGR) mixtures when properly matched to fuel spray characteristics, a multiple-injection strategy, and a high-energy ignition system. Other challenges specific to meeting exhaust emissions requirements with a high-efficiency engine are also explored in this study. In this Part 1 of the paper, the single-cylinder combustion system design, analysis, and testing is described. Multi-cylinder engine system design, analysis and development is described in Part 2 of the paper [<span class="xref">1</span>].</div></div>

Numerical investigation of Miller cycle with EIVC and LIVC on a high compression ratio gasoline engine.

Previous studies have shown that increase compression ratio (CR) is an effective way to improve thermal efficiency of gasoline engine without changing the mechanical structure and working cycle, however, it is limited by engine knock when increasing the intake boosting under high load operation. This study aimed to solve the knock problem of gasoline engine with higher CR by application of Miller cycle, which can be implemented by either early or late intake valve closing (EIVC or LIVC). Therefore, in this paper, based on the engine with CR of 13.5 and electromagnetic valves train (EMVT), a comparative study was carried out to investigate the effects of EIVC and LIVC on engine performance, by theoretical modeling and calculation. The results show that, at high load, EIVC strategy is more preferred than LIVC owing to its lower total power consumption, which can improve the indicated mean effective pressure (IMEP) by 0.0371 bar, while enhance turbulence intensity and improve combustion. And at part load, the advantage for EIVC declines gradually, nevertheless, it can still sensitively adjust the EGR rate and thus reduce NOx. This results of quantitative analysis about two Miller cycles can provide valuable reference for engine designers and researchers.

Research of the possibility of increasing the detonation stability of a marine dual-fuel diesel engine

This article examined the environmental and economic prerequisites for the widespread use of dual-fuel diesel engines in the global fleet. A brief overview of the existing cycles used in marine dual-fuel diesel engines was made.. The prospects and problems of their further development were also considered. The basis of the article is the study of a possible increase in the detonation resistance of dual-fuel diesels through the use of a strong Miller cycle. The research was conducted using the software of the leading company in the field of design and engine building AVL List GmbH. As a prototype, the serial marine engine MAN 8L51/60DF was adopted, and the model is based on data obtained during three years of its operation

Experimental investigation of the effects of Miller timing on performance, energy and exergy characteristics of two-stage turbocharged marine diesel engine

In order to analyze and improve the thermodynamic process of marine diesel engines and improve energy efficiency, the experimental study of the influence of Miller timing on the performance, energy and exergy of marine diesel engines was realized by redesigning the camshaft and adjusting the two-stage turbocharging system. The research results shown that with the increase of Miller timing, the average pumping loss decreased, the peak HRR and the maximum combustion temperature decreased, and the crank angle corresponding to the peak HRR was delayed backward, resulting in a backward delay of the 50% point, the 10–90% combustion duration was extended, and the EER, EEE and BTE first increased and then decreased. Secondly, the Miller cycle combined with two-stage turbocharging could make marine diesel engines reduced NOx emissions by more than 25% with a small effect on the BSFC. In addition, as the Miller timing increased, the proportion of exhaust energy, heat transfer energy to coolant, mechanical loss, and combustion loss all decreased, while the proportion of brake output work first increased and then decreased. The change trends of exergy terms with load and Miller timing were similar to their corresponding energy. It was worth noting that the exhaust exergy accounted for about 40% of the exhaust energy, while the heat transfer exergy to coolant accounted for only 13% of the heat transfer energy to coolant, indicating that the exhaust energy quality was higher, the utilization potential was greater, and the efficient utilization interval was concentrated in the high load area.

Numerical investigation on combustion system optimization of stoichiometric operation natural gas engine based on knocking boundary extension

In this work, a computational fluid dynamic model is established based on a natural gas engine and experimental results, and the combustion system of stoichiometric operation natural gas engine (intake manifolds and combustion chambers) is optimized based on knocking boundary extension under high load condition. In addition, the effects of Miller cycle and EGR on the combustion and knocking boundary extension are also explored. Results reveal that the intensity and distribution of in-cylinder turbulent kinetic energy will significantly affect the flame development and combustion characteristics. The combination of spiral intake manifold + K15 chamber shows better performance in antiknock because of the enhanced in-cylinder airflow motion and flame development. In addition, the Miller cycle with the late intake valve close timing of 20 °CA can effectively extend the antiknock performance of the engine under the current operating operation. EGR has limited effect on turbulent kinetic energy, and the primary affective mechanism of EGR on knocking is mainly achieved by changing the in-cylinder temperature during the combustion process. It is found that with tangential and spiral intake manifold + K15 chamber, combined with late intake valve closing (20 °CA) and EGR (30%), the indicated mean effective pressure (IMEP) can be increased by 5.21% within the knocking boundary compared to that of the original combustion system.

Research on the fuel consumption conservation potential of ADAS on passenger cars

Under the trend of automobile electrification, network connection, and intelligence, EU and USA have carried out fuel-saving research and initiatives on ADAS and CAV. The eCoMove project has aimed at economically optimal driving control and traffic management; MAVEN discusses the technical path of GLOSA (Green Light Optimal Speed Advisory) and ecological auto-driving EAD (Eco-Autonomous Driving) by smoothing the vehicle speed. The American NEXTCAR project contains multiple projects. When supplemented with DSF (Dynamic Skip Fire) and 48V technology, the road test led by Ohio State University resulted in a 15% fuel saving rate. Platoon and optimizing intersection signal lights can offer vehicles a more fuel-efficient condition; slope energy utilization, HEV SOC active management, cold storage evaporator, coasting, 48V and mDSF (miller cycle Dynamic Skip Fire) fuel-saving potential has been fully utilized.

Improving the performance of the passive pre-chamber ignition concept for spark-ignition engines fueled with natural gas

Passive pre-chamber ignition concept has been proven to be an excellent strategy to increase the ignition energy and enhance the combustion velocity even when spark-ignition engines are operating in diluted conditions. Other benefits of this system are the increased combustion stability and combustion efficiency, reducing hydrocarbons and carbon monoxide emissions. However, these advantages are limited at some operation conditions such as low engine load or diluted conditions since both the energy available in the pre-chamber and the scavenge of combustion products are compromised. In this framework, numerical studies using two different computational tools, based on one-dimensional modeling, are utilized to gain knowledge about the governing parameters and to improve the design of a pre-chamber when the engine operates at these restrictive conditions. In particular, the impact of the pre-chamber volume, the total cross sectional area of the holes and tangential angle of the nozzles have been numerically evaluated. Different pre-chamber designs were proposed and experimentally tested in a single-cylinder, high compression ratio turbocharged spark-ignition engine fueled with compressed natural gas and operating on Miller cycle. Results give valuable insight into the key aspects of the internal geometry and some relevant design paths to follow for a suitable pre-chamber definition.

Research of NOx Reduction on a Low-Speed Two-Stroke Marine Heavy-Fuel Oil Engine

AbstractMany methods have been developed to reduce emissions from marine heavy-fuel oil (HFO) engines. The effects of exhaust gas recirculation (EGR), Miller cycle, humid air motor (HAM), and start...

Investigation on the EGR effect to further improve fuel economy and emissions effect of Miller cycle turbocharged engine

Miller cycle and EGR have been proved to be the effective ways to improve the engine performance. In order to realize Miller cycle with high compression ratio, the piston and intake cam profile were redesigned for a turbocharged GDI engine. In-cylinder pressure was taken to clarify the difference of working cycles between Miller and Otto cycle. At full load, Miller engine with high compression ratio leads to the knock at a low speed. But at a high speed, engine can operate nearer stoichiometric conditions and get better economy. At partial load, engine can adopt higher compression ratio and intake pressure. The reduction of pumping losses and exhaust gas energy have become the key factors to improve fuel economy. On the basis, LP cooled EGR was introduced to study the further effect on Miller cycle. In-cylinder temperature is reduced due to the EGR dilution and heat capacity effect. Thus, engine can adopt larger ignition advance angle to obtain better combustion phase. For emission analysis, lower temperature can effectively reduce NOx emissions. However, because of the extended combustion duration and complex piston shape, insufficient combustion will result in the increase of THC. The constant excess-air coefficient makes the CO emissions almost unchanged.

A methodology for the assessment of the knock mitigation potential of a port water injection system

The introduction of innovative spark ignition engines featuring extreme downsizing, high compression ratio and Miller Cycle has led to an increasing interest in water injection as one of the most promising countermeasures to reduce the knock risk. In this paper, the potential of a Port Water Injection (PWI) system integrated in a state-of-the-art turbocharged downsized spark ignition engine was investigated through both numerical simulations and experiments. A 3D-CFD water spray model, properly calibrated against experimental data, was used to predict water spray evaporation and to evaluate the engine oil dilution risk. On the basis of the 3D CFD analysis, two new indexes were specifically defined and correlated with experimental data to evaluate the effectiveness of water injection in terms of knock mitigation and the risk of engine oil dilution.After assessing their reliability by means of a sensitivity analysis on both calibration parameters and hardware configurations, the proposed indexes were successfully exploited to perform a preliminary design and optimization of the water injection system.

NOx Emission Reduction Technology for Marine Engine Based on Tier-III: A Review

The development of maritime trade has greatly promoted the development of diesel engines. However, with the increasingly serious environmental problems, more and more attention has been paid to the exhaust emissions of high-power marine diesel engines. The restrictions on SOx have been implemented globally, and the limitation of the NOx will be the next priority. This paper illustrates (a) Principle and research progress of NOx emissions-reduction technology of marine diesel engine; (b) Summary of advantages and disadvantages among various reduction technologies and their reduction effects; (c) The application effect of mainstream technology on board. Firstly, since exhaust gas recirculation (EGR) can achieve Tier-III directly from Tier-I without considering the increased fuel consumption. It is deemed as the most promising technology to reduce emissions by controlling combustion condition. However, EGR has shortcomings of excessive increase in fuel consumption and generation of waste water, which need to be solved immediately. Secondly, selective catalytic reduction (SCR) is the most effective and straightforward means to achieve Tier-III. Despite of the continuous optimization of SCR unit volume, the problem of scrap catalyst seriously limits its wide application. How to match the supercharger more efficiently is a key factor in choosing between high and low pressure SCR. Thirdly, nature gas (NG) engines are capable of achieving a reduction in NOx, but in order to meet the requirements of Tier-III, it still needs to be assisted by other technologies. The emissions of hydrocarbon (HC) and CO in NG engines are huge defects that must be solved. Lastly, technologies such as the Miller cycle, Two-stage supercharging and mixed-water combustion can also reduce emissions but were rarely used alone. These technologies can be combined with EGR, SCR and NG engines to optimize the engines’ economy and emission characteristics.

Analysis and Multi-Parametric Optimisation of the Performance and Exhaust Gas Emissions of a Heavy-Duty Diesel Engine Operating on Miller Cycle

A major issue nowadays that concerns the pollution of the environment is the emissions emerging from heavy-duty internal combustion engines. Such concern is dictated by the fact that the electrification of heavy-duty transport still remains quite challenging due to limitations associated with mileage, charging speed and payload. Further improvements in the performance and emission characteristics of conventional heavy-duty diesel engines are required. One of a few feasible approaches to simultaneously improve the performance and emission characteristics of a diesel engine is to convert it to operate on Miller cycle. Therefore, this study was divided into two stages, the first stage was the simulation of a heavy-duty turbocharged diesel engine (4-stroke, 6-cylinder and 390 kW) to generate data that will represent the reference model. The second stage was the application of Miller cycle to the conventional diesel engine by changing the degrees of intake valve closure and compressor pressure ratio. Both stages have been implemented through the specialist software which was able to simulate and represent a diesel engine based on performance and emissions data. An objective of this extensive investigation was to develop several models in order to compare their emissions and performances and design a Miller cycle engine with an ultimate goal to optimize diesel engine for improved performance and reduced emissions. This study demonstrates that Miller cycle diesel engines could overtake conventional diesel engines for the reduced exhaust gas emissions at the same or even better level of performance. This study shows that, due to the dependence of engine performance on complex multi-parametric operation, only one model achieved the objectives of the study, more specifically, engine power and torque were increased by 5.5%, whilst nitrogen oxides and particulate matter were decreased by 30.2% and 5.5%, respectively, with negligible change in specific fuel consumption and CO2, as average values over the whole range of engine operating regimes.

Multi-criteria performance analysis of dual miller cycle – Organic rankine cycle combined power plant

Performance improvement in power generation systems have been current issues in the recent years. Combined power plants including Organic Rankine Cycle are promising for performance improvement to generate efficient energy. This study has examined a way of optimum conditions for both efficiency and exhaust emissions by introducing a combined plant of Dual Miller Cycle-Organic Rankine Cycle focused on diesel engine applications. Three different types of working fluids with appropriate characters are used in the Organic Rankine Cycle. Effective efficiency, effective power, exergy destruction, effective power density, ecological coefficient of performance, exergetic efficiency and effective ecological power density are investigated for both Dual Miller Cycle and Dual Miller Cycle – Organic Rankine Cycle combined plant configurations. The impacts of engine design and working parameters such as cycle temperature ratio, cycle pressure ratio, engine speed, cylinder dimensions and equivalence ratio on the performance parameters are investigated. Octafluorocyclobutane and pentafluoropropane have been used as working fluids of Organic Rankine Cycle. The results showed that pentafluoropropane is the best suitable organic fluid. The effective efficiency, ecological coefficient of performance and exergy destruction have been found as 45.42%, 74.14% and 12.4 kW, respectively. The results of this examination have shown optimum values of the engine design and operating parameters in terms of thermo-ecological feasibility which also reduces harmful ship emissions like nitrous oxides.

The Virtual Engine Development for Enhancing the Compression Ratio of DISI-Engines Combining Water Injection, Turbulence Increase and Miller Strategy

<div class="section abstract"><div class="htmlview paragraph">The increase in efficiency is the focus of current engine development by adopting different technologies. One limiting factor for the rise of SI-engine efficiency is the onset of knock, which can be mitigated by improving the combustion process. HCCI/SACI represent sophisticated combustion techniques that investigate the employment of pre-chamber with lean combustion, but the effective use of them in a wide range of the engine map, by fulfilling at the same time the need of fast load control are still limiting their adoption for series engine. For these reasons, the technologies for improving the characteristics of a standard combustion process are still largely investigated. Among these, water injection, in combination with the Miller cycle, offers the possibility to increase the knock resistance, which in turn enables the rise of the engine geometric compression ratio.</div><div class="htmlview paragraph">The need for reducing measurements and executing specific tests are mandatory to make still the IC-Engines competitive to other powertrain solutions. The implementation of water injection can be realized through different engine layouts and injection strategies. Besides, the results of injecting water are strictly dependent on the base engine features and load point. Therefore, virtual development becomes essential to study many engine configurations.</div><div class="htmlview paragraph">In this work, indirect and direct water injection strategies are considered to increase the engine knock limit. The investigation is conducted with the 3D-CFD-Tool QuickSim, which was developed at IVK/FKFS Stuttgart. The implementation of water physics, the extension of simulation domain up to full engine (including intake and exhaust system), and the possibility to analyze multiple cycles allow understanding the effects of water and valve timing on engine behavior. The simulation validation was accomplished through the experiments carried out on a single-cylinder engine, equipped with indirect and direct water injections. The set-up of a virtual test bench is exploited to increase the engine geometric compression ratio, by combining Miller valve timing strategies with water injection. Particularly the investigation of a water-optimized injector targeting and position for the direct water injector enables increasing the turbulence within the cylinder and compensating for the effect of early intake valve closure. With the support of a newly developed knock model, the effect of water and the modified valve overlap on the combustion process is analyzed, to increase engine efficiency and propose a new approach for future powertrain engine development process.</div></div>

Effects of miller cycle strategies on combustion characteristics and knock resistance in a spark assisted compression ignition (SACI) engine

Spark assisted compression ignition (SACI) is a proven strategy to extend the operation load range and control the combustion phase of homogeneous charge compression ignition (HCCI) with high thermal efficiency. However, the occurrence of abnormal combustion, such as engine knock, limits SACI to operate in a wider load range. In this paper, the effects of Miller cycle on knock suppression effect and combustion characteristics are experimentally studied using a single-cylinder gasoline engine. Two types of Miller cycle tests under different boundary conditions are conducted: one is Miller cycle with constant negative valve overlap (NVO), and the other is Miller cycle with variable NVO while maintaining intake charge. The results show that different Miller cycle strategies can be used to adjust the ratio of fuel consumed by flame propagation and auto-ignition, and it is effective to optimize the combustion process in SACI to suppress engine knock while maintaining high efficiency. The results show that thermal efficiency of the optimized condition increased by 2.75% compared to the baseline. Besides, split injection strategy is also studied and the results show that a proper second injection timing (180-100 CAD BTDC) could moderate engine knock.

Exploring the high load potential of diesel–methanol dual-fuel operation with Miller cycle, exhaust gas recirculation, and intake air cooling on a heavy-duty diesel engine

Legislations concerning emissions from heavy-duty diesel engines are becoming increasingly stringent. This requires conventional diesel combustion to be compliant using costly and sophisticated aftertreatment systems. Preferably, diesel–methanol dual-fuel is one of the suitable alternative combustion modes as it can potentially reduce the formation of nitrogen oxide and soot emissions which characterised the diesel mixing-controlled combustion. This is primarily due to the high latent heat of vaporisation and oxygen content of the methanol fuel. At high engine loads, however, the potential of diesel–methanol dual-fuel operation is constrained by the excessive combustion pressure rise rate and peak in-cylinder pressure, which limits both the engine efficiency and the percentage of methanol that can be used. For the first time, experimental studies were conducted to explore advanced combustion control strategies such as Miller cycle, exhaust gas recirculation, and intake air cooling for improving upon high load diesel–methanol dual-fuel combustion. Experiments were carried out at 1200 r/min and 18 bar indicated mean effective pressure on a single-cylinder heavy-duty diesel engine, which equipped with a high pressure common rail diesel injection, a methanol port fuel injection, and a variable valve actuation system on the intake camshaft. Results showed that the methanol energy fraction of a conventional diesel–methanol dual-fuel operation with a baseline intake valve closing timing was limited to 28%. This was due to the high combustion temperature at a high load which advanced the ignition timing of the premixed charge, resulting in high levels of pressure rise rate. The application of lower effective compression ratio and intake air temperature ( Tint) effectively decreased the compression temperature, which successfully delayed the ignition timing of the premixed charge. This allowed for a more advanced diesel injection timing to achieve improvement in the thermal efficiency and potentially enabled a higher methanol substitution ratio. Although the introduction of exhaust gas recirculation demonstrated very slight impact on the ignition timing of the premixed charge, a higher net indicated efficiency was observed due to a relatively lower local combustion temperature which reduced heat transfer loss. Moreover, the optimised diesel–methanol dual-fuel operation allowed a higher methanol energy fraction of 40% to be used at an effective compression ratio of 14.3 and Tint of 305 K and achieved the highest net indicated efficiency of 47.4%, improving by 3.7% and 2.6%, respectively, when compared to the optimised conventional diesel combustion (45.7%) and conventional diesel–methanol dual-fuel (46.2%). This improvement was accompanied with a reduction of 37% in nitrogen oxide emissions and little impact on soot emissions in comparison with the conventional diesel combustion.

Experimental study on the effects of the Miller cycle on the performance and emissions of a downsized turbocharged gasoline direct injection engine

The Miller cycle has been proven to be an effective way to improve the thermal efficiency for gasoline engines. However, it may show insufficient power performance at certain loads. In this study, the objective is to exploit the advantages of the Miller-cycle engines over the original Otto-cycle engines. Therefore, a new camshaft profile with early intake valve closure was devised, and two various pistons were redesigned to obtain higher compression ratio 11.2 and 12.1, based on the original engine with compression ratio 10. Then, a detailed comparative investigation of the effects of Miller cycle combined with higher compression ratio on the performance and emission of a turbocharged gasoline direct injection engine has been experimentally carried out based on the engine bench at full and partial loads, compared to the original engine. The results show that, at full load, for a turbocharged gasoline direct injection engine utilizing the Miller cycle, partial maximum power is compromised about 1.5% while fuel consumption shows a strong correlation with engine speed. At partial load, since the Miller effect can well reduce the pumping mean effective pressure, thus improves the fuel economy effectively. In addition, the suppression of the in-cylinder combustion temperature induced by the lower effective compression ratio contributes to the reduction of nitrogen oxide emission greatly. However, the total hydrocarbon emission increases slightly. Therefore, a combination of the Miller cycle and highly boosted turbocharger shows great potential in further improvement of fuel economy and anti-knock performance for downsized gasoline direct injection engines.