Compression Ratio and Derived Cetane Number Effects on Gasoline Compression Ignition Engine Running with Naphtha Fuels
In the context of stringent future emission standards as well as the need to reduce emissions of CO2 on a global scale, the cost of manufacturing engines is increasing. Naphtha has been shown to have beneficial properties for its use as a fuel in the transportation sector. Well to tank CO2 emissions from the production of Naphtha are lower than any other fuel produced in the refinery due to its lower processing requisites. Moreover, under current technology trends the demand for diesel is expected to increase leading to a possible surplus of light fuels in the future. Recent research has demonstrated that significant fuel consumption reduction is possible based on a direct injection gasoline engine system, when a low quality gasoline stream such as Naphtha is used in compression ignition mode. With this fuel, the engine will be at least as efficient and clean as current diesel engines but will be more cost effective (lower injection pressure, HC/CO after-treatment rather than NOx).
Increasing Efficiency in Gasoline Powertrains with a Two-Stage Variable Compression Ratio (VCR) System
Downsizing in combination with turbocharging currently represents the main technology trend for meeting CO2 emissions with gasoline engines. Besides the well-known advantages of downsizing the compression ratio has to be reduced in order to mitigate knock at higher engine loads along with increased turbocharging demand to compensate for the reduction in power. Another disadvantage occurs at part load with increasing boost pressure levels causing the part load efficiencies to deteriorate. The application of a variable compression ratio (VCR) system can help to mitigate these disadvantages. The 2-stage VCR system with variable kinetic lengths entails variable powertrain components which can be used instead of the conventional components and thus only require minor modifications for existing engine architectures. The presented variable length connecting rod system has been continuously developed over the past years.
Reducing Part Load Pumping Loss and Improving Thermal Efficiency through High Compression Ratio Over-Expanded Cycle
In vehicle application, most of time gasoline engines are part load operated, especially in city traffic, part load operation covers most common operation situations, however part load performances deteriorate due to pumping losses and low thermal efficiency. Many different technologies have been applied to improve part load performances. One of them is to adopt over-expanded (Atkinson/Miller) cycle, which uses late/early intake valve closing (LIVC/EIVC) to reduce pumping losses in part load operation. But over-expanded cycle has an intrinsic drawback in that combustion performance deteriorates due to the decline in the effective compression ratio (CR). Combining with high geometry CR may be an ideal solution, however there is a trade-off between maintaining a high CR for good part load fuel consumption and maintaining optimal combustion phasing at higher load.
Design of Affordable Multi-Cylinder Variable Compression Ratio (VCR) Engine for Advanced Combustion Research Purposes
The legislative limitations of vehicle exhaust emission together with the current and prospective growth of road transportation intensity calls for continuous effort to improve vehicle powertrains that must be done both in design and technology domains. As the powerful remedy, a new generation of combustion engines fitted with the variable compression ratio (VCR) feature is considered. VCR technology can extend the range of clean engine operations such as HCCI or other advanced combustion strategies, enabling reduction of all exhaust harmful compounds simultaneously. There are several known technologies for performing variable compression ratio in combustion engines; some of them have been successfully tested in engine research rigs and/or vehicle prototypes. In the paper the proposal of application of variable compression ratio technology in four-cylinder, water-cooled engine is presented.
The main focus of this work is to study variable compression ratio engines. The study begins with an analyze of the benefits of a variable compression ratio for Otto Cycle engines including mechanisms that already exist, the usual constructive solutions and the project criteria adopted in the area. The kinematic and dynamic model of SVC Saab mechanism, using Newton-Euler equations, is developed. Also, the cylinder pressure curve is important to be determined. The pressure curve and the cylinder volume function will be evaluated analytically considering the variation of the compression ratio. Furthermore, in order to know the influence of the design parameters in kinematic and dynamic, some sensitivity analysis of the engine mechanism will be performed. Finally, a computational implementation of the engine mechanisms analyzed will be done in Matlab language. This implementation includes the crank mechanisms and its kinematic and dynamic analyzes.
A dual piston Variable Compression Ratio (VCR) engine has been newly developed. This compact VCR system uses the inertia force and hydraulic pressure accompanying the reciprocating motion of the piston to raise and lower the outer piston and switches the compression ratio in two stages. For the torque characteristic enhancement and the knocking prevention when the compression ratio is being switched, it is necessary to carry out engine controls based on accurate compression ratio judgment. In order to accurately judge compression ratio switching timing, a control system employing the Hidden Markov Model (HMM) was used to analyze vibration generated during the compression ratio switching. Also, in order to realize smooth torque characteristics, an ignition timing control system that separately controls each cylinder and simultaneously performs knocking control was constructed.
Two-Stage Variable Compression Ratio with Eccentric Piston Pin and Exploitation of Crank Train Forces
By variation of the compression ratio the fuel consumption of high boosted gasoline engines can be reduced, due to operating with higher compression ratios at low load compared to an engine with fixed compression ratio. The two-stage VCR-system enables a high share of fuel saving potential relative to full variable systems. Considering a low cost manufacturability and a beneficial integratability into common engine architectures the length-adjustable conrod using an eccentric piston pin in the small eye has proved as the best concept. The adjustment is performed by a combination of gas and mass forces. This article describes the design of such a two-stage VCR-system as well as the functional testing under motored and fired engine operating conditions.
There was concern that a variable compression ratio engine fitted with a multi-link mechanism might produce louder booming noise due to the inertial forces caused by the lateral swing of the links. Accordingly, a relational expression between the inherent characteristics of the links was found to counterbalance those forces. As a result, it was found that a multi-link VCR engine designed on the basis of this link concept showed lower levels of horizontal excitation forces similar to the reduction of vertical forces. This suggests that, even without any add-on devices like a balance shaft, the engine can achieve the same level of booming noise performance as conventional engines. In addition, this new link concept obtained as a result of this study is expected to be effective in reducing higher-order interior noise as well.
A Dual Piston Variable Compression Ratio (VCR) engine has been newly developed. In order to ensure the strength of the Dual Piston, the design guidelines were established. There are two advantages of this design. One is the compactness and the compatibility with a mass production engine block. Another is less power consumption required during compression ratio switching. However, the durability is a challenge for this design because of the impact load during the switching driven by the inertial force of a reciprocating piston. In order to achieve a durable configuration, it was necessary to consider the dynamics of the stress after impact, from analysis of the impacting process during the switching. The analysis of stress and deformation mode was improved in accuracy by using Computer Aided Engineering (CAE) in the designing process.
A Multiple Factor Simulation and Emulation Approach to Investigate Advanced Air Handling Systems for Future Diesel Engines
To realize the required reductions in vehicle CO2 emissions pursued both voluntarily by the ACEA and possible legislatively by the EC, there is a move to increase the specific power output and thus efficiency of the automotive diesel engine. To this end there is a move towards engine downsizing and the adoption of significantly higher boost levels from air handling systems fitted to the engine. This task is complex and can often result in multiple configurations and iterations of prototype air handling hardware before performance criteria are met. The work detailed in this paper describes the modeling of high boost engine configurations using cycle simulation and a black box engine approach, the interactions between boost pressures; compression ratio and valve timing and duration have been investigated and will be described. Response surface models for engine performance have been developed such that rapid optimization of the engine air charge handling systems can be performed.
A variable compression ratio (VCR) technology, that has a new piston-crankshaft mechanism with multi links, has been patented and developed by Nissan for some years (This technology has been detailed in previous SAE paper 2003-01-0921 and 2005-01-1134). This paper will present the use of this VCR technology for Diesel engine. The objective set with the use of VCR for Diesel engine is mainly to reduce as much as possible engine out emission to prepare for long-term, more strict emission standards. Results presented will include the description of the 2l Diesel VCR engine and its VCR mechanism adapted to Diesel constraints. Combustion tests have been performed with the use of HCCI (Homogeneous Charge Compression Ignition) combustion. This technology is still in a research phase in Renault: the adaptation of VCR technology to a Diesel engine consists in the modification of several parts with the addition of lower links, control links and control shaft.
The concept of Extended Expansion (EE) in Internal Combustion (IC) engines though very old is gaining importance now a days due to the scope it possess in areas of fuel economy and engine emissions. In this paper, a single cylinder engine with variable expansion to compression ratio (ER/CR) was analyzed for its performance when copper coating is done on the surface of cylinder head - piston combination. Copper element that can promote pre-flame reaction was coated on the surface of cylinder head and piston. ER/CR ratio was varied with Late Intake Valve Closing Time (LIVCT) and the load was controlled with LIVCT-VCR combination. LIVCT was achieved by Electronic Valve Timing (EVT) system using solenoid valve. Due to the catalytic effect of copper coating it was found that for the copper coated EE engine, the brake thermal efficiency was improved by 4% compared to standard engine operating at same conditions.
Thermodynamic Cycles of Internal Combustion Engines for Increased Thermal Efficiency, Constant-Volume Combustion, Variable Compression Ratio, and Cold Start
An internal-combustion engine platform that may operate on a portfolio of cycles for increased engine expansion ratio, combustion under constant volume, variable compression ratio, and cold start is introduced. Through unique thermodynamic cycles, the engine may be able to operate on a much greater expansion ratio than the compression ratio for a significantly improved thermal efficiency. This improvement is attained without involving a complex mechanical structure or an enlarged engine size, and at the same time without reducing the compression ratio. The engine with these features may serve as an alternative to the Atkinson cycle engine or the Miller cycle engine. Additionally, based on the same engine platform, the engine may operate on other cycles according to the load conditions and environmental considerations. These cycles include those for combustion under constant volume, variable compression ratio under part load conditions, and cold start for alternative fuels.
A multiple-link variable compression ratio (VCR) mechanism is suitable for a long-stroke engine by providing the following characteristics: (1) a nearly symmetric piston stroke and (2) an upper link that stays vertical around the time of the maximum combustion pressure. These two characteristics work to reduce force inputs to the piston. The maximum inertial force around top dead center is reduced by the effect of the first characteristic. The second characteristic is effective in reducing piston side thrust force and helps ease piston pin lubrication. Because of the combined effect of these characteristics, the piston skirt can be made smaller and the piston pin can be shortened. That makes it possible for the piston skirt and piston pin to move between the counterweights, resulting in a downward extension of the piston stroke. As a result, a longer-stroke engine mechanism can be achieved without making the cylinder block taller.
Some automakers have been studying variable compression ratio (VCR) technology as one possible way of improving fuel economy. In previous studies, we have developed a VCR mechanism of a unique multiple-link configuration that achieves a piston stroke characterized by semi-sinusoidal oscillation and lower piston acceleration at top dead center than on conventional mechanisms. By controlling compression ratio with this multiple-link VCR mechanism so that it optimally matches any operating condition, the mechanism has demonstrated that both lower fuel consumption and higher output power are simultaneously possible. However, it has also been observed that fuel consumption does not reduce further once the compression ratio reached a certain level. This study focused on the fact that the piston-stroke characteristic obtained with the multiple-link mechanism is suitable to a longer stroke.
The effects of variable compression ratio (CR) and fuel composition on thermal efficiency were investigated in a homogeneous charge compression ignition (HCCI) engine using blends of n-heptane and toluene with research octane numbers (RON) of 0 to 90. Experiments were conducted by performing CR sweeps at multiple intake temperatures using both unthrottled operation, and constant Φ conditions by throttling to compensate for varying air density. It was found that CR is effective at changing and controlling the HCCI combustion phasing midpoint, denoted here as CA 50. Thermal efficiency was a strong function of CA 50, with overly advanced CA 50 leading to efficiency decreases. Increases in CR at a constant CA 50 for a given fuel composition did, in most cases, increase efficiency, but the relationship was weaker than the dependence of efficiency on CA 50.
The use of Jojoba Methyl Ester as a pilot fuel was investigated for almost the first time as a way to improve the performance of dual fuel engine running on natural gas or LPG at part load. The dual fuel engine used was Ricardo E6 variable compression diesel engine and it used either compressed natural gas (CNG) or liquefied petroleum gas (LPG) as the main fuel and Jojoba Methyl Ester as a pilot fuel. Diesel fuel was used as a reference fuel for the dual fuel engine results. During the experimental tests, the following have been measured: engine efficiency in terms of specific fuel consumption, brake power output, combustion noise in terms of maximum pressure rise rate and maximum pressure, exhaust emissions in terms of carbon monoxide and hydrocarbons, knocking limits in terms of maximum torque at onset of knocking, and cyclic data of 100 engine cycle in terms of maximum pressure and its pressure rise rate.
A Study of a Compression Ratio Control Mechanism for a Multiple-Link Variable Compression Ratio Engine
An engine compression ratio control system, consisting of a multiple-link mechanism, must be capable of both varying and holding the compression ratio under a condition of torque fluctuations caused by the cylinder pressure and inertial force from moving parts. A compact system design has been achieved by taking advantage of the features of the multiple-link mechanism, including the use of cylinder pressure assist for easy operation when high speed response is required, and using the holding device which has less energy consumption to maintain the compression ratio.
Characteristics of Combustion Stability and Emission in SCCI and CAI Combustion Based on Direct-Injection Gasoline Engine
Emissions remain a critical issue affecting engine design and operation, while energy conservation is becoming increasingly important. One approach to favorably address these issues is to achieve homogeneous charge combustion and stratified charge combustion at lower peak temperatures with a variable compression ratio, a variable intake temperature and a trapped rate of the EGR using NVO (negative valve overlap). This experiment was attempted to investigate the origins of these lower temperature auto-ignition phenomena with SCCI and CAI using gasoline fuel. In case of SCCI, the combustion and emission characteristics of gasoline-fueled stratified-charge compression ignition (SCCI) engine according to intake temperature and compression ratio was examined. We investigated the effects of relative air/fuel ratio, residual EGR rate and injection timing on the CAI combustion area.
The concept of the Early or late Intake valve closing cycle has been examined over the years as a technique for improving fuel economy in conjunction with the use of a three-way catalyst for excellent exhaust emission performance. With this concept, the intake valve closing (IVC) timing is set either before or after bottom dead center. With the emergence of continuously variable valve timing and lift (VEL) systems in recent years, the Early IVC cycle has become a more familiar concept. However, the Early IVC cycle has an intrinsic drawback in that, although pumping losses decrease when charging efficiency is reduced in connection with IVC control, combustion performance deteriorates due to the decline in the effective compression ratio. In recent years, full-scale research has been undertaken on variable compression ratio systems as a new type of variable engine mechanism separate from variable valving.
This paper describes the demands and potentials of current and future gasoline combustion systems regarding the fuels gasoline, natural gas, and Hydrogen. At first, fuel specifications that are crucial for the spark ignition process are compared. These are compared with the requirements of the combustion system. Potentials for the compensation of power loss, efficiency improvement and emission reduction using alternative fuels are discussed taking into account fuel-specific properties. While full load drawbacks with natural gas compared with gasoline can be reduced to less than 5% by combustion system tuning, Hydrogen operation with port injection leads to reductions of about 25 to 30%. These drawbacks can be compensated with boosting where both methane and Hydrogen are qualified due to their burning characteristics. Compared with λ=1 operation especially Hydrogen offers efficiency benefits of up to 30% in a wide mapping range due to quality control.
Internal combustion engine knock has limited compression ratios of spark ignition engines for most of the history of gasoline engines. This limitation continues to exist today. While knock is generally a low engine speed, high load phenomenon, this operating condition is infrequently used by many vehicle operators, and if the engine is brought to this operating condition generally little time is spent in this knock prone condition. This study seeks to investigate the transition into knock due to throttle changes from part to full load. The experimental results using a CFR engine operating on iso-octane fuel show that knock is delayed by at least one high load engine cycle after the throttle is opened. Optimization of spark timing to account for this effect provides for the best increase of engine load without audible knock occurring.
The objective of this program was to improve the HCCI combustion process on a single-cylinder VCR engine by calibrating engine and HCCI operation specific factors such as EGR flow rates, intake air pressure, intake air temperature, compression ratio, etc. Due to the large number of factors to be investigated, a statistical design of experiments method (DoE) was utilized in order to reduce the number of test combinations in the calibration test matrix and, thus, the duration of the engine calibration task. Upon completion of the HCCI engine calibration, the engine was operated through a steady-state test matrix representing vehicle certification test cycles. Weighting factors for each of the test points were applied to estimate the engine performance and emissions in respect to certification requirements.
Potential Benefits in Heavy Duty Diesel Engine Performance and Emissions from the Use of Variable Compression Ratio
Worldwide demand for reduction of automotive fuel consumption and carbon dioxide emissions results in the introduction of new diesel engine technologies. A promising technique for increasing the power density of reciprocating engines, improving fuel economy and curtailing engine exhaust emissions is the use of variable compression ratio (VCR) technology. Several automotive manufacturers have developed prototype vehicles equipped with VCR gasoline engines. The constructive pattern followed to alter the compression ratio varies with the manufacturer. The implementation of VCR technology offers two main advantages: the reduction of CO2 emissions due to optimal combustion efficiency in the entire range of engine operating conditions and the increase of power concentration due to high boosting of a small engine displacement (i.e., engine downsizing).
The authors have previously proposed an engine system that uses a new piston-crank system incorporating a multiple-link mechanism to vary the piston's motion at top dead center and thereby obtain the optimum compression ratio matching the operating conditions. This multiple-link variable compression ratio (VCR) mechanism can be installed without increasing the engine size or weight substantially by selecting a suitable type of link mechanism and optimizing the detailed specifications. Previous papers by the authors have made clear the features of the VCR mechanism that facilitates continuously variable control of the compression ratio . It was shown that engine friction attributable to piston-side thrust can be reduced through an upright orientation of the upper link in the expansion strokes.
Variable compression engines are a mean to meet the demand on lower fuel consumptions. A high compression ratio results in high engine efficiency, but also increases the knock tendency. On conventional engines with fixed compression ratio, knock is avoided by retarding the ignition angle. The variable compression engine offers an extra dimension in knock control, since both ignition angle and compression ratio can be adjusted. A vital question is thus what combination of compression ratio and ignition angle should be used to achieve maximum engine efficiency. Fuel optimal control of a variable compression engine is studied and it is shown that a crucial component is the model for the engine torque. A model for the produced work that captures the important effects of ignition and compression ratio is proposed and investigated. The main task for the model is to be a mean for determining the fuel optimal control signals, for each requested engine torque and speed.
Experimental Investigations on Performance and Emission Characteristics of CNG in a Spark Ignition Engine
In this study, systematical experiments were carried out to analyze the performance and emission characteristics of CNG using a single cylinder, variable compression ratio, spark ignition engine. Studies were also carried out to evaluate optimum spark timing at different equivalence ratios for the maximum brake torque. Effects of compression ratio on fuel consumption and exhaust emissions were also studied. Compressed natural gas showed 3-5% higher thermal efficiency and 15% lower fuel consumption as compared to gasoline. Also CO emissions were lower by 30-80% in rich zone and NOx emissions were lower by 12% at equivalence ratio of 1.0. At 50% throttle opening, MBT timings for natural gas were 4 to 6 degree more advanced depending on equivalence ratios as compared to gasoline. Increasing the compression ratio from 8 to 10 and 12 improved the thermal efficiency by 3 and 8 percent respectively. However, NOx emissions were increased by 15% and 40% respectively.
The MCE-5 VCR engine includes a gear mechanism, which transmits the piston motion to the crankshaft via the connecting rod. This arrangement reaches main VCR engines requirements in respect of conventional engines characteristics such as piston kinematics, combustion chamber shape, or vehicle integration. In addition, the MCE-5 design includes a roller-guided piston, which allows for a significant friction losses reduction while ensuring a great compatibility with high supercharging pressures on high mileage. To ensure a safe operation and to meet all operational requirements, different studies have been carried out to design appropriate safe high loaded gears. Several hundred hours of tests have been carried out on test bench under various operating conditions to validate the gear fatigue resistance and lubrication of the gear MCE-5 device. Positive results lead to various gear improvements and mass production perspectives.
Operating Conditions Using Spark Assisted HCCI Combustion During Combustion Mode Transfer to SI in a Multi-Cylinder VCR-HCCI Engine
The Homogenous Charge Compression Ignition (HCCI) operating range in terms of speed and load does not cover contemporary driving cycles, e.g. the European driving cycle EC2000, without increased engine displacement, supercharging, or without excessive noise and high NOx emissions. Hence, the maximum achievable load with HCCI is too low for high load vehicle operation and a combustion mode transfer from HCCI to spark ignited (SI) has to be done. At some operating conditions spark assisted HCCI combustion is possible, which makes a mixed combustion mode and controlled combustion mode transfers possible. The mixed combustion region and the operating conditions are investigated in this paper from lean SI limit to pure HCCI without SI assistance. Parameters as compression ratio, inlet air pressure, inlet air temperature, and lambda are used for controlling the mixed combustion mode. A strategy for closed-loop combustion mode transfer is discussed.
The potential of dual stage turbocharging and Miller Cycle for a six cylinders in line, 13 litres displacement, HD diesel engine was analysed in this work, by means of a 1-D engine simulation fluid dynamic code, coupled with a multi-zone combustion model for NOx and PM prediction. After a detailed validation process, based on an extensive experimental data set, the engine model was then used to predict the effects on engine performance and emission characteristics of different combinations of dual stage turbochargers, engine compression ratio values and intake valve lift profiles. The potential for an appreciable increase in the engine power, with a slight decrease in the specific fuel consumption and a remarkable decrease of NOx specific emissions was demonstrated.