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Technical Paper

Measurements and Modeling of Residual Gas Fraction in SI Engines

The residual gas in SI engines is one of important factors on emission and performance such as combustion stability. With high residual gas fractions, flame speed and maximum combustion temperature are decreased and there are deeply related with combustion stability, especially at Idle and NOx emission at relatively high engine load. Therefore, there is a need to characterize the residual gas fraction as a function of the engine operating parameters. A model for predicting the residual gas fraction has been formulated in this paper. The model accounts for the contribution due to the back flow of exhaust gas to the cylinder during valve overlap and it includes in-cylinder pressure prediction model during valve overlap. The model is derived from the one dimension flow process during overlap period and a simple ideal cycle model.
Technical Paper

Reducing Exhaust HC Emission at SI Engine Using Continuous and Synchronized Secondary Air Injection

The effect of secondary air injection (SAI) on exhaust hydrocarbon (HC) emission has been investigated in a spark-ignition (SI) single cylinder engine operating at steady-state cold condition. Both continuous SAI and synchronized SAI, which corresponds to intermittent secondary air injection to exhaust port, are tested. Oxidation characteristics of HC are monitored with a FID analyzer and exhaust gas temperatures with thermocouples. Effects of exhaust air-fuel ratio (A/F), location of SAI, and engine-A/F have been investigated. Results show that HC reduction rate increases as the location of SAI is closer to the exhaust valve for both synchronized and continuous SAIs. HC emission decreases with increasing exhaust-A/F when exhaust-A/F is rich, and is relatively insensitive when exhaust-A/F is lean. In synchronized SAI, SAI timing has significant effect on HC reduction and exhaust gas temperature. Optimum SAI timing observed is ATDC 100° and 230°.
Technical Paper

Reduced Chemical Kinetic Model of DME for HCCI Combustion

Homogeneous Charge Compression Ignition combustion engines could have a thermal efficiency as high as that of conventional compression-ignition engines and the production of low emissions of ultra-low oxides of NOx and PM. HCCI engines can operate on most alternative fuels, especially, dimethyl ether which has been tested as possible diesel fuel for its simultaneously reduced NOx and PM emissions. However, to adjust HCCI combustion to practical engines, the main problem about the HCCI engine must be solved; control of its ignition timing and burn rate over a range of engine speeds and loads. Detailed chemical kinetic modeling has been used to predict the combustion characteristics. But it is difficult to apply detailed chemical kinetic mechanism to simulate practical engines because of its high complexity coupled with multidimensional fluid dynamic models. Thus, reduced chemical kinetic modeling is desirable.
Technical Paper

Computational and Optical Investigation of Liquid Fuel Film on the Cylinder Wall of an SI Engine

The liquid fuel film on the cylinder liner is believed to be a major source of engine-out hydrocarbon emissions in SI engines, especially during cold start and warm-up period. Quantifying the liquid fuel film on the cylinder liner is essential to understand the engine-out hydrocarbon emissions formation in SI engines. In this work, the fuel film formation model was developed to investigate the distribution of wall fuel film on the cylinder wall of an SI engine. By integrating the continuity, momentum, and energy equations along the direction of fuel film thickness the simulation of the fuel film formation was carried out in the test rig. Spray impingement and fuel film models were incorporated into the computational fluid dynamics code, STAR-CD to calculate fuel film thickness and distribution of fuel film on the cylinder wall. With a laser-induced fluorescence method, the two-dimensional visualization of liquid fuel films was carried out to validate the simulation results.
Technical Paper

The Effects of Crevices on the Engine-Out Hydrocarbon Emissions in SI Engines

To understand the effects of crevices on the engine-out hydrocarbon emissions, a series of engine experiments was carried out with different piston crevice volumes and with simulated head gasket crevices. The engine-out HC level was found to be modestly sensitive to the piston crevice size in both the warmed-up and the cold engines, but more sensitive to the crevice volume in the head gasket region. A substantial decrease in HC in the cold-to-warm-up engine transition was observed and is attributed mostly to the change in port oxidation.
Technical Paper

Visualization of Mixture Preparation in a Port-Fuel Injection Engine During Engine Warm-up

The fuel injection process in the port of a firing 4-valve SI engine at part load and 25°C head temperature was observed by a high speed video camera. Fuel was injected when the valve was closed. The reverse blow-down flow when the intake valve opens has been identified as an important factor in the mixture preparation process because it not only alters the thermal environment of the intake port, but also strip-atomizes the liquid film at the vicinity of the intake valve and carries the droplets away from the engine. In a series of “fuel-on” experiments, the fuel injected in the current cycle was observed to influence the fuel delivery to the engine in the subsequent cycles.
Technical Paper

A Quasi-Dimensional Model for Prediction of In-Cylinder Turbulence and Tumble Flow in a Spark-Ignited Engine

Improving fuel efficiency and emission characteristics are significant issues in engine research. Because the engine has complex systems and various operating parameters, the experimental research is limited by cost and time. One-dimensional (1D) simulation has attracted the attention of researchers because of its effectiveness and relatively high accuracy. In a 1D simulation, the applied model must be accurate for the reliability of the simulation results. Because in-cylinder turbulence mainly determines the combustion characteristics, and mean flow velocity affects the in-cylinder heat transfer and efficiency in a spark-ignited (SI) engine, a number of sophisticated models have been developed to predict in-cylinder turbulence and mean flow velocity. In particular, tumble is a significant factor of in-cylinder turbulence in SI engine.
Technical Paper

An Experimental Investigation of Injection and Operating Strategies on Diesel Single Cylinder Engine under JP-8 and Dual-Fuel PCCI Combustion

The alternative fuel jet propellant 8 (JP-8, NATO F-34) can be used as an auto-ignition source instead of diesel. Because it has a higher volatility than diesel, it provides a better air-fuel premixing condition than a conventional diesel engine, which can be attributed to a reduction in particulate matter (PM). In homogeneous charged compression ignition (HCCI) or dual-fuel premixed charge compression ignition (PCCI) combustion or reactivity controlled compression ignition (RCCI), nitrogen oxides (NOx) can also be reduced by supplying external exhaust gas recirculation (EGR). In this research, the diesel and JP-8 injection strategies under conventional condition and dual-fuel PCCI combustion with and without external EGR was conducted. Two tests of dual-fuel (JP-8 and propane) PCCI were conducted at a low engine speed and load (1,500 rpm/IMEP 0.55 MPa). The first test was performed by advancing the main injection timing from BTDC 5 to 35 CA to obtain the emissions characteristics.
Technical Paper

An Investigation into the Operating Strategy for the Dual-Fuel PCCI Combustion with Propane and Diesel under a High EGR Rate Condition

In this work, the operating strategy for diesel injection methods and a way to control the exhaust gas recirculation (EGR) rate under dual-fuel PCCI combustion with an appropriate ratio of low-reactivity fuel (propane) to achieve high combustion stability and low emissions is introduced. The standards of combustion stability were carbon monoxide (CO) emissions below 5,000 ppm and a CoV of the indicated mean effective pressure (IMEP) below 5 %. Additionally, the NOx emissions was controlled to not exceed 50 ppm, which is the standard of conventional diesel combustion, and PM emissions was kept below 0.2 FSN, which is a tenth of the conventional diesel value without a diesel particulate filter (DPF). The operating condition was a low speed and load condition (1,500 rpm/ near gIMEP of 0.55 MPa).
Technical Paper

Development of Engine Control Using the In-Cylinder Pressure Signal in a High Speed Direct Injection Diesel Engine

Emissions regulations are becoming more severe, and they remain a principal issue for vehicle manufacturers. Many engine subsystems and control technologies have been introduced to meet the demands of these regulations. For diesel engines, combustion control is one of the most effective approaches to reducing not only engine exhaust emissions but also cylinder-by-cylinder variation. However, the high cost of the pressure sensor and the complex engine head design for the extra equipment are stressful for the manufacturers. In this paper, a cylinder-pressure-based engine control logic is introduced for a multi-cylinder high speed direct injection (HSDI) diesel engine. The time for 50% of the mass fraction to burn (MFB50) and the IMEP are valuable for identifying combustion status. These two in-cylinder quantities are measured and applied to the engine control logic.
Technical Paper

Study of a Stratification Effect on Engine Performance in Gasoline HCCI Combustion by Using the Multi-zone Method and Reduced Kinetic Mechanism

A gasoline homogeneous charged compression ignition (HCCI) called the controlled auto ignition (CAI) engine is an alternative to conventional gasoline engines with higher efficiency and lower emission levels. However, noise and vibration are currently major problems in the CAI engine. The problems result from fast burning speeds during combustion, because in the CAI engine combustion is controlled by auto-ignition rather than the flame. Thus, the ignition delay of the local mixture has to vary according to the location in the combustion chamber to avoid noise and vibration. For making different ignition delays, stratification of temperature or mixing ratio was tested in this study. In charge stratification, which determines the difference between the start of combustion among charges with different properties, two kinds of mixtures with different properties flow into two intake ports.
Technical Paper

An On-Line Model for Predicting Residual Gas Fraction by Measuring Intake/Exhaust and Cylinder Pressure in CAI Engine

CAI (Controlled Auto Ignition) combustion is already well known to be advantageous over conventional cycles in that it facilitates higher engine efficiency and has low emission characteristics. The CAI combustion process is mainly governed by in-cylinder RGF (Residual Gas Fraction), therefore achieving good control of in-cylinder RGF is essential in the development of CAI combustion engine. Usually, in-cylinder RGF controlled via low lift cam, short valve duration and negative valve overlap. More importantly on the other hand, accurate and instantaneous prediction of RGF must be done as a prerequisite to control. However, on-line prediction of RGF is not always practical due to the requirement of expensive fast response exhaust gas analyzers in the empirical case or otherwise due to theoretical models which are just too slow for application by means of simulation solving. In this paper, a newly enhanced theoretical model for predicting on-line in-cylinder RGF is introduced.
Technical Paper

Analysis of Cyclic Variation and the Effect of Fuel Stratification on Combustion Stabilityin a Port Fuel Injection (PFI) CAI Engine

CAI engine is well known to be advantageous over conventional SI engines because it facilitates higher engine efficiency and lower emission (NOx and smoke). However, its limited operation range, large cyclic variation, and difficulty in heat release control are still unresolved obstacles. Previous studies showed that a high load range of the CAI engine is limited mainly by the combustion noise caused by a stiff pressure rise (knock), and that a low load range is also limited by the combustion instability caused by the high dilution of residual gas. In this study, the characteristics of each cycle were analyzed to find the cause of the cycle variation at the high load limit of CAI operation. Moreover, to improve combustion stability, we tested the in-cylinder fuel stratification by applying nonsymmetrical fuel injection to the intake port. Experiments were performed on a PFI single cylinder research engine equipped with dual CVVT and low lift (2 mm) cam shaft with NVO strategy.
Technical Paper

Laminar Flame Speed Characteristics and Combustion Simulation of Synthetic Gas Fueled SI Engine

As the real-time supplying of hydrogen-rich gas becomes possible by the advances in the on-board fuel reforming technologies, utilizations of synthetic gas in IC engines are actively studied. However, due to the lack of fundamental studies on the combustion characteristics of synthetic gas, there is no precedent for the simulation of combustion process in synthetic gas fueled SI engine. In this study, the laminar flame speeds of synthetic gas and its mixture with iso-octane were calculated under extensive initial conditions of 3,575 points derived by combinations of temperature, pressure, fraction of lower heating value of synthetic gas and air-excess ratio variations.
Technical Paper

The Development of Real-time NOx Estimation Model and its Application

To meet the stringent emission regulations on diesel engines, engine-out emissions have been lowered by adapting new combustion concepts such as low-temperature combustion and after-treatment systems have also been used to reduce tailpipe emissions. To optimize the control of both in-cylinder combustion and the efficiency of an after treatment system to reduce NOx, the amount of real-time NOx emissions should be determined. Therefore, in previous studies, the authors developed a real-time NO estimation model based on the in-cylinder pressure and the data available from the ECU during engine operation. The model was evaluated by comparing its results with a CFD model, which agreed well. Then, the model was implemented on an embedded system which allows real-time applications, and was verified on a 2.2-liter diesel engine. The model showed good agreement with the experimental results at various steady-state conditions and simple transient conditions.
Technical Paper

Emission Reduction using a Close Post Injection Strategy with a Modified Nozzle and Piston Bowl Geometry for a Heavy EGR Rate

As EURO-6 regulations will be enforced in 2014, the reduction of NOx emission while maintaining low PM emission levels becomes an important topic in current diesel engine research. EGR is the most effective way to reduce the NOx emission because EGR has a dilution and thermal effect as a means to reduce the oxygen concentration and combustion temperature. Although EGR is useful in reducing the NOx emission, it suffers from a higher level of CO and THC emissions, which indicates a low combustion efficiency and poor fuel consumption. Therefore, in this research, a close post injection strategy, which is implemented using main injection and post injection, is introduced to improve combustion efficiency and to reduce PM emission under a high EGR rate. In addition, a modified hardware configuration using a double-row nozzle and a two-staged piston bowl geometry is adapted to improve the effect of the close post injection.
Technical Paper

Study on the Effects of the In-Cylinder EGR Stratification on NOx and Soot Emissions in Diesel Engines

Much research has been devoted to reducing NOx and soot emissions simultaneously in diesel engines. The low temperature combustion (LTC) concept has the potential to reduce these emissions at the same time, but it has limitations to its commercialization. In-cylinder EGR stratification is another combustion concept meant to reduce both types of emissions simultaneously using non-uniform in-cylinder EGR gas distribution. The EGR stratification concept uses a locally high EGR region of the in-cylinder so that the emissions can be reduced without increasing the overall EGR rate. In this study, the EGR stratification concept was improved with a CFD-based analysis. First, a two-step piston was developed to maximize the stratified EGR effect. Then, the feasibility of combustion and emission control by stratified EGR was evaluated under cases of artificially distributed EGR stratification and conventional diesel engine conditions.
Technical Paper

A Study of Emissions Reduction through Dual-Fuel Combustion with Propane in a Compression Ignition Engine

Novel Diesel combustion concepts such as premixed charge compression ignition (PCCI) and reactivity controlled compression ignition (RCCI) promise lower NOx and PM emissions than those of conventional Diesel combustion. RCCI, which can be implemented using low-reactivity fuels such as gasoline or gases and high-reactivity fuels such as Diesel, has the potential to achieve extremely low emissions and improved thermal efficiency. However, to achieve RCCI combustion, a higher boost pressure than that of a conventional engine is required because a high EGR rate and a lean mixture are necessary to achieve a low combustion temperature. However, higher boost pressures can cause damage to intake systems. In this research, the addition of gaseous fuel to a CI engine is investigated to reduce engine emissions, mainly NOx and PM emissions, with the same IMEP level. Two different methods were evaluated.
Technical Paper

Numerical Analysis of Pollutant Formation in Direct-Injection Spark-Ignition Engines by Incorporating the G-Equation with a Flamelet Library

Direct-injection spark-ignition (DISI) engines are regarded as a promising technology for the reduction of fuel consumption and improvement of engine thermal efficiency. However, due to direct injection, the shortened fuel-air mixing duration leads to a spatial gradient of the equivalence ratio, and these locally rich regions cause the formation of particulate matter. In the current study, numerical investigations on pollutant formation in a DISI engine were performed using combined flamelet models for premixed and diffusion flames. The G-equation model for partially premixed combustion was improved by incorporating the laminar flamelet library. Gasoline fuel was represented as a ternary mixture of gasoline surrogate and its laminar flame speeds were obtained under a wide range of engine operating conditions.
Technical Paper

The Efficiency and Emission Characteristics of Dual Fuel Combustion Using Gasoline Direct Injection and Ethanol Port Injection in an SI Engine

Ethanol, one of the most widely used biofuels, has the potential to increase the knock resistance of gasoline and decrease harmful emissions when blended with gasoline. However, due to the characteristics of ethanol, a trade-off relationship between knock tolerance and BSFC exists which is balanced by the blending ratio of gasoline and ethanol. Furthermore, in a spark-ignited engine, it is reported that the blending ratio that maximizes thermal efficiency varies based on the engine operating conditions. Therefore, an injection system that can deliver gasoline and ethanol separately is needed to fully exploit the benefit of ethanol. In this study, PFI injectors and a DI injector are used to deliver ethanol and gasoline, respectively. Using the dual fuel injection system, the compression ratio was increased from 9.5 to 13.3, and the knock mitigation characteristics at the full load condition were examined.