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Journal Article

Very Lean and Diluted SI Combustion Using a Novel Ignition System with Repetitive Pulse Discharges

A newly developed small-sized IES (inductive energy storage) circuit with semiconductor switch at turn-off action is successfully applied to an ignition system of a small gasoline internal combustion engine. This IES circuit can generate repetitive nanosecond pulse discharges. An ignition system using repetitive nanosecond pulse discharges is investigated as an alternative to a conventional spark ignition system. The present study focuses on the extension of the operational limits for lean and diluted combustion using the repetitive nanosecond pulse discharges. First, in order to investigate the flame kernel formation process when the repetitive nanosecond pulse discharges are used, the initial flame kernel is observed using Schlieren photography with a high speed camera. As a result, the flame kernel generated by repetitive pulse discharges is larger than by a conventional ignition system.
Technical Paper

Numerical Analysis of Combustion and Flow Inside a Small Rotary Engine for Developing an Unmanned Helicopter

For a disaster relief and automatic inspections, an unmanned helicopter is strongly expected. To develop this, a very high power density source is required. A Wankel-type rotary engine can be the best candidate for the power source. In this study, the development of a very small rotary engine with a displacement of 30 cc is targeted. In order to improve the combustion efficiency, gas exchange and stable ignition, a multi dimensional simulation inside the combustion chamber was carried out. At first, the effect of volumetric efficiency on the maximum power is mentioned. Secondly, the effect of scavenging efficiency is discussed. Thirdly, a blow off through a plug hole is described. The position of plug hole was found important to reduce the blow off amount. Finally, the effect of combustion speed on the engine performance is predicted. As a result, the proposed design will be tested using a proto-type engine.
Technical Paper

Measurement of the Local Gas Temperature at Autoignition Conditions Inside the Combustion Chamber Using a Two-Wire Thermocouple

The phenomenon of autoignition is an important aspect of HCCI and knock, hence reliable information on local gas temperature in a combustion chamber must be obtained. Recently, several studies have been conducted by using laser techniques such as CARS. It has a high spatial resolution, but has proven difficult to apply in the vicinity of combustion chamber wall and requires special measurement skills. Meanwhile, a thermocouple is useful to measure local gas temperature even in the vicinity of wall. However, a traditional one-wire thermocouple is not adaptable to measure the in-cylinder gas temperature due to slow response. The issue of response can be overcome by adopting a two-wire thermocouple. The two-wire thermocouple is consisted of two fine wire thermocouples with different diameter hence it is possible to determine the time constant using the raw data from each thermocouple.
Technical Paper

Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency

In reciprocating internal combustion engines, the Otto cycle indicates the best thermal efficiency under a given compression ratio. To achieve an ideal Otto cycle, combustion must take place instantaneously at top dead center, but in fact, this is impossible. Meanwhile, if we allow slower piston motion around top dead center, combustion will be promoted at that period; then both the in-cylinder pressure and degree of constant volume will increase, leading to higher thermal efficiency. In order to verify this hypothesis, an engine with slower piston motion around top dead center, using an ideal constant volume combustion engine, was built and tested. As anticipated, the degree of constant volume increased. However, thermal efficiency was not improved, due to increased heat loss.
Technical Paper

Development of a Novel Ignition System Using Repetitive Pulse Discharges: Ignition Characteristics of Premixed Hydrocarbon-Air Mixtures

A newly developed small-sized IES (inductive energy storage) circuit with static induction thyristor at turn-off action was successfully applied to an ignition system. This IEC circuit can generate repetitive nanosecond pulse discharges. In this paper, the ignition system using repetitive nanosecond pulse discharges was investigated as an alternative to conventional spark ignition systems. The experiments were conducted using spherically expanding flame configuration for CH4 and C3H8-air mixtures under various conditions. In conclusions, the ignition system using repetitive nanosecond pulse discharges was found to extend lean flammability limits compared with conventional spark ignition systems. In addition, the ignition system using repetitive nanosecond pulse discharges could shorten ignition delay time.
Journal Article

Development of a Novel Ignition System Using Repetitive Pulse Discharges: Application to a SI Engine

A newly developed small-sized IES (inductive energy storage) circuit with semiconductor switch at turn-off action was successfully applied to an ignition system. This IES circuit can generate repetitive nanosecond pulse discharges. An ignition system using repetitive nanosecond pulse discharges was investigated as an alternative to conventional spark ignition systems. Experiments were conducted using spherically expanding flame configuration for CH4 and C3H8-air mixtures under various conditions. The ignition system using repetitive nanosecond pulse discharges was found to improve inflammability of lean combustible mixtures, such as extended flammability limits, shorted ignition delay time, with increasing the number of pulses. The authors seek for the mechanisms for improving the inflammability in more detail to optimize ignition system, and verify the effectiveness of IES circuit in EGR condition, for real engine use.
Technical Paper

An Experimental Study of a Gasoline HCCI Engine Using the Blow-Down Super Charge System

The objective of this study is to extend the high load operation limit of a gasoline HCCI engine. A new system extending the high load HCCI operation limit was proposed, and the performance of the system was experimentally demonstrated. The proposed system consists of two new techniques. The first one is the “Blow-down super charging (BDSC) system”, in which, EGR gas can be super charged into a cylinder during the early stage of compression stroke by using the exhaust blow-down pressure wave from another cylinder phased 360 degrees later/earlier in the firing order. The other one is “EGR guide” for generating a large thermal stratification inside the cylinder to reduce the rate of in-cylinder pressure rise (dP/dθ) at high load HCCI operation. The EGR guides consist of a half-circular part attached on the edge of the exhaust ports and the piston head which has a protuberant surface to control the mixing between hot EGR gas and intake air-fuel mixture.
Technical Paper

A Study on New Combustion Method of High Compression Ratio Spark Ignition Engine

A new combustion method of high compression ratio SI engine was studied and proposed in order to achieve higher thermal efficiency of SI engine comparable to that of CI engine. Compression ratio of SI engine is generally restricted by the knocking phenomena. A combustion chamber profile and a cranking mechanism are studied to avoid knocking with high compression ratio. Since reducing the end-gas temperature will suppress knocking, a combustion chamber was considered to have a wide surface at the end-gas region. However, wide surface will lead to high heat loss, which may cancel the gain of higher compression ratio operation. Thereby, a special cranking mechanism was adopted which allowed the piston to move rapidly near TDC. Numerical simulations were performed to optimize the cranking mechanism for achieving higher thermal efficiency. An elliptic gear system and a leaf-shape gear system were employed in the simulations.
Technical Paper

A Trial of Improving Thermal Efficiency by Active Piston Control -Speed Control Effect of Combustion Chamber Volume Variation on Thermal Efficiency-

In reciprocating internal combustion engines, the piston stops in a moment at top dead center (TDC), so there exists a necessary time to proceed combustion. However more slowing piston motion around TDC, does it have a possibility to produce the following effects? The slowed piston motion may expedite combustion proceed and increase cylinder pressure. This may lead to an increase of degree of constant volume. As a result, thermal efficiency may be improved. In order to verify this idea, two types of engines were tested. The first engine attained high cylinder pressure as expected. The P-V diagram formed an almost ideal Otto cycle. However, this did not contribute to the improvement in the thermal efficiency. Then the second engine with further slower piston motion by active piston control was tested in order to examine the above reason.
Technical Paper

Evaluation of a Concept for DI Gasoline Combustion Using Enhanced Gas Motion

A direct injection gasoline engine system which employs a unique combustion system with enhanced gas motion is evaluated. Enhanced gas motion is produced by employing both a moderately strong swirl flow and a cavity in the piston. Advantages of this system are that the injection timing or spark timing need not be controlled severely and that since the injection timing can be set at near the intake BDC, time for evaporation can be gained to reduce soot emissions. Problems to be improved are that the Nox emissions level is worse than other lean burn systems and full load operation is not evaluated. According to the numerical calculations, the problems may be solved by enhancing the in-cylinder gas motion with axial stratification of swirl intensity at intake BDC; strong swirl near the cylinder head and weak swirl near the piston surface.
Technical Paper

Improvement in Thermal Efficiency of Lean Burn Pre-Chamber Natural Gas Engine by Optimization of Combustion System

To understand the mechanism of the combustion by torch flame jet in a gas engine with pre-chamber and also to obtain the strategy of improving thermal efficiency by optimizing the structure of pre-chamber including the diameter and number of orifices, the combustion process was investigated by three dimensional numerical simulations and experiments of a single cylinder natural gas engine. As a result, the configuration of orifices was found to affect the combustion performance strongly. With the same orifice diameter of 1.5mm, thermal efficiency with 7 orifices in pre-chamber was higher than that with 4 orifices in pre-chamber, mainly due to the reduction of heat loss by decreasing the impingement of torch flame on the cylinder linear. Better thermal efficiency was achieved in this case because the flame propagated area increases rapidly while the flame jets do not impinge on the cylinder wall intensively.
Technical Paper

1-D Simulation Model Developed for a General Purpose Engine

In recent years, improvements in the fuel economy and exhaust emission performance of internal combustion engines have been increasingly required by regulatory agencies. One of the salient concerns regarding general purpose engines is the larger amount of CO emissions with which they are associated, compared with CO emissions from automobile engines. To reduce CO and other exhaust emissions while maintaining high fuel efficiency, the optimization of total engine system, including various design parameters, is essential. In the engine system optimization process, cycle simulation using 0-D and 1-D engine models are highly useful. To define an optimum design, the model used for the cycle simulation must be capable of predicting the effects of various parameters on the engine performance. In this study, a model for predicting the performance of a general purpose SI (Spark Ignited) engine is developed based on the commercially available engine simulation software, GT-POWER.
Journal Article

The Effects of Ignition Environment and Discharge Waveform Characteristics on Spark Channel Formation and Relationship between the Discharge Parameters and the EGR Combustion Limit

In order to realize the high compression ratio and high dilution combustion toward improvement in thermal efficiency, the improvement in stability of ignition and initial phase of combustion under the high gas flow field is the major challenge. In terms of the shift on the higher power side of the operating point by downsizing and improvement of real world fuel consumption, the improvement of ignitability is increasingly expected in the wide operating range also including high load and high engine speed region. In this study, the effects of the gas pressure, gas flow velocity near the spark gap at ignition timing, and discharge current characteristics on spark channel formation were analyzed, focusing on restrike event and spark channel stretching in the spark channel formation process. And the relationship between the average discharge current until 1 ms and the EGR combustion limit was considered.
Journal Article

A Study of Low Speed Preignition Mechanism in Highly Boosted SI Gasoline Engines

The authors investigated the reasons of how a preignition occurs in a highly boosted gasoline engine. Based on the authors' experimental results, theoretical investigations on the processes of how a particle of oil or solid comes out into the cylinder and how a preignition occurs from the particle. As a result, many factors, such as the in-cylinder temperature, the pressure, the equivalence ratio and the component of additives in the lubricating oil were found to affect the processes. Especially, CaCO3 included in an oil as an additive may be changed to CaO by heating during the expansion and exhaust strokes. Thereafter, CaO will be converted into CaCO3 again by absorbing CO2 during the intake and compression strokes. As this change is an exothermic reaction, the temperature of CaCO3 particle increases over 1000K of the chemical equilibrium temperature determined by the CO2 partial pressure.
Technical Paper

Analyses of Cycle-to-Cycle Variation of Combustion and In-Cylinder Flow in a Port Injection Gasoline Engine Using PIV and PLIF Techniques

Reduction in the cycle-to-cycle variation (CCV) of combustion in internal combustion engines is required to reduce fuel consumption, exhaust emissions, and improve drivability. CCV increases at low load operations and lean/dilute burn conditions. Specifically, the factors that cause CCV of combustion are the cyclic variations of in-cylinder flow, in-cylinder distributions of fuel concentration, temperature and residual gas, and ignition energy. However, it is difficult to measure and analyze these factors in a production engine. This study used an optically accessible single-cylinder engine in which combustion and optical measurements were performed for 45 consecutive cycles. CCVs of the combustion and in-cylinder phenomena were investigated for the same cycle. Using this optically accessible engine, the volume inside the combustion chamber, including the pent-roof region can be observed through a quartz cylinder.
Technical Paper

Heat Transfer Analysis in a Diesel Engine Based on a Heat Flux Measurement Using a Rapid Compression and Expansion Machine

To investigate the heat transfer phenomena inside the combustion chamber of a diesel engine, a correlation for the heat transfer coefficient in a combustion chamber of a diesel engine was investigated based on heat flux measured by the authors in the previous study(8) using the rapid compression and expansion machine. In the correlation defined in the present study, thermodynamically estimated two-zone temperatures in the burned zone and the unburned zone are applied. The characteristic velocity given in the correlation is related to the speed of spray flame impinging on the wall during the fuel injection period. After the fuel injection period, the velocity term of the Woschni’s equation is applied. It was shown that the proposed correlation well expresses heat transfer phenomena in diesel engines.
Technical Paper

Effect of Coolant Water and Intake Air Temperatures on Thermal Efficiency of Gasoline Engines

An optimization of thermal management system in a gasoline engine is considered to improve thermal efficiency by minimizing the cost increase without largely changing the configuration of engine system. In this study, the influence of water temperature and intake air temperature on thermal efficiency were investigated using an inline four-cylinder 1.2L gasoline engine. In addition, one-dimensional engine simulations were conducted by using a software of GT-SUITE. Brake thermal efficiency for different engine speeds and loads could be quantitatively predicted with changing the cooling water temperature in the cylinder head. Then, in order to predict the improvement of the fuel consumption in actual use, vehicle mode running simulation and general-purpose engine transient mode simulation were carried out by GT-SUITE. As a result, it was found that by controlling the temperatures of the cooling water and intake gas, thermal efficiency can be improved by several percent.
Technical Paper

The Effect of In-Cylinder Flow and Mixture Distributions on Combustion Characteristics in a HCCI Engine

It has been widely known that thermal and fuel stratifications of in-cylinder mixture are effective to reduce in-cylinder pressure rise rate during high load HCCI operations. In order to optimize a combustion chamber design and combustion control strategy for HCCI engines with wide operational range, it is important to know quantitatively the influence of the temperature and fuel concentration distributions on ignition and heat release characteristics. At the same time, it is important to know the influence of in-cylinder flow and turbulence on the temperature and fuel concentration distributions. In this study, a numerical simulation of HCCI combustion were conducted to investigate the effects of the in-cylinder flow and turbulence, and the distributions of temperature on ignition and combustion characteristics in HCCI combustion.
Technical Paper

Effects of Coolant Temperature and Fuel Properties on Soot Emission from a Spark-ignited Direct Injection Gasoline Engine

Effects of measurement method, coolant temperature and fuel composition on soot emissions were examined by engine experiments. By reducing the pressure fluctuation in the sampling line, the measured soot emissions with better stability and reproducibility could be obtained. With lower coolant temperatures, larger soot emissions were yielded at much advanced fuel injection timings. Compared to gasoline, soot emissions with a blend fuel of normal heptane, isooctane and toluene were significantly decreased, suggesting the amounts of aromatic components (toluene or others) should be increased to obtain a representative fuel for the predictive model of particulate matter in SIDI engines.
Technical Paper

Fuel Stratification Using Twin-Tumble Intake Flows to Extend Lean Limit in Super-Lean Gasoline Combustion

To drastically improve thermal efficiency of a gasoline spark-ignited engine, super-lean burn is a promising solution. Although, studies of lean burn have been made by so many researchers, the realization is blocked by a cycle-to-cycle combustion variation. In this study, based on the causes of cycle-to-cycle variation clarified by the authors’ previous study, a unique method to reduce the cycle-to-cycle variation is proposed and evaluated. That is, a bulk quench at early expansion stroke could be reduced by making slight fuel stratification inside the cylinder using the twin-tumble of intake flows. As a result, the lean limit was extended with keeping low NOx and moderate THC emissions, leading to higher thermal efficiency.