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Technical Paper

Turbulence and Cycle-by-Cycle Variation of Mean Velocity Generated by Swirl and Tumble Flow and Their Effects on Combustion

Combinations of swirl flow and tumble flow generated by 13 types of swirl control valve were tested by using both impulse steady flow rig and LDV. Comparison between the steady flow characteristics and the result of LDV measurement under motoring condition shows that tumble flow generates turbulence in combustion chamber more effectively than swirl flow does, and that swirling motion reduces the cycle by cycle variation of mean velocity in combustion chamber which tends to be generated by tumbling motion. Performance tests are also carried out under the condition of homogeneous charge. Tumble flow promotes the combustion speed more strongly than expected from its turbulence intensity measured by LDV. It is also shown that lean limit air fuel ratio does not have a strong relation with cycle variation of mean velocity but with turbulence intensity.
Technical Paper

A Study of Cycle-to-Cycle Variations in SI Engines Using a Modified Quasi-Dimensional Model

This paper describes the use of a modified quasi-dimensional spark-ignition engine simulation code to predict the extent of cycle-to-cycle variations in combustion. The modifications primarily relate to the combustion model and include the following: 1. A flame kernel model was developed and implemented to avoid choosing the initial flame size and temperature arbitrarily. 2. Instead of the usual assumption of the flame being spherical, ellipsoidal flame shapes are permitted in the model when the gas velocity in the vicinity of the spark plug during kernel development is high. Changes in flame shape influence the flame front area and the interaction of the enflamed volume with the combustion chamber walls. 3. The flame center shifts due to convection by the gas flow in the cylinder. This influences the flame front area through the interaction between the enflamed volume and the combustion chamber walls. 4. Turbulence intensity is not uniform in cylinder, and varies cycle-to-cycle.
Technical Paper

Three-Dimensional Computation of the Effects of the Swirl Ratio in Direct-Injection Diesel Engines on NOx and Soot Emissions

Three-dimensional computation has been applied to analyze combustion and emission characteristics in direct-injection diesel engines. A computational code called TurboKIVA was used to investigate the effects of the swirl ratio, one of the fundamental factors related to combustion control, on combustion characteristics and NOx and soot emissions. The code was first modified to calculate soot formation and oxidation and the precise behavior of fuel drops on the combustion chamber wall. As a result of improving calculation accuracy, good agreement was obtained between the measured and predicted pressure, heat release rate and NOx and soot emissions. Using this modified version of TurboKIVA, the effects of the swirl ratio on NOx and soot emissions were investigated. The computational results showed that soot emissions were reduced with a higher swirl ratio. However, a further increase in the swirl ratio produced greater soot emissions.
Technical Paper

Combustion Chamber Deposit Effects on Hydrocarbon Emissions from a Spark-Ignition Engine

A dynamometer-mounted four-cylinder Saturn engine was used to accumulate combustion chamber deposits (CCD), using an additized fuel. During each deposit accumulation test, the HC emissions were continuously measured. The deposit thickness at the center of the piston was measured at the beginning of each day. After the 50 and 35-hour tests, HC emissions were measured with isooctane, benzene, toluene, and xylene, with the deposited engine, and again after the deposits had been cleaned from the engine. The HC emissions showed a rapid rise in the first 10 to 15 hours and stabilization after about 25 hours of deposit accumulation. The HC increase due to CCD accumulation accounted for 10 to 20% of the total engine-out HC emissions from the deposit build-up fuel and 10 to 30% from benzene, isooctane, toluene, and xylene, making CCDs a significant HC emissions source from this engine. The HC emissions stabilized long before the deposit thickness.
Technical Paper

Early Spray Development in Gasoline Direct-Injected Spark Ignition Engines

The characteristics of the early development of fuel sprays from pressure swirl atomizer injectors of the type used in direct injection gasoline engines is investigated. Planar laser-induced fluorescence (PLIF) was used to visualize the fuel distribution inside a firing optical engine. The early spray development of three different injectors at three different fuel pressures (3, 5, and 7 MPa) was followed as a function of time in 30 μsec intervals. Four phases could be identified: 1) A delay phase between the rising edge of the injection pulse and the first occurrence of fuel in the combustion chamber, 2) A solid jet or pre-spray phase, in which a poorly atomized stream of liquid fuel during the first 150 μsec of the injection. 3) A wide hollow cone phase, separation of the liquid jet into a hollow cone spray once sufficient tangential velocity has been established and 4) A fully developed spray, in which the spray cone angle is narrowed due to a low pressure zone at the center.
Technical Paper

Experimental Investigation of Smoke Emission Dependent upon Engine Operating Conditions

Smoke is emitted in diesel engines because fuel injected into the combustion chamber burns with insufficient oxygen. The emission smoke from diesel engines is a very important air pollution problem. Smoke emission, which is believed to be largely related to the diffusion combustion in diesel engines, results from pyrolysis of fuel not mixed with air. Therefore, the smoke emission is dependent on diffusion combustion phenomena, which are controlled by engine parameters. This paper presents an analysis of combustion by relating the smoke emission with heat release in diesel engines. An analysis is made of the diffusion combustion quantity, the smoke emission, and the fraction of diffusion combustion as related to the engine parameters which are air-fuel ratio, injection timing, and engine speed.
Technical Paper

Development of a New Compound Fuel and Fluorescent Tracer Combination for Use with Laser Induced Fluorescence

Laser induced fluorescence (LIF) is a useful method for visualizing the distribution of the air-fuel ratio in the combustion chamber. The way this method is applied mainly depends on the fluorescent tracer used, such as biacetyl, toluene, various aldehydes, fluoranthene or diethylketone, among others. Gasoline strongly absorbs light in the UV region, for example, at the 248-nm wavelength of broadband KrF excimer laser radiation. Therefore, when using this type of laser, iso-octane is employed as the fuel because it is transparent to 248-nm UV light. However, since the distillation curves of iso-octane and gasoline are different, it can be expected that their vaporization characteristics in the intake port and cylinder would also be different. The aim of this study was to find a better fuel for use with LIF at a broadband wavelength of 248 nm. Three tasks were undertaken in this study.
Technical Paper

Effects of NOx and Unburned Gasoline on Low Temperature Sludge Formation in Engine Oil

It is generally known that NOx reacts with unburned gasoline, olefins in particular, to form sludge precursors. In this study, the authors investigated the process by which NOx and unburned gasoline mix into the engine oil and analyzed the mechanism whereby stop and go driving accelerates sludge formation. It has been found that NOx detected in the engine oil as nitrite ions mixes into the oil in the crankcase. The NOx concentration in the engine oil increases rapidly when the crankcase gas temperature is nearly equal to the dew point of the water vapor in the crankcase. Unburned gasoline is mainly absorbed into the oil through the oil film on the cylinder walls and the oil in the ring grooves. During low-temperature engine operation in stop-go driving (i.e., when the vehicle is stopped), NOx and unburned gasoline are absorbed into the engine oil and, in high-temperature engine operation (i.e., when the vehicle is moving), NOx and unburned gasoline are released from the oil.
Technical Paper

Development of Four Cylinder SR Engine

The SR engine is a new medium-size, all aluminum (cylinder block, head, rocker cover and oil pan) in-line 4-cylinder gasoline powerplant developed as a replacement for CA engine in Nissan's compact passenger cars. The development aim set for this engine was to achieve excellent power output and ample torque in the middle-and high-speed ranges, as well as a clear, linear engine sound up to the red zone. These performance targets have been achieved through the use of the 4-valve-per-cylinder DOHC design featuring a Y-shaped valve rocker arm system. This system allows a straight intake port for high power output and a narrow valve angle for a compact combustion chamber. The result is ample torque output as well as good fuel economy.
Technical Paper

Factors Limiting the Improvement in Thermal Efficiency of S. I. Engine at Higher Compression Ratio

An analysis of the factors that limit the improvement in thermal efficiency at higher compression ratios was performed with both thermodynamic calculation and experiment. The results showed that the major factors were cooling loss and unburned fuel. Both of these factors increase with smaller swept volume, larger S/V ratio combustion chamber, and lower engine speed and load. These effects explain the observation that thermal efficiency peaks at relatively low compression ratio.
Technical Paper

An Analytical Study on Knocking Heat Release and its Control in a Spark Ignition Engine

In this study the relationship between the timing for the onset of autoignition and the amount of mixture fraction burned by autoignition and the resulting knock intensity is investigated using a combination of high-speed laser shadowgraphy and thermodynamic calculations. It is made clear that over 40 percent of the entire mixture burns due to autoignition in a crank angle of less than five to eight degrees when an engine is operated under a heavy knocking condition. This burn rate is about ten times higher than that of combustion seen in a normally propagating flame. This abrupt heat release causes an oscillation in cylinder gases, resulting in a knocking sound. The experimental procedure is applied to examine the effect of a squish combustion chamber on suppressing knock. The results indicate that, when autoignition occurs in the squish area, an amount of mixture burned by autoignition is small, resulting in lower knock intensity.
Technical Paper

Development of a New 12 Valve 4 Cylinder Engine

The 1.5 1 GA15 engine is a new inline 4 cylinder engine. The GA15 fully meets the major development objectives of sufficient torque at low and middle engine speeds, high power output, good fuel economy and quiet engine operation. Its structure features a compact combustion chamber with a small bore and long stroke, aerodynamic intake ports, a stiff engine cylinder block with a deep skirt and bearing beam, a newly designed silentrunning chaine, and pistons with full floating pins. High quality was achieved by adopting the latest methods in its development: vibration analysis of the assembled engine and transmission, FEM model, rigidity analysis of the cylinder block and head, and analysis of air flow in the intake port and movement of the timing chain.
Technical Paper

Effect of Gasoline Composition on Engine Performance

In order to clarify the effect of each gasoline component on engine performance during warm-up, changes in the air-fuel ratio and quantity of wall flow (liquid gasoline on the induction port) were measured using ordinary gasolines and model gasolines consisting of a blend of several hydrocarbons and MTBE (methyl-tertiary-butyl-ether). The unburned air-fuel mixture in a combustion chamber was sampled via a solenoid valve and analyzed by gas chromatography to investigate the vaporization rate of each component. The results show that MTBE has an important effect on driveability because it contains oxygen and easily vaporizes, resulting in a lean mixture in the transient state. The popular driveability index, T50 (50% distillation temperature), does not provide an adequate means of evaluating MTBE-blended gasoline.
Technical Paper

Predicting the Effects of Air and Coolant Temperature, Deposits, Spark Timing and Speed on Knock in Spark Ignition Engines

The prediction of knock onset in spark-ignition engines requires a chemical model for the autoignition of the hydrocarbon fuel-air mixture, and a description of the unburned end-gas thermal state. Previous studies have shown that a reduced chemistry model developed by Keck et al. adequately predicts the initiation of autoignition. However, the combined effects of heat transfer and compression on the state of the end gas have not been thoroughly investigated. The importance of end-gas heat transfer was studied with the objective of improving the ability of our knock model to predict knock onset over a wide range of engine conditions. This was achieved through changing the thermal environment of the end gas by either varying the inlet air temperature or the coolant temperature. Results show that there is significant heating of the in-cylinder charge during intake and a substantial part of the compression process.
Technical Paper

Effect of Intake Valve Deposits and Gasoline Composition on S.I. Engine Performance

Valve deposits in gasoline engines increase with time, absorbing fuel during acceleration and releasing fuel during deceleration. Valve deposits insulate the heat release from the cylinder and this phenomenon is the cause of bad fuel vaporization. In this way, the deposits greatly affect the driveability and exhaust emissions. Using a 3.OL MPI(Multipoint Injection) engine, we measured the quantity of fuel that deposits at the intake port, and the throttle response (using a wall-flow meter made by Nissan Motor Co.1), 2) to study the deposits effect on driveability and exhaust emissions at a low temperature. The deposits were formed on the intake valve surface (about 8.0 on the CRC deposit rating scale) through 200 hours of laboratory engine stand operation. At low temperature, C9 and C10 hydrocarbons tend to stick to the intake port surface and intake valve as “wall-flow”; this is one cause of bad driveability.
Technical Paper

Numerical Simulation System for Analyzing Fuel Film Flow in Gasoline Engine

A new numerical simulation system has been developed which predicts flow behavior of fuel film formed on intake port and combustion chamber walls of gasoline engines. The system consists of a film flow model employing film thickness as a dependent variable, an air flow model, and a fuel spray model. The system can analyze fuel film flow formed on any arbitrary three-dimensional configuration. Fuel film flow formed under a condition of continuous intermittent fuel injection and steady-state air flow was calculated, and comparison with experimental data showed the system possessing ability of qualitative prediction.
Technical Paper

Effects of Combustion Chamber Insulation on the Heat Rejection and Thermal Efficiency of Diesel Engines

Experiments were conducted with 4-cylinder and single-cylinder direct injection diesel engines to examine the effects of combustion chamber insulation on heat rejection and thermal efficiency. The combustion chamber was insulated by using a silicon nitride piston cavity that was shrink-fitted into a titanium alloy crown. The effect of insulation on heat rejection was examined on the basis of heat release calculations made from cylinder pressure time histories. High-speed photography was used to investigate combustion phenomena. The results showed that heat rejection was influenced by the combustion chamber geometry and swirl ratio and that it was reduced by insulating the combustion chamber. However, because combustion deteriorated, it was not possible to obtain an improvement in thermal efficiency equivalent to the reduction in heat rejection.
Technical Paper

Sources of Hydrocarbon Emissions from a Small Direct Injection Diesel Engine

The purpose of this paper is to clarify the mechanisms of unburnt hydrocarbon (HC) emissions from a small direct - injection (DI) diesel engine. HC emission levels of small DI diesel engines are considerably higher than those of corresponding indirect - injection (IDI) diesel engines, even when sacless injection nozzles that are effective in reducing HC emissions are installed on them. In this study, analytical engine tests were performed to evaluate the relative significance of various potential sources of HC emissions from a small DI diesel engine equipped with sacless type injectors.
Technical Paper

Photographic and Performance Studies of Diesel Combustion With a Rapid Compression Machine

Photographic and performance studies with a Rapid Compression Machine at the Massachusetts Institute of Technology have been used to develop insight into the role of mixing in diesel engine combustion. Combustion photographs and performance data were analyzed. The experiments simulate a single fuel spray in an open chamber diesel engine with direct injection. The effects of droplet formation and evaporation on mixing are examined. It is concluded that mixing is controlled by the rate of entrainment of air by the fuel spray rather than the dynamics of single droplets. Experimental data on the geometry of a jet in a quiescent combustion chamber were compared with a two-phase jet model; a jet model based on empirical turbulent entrainment coefficients was developed to predict the motion of a fuel jet in a combustion chamber with swirl. Good agreement between theory and experiment was obtained.
Technical Paper

Ignition, Combustion, and Exhaust Emissions of Lean Mixtures in Automotive Spark Ignition Engines

Misfire and cycle-to-cycle combustion variation are both serious problems in securing good engine performance and low exhaust emissions in the case of using extremely lean mixtures. Making some modifications in the ignition system and in the combustion chamber, and increasing the mixture turbulence, we examined their effects upon the lean limit, the engine performance, and the exhaust emissions. It was found that gap width and gap projection of a spark plug and spark energy as well as mixture turbulence had a great effect on extending the lean limit and improving engine performance with lean mixtures. A compact combustion chamber is preferable for lean mixture operation. Smooth operation of the engine can be maintained even at retarded spark timing by applying the above-mentioned items and providing hot intake air to the engine. Consequently, exhaust emissions, including hydrocarbons and oxides of nitrogen, can be substantially reduced.