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A three-dimensional model for premixed- charge naturally-aspirated rotary engine combustion is used to identify combustion chamber geometries that could lead to increased indicated efficiency for a lean (equivalence ratio =0.75) natural gas/air mixture. Computations were made at two rpms (1800 and 3600) and two loads (approximately 345 Kpa and 620 Kpa indicated mean effective pressure). Six configurations were studied. The configuration that gave the highest indicated efficiency has a leading pocket with a leading deep recess, two spark plugs located circumferentially on the symmetry plane (one after the minor axis and the other before), a compression ratio of 9.5, and an anti-quench feature on the trailing flank.

Recent measurements by of gas intake flows and exhaust pressure in a motored, ported, single-cylinder engine with strong swirl and roll have been used as boundary conditions to a three-dimensional, transient computer simulation of the flow within the cylinder. For each condition, the calculation is continued over several engine cycles until the periodic solution is obtained. The computed TDC tangential velocity and turbulence intensity are then compared with measured ones. A technique is described to evaluate scavenging efficiency, the fraction of charge that remains in the cylinder over later cycles and the degree of mixedness of fresh and residual charge. For this motored ported engine, it is found that the scavenging efficiency is very low (19.4% at 1200 RPM) and the inflow from the exhaust ports is very significant. For practical ported engines with combustion, the scavenging efficiency is much higher but the inflows from exhaust ports are still expected to be significant.

A three-dimensional model for a direct injection stratified-charge rotary engine has been employed to study two modifications to the pocket geometry of the engine. In one modification, a pocket is located towards the leading edge of the rotor and is shown to produce recirculation within the pocket and faster burning. In the second modification, a two pocket rotor with two injectors and two spark plugs is studied. It appears that this should result in better utilization of the chamber air. It also appears that both modifications rhould result in higher efficiency of the direct-injected stratifiedcharge rotary engine. However extensive computations are required before a final conclusion is reached and before specific recommendations can be made.

For practical, extensive 3-D computations for engine improvements, each physical submodel needs to be the simplest that is compatible with the accuracy of all other physical submodels and of the numerics. The addition of one progress variable controlled by one Arrhenius term is shown to be adequate to reproduce Diesel ignition delay in 2-D and 3-D computations. The rest of the model is that used for years by the authors to optimize combustion in reciprocating and rotary engines with premixed and non-premixed charges, including all of its model constants. This minimal Diesel autoignition submodel reproduces well trends and magnitudes of ignition delay versus chamber temperature and pressure. As in experiments, it is found that multiple ignition sources develop in rapid succession at various locations around the fuel spray after the first ignition event.

The exciplex fluorescence technique has been used to separately visualize liquid and vapor phase fuel in engines since its development by Melton. However, as a fluorescence technique it has the potential to be quantitative and the underlying assumptions have been outlined by Melton. An initial quantitative application of the TMPD/naphthalene system, based on these assumptions, applied to a hollow-cone spray in a two-stroke engine, indicated that it substantially over-estimates the concentration of fuel vapor about TDC. The reasons for the discrepancy were investigated and it was concluded that a major factor is the effect of temperature on the photophysics of the species involved. Thus the absorption spectra of the exciplex dopants were determined at temperatures up to 700 K. These experiments showed that the increase in absorption with temperature above 500 K is responsible for the failure of the earlier calibration.

Results are presented of two-dimensional computations of air-assist fuel injection into engines with bowl-in-piston and bowl-in-head, with and without swirl and for early and late injection but without combustion. The general finding is that swirl tends to destroy the head vortex of the air/fuel jet and results in a faster collapse of the spray cone toward its axis. Faster collapse is also promoted by high density of the chamber gas (e.g. late injection) and bowl-in-head design (limited availability of chamber gas around the spray, presence of walls and delayed influence of squish by the injector). With enhanced collapse, fuel-rich regions are formed around the axis and away from the injector. With reduced collapse, the radial distribution of the fuel is more uniform. Thus swirl tends to lead to both slower vaporization and richer vapor mixtures. Also, with strong swirl the rich mixtures tend to end up by the injector; without swirl, by the piston.

Efforts are reported to reproduce the distribution of liquid and vapor fuel from a pulsating hollow-cone liquid-only injector measured by the planar exciplex technique within the head cup of a motored ported single-cylinder engine operated at 1600 rpm with high swirl and a squish ratio of 75%. The injector, cup and cylinder were coaxial. The measurements show that shortly after the beginning of the injection the maximum liquid and vapor fuel concentrations are along the axis but also that the spray achieves substantial radial and axial penetrations. The engine flowfield without injection had previously been characterized by LDV and PIV and so had been the injector and its spray in constant pressure environments so that little arbitrariness was left in reproducing the spray in the engine. Two spray models were used. In one the large drops produced by the break up of the liquid sheet were introduced into the numerical field at the injector exit nearly with the poppet seat angle.

In a previous study, the authors compared the fuel-air mixing characteristics of gas jets and sprays in Diesel engine environments in the absence of combustion. A three-dimensional model for flows and sprays was used. It was shown that mixing was slower in gas jets relative to fast-evaporating sprays. In this study, which is an extension of the previous one, the direct-injection of gasesous methane, gaseous tetradecane and liquid tetradecane are studied using the same three-dimensional model. This study concentrates on combustion. It is shown that the fuel-air mixing rate and hence the burning rate are initially slower with gas injection.

Combustion in a divided-chamber, stratified-charge engine is considered and flame and pressure results obtained with a two-dimensional, unsteady model are compared with corresponding engine data. The model is applied to eight engine conditions differing in speed, load and size of the prechamber orifice. The model employs one overall chemical reaction rate, the k-ε representation of turbulence, and a wall heat loss proportional to the heat release. The computed results are shown to be in good agreement with the experimental ones in spite of the complexity of the problem and the early stages of detailed model validation studies. They are also shown to compare somewhat better than earlier ones obtained with an ad-hoc jet turbulence model. Both studies prove the importance of the prechamber jet to the overall combustion process for the particular engine investigated.

Through numerical solutions of appropriate two-dimensional unsteady conservation equations, it is found that transient laminar incompressible jets, turbulent incompressible jets and non-vaporizing and vaporizing sprays reach steady state around the back of their head vortices. Thus, for many applications, the stems of such transient jets can be considered in steady state and steady state information can be applied to large fractions of transient jets. Equations are given for the penetration rate of such jets. In particular, an equation is obtained for the penetration rate of the tip of sprays that compares favorably with measured ones. Also, scaling laws are reported that are obeyed by each of the three families of jets. Since the laws are somewhat different, no precise similarity exists among the three types of jet.

Atomization and full-cone sprays from single cylindrical orifices are considered. The following subjects are reviewed: the structure of the breakup region; the structure of the far field; modern models that, given the outcome of the breakup process, compute the steady and transient of sprays; some comparisons with detailed measurements; and some practical applications. The following conclusions are reached: the spray breakup and the development regions are the most relevant in engine applications; the inner structure of the breakup region is still largely unknown; two- and three-dimensional spray models are available but remain mostly untested, particularly in their vaporization and combustion components, in part because of a lack of accurate measurements in controlled engine-type environments; engine applications of such models are, nonetheless, recommended for very valuable learning, interpretative, and exploratory studies, but not for predictions.

A three-dimensional model for flows and combustion in reciprocating and rotary engines is applied to a direct-injection stratified-charge rotary engine to identify the main parameters that control its burning rate. It is concluded that the orientation of the six sprays of the main injector with respect to the air stream is important to enhance vaporization and the production of flammable mixture. In particular, no spray should be in the wake of any other spray. It was predicted that if such a condition is respected, the indicated efficiency would increase by some 6% at higher loads and 2% at lower loads. The computations led to the design of a new injector tip that has since yielded slightly better efficiency gains than predicted.

A three-dimensional combustion model of a direct-injection stratified-charge rotary engine is used to identify modifications that might lead to better indicated efficiency. The engine, which has a five-hole main injector and a pilot injector, is predicted to achieve better indicated efficiency if a two-hole ‘rabbit-ear’ pilot injector is used instead of its present single-hole pilot injector. This rabbit-ear arrangement is predicted to increase the surface area of the early flame (on account of better distribution of the fuel), and thereby result in an increased overall burning rate. Computations were made at high and low engine speeds and loads, encompassing the practical operating range. It is concluded that the modified pilot injector will increase indicated efficiency by about 5% within the computed operating range.

A recently developed laser Doppler velocimetry system for making two-point spatial correlation measurements of velocity fluctuations has been applied to the turbulent flow field of an IC engine. Fluctuation integral length scales have been measured within the clearance volume of a ported, single cylinder engine with a disc-shaped chamber and a compression ratio of 8.0. The engine was motored at 600 rpm and the engine flow field had a swirl ratio at top dead center of approximately 4. These measurements were made at the center of the clearance height at three-quarters of the cylinder radius. The integral length scale was found to reach a minimum of approximately one-fifth of the clearance height near IDC. Comparison of the results obtained using this technique with the integral length scales measured in engines by other authors using different methods gives agreement to within a factor of two.

Laser Doppler velocimetry has been used to make cycle-resolved velocity and turbulence measurements in a homogeneous-charge, spark-ignition engine. The engine had a ported intake and disc-shaped chamber with a compression ratio of 8 to 1. It was operated at a speed of 1200 rpm and with a TDC swirl number of 4. A stoichiometric propane-air mixture was used, and ignition was near the wall. The velocity measurements were made at three spatial locations at the midpoint of the clearance height. Tests were made to determine whether the presence of the flame affected the accuracy of the velocity measurements. It was found that the ensemble-averaged mean velocity shows a small deviation, and the rms fluctuation intensity is significantly influenced, but the effects appear to be confined to the flame zone. Data rates were sufficiently high in the preflame and postflame regions to determine the velocity history in each cycle (cycle resolved).

The combustion chamber pressure computed with a three-dimensional model is compared with the measured one in a rotary engine fueled with mixtures of natural gas and air. The rotary engine has a rotor displacement of 654 cm3, a compression ratio of 9.4 and uses 2 ignition sparks. The model incorporates a k-ϵ submodel for turbulence, wall function submodels for turbulent wall boundary layer transport, and a hybrid laminar/mixing controlled submodel for species conversion and energy release. Nine cases are considered that cover a wide range of engine operating conditions: rpm of 2503-5798, volumetric efficiency of 35.7-100.5% and equivalence ratio of 0.59-1.15. In all cases the computed and measured pressures agree within 12%.

Laser Doppler velocimetry has been used to make cycle-resolved velocity and turbulence measurements in a homogeneous-charge, spark-ignition engine. The engine had a ported intake and disc-shaped chamber with a compression ratio of 7.5 to 1. It was operated at a speed of 1200 rpm and with a TDC swirl number of 4. A stoichiometric propane-air mixture was used, and ignition was near the wall. Measurements of the tangential velocity component were made in both firing and non-firing cycles at nine spatial locations along a radius 180 degrees downstream of the spark. The radial velocity component was also measured at four of the locations. All measurements were made in the center of the clearance height. Tangential component measurements were made as close as 0.5mm from the cylinder wall, and the radial component was measured as close as 1.5mm from the wall.

Cycle-resolved LDV measurements of tangential and radial velocities were made in a ported engine within four piston cups. One cup was centered circular, one off-center circular, one centered square and one off-center square. The engine speed was 1200 rpm, the compression ratio 10.8, the squish area 75% and the TDC swirl ratio 4 for a pancake chamber. The velocity measurements were made at four depths in two axial sections. Near TDC in the centered circular cup, the profile of the ensemble-averaged tangential velocity tends to solid body with a swirl ratio of 12.5. In the centered square cup, the same velocity tends to solid-body profile along the short section and to top-hat profile along the outer part of the long section. The corresponding TDC swirl ratios are 11.3 and 5.5 due to mass conservation. The trends are similar but more complex in the off-center circular and square cups. In the centered circular cup, the swirl center is close to the cylinder axis near TDC.

Flame fronts were examined in a premixed-charge, spark-ignition, ported engine using a two-dimensional visualization technique with 10 nanoseconds time resolution and 200 microns best spatial resolution. The engine had a pancake chamber, a compression ratio of 8, a TDC swirl number of 4 and was operated at 300 to 3000 rpm with stoichiometric and lean propane/air mixtures. The measurements were made far from, and near to, the cylinder wall. A pulsed laser sheet was passed through the engine and the light scattered by sub-micron TiO2 or ZrO2 seeding particles was collected by a 100 x 100 diode array with fields of view of 1 cm x 1 cm, 2 cm x 2 cm, and 9 cm x 9 cm. The thickness of the flame front is as small as, or smaller than, the 200 micron best resolution of the measurements thus confirming that premixed-charge engine turbulent flames generally appear to be wrinkled laminar flames.

Laser Doppler velocimetry was used to make cycle-resolved velocity and turbulence measurements under motoring and firing conditions in a ported homogeneous charge S.I. engine. The engine had a flat pancake chamber with a compression ratio of 7.5. In one study, the effect of the intake velocity on TDC turbulence intensity was measured at 600, 1200, and 1800 rpm with three different intake flow rates at each speed. The TDC swirl ratio ranged from 2 to 6. The TDC turbulence intensities were found to be relatively insensitive to the intake velocity, and tended to scale more strongly with engine speed. For the combustion measurements, the engine was operated at 600, 1200, and 2400 rpm on stoichiometric and lean propane-air mixtures. Velocity measurements were made in swirling and non-swirling flows at several spatial locations on the midplane of the clearance height. The TDC swirl ratio was about 4. The measurements were made ahead, through, and behind the flame.