Lean-burn Spark Ignition Research Articles

A Systematic Air-fuel Ratio Control Strategy for Lean-burn SI Engines

In this paper, we present a new synthesis method to control air-fuel ratio (AFR) in lean-burn spark ignition engines to maximize the fuel economy. The major challenge in the control of AFR for lean-burn engines is the large time-varying delay in the control loop which restricts the application of conventional control techniques. Time-varying delay in the system dynamics is first approximated by the Padé approximation which renders the system dynamics into non-minimum phase characteristics. Application of dynamic compensators is invoked to retrieve unstable internal dynamics. The associated error dynamics is utilized to construct a PID controller combined with a dynamic compensator to track the desired AFR command using the feedback from the universal exhaust gas oxygen (UEGO) sensor. The proposed method is shown to achieve desired dynamic properties independent of the matched and unmatched disturbances. Results of applying the proposed method to experimental data demonstrate the closed-loop system stability and performance against time-varying delay, canister purge disturbances and measurement noise.

Research on the Combustion and Emission Characteristics of Lean Burn CNG Engine

The study on combustion characteristics of different excess air ratios (λ) and ignition timings was conducted on a lean-burn spark-ignition CNG engine, and the corresponding emission characteristics was also analyzed. The results show that the curve of cylinder pressure moves towards TDC, and the CoVimep decreases with the advance of ignition timing. With the excess air ratio increasing, the CoVimep increases while the cylinder pressure decreases firstly, and then increases. The flame development duration increases with the ignition timing advanced, but the rapid flame duration decreases, the total combustion duration basically keep constant, and all of them increase with the increasement of excess air ratio. Best fuel economy was gotten at λ=1.2. With the excess air ratio increasing, the CO and NMHC emissions decrease, the NOx emission firstly reaches to a peak value, which is about 3000ppm at λ=1.2, and then decreases. Both of the THC and CH4 emissions have opposite trends to NOx emission. All the emissions mentioned above increases with the advance of ignition timing.

Modelling of passive ammonia selective catalytic reduction system for lean-burn spark ignition direct injection engines

Lean-burn spark ignition direct injection (SIDI) engines offer potential fuel economy savings; however, the lack of simple and cost-effective lean nitrogen oxide (NO x) aftertreatment systems hinder their broad application. Lean NO x traps (LNT) and urea selective catalytic reduction (SCR) catalysts have been widely investigated as possible lean gasoline aftertreatment technologies, but with limited acceptance. Passive ammonia SCR operation is a promising low-cost alternative for lean NO x aftertreatment with SIDI engines. The aftertreatment system includes close-coupled three-way catalyst (TWC) converters and one or more under-floor SCR catalysts. Ammonia (NH3) is formed on the TWC with short pulses of rich engine operations and the generated NH3 is then stored on the under-floor SCR catalysts. During subsequent lean operations, the NO x that breaks through the TWC converters is reduced to nitrogen by NH3 stored on the SCR catalysts. This paper provides an overview of system level analysis as a complement to experimental testing in the development of a passive NH3 SCR-based aftertreatment system. Various aftertreatment architectures (SCR type, the order, and location of the catalysts) were efficiently evaluated and screened, to achieve the best trade-off between engine back-pressure, catalyst warm-up, emission constraints, and acceptable thermal ageing under high-speed/load conditions.

Effect of hydrogen addition on lean burn performance of a spark-ignited gasoline engine at 800 rpm and low loads

To reduce the fuel consumption and emissions of spark-ignited (SI) engines, hydrogen enrichment was used to improve the performance of a lean burn SI engine operating at low speed and load conditions. A hydrogen port-injection system was mounted on the intake manifolds to introduce hydrogen into the intake ports sequentially while keeping the original gasoline injection system unchanged. A hybrid electronic control unit (HECU) was adopted to control injection timings and durations of gasoline and hydrogen, accomplishing four excess air ratios of 1.00, 1.18, 1.43 and 1.67 and three hydrogen volume fractions in the intake of 3%, 5%, 8%. The experimental results showed that engine brake thermal efficiency and torque output were increased, combustion durations were shortened, cyclic variation and HC emissions were reduced, but NOx emissions were increased with the increase of hydrogen addition. CO emission was also reduced under lean conditions with hydrogen enrichment.

Numerical simulation of charge stratifications to improve combustion and NO formation of lean-burn SI engines

The influences of charge stratification on spark ignition (SI) engine combustion and NO emission were analyzed using a phenomenological model. The mixture in the cylinder was divided spherically into three parts: a central core with a stoichiometric air-fuel charge, a dilution region without any combustible charge, and a mixing region lying between the core and the dilution region. Three mixture stratification parameters such as the extent of dilution in the mixing region, the extent of combustible charge in the mixing region, and the gradient of stratification in the mixing region were investigated. The results indicate that the extent of combustible charge in the mixing region could reduce in-cylinder NO formation significantly, compared with the extent of dilution in the mixing region. As long as the degree of dilution in the mixing region is within the dilution limit of the combustible charge, the gradient of dilution has little effect on combustion and NO formation.

Experimental Study on the Operating Range Restrictions of a Lean-burn Turbocharged SI Natural Gas Engine

An experimental investigation aiming at extending the operating range of lean-burn turbocharged spark-ignition (SI) CNG engine was conducted on a six-cylinder throttle body injection CNG engine. Cylinder pressure was recorded by using a high sampling frequency transducer, thus allowing the analysis of pressure characteristics introduced by knock. Coefficient of variations (COV) in indicated mean effective pressure (IMEP) was used to define lean misfire. The effects of various engine parameters on an engine’s lean burn capability, knock limit, and preturbo temperature limit were examined at various operating conditions. The results indicate that a lean-burn SI CNG engine’s operating range is sensitively affected by the mechanical structure of the engine, excess air ratio, spark timing, load, engine speed, intake temperature, ambient humidity, etc. For the turbocharged engine, preturbo temperature is also a key consideration in extending the operating range of the engine.

Linear Parameter-Varying Lean Burn Air-Fuel Ratio Control for a Spark Ignition Engine

Maximization of the fuel economy of the lean burn spark ignition (SI) engine strongly depends on precise air-fuel ratio control. A great challenge associated with the air-fuel ratio feedback control is the large variable time delay in the exhaust system. In this paper, a systematic development of an air-fuel ratio controller based on post lean NOx trap (LNT) oxygen sensor feedback using linear parameter-varying (LPV) control is presented. Satisfactory stability and disturbance rejection performance is obtained in the face of the variable time delay. The LPV controller is simplified to an explicit parameterized gain scheduled lead-lag controller form for the ease of implementation. A Ford F-150 truck with a V8 4.6 l lean burn engine was used to demonstrate the LPV air-fuel ratio control design. Both simulation and experimental results demonstrate that the designed controller regulates the tailpipe air-fuel ratio to the preset reference for the full engine operating range.

OH Fluorescence Images of Pulsed Flame Jet in a Swirl Flow

Lean-burn spark ignition engines have the attribute of minimizing the emission of NOx and, concomitantly, maximizing fuel economy. For combustion in lean fuel-air mixtures, achievement of adequate reliability of ignition and sufficiently high burning rate requires special devices. The most effective among them is the injection of active radicals by means of the Pulsed Flame Jet (PFJ) ignition system. Presented here is an experimental proof of the action of OH radical produced by such an ignition system in a swirl flow with varying the spark discharge mode in the cavity of the PFJ igniter. The measuring apparatus used for this purpose was based on the Planar Laser-Induced Fluorescence (PLIF) method, and the effects of the spark discharge mode in the cavity of the PFJ igniter on the variation of OH fluerencence area in the jet and its intensity were revealed quantitatively.

The relative effects of fuel concentration, residual-gas fraction, gas motion, spark energy and heat losses to the electrodes on flame-kernel development in a lean-burn spark ignition engine

The potential of lean combustion for the reduction in exhaust emissions and fuel consumption in spark ignition engines has long been established. However, the operating range of lean-burn spark ignition engines is limited by the level of cyclic variability in the early-flame development stage that typically corresponds to the 0-5 per cent mass fraction burned duration. In the current study, the cyclic variations in early flame development were investigated in an optical stratified-charge spark ignition engine at conditions close to stoichiometry [air-to-fuel ratio (A = 15] and to the lean limit of stable operation (A/F = 22). Flame images were acquired through either a pentroof window (‘tumble plane’ of view) or the piston crown (‘swirl plane’ of view) and these were processed to calculate the intra-cycle flame-kernel radius evolution. In order to quantify the relative effects of local fuel concentration, gas motion, spark-energy release and heat losses to the electrodes on the flame-kernel growth rate, a zero-dimensional flame-kernel growth model, in conjunction with a one-dimensional spark ignition model, was employed. Comparison of the calculated flame-radius evolutions with the experimental data suggested that a variation in A/F around the spark plug of Φ(A/F) ≈ 4 or, in terms of equivalence ratio Φ, a variation in Φ ≈ 0.15 at most was large enough to account for 100 per cent of the observed cyclic variability in flame-kernel radius. A variation in the residual-gas fraction of about 20 per cent around the mean was found to account for up to 30 per cent of the variability in flame-kernel radius at the timing of 5 per cent mass fraction burned. The individual effect of 20 per cent variations in the ‘mean’ in-cylinder velocity at the spark plug at ignition timing was found to account for no more than 20 per cent of the measured cyclic variability in flame kernel radius. An individual effect of similar level was attributed to the heat losses to the electrodes, and a much lower effect than that, namely 5-10 per cent at most, could be attributed to cyclic variations in spark-energy release traces. Even when the variations in mean velocity were coupled to both the effects of heat losses to the electrodes and spark-energy release variations, the combined effect was found to account for no more than 40-50 per cent of the observed cyclic variability in flame-kernel radius at the 5 per cent mass-fraction burned timing of 40 ° crank angle after ignition timing.

Fuel Effect on Exhaust Emission of a Lean-Burn Spark-Ignition Engine.

Detailed composition of exhaust emission from various gaseous fuels and gasoline was obtained including unregulated species by using an FTIR analyser, and dependency of the composition mainly on equivalence ratio was compared paying emphasis on relation between NOx and unburned species. The results show that NO emission is increased when liquified petroleum gas and propane were used compared with the other gaseous fuels, and that NO2 concentration becomes comparable with NO when the charge mixture comes close to its lean limit. Trends of NOx and unburned hydrocarbon emissions are quantitatively similar for the gas fuels and gasoline, however, there are some differences on constituents in the unburned species.

Combustion characterization of natural gas in a lean burn spark-ignition engine

Lean burn natural gas fuelled spark-ignition engines are particularly attractive regarding environmental performance. Nevertheless, few data exist concerning lean combustion in gas engines in terms of ignition delay, combustion duration and combustion rate. Such data are necessary when using thermodynamic models to predict energy and environmental performance. The present study proposes a contribution to such combustion characterization as a function of spark timing, air excess and engine load. This work is based on experimental cylinder pressure measurements in an open chamber engine on the one hand, and on a one-zone thermodynamic model used as a heat release analysis tool on the other hand. Results obtained, such as mass fraction burned, ignition delay and combustion duration, allow identification of the parameters used in the Wiebe function, a semi-empirical law that is frequently used to describe the combustion in predictive models.

Study on NOx formation and reduction of lean-burn spark ignition gas engine

Study on NOx formation and reduction of lean-burn spark ignition gas engine

Optimizing local charge stratification in a lean-burn spark ignition engine

Gas pressure and local gas velocities have been measured in a single-cylinder spark ignition engine operating at low load and 1000 r/min and the results have characterized the extent to which combustion was enhanced by the injection of a small quantity of a mixture of propane vapour and air towards the spark plug in an otherwise quiescent chamber filled with a homogeneous lean propane/air charge. The effects of the locally generated mean flow/turbulence and equivalence ratio on combustion were examined separately by first injecting a mixture of equivalence ratio identical to that of the homogeneous charge and then a slightly rich mixture into homogeneous charges of lower equivalence ratios. The results show the advantageous effect of jet-induced local turbulence for overall air—fuel ratios between 17 and 24 with a maximum gain in peak pressure of 55 per cent at an air—fuel ratio of 20. The local injection of a rich mixture, in addition to increasing the gain in peak pressure from 30 to 50 per cent at an air—fuel ratio of 24, has extended the lean limit of the engine to 29. The timing of ignition relative to the end of injection, which varied as a function of the injection pressure, was found to have a strong effect on the peak combustion pressure so that, for example, a reduction of 8°(CA) in the time between the spark and the end of injection resulted in a 25 per cent reduction in combustion pressure at an air—fuel ratio of 22. The average flame speed was increased by local injection at all equivalence ratios; for example, a value of 7 m/s was obtained with local injection at an equivalence ratio of 0.7 which is equivalent to the flame speed measured with a homogeneous charge at the much higher equivalence ratio of 0.9.