Gautam T. Kalghatgi

Blow-Out Stability of Gaseous Jet Diffusion Flames. Part I: In Still Air

Abstract A universal non-dimensional formula that describes the blow-out stability limit of gaseous jet diffusion flames in still air has been found experimentally. Its validity has been established over a wide range of parameters that affect the blow-out limit. Its extrapolation to cases where the burner exit flow is choked suggests that for a given gas there is a critical burner diameter above which a stable flame can exist at any flow rate.

Pre-ignition and ‘super-knock’ in turbo-charged spark-ignition engines

Earlier studies of pre-ignitions at hot surfaces are first reviewed. The concept of a critical radius of a hot pocket of gas, closely related to the laminar flame thickness, that is necessary to initiate a propagating flame, has been used successfully to predict relative tendencies of different fuel–air mixtures to pre-ignite. As the mixture is compressed, the thickness of potential laminar flames decreases, and when this becomes of the order of the thermal sheath thickness at the hottest surface, pre-ignition can occur there, creating a propagating flame. Measured engine pre-ignition ratings are shown to correlate well with laminar flame thicknesses. Predictions are made concerning the effects of changes in intake temperature and pressure on the pre-ignition of different fuels. A growing current concern is occasional gas-phase, autoignitive, pre-ignitions that can occur in turbo-charged engines, giving rise to very severe autoignition and knock. It is concluded from the evidence of engine pressure records and autoignition delay times of the mixtures that such pre-ignitions have not arisen from autoignition of the fuel, but of a mixture with a smaller autognition delay time than stoichiometric n-heptane–air. One possibility is that autoignition occurs at hot spots containing some lubricating oil. It is shown that such pre-ignitions, particularly with catalytic enhancement, could initiate a propagating flame, rather than autoignitive propagation. In the later, much more severe autoignition arising after pre-ignition, autoignitive propagation velocities at a hot spot are estimated from computed values of the ignition delay times and assumed reactivity gradients in the fuel–air mixture at the hot spot. The severity of the associated pressure pulse is dependent upon the ratios ξ , of the acoustic speed to the localised autoignitive velocity, and ε , of the residence time of the acoustic wave in the hot spot to the short excitation time in which most of the chemical energy is released. The regime in which a localised detonation can be generated in the hot spot is defined by a peninsula on a plot of ξ against ε . A locus is plotted on this figure corresponding to the growing measured intensities of the engine knock as the pressure increases. This is based on computed autoignition delays and excitation times, for an appropriate surrogate fuel. These changes are characterised by ξ tending to unity and ε tending to ever-higher values, with increasingly intense, localised, developing detonations.

Fuel Octane Effects in the Partially Premixed Combustion Regime in Compression Ignition Engines

Previous work has showed that it may be advantageous to use fuels of lower cetane numbers compared to today’s diesel fuels in compression ignition engines. The benefits come from the longer ignition delays that these fuels have. There is more time available for the fuel and air to mix before combustion starts which is favourable for achieving low emissions of NOx and smoke though premixing usually leads to higher emissions of CO and unburned hydrocarbons. In the present work, operation of a single-cylinder lightduty compression ignition engine on four different fuels of different octane numbers, in the gasoline boiling range, is compared to running on a diesel fuel. The gasoline fuels have research octane numbers (RON) of 91, 84, 78, and 72. These are compared at a low load/low speed condition (4 bar IMEP / 1200 rpm) in SOI sweeps as well as at a higher load and speeds (10 bar IMEP / 2000 and 3000 rpm) in EGR sweeps. There is a NOx advantage for the 91 RON and 84 RON fuels at the lower load. At the higher load, NOx levels can be reduced by increasing EGR for all gasolines while maintaining much lower smoke levels compared to the diesel. In the conditions studied, the optimum RON range might be between 75 and 85.

Low NOx and Low Smoke Operation of a Diesel Engine Using Gasoline-Like Fuels

Much of the technology in advanced diesel engines, such as high injection pressures, is aimed at overcoming the short ignition delay of conventional diesel fuels to promote premixed combustion in order to reduce NOx and smoke. Previous work in a 2 litre single cylinder diesel engine with a compression ratio of 14 has demonstrated that gasoline fuel, because of its high ignition delay, is very beneficial for premixed compression ignition compared to a conventional diesel fuel. We have now done similar studies in a smaller — 0.537 litre — single cylinder diesel engine with a compression ratio of 15.8. The engine was run on three fuels of very different auto-ignition quality — a typical European diesel fuel with a cetane number (CN) of 56, a typical European gasoline of 95 RON and 85 MON with an estimated CN of 16 and another gasoline of 84 RON and 78 MON (estimated CN of 21). The previous results with gasoline were obtained only at 1200 rpm — here we compare the fuels also at 2000 rpm and 3000 rpm. At 1200 rpm, at low loads (∼4 bar IMEP) when smoke is negligible, NOx levels below 0.4 g/kWh can be easily attained with gasoline without using EGR while this is not possible with the 56 CN European diesel. At these loads, the maximum pressure rise rate is also significantly lower for gasoline. At 2000 rpm, with 2 bar absolute intake pressure, NOx can be reduced below 0.4 g/kWh with negligible smoke (FSN <0.1) with gasoline between 10 and 12 bar IMEP using sufficient EGR while this is not possible with the diesel fuel. At 3000 rpm, with the intake pressure at 2.4 bar absolute, NOx of 0.4 g/KWh with negligible smoke was attainable with gasoline at 13 bar IMEP. Hydrocarbon and CO emissions are higher for gasoline and will require after-treatment. High peak heat release rates can be alleviated using multiple injections. Large amounts of gasoline, unlike diesel, can be injected very early in the cycle without causing heat release during the compression stroke and this enables the heat release profile to be shaped.Copyright

Blow-Out Stability of Gaseous Jet Diffusion Flames Part II: Effect of Cross Wind

Abstract A non-dimensional stability curve that describes the blow-out stability of diffusion flames in a cross-wind, for different gases, has been established experimentally. For a given burner and a given gas, if the cross-wind speed is greater than a limiting value, a stable flame is not possible. For cross-wind speeds less than this limit, there are usually two blow-out limits which are on either side of the blow-out limit in still air.

Combustion Limits and Efficiency in a Homogeneous Charge Compression Ignition Engine

Abstract Combustion phasing is characterized by CA50, the crank angle position where 50 per cent of the total heat release occurs. CA50 = (a + b) [OI - OI0] where OI is the octane index defining the fuel auto-ignition quality given by OI = (1 - K) RON + K MON = RON - KS and S is the sensitivity of the fuel, (RON - MON). K, (a + b), and OI0, which is the requirement of the engine, can be estimated from in-cylinder thermodynamic parameters and engine operating conditions. If OI is much smaller or greater than OI0, combustion will reach, respectively, the ‘knock limit’ with unacceptably high rate pressure rise or the ‘instability limit’ with unacceptably high cyclic variation. Nineteen different gasoline-like fuels with RON > 60 have been tested at many different operating conditions in a single-cylinder homogeneous charge compression ignition (HCCI) engine. This generates a data set across which the correlations between CA50 and other operating parameters such as the equivalence ratio and the inlet pressure Pin are broken. Regression analysis shows that the maximum pressure rise rate (MPRR) increases with increasing φ and Pin and decreases with increasing CA50 and S if other parameters are kept constant. X = 1.55CA50 - 52 can be used to discriminate between conditions of high and low cyclic variation. For X < 0, the coefficient of variation (COV) of indicated mean effective pressure (i.m.e.p.) is less than 10 per cent in about 90 per cent of the cases, but if X>0, COV of i.m.e.p. is less than 10 per cent in only about 12 per cent of the cases. The i.m.e.p. increases with fuel energy content per cycle. The indicated fuel conversion efficiency ηi increases with increase in i.m.e.p. With other parameters kept constant, ηi decreases with increasing φ, intake air density, and intake temperature and increases with increasing CA50 and maximum pressure. An HCCI engine can be operated with an acceptable noise level and cyclic variation only within a narrow range of CA50 between about –5 and +10° from top dead centre.

Combustion Chamber Deposits in Spark-Ignition Engines: A Literature Review

Deposits, derived primarily from the fuel but with some contribution from the oil are formed inside the combustion chamber of a spark-ignition engine with use. The growth of combustion chamber deposits (CCD) is a dynamic and, to an extent, reversible process which at any given time reflects the balance between the formation and removal processes. Engine surface temperature is the most important parameter that affects their formation and changes in engine operation which tend to increase surface temperature, reduce deposit growth. At a fixed temperature, less volatile fuels tend to form more deposits than more volatile fuels. Some detergent additive packages tend to increase CCD levels. CCD reduce the heat lost to the coolant and increase charge temperature thereby increasing flame propagation rates but reducing volumetric efficiency; they might also affect the final phase of combustion by as yet undefined chemical means. This is reflected as an increase in octane requirement and NOx and a reduction in maximum power but an improvement in fuel economy and a reduction in CO2 emissions. They might also lead to higher HC emissions but not necessarily always since there might be competing mechanisms which come into play. CCD effect on CO emissions is not clearly established. They can also cause other interference problems like carbon rap. It is not known to what extent engine performance is affected by small changes in CCD levels. There is a large variation in deposit growth and its response to changes in fuel, additives and engine operating conditions across the combustion chamber and between different engines. Similarly, the performance of different engines will be affected to different extents by the deposits. While assessing the effects of different fuels or additives on engine performance and emissions through their effects on CCD, the simultaneous effects on other aspects such as inlet valve deposits which might have their own effects on engine operation, should also be considered. The paper reviews the literature on these topics.

The visible shape and size of a turbulent hydrocarbon jet diffusion flame in a cross-wind

Abstract The results of an extensive wind-tunnel study into the shapes and sizes of hydrocarbon jet diffusion flames in a horizontal cross-wind are presented. The shape of a turbulent diffusion flame in a cross-wind can be described by the frustum of a cone, which, in turn, can be defined by five different parameters of shape. When the burner axis is normal to the wind, each shape parameter can be related to the burner diameter, the burner exit velocity, the cross-wind speed, and the density of the burner gas by one equation. When the burner axis is not normal to the wind, the experimental results still follow an identifiable pattern and can be used to estimate flame shapes and sizes.

Octane Appetite Studies in Direct Injection Spark Ignition (DISI) Engines

The anti-knock or octane quality of a fuel depends on the fuel composition as well as on the engine design and operating conditions. The true octane quality of practical fuels is defined by the Octane Index, Ol = (1-K)RON + KMON where K is a constant for a given operating condition and depends only on the pressure and temperature variation in the engine (it is not a property of the fuel). RON and MON are the Research and Motor Octane numbers respectively, of the fuel. Ol is the octane number of the primary reference fuel (PRF) with the same knocking behaviour at the given condition. In this work a wide range of fuels of different RON and MON were tested in prototype direct injection spark ignition (DISI) engines with compression ratios of 11 and 12.5 at different speeds up to 6000 RPM. Knock Limited Spark Advance (KLSA) was used to characterize the anti-knock quality of the fuel. Experiments were also done using two cars with DISI engines equipped with knock sensor systems. The anti-knock quality of a fuel in the car is inferred from the power/acceleration performance, which changes in response to knock. RON is dominant for fuel anti-knock quality at all engine speeds. Moreover, for low and moderate engine speeds, frequently used on the road, for a given RON, lower MON results in better fuel anti-knock quality.

Optimizing Engine Concepts by Using a Simple Model for Knock Prediction

This licentiate thesis concerns the modeling of spark ignition engine combustion for use in one dimensional simulation tools. Modeling of knock is of particular interest when modeling turbocharged engines since knock usually limits the possible engine output at high load. The knocking sound is an acoustic phenomenon with pressure oscillations triggered by autoignition of the unburned charge ahead of the propagating flame front and it is potentially damaging to the engine. To be able to predict knock it is essential to predict the temperature and pressure in the unburned charge ahead of the flame front. Hence, an adequate combustion model is needed. The combustion model presented here is based on established correlations of laminar burning velocity which are used to predict changes in combustion duration relative to a base operating condition. Turbulence influence is captured in empirical correlations to the engine operating parameters spark advance and engine speed. This approach makes the combustion model predictive in terms of changes in gas properties such as mixture strength, residual gas content, pressure and temperature. However, a base operating condition and calibration of the turbulence correlations is still needed when using this combustion model. The empirical models presented in this thesis are based on extensive measurements on a turbocharged four cylinder passenger car engine. The knock model is simply a calibration of the Arrhenius type equation for ignition delay in the widely used Livengood-Wu knock integral to the particular fuel and engine used in this work.