CFR Engine Research Articles

The action of ignition improvers in diesel fuels

The effectiveness of i-propyl nitrate, n-butoxyethyl nitrate, di-t-butyl peroxide and i-octyl nitrate in increasing the cetane number of three diesel fuels (40, 45 and 53 CN) has been examined using the standard test conditions for a CFR engine. When the results for any two of the additives are compared in terms of the increase in cetane number (ΔCN) produced by a given concentration, it is found that the results for all three fuels lie on the same straight line, i.e. for any two fuels 1 and 2 and two additives A and B at the same concentration (ΔCN)1,A/(ΔCN)1,B=(ΔCN)2,A/(ΔCN)2,B It follows that for a given concentration of additive, the ratio of the ΔCN's produced in two different fuels should be the same for all additives, and the experimental results confirm that this is the case.

Storage and Partial Oxidation of Unburned Hydrocarbons in Spark-Ignited Engines - Effect of Compression Ratio and Spark Timing

Abstract The concentrations of the individual hydrocarbon species in the exhaust gases from a single-cylinder CFR engine have been measured. The effects of compression ratio and spark timing on these emissions have been observed using propane, isooctane (2,2,4-trimethylpentanerpar; and indolene clear fuels. The results show that total hydrocarbon emissions increase rapidly as the compression ratio increases and decrease sharply as the spark timing is retarded relative to MBT at a constant compression ratio. Changes in the concentration of unburned fuel in the exhaust contribute the most to these variations. These observations are consistent with the predicted increase in crevice and oil film storage of unburned hydrocarbons at higher compression ratio because of the higher peak pressures. They also agree with the predicted increase in post-flame burnup of stored hydrocarbons in the hotter cylinder gases encountered during low compression ratio or retarded spark operation.

A Model for the Lean Misfire Limit in Spark-Ignition Engines

Abstract The combined effect of compression ratio, engine speed, intake air temperature, nitrogen diluent an spark-timing on the lean misfire limit is investigated experimentally for a CFR engine with propane as a fuel. The misfire limit is defined as the point where 5 to 8 cycles out of 1,000 consecutive cycles have firing pressure traces equal to the motoring presssure trace. A balance between energy produced in the entrained volume of the developing flame and the energy diffused to unburnt gas is proposed as a model for the flame initiation. The initial conditions for the turbulent flame extinction process are obtained from a k∊ model which includes the effect of compressibility

Performance of Ethanol Blends in Gasoline Engines

ABSTRACT Astudy was performed with varying amounts of etha-nol in the fuel of a CFR engine at knock limited com-pression ratio and maximum power spark advance. Ethanol concentrations between 10 and 25 percent in 85 octane gasoline were tested for performance, fuel con-sumption, and exhaust emissions. For operation at knock limits and a constant equivalence ratio, the use of ethanol gave a reduction in petroleum usage 10 percent greater than the volumetric percentage of ethanol in the blend. With increasing ethanol concentration, BMEP and BSHC increase, BSNO and BSCO remain constant, and exhaust temperature decreases.

The Effect of Oil Layers on the Hydrocarbon Emissions from Spark-Ignited Engines

Abstract Measured amounts of oil were added to the engine cylinder of a single-cylinder CFR engine to determine the effect of oil layers on exhaust hydrocarbon emissions. The exhaust hydrocarbon concentration increased in proportion to the amount of oil added when the engine was fueled on isooctane. Addition of 0·6 cm3 of oil produced an increase of 1000 ppmC in exhaust hydrocarbon emissions at a coolant temperature of 320°K. Gas chromatographic analysis of the exhaust determined that fuel and fuel oxidation species, not oil oxidation products, were responsible for most of the increase. Similar experiments performed with propane fuel showed no increase in exhaust emissions when oil was added to the cylinder. These measurements have determined that the increase in tailpipe hydrocarbon concentration consists of fuel related species and is proportional to both the amount of oil added and the solubility of the fuel in the oil. Thus, we believe that the principal source of this increase in exhaust hydrocarbon ...

Effects of organic sulphur compounds on the ignition of unleaded and leaded hydrocarbon fuels

Measurements have been made of the effects of different sulphur compounds on the ignitionparameters of pure hydrocarbon fuels and of binary mixtures thereof both in a CFR engine and under laboratory conditions. In general, the sulphur compounds added promote the combustion of the fuels, particularly in the presence of lead compounds. The sulphurlead antagonism increases with the extent of the preflame reactions of the hydrocarbons, i.e. from aromatics through alkenes to alkanes. The effectiveness of the sulphur compounds as combustion promoters increases with their ease of ignition except with thiophenol where special effects may be attributed to the action of its oxidation products. Engine and laboratory experiments can be meaningfully compared only if they are carried out on a single base fuel.

Plasma jet ignition in a lean-burn CFR engine

The parameters controlling plasma jet ignition are explored in a lean burn CFR engine. The effects of plasma energy and its duration, the plasma cavity size and the plasma medium are investigated. Premixed charge of methane air is used throughout. With the engine running at 800 RPM and at an equivalence ratio of 0.7, increased plasma energy and reduced energy deposition time result in improved performance. Tests with several sizes of plasma cavity reveal that for a given plasma energy and duration, an optimum size for best performance can be determined. When the plasma medium is the fuel itself, it was found that both degradation and improvement could be obtained depending on the plasma cavity size. For the range of designs used here, it was found that the best performance could beobtained when the plasma cavity is 150 mm3 with the plasma medium being the same as the engine charge. In this case a constant specific fuel consumption of 0.12 gm/Kw-s at equivalence ratios of 0.65–0.80 was obtained. Emissions data on CO, HC and NOx show consistent trends and confirm the superiority of the 150 mm3 plasma unit.

A Fundamental Model for Predicting Fuel Consumption, NOxand HC Emissions of the Conventional Spark-Ignited Engine

Abstract A model of the four-stroke S.I. engine cycle has been developed which predicts fuel consumption, NOx and HC emissions as a function of engine design and operating conditions. The model is primarily thermodynamic in nature containing no formal spatial dependence. The major new features of the model are: first, a treatment of heat transfer which confines heat losses to a boundary layer region surrounding a central adiabatic core; second, an integral boundary-layer analysis of in-cylinder burnup of quenched hydrocarbons; and third, a calculation of exhaust port HC oxidation which considers the temperature history of each element of gas leaving the cylinder. The main adjustable parameters of the model relate to the rate of heat transfer and the ratio of the two-plate quench to the single-wall thickness. An extensive comparison of model predictions with experimental CFR engine data is presented. The results show excellent agreement between predicted and experimental fuel consumption and NOx emissions....

The Effect of Hydrogen Addition on Ignition Delays and Flame Propagation in Spark Ignition Engines

Abstract The results of an experimental investigation of the effect of supplemental hydrogen (up to 30 percent of the total fuel energy) on the combustion process in a CFR engine are reported. The hydrogen was added under otherwise constant conditions so that chemical properties were varied under constant hydrodynamic conditions. Calibrated cylinder pressure traces, averaged over many cycles, were incorporated into a two-zone thermodynamic analysis to determine the mass fraction burned as a function of crankangle. The techniques employed enabled changes in the induction period and combustion duration of the order of 0.1 ms to be resolved. The added hydrogen resulted in significant reductions in ignition delay or induction times, especially in lean mixtures. Reductions were greater with increased fractions of hydrogen. Once a turbulent flame was well established, the hydrogen had a relatively small effect on the burning rate. The results are consistent with a description of the combustion process which inc...

A fundamentally based model of knock in the gasoline engine

A mathematical model has been developed to simulate the occurrence of knock in the gasoline engine. A rapid-compression machine was used to obtain experimental information on the autoignition characteristics of a suitable range of sensitive and non-sensitive reference fuels. A generalised chemical reaction scheme which embodies the essential kinetic features of the degenerate branched chain reactions responsible for autoignition was then refined to give an accurate simulation of these observations. When incorporated in a computerised engine cycle simulation this chemical model predicts the development of end-gas reactions and the occurrence of knock. The model has been used to investigate the origin of severity in the CFR engine. In confirming that high end-gas temperatures are of major importance in de-rating sensitive fuels under ‘severe’ engine conditions this study gives us confidence in our approach to engine simulation, and especially in the chemical aspect of the model.

Flame-quench distance measurements in a CFR engine

A method has been devised which uses ion probe detection of flames burning through a gap inside the combustion chamber to provide rapid and convenient measurement of two-surface flame-quench distances in an operating engine. This method has been used to establish that such quench distances range from about 0.01 cm to > 0.2 cm, depending in decreasing order on equivalence ratio, manifold pressure, wall temperature, spark timing, and fuel type. A comparison between these experimental findings and the results from one- and two-surface measurements performed by others indicates that two-surface quench values are 2.5 to 12 times greater than one-surface values. The steep dependence on lean equivalence ratio of the measured two-surface quench values imitates to some extent the known behavior of lean hydrocarbon emissions (HCE), but more detailed single-surface quenching data is now needed to fully appreciate the mechanism involved in exhaust HCE.

An Investigation of Nonequilibrium Effects in an Internal Combustion Engine

A theoretical analysis of the basic chemical kinetic processes which lead to the formation and partial decomposition of nitric oxide and carbon monoxide has been undertaken. A mathematical model of an Otto cycle engine has been formulated, programmed for the CDC 6600 computer, and used to determine species concentrations and thermodynamic properties at any point in the engine cycle. The validity of this model has been confirmed by the results of an experimental program conducted on a typical multicylinder automotive engine, and data available for a single cylinder CFR engine. In addition, the computerized model was used to describe the chemical and physical processes upon which existing control techniques for nitric oxide are based, and to investigate the potential of a new concept, fuel-rich combustion and secondary air injection, for minimizing nitric oxide and carbon monoxide emissions. A unique concept in controlling undesirable engine emissions, i.e., rich combustion followed by secondary air injection during the engine expansion, was shown to result in significant reductions in nitric oxide and carbon monoxide emissions without incurring an appreciable loss in thermal efficiency as compared to stoichiometric operation.

Temperature Measurement of Flame in a Gasoline Engine : 1st Report, Measurement of Average Temperature

One method to measure the instantaneous flame temperature within the cylinder of a gasoline engine by means of the absolute radiation method is proposed. The radiation energy of D-line emitted from a trace of sodium, which is added to fuel, is measured by an optical-electronic system. Then, the measured energy is converted into the equivalent black body temperature. This temperature may be considered as the flame temperature which is averaged through an optical path in the luminous flame zone. In the process of conversion, the half-width of D-line is determined experimentally and the number of Na atoms radiating in the optical path is evaluated theoretically. As the optical system, a quartz window, 4 limiting diaphragms, an interference filter and a fiber-scope are arranged in order. And a light beam having 1 mm in diameter is introduced to a photo-multiplier by means of the fiberscope. By this technique, the flame temperature in CFR engine is measured under various operating conditions.

Temperature Measurement of Flame in a Gasoline Engine : 2nd Report, Measurement of Temperature Distribution

The temperature measured in the 1st Report is the one which is averaged through an optical path in the luminous flame zone. In the first step of this paper, the physical meaning of this average temperature is discussed. For this purpose, the flame is divided into many layers and the radiation from the whole flame is considered as the result composed of radiation and absorption of each layer. In the second step, based upon the above consideration, the temperature distribution in a flame is measured. To divide the flame into many layers, and to measure the radiation effect of each layer, a light-intercepter is inserted into the flame and displaced successively across the flame. By this technique, a change of the flame temperature distribution versus crank angle is measured on a CFR engine. As one of the results, Hopkinson's effect is ascertained. Namely, at the instant when the flame spreads over the combustion chamber, it is found that the temperature of gas around the park plug is about 300°C higher than that of gas near the cylinder wall.

Experimental and Theoretical Study of Nitric Oxide Formation in Internal Combustion Engines

Abstract The nonequilibrium formation of nitric oxide within the internal combustion engine cylinder is examined. A thermodynamic model which predicts the properties of the burnt and unburnt gases during the combustion process is developed. A set of reactions which govern the formation of nitric oxide is proposed, and rate equations for nitric oxide concentrations as a function of time in the post-flame gases are derived. The results of time-resolved measurements carried out on a CFR engine are described, where emitted light intensities at wavelengths selected to record radiation from the CO + O and NO + O continua were used to determine the nitric oxide concentration. The comparison between theoretical and experimental results for fuel-lean mixtures confirms that the important features of the model presented are correct.

Variation of the Flame Propagatin Time in a Spark Ignition Engine

Effects of operating conditions of a CFR engine on variations of flame travel time were studied statistically. Cyclic variations of falme travel time showed some departure form a normal distribution, but those of the "apparent flame propagation velocity", the distance of flame travel divided by the flame travel time, were shown to follow a normal one in general.The mean apparent flame propagation velocity υ^-f was found to have linear correlation with the standard deviation σ. It was shown also that υ^-f/SL (SL : laminar burning velocity) was directly proportional to σ/SL in all the results obtained at different inlet mixture temperatures, and to Harrow's Reynolds number of the mixture before combustion over a wide range.An explanation for actual pattern of cyclic distribution of flame travel was made by incorporating the effects of cyclic variation of mixture stregth and the flame velocity due to the turbulence in the combustion chamber.

Statistical Study on Variations of Flame Travel Time in a Spark Ignition Engine

Flame travel times of some base stocks of commercial gasolines were statistically studied using a CFR engine and a multi-channel flame counter. Two kinds of fuels, catalytically reformed naphtha (RFT) and a 50/50 mixture of it with straight-run light naphtha (RFT/LN) were used throughout the work, and in some cases, isooctane, toluene, xylene and cracked naphtha were used for comparison. Operating conditions of the engine varied were air-fuel ratios from 10:1 to 16:1, inlet mixsture temperatures of 50°, 100° and 150°C, and throttle openings of 90° (full throttle), 26° and 15°. Engine speed was kept constant at 900rpm, and the ignition timing was set at the best power.RFT and RFT/LN seemed to have the minimum flame travel time at a rich air-fuel ratio of 11:1. The flame travel time of RFT was longer than that of RFT/LN and the extent of cyclic disperison of the former was larger than that of the latter.The effect of inlet mixture temperature on the time of flame travel was small for RFT and only slightly noticeable for RFT/LN. The extents of cyclic dispersion at high inlet temperatures of 100° and 150°C for both fuels were not much different from those obtained at 50°C.As the throttle valve was closed, the flame travel time became longer, the extent of cyclic dispersion increased, and their sensitivity to air-fuel ratio become significant.As found in our previous work, where the apparent flame propagation velocity, defined as the distance of flame travel divided by the flame travel time, followed a normal distribution, the results obtained in the present work were analysed using the apparent mean flame propagation velocity Vf and the standard deviation σf and the coefficient of variation σf/Vf.The relationship between Vf/SL (SL=laminar burning velocity) and Reynolds number was discussed and a possibility of similar linear correlation as found for isooctane in the previous work was shown. The correlation of Vf versus σf was also discussed.

Pre-flame reaction products of isooctane formed in a motored engine

Qualitative and quantitative analyses have been made of the more stable reaction products of clear and leaded isooctane formed in a motored single-cylinder CFR engine. By varying the compression ratio (CR) of the engine, oxidative reactions could be followed from initial reaction to incipient autoignition. Cool flame characteristics of the fuels were followed and peak cycle pressures were measured. Techniques were developed to collect, separate, measure and identify the complex mixtures of reaction products. The combination of capillary gas-liquid chromalography (GLC) and time-of-flight mass spectrometry proved to be particularly valuable in identifying minor reaction products. Twenty compounds were identified, accounting for approximately 98% w of the products observed Isobutene, the isomers of diisobutylene, two C 8 cyclic ethers, and a C 8 aldehyde were the predominant reaction intermediates detected. Tetraethyllead (TEL) markedly reduced the intensity of the cool flame observed and, in general, inhibited the degradation of isooctane at all severity levels studied. At incipient autoignition, TEL reduced the concentration (per unit volume) of detected peroxides to a significantly greater extent than any other product or class of products measured. Thus, the main action of TEL appears to be the deactivation of peroxides and peroxy chain-branching intermediates.

The Effects of Operating Conditions of the Engine on Knocking Characteristics of Gasolines

Knocking characteristics of some stock components of gasoline were studied under varying operating conditions by the CFR engine. The antiknock characteristics of fuels, clear or with antiknock compounds, were studied by the incipient knock limited compression ratio, which was detected from the knock characteristic dp/dt traces of the combustion chamber pressure on the cathode ray oscilloscope. Fuels used were catalytically reformed naphtha, cracked naphtha, straight-run light naphtha and some primary reference fuels.It was shown that the lead responses of reformed naphtha and cracked naphtha were greater at richer mixture strength and at more retarded ignition timing.It was found that the relationship between the incipient knock limited compression ratio and the lead concentration at any operating conditions at the air-fuel ratio of 14.0 could be expressed by the same type of empirical equations for octane number and lead concentration.The effect of inlet-mixture temperature was analysed using the knock limited compression density at the end of compression stroke.Making use of a bled manifold, the effects of volatilities of lead alkyl compounds on its distribution in the front end of fuels were observed.

The Effects of Operating Conditions of the Engine on Antiknock Properties of Gasolines (3)

Antiknock qualities of clear catalytically reformed naphtha and with tetraethyl lead (TEL) and tetramethyl lead (TML) were studied under various operating conditions of the CFR engine. The operating variables investigated were engine speeds of 600, 900 and 1, 200rpm, ignition timing of 5° to 35° BTDC, air-fuel ratio between 10 and 16, and inlet mixture temperatatures of 50° to 150°C. The lead response was evaluated from increase of incipient knock limited compression ratio.The effect of TEL or TML varied with engine operating conditions, but generally, lead response in catalytically reformed naphtha was greater at richer mixture and at retarded timing of ignition.Because the relative effectiveness of TML to TEL varied considerably, depending on operating conditions and lead concentrations, it was difficult to state absolutely which was more effective than the other. It was found that the relationship between the incipient knock limited compression ratio and lead concentration was expressed by the same type of empirical equation as found on octane number and lead concentration.