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A fuel volatility model based on the major species present in the fuel has been formulated. The model accurately predicts the ASTM distillation curves and Reid Vapor Pressure for hydrocarbon fuels. The model may be used to assess the fuel effects on the extent of evaporation and the vapor composition in the mixture preparation process.

This paper provides an overview of spark-ignition engine unburned hydrocarbon emissions mechanisms, and then uses this framework to relate measured engine-out hydrocarbon emission levels to the processes within the engine from which they result. Typically, spark-ignition engine-out HC levels are 1.5 to 2 percent of the gasoline fuel flow into the engine; about half this amount is unburned fuel and half is partially reacted fuel components. The different mechanisms by which hydrocarbons in the gasoline escape burning during the normal engine combustion process are described and approximately quantified. The in-cylinder oxidation of these HC during the expansion and exhaust processes, the fraction which exit the cylinder, and the fraction oxidized in the exhaust port and manifold are also estimated.

An analysis method for characterizing fuel behavior during spark-ignition engine starting has been developed and applied to several sets of start-up data. The data sets were acquired from modern production vehicles during room temperature engine start-up. Two different engines, two control schemes, and two engine temperatures (cold and hot) were investigated. A cycle-by-cycle mass balance for the fuel was used to compare the amount of fuel injected with the amount burned or exhausted as unburned hydrocarbons. The difference was measured as “fuel unaccounted for”. The calculation for the amount of fuel burned used an energy release analysis of the cylinder pressure data. The results include an overview of starting behavior and a fuel accounting for each data set Overall, starting occurred quickly with combustion quality, manifold pressure, and engine speed beginning to stabilize by the seventh cycle, on average.

The direct injection spark-ignition engine is the only internal combustion engine with the potential to equal the efficiency of the diesel and to tolerate a wide range of fuel types and fuel qualities without deterioration of performance. However, this engine has low combustion efficiency and excessive hydrocarbon emissions when operating at light load. In this paper, potential sources of hydrocarbon emissions during light load operation are postulated and analyzed. The placement of fuel away from the primary combustion process in conjunction with a lack of secondary burnup are isolated as important hydrocarbon emissions mechanisms. Analyses show that increasing cylinder gas temperatures can improve secondary burnup of fuel which would reduce hydrocarbon emissions. Practical means to achieve this include higher compression ratio and use of ceramic parts in the combustion chamber.

The engine-out (EO) total and speciated hydrocarbon emissions data from the Auto-Oil Program Phase II Heavy Hydrocarbon Study had been analyzed. The methodology was to first investigate the stabilized EO emissions (Bag 2) of a specific vehicle (Vehicle 04B, a 1989 Model Year Ford Taurus); then the vehicle-to-vehicle differences in Bag2 emissions were considered. Finally, the differences in the Bag2 and the starting/warm-up EO emissions (Bag1) were examined. The speciated emissions may be interpreted as a “feed-through” part due to the unreacted fuel species, and an “offset” part due to the decomposition products. The significant non-fuel emitted species were methane and the olefins. The HC emissions for vehicles with different total emissions were similar in species composition. For both the total and speciated emissions, there was no substantial difference between the Bag1 and Bag2 values for Vehicle 04B.

The oxidation chemistry of paraffins, aromatics, olefins and MTBE were examined. Detailed chemical kinetics calculations were carried out for oxidation of these compounds in the engine cycle. The oxidation rates are very sensitive to temperature. At temperatures of over 1400 K (depending on the fuel), all the hydrocarbons are essentially oxidized for typical residence time in the engine. Based on the kinetics calculations, a threshold temperature is defined for the conversion of the fuel species to CO, CO2, H2O and partially oxidized products. The difference in the survival fraction between aromatics and non-aromatics is attributed to the higher threshold temperature of the aromatics.

The chemistry of unburned hydrocarbon oxidation in SI engine exhaust was modeled as a function of temperature and concentration of unburned gas for lean and rich mixtures. Detailed chemical kinetic mechanisms were used to model isothermal reactions of unburned fuel/air mixture in an environment of burned gases at atmospheric pressure. Simulations were performed using five pure fuels (methane, ethane, propane, n-butane and toluene) for which chemical kinetic mechanisms and steady state hydrocarbon (HC) emissions data were available. A correlation is seen between reaction rates and HC emissions for different fuels. Calculated relative amounts of intermediate oxidation products are shown to be consistent with experimental measurements.

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.

The purpose of this work was to develop an understanding of how liquid fuel transported into the cylinder of a port-fuel-injected gasoline-fueled SI engine contributes to hydrocarbon (HC) emissions. To simulate the liquid fuel flow from the valve seat region into the cylinder, a specially designed fuel probe was developed and used to inject controlled amounts of liquid fuel onto the port wall close to the valve seat. By operating the engine on pre-vaporized Indolene, and injecting a small amount of liquid fuel close to the valve seat while the intake valve was open, we examined the effects of liquid fuel entering the cylinder at different circumferential locations around the valve seat. Similar experiments were also carried out with closed valve injection of liquid fuel at the valve seat to assess the effects of residual blowback, and of evaporation from the intake valve and port surfaces.

A research program designed to measure the contribution from fuel absorption in the thin layer of oil, lubricating the cylinder liner, to the total and speciated HC emissions from a spark ignition engine has been performed. The logic of the experiment design was to test the oil layer mechanism via variations in the oil layer thickness (through the lubricant formulations), solubility of the fuel components in the lubricants, and variations in the crankcase gas phase HC concentration (through crankcase purging). A set of preliminary experiments were carried out to determine the solubility and diffusivity of the fuel components in the individual lubricants. Engine tests showed similar HC emissions among the tested lubricants. No consistent increase was observed with oil viscosity (oil film thickness), contrary to what would be expected if fuel-oil absorption was contributing significantly to engine-out HC. Similarly, no effect of crankcase purging could be observed.

A one-dimensional model has been developed for the species and energy transfer over a thin (0.1-0.5 mm) layer of liquid fuel present on the wall of a spark-ignition engine. Time-varying boundary conditions during compression and flame passage were used to determine the rate of methanol vaporization and oxidation over a mid-speed, mid-load cycle, as a function of wall temperature. The heat of vaporization and the boiling point of the fuel were varied about a baseline to determine the effect of these characteristics, at a fixed operating point and lean conditions (ϕ = 0.9). The calculations show that the evaporation of fuels from layers on cold walls starts during flame passage, peaking a few milliseconds later, and continuing through the exhaust phase.

In order to understand how unburned hydrocarbons emerge from SI engines and, in particular, how non-fuel hydrocarbons are formed and oxidized, a new gas sampling technique has been developed. A sampling unit, based on a combination of techniques used in the Fast Flame Ionization Detector (FFID) and wall-mounted sampling valves, was designed and built to capture a sample of exhaust gas during a specific period of the exhaust process and from a specific location within the exhaust port. The sampling unit consists of a transfer tube with one end in the exhaust port and the other connected to a three-way valve that leads, on one side, to a FFID and, on the other, to a vacuum chamber with a high-speed solenoid valve. Exhaust gas, drawn by the pressure drop into the vacuum chamber, impinges on the face of the solenoid valve and flows radially outward.

This review summarizes the latest developments in diesel emissions regarding regulations, engines, NOx (nitrogen oxides) control, particulate matter (PM) reductions, and hydrocarbon (HC) and CO oxidation. Regulations are advancing with proposals for 70% tightening of fleet average light-duty (LD) criteria emissions likely to be proposed in California for ~2016-22. CO₂ regulations in both the heavy- and light-duty sectors will also tighten and impact diesel engines and emissions, probably long into the future. Engine technology is addressing these needs. Light-duty diesel engines are making incremental gains with combustion enhancements that allow downsizing for CO₂ savings. Heavy-duty (HD) engine show trade-offs between hardware recipes, exhaust deNOx control, and fuel consumption.

The effect of engine operating parameters (speed, spark timing, and fuel-air equivalence ratio [Φ]) on hydrocarbon (HC) oxidation within the cylinder and exhaust system is examined using propane or isooctane fuel. Quench gas (CO2) is introduced at two locations in the exhaust system (exhaust valve or port exit) to stop the oxidation process. Increasing the speed from 1500 to 2500 RPM at MBT spark timing decreases the total, cylinder-exit HC emissions by ∼50% while oxidation in the exhaust system remains at 40% for both fuels. For propane fuel at 1500 rpm, increasing Φ from 0.9 (fuel lean) to 1.1 (fuel rich) reduces oxidation in the exhaust system from 42% to 26%; at 2500 RPM, exhaust system oxidation decreases from 40% to approximately 0% for Φ = 0.9 and 1.1, respectively. Retarded spark increases oxidation in the cylinder and exhaust system for both fuels. Decreases in total HC emissions are accompanied by increased olefinic content and atmospheric reactivity.

An experimental study was performed in a firing SI engine at conditions representative of the warmup phase of operation in which liquid gasoline films were established at various locations in the combustion chamber and the resulting impact on hydrocarbon emissions was assessed. Unique about this study was that it combined, in a firing engine environment, direct visual observation of the liquid fuel films, measurements of the temperatures these films were subjected to, and the determination from gas analyzers of burned and unburned fuel quantities exiting the combustion chamber - all with cycle-level resolution or better. A means of deducing the exhaust hydrocarbon emissions that were due to the liquid fuel films in the combustion chamber was developed. An increase in exhaust hydrocarbon emissions was always observed with liquid fuel films present in the combustion chamber.

A one-dimensional model for crevice HC post-flame oxidation is used to calculate and understand the effect of operating parameters and fuel type (propane and isooctane) on the extent of crevice hydrocarbon and the product distribution in the post flame environment. The calculations show that the main parameters controlling oxidation are: bulk burned gas temperatures, wall temperatures, turbulent diffusivity, and fuel oxidation rates. Calculated extents of oxidation agree well with experimental values, and the sensitivities to operating conditions (wall temperatures, equivalence ratio, fuel type) are reasonably well captured. Whereas the bulk gas temperatures largely determine the extent of oxidation, the hydrocarbon product distribution is not very much affected by the burned gas temperatures, but mostly by diffusion rates. Uncertainties in both turbulent diffusion rates as well as in mechanisms are an important factor limiting the predictive capabilities of the model.

The extent of oxidation of hydrocarbons desorbing from the oil layer has been measured directly in a hydrogen-fueled, spark-ignited engine in which the lubricant oil was doped with a single component hydrocarbon. The amount of hydrocarbon desorbed and oxidized could be measured simultaneously as the dopant was only source of carbon-containing species. The fraction oxidized was strongly dependent on engine load, hydrogen fuel-air ratio and dopant chemical reactivity, but only modestly dependent on spark timing and nitrogen dilution levels below 20 percent. Fast FID measurements at the cylinder exit showed that the surviving hydrocarbons emerge late in the exhaust stroke.

A time resolved study of the unhurned hydrocarbons in the cylinder of a spark ignition engine has been made. A fast acting needle value was used to sample the gas near the cylinder wall opposite the spark plug. The volume sampled was measured by water displacement and the total hydrocarbon mole fraction was measured with a flame ionization detector. Measurements were made as a function of crank angle over the entire engine cycle for a range of equivalence ratios, inlet pressures, spark advances, inlet temperatures, and EGR fractions. Average hydrocarbon concentrations in the exhaust were also measured. Two possible sources of post combustion hydrocarbon in the cylinder were considered: thin wall quench layers and fine crevices into which a flame cannot propagate. The results suggest that crevices were the source of the hydrocarbon. Models for predicting hydrocarbon from both quench layers and crevices were developed and are presented.

The fuel effects on HCCI operation in a spark assisted direct injection gasoline engine are assessed. The low load limit has been extended with a pilot fuel injection during the negative valve overlap (NVO) period. The fuel matrix consists of hydrocarbon fuels and various ethanol blends and a butanol blend, plus fuels with added ignition improvers. The hydrocarbon fuels and the butanol blend do not significantly alter the high or the low limits of operation. The HCCI operation appears to be controlled more by the thermal environment than by the fuel properties. For E85, the engine behavior depends on the extent that the heat release from the pilot injected fuel in the NVO period compensates for the evaporative cooling of the fuel.

A critical evaluation of the current ASTM method of rating diesel fuels, and of the available non-engine techniques for the estimation of cetane nunbers of diesel fuels is presented. The relationship between ignition quality and fuel composition is reviewed and it is shown that each member of an homologous series of hydrocarbons does not have the same ignition characteristics as the other members of the series. It is emphasized that the belief that paraffins have relatively high cetane ratings as compared to aromatics and cycloparaffins is not always correct. The basic flaw in the cetane index correlations, which use the easily measurable physical properties of the fuels as independent parameters, is explained. A fuel data base has been used to compare the different correlations.