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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.

The loss mechanisms associated with engine downsizing, boosting and compression ratio change are assessed. Of interest are the extents of friction loss, pumping loss, and crevice loss. The latter does not scale proportionally with engine size. These losses are deconstructed via a cycle simulation model which encompasses a friction model and a crevice loss model for engine displacement of 300 to 500 cc per cylinder. Boost pressure is adjusted to yield constant torque. The compression ratio is varied from 8 to 20. Under part load, moderate speed condition (1600 rpm; 13.4 Nm/cylinder brake torque), the pumping work reduces significantly with downsizing while the work loss associated with the crevice volume increases. At full load (1600 rpm; 43.6 Nm/cylinder brake torque), the pumping work is less significant. The crevice loss (normalized to the fuel energy) is essentially the same as in the part load case. The sensitivities of the respective loss terms to downsizing are reported.

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.

The first 3 cycles in the cold crank-start process at 20°C are studied in a GDI engine. The focus is on the dependence of the HC and PM/PN emissions of each cycle on the injection strategy and combustion phasing of the current and previous cycles. The PM/PN emissions per cycle decrease by more than an order of magnitude as the crank-start progresses from the 1st to the 3rd cycle, while the HC emissions stay relatively constant. The wall heat transfer, as controlled by the combustion phasing, during the previous cycles has a more significant influence on the mixture formation process for the current cycle than the amount of residual fuel. The results show that the rise in HC emissions caused by the injection spray interacting with the intake valves and piston crown is reduced as the cranking process progresses. Combustion phasing retard significantly reduces the PM emission. The HC emissions, however, are relatively not sensitive to combustion phasing in the range of interest.

This paper describes the development of a spark ignition engine operating in a stratified-EGR mode at part load. The concept is to reduce the pumping loss with high levels of EGR while maintaining stable combustion via charge stratification. Since the engine operates stoichiometrically, the ability to control NOx emissions by the three-way catalyst is retained. The configuration of introducing the stoichiometric fresh mixture to the center portion of the combustion chamber with the EGR gas on the two sides is visualized in a transparent engine using planar laser-induced fluorescence (PLIF) and Mie scattering. Visualization results showed that the stratification between air/fuel mixture and EGR gas was relatively well established during the intake stroke. There was, however, significant mixing in the late part of the compression stroke.

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.

A strategy to facilitate the mixture preparation process in PFI engines is to delay the Intake Valve Opening (IVO) by shifting the cam phasing so that the cylinder pressure is sub-atmospheric when the valve opens. The physics of the effect are discussed in terms of the pressure differential between the manifold and the cylinder, and the resulting flow and charge temperature history. The effect was evaluated by measuring the equivalence ratio of the trapped charge and the exhaust HC emissions in the first cycle of cranking in a 2.4L engine. When the IVO timing was changed from 18° BTDC to 21° ATDC, the in-cylinder fuel equivalence ratio increased by approximately 10%. This increase was attributed mainly to the enrichment of the charge by displacing the leaner mixture at the top of the cylinder in the period between BDC and IVC. The exhaust HC, however, increased by 40%. No conclusive explanation was established for this increase in HC emissions.

The interaction of intake port gas flow with the fuel spray in a port-fuel-injection engine is studied to see whether there are opportunities to facilitate the mixture preparation process and to improve the HC emissions through this interaction. The operating regime of interest is the fast idle period in a cold start. For single pulse injection, the HC emissions were not sensitive to injection details for closed-valve injection; emissions increased with open-valve injection. Then a split injection strategy was used in which the fuel was divided into two pulses. The first pulse was delivered during valve-closed; the second pulse was injected in the back flow period. Under cold-valve conditions, a small benefit (compared to close valve injection) was obtained with a second pulse fuel of 25%: 6% decrease in Specific HC emissions and 4.5% increase in the fuel delivery fraction.

The effects of highly heated fuel on diesel combustion were studied experimentally in a rapid compression machine. A pure fuel, dodecane, heated up to and beyond its critical temperature, was injected into a diesel combustion chamber with the air charge at a compression ratio of 18.2 to 1. The ignition delay was found to decrease with the increase of fuel temperature. The delay decreased to almost zero (within the limit of the accuracy of the instrumentation) at fuel temperatures above 600K. This decrease of delay was explained in terms of a thermal ignition model. For the short ignition delay combustions, the premixed burning phase could not be detected from the heat release data. The mixing controlled burning phases of the heated and unheated fuels however, were not much different; in particular, there was no rapid mixing phenomenon when the fuel temperature was above critical.

A method for determining the flame contour based on the flame arrival time at the fiber optic (FO) spark plug and at the head gasket ionization probe (IP) locations has been developed. The experimental data were generated in a single-cylinder Ricardo Hydra spark-ignition engine. The head gasket IP, constructed from a double-sided copper-clad circuit board, detects the flame arrival time at eight equally spaced locations at the top of the cylinder liner. Three other IP's were also installed in the cylinder head to provide additional intermediate data on flame location and arrival time. The FO spark plug consists of a standard spark plug with eight symmetrically spaced optical fibers located in the ground casing of the plug. The cylinder pressure was recorded simultaneously with the eleven IP signals and the eight optical signals using a high-speed PC-based data acquisition system.

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.

The heat transfer characteristics of impinging diesel sprays were studied in a Rapid Compression Machine. The temporal and spatial distributions of the heat transfer around the impingement point -were measured by an array of high frequency response surface thermocouples. Simultaneously, the flow field of the combusting spray was photographed with high speed movie through the transparent head of the apparatus. The results for the auto-ignited fuel sprays were compared to those of non-combusting sprays which were carried out in nitrogen. The values of the heat flux from the combusting sprays were found to be substantially different from those of the non-combusting sprays. The difference was attribute to the radiative heat transfer and the combustion generated bulk, motion and small scale turbulence.

The residual gas fraction prior to ignition at the vicinity of the spark plug in a single cylinder, two-valve spark ignition engine was measured with a fast-response flame ionization hydrocarbon detector. The technique in using such an instrument is reported. The measurements were made as a function of the intake manifold pressure, engine speed and intake/exhaust valve-overlap duration. Both the mean level of the residual fraction and the statistics of the cycle-to-cycle variations were obtained.

The effects of the directed port flow produced by a Charge-Motion-Control-Valve (CMCV) on mixture preparation in a Port-Fuel-Injection engine were assessed under conditions typical of fast idle in a cold start process. The port fuel was found to comprise two components: a “valve” puddle (at the vicinity of the valve) that built up quickly, and that was mainly responsible for the delivery of the fuel to the cylinder charge; a “port” puddle located significantly upstream. The latter was mainly created by the reverse back flow process and built up slowly. Although the fuel amounts in these two components were roughly the same, the latter did not significantly interact with the fuel transport to the cylinder charge. The CMCV only weakly affected the purging or filling time of the valve puddle, hence the dynamics of the fuel delivery process was not materially affected.

The flow and heat transfer phenomena in the intake port of a spark ignition engine with port fuel injection play a significant role in the mixture preparation process, especially at part load. The backflow of the hot burned gas from the cylinder into the intake port when the intake valve is opened breaks up any liquid film around the inlet valve, influences gas and wall temperatures, and has a major effect on the fuel vaporization process. The backflow of in-cylinder mixture with its residual component during the compression stroke prior to inlet valve closing fills part of the port with gas at higher than fresh mixture temperature. To quantify these phenomena, time-resolved measurements of the hydrocarbon concentration profile along the center-line of the intake port were made with a fast-response flame ionization detector, and of the gas temperature with a fine wire resistance thermometer, in a single-cylinder engine running with premixed propane/air mixture.

The liquid fuel entry into the cylinder and its subsequent behavior through the combustion cycle were observed by a high speed CCD camera in a transparent engine. The videos were taken with the engine firing under cold conditions in a simulated start-up process, at 1,000 RPM and intake manifold pressure of 0.5 bar. The variables examined were the injector geometry, injector type (normal and air-assisted), injection timing (open- and closed-valve injection), and injected air-to-fuel ratios. The visualization results show several important and unexpected features of the in-cylinder fuel behavior: 1) strip-atomization of the fuel film by the intake flow; 2) squeezing of fuel film between the intake valve and valve seat at valve closing to form large droplets; 3)deposition of liquid fuel as films distributed on the intake valve and head region. Some of the liquid fuel survives combustion into the next cycle.

Gasoline HCCI engine has the potential of providing better fuel economy and emissions characteristics than the current SI engines. However, management of HCCI operation for a vehicle is a challenging task. In this paper, the issues of mode transitions between the Spark Ignition and HCCI regimes, and the dynamic nature of the load trajectory within the HCCI regime are considered. Then the phenomena encountered in these operations are illustrated by the data from a single-cylinder engine with electromagnetic-variable-valve timing control. Mode transitions from the SI to HCCI regime may be categorized as robust and non-robust. In a robust transition, every intended HCCI cycle is successful. In a non-robust transition, one or more intended HCCI cycles misfire, although the cycles progress to a satisfactory HCCI operating point in steady state. (The spark ignition was kept on so that the engine could recover from a misfired cycle.)

The mixture preparation and hydrocarbon (HC) emissions behaviors for a single-cylinder port-fuel-injection SI engine were examined in an engine/dynamometer set up that simulated the first cycle of cranking. The engine was motored continuously at a fixed low speed with the ignition on, and fuel was injected every 8 cycles. Unlike the real engine cranking process, the set up provided a well controlled and repeatable environment to study the cranking process. The parameters were the Engine Coolant Temperature (ECT), speed, and the fuel injection pulse width. The in-cylinder and exhaust HC were measured simultaneously with two Fast-response Flame Ionization Detectors. A large amount of injected fuel (an order of magnitude larger than the normal amount that would produce a stoichiometric mixture in a warm-up engine) was required to form a combustible mixture at low temperatures.

Knock in a HCCI engine was examined by comparing subjective evaluation, recorded sound radiation from the engine, and cylinder pressure. Because HCCI combustion involved simultaneous heat release in a spatially large region, substantial oscillations were often found in the pressure signal. The time development of the audible signal within a knock cycle was different from that of the pressure trace. Thus the audible signal was not the attenuated transmission of the cylinder pressure oscillation but the sound radiation from the engine structure vibration excited by the initial few cycles of pressure oscillation. A practical knock limited maximum load point for the specific 2.3 L I4 engine under test (and arguably for engines of similar size and geometry) was defined at when the maximum rate of cycle-averaged pressure rise reached 5 MPa/ms.