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Multi-dimensional numerical simulation of the combustion process in spark ignition engines were performed using the Coherent Flame Model (CFM) which is based on the flamelet assumption. The CFM uses a balance equation for the flame surface area to simulate flame surface advection, diffusion, production and destruction in a turbulent reacting flow. There are two model constants in CFM, one associated with the modeling of flame surface production and the other with the modeling of flame surface destruction. Previous experimental results on two test engines charged with propane-air mixtures were used to compare with the computations for different engine speeds, loads, equivalence ratios and spark plug locations. Predicted engine cylinder pressure histories agree well with the experimental results for various operating conditions after the model constants were calibrated against a reference operating condition.

The results of continuing investigations of a new type of ignitor, the railplugs are reported. Previous studies have shown that railplugs can produce a high velocity jet of plasma. Additionally, railplugs have the potential of assuring ignition under adverse conditions, such as for very dilute mixtures, because the railplug plasma is both hotter and has a larger mass than the plasma generated by a spark plug. In this paper, engine data are presented to demonstrate the improved dilution tolerance obtainable with railplugs. Data acquired using a railplug are compared to results obtained using a conventional spark plug and a spark plug with a wide spark gap, both using an inductive ignition system. The present results affirm earlier, preliminary findings that railplugs can extend the dilution limit and produce faster combustion.

The results of continuing investigations of a new type of ignitor, the railplug, are reported. Previous studies have shown that railplugs can produce a high velocity jet that is driven both by electromagnetic and thermal forces and that the jet velocity is strongly affected by the railplug geometry and by the electronics characteristics of the follow-on circuit. The present research was intended to provide insights about both: 1) how to match the electronics characteristics to a given geometry and 2) how the geometry affects the jet velocity. It is found that faster current rise times result in higher plasma velocities but current pulses that are too short result in rapid deceleration of the plasma while it is still within the railplug. It is also found that a fundamental geometric parameter is the ratio of the inductance gradient to the volume trapped within the railplug: the larger L′/V, the faster the resulting combustion process.

Researchers in the field of turbulent combustion have found fractal geometry to be a useful tool for describing and quantifying the nature of turbulent flames. This paper describes and compares several techniques for the fractal analysis of two dimensional (2-D) turbulent flame images. Four methods of fractal analysis were evaluated: the Area Method, the Box Method, the Caliper Method, and the Area-Caliper Method. These techniques were first applied to a computer-generated fractal image having a known fractal dimension and known cut-offs. It was found that a “window” effect can cause the outer cut-off to be underestimated. The Caliper Method was found to suffer from noise arising from the statistical nature of the analysis. The Area-Caliper Method was found to be superior to the other methods. The techniques were applied to two types of flame images obtained in a spark ignition engine: Mie scattering from particles seeded in the flow and laser induced fluorescence of OH.

The results of continuing investigations of a new type of ignitor, the railplug, are reported. Previous studies have shown that railplugs can produce a high velocity jet of plasma. Additionally, railplugs have the potential of assuring ignition under adverse conditions, such as cold start of an IDI diesel engine, because the railplug plasma can force ignition in the combustion chamber rather than relying on autoignition under cold start conditions. In this paper, engine data are presented to demonstrate the improved cold starting capability obtainable with railplugs. Data acquired using a railplug are compared to results obtained using no assist and using glow plugs. The engine used for this investigation will not start without glow plugs (or some starting aid) at temperatures below O°C, and the manufacturer's specification of the cold start limit for this engine using glow plugs is -24°C. Railplugs are able to initiate combustion at -29°C in one to two seconds with no preheating.

A new type of ignitor, the railplug, shows promise of extending the dilution limits for spark ignition engines. While much of the effort expended in our study of railplugs has focused upon demonstrating their effectiveness, it is recognized that railplug durability is presently not acceptable for production engine applications. The goal of the present study was to examine the factors that affect durability. The results of two types of investigations are reported. The effects of rail materials, pressure, delivered energy, and voltage at constant delivered energy on electrode erosion rates were studied for repeated firings in air at constant pressure. Railplug durability in a four-stroke SI engine was also evaluated, including examination of the effects of delivered energy, current pulse characteristics, and materials.

We present a multidimensional numerical model that calculates turbulent premixed flame propagation, assuming the flames have fractal geometries. Two scaling transformations, previously developed for laminar flames, are used to incorporate the fractal burning model in KIVA-II1, a numerical hydrodynamics code for chemically reactive flows. In this work the model is implemented for propane/air mixtures. For applications to internal combustion engines, we have also developed a fractal model for early flame kernel growth. Our multidimensional model can be used in experimental comparisons to test postulated fractal parameters, and we begin this task by comparing calculated results with measurements of propane/air combustion in a spark ignition engine. Good agreement is obtained between computed and measured flame positions and pressures in all cases except a low engine speed case.

Lean mixture combustion might be an important feature in the next generation of SI engines, while diluents (internal and external EGR) have already played a key role in the reductions of emissions and fuel consumption. Lean burn modeling is even more important for engine modeling tools which are sometimes used for new engine development. The effect of flame strain on flame speed is believed to be significant, especially under lean mixture conditions. Current quasi-dimensional engine models usually do not include flame strain effects and tend to predict burn rate which is too high under lean burn conditions. An attempt was made to model flame strain effects in quasi-dimensional SI engine models. The Ford model GESIM (stands for General Engine SIMulation) was used as the platform. A new strain rate model was developed with the Lewis number effect included.

In premixed turbulent combustion models, two mechanisms have been used to explain the increase in the flame speed due to the turbulence. The newer explanation considers the full range of turbulence scales which wrinkle the flame front so as to increase the flame front area and, thus, the flame propagation speed. The fractal combustion model is an example of this concept. The older mechanism assumes that turbulence enables the penetration of unburned mixtures across the flame front via entrainment into the burned mixture zone. The entrainment combustion or eddy burning model is an example of this mechanism. The results of experimental studies of combustion regimes and the flame structures in SI engines has confirmed that most combustion takes place at the wrinkled flame front with additional combustion taking place in the form of flame fingers or peninsulas.

A reduced autoignition mechanism for iso-octane has been developed by identifying paths to formation of the stable species measured during motoring knock experiments and eliminating paths to formation of species that were not measured. The resulting mechanism includes low-, intermediate-, and high-temperature reactions and consists of 103 species and 131 reactions. This mechanism differs from detailed models not only in the number of reactions and species, but most importantly, in the nature and rates of the degenerate chain branching reactions. To implement this mechanism, a knock subroutine has been added to a quasidimensional spark ignition engine model that accounts for heat losses, blowby, etc. Thus, errors in the reactivity predictions can be assigned almost exclusively to the kinetics. Numerical predictions of the exhaust composition during motoring knock are compared with experimental measurements as a function of compression ratio for several operating conditions.

Because only the unburned gases in the crevices can contribute to hydrocarbon emissions, a model was developed that can be used to determine the temporal and spatial histories of both burned gas and unburned gas flow into and out of the piston-liner crevices. The burned fraction in the top-land is primarily a function of engine design. Burned gases continue to get packed into the inter-ring volume until well after the end of combustion and the unburned fuel returned to the chamber from this source depends upon both the position of the top ring end gap relative to the spark plug and of the relative positions of the end gaps of the compression rings with respect to each other. Because the rings rotate, and because the fuel that returns to the chamber from the inter-ring crevice dominates the sources between BDC and IVO when conditions are unfavorable to in-cylinder oxidation, these represent two sources of variability in the HC emissions.

Fuel transport was visualized within the cylinder of a port injected four-valve SI engine having a transparent cylinder liner. Measurements were made while motoring at 250 rpm to simulate cranking conditions prior to the first firing cycle, and at 750 rpm to examine the effects of engine speed. A production GM Quad-4 cylinder head was used, and the stock single-jet port fuel injector was used to inject indolene. A digital camera was used to capture back-lighted images of cylinder wall wetting for open and closed intake valve injection. In addition, two-dimensional planar imaging of Mie scattering from the indolene fuel droplets was used to characterize the fuel droplet distribution as a function of crank angle for open and closed intake valve injection. LDV was used to measure the droplet and air velocities near the intake valves during fuel induction. It was found that with open-valve injection a large fraction of the fuel impinged on the cylinder wall opposite the intake valves.

The Rotating Liner Engine (RLE) is an engine design concept where the cylinder liner rotates in order to reduce piston assembly friction and liner/ring wear. The reduction is achieved by the elimination of the mixed and boundary lubrication regimes that occur near TDC. Prior engines for aircraft developed during WW2 with partly rotating liners (Sleeve Valve Engines or SVE) have exhibited reduction of bore wear by factor of 10 for high BMEP operation, which supports the elimination of mixed lubrication near the TDC area via liner rotation. Our prior research on rotating liner engines experimentally proved that the boundary/mixed components near TDC are indeed eliminated, and a high friction reduction was quantified compared to a baseline engine. The added friction required to rotate the liner is hydrodynamic via a modest sliding speed, and is thus much smaller than the mixed and boundary friction that is eliminated.

Cooled exhaust gas recirculation (EGR) is widely used in diesel engines to control engine-out NOx (oxides of nitrogen) emissions. A portion of the exhaust gases is re-circulated into the intake manifold of the engine after cooling it through a heat exchanger. EGR cooler heat exchangers, however, tend to lose efficiency and have increased pressure drop as deposit forms on the heat exchanger surface due to transport of soot particles and condensing species to the cooler walls. In this study, condensation of water vapor and hydrocarbons at the exit of the EGR cooler was visualized using a fiberscope coupled to a camera equipped with a complementary metal oxide semiconductor (CMOS) color sensor. A multi-cylinder diesel engine was used to produce a range of engine-out hydrocarbon concentrations. Both surface and bulk gas condensation were observed with the visualization setup over a range of EGR cooler coolant temperatures.

Diesel engine developers are continually striving to reduce harmful NOx emissions through various calibration and hardware strategies. One strategy being implemented in production Diesel engines involves utilizing cooled exhaust gas recirculation (EGR). Although there is a significant NOx reduction potential by utilizing cooled EGR, there are also several issues associated with it, such as EGR cooler fouling and a reduction in cooler effectiveness that can occur over time. The exact cause of these issues and many others related to cooler fouling are not clearly understood. One such unanswered issue or phenomenon that has been observed in both field tested and lab tested EGR coolers is that of a recovery in EGR cooler effectiveness after a shutdown or after cycling between various conditions.

A new electronic sensor has been developed to measure the time-resolved concentration of carbonaceous particulate matter (PM) emitted in engine exhaust. One application of the sensor could be to provide cycle-resolved feedback on the carbonaceous PM concentration in the exhaust to the engine control unit (ECU), thereby enabling real-time control of engine operating parameters to lower PM emissions. Another promising application is to monitor the performance of particulate traps. The sensor was tested in exhaust flows from a single cylinder diesel engine and from a steady-state acetylene diffusion flame in a flow tunnel. Steady-state engine measurements were made at constant speed and variable load, and transient measurements were performed during engine start-up and accelerations. The sensor response was compared with an opacity meter and with gravimetric filter measurements.

The Rotating Linear Engine (RLE) derives improved fuel efficiency and decreased maintenance costs via a unique lubrication design, which decreases piston assembly friction and the associated wear for heavy-duty natural gas and diesel engines. The piston ring friction exhibited on current engines accounts for 1% of total US energy consumption. The RLE is expected to reduce this friction by 50-70%, an expectation supported by hot motoring and tear-down tests on the UT single cylinder RLE prototype. Current engines have stationary liners where the oil film thins near the ends of the stroke, resulting in metal-to-metal contact. This metal-to-metal contact is the major source of both engine friction and wear, especially at high load. The RLE maintains an oil film between the piston rings and liner throughout the piston stroke due to liner rotation. This assumption has also been confirmed by recent testing of the single cylinder RLE prototype.

Cylinder liner rotation (Rotating Liner Engine, RLE) is a new concept for reducing piston assembly friction in the internal combustion engine. The purpose of the RLE is to reduce or eliminate the occurrence of boundary and mixed lubrication friction in the piston assembly (specifically, the rings and skirt). This paper reports the results of experiments to quantify the potential of the RLE. A 2.3 L GM Quad 4 SI engine was converted to single cylinder operation and modified for cylinder liner rotation. To allow examination of the effects of liner rotational speed, the rotating liner is driven by an electric motor. A torque cell in the motor output shaft is used to measure the torque required to rotate the liner. The hot motoring method was used to compare the friction loss between the baseline engine and the rotating liner engine. Additionally, hot motoring tear-down tests were used to measure the contribution of each engine component to the total friction torque.

Ignition of extremely lean or dilute mixtures is a very challenging problem. Therefore, it is essential for the engine development engineer to understand the fundamentals and limitations of existing ignition systems. This paper presents a new railplug ignition concept, a high-energy ignition system, which can enhance ignition of very lean mixtures by means of its high-energy deposition and high velocity jet of the plasma. This paper presents initial results of tests using an inductive ignition system, a capacitor discharge ignition system, and a railplug high-energy ignition system. Discharge characteristics, such as time-resolved voltage, current, and luminous emission were measured. Spark plug and railplug ignition are compared for their effects on combustion stability of a natural gas engine. The results show that railplugs have a very strong arc-phase that can ensure the ignition of very dilute mixtures.

As natural gas engines are designed to operate leaner and with increased boost pressure, durability of the spark plugs becomes problematic. Among the various new ignition devices that have been considered to solve some of the problems facing spark plugs, railplugs appear to hold clear advantages in some areas. There are two types of railplugs: coaxial rail and parallel rail. This paper reports the results of an experimental study of various parameters that affect the performance of parallel railplugs. Their performance was quantified by the distance that the arc traveled along the rails from the initiation point. Travel along the rails is thought to be an important performance metric because rail-travel limits excessive local wear and produces a distributed ignition source which can potentially reduce mixture inhomogeneity induced ignition problems.