Search Results

A comprehensive methodology for predicting the transient thermal response of spark-ignition engines subject to time-varying boundary conditions is presented. The approach is based on coupling a cycle-resolved quasi-dimensional simulation of in-cylinder thermodynamic events with a resistor-capacitor (R-C) thermal network of the various component and fluid interactions throughout the engine and exhaust system. The dynamic time step of the thermal solution is limited by either the frequency of the prescribed time-dependent boundary conditions or by the minimum thermal time constant of the R-C network. To demonstrate the need for fully-coupled, transient thermodynamic and heat transfer solutions, model behavior is first explored for step-change and staircase variations of engine operating conditions.

A computer simulation of the Homogenous Charge Compression Ignition (HCCI) four-stroke engine has been developed for combustion and performance studies. The simulation couples models for mass, species, and energy within a zero-dimensional framework. The combustion process is described via a user-defined chemical kinetic mechanism. The CHEMKIN libraries have been used to formulate a stiff chemical kinetic solver suitable for integration within a complete engine cycle simulation, featuring models of gas exchange, turbulence and wall heat transfer. For illustration, two chemical kinetics schemes describing hydrogen and natural gas chemistry have been implemented in the code. The hydrogen scheme is a reduced one, consisting of 11 species and 23 reactions. The natural gas chemistry is described via the GRI-mechanism 3.0 that considers 53 species and 325 reactions, including NOx chemistry.

A global, systems-level model which characterizes the thermal behavior of internal combustion engines is described in this paper. Based on resistor-capacitor thermal networks, either steady-state or transient thermal simulations can be performed. A two-zone, quasi-dimensional spark-ignition engine simulation is used to determine in-cylinder gas temperature and convection coefficients. Engine heat fluxes and component temperatures can subsequently be predicted from specification of general engine dimensions, materials, and operating conditions. Emphasis has been placed on minimizing the number of model inputs and keeping them as simple as possible to make the model practical and useful as an early design tool. The success of the global model depends on properly scaling the general engine inputs to accurately model engine heat flow paths across families of engine designs. The development and validation of suitable, scalable submodels is described in detail in this paper.

This study presents a systematic methodology for performing transient heat release analysis in a diesel engine. Novel techniques have been developed to infer the mass of air trapped in the cylinder and the mass of fuel injected on a cycle-by-cycle basis. The cyclic mass of air trapped in the cylinder is found accounting for pressure gradients, piston motion and short-circuiting during the valve overlap period. The cyclic mass of fuel injected is computed from the injection pressure history. These parameters are used in conjunction with cycle-resolved pressure data to accurately define the instantaneous thermodynamic state of the mixture. This information is used in the calculation and interpretation of transient heat release profiles.

A prototype chromel-alumel overlapping thin-film thermocouple (TFTC) has been developed for transient heat transfer measurements in ceramic-coated combustion chambers. The TFTC has been evaluated using various metallurgical techniques such as scanning electron microscopy, energy dispersive x-ray detection, and Auger electron spectroscopy. The sensor was calibrated against a standard thermocouple in ice, boiling water, and a furnace at 1000°C. The microstructural and chemical analysis of the thin-films showed the alumel film composition was very similar to the bulk material, while the chromel film varied slightly. An initial set of ceramic plug surface temperatures was taken while motoring and firing the engine at 1900 rpm to verify thermocouple operation. The data shows a 613 K mean temperature and a 55 K swing for the ceramic surface compared with a 493 K mean temperature and a 20 K swing for the metal surface at the same location.

A telemetry linkage system has been developed for piston temperature measurements in a direct-injection diesel engine. In parallel with the development of the telemetry linkage system, fast response thermocouples were installed at three piston locations - two on the bowl surface and one on the crown surface. A novel design was used to achieve electrical continuity between the piston and the connecting rod by means of a flexible steel strap pivoted on the piston skirt. The telemetry linkage system was then used to transport the electrical wires from the thermocouples to the external data acquisition system. A series of tests was run to determine the effects of location and load on piston surface temperatures. Surface temperature profiles varied substantially among the three locations, reflecting the differences in the combustion and heat flow characteristics of their surrounding regions.

In this paper, the available correlations proposed in the literature for the gas-side heat transfer in the intake and exhaust system of a spark-ignition internal combustion engine were surveyed. It was noticed that these only by empirically fitted constants. This similarity provided the impetus for the authors to explore if a universal correlation could be developed. Based on a scaling approach using microscales of turbulence, the authors have fixed the exponential factor on the Reynolds number and thus reduced the number of adjustable coefficients to just one; the latter can be determined from a least squares curve-fit of available experimental data. Using intake and exhaust side data, it was shown that the universal correlation The correlation coefficient of this proposed heat transfer model with all available experimental data is 0.845 for the intake side and 0.800 for the exhaust side.

Exhaust gas recirculation (EGR) coolers are commonly used in diesel engines to reduce the temperature of recirculated exhaust gases in order to reduce NOx emissions. The presence of a cool surface in the hot exhaust causes particulate soot deposition as well as hydrocarbon and water condensation. Fouling experienced through deposition of particulate matter and hydrocarbons results in degraded cooler effectiveness and increased pressure drop. In this study, a visualization test setup is designed and constructed so that the effect of water condensation on the deposit formation and growth at various coolant temperatures can be studied. A water-cooled surrogate rectangular channel is employed to represent the EGR cooler. One side of the channel is made of glass for visualization purposes. A medium duty diesel engine is used to generate the exhaust stream.

Since the early inception of the adiabatic diesel engine in 1974, marked progress has taken place as a result of research efforts performed all over the world. The use of ceramics for heat engines in production applications has been limited to date, but is growing. Ceramic use for production heat engine has included: combustion prechambers, turbochargers, exhaust port liners, top piston ring inserts, glow plugs, oxygen sensors; and additional high temperature friction and wear components. The potential advantages of an adiabatic engine vary greatly with specific application (i.e., commercial vs. military, stationary vs. vehicular, etc.), and thus, a better understanding of the strengths and weaknesses (and associated risks) of advanced adiabatic concepts with respect to materials, tribology, cost, and payoff must be obtained.

The high temperature tribology issue for uncooled Low Heat Rejection (LHR) diesel engines where the cylinder liner piston ring interface exceeds temperatures of 225°C to 250°C has existed for decades. It is a problem that has persistently prohibited advances in non-watercooled LHR engine development. Though the problem is not specific to non-watercooled LHR diesel engines, it is the topic of this research study for the past two and one half years. In the late 1970s and throughout the 1980s, a tremendous amount of research had been placed upon the development of the LHR diesel engine. LHR engine finite element design and cycle simulation models had been generated. Many of these projected the cylinder liner piston ring top ring reversal (TRR) temperature to exceed 540°C[1]. In order for the LHR diesel to succeed, a tribological solution for these high TRR temperatures had to be developed.

The present study introduces a modeling approach for investigating the effects of valve events and gas exchange processes in the framework of a full-cycle HCCI engine simulation. A multi-dimensional fluid mechanics code, KIVA-3V, is used to simulate exhaust, intake and compression up to a transition point, before which chemical reactions become important. The results are then used to initialize the zones of a multi-zone, thermo-kinetic code, which computes the combustion event and part of the expansion. After the description and the validation of the model against experimental data, the application of the method is illustrated in the context of variable valve actuation. It has been shown that early exhaust valve closing, accompanied by late intake valve opening, has the potential to provide effective control of HCCI combustion.

Stringent fuel economy and emissions regulations have driven development of new mixture preparation technologies and increased spark-ignition engine complexity. Additional degrees of freedom, brought about by devices such as cam phasers and charge motion control valves, enable greater range and flexibility in engine control. This permits significant gains in fuel efficiency and emission control, but creates challenges related to proper engine control and calibration techniques. Accurate experimental characterization of high degree of freedom engines is essential for addressing the controls challenge. In particular, this paper focuses on the evaluation of three experimental residual gas fraction measurement techniques for use in a spark ignition engine equipped with dual-independent variable camshaft phasing (VVT).

This paper describes the measurement of in-cylinder engine friction using the instantaneous IMEP method. This method has been applied to measure in-cylinder friction force in a modern, low friction design production spark ignited engine. An improved mechanical telemetry system has been developed to implement this method. The telemetry system continues to provide excellent data even after 50+ hours of operation at speeds as high as 2000 rpm. Investigated in this study were the primary sources of error associated with this technique. Also presented are the steps taken to minimize the effects of these errors. The refined technique has been subsequently used to obtain piston assembly friction data for both motoring and a limited number of firing cases. The effects of design parameters and operating conditions were investigated.

This paper investigates the effects of manufacturing variations in fuel injectors on the engine performance with emphasis on emissions. The variations are taken into consideration within a Reliability-Based Design Optimization (RBDO) framework. A reduced version of Multi-Zone Diesel engine Simulation (MZDS), MZDS-lite, is used to enable the optimization study. The numerical noise of MZDS-lite prohibits the use of gradient-based optimization methods. Therefore, surrogate models are developed to filter out the noise and to reduce computational cost. Three multi-objective optimization problems are formulated, solved and compared: deterministic optimization using MZDS-lite, deterministic optimization using surrogate models and RBDO using surrogate models. The obtained results confirm that manufacturing variation effects must be taken into account in the early product development stages.

The two newest internal combustion engine technologies which have demonstrated the most promise in the last 25 years are the direct-injection stratified-charge (DISC) rotary engine [1] and the adiabatic diesel engine [2]. The (DISC) engine is particularly attractive for aviation applications [3] because of its light weight, multi-fuel capability and potential for low fuel consumption. However, one disadvantage with the DISC engine is the weight and size of the liquid cooling system. NASA Lewis Research Center has supported a technology enablement program to advance the DISC rotary engine for general aviation applications and recognizes the importance of improvement in fuel consumption and reductions in the coolant system weight [4].

Exhaust gas rebreathing is considered to be a practical enabler that could be used in HCCI production engines. Recent experimental work at the University of Michigan demonstrates that the combustion characteristics of an HCCI engine using large amounts of hot residual gas by rebreathing are very sensitive to engine thermal conditions. This computational study addresses HCCI engine operation with rebreathing, with emphasis on the effects of engine thermal conditions during transient periods. A 1-D cycle simulation with thermal networks is carried out under load and speed transitions. A knock integral auto-ignition model, a modified Woschni heat transfer model for HCCI engines and empirical correlations to define burn rate and combustion efficiency are incorporated into the engine cycle simulation model. The simulation results show very different engine behavior during the thermal transient periods compared with steady state.

We have developed a methodology for analysis of Premixed Charge Compression Ignition (PCCI) engines that applies to conditions in which there is some stratification in the air-fuel distribution inside the cylinder at the time of combustion. The analysis methodology consists of two stages: first, a fluid mechanics code is used to determine temperature and equivalence ratio distributions as a function of crank angle, assuming motored conditions. The distribution information is then used for grouping the mass in the cylinder into a two-dimensional (temperature-equivalence ratio) array of zones. The zone information is then handed on to a detailed chemical kinetics model that calculates combustion, emissions and engine efficiency information. The methodology applies to situations where chemistry and fluid mechanics are weakly linked.

Five small two-stroke engine designs were tested at different air/fuel ratios, under steady state and transient cycles. The effects of combustion chamber design, carburetor design, lean burning, and fuel composition on performance, hydrocarbon and carbon monoxide emissions were studied. All tested engines had been designed to run richer than stoichiometric in order to obtain satisfactory cooling and higher power. While hydrocarbon and carbon monoxide emissions could be greatly reduced with lean burning, engine durability would be worsened. However, it was shown that the use of a catalytic converter with acceptably lean combustion was an effective method of reducing emissions. Replacing carburetion with in-cylinder fuel injection in one of the engines resulted in a significant reduction of hydrocarbon and carbon monoxide emissions.

This paper investigates theoretically the effects of heat transfer characteristics, such as crank-angle phasing and wall temperature swings, on the thermodynamic efficiency of an IC engine. The objective is to illustrate the fundamental physical basis of applying thin thermal barrier coatings to improve the performance of military and commercial IC engines. A simple model illustrates how the thermal impedance and thickness of coatings can be manipulated to control heat transfer and limit the high temperatures in engine components. A friction model is also included to estimate the overall improvement in engine efficiency by the proper selection of coating thickness and material.

Homogeneous charge compression ignition (HCCI) has received much attention in recent years due to its ability to reduce both fuel consumption and NO emissions compared to normal spark-ignited (SI) combustion. However, due to the limited operating range of HCCI, production feasible engines will need to employ a combination of combustion strategies, such as stoichiometric SI combustion at high loads and leaner burn spark-assisted compression ignition (SACI) and HCCI at intermediate and low loads. The goal of this study was to extend the high load limit of HCCI into the SACI region while maintaining a stoichiometric equivalence ratio. Experiments were conducted on a single-cylinder research engine with fully flexible valve actuation. In-cylinder pressure rise rates and combustion stability were controlled using cooled external EGR, spark assist, and negative valve overlap. Several engine loads within the SACI regime were investigated.