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Technical Paper

Steady-State Local Heat Flux Measurements in a Straight Pipe Extension of an Exhaust Port of a Spark Ignition Engine

Experiments were carried out on a straight pipe extension of an exhaust port of a multi-cylinder, spark-ignition engine to investigate the axial variation of the steady-state surface heat transfer. Local, steady-state, surface heat flux measurements were made at five different stations on the test section. Based on an optimization procedure developed in this study, the heat-flux measurements obtained for axial distances x / D > 2, were found to be correlated very well (R2 = 0.95) by an equation in the form of an entrance length correction, which is a function solely of x / D, multiplied by the Sieder-Tate convective heat transfer correlation; a correlation valid for fully-developed, steady-state, turbulent, pipe flows. Most importantly, this paper provides strong evidence that the observed heat transfer augmentation in the engine exhaust system is due solely to entrance effects and not due to flow fluctuations, which was the accepted cause.
Technical Paper

Characterization of Autoignition in a Knocking SI Engine Using Heat Release Analysis

In this paper, we investigate the effects of autoignition on the heat release characteristics of a spark-ignition (SI) engine, under knocking conditions. In a normal, flame-propagation combustion, the heat release rate increases smoothly to a maximum, and then progressively decreases as the entire mixture is consumed. When autoignition occurs, the heat release rate profile shows a departure from its normal profile: since autoignition results in an explosive combustion, an abnormal rapid increase in heat release rate is generated, with significantly higher peak heat release rates and faster fuel consumption. Three distinct heat-release-rate profiles for autoignition can be identified at different engine speeds, which differ in the phasing of the sudden increase in release rate due to autoignition, relative to the peak release rate due to normal combustion.
Technical Paper

Thermal Studies in the Exhaust Manifold of a Turbocharged V6 Diesel Engine Operating Under Steady-State Conditions

This study employs experimental and computational methods to investigate the thermal state of the exhaust manifold of a multi-cylinder turbocharged diesel engine operating under steady-state conditions. The local skin temperatures and surface heat fluxes varied significantly throughout the external surface of the manifold. The augmentation of the local heat flux with increasing load and engine speed may be represented solely by the increase in the fuel mass flow rate. The results of the 1D simulation are in good agreement with the measurements of the exit gas temperatures, skin temperatures, and surface heat fluxes.
Technical Paper

Relative Contributions of Intake and Exhaust Tuning on SI Engine Breathing - A Computational Study

This study examines the contributions and interactions of intake and exhaust tuning on a 4-stroke single-cylinder engine for various engine speeds and valve timings. The parametric study was performed using a 1-D engine simulation model, the combustion sub-model of which was calibrated based on experimental pressure data. Mechanisms by which tuning changes the volumetric efficiency of an engine were studied. Simulation results are compared with established empirical correlations which predict pipe lengths for maximum volumetric efficiency. It was found that intake tuning has a more dominant role in the breathing capability of the engine compared to exhaust tuning and that both are independent from each other. Valve timing was found to have no effect on intake tuning characteristics but to affect exhaust tuning.
Technical Paper

Investigation of the Effects of Autoignition on the Heat Release Histories of a Knocking SI Engine Using Wiebe Functions

In this paper, we develop a methodology to enable the isolation of the heat release contribution of knocking combustion from flame-propagation combustion. We first address the empirical modeling of individual non-autoigniting combustion history using the Wiebe function, and subsequently apply this methodology to investigate the effect of autoignition on the heat release history of knocking cycles in a spark ignition (SI) engine. We start by re-visiting the Wiebe function, which is widely used to model empirically mass burned histories in SI engines. We propose a method to tune the parameters of the Wiebe function on a cycle-by-cycle basis, i.e., generating a different Wiebe to suitably fit the heat release history of each cycle. Using non-autoigniting cycles, we show that the Wiebe function can reliably simulate the heat release history of an entire cycle, if only data from the first portion of the cycle is used in the tuning process.
Technical Paper

Intake-Valve Temperature and the Factors Affecting It

Steady-state temperature measurements were made at two locations on the back surface of the intake valves of one of the cylinders of a Saturn 1.9-L DOHC engine. The temperature locations were such that in the upstream location the thermocouple is subjected to the impingement of the fuel spray during the injection process, whereas in the downstream location the thermocouple is out of the main fuel spray. The measured intake valve temperature at the upstream location was significantly lower than that at the downstream location, which was attributed to the spray cooling effect. The intake valve temperature was found to increase with increasing load, speed and coolant temperature. As the air-fuel ratio changes the valve temperature exhibits a maximum at near stoichiometric compositions, which is attributed to convective heat transfer from the backflow of combustion gases during the valve-overlap period.
Technical Paper

Contributors to the Fuel Economy Advantage of DISI Engines Over PFI Engines

A methodology was developed, based on engine-simulation analysis and experiments, to evaluate quantitatively the contributions of the various factors on the fuel-economy advantage of direct-injection, spark-ignition (DISI) engines over corresponding port-fuel injection (PFI) engines. The fuel-economy comparison was based on a set of seven, steady-state test points, which simulate a 2400-kg vehicle powered by a 5.3-L V8 engine over the Federal Test Procedure (city cycle). The results show that the DISI engine has a 15% fuel-economy advantage over the corresponding PFI engine operating without EGR. The biggest positive contributor to this gain is the reduced pumping losses, which account for a 10% gain, followed by: favorable mixture properties due to lean/dilute operation with about a 7.5% gain, lower heat losses with a 2% gain, and higher compression ratio with a 3% gain.
Technical Paper

Combustion Characteristics of a Spray-Guided Direct-Injection Stratified-Charge Engine with a High-Squish Piston

This work describes an experimental investigation on the stratified combustion and engine-out emissions characteristics of a single-cylinder, spark-ignition, direct-injection, spray-guided engine employing an outward-opening injector, an optimized high-squish, bowled piston, and a variable swirl valve control. Experiments were performed using two different outward-opening injectors with 80° and 90° spray angles, each having a variable injector pintle-lift control allowing different rates of injection. The fuel consumption of the engine was found to improve with decreasing air-swirl motion, increasing spark-plug length, increasing spark energy, and decreasing effective rate of injection, but to be relatively insensitive to fuel-rail pressure in the range of 10-20 MPa. At optimal injection and ignition timings, no misfires were observed in 30,000 consecutive cycles.
Technical Paper

Intake-Valve Temperature Histories During S.I. Engine Warm-Up

The present study is an experimental investigation on the influence of engine operational parameters on the temperature history of intake valves. During the initial stage of the warm-up process, the temperature history of the intake valve followed an exponential behavior with a time constant that ranged from about 23 to 39 s for the test conditions examined. In contrast, the temperature history of the coolant varied linearly with time suggesting that the net heat input to the coolant is roughly constant during the initial stage of the engine warm-up process. After the initial transient phase that lasted about one minute, the temperature rise of the intake valve was quasi-steady. During this latter period, the measured intake valve temperature was predicted by the steady-state temperature correlation developed in an earlier study.
Technical Paper

Effects of Operational Parameters on Structural Temperatures and Coolant Heat Rejection of a S. I. Engine

This study reports the effects of various engine operational parameters and coolant conditions on the structural temperature distribution, and on the heat rejection to the coolant and to the lubricating oil of a 16-valve 4-cylinder engine. Included are comparisons of the cooling characteristics of the aqueous solution coolant with those of 100% ethylene glycol coolant. Lastly, an empirical equation for the coolant heat rejection of this engine is presented.
Technical Paper

Evaluation of the Potential of a Low-Heat-Rejection Diesel Engine to Meet Future EPA Heavy-Duty Emission Standards

The potential for Low-Heat-Rejection (LHR) engines to meet current and future emission standards is a critical consideration. For the most part, NOx emissions measured both at GMR and elsewhere have been significantly higher than for conventional diesel engines, and many of the control methods for NOx increase fuel consumption. In recent studies at General Motors Research Laboratories (GMR), the LHR engine has shown potential for significant reductions in smoke and particulate emissions. In this study, data acquired from a single-cylinder LHR engine having a 2.0-L displacement and a quiescent combustion system were combined with multicylinder engine mapping data for a 1988 production truck engine to form a simulated multicylinder LHR truck engine with turbo-charging and intercooling. These data were used as input to a simulation model of the EPA heavy-duty transient test schedule to estimate LHR engine emissions and fuel consumption.
Technical Paper

On the Performance and Emissions of an Uncooled Heavy-Duty Single-Cylinder Diesel Engine

In this study, which is a continuation of an earlier one, the performance and emissions of an uncooled 2.0-L single-cylinder open-chamber diesel engine were experimentally investigated further. In addition to the routine performance and emissions measurements, the heat-release characteristics, friction losses, and temperatures of combustion-chamber components were measured. The performance of the uncooled engine, which was significantly better than that in the earlier study, was as good as the performance of the water-cooled engine with the exception of high-load/low-speed conditions. For a given fuel-input energy, the uncooled engine had lower friction losses and lower pumping losses but also generally had lower thermal efficiency. The uncooled engine appeared to have less premixed combustion and substantially shorter heat-release duration than the water-cooled engine.
Technical Paper

Impact of Engine Design on Vehicle Heating System Performance

A global thermal model of a vehicle powertrain is used to quantify how different engine design and powertrain calibration strategies influence the performance of a vehicle heating system. Each strategy is evaluated on its ability to improve the warm-up and heat rejection characteristics of a small-displacement, spark-ignition engine while minimizing any adverse effect on fuel consumption or emissions. An energy audit analysis shows that the two strategies having the greatest impact on heating system performance are advancing the spark and forcing the transmission to operate in a lower gear. Changes in head mass, exhaust port diameter, and coolant flow rate influence the coolant warm-up rate but have relatively little effect on steady state heat transfer at the heater core.