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Technical Paper

Cycle-Averaged Heat Flux Measurements in a Straight-Pipe Extension of the Exhaust Port of an SI Engine

This paper presents an experimental study of the cycle-averaged, local surface heat transfer, from the exhaust gases to a straight pipe extension of the exhaust port of a four-cylinder spark-ignition (SI) engine, over a wide range of engine operating conditions, from 1000 rpm, light load, through 4000 rpm, full load. The local steady-state heat flux was well correlated by a Nusselt-Reynolds number relationship that included entrance effects. These effects were found to be the major contributor to the local heat transfer augmentation. The Convective Augmentation Factor (CAF), which is defined as the ratio of the measured heat flux to the corresponding heat flux for fully-developed turbulent pipe flow, was found to decrease with increasing Reynolds number and increasing axial distance from the entrance of the test section.
Technical Paper

Fuel Economy and Engine-Out Emissions from a Single-Cylinder Two-Valve Direct-Injection S.I. Engine Operating in the Stratified-Combustion Regime

This study is an experimental and computational investigation of the influence of injection timing, fuel spray orientation, and in-cylinder air motion on the combustion, fuel economy, and engine-out emissions of a single-cylinder, 2-valve, spark-ignition direct-injection (SIDI) engine, operating under stratified-charged conditions. For the best compromise between fuel consumption, combustion stability, engine-out hydrocarbon emissions and smoke, the engine required relatively retarded injection timings (in comparison to other charge- or wall-controlled DI engines), high swirl levels, and a spray orientation that is directed towards the intake-valve side and targets the ridge wall of the piston.
Technical Paper

The Influence of Mixture Preparation on the HC Concentration Histories from a S.I. Engine Running Under Steady-State Conditions

Exhaust-port hydrocarbon (HC) concentration measurements were made using a Fast Response Flame Ionization Detector (FRFID) in order to investigate the mechanisms by which mixture preparation affects engine-out HC emissions. The mixture preparation was varied by: (a) using fuels of different volatility, (b) varying the injection timing, and (c) decreasing the coolant temperature. The observed increases in HC emissions which resulted from lowering the coolant temperature or employing open valve injection are primarily attributed to the resulting increase in the in-cylinder liquid fuel, which is deposited mainly on the cylinder walls and in the piston crevices. The HC attributed to the liquid fuel deposited on cylinder walls exit the engine cylinder roughly in the middle of the exhaust process. On the other hand, the HC attributed to the liquid fuel stored in the piston crevices, and which represent the largest fraction, exit the cylinder during the end of the exhaust process.
Technical Paper

Effects of Mixture Preparation on HC Emissions of a S.I. Engine Operating Under Steady-State Cold Conditions

The effects on engine-out HC emissions of a premixed propane system, and three PFI systems employing different types of injectors and using Phase II gasoline were investigated on a four-cylinder DOHC spark-ignition engine. Cold conditions resulted in significant increases in engine-out HC emissions. Phase II gasoline caused much higher emissions of HC than propane fuel. The difference in the HC emissions from the two fuels increased dramatically with lowering the coolant temperature of the engine. At cold conditions, liquid fuel entering the combustion chamber appears to be the primary source of engine out HC emissions. At the coldest temperature tested the estimated percent contribution of in-cylinder liquid fuel to the observed increase of HC emissions was as much as 96%.
Technical Paper

Effects of Piston Crevice Geometry on the Steady-State Engine-Out Hydrocarbons Emissions of a S.I. Engine

This study investigated the effects of piston Crevice geometry on the steady-state engine-out hydrocarbons (HC) from a Saturn DOHC four-cylinder production engine. A 50% reduction in top-land height produced about 20-25% reduction in HC emissions, at part loads. The effect of top-land radial clearance on HC emissions was found to depend on the value of top-land height, which suggests a complex relation between flame propagation in the piston crevice and crevice geometry. For idle, increasing top-land clearance resulted in an increase in HC emissions. This trend is opposite to the trend at part load. A simple model was developed which predicts surprisingly well the contribution of piston crevices to HC emissions. It was estimated that for the test engine, piston crevices contribute about 50% of the engine-out hydrocarbons. Finally exhaust gas recirculation appears to decrease the sensitivity of HC emissions to crevice dimensions.
Technical Paper

The Effects of Fuel Preparation on Hydrocarbon Emissions of a S.I. Engine Operating Under Steady-State Conditions

This study investigated the effects of three fuel delivery systems (port fuel injection with production injectors, port fuel injection with air-assisted injectors and a premixed, prevaporized fuel system) on engine-out hydrocarbon (HC) emissions from a four-cylinder spark ignition engine. Comparative tests were run at three part-load conditions and a wide range of EGR. Other engine parameters examined were intake-air and coolant temperatures, and injection timing. The observed effects of injection timing on HC emissions were related to the intake-flow events, which, in turn, affect in-cylinder fuel evaporation and combustion.
Technical Paper

The Effects of Intake-Flow Configuration on the Heat-Release and Heat-Transfer Characteristics of a Single-Cylinder Four-Valve S.I. Engine

Local transient heat-flux measurements and heat-release analyses were employed to investigate the effects of introducing swirl or tumble fluid motion during the intake stroke on the combustion and heat-transfer characteristics of a single-cylinder spark-ignition engine. In general, swirl or tumble motion decreased the period of flame development and increased the peak rate of heat release, but, surprisingly, it increased the period of combustion. The latter increase was the result of comparatively low rates of fuel burning during the last stages of combustion. Swirl or tumble motion also significantly increased the local heat flux on the cylinder head. The highest peak heat flux was obtained for tumble motion. The observed increase in heat flux is attributed to the resultant increase in the mean velocity and in the turbulent intensity of the gases in the combustion chamber, which, in turn, augment the rate of heat release and the effective convective heat-transfer coefficient.
Technical Paper

Combustion and Heat Transfer Studies in a Spark-Ignited Multivalve Optical Engine

The application of sophisticated analytical techniques for the design of spark-ignition engines has brought about the need for detailed information on the heat transfer processes in these engines. This study utilized time-resolved heat-flux measurements, heat-release analysis and high-speed flame photography to investigate experimentally the combustion and heat-transfer characteristics of an optically accessible single-cylinder engine. The engine has a pent-roof shaped combustion chamber with two intake and two exhaust valves. The primary engine variable examined was the intake-flow configuration which was varied by means of shrouded valves. The measured local heat-flux histories on the combustion side of the head were found to have significant cycle-to-cycle and spatial variations, which are believed to be caused primarily by corresponding variations in combustion.
Technical Paper

Performance and Emissions Achievements with an Uncooled Heavy-Duty, Single-Cylinder Diesel Engine

In this study, the performance and emissions characteristics of an uncooled, thermally insulated diesel that utilized an optimized injector-tip configuration are examined. When the uncooled engine was compared to a conventional water-cooled engine at the same brake power and airflow, the uncooled engine had equal or superior fuel consumption, significantly higher nitric oxide emissions, and significantly lower smoke and particulate emissions. The dramatic reduction in smoke emitted by the uncooled engine was not observed in studies reported earlier. This smoke reduction is attributed to the high gas temperatures and increased rates of air-fuel mixing that augmented the rate of oxidation of the soot particles when the injector tip was optimized for the uncooled engine and airflow was adjusted to match that of the cooled engine. Heat-release analyses showed that the uncooled engine had less premixed combustion and significantly shorter combustion duration than the water-cooled engine.
Technical Paper

Effects of Injector-Tip Configuration on the Performance and Emissions of an Uncooled Diesel

The influence of the number and the size of the fuel-injector orifices and their opening pressure on the performance and emissions of an uncooled, thermally insulated diesel engine was experimentally investigated. Increasing the number of orifices was generally found to decrease the Brake Specific Fuel Consumption (BSFC) and smoke emissions but to increase the nitric-oxide (NO) emissions. Increasing the number of orifices resulted in a slight increase in premixed burning and in a substantial decrease in the duration of combustion. Increasing the orifice size increased the BSFC and smoke emissions but decreased the NO emissions. The heat-release characteristics were not significantly altered, however. Finally, increasing the opening pressure of the injector increased the BSFC and smoke emissions and decreased the NO emissions of the uncooled engine.
Technical Paper

A Two-Stage Heat-Release Model for Diesel Engines

A two-stage heat-release model was developed and applied to both a divided-chamber and an open-chamber diesel engine to determine the fuel burning rates and product temperatures from measured cylinder pressure-time profiles. Measured NO emission levels for several engine operating conditions were used to select the equivalence ratios of the two stages. Combustion in the first stage was taken to occur at a stoichiometric air-fuel ratio, while second-stage combustion was considered to occur at an equivalence ratio below the cylinder-averaged equivalence ratio. An empirical fit for the equivalence ratio of the second stage was determined. Good agreement between the results of this model and the corresponding single-stage model was obtained for heat-release and heat-transfer histories. The computed combustion temperatures for the rich stage were found to be consistently higher (7 to 22% on an absolute scale) than published flame-temperature measurements.
Technical Paper

The Influence of Partial Suppression of Heat Rejection on the Performance and Emissions of a Divided-Chamber Diesel Engine

Suppression of the heat rejection to the coolant was achieved by the use of an air-gap-insulated piston, an antechamber that was partially insulated by an air-gap, and high-temperature coolant (ethylene glycol at 120°C). In comparison to the standard configuration (STD) of the engine, the low-heat-rejection configuration (LHR) resulted in a small increase in brake thermal efficiency for light-load conditions, in a reduction in volumetric efficiency, in an increase in the exhaust energy, and in an increase in the heat rejection to the lubricating oil. Heat-release analysis performed on the two engines showed higher overall fuel burning rates, and consequently shorter combustion durations, in the LHR engine than in the STD engine. This is believed to cause the observed higher nitric oxide emissions. Also, the LHR engine was found to have higher hydrocarbon emissions but slightly lower particulate emissions.
Technical Paper

Evaluation of an Air-Gap-Insulated Piston in a Divided-Chamber Diesel Engine

An air-gap-insulated piston designed for reduced heat loss was evaluated by examining its influence on the coolant heat rejection, engine performance and exhaust emissions of a single-cylinder divided-chamber diesel engine. At 1000 and 1500 r/min engine speed, use of the low-heat-rejection (LHR) piston resulted in a reduction in total coolant heat rejection ranging from 3% at light load to 5-7% at full load, in a general reduction in hydrocarbons, carbon monoxide and smoke emissions, in an increase in oxides of nitrogen, and in a significant improvement in brake specific fuel consumption only at light loads. It was estimated that the LHR piston design reduced the piston-crown surface heat transfer by an amount equivalent to from 3.5% (full load) to 5.5% (light load) of the input fuel energy at 1000 r/min.
Technical Paper

Hydrocarbon Emissions from a Single-Cylinder, Divided-Chamber Diesel Engine

The potential sources of hydrocarbon (HC) emissions from a single-cylinder, divided-chamber diesel engine were investigated in this study. To evaluate the relative importance of these sources, the variations of HC emissions with engine speed, air-fuel ratio, combustion timing, intake-air temperature, coolant temperature and oil temperature were examined. Included in the diagnostics of this investigation were: ignition-delay measurements, combustion-chamber surface temperature measurements and heat-release modeling. Lubricating oil was found to contribute significantly to HC emissions. In addition, the results suggested that bulk quenching of flame and non-flame reactions is a primary source of HC emissions. On the other hand, lean mixing during the ignition delay period and wall interactions appear to be secondary sources.
Technical Paper

The Effects of Coolant Temperature on the Performance and Emissions of a Single-Cylinder Divided-Chamber Diesel Engine

Comparative experiments were performed on an experimental divided-chamber diesel engine for three coolant conditions: baseline (water at 82°C), high coolant temperature (glycol at 120°C) and a differential cooling condition where the antechamber was kept cold (water at 20°C) and the main chamber was kept hot (glycol at 120°C). High-temperature cooling was found to provide a significant brake-specific-fuel-consumption advantage at low-speed and low-load conditions and at very retarded combustion-timing conditions. In general, high coolant temperature caused an increase in hydrocarbon (HC) emissions. Lowering the antechamber surface temperature at the low-speed conditions was found to cause an increase in gaseous emissions and a reduction in smoke and particulate emissions.
Technical Paper

Thermal Loading of the Cylinder Head of a Divided - Chamber Diesel Engine

Time-averaged combustion chamber surface temperatures and surface heat fluxes were measured at three locations (one in the antechamber and two in the main chamber) on the head of a single-cylinder, divided-chamber diesel engine. In general the surface temperature and heat flux were found to increase with increasing engine speed, fuel-air ratio and intake-air temperature, decreasing coolant temperature and advancing combustion timing. At motored conditions the highest heat flux was at the antechamber location. This was caused by the high swirl flows present in the antechamber. In contrast, at all other conditions the highest heat flux was measured at the location in the main chamber near the valves. This was attributed to the convective action of the high-temperature stream of combustion gases exiting the antechamber during the expansion stroke. Lastly, the local surface heat flux measurements were correlated in terms of the air and fuel consumption rates of the engine.
Technical Paper

Gaseous and Particulate Emissions from a Single - Cylinder Divided-Chamber Diesel Engine

In this study, the effects of engine speed, air-fuel ratio, combustion timing, intake-air temperature, and coolant and oil temperature on exhaust gaseous emissions (nitric oxide, carbon monoxide and hydrocarbons) and particulate emissions (particulates, volatiles and smoke) were investigated in a single-cylinder, divided-chamber diesel engine. In addition, the trade-off behavior of the pollutants was investigated. To aid in the interpretation of the experimental findings, a single-chamber, single-zone heat release model utilizing experimental main-chamber pressure-time data was employed. The large increase in nitric oxide emission index caused either by increasing the air-fuel ratio or by advancing the combustion timing is attributed to the proportionally larger amounts of fuel that burn at near TDC conditions.
Technical Paper

An Experimental and Computational Investigation of the Flow in Diesel Prechambers

High-speed schlieren photography was employed to study air-fuel mixing and combustion in several two-dimensional prechambers that represented a 1980 GM-oldsmobile diesel prechamber and modifications thereof. The experiments were performed in a Rapid Compression Machine. A computational study was also undertaken to model these experiments using a two-dimensional computer code. The computational study also considered a Ricardo Comet V swirl chamber, in addition to the above chambers. The computations gave predictions of the full two-dimensional transient flow field during the compression stroke in the absence of injection or combustion. This study showed that the prechamber flow is dominated by the incoming jet. As a result the vortex, which is formed due to the nearly tangential jet entry, initially is confined to part of the chamber and not centered in the chamber.
Technical Paper

Inventory of Heat Losses for a Divided–Chamber Diesel Engine

A series of experiments was performed to characterize the various components of heat losses from a single-cylinder divided-chamber diesel engine. This investigation included studies (a) to determine the contribution of piston friction to the heat rejection to the coolant, (b) to measure the amount of heat rejected through the exhaust port to the coolant and (c) to evaluate the heat losses to the surroundings. The above measurements were used to evaluate the total heat losses to the combustion chamber by the working fluid during the engine cycle. These losses were then compared to the heat losses during the closed portion of the cycle (intake valve closing to exhaust valve opening) that were computed with the aid of pressure-time data.
Technical Paper

The Distribution of Heat Rejection from a Single-Cylinder Divided-Chamber Diesel Engine

The effects of engine speed, load, and injection timing on the distribution of heat rejection to the coolant were examined in a single-cylinder divided-chamber diesel engine. The cooling system was separated into four zones: cylinder liner, intake port, exhaust port, and ante-chamber. The fractions of the total amount of heat rejected to the coolant from the four cooling zones were moderately affected by load and injection timing, but were not affected by engine speed. Typical values of these fractions are: cylinder liner - 53%, exhaust port - 22%, antechamber -18%, and intake port - 7%. The total amount of heat rejected to the coolant increased with engine speed and load; injection timing had a smaller but significant effect. Finally, the heat rejected to each cooling zone was correlated with the rate of fuel consumption.