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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

Thermal Studies in the Exhaust System of a Diesel-Powered Light-Duty Vehicle

This paper is a continuation of an earlier paper, which examined the steady-state internal heat transfer in the exhaust system of a diesel powered, light-duty vehicle. The present paper deals with the heat transfer of the exhaust system during two types of transient testing, as well as, the estimation of the exhaust systems external heat transfer. Transient heat transfer was evaluated using: a simple fuel-step transient under constant speed and the New European Driving Cycle (NEDC). The thermal response of the external walls varied considerably for the various components of the exhaust system. The largest percent difference between the measured temperatures and the corresponding quasi-steady estimates were about 10%, which is attributed to thermal storage. Allowing for thermal storage resulted in an excellent agreement between measurements and analysis.
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

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

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.