Search Results

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.

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.

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.

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.

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.

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.

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.

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.

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.