Abstract
The methods and procedures adopted to reduce emissions by combustion engines frequently limit their performance. It is common sense today that since it is impossible to reduce current energy consumption levels, the most effective way to reduce the negative impact of energy use on the environment is to find ways to make the most effective use of this energy. A very effective way to limit the emission of nitrous gases (NOx) is to interfere directly in the engine combustion process by way of exhaust gas recirculation. This method, however, although proven to reduce NOx levels, leads to a considerable engine performance loss as the presence of inert gases reduces the flame front speed. New techniques and procedures are therefore needed that could take advantage of the proven benefits of exhaust gas recirculation without hindering and if possible actually improving - the engines thermal performance. This goal has been the subject of many recent studies. This paper looks at the effectiveness of increasing compression ratio and turbocharging as a means to restore the power and performance of engines in which exhaust-gas recirculation (EGR) has been performed, and at the effect of these procedures on NOx , CO2 , CO, and unburned hydrocarbon emissions.
This is an experimental study, conducted on a 1,297cc Otto Cycle engine mounted on a dynamometric bench, with volumetric displacement and compression ratio at 8,2:1. The instrumentation proposed makes it possible to gather information on the engines performance, the in-cylinder dynamic pressure curve, and the NOx , CO2 CO and unburned hydrocarbon emission levels. To evaluate the role played by the compression ratio a second cylinder head was used, resulting in a compression ratio of 8,9:1. For turbocharging purposes, the original head was used.
Three test procedures were used in this study. In the first procedure , the gas recirculation was increased while the load, the spark angle, and the fuel flow were kept unchanged. In the second approach, the recirculation was raised while keeping steady the engines speeds, and adjustments were made in the ignition point to match the maximum torque level, without detonation, maintaining a stechiometric fuel-air ratio. The first test prosedure was carried out on the three engine set-ups, while the second approach was only used under normal and turbocharging conditions. And the third procedure, with the turbocharged engine optimized to full load, tests were conducted with 75% and 50% of the full load, varying the recirculation ratio.
The results of this work indicate that the increase of the compression ratio is an effective way to correct the loss in the performance of the engine without jeopardizing the reduction in the emission of NOx resulting from the use of EGR. The increase in the compression ratio from 8,2:1 to 8,9:1, with a recirculation percentage of 6% leads to a 50% reduction in the emission of NOx, while maintaining a 10% gain of power in the engine.
The turbocharged engine presents great sensitivity to the recirculation. Very high recirculation levels may null the effect of the turbocharger. A proper selection of the recirculation percentages, together with an adjustment of the ignition angleand the air-fuel ratio, make it possible a great gain of performance with a substantial reduction in the emissions of NOx. Comparing the results with the ones of the original engine, it was achieved a reduction of 20%, 61% and 52% in the emissions of NOx, HC and CO respectively, an increase in power of 33% and a reduction in the specific consumption of 3% at 3000 rpm and recirculation ratio of 4,2%.
The advantages and disadvantages of the use of EGR in a turbocharged engine, under full load, are the same as under partial loads.