Abstract
Keywords: fuel economy, load control, variable valve lift and timing
A fundamental feature of the automobiles engines is the great variety of their working regimes. Amongst these, an important weight (80 - 90% of the vehicle´s operational life) corresponds to those of low part loads, specific to the urban traffic, thus having an overwhelming contribution to the engine´s fuel consumption. Although, the maximum thermal efficiency of the actual spark ignition engine is of about 35 %, in case of the vehicle´s urban use, it doesn´t exceed 10 - 15%; consequently, it results for the fuel consumption and toxic emissions large values. Therefore, in order to determine a significant improvement of the thermal efficiency, nowadays there are looked for the constructive solutions that act especially in the area of these working regimes.
Nowadays, European carmakers are facing the challenge of meeting their voluntary targets to increase fuel efficiency of the new cars for reasons of global warming and energy conservation. In the late 1990s, automotive manufacturers agreed to reduce carbon dioxide emissions of the new cars sold in Europe. They set a target of 140 g CO2/km by 2008 and another of 120 g/km by 2012. The first objective yields to limiting fuel consumption to 6.0 litres per 100 km for gasoline-driven vehicles, and to 5.3 litres per 100 km for Diesel vehicles. At the moment, the average CO2 emission is around 160 g/km and this has mainly been achieved by the rapid increase in diesel sales in the recent years. However, it has to be said, diesel is not the only solution.
Historically, engine selection for light vehicles is about choosing either the spark ignition engine, with low initial cost, high performance and poor fuel economy or the compression ignition engine, with its high initial cost, low performance and good fuel economy. Of the two technologies, spark ignition engine is however the most likely to attract investment over the next decade.
In the case of spark ignition engine, the quantitative load control by throttling the inlet pipe causes not only the increasing of the pumping work but also the decreasing of the real compression ratio. These drawbacks, caused by throttling load control method, are more important as the load level lowers and in the case of the bigger engines, they are even more amplified. Thus, it can be said that one of the most promising methods of increasing spark ignition engine´s fuel economy is the unthrottled load control. Nowadays, there are two methods intensively investigated in order to obtain the unthrottled operation: gasoline direct injection (GDI) featuring load control by means of lean burn possibilities (Yamaguchi, 1995) and variable valve actuation (VVA). Both techniques promise a significant improvement of fuel economy. However, only VVA still allows the use of the less expensive conventional exhaust gas after treatment, whereas lean GDI needs the more expensive extra treatment of nitrogen oxide (deNOx).
Recently, there has been much interest in the throttle-less operation by means of VVA, well epitomized by the BMW´s Valvetronic-Vanos system (Moss, 2003). The potential of such a technology has to be evaluated by a cost and consumption benefit tradeoff and all these, of course, without affecting the engine´s power level or reliability. Thus, as a response to these emerging problems, the authors are to be presented hereafter a VVA solution, used for low part loads efficiency improvement, which is equally able to run unthrottled.
This work is being developed thanks to the financial support of the Romanian Council for Scientific Research in the Higher Education (CNCSIS) and the French Agency for the Environment (ADEME).