Atkinson Cycle and Very High-Pressure Turbocharging: Increasing Internal Combustion Engine Efficiency and Power While Reducing Emissions

Topics: Internal combustion engine, Compression ratio, Gasoline Pages: 8 (2828 words) Published: September 10, 2012
ATKINSON CYCLE AND VERY HIGH-PRESSURE TURBOCHARGING: INCREASING INTERNAL COMBUSTION ENGINE EFFICIENCY AND POWER WHILE REDUCING EMISSIONS Gheorghiu, Victor* Hamburg University of Applied Sciences Berliner Tor 21, 20099 Hamburg, Germany Keywords: Atkinson Cycle, Ultra-Downsizing, High Pressure Turbocharging, CO2 and NOx Reduction.

ABSTRACT The Downsizing of internal combustion engines (ICE) is already recognized as a very suitable method for the concurrent enhancement of the indicated fuel conversion efficiency (IFCE) and lowering of the CO2 and NOx emissions [1], [2]. In this report the Ultra-Downsizing is introduced as a still higher development stage of ICE. The Ultra-Downsizing will be implemented here by means of strict Atkinson cycles, using asymmetrical crank mechanisms, combined with a very intensive multistage high-pressure turbocharging with intensive intercooling. This will allow an increase of ICE performance while keeping the thermal and mechanical strain strength of engine components within the current usual limits. INTRODUCTION The scarcity of the available oil and gas reserves and the global warming phenomenon urge together the automotive industry toward a decrease in fuel consumption and thus a reduction of CO2 emissions. These factors will also determine the future R&D trends for ICE. Downsizing of ICE means the simultaneous decreasing of the displaced volume (usually by reducing the number of cylinders) and increasing of the indicated mean pressure (IMEP) by means of turbocharging [1], [2]. This allows the preservation of power and torque performance while decreasing the engine size. Thereby a) the mechanical and thermal losses are reduced, b) the engine becomes lighter, leading to a drop in the overall weight of the vehicle, and c) the engine operates more time within its optimum fuel consumption zone. The advantages offered by a) and b) hold true even for ICE used in hybrid propulsion systems, while the advantage c) is already a feature of the full-hybrid vehicles. The level of downsizing determines the strength of the thermal and mechanical strains of the engine components. In order to avoid exceeding the usual limits, either the boost pressure or the volumetric compression ratio (VCR) must be reduced accordingly. As consequence, the whole potential of downsizing are not achieved and the IFCE and IMEP remain at low level. The current ICEs have classical (symmetrical) crank mechanisms (i.e. with compression and expansion strokes of equal length) and follow the Seiliger cycles. Real implemented Atkinson cycles require unequal strokes featuring a shorter compression stroke, which leads to a higher IFCE [3]. Atkinson cycles have been used so far mostly with symmetrical crank mechanisms, where the intake valves are closed very late in the cycle [4]. Thus, a part of the charge sucked into cylinder is push back to the intake pipes and the effective compression stroke is decreased. This quasi implementation of Atkinson cycles shows no noticeable improvements of the IFCE and, hence, it will not be discussed in the course of this paper [3].

Real Atkinson cycles can be implemented only with the help of asymmetrical crank mechanisms. This allows to use concurrent very high boost pressures (to increase the IMEP) and higher VCR (to enhance the IFCE) and to set them much more independently of each other compared to Seiliger cycles [3]. Because an important part of the fresh charge compression takes place beyond the cylinder, the high compressed fresh charge can be cooled intensively before it is sucked in cylinder. The following moderate compression in the cylinder (i.e. with relative lower VCR) lead to lower temperature peaks during the combustion process and, consequently, to less NOx emissions. This approach has already been proved in several previous theoretically investigations based on ideal Seiliger and Atkinson cycles [3]. These investigations did not take into consideration the effect of heat exchange...
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