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20The current demand for fuel efficient and lightweight powertrains, particularly for application 21 in downsized and hybrid electric vehicles, has renewed the interest in two-stroke engines. In this 22 framework, an overhead four-valve spark-ignition gasoline engine was modified to run in the two-23 stroke cycle. The scavenging process took place during a long valve overlap period around bottom 24 dead centre at each crankshaft revolution. Boosted intake air was externally supplied at a constant 25 pressure and gasoline was directly injected into the cylinder after valve closure. Intake and 26 exhaust valve timings and lifts were independently varied through an electrohydraulic valve train, 27 so their effects on engine performance and gas exchanging were investigated at 800 rpm and 28 2000 rpm. Different exhaust backpressures were also evaluated by means of exhaust throttling. Air 29 trapping efficiency, charging efficiency and scavenge ratio were calculated based on air and fuel 30 flow rates, and exhaust oxygen concentration at fuel rich conditions. The results indicated that 31 longer intake and exhaust valve opening durations increased the charge purity and hence torque 32 at higher engine speeds. At lower speeds, although, shorter valve opening durations increased air 33 trapping efficiency and reduced the estimated supercharger power consumption due to lower air 34 short-circuiting. A strong correlation was found between torque and charging efficiency, while air 35 trapping efficiency was more associated to exhaust valve opening duration. Introduction 61The concept of engine downsizing has been adopted as the most feasible solution to four-62 stroke spark-ignition engines to attend upcoming CO 2 emission legislations. Following this 63 principle the engine operation region is shifted towards higher loads by decreasing its swept 64 volume and the number of cylinders. Therefore, large improvements in fuel consumption are 65 obtained from lower pumping losses at higher intake pressures. To ensure the same full load 66 performance of larger engines, supercharging and/or turbocharging is used to improve the charge 67 density [1]. The increased engine load and hence higher in-cylinder temperature/pressure results 68 in greater structural and thermal stresses. The occurrence of knocking combustion becomes also 69 more often due to more severe compression of the end-gas [2], as well as higher probability of 70 low-speed pre-ignition [3][4]. Conversely, two-stroke engines have the inherent advantage of 71 doubled firing frequency compared to four-stroke engines, so the same output torque can be 72 achieved with one half the indicated mean effective pressure (IMEP). The lower engine load, and 73 consequently reduced structural robustness required, has made the two-stroke cycle mostly used 74 for light weight vehicles. Its application as range extender in hybrid electric vehicles has been also 75 evaluated for such primarily reason [5]. In small engines the use of intake and exhaust ports is 76 rather att...

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