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As in the standard American Society for Testing and Materials (ASTM) procedure which is used to evaluate the fuel Octane Number (ON), some signal properties are considered, while others are neglected, it happens that different pressure signals of the sensor, obtained from different fuels and operating conditions, can lead to the same Knock Intensity index (KI) value, even though the knock behavior is not the same. Therefore the aim of this work was to analyze the standard signal processing chain of the Cooperative Fuel Research engine (CFR) (from the pressure sensor to the knock-meter display) and its effects on the value of the KI, for different fuels and operating conditions.

Experimental tests have been carried out on a single-cylinder crankcase-scavenged two-stroke engine, with both indirect and direct gasoline injection, in order to compare the results obtained with these two different fuel-feeding systems. Engine operating conditions were chosen like those of a typical aeronautical application. They were determined using a theoretical method, that is by computing the power of an aircraft, that is necessary for a steady-state flight at different aircraft velocities. This power curve turned out to be in good agreement with the “propeller load” that was experimentally found through preliminary bench tests, that is, the cubic characteristic, of power versus engine revolution speed, matching the maximum power of the engine. Brake specific fuel consumption (bsfc) and exhaust emission measurements were then carried out using bench tests along the “propeller load”.

As more stringent emission limits and low consumption requirements also involve s.i. 2-stroke engines, one of the most important design modifications that can cope with these constraints is to perform the scavenging process using pure air, which means not only fuel-free air but also oil-free air. A new single-cylinder prototype engine, equipped with a gasoline direct injection (GDI) apparatus has therefore been designed and built. In order to reduce manufacturing costs, this prototype was obtained by modifying a mass-produced 4-stroke 4-cylinder automotive engine. Apart from the replacement of the original indirect fuel feeding system with GDI, two more remarkable features should be pointed out: the use of a force-fed lubrication system, like those used in current 4-stroke engines and, as a consequence, the use of an external scavenging pump.

Preliminary bench tests have been carried out on an original-design 2-stroke single-cylinder prototype engine, which is equipped with an electronically controlled gasoline direct injection apparatus. The main design and operating features of the engine concern: a unidirectional airflow during the scavenging process (from the inlet ports near the BDC to overhead cam-actuated valves), an external air pump (a Roots volumetric type driven by an electric motor) and a force-feed lubrication system, like those usually exploited in mass-produced 4-stroke engines. Experimental bench tests were carried out under low load, intermediate rotational speed operating conditions. The performances were compared to those obtained from a commercial crankcase-scavenged 2-stroke engine using an indirect injection fuel feeding system. Encouraging results were obtained as far as fuel consumption and pollutant emissions are concerned.