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An electronic instrument for the measurement of fuel consumption in reciprocating internal combustion engines for light aircraft has been designed, manufactured and tested. The operating principle of the measuring device is based on the simple, theoretically supported and experimentally verified observation that the fuel mass flow rate is almost exactly proportional to the product of the intake manifold air pressure “pc” and the engine revolution speed “n”. Therefore, only two sensors are needed, and no fuel pipe cutting is required for installation and operation. This feature represents a major point in favor of simplicity, reliability and safety. The aim of the instrument is to provide a fuel consumption indication which can be used during cruising. The instrument is not intended as a replacement for the usual on-board fuel level gauge, but can be used to integrate the flight information with the overall and instantaneous fuel consumption data.

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.

An original instrument for fuel consumption measurement in reciprocating internal combustion engines for light aircraft has been developed and built. It is based on the detection of two parameters: the engine rotational speed and the manifold pressure. The aim of the instrument is to provide a fuel consumption indication which can be used during cruising. The instrument is not intended to replace the usual on board fuel level gauge, but can be used to integrate the flight information with the overall and instantaneous fuel consumption data, and with the cruising range indication, leading to a significant increase in flight safety. Some results of fuel consumption measurements from experimental tests are here presented and discussed. Such results were first obtained with the instrument installed on the engine during bench tests.

The performance of two-stroke spark-ignition engines is greatly influenced by the scavenging process The variation of the crankcase clearance volume has here been investigated as a method for engine-load reduction. This method allows the reduction of the load without throttling or only by partial throttling with a theoretical increase of the engine efficiency. A comparison of two methods (air throttling and crankcase clearance volume variation) has therefore been carried out. The reduction of pumping work, due to the use of the variable crankcase clearance volume, has not always caused a consequent reduction of the specific fuel consumption. This is mainly due to deterioration of the scavenging process and to the occurrence of pre-ignition which occur above all at light loads.

An original 2-stroke prototype engine, equipped with an electronically controlled gasoline direct-injection apparatus, has been tested over the last few years, and the performances of these tests have been compared with those obtained using a commercial crankcase-scavenged 2-stroke engine. Very satisfactory results have been obtained, as far as fuel consumption and unburned hydrocarbons in the exhaust gas are concerned. Large reductions in fuel consumption and in unburned hydrocarbons have been made possible, because the injection timing causes all the injected gasoline to remain in the combustion chamber, and thus to take part in the combustion process. Moreover, a force-feed lubrication system, like those usually exploited in mass-produced 4-stroke engines, has been employed, because of the presence of an external pump. In fact, it is no longer necessary to add oil to the gasoline in the engine, as the gasoline does not pass through the crankcase volume.