Abstract
Keywords: LPG, diesel engine, combustion, pollutants, cetan number
The legislation concerning the pollutants emissions norms for diesel engine, especially for NOx and PM emissions, imposes the use of different types of alternative fuels. The LPG is an alternative fuel with real future opportunities due to the following aspects: the NOx emission level is lower; the PM emission is indistinguishable; CO2 emission decreases or is maintained to the same level; the engine power is the same; the automotive autonomy increase; the HC and CO emissions level decreases; LPG protects the particles filter and environment because don't contains sulphur; the infrastructure for LPG distribution already exists.
Is advantageous to use LPG (the third common fuel used especially for automotive spark ignition engine) and in diesel engine in order to reduce the pollutants emissions levels and to increase thermal efficiency which is directly related with CO2 emission level. From this point of view is more advantageous to use LPG at diesel engine than SI engine, but is very difficult to use only liquid LPG direct injection because of its higher autoignition endurance (CN = -2....-3). In this case is necessary to use peroxides additives for LPG but the explosion danger is very high. Another impediment appears at the liquid LPG direct injection: poor oiling properties for LPG comparative to diesel fuel. This deficiency is eliminated by additives use or by special covering of the mobile components of injection system, inside surface of the pump cylinder and injector nozzle with fluorine-polymeric substances. These disadvantages are removed through the use of the diesel gas method proposed by authors: the LPG is injected in gaseous phase in the front of the inlet valve and one small quantity of diesel fuel is injected into combustion chamber assuring the ignition of the air-LPG mixture. The injection of LPG in the front of the inlet valve provides a precise LPG dose control and all the injectors are electronic actuated. The engine keeps up its classic diesel fuelling system which is adjusted for dual fuelling. The electronic control unit for the LPG injection system (EU-LPG) is attached by the engine Electronic Control Unit (ECU) and the engine electronic control is provided by the communication between those two electronic control units. This fuelling method is very easy to imply to the existing engines or to the new engines.
The paper presents some theoretical results obtained by authors from a research program developed for an automotive diesel engine with 1500 cm3 displacement fuelled by diesel-gas method. Are shown the influences of LPG dose on energetically and polluting engine performances.