Abstract
Keywords dual fuel, power mode, additive mode, pilot injection, pilot injection parameters
Abstract
A Diesel engine, equipped with a Common Rail injection system, has been tested operating in dual-fuel mode, using Diesel fuel and natural gas simultaneously. The natural gas was premixed with the engine intake air and used in two operating modes: to provide the full power required by the engine, thus engine combustion and to reduce pollutants emissions. During tests, engine efficiency, in terms of fuel consumption, and pollutant emissions, in terms of Nox, opacity, CO and total hydrocarbons, have been measured.
In the first operating mode, the main injection was eliminated and the liquid fuel was injected only during the pilot injection, in order to ignite the premixed charge of air and natural gas. For this operating mode, the dual-fuel combustion shows a higher ignition delay and a lower heat release rate during the diffusive combustion phase with respect to the normal combustion, obtained using only the Diesel fuel. During the tests, pilot injection timing has been varied, and its effect on engine behaviour has been analysed. Generally, HC, CO and Nox emissions levels at the exhaust are higher, when pilot advance increases. In terms of particulate emissions, this operating mode shows a better behaviour with respect to the full Diesel fuel operation, thanks to the lower injection quantity of liquid fuel, only limited to the pilot injection.
Using the natural gas in the engine, as an additive fuel, while performing the Diesel fuel main injection, leads to maintain practically unchanged engine efficiency, respect to the traditional Diesel fuel operation mode. Concerning the emissions levels at the exhaust, the use of small quantities of gas (10-30% respect to the total fuel energy) improves the Nox-soot trade-off; however, at the same time, HC and CO emissions are characterized by higher values.