Gas-Diesel Dual-Fuel Combustion in a Heavy-Duty Engine
Licentiate thesis, 2015
Global efforts have been devoted to developing different kinds of alternative fuels in order to reduce the dependence on conventional petroleum fuels. Among these alternative fuels, the compressed natural gas (CNG) is a pop- ular choice because of its relatively clean combustion and global availability at attractive prices. Biogas is another important alternative fuel due to its renewable characteristics. Fossil CNG fuel normally consists of four com- ponents: methane, ethane, propane and n-butane. Methane as the primary fuel component can represent up to 90% of the fuel content. The CNG com- position varies from region to region. Biogas normally consists of methane and carbon dioxide. The latter needs to be removed prior to using biogas as a fuel in combustion engines.
One of the most attractive approaches for using gas (CNG/biogas) in heavy- duty engines is the dual-fuel mode, which can be realized by small modifi- cations of current heavy-duty engines. The secondary fuel in this mode is Diesel, which is used as an ignition source. In the conventional dual-fuel scenario, gas is port-injected into the intake manifold and premixed with air before entering the cylinder. A Diesel pilot is directly injected into the cylinder to initiate combustion at the end of the compression stroke.
Reactivity Controlled Compression Ignition (RCCI), one of the low temper- ature combustion concepts, can also be used with gas/Diesel combinations. Gas can also be port-injected into the intake manifold as in the conventional dual-fuel scenario, but in the RCCI approach, Diesel needs to be directly injected into the cylinder much earlier in the compression stroke than in the conventional dual-fuel scenario.
Experiments were carried out for both the conventional dual-fuel and RCCI scenarios. In the conventional dual-fuel scenario, the influence of the CNG supplement ratio on engine performance and emissions was explored at two different load points. The results indicated improved performance and emis- sions at intermediate load compared to those at low load. For the RCCI scenario, the effects of different parameters on the engine performance and emissions were investigated, including the SOI timing, Diesel injection quan- tity, EGR, compression ratio, engine speed and load. The latter test cam- paign revealed that a high indicated thermal efficiency (over 50%) could potentially be achieved with the RCCI concept.
Besides the experimental investigation, dual-fuel simulation work was also performed. To simulate dual-fuel combustion, a new numerical model in- corporating two coupled combustion modes was developed: the Diesel com- bustion mode modeled as a partially premixed reactor and the flame prop- agation mode based on the turbulent flame propagation model. This com- bustion model was implemented in the updated KIVA-3V CFD engine code. Validation of this 3D dual-fuel combustion model was carried out for three different cases by comparing simulation and experimental results. The com- parison showed reasonable good agreement between experiment and simu- lation for most aspects of engine performance but discrepancies regarding the onset of ignition delay and emissions.