A Combustion Concept for Future Heavy-Duty Diesel Engines
Doctoral thesis, 2008
The Diesel engine has been intensively developed by manufacturers and researchers since 1892, when it was invented. However, the direction of its development has shifted in recent years.
Environmental issues are becoming increasingly important in our society, prompting more efforts to be put into making the engines cleaner. Ways in which this can be achieved include the use of exhaust filters or catalysts, and improvements to the combustion system. Manufacturers
are being obliged by emissions legislation to make strenuous efforts to develop cleaner engines.
In order to create a mode of diesel combustion that reduces both NOx and PM good fuel distribution in an appropriate gaseous mixture is needed; which can be promoted by long ignition delay and appropriate injection strategies. The temperature (both in terms of distribution and
local maxima) also has a strong effect on the emissions. The local gas temperature during combustion can be altered by changing the local equivalence ratio and the charge gas
composition. The work presented in this thesis is focused on new concepts of diesel combustion aimed at reducing nitric oxide and particulate matter (PM) emissions, while keeping fuel consumption low. In the studies underlying this thesis, engine experiments, including optical measurements, were performed in a single-cylinder research engine with a geometry based on the Volvo Powertrain D12C heavy-duty engine at Chalmers University of Technology. Several
hardware modifications were made on the research engine. A low compression ratio piston was used since this lowers the compression temperature in the cylinder and thus lengthens the delay between the start of injection and ignition. The common rail injection system gives flexibility in
terms of injection timing, and the injector nozzle chosen, which has eight orifices with small diameter, together with high injection pressure, provides good break-up and evaporation.
Multiple injections combined with EGR and high charge pressures have the potential to reduce
engine-out emissions without increasing fuel consumption. The flexibility is very large when applying multiple injections and high EGR levels, thus it is impossible to state when a global optimum is reached, and hence there will be endless scope for further development. A new way to realize a two-regime combustion concept has been proposed; low temperature lean and low temperature rich regime - PPDC (Partly Premixed Diffusion Combustion). By using PPDC one
can exploit the advantages of both HCCI combustion and conventional diffusion controlled Diesel combustion. A non-igniting premix charge reduces the emissions while a main charge controls the ignition timing, and thus fuel consumption.