Clean, Controlled DI Diesel Combustion Using Dilute, Cool Charge Gas and a Short-Ignition-Delay, Oxygenated Fuel

The influence of charge-gas dilution and cooling on incylinder combustion processes and engine-out smoke and NO_x emissions was experimentally investigated in an optically accessible heavy-duty diesel engine using diethylene glycol diethyl ether (DGE) fuel, a viable diesel oxygenate. In-cylinder pressure and natural luminosity were measured simultaneously with engine-out smoke and NO_x while varying the temperature, density, and nitrogen dilution of the charge gas; the quantity of fuel injected; and the injection timing. Measurements obtained using DGE at a reduced charge-gas temperature and with nitrogen dilution were compared to measurements obtained at typical diesel operating conditions using DGE and using a diesel reference fuel.

The results demonstrate that low-temperature combustion with near-zero engine-out smoke and NO_x emissions can be achieved using a traditional directinjection strategy combined with charge-gas dilution, cooling, and use of a short-ignition-delay, oxygenated fuel. Engine-out NO_x emissions are 0.17 g/ihp-hr at 12 bar IMEP and 1200 rpm. The variation of exhaust NO_x with charge-gas nitrogen dilution is correlated with calculated equilibrium NO_x levels in nitrogen-diluted adiabatic flames, suggesting that the measured NO_x reductions are the result of reduced flame temperatures. This type of low-temperature combustion offers the emissions benefits of homogeneous charge compression ignition (HCCI) without the corresponding combustion-phasing control problems, since ignition timing is controlled by fuel-injection timing. Cylinder-pressure characteristics are relatively insensitive to injection timing, intake temperature, or dilution level. Combustion phasing is easily optimized for efficiency without serious constraints due to cylinder-pressure limitations. Furthermore, the present strategy has the potential to be used throughout the entire load range, eliminating complications associated with the transition between a low-temperature combustion mode at low loads and a more-traditional mode at high loads.