Miller Cycle and Internal EGR in Diesel Engines Using Alternative Fuels

The Single Cylinder Research Engine (SCRE) at the Institute of Internal Combustion Engines and Powertrain Systems is equipped with a variable valve train that allows to switch between regular intake valve lift and early intake valve closing (Miller). On the exhaust side, a secondary valve lift on each valve is possible with adjustable back pressure and thus the possibility of realising internal EGR. In combination with alternative fuels, even if they are Drop-In capable as HVO, properties differ and can influence the emission and efficiency behaviour. The investigations of this paper are focusing on regenerative Drop-In fuel (HVO), fossil fuel (B7), and an oxygenate (OME), that needs adaptions at the engine control unit, but offers further emission potential. By commissioning a 2-stage boost system, it is possible to fully equalize the air mass in Miller mode compared to the normal valve lift. This enables a comprehensive analysis of the behavior of the fuels under different boundary conditions. In addition to the boost pressure, the exhaust gas pressure and engine speed are varied and analyzed with regards to emissions and efficiency. For this purpose, a combustion analysis is carried out and analyzed based on cylinder pressure data to work out the causes of the respective effects. One expected effect is a NOx reduction in Miller mode with the same air mass due to reduced effective compression, without significant efficiency losses due to the constant expansion. In the investigations this effect is clearly visible and therefore represents great potential for reducing NOx emissions. The results clearly show the influence of the fuel that is used. Even a drop-in fuel such as HVO shows potential that can be utilised to reduce emissions and increase efficiency.