LP EGR and IGR Compromise on a GDI Engine at Middle Load 2013-01-0256
Burned gas recirculation is emerging as a promising technology to reduce fuel consumption without compromising performance in turbocharged spark ignited engines. This recirculation can be done internally through Internal Gas Residual (IGR) using Variable Valve Timing (VVT) or externally through classical Exhaust Gas Recirculation circuit (EGR). Both have a large impact on combustion. The purpose of the paper is to give clues to get the best compromise at moderate load between these two technologies in terms of fuel consumption.
This experimental work was performed on a Gasoline Direct Injection (GDI) engine, 2.0L displacement, dual independent VVT, equipped with a Low Pressure, cooled and catalyzed EGR loop (LP EGR). The load region covers 6 to 10 bar Indicated Mean Effective Pressure (IMEP). EGR rates obtained vary between 0 and 15%. IGR variation is obtained by using the VVT in order to vary the valve overlap. IGR rates vary from 4 to 8%. They are estimated using the thermodynamic properties of intake and exhaust gases.
In-depth analysis of IGR and LP EGR effects is performed using an original method of split of losses. This method helps identifying what are the main causes of efficiency losses in the thermodynamic cycle. It has proven to be very useful to weigh pros and cons of IGR and LP EGR.
Results show firstly that LP EGR has a positive effect on Indicated Specific Fuel Consumption (ISFC) once knock is present, as it pushes back the knock limit, therefore improving the combustion phasing. For example at 2000 rev/min 10 bar IMEP, up to 4% gains on ISFC can be achieved with EGR (depending on VVT positions). Secondly, fuel consumption is lowered by 12 g/kW.h by optimizing VVT positions at 2000 rev/min 10 bar. This corresponds to the benefit of reducing the IGR rate from 6 to 4%. A further gain of 3 g/kW.h is obtained adding 10% EGR. Thirdly, the increase of IGR rate from 5 to 8% at 1500 rev/min and 8 bar IMEP increases the ISFC by 8%. This is mainly due to the negative effect of IGR on combustion phasing and duration. IGR increases the temperature at the end of compression stroke, therefore increasing the knock sensitivity.