External Exhaust Gas Recirculation (EGR) has been used on diesel engines for decades and has also been used on gasoline engines in the past. It is recently reintroduced on gasoline engines to improve fuel economy at mid and high engine load conditions, where EGR can reduce throttling losses and fuel enrichment. Fuel enrichment causes fuel penalty and high soot particulates, as well as hydrocarbon (HC) emissions, all of which are limited by emissions regulations.
Under stoichiometric conditions, gasoline engines can be operated at high EGR rates (> 20%), but more than diesel engines, its intake gas including external EGR needs extreme cooling (down to ~50°C) to gain the maximum fuel economy improvement. However, external EGR and its problems at low temperatures (fouling, corrosion & condensation) are well known. To avoid this, a coated Gasoline Particulate Filter (cGPF) is introduced in the EGR loop to clean the EGR gas by removing particulates, HC, NOx and CO, such that only CO2, N2 and H2O remain. After cooling the cleaned EGR gas, water condensation is clear without particulates and with neutral pH-values, otherwise being dirty and acidic.
These characteristics allow cleaned EGR to cool down with less fouling and corrosion risks. Using cooled clean EGR, gasoline combustion temperature is reduced significantly, which reduces engine knocking, engine-out soot particulates, engine heat losses and fuel consumption, all at the same time.
Engine testing with cooled clean EGR at high loads confirms fuel consumption is reduced by more than 10% while particulate numbers is reduced by more than 75%. Engine knocking is suppressed and exhaust temperature is noticeably reduced, which allows for a significant increase of engine compression ratio to further improving fuel economy across the entire engine map. However, to ensure these benefits an effective EGR cooling system is required.
The technology for Cooled and Cleaned EGR, CleanEGR™, can be applied to Gasoline engines at a competitive cost in the near future. And rather than trade-off fuel economy improvement (CO2 reduction) versus particulate emissions, it benefits both, which are often thought of as competing. While the tests in this presentation are performed with 91 octane fuel, CleanEGR offers the economical operation of 87 octane fuel in some regions as well.