|Elements of the dual injection system. Red is for the high-pressure direct injection system, blue for the low pressure MPI system. Credit: Audi. Click to enlarge.|
The 2.0-liter EA888 Gen3 engine featured in Audi’s all-road shooting brake plug-in hybrid concept unveiled at the North American International Auto Show this week (earlier post) features—as does its production ilk—a dual injection system that combines direct injection with indirect injection into the intake manifold. In part-load operation, the indirect injection supplements direct gasoline injection to improve fuel economy and to reduce the output of particulates from the engine. (Earlier post.)
Audi says the approach is sufficient to meet Euro 6 particle limits without the use of a particulate filter. (Earlier post.) The EU has set a standard for 6 × 1012 number/km limits between 2014 and 2017, tightening to 6 × 1011 number/km. (Earlier post.)
|Particle number vs. particle mass for various LDV engine technologies. Source: MECA, Ford Motor Company. Click to enlarge.|
GDI technology provides fuel efficiency benefits from charge air cooling, more facile turbocharging, and the downsizing that these permit. The drawback, as noted in a 2013 study by Ford researchers on GDI particulates, is that direct injection of fuel into the combustion cylinder risks liquid fuel impingement onto the piston and cylinder surfaces and inhomogeneous air fuel mixing; consequentially PM formation is higher than in conventional PFI gasoline engines.
A 2012 report on particulates from gasoline vehicles by the European Joint Research Commission found that gasoline direct injection (GDI) vehicles consistently emit a very high number of particles, with the actual emission levels even approaching those of conventional diesels in some cases.
Lean-burn GDI vehicles are generally higher particle emitters than GDIs running on stoichiometric fuel mixtures. Still, the study found, the lowest emitting stoichiometric GDI vehicle exceeded the diesel Euro 5 limit (6×1011 #/km) by more than 100%.
The JRC report noted that it was not clear if OEMs could meet GDI particle limits via engine improvements, or whether this will require the introduction of a Gasoline Particulate Filter (GPF)—a solution that would add to the cost of the already more costly GDI engine itself.
Port Fuel Injection (PFI) engines, on the other hand, have no trouble in meeting the limits, although the number of particles emitted varies with driving behavior.
As described in a paper published in MTZ on the development of the 1.8L TFSI EA888 Gen3 variant, the dual injection system opens up new levels of freedom for engine applications; mixture formation can be carried out by a combination of up to three direct injection events and the indirect injection.
Various assessment criteria are used to co-ordinate the different injection modes, including:
- efficiency, knocking;
- emissions, especially particle mass and number;
- manifold wall condensation, fuel in the engine oil; and
- smooth running.
The system combines high pressure direct injection and the lower pressure MPI system. The MPI valves are supplied with fuel via a flush connection through the high pressure fuel pump (HPP) in order to guarantee internal cooling of the HPP during MPI operation.
A choke is built into the flush connection to minimize the pulsations transferred to the MPI fuel rail by the HPP. The MPI fuel rail has an integrated low-pressure sensor to regulate the injection volume; the MPI valves are integrated in the Variable Tumble System (VTS) flange.
M. Matti Maricq, Joseph J. Szente, Jack Adams, Paul Tennison, and Todd Rumpsa (2013) “Influence of Mileage Accumulation on the Particle Mass and Number Emissions of Two Gasoline Direct Injection Vehicles”, Environmental Science & Technology 47 (20), 11890-11896 doi: 10.1021/es402686z
Athanasios Mamakos, Christos Dardiotis, and Giorgio Martini (2012) “Assessment of particle number limits for petrol vehicles” (JRC report)
Thomas Heiduk, Michael Kuhn, Maximilian Stichlmeir, Florian Unselt (2011) “The new 1.8L TFSI Engine from Audi part 2: Mixture Formation, Combustion Method and Turbocharging,” MTZ Volume 72, Issue 7-8, pp 58-64 doi: 10.1365/s38313-011-0078-1