GDI-Motoren: Minimize emissions and increase performance under pressure

It is expected that around 40 million engines with gasoline direct injection will be sold by 2025. From this point of view, it is astonishing that these engines emit more dangerous particulate matter than engines with prechamber injection or even the latest diesels with particulate filters.

The potential increase in market share means that GDI particulate emissions - while comparatively low compared to unfiltered diesels - are now coming under greater scrutiny from manufacturers and regulators.
To reduce these emissions while improving performance, engineers are currently exploring new designs and concepts, including increasing gasoline pressure, alternative fuels and emissions reduction systems.
According to Matti Maricq, technical director of Chemical Engineering and Emissions after Treatment at Ford's Research and Innovation Center in Dearborn, injecting fuel directly into the cylinder creates a clean-burning explosion that wastes little fuel and releases more power.
During this process, gasoline is supplied directly where the combustion chamber is hottest. This enables a more thorough, uniform and gentle combustion.
Clean-burning GDI engines emit hazardous particulates
However, due to incomplete fuel volatilization, partially fuel-rich zones, and "wetting" of pistons and cylinder surfaces, GDI engines produce undesirable particulates. Most emissions typically occur during cold start and high load transition situations during warm-up. However, this can vary depending on load, driving cycle phase and driver demands.
Although "green" critics remain skeptical of so-called "engine management" methods because they perceive them to be unreliable compared to exhaust filters, most OEMs and component suppliers expect engineering changes and improved designs to ultimately prove more cost-effective and equally reliable.
The current state of development suggests that higher fuel pressures, possibly near 40MPa, along with new high-precision injectors will greatly improve future GDI systems. To further optimize the system, engineers at the injector will continue to refine aspects of timing, targeting, metering and atomization.
A recent study published by SAE noted that increasing fuel system pressure improves blend homogeneity and reduces diffusion flaming. This significantly reduces particulate emissions under homogeneous combustion in GDI engines.
Furthermore, as a result of the improved intake movement, a further reduction in particulate emissions was achieved at fuel pressures between 20 MPa to 40 MPa.
Combustion data show that increasing fuel pressure has a great influence on reducing combustion emissions and optimizes fuel consumption.
Accurate measurement of fuel pressure
Nevertheless: For a GDI system to work optimally, it is important that the fuel pressure in the common rail (CR) is measured correctly during the design and test phase so that the ECU can be mapped accordingly.
Measuring CR fuel pressure is the key to lower particulate emissions. The direct injection pressure is measured with sensors and the signals are used to determine the pump speed and/or volume.
Most direct injection systems use piezoresistive pressure sensors on the low-pressure side of the system. When pressure is applied, the silicone chip element generates a measurable electrical voltage. It increases when the pressure increases.
On the high pressure side, sensors usually use a metal diaphragm on a resistive bridge. When pressure is applied, the bridge produces a change in resistance, which manifests itself in a change in the applied voltage. The electronic control module converts the voltage into a calculated pressure - usually with an accuracy of ± 2%.
To maintain the correct pressure, the electronic control module pulses the low pressure pump. The system typically features a controller and no return lines. Some systems even have built-in temperature sensors in the lines that are used to calculate the density of the fuel so that the fuel trim can be matched to the amount of energy in the fuel.
To ensure accurate measurement of line pressure, it is important to use high-precision pressure transmitters to map CR pressure under all engine and load conditions. Any error during this process may result in incorrect modulation of the CR pressure. The result is serious deviations.
With the introduction of the harmonized driving cycle, OEMs are under renewed pressure to meet the emission levels targeted by authorities. The GDI gasoline engines will be at the forefront of a new generation of green technologies. Still, for this technology to meet future regulations, particulate emissions must be reduced - in large part by closely monitoring CR fuel pressure.