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The evolution of a soot cloud forming inside a GDI engine cylinder per angle of crankshaft rotation, as simulated by the FORTÉ CFD model. (Image courtesy Reaction Design; for more images click on the small arrow at the upper right corner of this image box.)

Attacking GDI engine particulate emissions

It may be surprising to learn that the modern gasoline direct-injection (GDI) engines in today’s passenger cars can emit more hazardous fine particulate matter than a port fuel-injected engine (PFI), or even the latest heavy-duty diesels equipped with a particulate filter. And the potential impact to public health from these particulates is driving new developments in fuel delivery, controls, and combustion strategies.

Cars with downsized, turbocharged GDI engines such as Ford’s popular EcoBoost families have become automakers’ most effective tool in meeting stricter government fuel economy standards. BMW, Daimler and Kia are already using GDI technology in most of their production vehicles while Robert Bosch sees GDI as an ideal basis for future hybrids, 48-volt, strong or plug-in. So it’s no wonder that GDI engines are to be the fastest-growing market segment in propulsion during the coming decade, with some 40 million units in use by 2025, according to the research firm IHS.

Such growth means that GDI particulate emissions, though low compared to those of an unfiltered diesel, is now an emerging issue that has researchers examining various approaches to mitigating the problem including new combustion design and engineering concepts, alternate fuels and emissions controls.

Unhealthy soot

Confirmation of the hazard comes via a recent study conducted by researchers at Oak Ridge National Laboratory’s (ORNL) Fuels, Engines and Emissions Research Center who found that sample GDI engines emit five to 10 times more particulate matter than their PFI counterparts.

“The tradeoff for fuel economy is higher particulate matter emissions,” said ORNL senior R&D team leader John Storey. “The particulate size ranges from 5 to 5000 nm in diameter and they can include very heavy, low volatility hydrocarbons and tars.”

These carbon-based agglomerates can irritate the eyes, nose, throat and lungs, contributing to respiratory and cardiovascular illnesses and even premature death especially among the vulnerable: children, the elderly and those with respiratory conditions.

The particles that are released by GDI engines are smaller and more varied in size than diesel particles, Storey noted. And since these ultrafine particles (UFPs) are just on the heavy end of smoke size-wise, they can penetrate deeper into lungs, thus posing greater health risks. Public health authorities are growing concerned about UFP risks in urban areas and near busy highways and major roads.

The California ARB LEV-3 limits and U.S. EPA Tier 3 standard for particulate mass (PM) emissions start this year, said Cary Henry, principal engineer for aftertreatment technology at Southwest Research Institute in San Antonio. For cars, that means PMs must go from releasing less than 10 mg/mi to 3 mg/mi during a 2017-to-2021 phase-in period, and then down to 1 mg/mi beginning in 2025—a 90% reduction. Engine emissions tend to change with internal wear, so the goal is to maintain these levels over a vehicle’s 150,000-mi (93,200-km) lifetime.

In Europe, a 5 mg/km (3.1 mg/mi) PM emission limit for GDI engines took effect in 2009 with the Euro 5 standard. The first restrictions for particulate number (PN) emissions—considered more difficult to achieve than PM targets—come into effect this year with Euro 6, Henry explained. The latter initially limits PN totals to 6 × 1012 number/km, and then in late 2017 falls an order of magnitude to 6 × 1011 number/km. In the U.S., adopting PN standards is under debate.

GDI pros and cons

Injecting fuel directly into the cylinder enables a clean-burning explosion that wastes little fuel and delivers greater power, said Matti Maricq, technical leader in chemical engineering and emissions aftertreatment at Ford's Research and Innovation Center in Dearborn. Gasoline is sprayed directly where the combustion chamber is the hottest (rather than in the air intake), allowing for a more thorough, even and leaner burn.

But because of what is thought to be incomplete fuel volatilization, partially fuel-rich zones and the impingement of fuel on piston and cylinder surfaces, GDI engines produce a certain amount of particulate matter. Most emissions typically occur during cold starts and high load transients during warming, but output varies according to the load, the drive-cycle phase and driver demands.

Until tougher environmental regulations took effect, there was little incentive to replace PFI technology, Maricq said. “That’s where you spray the fuel with a closed intake port. The fuel evaporates and has plenty of time to mix with the air,” enabling more complete, stoichiometric combustion—a chemically balanced reaction that “creates almost no soot.”

In the early 2000s the auto industry recognized that “GDI could rapidly effect CO2 emissions on a large scale,” so automakers have been switching over to it ever since, he told Automotive Engineering, noting that GDI technology continues to evolve. Early versions, Maricq said, used a stratified injection strategy which included late fuel injection and charge air motion to produce a stoichiometric air/fuel mixture in the vicinity of the spark plug and, thereby, reduce pumping losses at light loads.

The downside of stratified GDI and the associated ‘wall-guided direct injection’ delivery is a tendency for liquid fuel droplets to splash onto the piston and cylinder surfaces and in homogeneous air/fuel mixing, both of which form particulates. Instead, more recent development of GDI technology has focused on early injection, what is called homogeneous operation, in which ‘spray-guided direct injection’ directs fuel straight down the cylinder, minimizing particulate formation.

Cutting particle emissions

The GDI particulate problem can be mitigated in many ways, of course, but probably the most brute-force approach is Audi’s dual PFI/GDI system which it introduced at the 2014 North American International Auto Show. The plug-in hybrid concept features a primary GDI supplemented with indirect-injection PFI to lower particulate output during part-load operation enough to meet Euro 6 limits.

Another straightforward measure is aftertreatment—to mimic diesel engines and install gasoline particulate filters. Henry said that filters have been shown to lower PN emissions by 80% to 90% and meet Euro 6 limits. It’s no surprise that carmakers have so far avoided filters, which would add additional costs (around $50 to $100 per vehicle) and may reduce engine efficiency.

Although "green" critics call the alternative approach, so-called “engine management” methods, unreliable compared to exhaust filters, most OEMs and component suppliers expect that combustion design and engineering changes will prove more cost-efficient and eventually equally effective.

Technology managers at Bosch and Delphi indicate that new ultra-precision, non-thermal laser drilling techniques for making injector nozzle holes that deliver fuel in a more ideal fashion will greatly improve future GDI systems. Specialists will steadily optimize injector timing, targeting, metering and atomization as well as the point of injection to achieve this goal. Higher injection pressures will also contribute, they said.

Other potential solutions might be found in low-temperature combustion and cooled-EGR concepts that can cut GDI particle emissions as can using more ethanol in the gasoline which adds oxygen that inhibits soot formation, Henry said.

Maricq said that he expects that the GDI particulate problem will eventually yield to such measures as modified fuels or multiple injections per cycle, "which theoretically lets you tune when the fuel gets in there, so not too much gets there at once.”

“But it’s still early days,” he cautioned. “We don’t yet have a good handle on all the factors, all the knobs that engineers could turn, at least enough for us to confidently choose the sweet spot for addressing the problem.”

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