Unintended Consequences Drive GDI Engines to Your Shops - Part 5
Let’s “turn up the heat” with an overview of GDI’s closely-connected:
1. Fuel injectors
2. Combustion chambers
In gasoline direct injection (GDI) engines, gasoline injector tips are directly exposed to combustion chamber heat, pressure and contaminants. As a result, according to VehicleMD, GDI fuel injector deposits are similar to combustion chamber carbon deposits. Meaning they’re harder, more tenacious, more difficult to remove and more benefitting from preventive maintenance.
In the past, techs were familiar with port fuel injection (PFI) where fuel injectors, located in lower-temperature intake runners, sprayed gasoline at 40–60 pounds per square inch during the relatively lengthy period of intake valve closure.
That’s all changed with GDI.
Bosch reported, “As a result of increasingly strict emission laws and demand for low fuel consumption…technical components require innovative concepts and ideas.”
· To overcome combustion chamber pressures, GDI injectors spray gasoline at more than 2,000 psi, requiring high-pressure fuel pumps and steel fuel rail walls, often 1/8-inch thick.
· When replacing a failed GDI injector, according to the Automotive Aftermarket Suppliers Association, “…professionals must also be aware that many [GDI injector] manufacturers might require a complete fuel rail replacement when a single fuel injector is replaced for safety reasons.”
· GasItaly reported, “The fuel requirements at idle can drop the [GDI fuel injector] opening time to just 0.4 milliseconds.” So electromagnet injectors, popular in PFI engines, are typically replaced with piezoelectric injectors using wafer-like slices of piezo material in a long stack that expands slightly to activate the injector when electrically charged.
· Society of Automotive Engineers (SAE) Paper 2014-01-1209 added, “Multiple injections…renders the injector technology a primary enabler for advancing the GDI engine combustion characteristics.”
Are these innovative concepts significant for service providers?
Bosch also reported, “[GDI] does change the repair process and maintenance program.”
See article No. 1 in this series from March 2015 for the numbers; all of the current 5 million EcoBoost engines use GDI technology.
Plus, original equipment manufacturer (OEM) redesigns increase the need for updates. For example, the 2017 3.5-liter Ecoboost V6 will share only a name and displacement with the current engine. Ford reports a clean-sheet redesign including a new block, cylinder heads, turbos and intake systems.
Diagnosis also requires understanding increased deposit sensitivity of today’s engines, with Underhood Service reporting, “Today’s engines operate on a ragged edge between optimal efficiency and a misfire.” Underhood Service writer Larry Carley addressed sensitivity to deposit restriction, “Only an 8-10 percent restriction in a single [GDI] fuel injector can be enough to cause a misfire.” See Fig. 1 for a GDI injector tip that needed deposit control maintenance long ago.
Why are these GDI deposit problems happening when direct injection has served diesel engines for more than a century? The answer begins with gasoline/gasohol properties, including the absence of diesel’s oil protection. With reputable sources detailing how gasoline and gasohol properties contribute to GDI owner problems and service provider headaches, research can address many GDI customer issues.
Also, motorists need to understand that vehicle age plays significantly into GDI’s “unintended consequences.” Auto Guide asked, “What will GDI mean to people that hold on to vehicles beyond 10 years?” That’s an important question. A 2015 Information Handling Services automotive survey reports that the average car on US roads is a record-high 11.5 years old.
For smaller, lighter GDI engines to provide performance equal to larger, heavier PFI engines while reducing fuel usage and emissions, GDI fuel injectors must spray more gasoline under higher pressure in shorter bursts into the combustion chamber. Here it must instantly swirl/tumble and mix with air (variably, depending on stratified vs. homogeneous combustion operating mode), vaporize and combust. Any disruption of these critical functions can require costly repairs or cause catastrophic engine failure.
Does typical driving increase deposits or maintenance needs?
SAE Paper 1999-01-1498 reported on GDI engines tested at 12,489-mile intervals to determine the effect of mileage accumulation on deposit formation with driving cycles: “… representing a realistic mix of driving conditions (idle, urban, highway)…The program showed that engine fuel system deposits, including specifically those on…combustion chamber and injectors are formed in higher amounts in the GDI engine than in the PFI engine.”
Another challenge with unintended consequences is to improve fuel economy and reduce emissions, GDI combustion often involves very lean air-fuel ratios.
Quoting from the International Journal of Advanced Scientific and Technical Research, “The stoichiometric air-fuel ratio for gasoline is 14.7:1 by weight (mass), but [the GDI] ultra lean mode can involve ratios as high as 65:1.”
Lean air-fuel ratios reduce tolerance for combustion chamber deposits (CCD), increased by GDI’s unburned fuel and other issues.
SAE Paper 2014-01-2885 reported, “CCDs disturb the turbulent motion of the charge intended to improve the air-fuel mixture in the cylinder.” SAE’s 2014 Technical Session article added, “The in-cylinder tumble intensity of the GDI engine is crucial to combustion stability and thermal efficiency…”
Note the need for preventive maintenance to address deposit interference with critical GDI engine functions.
Consumer Reports stated, “Some [GDI] carmakers…have issued technical service bulletins to their dealers recommending drivers use only name-brand detergent gasoline — without ethanol additives — and that they periodically add a fuel-system cleaner when they refuel.”
Another challenge for GDI diagnostics is that OEMs complicate GDI diagnosis with distinctly different designs for fuel charge combustion, including (see Fig. 2):
· Spray guided
· Wall guided
· Air guided
This is further complicated when GDI engines switch between stratified combustion, to improve start-up and low-speed combustion of lean air-fuel ratios, and homogeneous combustion, for high-speed and high-load burn efficiency.
For more unintended consequences of GDI, see our next article detailing super-knock, low-speed pre-ignition, which is known to destroy engines, and a key subject, oil system deposits.