Direct injection: A Q&A with Southwest Research Institute

October 4, 2016 By    
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Southwest Research Institute conducted tests to determine the feasibility of running a modern turbocharged gasoline direct injected engine on propane. Photo courtesy of Southwest Research Institute.

LP Gas wrote about direct injection technology and its impact on the propane industry in the autogas-themed October print edition. As a component to that piece, the magazine also connected with Michael G. Ross, program manager in the Engine, Emissions and Vehicle Research Division at San Antonio-based Southwest Research Institute, to gain another perspective on the technology. Here is our Q&A with him:

LPG: Could you please describe direct injection technology?
Ross: Direct injection describes engines in which the fuel is injected directly into the combustion chamber rather than the intake port. All diesel engines and a growing percentage of gasoline engines are direct injected. In gasoline engines, direct injection cools the air-fuel charge in the combustion chamber, reducing the tendency for knock. This allows manufacturers to increase the compression ratio for a 3 to 5 percent improvement in fuel efficiency.

Gasoline direct injection (GDI) systems operate at much higher pressures than traditional port fuel injectors, up to about 3,000 psi, which is sufficient to allow direct injection of liquid propane using the gasoline fuel injectors. The high octane rating of propane, particularly in turbocharged GDI engines, permits spark timing and fueling to be optimized for significantly better fuel efficiency under high-load operating conditions, such as acceleration and towing.

Illustration courtesy of the Southwest Research Institute

As the illustration shows, an injector introduces fuel directly into the combustion chamber of a gasoline direct injection engine. Southwest Research Institute recently conducted tests to determine the feasibility of running a modern turbocharged gasoline direct injection engine on propane. Illustration courtesy of the Southwest Research Institute

LPG: What is Southwest Research Institute’s involvement in this?
Ross: Southwest Research Institute has performed a series of projects funded by the Propane Education & Research Council (PERC) to demonstrate the feasibility and benefits of using propane in gasoline direct injected engines. In all of the projects, stock GDI fuel injectors were used with the OEM engine controller.

In the most recent project, the OEM control settings for fuel injection timing, spark timing and fuel enrichment were modified to optimize efficiency and emissions when operating a popular turbocharged GDI engine on propane. The testing showed that fuel injection timing needed to be advanced relative to gasoline settings for the best fuel efficiency and carbon monoxide emissions when operating on propane. More importantly, at half-load and above when the engine was knock-limited on gasoline, propane eliminated the need for retarded spark timing and excessive fuel enrichment, resulting in a 5 to 19 percent reduction in fuel consumption at high engine loads without exceeding OEM exhaust temperature limits.

LPG: How does this differ from the conversion kits already on the market?
Ross: All current propane liquid port injection or vapor injection kits require the installation of unique propane-specific fuel injectors and fuel rails, and most kits require aftermarket electronic controllers and associated wiring harnesses. All of these additional low-volume, propane-specific parts increase the cost for propane conversions.

Using the stock gasoline direct injection system for propane eliminates the extra cost of parts and installation of special fuel injectors and fuel rails. It also eliminates the need for an additional fuel controller, provided the conversion kit supplier has the capability of reprogramming the OEM controller. Using the OEM controller for propane also allows fuel injection timing, spark timing and fuel enrichment to be optimized to minimize fuel consumption and emissions. Current aftermarket controllers generally do not modify the gasoline fuel enrichment schedule or ignition timing, so the engine’s thermal efficiency is typically the same or slightly worse when running on propane.

LPG: How long has direct injection been under development?
Ross: Research in direct injection of LPG in spark ignition engines goes back to at least 2000 and possibly earlier but has accelerated since the introduction of gasoline direct injected vehicles over the past 10 years. LPG conversion kits for GDI vehicles have been available in Europe for several years from all the major LPG conversion system manufacturers, but only recently have efforts been made to certify these kits for the U.S. market.

LPG: Could you please describe the process of the U.S. Environmental Protection Agency (EPA) certification and the impact of furthering direct injection technology? Is California Air Resources Board (CARB) certification in play?
Ross: EPA and CARB requirements for certification of direct injected propane vehicles are no different from current port injected vehicles and generally require vehicles to meet the same emissions standards as the gasoline version. CARB certification is more complicated and costly due to California’s unique on-board diagnostic requirements.

LPG: How would you describe the marketing/sales prospects of this technology?
Ross: The potential market for direct injection propane systems is increasing rapidly as more gasoline direct injected engines are introduced. For example, the majority of current light-duty pickup trucks sold in the U.S. now have direct injected gasoline engines, and it is very likely that gasoline direct injection will be introduced in heavy-duty pickups within the next two years.

LPG: What does this technology mean for propane marketers? Presumably more load of LPG can be marketed?
Ross: The potential for lower-cost, better-performing propane direct injected vehicles should lead to higher sales of propane autogas vehicles, and therefore, higher LPG load.

LPG: Can these systems be installed at the LPG marketer/mechanic level? Or does direct injection come directly from OEM assembly lines?
Ross: For OEMs, using the same fuel injectors and engine controllers as the gasoline version would make it relatively easy to build propane vehicles online, rather than requiring offline conversions. Aftermarket conversions would also be possible by LPG marketers using EPA/CARB-certified conversion kits.

LPG: What advice do you have for propane marketers interested in becoming involved with direct injection?
Ross: Propane marketers interested in direct injection should check with conversion kit suppliers regarding the availability of EPA-/CARB-certified conversion kits for direct injected vehicles. PERC is also a good source of information regarding the status of direct injection R&D.

LPG: What is the significance of the Department of Energy grant? How will this assist in furthering direct injection?
Ross: Through efforts of the National Propane Gas Association, Congress allocated $5 million to the Department of Energy (DOE) specifically for research and development of propane direct injection. The DOE is expected in early 2017 to announce funding awards for one or two projects to develop advanced direct injected propane engines with project durations of two to three years.

LPG: How do vehicle owners benefit by using direct injection?
Ross: One of the benefits of using propane in direct injected engines is reduced emissions. One of the side effects of gasoline direct injection is higher particulate emissions compared with port-injected gasoline engines. Published research studies have shown that LPG reduces particulate emissions by more than 90 percent in direct injected gasoline engines while also reducing greenhouse gas emissions by approximately 10 to 12 percent.

In the short run, propane conversions of direct injected vehicles are likely to have about the same cost and performance as current liquid port injected propane conversions. However, it is important to have these kits available so that propane can be used in popular direct injected fleet vehicles.

The real breakthrough will come when OEMs use direct injection to introduce advanced high-efficiency propane engines that will significantly reduce the overall cost of ownership compared with gasoline, diesel or natural gas options. These engines will have diesel-like torque and efficiency, but much less complicated emission control equipment and lower maintenance requirements than diesel engines. Particularly for medium-duty trucks and buses, these advanced propane direct injected engines could be game-changing.


More: Using autogas in direct injection fuel systems

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