Last month, the Engine Technology Forum explored the viability of propane as an alternative fuel in internal combustion (IC) engines in its webinar, 鈥淗ow Propane is Evolving for the Future.鈥�

鈥淧ropane as a fuel offers several benefits, including a high octane rating and widespread fueling and availability by over 3,500 distributors,鈥� the Engine Technology Forum said in a press release following the event. 鈥淚t generally has lower emissions compared to gasoline or diesel, and those emissions can be cost-effectively controlled by utilizing a three-way emissions catalyst similar to gasoline.鈥�

Aftertreatment Differences

Gavin Hale, director of product development and power generation for the Propane Education & Research Council (PERC), addressed the differences between diesel and propane aftertreatment systems during the webinar, saying the biggest opportunity for engine makers and vehicle OEMs vis-脿-vis propane is regarding simplified exhaust aftertreatment.

Fueling a propane delivery truck. (Photo: Propane Education & Research Council)

鈥淭hey get to reduce emissions with a fuel that鈥檚 available and retain most of the design content of the truck,鈥� he said, 鈥渨hich is key, because a lot of other alternative energy sources mean a big tear-up for the OEMs.鈥�

Describing the current diesel engine aftertreatment technology, Hale cited a Bosch SCR system for low-nitrogen oxide (NOx) heavy-duty diesel engines published as part of by the California Air Resources Board (CARB). He noted that the components of that aftertreatment system include primary and secondary diesel oxidation catalysts (DOC), selective catalytic reduction (SCR), ammonia slip catalysts (ASC), fuel burners, NOx sensors and a diesel particulate filter (DPF).

鈥淵our aftertreatment is way more complicated than your engine, really,鈥� he said. 鈥淚t becomes the dominant factor.鈥�

By contrast, Hale said aftertreatment for a propane-powered IC engine is considerably simpler.

鈥淲ith propane today, all you need is a three-way catalytic converter, similar to the ones that you have on your car today. For the 2027 regulations, probably two catalytic converters.鈥�

He added that Phase 3 GHG limits might require slight modifications.

鈥淭his really stays the key recipe, with the addition of SCR potentially in the exhaust aftertreatment,鈥� Hale said. 鈥淏ut that depends on the balance of technology that we鈥檙e able to deploy in-cylinder to drive the best efficiency in the engine rather than relying on the aftertreatment to catch everything.鈥�

Examining Direct Injection

According to Allen Schaeffer, executive director of the Engine Technology Forum, innovation is also key to making propane viable as an alternative fuel in on- and off-highway applications. One such innovation is direct injection (DI) of propane. In the press release, he said DI technology has 鈥渢he potential to be a game-changer for expanding the use of propane technology to more applications.鈥�

Manufacturing, engineering and testing company Katech Engineering and fuel systems specialist Stanadyne partnered to explore propane DI in IC engines. They presented their findings during the webinar.

Eric Suits, lead engineer for Katech, said it was worth exploring direct injection of propane for several reasons. He noted that with regard to gasoline engines, direct injection makes up a large part of the light-duty vehicle market, with 73 percent of model-year 2023 vehicles using the technology, according to from the U.S. Dept. of Energy.

鈥淚t鈥檚 already highly accepted and has its natural benefits and efficiency and improved emissions, which is why the percentages continue to go up of the OEMs currently using it,鈥� Suits said, adding that when coupled with propane鈥檚 high octane rating (105) and low emissions, 鈥渂ringing those two technologies together was an obvious area.鈥�

There were, however, obstacles to overcome. One of these is vapor lock, as propane can exist in either gaseous or liquid form depending upon temperature and pressure, Suits said. If en route to the fuel pump the propane is heated to a certain point and there isn鈥檛 a corresponding increase in pressure, it will vaporize and cause a vapor lock situation.

Additionally, Suits said propane direct injection had not been especially well researched. 鈥淭here have been a few articles written here and there, but from a feasibility standpoint, no one had really looked at tackling it.鈥�

DI Testing

Katech began the DI project with non-firing tests to benchmark current gasoline direct injection (GDI) components on the market, such as injectors and pumps, Suits said, to determine what shortcomings those components had when using them with propane. This led to firing tests intended to benchmark gasoline against propane, with the results given to Stanadyne so the company could determine the specific characteristics required for use with propane.

The General Motors L8T 6.6L GDI test engine was updated with the Stanadyne components and operated on LPG direct injection. (Photo: Stanadyne)

Once it developed the necessary fuel system components, Stanadyne proved out the concept using a General Motors 6.6L L8T V-8 spark-ignited engine as its base engine platform, according to Srinu Gunturu, chief engineer for Stanadyne. He said Stanadyne only changed a few of the hardware components for use with propane fuel.

鈥淥n the engine, it鈥檚 only the fuel pump and injector,鈥� he said, 鈥渁nd the engine was used as-is.鈥�

Gunturu said Stanadyne had to improve the volumetric efficiency of the pump and make changes to the injector for a higher fuel flow to deliver the right amount of fuel to the engine.

鈥淲e added a secondary inlet to be able to make sure that we don鈥檛 have that vapor lock inside the pump, especially in the plunger and the sleeve area,鈥� he said.

Suits said engine tests were run at a lambda of 1.0, adding it was important from an emissions standpoint.

鈥淐ome 2027 regulations, enrichment during wide-open testing won鈥檛 be allowed,鈥� he said. 鈥淎nd with propane, we鈥檙e able to easily do that without EGT (exhaust gas temperature) limits or concerns, and it lets the catalyst work as it should.鈥�

DI Test Results

The tests run by Stanadyne and Katech went beyond simply examining engine and component performance.

鈥淲e ran some durability up to 250 hours on the engine to prove out that this DI engine is promising for the propane systems,鈥� Gunturu said.

Suits added that the durability testing included on- and off-highway test cycles as well as U.S. Environmental Protection Agency (EPA) and CARB test cycles.

Following the durability testing, Stanadyne examined its fuel system components.

鈥淥n the pumps, we haven鈥檛 seen any change in the pump performance after running for the 250 hours on the propane鈥� Gunturu said. There was, however, some change in performance in the fuel injector, he said.

鈥淲e saw some wear due to lift change in our injectors,鈥� he said. 鈥淎nd we were able to identify what has changed on those 鈥� what caused the lift change. We were able to put some fixes.鈥�

Gunturu said close examination revealed wear between the armature and retainer, which Stanadyne attributed to the decrease in lift.

鈥淲e did have a design fix already implemented to address this change in the lift and wear of the armature and the retainer,鈥� he said. 鈥淎lso, we are already planning to do further testing.鈥� Stanadyne is working with OEMs to prove out these corrective actions, Gunturu said.

Suits said that in terms of the engine, the durability testing revealed no significant mechanical wear.

鈥淥ften early on, maybe the wording 鈥� 10, 20, 30 years on propane 鈥� was you need to upgrade the valve seats, you need to upgrade this because it鈥檚 propane and there鈥檚 no liquid going over the valves,鈥� Suits said. 鈥淏ut modern direct-injected engines account for that. The materials and technologies with that have advanced. So, there was no valve recession, nowhere on the engine side while running on propane, even in a quite stringent test profile.鈥�

Regarding vapor lock, Suits said that Katech successfully addressed that as part of the project.

鈥淥ne of the big areas Katech focused on, with Stanadyne supplying the hardware, was building a hardware and software system that delivered the propane correctly to the pump and never allowed for a vapor lock condition,鈥� he said.

Via some valves and control systems, Suits said Katech was 鈥渁ble to deliver liquid propane to the pump at all test conditions.鈥�

Research program reveals viability of LPG direct injection in medium-duty engines Funded by PERC and conducted by Katech Engineering, the project used software as well as specially designed hardware from Stanadyne to help overcome vapor lock.