Liquefied petroleum gas (LPG) is an alternative fuel option that combines reduced CO2 emissions with lower fuel cost. Increasingly automakers are offering LPG versions of certain vehicle models, typically with a dual-fuel (bi-fuel) engine. Other gasoline vehicles are converted to dual-fuel combustion.
At the 2011 Frankfurt Motor Show the supplier Mann+Hummel presented a filtration technology that is used in vehicles with liquified gas injection. Filtering LPG in its liquid form poses some challenges as the system pressure can go up to 25 bar (363 psi).
“At this level the filter system is exposed to substantial mechanical forces,” said Dr. Harald Banzhaf, head of liquid filtration elements R&D at Mann+Hummel in Ludwigsburg, Germany. Reliably filtering liquified gas is absolutely necessary to protect the gas pump and injectors.
Typically, LPG is converted from its liquid form in the tank to the gaseous form before it is injected. Systems that inject the gas as an actual liquid have been around since the mid-1990s; the Dutch company Vialle coined the name Liquid Propane Injection (LPI) for this principle.
In Australia, GM Holden offers LPI technology as an option in some of its cars. And in March 2011, Hyundai applied for an LPI patent (application no. 12/621199; http://www.freepatentsonline.com/y2011/0048376.html).
Such LPI systems exploit two major benefits. First, the amount of fuel that is injected into the intake manifold can be controlled more precisely as a liquid than with as a gas. Probably even more importantly, the evaporation cooling effect caused by the expanding propane-butane mix reduces the charge-air temperature, which in turn permits higher compression ratios and improves the energy density in the combustion chamber.
Both benefits help to make the transition from petrol to liquid propane injection mode less noticeable to the driver. In addition to that the lower temperature level can help to reduce problems such as burnt valves.
In terms of energy efficiency, LPI makes sense; when the LPG is converted to its gaseous form in an LPG engine the evaporation cooling must not cause the converter to freeze up. It needs to be heated, which can be done by using warm coolant. In other words, the evaporation cooling is wasted because it occurs at the wrong place where more energy has to be dissipated to compensate for the effect.
“The challenge we were entrusted with by our new Australian customer was to come up with a filtration technology that is more efficient than existing filters but retains a low level of differential pressure,” Banzhaf explained. “A higher differential pressure would consume more energy which would have an impact on the fuel consumption. One of our targets was to actually save energy while improving filtration.”
The filtration technology that will now be installed by the Australian OEM was developed at the Mann+Hummel site in Korea, where it is also manufactured before it gets shipped to the supplier’s Sydney site.
The filter element is a composite of an ultra-fine non-woven glass-fiber medium supported by an outer metal mesh skeleton. Banzhaf noted that the glass fiber has one big advantage.
"During operation no filter cake will build up on the surface. Instead the particles are transported into the fiber mesh where they are held back deep inside the chaotic filter media structure," he explained. "As a consequence many liquid paths remain open for the gas flow through the filter medium. Therefore the differential pressure level of a glass-fiber filter medium will remain constant for a long time.” In the current application the filter is designed for a 60,000 km (37,300 mi) lifetime.
The glass-fiber filter mats are difficult to handle because they are easily compressed during manufacturing. This is a particular challenge with composite materials, Banzhaf noted. Part of the manufacturing know-how required to avoid glass fiber compression during production was developed at two German R&D sites of the supplier.
The new filter element has not only been tested to 25 bar as requested by the OEM. During the approval by the Australian Gas Association the filter element was successfully taken up to 45 bar (650 psi), said Banzhaf. It has been certified to the rigorous AZ/NZS 1425:2007 standards.
The filter retains 92% of particles down to 5 microns and above 99% of particles down to 10 microns—nearly the fineness required for a modern common rail diesel injection system filter, Banzhaf noted.