The Good and The Bad About LPG Vehicle Fuels

BELL PERFORMANCE FUEL ISSUES SERIES: THE GOOD AND THE BAD ABOUT LPG

EXECUTIVE SUMMARY

LPG in the United States is used mostly in home/industrial heating and cooking systems and large municipal transportation fleets. In both applications, LPG offers cleaner combustion at an inexpensive fuel cost. Although it is a much cleaner burning fuel than diesel or gasoline, contrary to long-held belief, LPG is not entirely without its fuel-related performance issues.

INTRODUCTION

LPG, CNG and natural gas are all gaseous fuels composed of some of the same components – propane and butane. LPG and “propane” are terms used interchangeably in the United States, but tend to mean the same thing – a blend of 70% propane and 30% butane, with a few trace ingredients added (like a mercaptan-type smell agent so you can smell a gas leak). There can be minor differences in LPG blends sold between different states, including California, which has a couple of different specifications to dictate the ratio of propane and butane in the overall gas.

In crude oil refining, LPG is the first component that comes off the distillation tower, so it is always produced when crude oil is refined to make gasoline and diesel. At normal temperatures, LPG is a gas but is usually cooled and compressed into cylinders for storage. Compressed LPG has an energy value 270 times of the same volume of gaseous LPG.

LPG use is more popular in Europe and Asia than in the United States, as foreign consumers tend to be more ahead of the curve than US consumers for environmental concerns and the distribution system is more developed. Use of LPG in the United States is confined mainly to home heating systems, industrial applications (like forklifts and industrial boiler power generation), large municipal bus and transportation fleets, and small consumer product use like gas grills. LPG-powered transportation (cars, trucks and scooters) is much more common in Europe and Asia.

For the United States to catch up in LPG use, distribution infrastructure will have to greatly improve – it is much more common to find an LPG filling station in Europe than it is in the United States. Expanding third world countries like China (now the 2nd largest economy in the world, passing Japan) and India rely heavily on LPG to fuel the transportation needs of their citizenry, as well as to satisfy the growing global pressure to be more environmentally friendly. In response to this pressure, you have the Chinese centralized government mandating a 25% reduction in pollution emissions within the next five years. While the US has gravitated towards ethanol and biodiesel renewable fuel blends to meet these demands, LPG fuels are central to plans to meet the mandates in these foreign countries experiencing explosive growth. GOOD POINTS ABOUT LPG FUELS

LPG molecules are small – 3 and 4-carbon chains compared to the 8-18+ carbon-chain molecules found in gasoline and diesel – and burn very cleanly compared to gasoline and diesel. This better combustion means less carbon buildup in the engine and longer life for both spark plugs and lubricating oil. This is a plus for all kinds of consumers who need to have maximum fuel value from their vehicles, large and small.

Fuel-combustion emissions are also pretty low for LPG compared to the popular automotive fuels – unburned hydrocarbon and particulate emissions are very very low, as are sulfur gas emissions and NOx (nitrogen oxide) emissions, which are both precursors for smog in polluted urban areas. Evaporative emissions (such as what happens when fuel is dispenses – some of the fuel escapes into the air) are very low due to the necessary closed LPG fuel delivery systems at dispensing stations.

LPG has a lower gross sulfur content than gasoline, reducing the potential for corrosion in storage systems.

These facts make LPG a favorite fuel for environmentally-conscious consumers, including local governments who face mandates from their constituency and higher up in the governmental ladder to be more “green”. All of this adds up to the fact that LPG fuels are great for the environment, apart from the carbon emissions they generate in the form of CO2.

LPG FUEL ISSUES

Because LPG is so clean burning most consumers and even some gas fuel distribution professionals are unaware that LPG fuels aren’t without their fuel-related problems or room for improvement.

Vehicle Fuel Availability in the United States

Very few LPG-only consumer vehicles are sold in the USA each year. You can purchase conversion kits that will turn your regular gasoline-powered vehicle into a dual flex-fuel vehicle that can switch between gasoline and LPG. Conversion kits cost $2000-$4000. Unfortunately, even if you choose to do this, the simple availability of vehicle LPG fueling stations is a prohibitive problem in the US for further expansion of this kind of green fuel. California has the most vehicle “propane” filling stations – about 600 of them. Across the nation, only 3% of the LPG used is sold for vehicle use.

Mileage Range for Vehicles

Consumers in the United States who do have flex-fuel LPG vehicles can face mileage range issues with their LPG tanks. Remember that compressed LPG has a different density than gasoline or diesel. A typical LPG car like the converted 2008 Honda Civic can get as much as 36 mpg. That’s great mileage. Unfortunately, because of the nature of LPG compression, despite the 36 mpg (24 mpg city), the Civic can only go about 185 miles on a full tank of LPG fuel.

Deposit Buildup from Refinery-Sourced Impurities

LPG taken straight from the original source (the ground) is a clean mixture of propane, butane and other hydrocarbon gases and does not contain double-bonded olefin impurities like propene (propane with a double-bond). The pure LPG fuel without impurities burns very cleanly to produce tremendous heat energy (21,000 BTUs per pound) with a minimum of deposits and harmful emissions like carbon monoxide.

Unfortunately a portion of the LPG typically available in the marketplace has been produced through cracking methods at a refinery (to maximize the more profitable products of gasoline and diesel) and contains unstable molecules called alkenes and olefins. Cracking processes involve the chemical splitting of longer molecules into shorter ones – the refinery will split the heaviest molecules (like heavy fuel oil) to create more gasoline and diesel. While doing this, additional supplies of LPG are produced alongside this. But these additional yields of LPG are not pure LPG fuel, because they contain cracked molecules from being part of the other cracking process.

These cracked LPG fuels contain alkenes (molecules with reactive un-saturated double-bonds) which tend to react with each other, water and other molecules from the fuel and environment (S, O, N) to produce longer-chain polymers, which end up as heavy-end deposits. These unstable molecules contain double-bonds which cause them to react with other molecules in the fuel and the surrounding environment (including oxygen and sulfur) to form long polymer molecules. These deposits, because they are longer chains, are heavier and do not dissolve well in LPG fuels, as propane is a poor solvent for such species. Hence they will precipitate out of the fuel before and during vaporization. Once this happens, these deposits cannot be re-dissolved into the fuel.

These heavy deposits will build up in vaporizers and converters, the vaporizer lines, the fuel injectors and injector metering orifices and the burners. This is true whether it’s a vehicle or a home heating system powered by propane. Due to the heavy nature of the deposits, they tend to stay and build up in the areas where they first fall.

The Effect of LPG Deposits on Performance

Once these areas experience deposit buildup, it affects the fuel flow rate and the air/fuel mixing, making the system much less efficient and raising the emissions produced. You can also see a progressive reduction in the fuel efficiency of the system, whether heating or automotive. Not only is this because deposits cause the fuel to burn less efficiency, but deposits can also have a sponge-like effect, soaking up LPG fuel and releasing it more slowly over time, which deviates from how the system was designed to operate.

In a vehicle that burns LPG for fuel, deposit buildup from olefins causes the octane requirement by the engine to increase. This is because these deposits also build up in the combustion chamber and the cylinder spaces, changing the volume within the cylinder, which is partially what determines octane requirement for correct firing of the fuel at the correct cylinder position. LPG fuel typically has a very high octane rating, 96-100. So the effect of deposits on this rating is actually amplified and greater in an LPG-burning engine than a regular gasoline engine designed to perform well on 87 or 89 octane gasoline.

A system with deposit buildup may typically experience difficulty starting up in cold weather. As deposits build up in a furnace or vehicular system, they start to affect how well the fuel vaporizes and how well it flows through the fuel delivery system. This is especially an issue with LPG in the winter, which must start up immediately. These kind of deposits will hinder how well this fuel vaporizes and performs in cold weather. This can be an issue if the fuel vaporizer gets fouled with deposits – the fuel doesn’t vaporize well enough to perform as it should.

Equipment Wear

When used in vehicular applications (cars and trucks), LPG proves to be a dry fuel that doesn’t provide the same kind of lubrication for critical fuel delivery parts that liquid fuels can. Over time, LPG drivers tend to find excessive wear on certain critical engine parts. Valves, injectors and compression rings can be the most common parts affected.

In furnace systems, some built-up deposits can be abrasive and may break off in the turbulent air flow of the fuel delivery system. When this happens, they can wear on metal surfaces and cause damage. This typically happens slowly over a long period of time, but can end up leaving you with costly repair bills for system maintenance.

To prevent excessive wear on vehicular and industrial or home furnace system parts, it can be beneficial to treat the LPG fuel with a lubricant. Water and Moisture Collection

Contrary to perception, water and moisture can get into the LPG fuel via the supply chain. When this happens, fuel fouling can result as the fuel undergoes oxidation while reaction with the oxygen in the water. Oxidation reactions cause the pure propane/butane molecules to react and polymerize (stick together), forming heavy deposits that can sink to the bottom of the tank or be carried with the LPG fuel into downstream storage containers where they end up accumulating and causing deposits in the vehicles and furnace systems that ultimately burn the fuel.

CONCLUSION

LPG and other compressed gases are excellent choices for consumers looking for a clean fuel that is relatively friendly for the environment, burning with low emissions and soot output. Vehicles running on LPG fuel (if you can find them) have low emissions and get high miles per gallon (although the volume of LPG which can be stored in a vehicle is less than a gasoline or diesel fuel tank capacity). LPG’s problems are pretty minor compared to biodiesel or ethanol and can be resolved pretty easily if the fuel supplier additizes the LPG fuel.