Introduction
With the rising desire to attain more miles per gallon of fuel, car manufacturers are now focusing their attention on lean burn engine technologies. On its part, the American government demonstrated its support for lean burn engine technology when it ratified the Energy Policy Act of 2005.
In essence, this act qualified cars, trucks, and sports utility vehicles that had engines using the lean burn technology for tax rebates amounting to as much as $2000 under the stipulation of that act. This research paper will examine the technology behind lean burn engines, its impact on the environment and its future prospects (Tobias, et al. 2006).
The first lean burn technology for automobile engines was introduced in the market nearly four decades ago by Chrysler. The technology comprised of a set of sensors and electronics that would generally regulate the movement of the spark plug under a range of driving circumstances. This technology was meant to increase the performance of the engine and utilize the fuel used per mile.
Initially, the automotive challenge that the inventors of lean burn technology wanted to overcome was the throttling losses. Naturally, an automobiles car must have enough space to provide the required power for acceleration but still be able to operate below its possible output when driving at high speed and this was hard to achieve using the lean burn technology (Tobias, et al. 2006).
At the present, a large percentage of cars utilize a form of technology commonly known as the four-stroke combustion cycle to convert fuel into motion. This kind of technology is also referred to as the Otto cycle, in respect of Nikolaus Otto, who discovered it in 1867. The main problem of this kind of engine technology is that it consumes much fuel and environmentalists consider it harmful to the ozone layer.
On its part, the lean burn engine technology utilizes refined ignition systems along with advanced emission technologies. This combination allows cars using this technology to attain excellent mileage performance and causes minimal threat to the environment (Tobias, et al. 2006).
Lean Burn Engine Technology
Currently, internal combustion engines that use natural gas are commanding more attention as interest grows in manufacturing automobile engines that are both efficient and environmentally friendly. In response to this interest, automobile manufacturers have introduced natural gas automobile engine sets that feature “lean-burn” technology.
The technology is referred to as lean because unlike the convectional engine technology it uses excess air along with the fuel. The use of this kind of technology has brought about two positive effects.
First, the excess air used in lean burn technology reduces the temperature of the combustion process and this lowers the amount of oxides of nitrogen (NOx) produced by nearly half, compared to a conventional gasoline engine. Second, since there is also excess oxygen available, the combustion process is more efficient and more power is produced from the same amount of fuel (Tobias, et al. 2006).
The Combustion Process
Any air/fuel reaction requires an energy source to initiate combustion. In the conventional petroleum engines, the combustion process is initiated by the spark plug. In lean-burn gas engines, the combustion process is initiated by pre-mixing the air and fuel upstream of the turbocharger before being introduced into the cylinder.
This creates a more homogenous mixture in the combustion chamber and reduces the occurrence of “knocking” or detonation. To prevent either knocking or misfiring, the combustion process must be controlled within a narrow operating window.
In lean burn engine technology, the charge air temperatures together with air to fuel ratio are closely monitored. This is done by the microprocessor- based engine controller, which regulates the fuel flow and air/gas mixture and ignition timing (Cutter Information Corp., 1992).
Unlike the conventional petroleum engine, the design of the lean-burn engine incorporates a simple open combustion chamber housed in the piston crown.
The shape of the piston crown introduces turbulence in the incoming air/fuel mixture that promotes complete combustion by thoroughly exposing it to the advancing flame front. The flame plate of the cylinder head is regular (flat) and the spark plug is centrally located. The air and gas fuel are correctly mixed under the control of the engine management system (Cutter Information Corp., 1992).
Improving efficiency of the System
Although lean burn engines that use natural gas have been in existence for some time now, there is room for improvement in terms of energy efficiency and emissions reduction. In fact, enhancing the efficiency of lean burn gas engines remains one of the most likely and cost-effective approaches to enhancing vehicle fuel economy over the next three decades.
According to automobile experts, the United States has the potential of reducing its transportation fuel usage by as much as 40% through the production of engines that use the lean burn gas technology (Automotive News, 1992). If this is achieved, it will definitely lead to better economic, environmental, and energy security.
By using lean burn engine technology in hybrid and plug-in hybrid electric vehicles, it is possible to attain even greater fuel saving benefits (EIA, 2008).
Ideally, one way of increasing the efficiency of the lean burn engine technology is by focusing on complementary technology pathways involving shared partnerships with vehicle and engine manufacturers, suppliers, national laboratories, and learning institutions.
Another strategy that can increase the efficiency of automobile engines using the lean burn technology is by inventing new combustion energy than the currently available natural gas usage.
However, this should have minimal emissions in order to enable the engines to meet the required emission regulations without having to use the complex and costly equipment that is currently required. The invention of a cheap technology would definitely ensure that more people buy vehicles fitted with lean burn engines since the current costs have been a hindrance to many people (EIA, 2008).
Ideally, one way of achieving all these strategies is by having well balanced research and development efforts ranging from basic research to prototype demonstration. This should be informed at every stage by industry collaborators whose role is to help in identifying the critical barriers to the commercialization of this technology.
The existing public laboratories and universities should also increase their efforts in understanding how the combustion process can be enhanced to maximize its performance and keep emissions at minimal (Pollack, 1992).
Environmental Impact of Lean Burn Engine Technology
Motor vehicles continue to be the dominant source of air pollution despite tremendous advances in engine technology and pollution control. In industrialized countries, even as cleaner vehicles are replacing older ones, and as transportation emissions are beginning to decline, vehicles are still the major source of air pollution.
Meanwhile, in the developing world, vehicle numbers are growing exponentially and without strict control standards in place, emissions from transportation sources are becoming an increasingly urgent concern (Tobias, et al. 2006).
There have been repeated calls for gasoline vehicles to raise fuel economy, and thus decrease CO2 emissions. Lean-burn engine designs that use natural gas reduce fuel consumption by 15–20%. In order not to trade off higher fuel efficiency for increased pollutant emissions, lean-burn engine designs will require new aftertreatment technology for control of Nitrogen Oxides (NOx) emissions.
NOx storage traps, the most efficient existing NOx control technology for lean-burn engines, are much more dramatically impacted by fuel sulfur than other form of engine technologies. Because higher sulfur levels reduce the effectiveness of the traps and necessitate increased fuel consumption, ultralow sulfur fuel is the key enabler for increasing the efficiency of vehicles using the lean burn engine technology (Tobias, et al. 2006).
The lean-burn engine increases the ratio of air to fuel, thus reducing fuel use. Lean-burn engines provide an automatic benefit for CO and HC control, which are formed in smaller amounts and can be more easily oxidized in the oxygen-rich exhaust.
The challenge comes with control of NOx in an oxygen-abundant environment. NOx storage traps face fewer technical challenges in natural gas engines than in diesel engines because combustion temperatures are easier to control. NOx storage traps demonstrate over 90% efficiency in storage and conversion of NOx to N2, but require virtually sulfur-free fuels for efficient use and this has not yet been achieved (Tobias, et al. 2006).
Storage traps operate by incorporating basic oxides into the catalyst, which in turn reacts with the oxidized NO2 in the presence of excess O2 to form fairly stable nitrates. NOx can be stored in this way during lean combustion (excess oxygen) conditions.
As the storage medium approaches saturation, or whenever acceleration occurs, the engine will burn fuel-rich, generating CO and HC gases. This triggers the release of NO2, which reacts, as in a three way canisters to oxidize CO and HC to CO2 and H2O while simultaneously being reduced to N2 (Tobias, et al. 2006).
One of the results of this technology is significantly reduced emissions in the exhaust. Most of the new lean-burn automobile engines have NOx emissions as low as.85 grams/BHP-hr, and produce low amounts of hydrocarbons (HC), carbon monoxide (CO) and particulate matter (PM).
This allows the engine sets to meet the most stringent air quality regulations without after-treatment devices in the exhaust stream. For even lower emissions, lean-burn automobile engine sets are frequently coupled with integrated after-treatment options such as Selective Catalytic Reduction (SCR) and Oxidation Catalysts, resulting in NOx levels at or below 0.15 grams/BHP-hr.
With these after-treatment options, the lean gas automobile engines have been shown to meet the most stringent prime power emissions regulations anywhere in the world (Tobias, et al. 2006).
As it has already been noted, the lean burn engine technology will enable huge leaps in fuel efficiency, translating directly into reductions in CO2 emissions in automobiles. If properly utilized, this technology could dramatically reduce both greenhouse gas and conventional pollutant emissions.
Additionally, research continues on how to make zero-emission transportation technologies, such as the hydrogen fuel cell economically viable. Sulfur is a major obstacle for technologies to both reduce and eliminate greenhouse gas emissions. The reduced fuel use in lean burn engine technology means that low sulfur is generated and this is definitely a good thing for the environment (Johnson, 1992).
Sulfur levels in fuel have a range of direct and indirect impacts on greenhouse gas emissions. Sulfur prevents the efficient functioning of certain types of catalysts, which in turn translates into higher methane emissions from oxidation catalysts and higher CO2 emissions from more advanced technologies.
Sulfur also impedes the viability of emissions control technologies in several arenas. In addition, fuel cells—the most promising long-term solution for eliminating greenhouse gas emissions—will require sulfur-free fuels in order to function. This goal is attainable if the use of lean burn engine technology in automobiles is fully adopted (Johnson, 1992).
Fuel flexibility
Another advantage of the lean-burn engine technology with full-authority electronic engine controls is the ability to operate on gas with a wide range of quality. A measurement called the Methane Number (MN) is used to determine gas suitability as an engine fuel. Most natural gas has an MN from 70 to 97, and pipeline quality gas typically has an MN of about 75.
Resource recovery gas from landfills or sewage treatment facilities is typically of lower quality, but is often suitable for use in lean-burn engines. Most of the lean-burn automobile engines operate on gas with an MN of 50 or greater, providing excellent fuel flexibility.
However, gas with a MN below 70 may require the engines output to be derated. Lean-burn gas engine sets are setting a new standard for fuel efficiency, high power output for their size, and for low emissions.
In regions with supplies of natural gas, these engine sets are providing highly reliable electric power for utility peaking, distributed generation, prime power and for combined heat and power systems (MacKenzie, Roger, & Donald, 1992).
The Future Prospects of Lean Burn Technology
Currently, there is an increased interest in lean-burn engine technologies, i.e. lean-burn natural gas engines, mainly due to their higher fuel efficiency compared to conventional petroleum engines (Greene, 2005). These engines work under excess oxygen and consequently produce oxygen-rich exhaust.
However, effectively reducing NOx in oxygen-rich exhaust is a challenging endeavor because the conventional three-way catalyst technology is not able to reduce NOx efficiently under any circumstances. Therefore, new catalytic systems have to be developed. Several approaches have been suggested and among the most promising is the so-called NOx storage reduction (NSR) concept.
Because of the three-way catalyst, the emission of NOx by natural gas engines has decreased significantly. However, this gain is counteracted by the continuing rise in vehicle numbers and distances traveled, particularly by the increase in the number of diesel engines that do not use the lean burn engine technology.
The latest quarterly pricing survey by PricewaterhouseCoopers (Greene, 2005) shows that cars using the lean burn engine technology accounted for 49% of the total European car market at the end of 2005. It is expected that the number of cars using the lean burn engine technology will continue to grow, potentially achieving 55-60% of the total market.
There are several reasons for the increased interest in lean burn engines that use natural gas. The lean-burn combustion of natural gas engines results in higher fuel efficiency compared to conventional petroleum engines and consequently significantly lower amounts of the greenhouse gas CO2.
In addition, natural gas has enhanced performance regarding CO and HC. Most governments in Westerns countries have been encouraging the use of lean burn engine technology. Tax regimes make natural gas cheaper than petroleum in order encourage the sales of lean burn technology engines (MacKenzie, Roger, & Donald, 1992).
The increased interest in lean-burn engine technology has prompted research on the development of a new catalyst that is capable of reducing NOx in excess O2 to meet future legislation. Selective catalytic reduction (SCR) of NOx using ammonia (NH3) or urea (CO (NH2)2) is a well-known process in industry as well as in stationary lean burn engine applications.
In the presence of O2, NH3 tends to react with NOx to form N2. Urea, which is safer in use than NH3, can be used as NH3 source and urea-based systems are already applied for heavy-duty trucks (Sanger, 2001). Higher fuel efficiency and reduced emissions of the greenhouse gas CO2 make lean-burn engines attractive compared to conventional petroleum engines.
However, controlling the exhaust NOx emission has been recognized as one of the most challenging aspects for lean-burn engine technology as the conventional three-way catalyst is not effective in reducing NOx in a lean exhaust due to the high oxygen level (Sanger, 2001).
A NOx storage/reduction (NSR) catalyst is among the most promising solutions to control NOx in lean exhaust. The NSR catalyst contains a storage component in order to store NOx under lean conditions. Catalyst regeneration is necessary as the storage capacity of the absorbing component gets saturated (Brunekreef, et al, 2010).
Regeneration takes place by the introduction of a short period of rich driving, as injections of extra fuel cause decomposition of stored NOx and subsequent reduction into N2. A critical aspect of the NSR technology is the catalyst deactivation by sulfur. A detailed understanding about the NOx storage and reduction mechanism is important for improving catalyst regeneration times and preventing catalyst deactivation.
There has also been increased funding on research to ensure that commercialization of lean burn engine technology will not produce unintended human health effects. The already conducted research on this field has revealed that lean burn engines using low sulfur (less than 15-ppm sulfur) fuel and equipped with catalyzed particulate matters does not cause lung inflammation or pose other health hazards.
This shows that the future prospects of lean burn engine technology is indeed bright just by looking at its environmental and financial factors and the increased funding from governments that want to develop an environmentally friendly ecosystem (Brunekreef, et al, 2010).
Conclusion
With the increased desire to gain more miles per gallon of fuel, car manufacturers are turning their attention to lean burn engine technologies. Over the last few years, the number of vehicles using engines made from lean burn technology has been on the rise. This has been enhanced by the efforts of various governments, which have been offering tax cuts to those purchasing cars fitted with this kind of engine technology.
Although there are numerous challenges that have prevented this technology from being fully adopted, current trends in the automobile industry show that these challenges can be overcome in the course of time. With most governments and organizations keen on preserving the environment, there is no doubt that the use of lean burn engine technology will gain more popularity in the coming years.
References
Automotive News. (1992). Cleaner Civic OK in California. Automotive News 19 (2), 8.
Brunekreef, B., et al. (2010). Air Pollution from Truck Traffic and Lung Function in Children Living Near Motorways. Epidemiology 8 (1), 298–303.
Cutter Information Corp. (1992). Lean Burn Takes a Step Forward. Global Environmental Change Report, 7, (1), 7.
EIA. (2008). The Transition to Ultralow-Sulfur Diesel Fuel: Effects on Prices and Supply. Washington: Energy Information Administration.
Greene, D. (2005). A Note on Implicit Consumer Discounting of Automobile Fuel Economy: Reviewing the Available Evidence. Transportation Research 17 (6), 491-500.
Johnson, R. (1992). Next Accord May Offer New Engine. Automotive News, 17 (1), 231-240.
MacKenzie, J., Roger, D. and Donald, C. (1992). The Going Rate: What it Really Costs to Drive. Washington: World Resources Institute.
Pollack, A. (1992). Running Half-Engine to Save Fuel. NY Times, 18 November, p. 16.
Sanger, D. (2001). Fuel Efficiency: New Japan Coup? New York Times, July 31, p. C1.
Tobias, H., et al. (2006). Chemical Analysis of Diesel Engine Nanoparticles Using a Nano-DMA/Thermal Desorption Particle Beam Mass Spectrometer. Environmental Science & Technology 35 (2), 2233–2243.