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Introduction
An HCCI engine ignites an air/fuel mixture by compressing it in the cylinder at low temperature, instead of using a spark plug. The ignition is spontaneous and does not propagate as a flame front. In a gas engine, the ignition is generated from a spark plug, and in a diesel engine, there are multiple flame fronts.
The concept of HCCI technology was introduced three decades back, but advancements took place in the past couple of years when major automakers and research groups across the world started working on building prototypes.
Why HCCI?
HCCI's primary advantage is more favorable fuel consumption, or better MPG, and generating inherently low emissions. The power produced in an HCCI engine is similar to spark ignition (SI) gasoline engines.
An HCCI engine, when combined with other advanced technologies, can provide up to 15 percent greater fuel economy than a comparable, non-HCCI engine by using the compression combustion process that operates at a much lower temperature than diesel combustion.
HCCI combustion takes place at lower temperatures helping to reduce NOx emissions. A diesel HCCI set up emits 30 percent of the NOx that a conventional diesel engine emits, which reduces the necessity of exhaust aftertreatment. By comparison, gas engines have inherently lower NOx emissions, and therefore still more aftertreatment is required for diesels to reduce NOx to the level of gasoline engines.
The technology is compatible with a wide variety of fuels, such as biofuels, diesel, gasoline, and E85 (ethanol). GM's findings reveal that ethanol is a suitable fuel for auto ignition that offers high octane with no smoke-producing tendency. In addition, ethanol produces easier combustion with low carbon accumulation.
Challenges
Despite several advantages, this technology needs to address the following challenges:
The biggest challenge in HCCI technology is controlling the temperature, pressure, and air/fuel mixture quantity to manage the combustion process efficiently without noise or imparting any damage to the engine.
An HCCI engine is best used with individual valve actuators; in other words, it is a camless valve train with each valve infinitely variable in timing and lift. However, this is an expensive, complex, and difficult-to-control technology. Therefore, camless engine technology has not made a mark in the market place.
Difficulty in controlling ignition timing and combustion duration
Has a narrow operating window under light to moderate loads. Can produce noise at high loads when transitioned from in and out of auto ignition, and causes misfire or partial burn at low loads
Vulnerable to temperature variations across the cylinders
Diesel HCCI technology needs to address the NOx emission issue, as EPA regulations are enforced for large and small diesel engines
Currently, auto ignition in HCCI technology occurs only at 4 to 5 bar, or 60 psi BMEP. But a modern engine at wide open throttle should operate at 200-300 psi.
Gasoline HCCI Engine
GM engineers say that gasoline HCCI engines will have 15 percent greater fuel efficiency than comparable conventional gasoline engines making the same HP. GM's first-generation HCCI production unit, likely introduced by 2012, is expected to start operating at stoichiometric mixtures and spark ignition mode at very light loads. It will then transition to HCCI mode at higher loads and intermittently run in spark ignition mode.
Compression ratios can be varied by valve timing in HCCI and modern gasoline engines. There are mechanical means of producing different compression ratios, by a variable length piston stroke, but these are very complex.
Diesel HCCI Engine
Diesel engines can achieve a 20 to 30 percent greater fuel economy than gasoline engines of comparable HP because of a higher compression ratio needed to ignite the air/fuel mixture and reduced throttling losses. Therefore, diesel HCCI engines could be more fuel efficient than gasoline HCCI engines. Also, diesel HCCI produces low NOx at low PM, whereas in a conventional diesel engine there is a trade off between NOx and PM.
In addition to a higher compression ratio, higher thermal efficiency, and reduced throttling losses, a diesel HCCI will generate greater mileage from the high volumetric energy density of the fuel; this adds a 10 -12 percent improvement in MPG just from the fact that diesel fuel is more carbon-rich.
HCCI is also referred to as “controlled auto ignition,” but experimenters have found that auto ignition is possible only at low to moderate loads. As the load increases, it switches over to spark ignition, which does not work with diesel fuel. Thus, a different fuel is required for an HCCI engine to function effectively. Octane gasoline is possibly the right fuel and experimenters are aiming to run HCCI engines with a conventional diesel fuel.
One drawback is the fact that diesel engines are roughly three times more expensive than gasoline. To combat this, and net the best of both worlds, an HCCI technology is being researched at GM that could combine the characteristics of both the gasoline and diesel engine.
Where is the HCCI technology heading?
As part of GM's August 2007 Powertrain technology show, the company brought a Saturn Aura and an Opel Vectra, each equipped with a 2.2L HCCI EcoTec four-cylinder engine. Initially, this engine could work in HCCI mode, but only for a short period of time. Thereafter, it continued to work in the basic spark ignition mode. The transition from the HCCI mode to spark ignition was accompanied with a ringing sound for two power cycles post transition.
In 2009, a mixed-mode HCCI engine with an external EGR system was developed that is capable of operating from idle up to 60 mph. Vehicles with fuel-efficient HCCI engines will likely be available at a much lower cost than a hybrid vehicle. GM forecasts low volume production of HCCI engines by 2012.
HCCI promises to be the cleanest gasoline engine. GM has further broadened the operating range of its 2.2L Ecotec HCCI engine currently being tested on a Chevrolet Malibu. This engine's speed/load map in HCCI mode now extends down to idle (800 rpm), which is around 200 rpm lower than its previous HCCI test. The upper limit is confined within 3000 rpm, as it produces higher combustion noise. More work is necessary in this area.
HCCI technology could be an advantageous option for extended-range electric vehicles such as the upcoming Chevrolet Volt, during part-load operation when the ICE is charging the battery.
The electronics within HCCI are considered to be the most sophisticated within the internal combustion engine family. The HCCI engine will have complete onboard diagnostics capability and assured 150,000-mile emissions compliance.
HCCI engines need to have an in-cylinder pressure sensor for each cylinder. Today, all engine prototypes have lab grade pressure sensors that cost around $1,000 each. These sensors are accurate but are not designed for a long life. So the key to commercialization is to have one in-cylinder pressure sensor that will last for the life of the engine.
The HCCI engines need to be properly controlled. As load becomes uneven, the HCCI cylinder needs to adjust accordingly. Researchers are in the process of developing sensors and controllers that will keep temperatures constant throughout the engine or compensate for temperature differences by adjusting pressure ratios within the cylinders.
Coordinating ignition in the cylinders is a challenge. This is because they are at different temperatures, and although they may have a one percent difference in their compression ratios, the difference greatly increases when a high volume of air/fuel mixture is compressed. In addition to temperature variations and different compression ratios, the other factors that affect combustion timing are the air:fuel ratio and intake pressure. These factors need to be synchronized to increase power and efficiency. Researchers are trying to design sensors that will communicate cylinder pressure and temperature to the engine. As soon as the engine knows that some of its cylinders need to be adjusted, either in temperature or pressure, in line with their neighbors, it will take steps to re-adjust them.
The EGR flow needs to be closely controlled in HCCI engines. At some point within the engine operating range, up to 60 percent of the cylinder charge is unburnable EGR. So, if so much oxygen is displaced, engine power will reduce but emissions output should be low as well. At low operating load, this is not a problem.
Conclusion
Technical breakthroughs are needed. Despite the company's financial crisis, GM prioritized the HCCI technology and is speedily moving towards production. It can be anticipated that gasoline HCCI is not likely to be ready for the market by 2012, and maybe even 2015. However, diesel-fuel HCCI or HCCI-like diesel engines (with no SI assist) will possibly be introduced by 2012.
An HCCI engine ignites an air/fuel mixture by compressing it in the cylinder at low temperature, instead of using a spark plug. The ignition is spontaneous and does not propagate as a flame front. In a gas engine, the ignition is generated from a spark plug, and in a diesel engine, there are multiple flame fronts.
The concept of HCCI technology was introduced three decades back, but advancements took place in the past couple of years when major automakers and research groups across the world started working on building prototypes.
Why HCCI?
HCCI's primary advantage is more favorable fuel consumption, or better MPG, and generating inherently low emissions. The power produced in an HCCI engine is similar to spark ignition (SI) gasoline engines.
An HCCI engine, when combined with other advanced technologies, can provide up to 15 percent greater fuel economy than a comparable, non-HCCI engine by using the compression combustion process that operates at a much lower temperature than diesel combustion.
HCCI combustion takes place at lower temperatures helping to reduce NOx emissions. A diesel HCCI set up emits 30 percent of the NOx that a conventional diesel engine emits, which reduces the necessity of exhaust aftertreatment. By comparison, gas engines have inherently lower NOx emissions, and therefore still more aftertreatment is required for diesels to reduce NOx to the level of gasoline engines.
The technology is compatible with a wide variety of fuels, such as biofuels, diesel, gasoline, and E85 (ethanol). GM's findings reveal that ethanol is a suitable fuel for auto ignition that offers high octane with no smoke-producing tendency. In addition, ethanol produces easier combustion with low carbon accumulation.
Challenges
Despite several advantages, this technology needs to address the following challenges:
The biggest challenge in HCCI technology is controlling the temperature, pressure, and air/fuel mixture quantity to manage the combustion process efficiently without noise or imparting any damage to the engine.
An HCCI engine is best used with individual valve actuators; in other words, it is a camless valve train with each valve infinitely variable in timing and lift. However, this is an expensive, complex, and difficult-to-control technology. Therefore, camless engine technology has not made a mark in the market place.
Difficulty in controlling ignition timing and combustion duration
Has a narrow operating window under light to moderate loads. Can produce noise at high loads when transitioned from in and out of auto ignition, and causes misfire or partial burn at low loads
Vulnerable to temperature variations across the cylinders
Diesel HCCI technology needs to address the NOx emission issue, as EPA regulations are enforced for large and small diesel engines
Currently, auto ignition in HCCI technology occurs only at 4 to 5 bar, or 60 psi BMEP. But a modern engine at wide open throttle should operate at 200-300 psi.
Gasoline HCCI Engine
GM engineers say that gasoline HCCI engines will have 15 percent greater fuel efficiency than comparable conventional gasoline engines making the same HP. GM's first-generation HCCI production unit, likely introduced by 2012, is expected to start operating at stoichiometric mixtures and spark ignition mode at very light loads. It will then transition to HCCI mode at higher loads and intermittently run in spark ignition mode.
Compression ratios can be varied by valve timing in HCCI and modern gasoline engines. There are mechanical means of producing different compression ratios, by a variable length piston stroke, but these are very complex.
Diesel HCCI Engine
Diesel engines can achieve a 20 to 30 percent greater fuel economy than gasoline engines of comparable HP because of a higher compression ratio needed to ignite the air/fuel mixture and reduced throttling losses. Therefore, diesel HCCI engines could be more fuel efficient than gasoline HCCI engines. Also, diesel HCCI produces low NOx at low PM, whereas in a conventional diesel engine there is a trade off between NOx and PM.
In addition to a higher compression ratio, higher thermal efficiency, and reduced throttling losses, a diesel HCCI will generate greater mileage from the high volumetric energy density of the fuel; this adds a 10 -12 percent improvement in MPG just from the fact that diesel fuel is more carbon-rich.
HCCI is also referred to as “controlled auto ignition,” but experimenters have found that auto ignition is possible only at low to moderate loads. As the load increases, it switches over to spark ignition, which does not work with diesel fuel. Thus, a different fuel is required for an HCCI engine to function effectively. Octane gasoline is possibly the right fuel and experimenters are aiming to run HCCI engines with a conventional diesel fuel.
One drawback is the fact that diesel engines are roughly three times more expensive than gasoline. To combat this, and net the best of both worlds, an HCCI technology is being researched at GM that could combine the characteristics of both the gasoline and diesel engine.
Where is the HCCI technology heading?
As part of GM's August 2007 Powertrain technology show, the company brought a Saturn Aura and an Opel Vectra, each equipped with a 2.2L HCCI EcoTec four-cylinder engine. Initially, this engine could work in HCCI mode, but only for a short period of time. Thereafter, it continued to work in the basic spark ignition mode. The transition from the HCCI mode to spark ignition was accompanied with a ringing sound for two power cycles post transition.
In 2009, a mixed-mode HCCI engine with an external EGR system was developed that is capable of operating from idle up to 60 mph. Vehicles with fuel-efficient HCCI engines will likely be available at a much lower cost than a hybrid vehicle. GM forecasts low volume production of HCCI engines by 2012.
HCCI promises to be the cleanest gasoline engine. GM has further broadened the operating range of its 2.2L Ecotec HCCI engine currently being tested on a Chevrolet Malibu. This engine's speed/load map in HCCI mode now extends down to idle (800 rpm), which is around 200 rpm lower than its previous HCCI test. The upper limit is confined within 3000 rpm, as it produces higher combustion noise. More work is necessary in this area.
HCCI technology could be an advantageous option for extended-range electric vehicles such as the upcoming Chevrolet Volt, during part-load operation when the ICE is charging the battery.
The electronics within HCCI are considered to be the most sophisticated within the internal combustion engine family. The HCCI engine will have complete onboard diagnostics capability and assured 150,000-mile emissions compliance.
HCCI engines need to have an in-cylinder pressure sensor for each cylinder. Today, all engine prototypes have lab grade pressure sensors that cost around $1,000 each. These sensors are accurate but are not designed for a long life. So the key to commercialization is to have one in-cylinder pressure sensor that will last for the life of the engine.
The HCCI engines need to be properly controlled. As load becomes uneven, the HCCI cylinder needs to adjust accordingly. Researchers are in the process of developing sensors and controllers that will keep temperatures constant throughout the engine or compensate for temperature differences by adjusting pressure ratios within the cylinders.
Coordinating ignition in the cylinders is a challenge. This is because they are at different temperatures, and although they may have a one percent difference in their compression ratios, the difference greatly increases when a high volume of air/fuel mixture is compressed. In addition to temperature variations and different compression ratios, the other factors that affect combustion timing are the air:fuel ratio and intake pressure. These factors need to be synchronized to increase power and efficiency. Researchers are trying to design sensors that will communicate cylinder pressure and temperature to the engine. As soon as the engine knows that some of its cylinders need to be adjusted, either in temperature or pressure, in line with their neighbors, it will take steps to re-adjust them.
The EGR flow needs to be closely controlled in HCCI engines. At some point within the engine operating range, up to 60 percent of the cylinder charge is unburnable EGR. So, if so much oxygen is displaced, engine power will reduce but emissions output should be low as well. At low operating load, this is not a problem.
Conclusion
Technical breakthroughs are needed. Despite the company's financial crisis, GM prioritized the HCCI technology and is speedily moving towards production. It can be anticipated that gasoline HCCI is not likely to be ready for the market by 2012, and maybe even 2015. However, diesel-fuel HCCI or HCCI-like diesel engines (with no SI assist) will possibly be introduced by 2012.
