HCCI is an alternative piston-engine combustion process that can provide efficiencies as high as compression-ignition, direct-injection (CIDI) engines (an advanced version of the commonly known diesel engine) while, unlike CIDI engines, producing ultra-low oxides of nitrogen (NOx) and particulate matter (PM) emissions. HCCI engines operate on the principle of having a dilute, premixed charge that reacts and burns volumetrically throughout the cylinder as it is compressed by the piston. In some regards, HCCI incorporates the best features of both spark ignition (SI) and compression ignition (CI), as shown in Figure 1. As in an SI engine, the charge is well mixed, which minimizes particulate emissions, and as in a CIDI engine, the charge is compression ignited and has no throttling losses, which leads to high efficiency. However, unlike either of these conventional engines, the combustion occurs simultaneously throughout the volume rather than in a flame front. This important attribute of HCCI allows combustion to occur at much lower temperatures, dramatically reducing engine-out emissions of NOx.
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Most engines employing HCCI to date have dual mode combustion systems in which traditional SI or CI combustion is used for operating conditions where HCCI operation is more difficult. Typically, the engine is cold-started as an SI or CIDI engine, then switched to HCCI mode for idle and low- to mid-load operation to obtain the benefits of HCCI in this regime, which comprises a large portion of typical automotive driving cycles. For high-load operation, the engine would again be switched to SI or CIDI operation. Research efforts are underway to extend the range of HCCI operation, as discussed in the body of this report.
B. What are the Advantages of HCCI?
The advantages of HCCI are numerous and depend on the combustion system to which it is compared. Relative to SI gasoline engines, HCCI engines are more efficient, approaching the efficiency of a CIDI engine. This improved efficiency results from three sources: the elimination of throttling losses, the use of high compression ratios (similar to a CIDI engine), and a shorter combustion duration (since it is not necessary for a flame to propagate across the cylinder). HCCI engines also have lower engine-out NOx than SI engines. Although three-way catalysts are adequate for removing NOx from current-technology SI engine exhaust, low NOx is an important advantage relative to spark-ignition, direct-injection (SIDI) technology, which is being considered for future SI engines.
Relative to CIDI engines, HCCI engines have substantially lower emissions of PM and NOx. (Emissions of PM and NOx are the major impediments to CIDI engines meeting future emissions standards and are the focus of extensive current research.) The low emissions of PM and NOx in HCCI engines are a result of the dilute homogeneous air and fuel mixture in addition to low combustion temperatures. The charge in an HCCI engine may be made dilute by being very lean, by stratification, by using exhaust gas recirculation (EGR), or some combination of these. Because flame propagation is not required, dilution levels can be much higher than the levels tolerated by either SI or CIDI engines. Combustion is induced throughout the charge volume by compression heating due to the piston motion, and it will occur in almost any fuel/air/exhaust-gas mixture once the 800 to 1100 K ignition temperature (depending on the type of fuel) is reached. In contrast, in typical CI engines, minimum flame temperatures are 1900 to 2100 K, high enough to make unacceptable levels of NOx. Additionally, the combustion duration in HCCI engines is much shorter than in CIDI engines since it is not limited by the rate of fuel/air mixing. This shorter combustion duration gives the HCCI engine an efficiency advantage. Finally, HCCI engines may be lower cost than CIDI engines since they would likely use lower-pressure fuel-injection equipment.
Another advantage of HCCI combustion is its fuel-flexibility. HCCI operation has been shown using a wide range of fuels. Gasoline is particularly well suited for HCCI operation. Highly efficient CIDI engines, on the other hand, cannot run on gasoline due to its low cetane number. With successful R&D, HCCI engines might be commercialized in light-duty passenger vehicles by 2010, and by 2015 as much as a half-million barrels of oil per day may be saved.
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Tests have also shown that under optimized conditions HCCI combustion can be very repeatable, resulting in smooth engine operation. The emission control systems for HCCI engines have the potential to be less costly and less dependent on scarce precious metals than either SI or CIDI engines.
HCCI is potentially applicable to both automobile and heavy truck engines. In fact, it could be scaled to virtually every size-class of transportation engines from small motorcycle to large ship engines. HCCI is also applicable to piston engines used outside the transportation sector such as those used for electrical power generation and pipeline pumping.
C. Why is R&D Important for HCCI?
Although stable HCCI operation and its substantial benefits have been demonstrated at selected steady-state conditions, several technical barriers must be overcome before HCCI can be widely applied to production automobile and heavy-truck engines. R&D will be required in several areas, including: controlling ignition timing over a wide range of speeds and loads, limiting the rate of combustion heat release at high-load operation, providing smooth operation through rapid transients, achieving cold-start, and meeting emissions standards. Overcoming these technical challenges to practical HCCI engines requires an improved understanding of the in-cylinder processes, an understanding of how these processes can be favorably altered by various control techniques, and the development and testing of appropriate control mechanisms.
As a result of recent research (see Section III A), the basic principles of HCCI are reasonably well understood. However, in practical engines the air/fuel charge is never completely homogeneous, and creating a charge with an even greater degree of stratification (temperature and/or mixture stratification) appears to have a strong potential for controlling combustion rates to enable high-load operation and for reducing hydrocarbon emissions. (For the remainder of this report, the term HCCI will also be used to refer to variants of HCCI, e.g., partially stratified (i.e., not fully homogeneous) charge compression ignition or SCCI). Research is required to understand how various fuel-injection techniques, methods for introducing EGR, and charge mixing techniques alter HCCI combustion through partial charge stratification. R&D efforts are also needed for the development of fuel-injection hardware and other mixing control techniques that may be required to achieve the desired changes to the in-cylinder processes (e.g., partial stratification). In addition, R&D efforts are needed to investigate control systems such as variable valve timing (VVT) and variable compression ratio (VCR). These controls have a strong potential for controlling HCCI timing, assisting with cold-start, controlling the engine through transients, and switching into and out of HCCI mode as may be necessary for some applications (see Section III B and Section V).
Finally, R&D efforts are needed for the development of sensors and control algorithms for closed-loop control (See Section V F.).
Because of the need to reduce worldwide fuel consumption, greenhouse gas emissions, and criteria air emissions (Federal Tier 2 standards are to be implemented in 2004), there is strong interest in HCCI worldwide. This combustion process stands out as a strong candidate for future automotive and truck engines that consume less fuel while producing substantially lower levels of smog-forming emissions. Japan and several European countries have aggressive R&D programs in HCCI including public and private sector components. Many of the leading developments to date have come from these countries. While HCCI research is ongoing within several public and private organizations in the U.S., there is a real possibility that the U.S. will lag in the development of HCCI, if U.S. research is not expanded. For the U.S. automobile and truck-engine industries to maintain their international competitiveness, R&D efforts are needed to develop practical HCCI engines.
SECTION II. BENEFITS AND CHALLENGES
A major advantage of HCCI combustion is its fuel-flexibility. Because HCCI engines can be fueled with gasoline, implementation of HCCI engines should not adversely affect fuel availability or infrastructure. (CIDI engines cannot be operated with gasoline due to its low cetane number.) With successful R&D, HCCI engines might be commercialized in light-duty passenger vehicles by 2010, and by 2015 as much as a half-million barrels of primary oil per day may be saved. Additional savings may accrue from reduced refining requirements for fuels for HCCI engines relative to gasoline for conventional SI technology. In addition to gasoline, HCCI operation has been shown for a wide-range of other fuels. Due to this fuel-flexibility, some HCCI applications (e.g., light-duty vehicles) could use gasoline, while other HCCI applications (e.g., heavy-duty trucks) could use diesel fuel.
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HCCI also has advantages as a potential low emissions alternative to CIDI engines in light-, medium- and heavy-duty applications. Even with the advent of effective exhaust emission control devices, CIDI engines will be seriously challenged to meet the U. S. Environmental Protection Agency (EPA) 2004 Tier 2 light-duty emission standards or the newly enacted 2007-2010 standards for trucks. This challenge is difficult to overcome because NOx and particulate matter emission controls often counteract each other. Moreover, CIDI emission control technologies are unproven, expensive, require the injection of fuel or other reductants into the exhaust stream for NOx reduction, and currently do not last the life of the engine. These emission control systems would also require the use of more expensive ultra-low-sulfur fuels (less than 15 ppm). In addition to emission control devices, expensive fuel injection equipment will be necessary to control emissions (some estimate fuel injection equipment will account for one-third of engine costs). Although the actual cost and fuel-consumption penalties of CIDI emission controls are uncertain, the use of HCCI engines or engines operating in HCCI mode for a significant portion of the driving cycle could significantly reduce the overall cost of operation, thus saving fuel and reducing the economic burden of lowering emissions.
As an alternative high-efficiency engine for light-duty vehicles, HCCI has the potential to be a low emissions alternative to CIDI and SIDI engines. Intensive efforts are underway to develop CIDI and SIDI engines for automotive applications to improve overall vehicle fuel efficiency; however, both CIDI and SIDI engines face several hurdles. As discussed in the preceding paragraph, the emission control devices to reduce NOx from CIDI engines have several problems. A similar situation exists for SIDI engines because achieving more efficient operation requires them to operate lean. Consequently, NOx emission control devices similar to those being developed for CIDI engines are required. In addition, the sulfur content of gasoline will be 30 ppm average and 80 ppm maximum as specified by the EPA Tier 2 light-duty vehicle emission standards, a level that may be too high for the long term durability of lean NOx emission control systems. It remains to be seen whether SIDI engines will be developed that can meet the Tier 2 emission standards. While HCCI engines have several inherent benefits as replacements for SI and CIDI engines in vehicles with conventional powertrains, they are particularly well suited for use in internal combustion (IC)-engine/electric series hybrid vehicles. In these hybrids, engines can be optimized for operation over a fairly limited range of speeds and loads, thus eliminating many of the control issues normally associated with HCCI, creating a highly fuel-efficient vehicle. In addition to the on-highway applications discussed above, it should be noted that the benefits of HCCI engines could be realized in most other internal combustion engine applications such as off-road vehicles, marine applications, and stationary power applications. The resulting benefits would be similar to those discussed above.
HCCI combustion is achieved by controlling the temperature, pressure and composition of the air/fuel mixture so that it autoignites near top dead center (TDC) as it is compressed by the piston. This mode of ignition is fundamentally more challenging than using a direct control mechanism such as a spark plug or fuel injector to dictate ignition timing as in SI and CIDI engines, respectively. While HCCI has been known for some twenty years, it is only with the recent advent of electronic engine controls that HCCI combustion can be considered for application to commercial engines. Even so, several technical barriers must be overcome before HCCI engines will be viable for high-volume production and application to a wide range of vehicles. The following describes the more significant challenges for developing practical HCCI engines for transportation. Greater detail regarding these technical barriers, potential solutions, and the R&D needed to overcome them are provided in Section V. Some of these issues could be mitigated or eliminated if the HCCI engine was used in a series hybrid-electric application, as discussed above.
1. Controlling Ignition Timing over a Range of Speeds and Loads
Expanding the controlled operation of an HCCI engine over a wide range of speeds and loads is probably the most difficult hurdle facing HCCI engines. HCCI ignition is determined by the charge mixture composition and its temperature history (and to a lesser extent, its pressure history). Changing the power output of an HCCI engine requires a change in the fueling rate and, hence, the charge mixture. As a result, the temperature history must be adjusted to maintain proper combustion timing. Similarly, changing the engine speed changes the amount of time for the autoignition chemistry to occur relative to the piston motion. Again, the temperature history of the mixture must be adjusted to compensate. These control issues become particularly challenging during rapid transients.
Several potential control methods have been proposed to provide the compensation required for changes in speed and load. Some of the most promising include varying the amount of hot EGR introduced into the incoming charge, using a VCR mechanism to alter TDC temperatures, and using VVT to change the effective compression ratio and/or the amount of hot residual retained in the cylinder. VCR and VVT are particularly attractive because their time response could be made sufficiently fast to handle rapid transients. Although these techniques have shown strong potential (see Section III B), they are not yet fully proven, and cost and reliability issues must be addressed.
2. Extending the Operating Range to High Loads
Although HCCI engines have been demonstrated to operate well at low-to-medium loads, difficulties have been encountered at high-loads. Combustion can become very rapid and intense, causing unacceptable noise, potential engine damage, and eventually unacceptable levels of NOx emissions. Preliminary research indicates the operating range can be extended significantly by partially stratifying the charge (temperature and mixture stratification) at high loads to stretch out the heat-release event. Several potential mechanisms exist for achieving partial charge stratification, including varying in-cylinder fuel injection, injecting water, varying the intake and in-cylinder mixing processes to obtain non-uniform fuel/air/residual mixtures, and altering cylinder flows to vary heat transfer. The extent to which these techniques can extend the operating range is currently unknown, and R&D will be required. Because of the difficulty of high-load operation, most initial concepts involve switching to traditional SI or CI combustion for operating conditions where HCCI operation is more difficult. This dual mode operation provides the benefits of HCCI over a significant portion of the driving cycle but adds to the complexity by switching the engine between operating modes.
3. Cold-Start Capability
At cold start, the compressed-gas temperature in an HCCI engine will be reduced because the charge receives no preheating from the intake manifold and the compressed charge is rapidly cooled by heat transferred to the cold combustion chamber walls. Without some compensating mechanism, the low compressed-charge temperatures could prevent an HCCI engine from firing. Various mechanisms for cold-starting in HCCI mode have been proposed, such as using glow plugs, using a different fuel or fuel additive, and increasing the compression ratio using VCR or VVT. Perhaps the most practical approach would be to start the engine in spark-ignition mode and transition to HCCI mode after warm-up. For engines equipped with VVT, it may be possible to make this warm-up period as short as a few fired cycles, since high levels of hot residual gases could be retained from previous spark-ignited cycles to induce HCCI combustion. Although solutions appear feasible, significant R&D will be required to advance these concepts and prepare them for production engines.
4. Hydrocarbon and Carbon Monoxide Emissions
HCCI engines have inherently low emissions of NOx and PM, but relatively high emissions of hydrocarbons (HC) and carbon monoxide (CO). Some potential exists to mitigate these emissions at light load by using direct in-cylinder fuel injection to achieve appropriate partial-charge stratification. However, in most cases, controlling HC and CO emissions from HCCI engines will require exhaust emission control devices. Catalyst technology for HC and CO removal is well understood and has been standard equipment on automobiles for many years. However, the cooler exhaust temperatures of HCCI engines may increase catalyst light-off time and decrease average effectiveness. As a result, meeting future emission standards for HC and CO will likely require further development of oxidation catalysts for low-temperature exhaust streams. However, HC and CO emission control devices are simpler, more durable, and less dependent on scarce, expensive precious metals than are NOx and PM emission control devices. Thus, simultaneous chemical oxidation of HC and CO (in an HCCI engine) is much easier than simultaneous chemical reduction of NOx and oxidation of PM (in a CIDI engine). In addition, HC and CO emission control devices are simpler, more durable, and less dependent on scarce, expensive precious metals.