06-05-2013, 02:46 PM
Homogeneous Charge Compression Ignition (HCCI) Technology
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Executive Summary
Conference Report 106-914, which accompanied the Department of Interior and Related Agencies
Appropriations Act, 2001, P.L. 106-291, requested that the Department of Energy’s Office of
Energy Efficiency and Renewable Energy (EERE) submit a report on Homogeneous Charge
Compression Ignition (HCCI) technology. The Conference report also urged the Department of
Energy to work with the National Research Council (NRC) to address the potential of HCCI
engines. The NRC Standing Committee to Review the Research Program for the Partnership for a
New Generation of Vehicles (PNGV) is conducting its seventh review of the PNGV program. On
December 7, 2000, the Four-Stroke, Direct-Injection (4SDI) Technical Team briefed the NRC
committee on the entire PNGV program. The 4SDI Technical Team included a presentation that
summarized the on-going HCCI program and many of the issues discussed in this report. The
NRC publication of its examination of the PNGV program is expected in June 2001.
INTRODUCTION
Conference Report 106-914, which accompanied the Department of Interior and Related Agencies
Appropriations Act, 2001, P.L.106-291, requested that the Department of Energy’s Office of
Energy Efficiency and Renewable Energy (EERE) submit a report on Homogeneous Charge
Compression Ignition (HCCI) technology. This report provides a comprehensive overview of the
current state-of-the-art in HCCI technology. Section II summarizes the benefits of HCCI and
some of the main challenges remaining to the development of commercially viable engines. Section
III presents recent developments in HCCI technology and discusses two commercial engines that
use HCCI during a portion of their operating range. Section IV describes the related research
currently being performed with support from DOE. Section V lists and describes the potential
future directions for HCCI R&D activities. Concluding remarks are provided in Section VI.
A. What is HCCI?
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.
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.
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).
Benefits
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.
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.