HCCI-Seminar Paper

Introduction

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. 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.

History

HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for combustion to occur. Another example is the "diesel" model aircraft engine.

ADVANTAGES

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. 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.

IMPORTANCE OF R&D IN 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 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 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. Finally, R&D efforts are needed for the development of sensors and control algorithms for closed-loop control

Because of the need to reduce worldwide fuel consumption, greenhouse gas emissions, and criteria air emissions, 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.

Benefits

A major advantage of HCCI combustion is its fuel-flexibility. Because HCCI engines can be fuelled 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  Environmental light-duty emission standards or 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, a level that may be too high for the long term durability of lean NOx emission control systems.. While HCCI engines have several inherent benefits as replacements for SI and CIDI engines in vehicles with conventional power trains, 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.

Advancements in Speed and Load Control

Combustion control is the biggest challenge to HCCI engines becoming a commercial success. For this reason, several methods have been proposed for achieving HCCI engine control over the wide range of operating conditions required for typical transportation-engine applications. Control technologies reported in the literature have demonstrated some degree of success, but further R&D efforts are required (see Section V). Some of the proposed methods include:

*Variable compression ratio (VCR): HCCI combustion is strongly affected by the compression ratio of the engine. Therefore, a VCR engine has the potential to achieve satisfactory operation in HCCI mode over a wide range of conditions because the compression ratio can be adjusted as the operating conditions change. Conditions change quickly in vehicular applications; consequently, a fast control system that modifies the compression ratio in fractions of a second is necessary. Several options have been studied to obtain VCR engines. One option is to mount a plunger in the cylinder head whose position can be varied to change the compression ratio . The compression ratio could also be varied by using an opposed-piston engine design having variable phase-shifting between the two crankshafts . SAAB has recently announced the development of another method that is based on a hinged, tilting cylinder arrangement . The DOE has sponsored a unique VCR engine design, which is being developed by Envera and tested at Argonne National Laboratory. Similar to the SAAB approach, the Envera approach varies the distance between the cylinder head and the crankshaft. However, unlike the SAAB approach, the VCR mechanism fits inside the crankcase and is expected to provide a faster response and require less energy. The Envera design will be tested this summer with publication of results to follow. While any of these systems or some other mechanism might succeed, only the variable-position plunger system has been demonstrated in an HCCI engine For these tests, the plunger was controlled by a hydraulic system allowing its position to be varied during engine operation. The data show that the VCR system is capable of controlling HCCI ignition timing to maintain optimal combustion phasing across a very wide range of intake temperatures and fuel types of varying octane number. Although transient operation and variations in speed and load were not reported, the results suggest that a VCR system with sufficiently fast response time is a strong candidate for HCCI engine speed and load control.  For these tests, the plunger was controlled by a hydraulic system allowing its position to be varied during engine operation. The data show that the VCR system is capable of controlling HCCI ignition timing to maintain optimal combustion phasing across a very wide range of intake temperatures and fuel types of varying octane number. Although transient operation and variations in speed and load were not reported, the results suggest that a VCR system with sufficiently fast response time is a strong candidate for HCCI engine speed and load control. VCR would add some cost and complexity to the engine. SAAB has announced plans to go into production with its VCR system on a conventional SI engine.

*Variable valve timing (VVT): VVT can be used to change the trapped compression ratio of the engine (i.e., the amount of compression after the gases are trapped by intake-valve closure), and therefore VVT can achieve a similar effect on HCCI combustion as varying the geometric compression ratio of the engine. An engine could be built with a high geometric compression ratio, with lower trapped compression-ratios being obtained by delaying the closing of the intake valve during the compression stroke. Engines with VVT have the added benefit of allowing changes in the temperature and composition of the incoming charge by retaining hot residual gases from the previous cycle in the cylinder. By varying the amount of hot residual, the temperature and mixture of the new charge can be adjusted. Increasing the temperature of the charge in this manner can be used to induce HCCI combustion even with relatively low geometric compression ratios or under cold-engine conditions. In addition, altering the charge composition with partial mixing of the residual could benefit combustion rate control as will be discussed in Section V. VVT could be implemented in an engine with mechanical, magnetic, or hydraulic valve actuators . Recently, researchers at Stanford University, using an electro-hydraulic VVT system, have shown that HCCI combustion can be induced in an engine with a relatively low (10:1) compression ratio . Stanford also showed that the VVT system could be used to control combustion timing and to switch between SI and HCCI operation from one cycle to the next ( see in Section IV D). Like VCR, a VVT system would add cost to the engine; however, several manufacturers already have VVT systems in production or are planning to go into production within the next year or two.

Results Using Different Fuels

One of the advantages of HCCI combustion is its intrinsic fuel flexibility. HCCI combustion has little sensitivity to fuel characteristics such as lubricity and laminar flame speed. Fuels with any octane or cetane number can be burned, although the operating conditions must be adjusted to accommodate different fuels, which can impact efficiency, as discussed below. An HCCI engine with VCR or VVT could, in principle, operate on any hydrocarbon or alcohol liquid fuel, as long as the fuel is vaporized and mixed with the air before ignition.The literature shows that HCCI has been achieved with multiple fuels. The main fuels that have been used are gasoline, diesel fuel, propane, natural gas, and single- and dual-component mixtures of the gasoline and diesel primary reference fuels (iso-octane and n-heptane, respectively). The applicability of these fuels to HCCI engines is discussed below. Other fuels (methanol, ethanol, acetone) have also been tried in experiments, but with inconclusive results.

*Gasoline: Gasoline has multiple advantages as an HCCI fuel. Gasoline also has a high octane number (87 to 92 in the U.S. and up to 98 in Europe), which allows the use of reasonably high compression ratios in HCCI engines. Actual compression ratios for gasoline-fueled HCCI engine data vary from 12:1 to 21:1 depending on the fuel octane number, intake air temperature, and the specific engine used (which may affect the amount of hot residual naturally retained). This compression-ratio range allows gasoline-fueled HCCI engines to achieve

relatively high thermal efficiencies (in the range of diesel-fueled CIDI engine efficiencies). A potential drawback to higher compression ratios is that the engine design must accommodate the relatively high cylinder pressures that can result, particularly at high engine loads Additional advantages of gasoline include easy evaporation, simple mixture preparation, and a ubiquitous refueling infrastructure.

* Diesel Fuel: Diesel fuel autoignites rapidly at relatively low temperatures but is difficult to evaporate. To obtain diesel-fuel HCCI combustion, the air-fuel mixture must be heated considerably to evaporate the fuel. The compression ratio of the engine must be very low (8:1 or lower) to obtain satisfactory combustion, which results in a low engine efficiency. Alternatively, the fuel can be injected in-cylinder, but without air preheating, temperatures are not sufficiently high for diesel-fuel vaporization until well up the compression stroke. This strategy often results in incomplete fuel vaporization and poor mixture preparation, which can lead to particulate matter and NOx emissions. However, one concept for direct injection of diesel fuel, involving late injection (after TDC) with high swirl, has been successful at thoroughly vaporizing and mixing the fuel before ignition at light loads. This mode of operation is used in the Nissan MK engine. In addition, diesel fuel has an extensive refueling infrastructure.

* Propane: Propane is an excellent fuel for HCCI. High efficiencies can be achieved with propane-fueled HCCI engines because propane has a high octane number (105). Because propane is a gaseous fuel, it can be easily mixed with air. Some infrastructure also exists for propane. Because it can be maintained as a liquid at moderate pressures, the amount of fuel that can be stored onboard a vehicle is comparable to what can be stored for typical liquid fuels.

* Natural Gas: Because natural gas has an extremely high octane rating (about 110), natural gas HCCI engines can be operated at very high compression ratios (15:1 to 21:1), resulting in high efficiency. However, similar to gasoline or propane, the engine design must accommodate the relatively high cylinder pressures that can result.