Automotive spark-ignited direct-injection gasoline engines

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Abstract

The development of four-stroke, spark-ignition engines that are designed to inject gasoline directly into the combustion chamber is an important worldwide initiative of the automotive industry. The thermodynamic potential of such engines for significantly enhanced fuel economy, transient response and cold-start hydrocarbon emission levels has led to a large number of research and development projects that have the goal of understanding, developing and optimizing gasoline direct-injection (GDI) combustion systems. The processes of fuel injection, spray atomization and vaporization, charge cooling, mixture preparation and the control of in-cylinder air motion are all being actively researched, and this work is reviewed in detail and analyzed. The new technologies such as high-pressure, common-rail, gasoline injection systems and swirl-atomizing gasoline fuel injectors are discussed in detail, as these technologies, along with computer control capabilities, have enabled the current new examination of an old objective; the direct-injection, stratified-charge (DISC), gasoline engine. The prior work on DISC engines that is relevant to current GDI engine development is also reviewed and discussed.

The fuel economy and emission data for actual engine configurations are of significant importance to engine researchers and developers. These data have been obtained and assembled for all of the available GDI literature, and are reviewed and discussed in detail. The types of GDI engines are arranged in four classifications of decreasing complexity, and the advantages and disadvantages of each class are noted and explained. Emphasis is placed upon consensus trends and conclusions that are evident when taken as a whole. Thus the GDI researcher is informed regarding the degree to which engine volumetric efficiency and compression ratio can be increased under optimized conditions, and as to the extent to which unburned hydrocarbon (UBHC), NOx and particulate emissions can be minimized for specific combustion strategies. The critical area of GDI fuel injector deposits and the associated effect on spray geometry and engine performance degradation are reviewed, and important system guidelines for minimizing deposition rates and deposit effects are presented. The capabilities and limitations of emission control techniques and aftertreatment hardware are reviewed in depth, and areas of consensus on attaining European, Japanese and North American emission standards are compiled and discussed.

All known research, prototype and production GDI engines worldwide are reviewed as to performance, emissions and fuel economy advantages, and for areas requiring further development. The engine schematics, control diagrams and specifications are compiled, and the emission control strategies are illustrated and discussed. The influence of lean-NOx catalysts on the development of late-injection, stratified-charge GDI engines is reviewed, and the relative merits of lean-burn, homogeneous, direct-injection engines as an option requiring less control complexity are analyzed. All current information in the literature is used as the basis for discussing the future development of automotive GDI engines.

Introduction

With the increasing emphasis on achieving substantial improvements in automotive fuel economy, automotive engineers are striving to develop engines having enhanced brake-specific fuel consumption (BSFC), and which can also comply with future stringent emission requirements. The BSFC, and hence the fuel economy, of the compression-ignition, direct-injection (CIDI), diesel engine is superior to that of the port-fuel-injected (PFI) spark-ignition engine, mainly due to the use of a significantly higher compression ratio, coupled with unthrottled operation. The diesel engine, however, generally exhibits a higher noise level, a more limited speed range, diminished startability, and higher particulate and NOx emissions than the spark ignition (SI) engine. Over the past two decades, attempts have been made to develop an internal combustion engine for automotive applications that combines the best features of the SI and the diesel engines. The objective has been to combine the specific power of the gasoline engine with the efficiency of the diesel engine at part load. Such an engine would exhibit a BSFC approaching that of the diesel engine, while maintaining the operating characteristics and specific power output of the SI engine.

Research has indicated that a promising candidate for achieving this goal is a direct-injection, four-stroke, spark-ignition engine that does not throttle the inlet mixture to control the load. In this engine, a fuel spray plume is injected directly into the cylinder, generating a fuel–air mixture with an ignitable composition at the spark gap at the time of ignition. This class of engine is designated as a direct-injection, stratified-charge (DISC) engine. This engine type generally exhibits an improved tolerance for fuels of lower octane number and driveability index, and a significant segment of the early work on prototype DISC engines focused on the inherent multi-fuel capability [1], [2], [3]. In a manner similar to that of the diesel, the power output of this engine is controlled by varying the amount of fuel that is injected into the cylinder. The induction air is not significantly throttled, thus minimizing the negative work of the pumping loop of the cycle. By using a spark plug to ignite the fuel as it mixes with air, the engine is provided with direct ignition, thus avoiding many of the requirements of autoignition quality that are inherent in fuels for the diesel engine. Furthermore, by means of the relative alignment of the spark plug and the fuel injector, overall ultra-lean-operation may be achieved, thus yielding an enhanced BSFC [4], [5], [6], [7], [8].

From a historical perspective, interest in these significant benefits has promoted a number of important investigations of the potential of DISC engines. Several detailed combustion strategies were proposed and implemented, including the Texaco Controlled Combustion System (TCCS) [9], MAN-FM of Maschinenfabrik Auguburg-Nurnberg [10], [11], [12], and the Ford programmed combustion (PROCO) system [13], [14]. These earlier systems were based upon engines having two valves per cylinder, with a bowl-in-piston combustion chamber. Late injection was achieved by utilizing a mechanical pump-line-nozzle fuel injection system from a diesel engine application. Unthrottled operation was obtained throughout most of the load range, with a BSFC that was competitive with an indirect-injection (IDI) diesel engine. A major drawback was that late injection timing was maintained even at full load due to the limitations of the mechanical fuel injection system. This provided smoke-limited combustion for air–fuel ratios richer than 20:1. The necessity of using diesel fuel injection equipment, coupled with the need for a turbocharger to provide adequate power output culminated in an engine that had cost and performance characteristics that were similar to those of a diesel engine, but which had poor part-load unburned hydrocarbon (UBHC) emissions. The combination of relatively poor air utilization and the use of fuel injection equipment that was limited in speed range meant that the engine specific power output was quite low. A discussion of the geometric configuration of these early systems is given in Section 6.1.

Many of the basic limitations that were encountered in the earlier work on DISC engines can now be circumvented. This is particularly true for the significant control limitations that existed for the direct-injection (DI) injectors of 15 years ago. New technologies and computer-control strategies are currently being invoked by a number of automotive companies to reexamine the extent to which the potential benefits of the gasoline direct injection (GDI) engine can be realized in production engines [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [276], [277], [278], [279], [280], [281], [282], [283], [284], [285], [286], [287], [288], [303], [306], [318]. These engines and combustion strategies are discussed in detail in 6.2 Mitsubishi combustion system, 6.3 Toyota combustion system, 6.4 Nissan combustion system, 6.5 Ford combustion system, 6.6 Isuzu combustion system, 6.7 Mercedes-Benz combustion system, 6.8 Mazda combustion system, 6.9 Audi combustion system, 6.10 Honda combustion system, 6.11 Subaru combustion system, 6.12 Fiat combustion system, 6.13 Renault combustion system, 6.14 Ricardo combustion system, 6.15 AVL combustion system, 6.16 FEV combustion system, 6.17 Orbital combustion system.

The information in this document will provide the reader with a comprehensive review of the mixture dynamics and combustion control strategies that may be utilized in four-stroke, spark-ignited, direct-injection, gasoline engines. The current state of knowledge, as exhibited in more than 370 key publications, many of which remain untranslated into English, is discussed in detail, and the critical research and development needs for the near future are identified.

The major difference between the PFI engine and the GDI engine is in the mixture preparation strategies, which are illustrated schematically in Fig. 1. In the PFI engine, fuel is injected into the intake port of each cylinder, and there is an associated time lag between the injection event and the induction of the fuel and air into the cylinder. The vast majority of current automotive PFI engines utilize timed fuel injection onto the back of the intake valve when the intake valve is closed. During cranking and cold starting, a transient film, or puddle, of liquid fuel forms in the intake valve area of the port. This causes a fuel delivery delay and an associated inherent metering error due to partial vaporization, making it necessary to supply amounts of fuel that significantly exceed that required for the ideal stoichiometric ratio. This puddling and time lag may cause the engine to either misfire or experience a partial burn on the first 4–10 cycles, with an associated significant increase in the UBHC emissions. Alternatively, injecting fuel directly into the engine cylinder totally avoids the problems associated with fuel wall wetting in the port, while providing enhanced control of the metered fuel for each combustion event, as well as a reduction in the fuel transport time. The actual mass of fuel entering the cylinder on a given cycle can thus be more accurately controlled with direct injection than with PFI. The GDI engine offers the potential for leaner combustion, less cylinder-to-cylinder variation in the air–fuel ratio and lower operating BSFC values. The UBHC emissions during a cold start are also potentially lower with direct injection, and the engine transient response can be enhanced. As a result of the higher operating fuel pressure of the GDI system, the fuel entering the cylinder is much better atomized than that of the PFI system, particularly under cold operating conditions, thus yielding much higher rates of fuel vaporization. The mean drop size is typically 16 microns SMD as compared to 120 microns SMD with the PFI system. It is important to note, however, that injection of fuel directly into the cylinder is not a guarantee that fuel film problems are not present. The wetting of piston crowns or other combustion chamber surfaces, whether intentional or unintentional, does introduce the important variable of transient wall film formation and evaporation.

The GDI concept does indeed offer many opportunities for circumventing the basic limitations of the PFI engine, particularly those associated with port wall wetting. The fuel film in the intake port of a PFI engine acts as an integrating capacitor, and the engine actually operates on fuel inaccurately metered from the pool in the film, not from the current fuel being accurately metered by the injector [38]. During a cold start, the fuel from more than 10 cycles must be injected to achieve a steady, oscillatory film of liquid fuel in the intake port. This means that the cold PFI engine does not fire or start on the first few cycles, even though fuel is being repetitively injected into the film pool. Control algorithms must be used to provide significant over-fueling if acceptable PFI start times are to be achieved, even though the catalyst temperature is below the light-off threshold at this condition and UBHC emissions will be increased. Thus it is not unusual for PFI systems to generate 90% of the total UBHC emissions in the US FTP emission test within the first 90 s [39].

The direct injection of gasoline into the cylinder of a four-stroke, gasoline, spark-ignition engine eliminates the integrating fuel film in the intake port. It is well established that the direct injection of gasoline with little or no cold enrichment can provide starts on the second cranking cycle [40], and can exhibit significant reductions in UBHC spikes during load transients. An excellent example of the comparison of the fuel quantity required to start GDI and PFI engines is provided in Fig. 2 [41]. It is quite evident that the GDI engine requires much less fuel to start the engine, and that this difference in the minimum fuel requirement becomes larger as the ambient temperature decreases.

Another limitation of the PFI engine is the requirement of throttling for basic load control. Even though throttling is a well-established and reliable mechanism of load control in the PFI engine, the thermodynamic loss associated with throttling is substantial. Any system that utilizes throttling to adjust load levels will experience the thermodynamic loss that is associated with this pumping loop, and will exhibit thermal efficiency degradation at low levels of engine load. Current advanced PFI engines still utilize, and will continue to require, throttling for basic load control. They also have, and will continue to have, an operating film of liquid fuel in the intake port. These two basic PFI operating requirements represent major impediments to achieving significant breakthroughs in PFI fuel economy or emissions. Continuous incremental improvements in the older PFI technology will be made, but it is unlikely that the long-range fuel economy and emission objectives can be simultaneously achieved. The GDI engine, in theory, has neither of these two significant limitations, nor the performance boundaries that are associated with them. The theoretical advantages of the GDI engine over the contemporary PFI engine are summarized as follows, along with the enabling mechanism:

  • Improved fuel economy (up to 25% potential improvement, depending on test cycle resulting from:

    • less pumping loss (unthrottled, stratified mode);

    • less heat losses (unthrottled, stratified mode);

    • higher compression ratio (charge cooling with injection during induction);

    • lower octane requirement (charge cooling with injection during induction);

    • increased volumetric efficiency (charge cooling with injection during induction);

    • fuel cutoff during vehicle deceleration (no manifold film).

  • Improved transient response.

    • less acceleration-enrichment required (no manifold film).

  • More precise air–fuel ratio control.

    • more rapid starting;

    • less cold-start over-fueling required.

  • Extended EGR tolerance limit (to minimize the use of throttling).

  • Selective emissions advantages.

    • reduced cold-start UBHC emissions;

    • reduced CO2 emissions.

  • Enhanced potential for system optimization.

The significantly higher injection pressures used in common-rail GDI injection systems as compared to PFI fuel systems increase both the degree of fuel atomization and the fuel vaporization rate, and, as a result, it is possible to achieve stable combustion from the first or second injection cycle without supplying excess fuel. Therefore, GDI engines have the potential of achieving cold-start UBHC emissions that can approach the level observed for steady running conditions. Takagi [23] reported that the cold-start UBHC emissions obtained with the Nissan prototype GDI engine are approximately 30% lower than that of an optimized PFI engine under comparable conditions. Another potential advantage of the GDI engine is the option of using fuel cutoff on deceleration. If implemented successfully, fuel cutoff can provide additional incremental improvements in both fuel economy and engine-out UBHC emission levels. For the PFI engine, which operates from an established film of fuel in the intake port, the cutoff of fuel during vehicle deceleration is not a viable option, as it causes a reduction or elimination of the liquid fuel film in the port. This generates very lean mixtures in the combustion chamber for a few cycles following the restoration of the load, generally resulting in an engine misfire.

It should be noted that design engineers, managers and researchers who must evaluate and prioritize the published information on the advantages of GDI engines over PFI engines should be aware of one area of data comparison and reporting that is disconcerting. In many papers the GDI performance is compared to PFI baselines that are not well defined, thus making it very difficult for the reader to make a direct engineering comparison between GDI and PFI performance. One extreme example is the comparison of GDI and PFI fuel economy data that was obtained using two different vehicles with two different inertial weights. An example of a more subtle difference is the evaluation of the BSFC reduction resulting from the complete elimination of throttling in a GDI engine, but not noting or subtracting the parasitic loss of a vacuum pump that would have to be added for braking and other functions. A number of published comparisons lie between these two extremes. The readers are cautioned to review all claims of comparative GDI/PFI data carefully as to the precise test conditions for each, and the degree to which the systems were tested under different conditions or constraints.

PFI engines do have some limited advantages over GDI engines due to the fact that the intake system acts as a pre-vaporizing chamber. When fuel is injected directly into the engine cylinder, the time available for mixture preparation is reduced significantly. As a result, the atomization of the fuel spray must be fine enough to permit fuel evaporation in the limited time available between injection and ignition. Fuel droplets that are not evaporated are very likely to participate in diffusion burning, or to exit the engine as UBHC emissions. Also, directly injecting fuel into the engine cylinder can result in unintended fuel impingement on the piston or the cylinder wall. These factors, if present in the design, can contribute to levels of UBHC and/or particulate emissions, and to cylinder bore wear that can easily exceed that of an optimized PFI engine. Some other advantages of PFI engines such as low-pressure fuel system hardware, higher power density at part load and the feasibility of using three-way catalysis and higher exhaust temperatures for improved catalyst efficiency present an evolving challenge to the GDI engine.

Although the GDI engine provides important potential advantages, it does have a number of inherent problems that are similar to those of the early DISC engines. The replacement of the PFI engine by the GDI engine as the primary production automotive powerplant is constrained by the following areas of concern:

  • difficulty in controlling the stratified charge combustion over the required operating range;

  • complexity of the control and injection technologies required for seamless load changes;

  • relatively high rate of formation of injector deposits and/or ignition fouling;

  • relatively high light-load UBHC emissions;

  • relatively high high-load NOx emissions;

  • high local NOx production under part-load, stratified-charge operation;

  • soot formation for high-load operation;

  • increased particulate emissions;

  • three-way catalysis cannot be utilized to full advantage;

  • increased fuel system component wear due to the combination of high-pressure and low fuel lubricity;

  • increased rates of cylinder bore wear;

  • increased electrical power and voltage requirements of the injectors and drivers;

  • elevated fuel system pressure and fuel pump parasitic loss.

The above concerns must be addressed and alleviated in any specific design if the GDI engine is to supplant the current PFI engine. If future emission regulations such as the ultra-low-emission-vehicle (ULEV), the super ultra-low-emission-vehicle (SULEV), and corporate average fuel economy (CAFE) requirements can be achieved using PFI engines without the requirement of complex new hardware, the market penetration rate for GDI engines will be reduced, as there will most assuredly be a GDI requirement for sophisticated fuel injection hardware, a high-pressure fuel pump and a more complex engine control system. An important constraint on GDI engine designs has been relatively high UBHC and NOx emissions, and the fact that three-way catalysts could not be effectively utilized. Operating the engine under overall lean conditions does reduce the engine-out NOx emissions, but this generally cannot achieve the minimum 90% reduction level that can be attained using a three-way catalyst. Much work is underway worldwide to develop lean-NOx catalysts, but at this time the attainable conversion efficiency is still much less than that of three-way catalysts. The excessive UBHC emissions at light-load also represent a significant research problem to be solved. In spite of these concerns and difficulties, the GDI engine offers an expanded new horizon for future applications as compared to the well-developed PFI engine.

In summary, the potential advantages of the GDI concept are too significant to receive other than priority status. The concept offers many opportunities for achieving significant improvements in engine fuel consumption, while simultaneously realizing large reductions in engine-out UBHC emissions. The current high-tech PFI engine, although highly evolved, has nearly reached the limit of the potential of a system that is based upon throttling and a port fuel film; however, the technical challenge of competing with, and displacing, a proven, evolved performer such as the PFI engine is not to be underestimated. Since the late 1970s, when a significant portion of the DISC engine work was conducted, the SI engine has continued to evolve monotonically as an ever-improving baseline. The fuel system has also evolved continuously from carburetor to throttle-body injection, then to simultaneous-fire PFI, and more recently to phased-sequential-fire PFI. The result is that today the spark-ignited PFI engine remains the benchmark standard for automobile powerplants [38]. In order to displace this standard, the practical development targets for future GDI engines, as compared to current best-practice PFI engines, are as follows [42]:

  • 15–20% reduction in BSFC over an integrated cycle;

  • compliance with stringent future emissions regulations;

  • specific power output that is comparable to PFI;

  • driveability during cold starts, warm-up and load transients that is comparable to PFI.

In addition to the above, reliable and efficient hardware and control strategies for gasoline direct injection technology will have to be developed and verified under field conditions. For the same reasons that port fuel injection gradually replaced carburetor and throttle-body injection, a GDI combustion configuration that will be an enhancement of one of the concepts outlined in this paper will emerge as the predominate engine system, and will gradually displace the sequential PFI applications.

Section snippets

Direct-injection gasoline fuel system

Unthrottled operation with the load controlled by the fuel quantity has been shown to be a very efficient operating mode for the internal combustion engine, as the volumetric efficiency is increased and the pumping loss is significantly reduced. This strategic approach is very successful in the diesel engine, as ignition occurs spontaneously at points within the combustion chamber where the mixture is well prepared for autoignition. The fixed location of the ignition source in the SI engine,

Flow structure

The transient in-cylinder flow field that is present during the intake and compression strokes of a GDI engine is one of the key factors in determining the operational feasibility of the system. The magnitude of the mean components of motion, as well as their resultant variations throughout the cycle, are of importance that is comparable to that of the fuel injection system. On a microscopic scale, a high level of turbulence is essential for enhancing the fuel–air mixing process; but

Combustion chamber geometry

The location and orientation of the fuel injector relative to the ignition source are critical geometric parameters in the design and optimization of a GDI combustion system. During high-load operation, the selected injector spray axis orientation and cone angle must promote good fuel–air mixing with the induction air in order to maximize the air utilization. For late injection, the spark plug and injector locations should ideally provide an ignitable mixture cloud at the spark gap at an

Fuel economy potential

A current strategic objective for the automotive application of the four-stroke, gasoline engine is a substantial improvement in fuel consumption while meeting the required levels of pollutant emissions and engine durability. The improvement of passenger car fuel economy represents a very important goal that will determine the future use of SI engines instead of the small, high-speed, diesel engines [45], [310], [332], [344], [345], [346], [350], [351], [363]. The thermal efficiency of the GDI

Early research engines

Operating with a single, fixed ignition location imposes very stringent requirements on the mixture preparation process in GDI engines, as it is very difficult to provide a combustible mixture at the spark gap over the entire engine operating map. That is why most combustion control strategies are primarily directed towards the specifics of preparing and positioning the air–fuel mixture. A stratified-mixture region of repeatable location and charge distribution is important for achieving the

Conclusion

The key areas of the theoretical potential and the current research and development status of the spark-ignition, four-stroke, direct-gasoline-injection engine have been discussed in detail in each of the sections of this treatise. It is quite evident from the technical literature worldwide that significant incremental gains in engine performance and emission parameters are indeed indicated and, to some degree, have been achieved. Specific technical issues related to BSFC, UBHC, NOx,

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