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The methodology put forth in this paper stems from the premise that the primary reason for the generation of major pollutants by diesel engines, particulates and nitric oxides, is associated with over-reliance upon diffusion flames to carry out the process of combustion. Specific means are, therefore, proposed to inhibit their formation. This consists of refinements involving the use of either hollow cone spray injectors or air blast atomizers. Concomitantly, the process of combustion is staged by either regulating the rate of injection or employing a number of consecutively activated injectors per cylinder under a microprocessor command, while regions of high temperature peaks are distributed throughout the charge and kept at a relatively low level by exploiting the large scale vortex structure of turbulent pulsed jets combined with residual gas recirculation.

Presented here is a reportage of the panel debate on the proposition: “Is there a future for internal combustion engines beyond the technologies of Otto and Diesel?,” held at the SAE 2001 Congress. This is preceded by a recount of all the panel discussions on the future of combustion in engines, which have taken place at the SAE Congresses since 1997. In a commentary following the reportage, a prospective view of the future is provided. It puts forth the concept that the technology, inherited over a hundred years ago from Otto and Diesel, by which the exothermic process of combustion is executed in an engine cylinder, can be advanced significantly by adopting the best that modern micro-electronic and MEMS technology can offer.

In the vein of the paper we presented at the last SAE Congress (SAE 970538), the evolution of the exothermic process of combustion (an event referred to popularly as ‘heat release’) in an engine is considered from the point of view of the utilization of fuel. Its consumption in the course of this process is expressed in a functional form, akin to that engendered for mathematical description of life. There are a number of such functions recorded in the literature and their salient features are revealed. Of particular relevance to fuel utilization in engines is a reverse form of the Vibe function (known in the English engine literature as the ‘Wiebe function’), which we call the fuel life function. Its parameters can be derived from numerical modeling of combustion in engines or from reduction of indicator diagram data.

In principle, the thermodynamic and thermochemical processes evolve with time, irrespectively of their spatial orientation. They are, therefore, specified in terms of ordinary differential equations with respect to time as the only independent variable. This feature is well reflected in the literature by the so-called zero-dimensional models. Current demands of technological progress impose much stricter requirements upon the precision of such calculations than ever before. A methodology for catering to them is presented. Its application is illustrated by the performance analysis of a Renault engine, operated at full and part loads, with particular emphasis placed upon the formation of major combustion-generated pollutants, NOx and CO, in a premixed-charge engine.

The mandate of the President's Clean Car Initiative to produce a car engine that will more than double the mileage per gallon of fuel and simultaneously eliminate pollutant emissions poses an unprecedented challenge to automotive industry. It ought to be met, it is claimed, by radical improvements in the execution of the exothermic process of combustion. The conventional combustion process involves the mode of flame traversing the charge (FTC). In this process control over the exothermic process is, in effect, non-existent. The exothermic process should be executed instead by a microprocessor controlled fireball mode of combustion (FMC) - the epitome of direct injection stratified charge (DISC) engine, featuring late injection and stratified combustion using a pulsed combustion jet (PCJ) system.

In order to advance the technology of combustion in engines, the execution of heat release, or, more precisely, the evolution of exothermic energy, should be treated as a manufacturing process: at the input are the reactants, the cylinder charge comprised of compressed air mixed with fuel and recirculated exhaust or, better, residual gas, and at the output are products, among which the most noteworthy are useful work and harmful pollutants. The conversion of reactants into products is carried out by the exothermic process of the oxidation of a hydrocarbon fuel. It is then the execution of this process that warrants particular attention, so that in the engine of the future it would be modulated by an electronic control system. Described in the paper is a rational method for engineering implementation of this concept, associated with the exploitation of the best that modern computer and control technologies have to offer.

The concept of the paper stems from the premise that the process of “heat release” in engines involves in essence the evolution and deposition of exothermic energy generated by combustion-events that can be governed promptly by a feedback, adaptive micro-electronic control system. The key to its realization is the principle of DISC (Direct Injection Stratified Charge) engine, implemented by a multi-jet system. The background and the salient features of such a system, referred to as a CCE (Controlled Combustion Engine), have been described in a companion paper (SAE 951961). Presented here are fundamental aspects of the model of the exothermic process and the intrinsic properties of its control system.

The refinement of heat release analysis stems from the recognition that a combustion system is intrinsically non-linear. Thus, as appropriate for such an entity, its properties are expressed in terms of a thermochemical phase (or state) space, of which the thermodynamic aspects are exposed on a so-called Le Chatelier diagram, providing the fundamental background for the development of micro-electronic control to attain the most effective utilization of fuel. Implementation of this method of approach is illustrated by the analysis of the exothermic process taking place in two typical internal combustion engines, spark-ignition and diesel.

The development of lean charge, fast burn engines depends crucially on enhanced ignition, since one can obtain thereby proper means for increasing the rate of burn in mixtures characterized notoriously by low normal burning speeds. Enhanced ignition involves not only high energies and long duration of ignition, but also a wide dispersion of its sources, so that combustion is carried out at as many sites throughout the charge as possible. Upon this premise, various ignition systems for I.C. engines, operating with premixed charge, are reviewed. The systems are grouped within the following categories: (1) high energy spark plugs; (2) plasma jet igniters; (3) photochemical, laser, and microwave ignition concepts; (4) torch cells; (5) divided chamber stratified charge engines; (6) flame jet igniters; (7) combustion jet ignition concepts; (8) EGR ignition system.

The main advantage of jet igniters lies in their ability to provide distributed ignition sources that are capable of initiating and enhancing combustion in lean mixtures. This is achieved principally by two mechanisms: the provision of a high concentration of free radicals, enhancing ignition at lower temperatures, and of the extended highly turbulent igniting surface, yielding larger flame front areas and hence increased burning rates. The paper describes experimental and analytical procedures that were developed to determine the performance of a jet igniter with particular emphasis on the fluid mechanic effects. This consists of the evaluation of the penetration depth of the jet as a function of the plasma plug geometry, and of the rate of burning in the jet-ignited charge, as well as the corresponding turbulent and laminar flame burning velocities. To de-emphasize the thermochemical effects, the plasma medium and energy were maintained without change for all the tests.

The paper reports a preliminary study of jets of active radicals used as igniters for lean mixtures. The jets were generated either by combustion or by electric discharge. Experiments were performed in a cylindrical steel vessel, 9 cm in diameter and 9 cm long, filled initially with either air or an ultra-lean (equivalence ratio: 0.5) methane-air mixture at atmospheric pressure and room temperature. Observations were made by schlieren photography, using a sub-microsecond spark discharge in air as a point light source. The gasdynamic properties of the jets were shown to be primarily governed by their initial velocity, while the particular process by which they were formed played, in this respect, a secondary role. The jets of radicals invariably appeared as turbulent plumes which were embedded in blast waves headed by hemispherical shock fronts.

This paper is based upon the premise that the best way to deal with current constraints imposed upon car engines is by technological advances designed to (1) minimize pollutant emissions, (2) maximize engine efficiency, and (3) optimize tolerance to a wider variety of fuels. A sense of direction for such advances is derived from a critical assessment of the fundamental advantages of reciprocating I.C. engines as prime movers for automobiles, and the review of their recent developments. On this basis it is shown that major impact in this respect could be made by control1ed combustion. Intrinsically, this should involve proper handling of active radicals, the essential elements of the combustion reaction. Practically, this can be achieved by a variety of means, such as charge stratification, exhaust gas recirculation, homogeneous lean burn, combined with enhanced ignition and enhanced auto-catalysis.

Performance of an array of plasma ignition systems has been studied in a CFR engine.This included a standard spark plug, an extended spark plug, a surface discharge plug, and two plasma jet ignitors, one with open cavity and the other with cavity provided with a jet forming orifice.For all the tests the engine was run at a compression ratio of 3:1, a wide open throttle, and minimum for best torque (MBT) ignition timing. In this way specific information was obtained on ignition delay, duration of the exothermic combustion process, engine efficiency, and pollutant emissions.The study demonstrated the effect of various ignition systems on engine performance as the lean operating limit is approached.

The problem of knock is traced back to the earliest scientific paper on combustion in premixed gases written by Mallard and Le Chatelier. The pioneering contributions of Ricardo, Kettering and Semenov are then put in proper perspective. Upon the recognition of the fact that this phenomenon has been, and still is, imposing the major technological constraint upon the automotive and oil industries, its various cures are reviewed. Essential features of combustion instability leading to its onset are then exposed, and the methodology is outlined for a rational attack upon the problem it poses.

A photographic study was conducted using an optical-access compression-expansion machine in order to reveal the mechanism of ignition and flame propagation initiated by plasma igniters. The tests included a jet igniter with inert cavity liner (quartz), a jet igniter with reactive cavity liner (paraffin), and a J-gap spark plug. Schlieren cinematographic records were obtained for each condition along with concomitant pressure traces. Basic features of ignition and combustion at lean limit were determined for each igniter. The spark plug permited lean operation down to an equivalence ratio of 0.7, after which mis-ignition occurred. Jet igniters provided an extension of lean limit to 0.5 in equivalence ratio. For jet igniters, these limits were imposed by either extinction of the flame or too slow burning rate, rather than by misfire.

Upon ‘harnessing the fire’ by the early pioneers, the quest for controlled combustion provided the major incentive for progress in engine technology. At first this involved primarily the problem of knock and later that of pollutant emissions. It appears that both can be solved by treating the engine cylinder not only as a source of power but also as a controllable chemical reactor. The principal concept whereby this can be accomplished involves multi-point ignition combined with charge dilution and stratification. Means for this purpose include jet ignition, product recirculation, and chemical additives. The most suitable for the realization of this concept is a direct injection two-stroke engine.

Pulsed Jet Combustion (PJC) is introduced here as a key element for engines where the progress of combustion is interactively controlled by a microprocessor system. Practical realization of PJC presented here involves the use of an 18 mm plug containing a cavity, where a rich mixture is ignited by a conventional spark discharge, closed by a tip with a suitable orifice to form the effluent stream. Its performance is determined by tests carried out in a constant volume vessel, simulating the enclosure of a CFR engine at 60 CAD with compression ratio of 7:1, using propane/air mixtures at equivalence ratios of an order of 0.6, in comparison to that of a flame traversing the charge, a so-called FTC mode, upon ignition by standard spark discharge under identical geometrical and initial thermochemical conditions. The results demonstrate the superiority of PJC for executing the exothermic process of combustion in a lean burn engine.

Square piston engine simulator experiments reveal flame kernel development and flame propagation for single event combustion. High speed color and black and white schlieren cinematography provide time and space resolved records for the combustion of intake manifold injected iso-pentane with air at a compression ratio of 6.6 to 1, engine speed of 900 rpm, spark and plasma ignition, and flat and squish pistons. Digitized schlieren records provide visual and quantitative data on the evolution of the volumetric burning rate. The numerical analysis is based on the zero Mach number model, a simplification based on the concept that the local speed of sound, as a consequence of the relatively high temperature, is practically infinite in comparison with the flow velocity. The results provide information on the propagation of the flame front and the velocity vector field that is first generated by the flame and later causes its deformation.

A square-piston engine simulator used at Berkeley to study both spark-ignited and diesel engine processes is described. The square piston configuration provides optical access to fluid mechanical and combustion processes through two fiat quartz windows used as cylinder walls. Results from three previous research projects are reviewed to illustrate the engine's capabilities. Since these studies, we developed and used a color schlieren cinematography system to record in-cylinder processes. Color schlieren movies of both spark-ignition and diesel combustion reveal the essential fluid mechanical and combustion features within the engine. For these movies, we redesigned the diesel fuel system and installed a new liquid fuel injection system for spark-ignited operation. By preventing fuel and soot condensation on the windows, these new fuel systems improved the quality of our Schlieren images.

The dynamic stage of combustion - the intrinsic process for pushing the compression polytrope away from the expansion polytrope to generate the indicator work output of a piston engine - was studied to reveal the influence of the air/fuel ratio on the effectiveness with which the fuel was utilized. The results of tests carried out for this purpose, using a 12 liter diesel engine, were reported last year [SAE 1999-01-0517]. Presented here is an analytic interpretation of the data obtained for part-load operation at 1200 and 1800 rpm. A solution is thus provided for an inverse problem: deduction of information on the dynamic features of the exothermic process of combustion from measured pressure record. Provided thereby, in particular, is information on the effectiveness with which fuel was utilized in the course of this process - a parameter reflecting the effect of energy lost by heat transfer to the walls.