Cellular Ceramics / 5.10

580

5.10

Porous Media in Internal Combustion Engines

Miroslaw Weclas

5.10.1

Introduction

Modern direct-injection (DI) combustion engines are characterized by low specific fuel consumption and high raw emission levels. Reduction of these emissions (especially NOx and soot particles) requires complex post-treatment techniques and engine-management systems. Thus, future combustion engines require new concepts with extreme reduction of raw emissions from the primary combustion process with at least unchanged or reduced fuel consumption. Advantageously, this would even be associated with a reduced need for expensive post-treatment of exhaust gases in future vehicles.

Conventional diesel engines work with an injection pressure of up to 2000 bar and local combustion temperatures can reach up to 2400 K. A problem is the time required to obtain a homogeneous mixture before ignition, and moreover the diffu- sion-controlled combustion process leads to a flame front and temperature gradient during combustion. These processes, each of which is of very complex nature, lead to the trade-off of the diesel engine: the formation of soot particles and NOx. Soot particles are formed in oxygen-lean regions and/or at peak temperatures of the combustion process, while NOx is formed in oxygen-rich regions. It is not possible in conventional combustion systems to decrease the amount of one of the components without increasing the amount of the other.

Current technologies such as electronically controlled high-pressure injection systems, variable valve control, exhaust gas recirculation (EGR), and combinations thereof, however, do not automatically solve the problem of engine emissions under all operational conditions. It is thus necessary to establish new concepts for mixture formation and combustion that allow development of future “clean reciprocating engines”.

Generally, a possible solution is a combustion system operating with a gaseous, homogeneous air/fuel (A/F) mixture (from very lean to nearly stoichiometric mixture compositions), characterized by a homogeneous combustion process in the cylinder. Such a system could also reduce the specific fuel consumption under conditions of near-zero combustion emissions. A possible route to achieve these conditions is the use of porous media in the combustion zone of a combustion engine. In this chapter the use of ceramic foams as porous media (PM) in the combustion

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

582 Part 5 Applications

Homogeneous combustion in internal combustion (IC) engines is defined as a process in which a 3D ignition of the homogeneous charge is followed by simultaneous heat release in the entire combustion chamber volume with a homogenous temperature field (Fig. 2). Besides volumetric ignition, which plays a dominant role, further aspects must be taken into account: 1) Combustion temperature, especially for close-to-stochiometric A/F compositions. It is necessary to lower this temperature from the adiabatic level to allow near-zero NOx emissions. 2) The service range of engine loads, that is, the extent to which the homogeneous combustion system can operate over a wide range of engine loads, from very lean to nearly stoichiometric A/F compositions. 3) A wide range of vehicle speed must also be considered. To satisfy the above conditions it is necessary to control the ignition timing under variable operational conditions, and to control the rate of heat release for different mixture compositions. Moreover, for low emissions it is necessary that the liquid fuel is completely vaporized prior to ignition.

Fig. 2 Conditions and requirements for homogeneous combustion in engines.

According to the definition given above, three steps of the mixture formation and combustion can be selected that define the ability of a given system to operate as a homogeneous combustion system: 1) homogenization of cylinder charge, 2) ignition conditions, and 3) heat-release process in the temperature field.

A critical factor of a system operating with homogeneous combustion is the ignition process. Among the four different ignition strategies – local ignition by spark plug, thermal self-ignition by compression, controlled (low-temperature chemical) autoignition, and 3D thermal PM self-ignition – the last three can provide a homogeneous combustion process.

A critical point is the control of ignition timing and heat-release rate under variable engine loads. This is of special importance in so-called homogeneous-charge compres- sion-ignition (HCCI) systems, which are currently under investigation [1–3]. The following aspects must be considered for further development of HCCI systems (in general for all homogeneous combustion systems) for a wide range of engine operational

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Table 1 Continued

A large specific surface area can be utilized for excellent interphase heat transfer in the PM volume, fuel (spray) distribution throughout the PM volume, and for very fast liquid vaporization in the PM volume. This is supported by excellent heat-trans- fer properties, especially by strong thermal radiation of the solid phase. Together with a high heat capacity and thermal resistance of the PM material, this kind of 3D structures can be used for realization of very clean and efficient homogeneous combustion in a PM volume. High porosity of porous structures (more than 80 %) leads very good properties for gas flow, spray, and flame throughout the PM volume. Direct electrical heating of porous reactors can be utilized for cold-start conditions and vaporization of liquid fuel, but also for afterburning and self-cleaning process. Especially for ceramic materials the method of inserting the PM reactor in engine

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components (e.g., engine head or piston top) may significantly limit the applicability of this technology to engines. Here further investigations and development of new materials are necessary. These requirements become critical if the porous medium is used directly for controlling the combustion process (as a so-called PM reactor) under high-pressure conditions. Structural features and properties of porous media, especially ceramic foams can be found in Chapters 2.1, 2.6, 3.1, and 4.3 of this book.

The following engine processes can be supported by the application of highly porous medium:

Four new concepts concerning applications of PM technology to mixture formation and combustion in IC engines can be considered:

. Combustion system with mixture formation and homogeneous combustion in the PM volume, so-called PM engine concept [9, 10].

. Mixture formation system, with heat recuperation, vaporization, and chemical recombination in PM volume, so-called MDI concept (mixture direct injection) [7].

. Intelligent engine concept based on the MDI system permitting homogeneous combustion conditions (in a free cylinder volume) over a wide range of engine operational conditions [7].

. Phased combustion system for conventional DI diesel engine, with temporal and spatial control of mixture composition by utilization of interaction between diesel jet and PM structure, so-called two-stage combustion.

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Fig. 3 Top: comparison of high-pressure fuel injection onto solid wall (left) and into porous medium (right). Common-rail diesel injection onto porous plate: experimental setup (middle) and jet dispersion after passing a ceramic foam plate (bottom).

Most of the PM technologies reported in the literature focus on internal heat recuperation in engines [11–22]. The main goal of such PM applications in internal combustion engines is to influence the thermal efficiency of the engine by internal heat recuperation. There are also concepts combining heat regeneration and catalytic reduction of toxic components, gaseous and particulate. Heat flux and energy recirculation in such engines have been described in detail in Ref. [11]. In this case the heat recuperator is attached to a rod and moves inside the cylinder, synchronized to the piston movement. The main advantage of such internal (in-cylinder) heat

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recuperation between burned gases and fresh air is high volumetric efficiency of the cylinder, which is necessary for high power density of the engine.

This kind of application of the porous medium to internal combustion engine deals with energy balance of the cycle. Engines with heat recuperation could realize much higher combustion temperatures which result in much higher NOx emissions.

The presented engine concept could also be extended by application of catalytic porous insert offering afterburning of combustion products such as particles. A similar concept was recently analyzed by Hanamura and Nishio [12]. Also in this engine the maximum combustion temperature is higher than adiabatic owing to heat recuperation in a porous medium.

Another application of porous-medium technology to internal combustion engines covers exhaust post-treatment systems, especially catalytic converters and particle filters. More information on application of porous-medium technology, especially to diesel particle filters, can be found in the literature [23–28] (see also Chapter 5.2 of this book).

5.10.4

The PM Engine Concept: Internal Combustion Engine with Mixture Formation and Homogeneous Combustion in a PM Reactor

Here PM engine is defined as an internal combustion engine in which a homogeneous combustion process is realized in a porous-medium volume. The following individual processes of the PM engine can be realized in the volume of a porous medium: internal heat recuperation, fuel injection, fuel vaporization, mixing with air, homogenization of charge, 3D thermal self-ignition, and homogeneous combus-

PM-Reactor

Fig. 4 Possible locations of PM reactor in PM engines: in head (left), in piston (middle), and in cylinder (right).

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Simultaneously, other processes can be performed in the cylinder volume and continued through exhaust, intake, and compression strokes (Fig. 5a). Near the TDC of compression (Fig. 5b) the valve in the PM chamber opens and the compressed air flows from the cylinder to the hot PM containing fuel vapor. Very fast mixing of both gases occurs before mixture ignition in the whole PM volume (Fig. 5c). The resulting heat release process occurs simultaneously in the entire PM volume. Three necessary conditions for a homogeneous combustion are fulfilled here: homogenization of charge in the PM volume, 3D thermal self-ignition in the PM volume, and volumetric combustion with a homogeneous temperature field in the PM volume. Additionally, the PM acts as a heat capacitor and controls the combustion temperature.

5.10.4.2

PM Engine with Open PM Chamber

Another possible realization of the PM engine is a combustion system with a permanent contact between working gas and PM reactor. In this case it is assumed that the PM combustion chamber is mounted in the engine head, as shown in Fig. 6. During the intake stroke (Fig. 6a) the PM heat capacitor has a weak influence on the in-cylinder thermodynamic air conditions. Also during early compression stroke, only small amount of air contact the hot PM. This heat exchange process (nonadiabatic compression) increases with continuing compression timing (Fig. 6b), and at the TDC the whole combustion air is entrapped in the PM volume. Near the TDC of compression the fuel is injected into the PM volume (Fig. 6c) and very fast fuel vaporization and mixing with air occur in the 3D structure of the PM. A 3D thermal

Fig. 6 Principle of PM engine operation with an open chamber; 1) intake valve, 2) exhaust valve, 3) fuel injector, 4) piston.