Combustion Systems

Hannu Jääskeläinen, Magdi K. Khair

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• Introduction
• Combustion Chamber Geometry
• In-Cylinder Flow
• Compression Ratio

Introduction

Combustion systems incorporate multiple factors that impact the combustion process. These include:

• the fuel injector nozzle,
• fuel injection characteristics,
• in-cylinder gas composition,
• in-cylinder flow characteristics,
• combustion chamber geometry,
• compression ratio and
• cylinder dimensions.

In all combustion systems, these factors must work together to ensure that the combustion process, whether conventional or advanced, achieves the required performance and emissions goals.

This paper discusses some aspects related to combustion chamber geometry, in-cylinder flow and compression ratio.

Combustion Chamber Geometry

Design Considerations

Diesel combustion is known to be very lean with A/F ratios of 25:1 at peak torque, 30:1 at rated speed/maximum power conditions, and over 150:1 at idle for turbocharged engines. Yet, this extra air does not enter into the combustion process. It is rather heated during combustion and exhausted—causing diesel exhaust to be lean. Even though the average air-fuel ratio is lean, if proper care is not taken in the design process, regions of the combustion chamber can be fuel rich and lead to excessive smoke emissions. A key objective in designing the combustion bowl then is to ensure that mixing of fuel and air is adequate to mitigate the impact of fuel rich regions and allow the engine to meet its performance and emissions targets. Turbulence in the air motion within the combustion bowl is found to be beneficial to the mixing process and can be used to achieve this goal. Swirl induced by the intake port can be enhanced or squish can be generated by the piston as it approaches the cylinder head to create more turbulence during the compression stroke through proper design of the bowl in the piston crown.

Combustion chamber design has the most significant impact on particulate emissions. It can also have an influence on unburned hydrocarbons and CO. While NOx emissions can be influenced by bowl design [3128], bulk gas properties play a very important role in their exhaust levels. However, because of the NOx/PM trade-off, combustion chamber designs have had to evolve as NOx emission limits have decreased—primarily to avoid the increases in PM emissions that would otherwise result.

A review of design considerations for combustion systems can be found in the literature [3489][3490].

K-Factor. An important parameter for optimizing a DI diesel combustion system is the proportion of available air participating in the combustion process [734][3489]. The K-factor, calculated as the ratio of the piston bowl volume to the clearance volume, is an approximate measure of the proportion of air available for combustion. Reducing engine displacement leads to a decrease in the relative K-factor and hence a tendency for deterioration in the combustion behavior. For a given displacement and at constant compression ratio, the K-factor can be improved by selecting a longer stroke. Selection of the bore-to-stroke for an engine can be influenced by the K-factor consideration and several others factors including: engine packaging, ports and valves, etc. A particularly key issue in setting the maximum bore-to-stroke ratio is the very challenging cylinder head packaging required to accommodate a four-valve per cylinder design and a common-rail fuel injection system with a centrally-located injector. Cylinder heads are complicated to design due to the many passages including water cooling, cylinder head hold down bolts, intake and exhaust ports, injectors, glow plugs, valves, valve stems, valve recesses and valve seats, as well as other passages such as those used for exhaust gas recirculation in some designs [735].

Open vs. Re-Entrant Combustion Chambers. Combustion chambers in modern direct injection diesel engines can be designated as either open or re-entrant. If the top opening of the bowl in the piston is smaller in diameter than the maximum bowl diameter, it is a re-entrant bowl. These bowls have a “lip”. If there is no lip it is an open combustion chamber [3490].

Types of Combustion Bowls

Mexican Hat Bowls

Diesel engine combustion chambers with a “Mexican hat” bowl, also known as the “Hesselman” chamber have been known since at least the 1920s [3126]. These open combustion chambers were commonly used until about 1990 in heavy-duty engines before the re-entrant bowl became more important. Figure 1 illustrates the general shape of this type of bowl. Note the straight sides at the outer periphery [3127]. This combustion chamber shape is intended for relatively advanced injection timing values were the bowl contains a bulk of the burning gases. It is not well suited for retarded injection timing strategies.

Figure 2 compares the soot emissions from a number of bowls including the Mexican hat bowl. Note that some of the alternative geometry chambers (re-entrant bowls) provide better soot oxidation at the engine conditions shown. Bowl shape had less influence on soot at higher engine speeds in this study [3128].

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