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Research Papers: Internal Combustion Engines

# Modeling the Effects of Variable Intake Valve Timing on Diesel HCCI Combustion at Varying Load, Speed, and Boost Pressures

[+] Author and Article Information
C. L. Genzale

Engine Research Center, Department of Mechanical Engineering, University of Wisconsin—Madison, 1500 Engineering Drive, Madison, WI 53705genzale@wisc.edu

S.-C. Kong

Department of Mechanical Engineering, Iowa State University, 3028 Black Engineering Building, Ames, IA 50011

R. D. Reitz

Engine Research Center, Department of Mechanical Engineering, University of Wisconsin—Madison, 1500 Engineering Drive, Madison, WI 53705

J. Eng. Gas Turbines Power 130(5), 052806 (Jun 11, 2008) (8 pages) doi:10.1115/1.2938270 History: Received October 21, 2005; Revised May 09, 2008; Published June 11, 2008

## Abstract

Homogeneous charge compression ignition (HCCI) operated engines have the potential to provide the efficiency of a typical diesel engine, with very low $NOx$ and particulate matter emissions. However, one of the main challenges with this type of operation in diesel engines is that it can be difficult to control the combustion phasing, especially at high loads. In diesel HCCI engines, the premixed fuel-air charge tends to ignite well before top dead center, especially as load is increased, and a method of delaying the ignition is necessary. The development of variable valve timing (VVT) technology may offer an important advantage in the ability to control diesel HCCI combustion. VVT technology can allow for late intake valve closure (IVC) times, effectively changing the compression ratio of the engine. This can decrease compression temperatures and delay ignition, thus allowing the possibility to employ HCCI operation at higher loads. Furthermore, fully flexible valve trains may offer the potential for dynamic combustion phasing control over a wide range of operating conditions. A multidimensional computational fluid dynamics model is used to evaluate combustion event phasing as both IVC times and operating conditions are varied. The use of detailed chemical kinetics, based on a reduced $n$-heptane mechanism, provides ignition and combustion predictions and includes low-temperature chemistry. The use of IVC delay is demonstrated to offer effective control of diesel HCCI combustion phasing over varying loads, engine speeds, and boost pressures. Additionally, as fueling levels are increased, charge mixture properties are observed to have a significant effect on combustion phasing. While increased fueling rates are generally seen to advance combustion phasing, the reduction of specific heat ratio in higher equivalence ratio mixtures can also cause noticeably slower temperature rise rates, affecting ignition timing and combustion phasing. Variable intake valve timing may offer a promising and flexible control mechanism for the phasing of diesel HCCI combustion. Over a large range of boost pressures, loads, and engine speeds, the use of delayed IVC is shown to sufficiently delay combustion in order to obtain optimal combustion phasing and increased work output, thus pointing towards the possibility of expanding the current HCCI operating range into higher load points.

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## Figures

Figure 1

Axisymmetric computational grid of piston bowl at top dead center

Figure 2

Effect of late IVC timing on cylinder pressure at base line operating condition (821rpm, mfuel=31.2mg, Tin=298K, and Pin=102.5kPa)

Figure 3

Effect of late IVC timing on heat release rate at base line operating condition (821rpm, mfuel=31.2mg, Tin=298K, and Pin=102.5kPa)

Figure 4

Effect of late IVC timing on 50% AHR points at varying fueling rates (821rpm, Tin=298K, and Pin=102.5kPa)

Figure 5

Effect of late IVC timing on 50% AHR points at varying boost pressures (821rpm, mfuel=31.2mg, and Tin=298K)

Figure 6

Effect of late IVC timing on 50% AHR points at higher load with varying boost pressures (821rpm, mfuel=62.4mg, and Tin=298K)

Figure 7

Effect of late IVC timing on 50% AHR points at high and low engine speeds (mfuel=31.2mg, Pin=102.5kPa, and Tin=298K)

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