Homogeneous Charge Compression Ignition of Methane

Homogeneous Charge Compression Ignition (HCCI) engines are being considered as an alternative to traditional spark- and compression-ignition engines. As the name implies, a homogeneous fuel/oxidant mixture is auto-ignited by compression with simultaneous combustion occurring throughout the cylinder volume. Combustion temperatures under lean burn operation are relatively low, resulting in low levels of NOx emission. Furthermore, the fuel’s homogeneous nature, as well as the combustion process itself, lead to low levels of particulate matter being produced.

Although HCCI combustion shows promise, the method has several recurring problems: an important one to be addressed is ignition timing. This example examines the HCCI of methane, investigating ignition trends as a function of initial temperature, initial pressure, and fuel additives.

This example solves the mass and energy balances describing the detailed combustion of methane in a variable-volume system. The large amount of kinetic and thermodynamic data required to set up the problem is easily available by importing the relevant files into the Reaction Engineering interface.

Model Definition

It is difficult to form the uniform mixtures required for HCCI with conventional diesel fuel. Natural gas fuels, on the other hand, readily produce homogeneous mixtures and have the potential to serve as HCCI fuels. This example considers the combustion of methane, as described by the GRI-3.0 mechanism, incorporating a detailed reaction mechanism of 53 species taking part in 325 reactions. The files describing the reaction kinetics and thermodynamics of the GRI-3.0 mechanism are available on the Internet (Ref. 1), and you can import the files into the Reaction Engineering interface.

Variable Volume Reactor

This model represents the combustion cylinder with a perfectly mixed batch system of variable volume, a predefined reactor type available with the Reaction Engineering interface. Figure 1 shows an engine cylinder and it includes the relevant parameters to calculate the instantaneous cylinder volume.

Figure 1: The volume of a combustion cylinder can be expressed as a function of time with the slider-crank relationship. The diagram shows the key geometric parameters. La is the length of the crank arm, Lc is the length of the connecting rod, D equals the cylinder diameter, and α is the crank angle.

The volume change as a function of time is described by the slider-crank equation:

where V is the cylinder volume (SI unit: m3), Vc is the clearance volume (SI unit: m3), CR equals the compression ratio, and R denotes the ratio of the connecting rod to the crank arm (Lc/La). Further, α is the crank angle (SI unit: rad), which is also a function of time

where N is the engine speed in rpm, and t is the time (SI unit: s).

The engine specifications are:


Engine specification	Variable name	value
Bore	D	13 cm
Stroke	S	16 cm
Connecting rod	Lc	26.93 cm
Crank arm	La	8 cm
Engine speed	N	1500 rpm
Compression ratio	CR	15

Equation 1 includes the clearance volume Vc which is calculated from

(1)

Vs is the volume swept by the piston during a cycle from the equation

Figure 2 shows the calculated cylinder volume as a function of the crank angle. The piston is initially at bottom dead center (BDC), corresponding to a crank angle of −180 degrees.

Figure 2: Cylinder volume as a function of crank angle. The crank angle is defined as being zero at top dead center (TDC).

Methane Combustion reaction

The kinetic and thermodynamic data for methane combustion is available in the form of CHEMKIN data input files. The files are imported into the Reaction Engineering interface within the Reversible Reaction Group and Species Thermodynamics (belongs to Species Group) features. This automatically sets up the mass and energy balances for a batch reactor of constant volume.

In this example methane is combusted under lean conditions, that is, supplying more than the stoichiometric amount of oxidizer. The stoichiometric requirement of the oxidizer (air) to combust methane is found from the overall reaction:

Assuming that the composition of air is 21% oxygen and 79% nitrogen, the stoichiometric air-fuel ratio is

(2)

The equivalence ratio relates the actual air-fuel ratio to the stoichiometric requirements

(3)

This model sets the equivalence ratio to Φ = 0.5.

From Equation 2 and Equation 3 it is possible to calculate the molar fraction of fuel in the reacting mixture as

and subsequently the initial concentration is

Results and Discussion

Figure 3 shows the cylinder pressure as a function of time when a methane-air mixture is compressed and ignites. The piston starts at bottom dead center (BDC) and reaches top dead center (TDC) after 0.02 s. At BDC the pressure is set to 1.5 × 105 Pa, Φ is 0.5, and the compression ratio is CR = 15. The initial temperature is varied from 400 K to 800 K.

Figure 3: Pressure traces illustrating the compression and ignition of fuel in an engine cylinder. The initial temperature varies between 400 K and 800 K.

Consistent with literature results, methane does not ignite at an initial temperature of 400 K (Ref. 2). Furthermore, the induction delay decreases with increasing initial temperature. The induction delay time can be evaluated from the pressure gradient. For instance, the induction delay is 0.0193 s when Tinit = 500 K.

Figure 4 illustrates the pressure traces as the initial pressure varies from 1 × 105 Pa to 3 × 105 Pa. The initial temperature is 500 K. An increase in pressure means an increase in the species concentrations in the fuel-air mixture, resulting in the expected advance in ignition times.

Figure 4: Increased initial gas pressure advances ignition times.

As mentioned, a significant challenge to the realization of HCCI engines is ignition control. In this regard, combustion at TDC is suggested as the optimum timing (Ref. 3). These results show that the inlet temperature of the fuel-air mixture is a potential tuning parameter for ignition. However, relatively high inlet temperatures are often required for proper timing. This adversely affects engine performance because the trapped mass as well as the volumetric efficiency decreases. An alternative that facilitates ignition is to mix small amounts of additives into the fuel-air mixture (Ref. 4). These additives chemically activate the reaction mixture even at relatively low temperatures. This approach alleviates the requirements of high intake temperatures. Figure 5 shows how small amounts of formaldehyde (CH2O) cause ignition at an initial temperature of 400 K, which is a temperature insufficient to induce combustion with a pure methane fuel.

Figure 5: Small amounts of formaldehyde stimulate ignition of the fuel-air mixture.

The increased reactivity observed in the presence of CH2O is explained by the opening of a new chemical pathway leading to the formation of hydroxyl radicals. Specifically, CH2O reacts with O2 to produce H2O2:

H2O2, in turn, decomposes to reactive OH radicals, which subsequently react violently with the fuel molecules to cause ignition:

The results in the following figures show the species molar fractions of CH2O, HO2, H2O2, and OH during the combustion of methane. Figure 6 shows molar fraction plots for the case when 0.13% CH2O is added to the fuel; Figure 7 is the equivalent species plots for the case when pure methane is combusted. In each case conditions are tuned to produce ignition near TDC, thus providing a reference point for comparing the species concentrations.

Figure 6: Selected species molar fractions as a function of crank angle. 0.13 molar percent CH2O is added to the reacting mixture, which is initially at 400 K and 1.5 bar.

Figure 7: Selected species molar fraction as a function of crank angle. Only methane is combusted. The initial temperature is 469 K and the initial pressure is1.5 bar.

The implications of the CH2O reaction path are apparent by comparing Figure 6 and Figure 7: CH2O stimulates the production of HO2 and H2O2, which in turn produce OH radicals in amounts critical to fuel ignition.

References

1. G.P. Smith, D.M. Golden, M. Frenklach, N.W. Moriarty, B. Eiteneer, M. Goldenberg, C.T. Bowman, R.K. Hanson, S. Song, W.C. Gardiner, Jr., V. V. Lissianski, and Z. Qin, GRI-Mech 3.0 home page, http://www.me.berkeley.edu/gri_mech/.

2. S.B. Fiveland and D.N. Assanis, “A four-stroke homogeneous charge compression ignition engine simulation for combustion and performance studies, “SAE Paper 2000-01-0332, 2000.

3. D.L. Flowers, S.M. Aceves, C.K. Westbrook, J.R. Smith, and R.W. Dibble, “Detailed Chemical Kinetic Simulation of Natural Gas HCCI Combustion: Gas Composition Effects and Investigation of Control Strategies,” J. Eng. Gas Turbine Power, vol. 123, no. 2, pp. 433–439, 2001.

4. M.H. Morsy, “Ignition control of methane fueled homogeneous charge compression ignition engines using additives,” Fuel, vol. 86, no. 4, pp. 533–540, 2007.

Chemical_Reaction_Engineering_Module/Ideal_Tank_Reactors/compression_ignition

Notes about the COMSOL Implementation

The kinetic and thermodynamic data required for this model are available on the Internet. Find the GRI-Mech 3.0 input files at (Ref. 1):

http://www.me.berkeley.edu/gri_mech/version30/text30.html.

Download the reaction mechanism and rate coefficient file (grimech30.dat), as well as thermodynamic data file (thermo30.dat) and store them on your computer so you can import these into the Reaction Engineering interface.

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