Early Injection Strategy for Low-emission Premixed-Combustion Engine: History
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Low-emission and high-efficiency have always been the targets for Internal Combustion Engine development. For diesel engines, homogeneous charge (aka. HCCI) and premixed charge (aka. PCCI) combustion modes provide both low-emission and high-efficiency simultaneously. To achieve these advanced combustion modes, early injection is needed as a relatively longer air-fuel mixing time is guaranteed. Several key parameters, such as the injection timing, pressure, angle, directly determine the final combustion process and thus the emission and efficiency performance. The pros and cons of these key parameters are discussed in detail here to provide a good review of the early-injection strategy.

  • early injection
  • HCCI
  • PCCI
  • Internal Combustion Engine
  • Diesel Engine
  • NOx
  • Soot

1. Advanced Combustion Modes: HCCI and PCCI

The conventional diesel combustion process can be classified into four major phases: ignition delay, premixed combustion, mixing controlled combustion, and the late burning phase[1]. During the premixed combustion phase, polyaromatic hydrocarbons, the precursors of soot, are quickly formed in the hot (1600–2000 K), fuel-rich combustion regions. Soot formation follows, filling the entire downstream jet cross-section. Near the peak heat release rate of premixed combustion, a diffusion flame forms in the periphery of the fuel-rich, high-temperature downstream regions of the jet. NOx emission forms in the hot (1800–2000 K) and near-stoichiometric mixtures in the periphery of the jet near the diffusion flame. So the conventional combustion phase regime encompasses both NOx and soot islands, as shown in Figure 1. This is not preferable while considering the more and more stringent emission regulations. Therefore, advanced combustion modes that could eliminate or avoid the fuel-rich and high-temperature environment are needed.

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Figure 1. φ-T diagram of conventional combustion, homogeneous charge compression ignition (HCCI) combustion, and premixed charge compression ignition (PCCI) combustion[2]. φ, equivalence ratio; T, temperature.

HCCI combustion was first proposed by Onishi et al.[3] and Noguchi et al.[4]. The main characteristic of HCCI is a (more or less homogeneous) premixed air-fuel mixture that undergoes auto-ignition as a result of compression. The auto-ignition allows the combustion of a very lean mixture, which helps to eliminate the fuel-rich region, resulting in low soot emission. The combustion temperature is significantly lower than that of conventional diesel combustion, which is beneficial for the reduction of NOx emissions. However, a major difficulty in HCCI is to get a homogeneous admixture of air and fuel. This is especially true for diesel engines, because the lower volatility of diesel fuels makes it more difficult to obtain a homogeneous mixture compared to gasoline fuels. Besides, the high cetane number of conventional diesel fuel results in large rates of pressure rise and difficulties in combustion phasing control[5][6][7][8][9].

PCCI combustion has been described as a middle path between conventional and HCCI combustion modes[10][11][12][13]. In HCCI combustion, there is the challenge of combustion phasing control and homogeneous mixture preparation. To overcome these problems, for PCCI combustion, only part of the fuel undergoes the HCCI type of clean combustion, while the remainder undergoes conventional combustion. Since the remaining fuel undergoes conventional combustion, the combustion phasing is still controlled by the injection timing. Also, only partial fuel is used to prepare the homogeneous mixture, so the mixture preparation for PCCI combustion is simpler than for HCCI combustion. The part of the premixed fuel results in peak equivalence ratios staying below the soot formation threshold. Further, high levels of EGR are often used to decrease the oxygen concentration and lower peak flame temperatures, resulting in the movement of the NOx island.

As given by Table 1, both HCCI and PCCI provide clear advantages out of the conventional diesel combustion mode regarding the soot and NOx emissions.

Table 1. Comparison of key characteristics of conventional diesel, HCCI, and PCCI combustion.

2. Early Injection Strategy Definition

The preparation of a homogeneous mixture is important for both HCCI and PCCI combustion. In order to allow enough time for fuel to mix with the air before combustion, the early injection strategy, by which the fuel is injected in an early stage of the compression stroke, has been applied widely in HCCI and PCCI diesel engines. The start of early injection is typically 20–200 before the top dead center(BTDC). Based on the characteristic of HCCI and PCCI combustion, the early injection strategy can be classified as a single injection and two-stage injection, as seen in Figure 2. For a two-stage injection, the first injection is also called the pilot injection, and the second injection is also called the main injection. Based on the injection timing, the early injection strategy can be divided into three patterns, as seen in Figure 3: The injection closest to TDC is defined as late; that farthest from TDC is defined as early, and the one in between is defined as middle[14]. The demarcation points of these three patterns in this paper are defined as 60, 40, and 20 BTDC, respectively.

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Figure 2. Single and two-stage early injection strategy.

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Figure 3. Early injection strategy divided by injection timing.

Most of these early injection strategies have a certain unavoidable influence on the fuel spray formation inside the combustion chamber, further affecting the combustion and emissions of HCCI and PCCI engines. In addition, using the early injection strategy will cause a wall-wetting problem. Because of the lower temperature of the gas and the density in the cylinder during the early injection period, the fuel spray will impinge on the cylinder wall or piston head due to the slow fuel vaporization rate and longer liquid penetration length. Wall-  wetting mainly leads to (1) low combustion eciency, (2) excessive soot/carbon monoxide (CO)/hydrocarbon (HC) emissions, and (3) (local) oil dilution[15][16]. Many methods, including limiting the injection angle, have been proposed to limit or reduce wall-wetting.

Overall, there are several key parameters in the early-injection process that define the final combustion and emission performances. The effects of fuel injection pressure, injection timing, and injection angle on engine performance and emissions are discussed in detail separately in the following sections.

3. Effects of Injection Pressure

Injection pressure could change the combustion and the emissions significantly as it directly determines the fuel spray, injection duration, and therefore the time to mix air and fuel into a homogenous mixture. However, it is not a straightforward approach to have lower emissions by simply increasing the injection pressure.

On the one hand, higher injection pressure results in shorter injection duration and longer premixing time before the onset of combustion. The increased injection pressure leads to better atomization of the fuel, which causes better air-fuel mixing and fewer fuel-rich regions. This results in a higher heat release peak, with a rapid burn rate and a shorter combustion duration, which is beneficial for engine thermal efficiency. In addition, the in-cylinder temperature for higher injection pressure is higher than that for low injection pressure. The better air-fuel mixing and higher in-cylinder temperature benefit the oxidation of soot, CO, and HC.

On the other hand, the length of spray penetration increases under higher injection pressure. This can cause serious spray-wall impingement, because both the in-cylinder temperature and pressure are low for the early injection duration. Fuel impingement on the piston head or cylinder wall leads to incomplete fuel vaporization and oxidation, which creates either over-rich or over-lean regions. This conflicts with the advantage of atomization with higher injection pressure mentioned above.

Similarly, the final NOx emission is also a result of these paradoxical effects. High in-cylinder temperature is usually observed with high fuel injection pressure, which promotes the formation of NOx. However, a better mixing process under high injection pressure makes it easier to achieve HCCI combustion, which brings low NOx emission.

Table 2. Variation of performance and emissions after increasing the injection pressure. BSFC, brake-specific fuel consumption. 

Author

Injection Pressure (bar)

Fuel

BSFC

NOx

HC

CO

Soot

Jeong et al.[17]

500–900

Diesel

na

na

na

Fang et al.[18] 

600/1000

Diesel

na

na

na

Shimazaki et al.[19]

300–1200

Diesel

↓↑

Kiplimo et al.[20]

800/1400

Diesel

Liu et al.[21]

600–1400

Diesel

na

na

na

na

Chen et al.[22]

1000–1400

Diesel

na

na

Siewert[23]

800–1600

Diesel

na

Park et al.[24]

400/1200

Bioethanol blends

Arun et al.[25]

200–240

Carbon black–water–diesel

na

↓↑

↓↑

↓↑

↓↑

Nanthagopal et al.[26]

200–240

Biodiesel

With reviewing multiple research studies and summarized in Table 2, the effects of fuel injection pressure on the emission of different species can be seen clearly. It can be found that for both engines, with increased injection pressure, the engine thermal efficiency improved. NOx emissions increased slightly due to the higher combustion temperature. The levels of soot, HC, and CO emissions were determined by the paradoxical effect of better atomization or more serious wall impingement. However, soot emissions were always reduced by increasing the injection pressure.

4. Effects of Injection Timing

On one hand, injecting the fuel at an earlier time prolongs the ignition delay and helps to create a more homogeneous mixture. The formed lean mixture is then burned at a low temperature, resulting in low NOx emissions.

On the other hand, the cylinder pressure and temperature are low under earlier injection timing, which leads to poor fuel evaporation and the wall-wetting problem. Under earlier injection conditions, the fuel-air mixture is mostly formed at the outside of the combustion chamber, and some local rich mixture regions are formed due to the wall-wetting issue. Moreover, the negative work during the compression stroke increases because of the earlier combustion event. These all deteriorate the combustion efficiency and increase the products of incomplete combustion. For HC and CO emissions, the impingement target is an important factor. For middle injection timing, spray wall impingement occurs on the piston head or the outside part of the combustion chamber. For early injection timing, the impingement occurs on the cylinder wall. The different temperature and flow motions of the piston head and cylinder wall directly affect the evaporation process of the wall film. In addition, research results show that when the impingement target is at the bowl–lip area, the fuel-air mixing can be better and low HC and CO emissions can be achieved.

For soot emission, the factor of injection timing has two opposite effects. Earlier injection timing means that longer premixing allows the mixture to reach a lower equivalence ratio for low temperature, which restrains the generation of soot. Moreover, a longer time for soot oxidation is also achieved with earlier injection. On the contrary, spray wall-wetting will form some local rich regions, especially in the crevices, which promotes soot generation. Furthermore, the temperature of these wall-wetting regions is generally lower, which prevents the oxidation of soot emission.

As discussed above, the early injection strategy contains single and two-stage injection modes. Thus, in this section, the effects of the injection timing are discussed separately based on the injection mode.

4.1 Single Early Injection Timing Effects

Benajes et al.[27] investigated the influence of injection timing on particle emissions with early fuel injection timing of low-temperature diesel combustion. The injection timing was set from 33 to 24 BTDC. Results showed that PM mass and particle number increased with advanced fuel injection timing; the number of particles larger than 50 nm was especially increased. This was mainly because the higher relative levels of liquid fuel deposition on the piston bowl surface formed a locally rich mixture, which promoted soot generation. HC and CO emissions were also increased due to the spray overshoot. Kiplimo et al.[20] studied the impact of injection timing on the performance and emissions of an HCCI diesel engine. The results indicated thermal efficiency and IMEP decreased with advanced injection timing. Injection timing earlier than 30BTDC resulted in higher smoke emissions. This could be affected by the fuel impinging on the piston surface and splashing to the crevices, causing a rich-fuel zone. NOx emissions were lower with earlier injection timing. With the injection timing advanced, CO and HC emissions increased dramatically, owing to the wall-wetting. Figure 4 gives a visualization of the interactions of fuel spray and the surface/wall under various injection timings. Kim and Lee[28] examined the influence of injection timing on the performance and NOx emissions of an HCCI diesel engine. Results showed that IMEP decreased rapidly as the injection timing was advanced beyond 20 BTDC. When the injection timing was set between 30 and 50 BTDC, IMEP was approximately half of that of conventional diesel combustion. NOx emissions were strongly affected by the injection timing. As the injection timing was advanced beyond 30 BTDC, NOx emissions fell near to nearly 0. Kook et al.[29] investigated the effect of injection timing on premixing and combustion in a single-cylinder diesel engine. The injection timing was varied from 50 to 200 BTDC. The results showed that the early injection produced negative work. This negative work decreased as the injection timing was advanced by 70 BTDC due to the extended ignition delay period and retarded combustion phasing, and this caused increased IMEP and thermal efficiency. However, at more advanced injection timing, IMEP and thermal efficiency started to decrease because of the decreased combustion temperature.