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Multiple Fuel Injection Strategies for Compression Ignition Engines: History
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Contributor: Tyler Simpson , Christopher Depcik

A significant volume of work is devoted to testing diesel fuel under a multiple fuel injection scenario whereas comparatively less is seen about neat biodiesel and biodiesel blends with diesel. Many mechanisms have been proposed to explain the reduction of exhaust emissions, combustion noise, and fuel consumption possible with multiple fuel injection events.

  • combustion
  • compression ignition
  • emissions
  • fuel injection

1. Summary of Multiple Fuel Injection Strategies Using Diesel Fuels

Starting with pilot injections, a broad range of injection timings and quantities for both the pilot and subsequent main injection events have been explored. This is in addition to a varying number of pilot injections per engine cycle. Pilot injection experimentation began with the goal of combustion noise and nitrogen oxides (NOx) emissions reduction, which has been successful while also being effective in reducing the brake specific fuel consumption (BSFC). The general thought was that noise could be decreased through lowering peak cylinder pressure rise rates [1][2][3]. The study by Warey et al. in 2015 [4] delved further into the noise reduction mechanism, showing that it only takes a pilot quantity of ~6% to reach significant reductions in combustion noise. This was mainly attributed to two possible mechanisms. First, the pressure rises and falls due to heat release and expansion can influence the relative pilot and main injection combustion phasing that may subsequently impede pressure fluctuations in a critical frequency range. Second, combustion phasing that causes destructive interference may be present only due to pressure fluctuations because of the heat release.
The original notion that NOx reduction could be attributed to a decrease in peak heat release rate has been confirmed many times [3][5][6][7][8][9]; however, a strong load dependency has also been reported [2][3][6][7][8][10][11]. Every pilot quantity has been explored that can still be considered a “pilot” injection (i.e., quantities less than 50%), and pilot injection timing has been tested as early as 90° Before Top Dead Center (BTDC) (i.e., the compression stroke) [12]. Significant early injection events often come with piston/cylinder wall impingement issues, especially at low engine speeds and loads with associated low turbulence. This causes oil dilution and spikes in hydrocarbons (HCs), carbon monoxide (CO), and particulate matter (PM) emissions [12][13][14][15]. In addition, pilot Start of Injection (SOI) has been investigated as late as 0° After Top Dead Center (ATDC) [16], with a main SOI as delayed as 10° ATDC. At low loads, optimal injection parameters for NOx abatement appears include a small pilot quantity of 10–15% [8][17][18][19][20].
There are countless reports of pilot/main dwell times that are favorable, but they are relative to the phasing of the injections with respect to piston location, as well as the number of pilot injections. Nevertheless, pilot/main dwells ranging from 2° to 80° have been reported [15][21]. At medium load, it has been reported that pilot influence starts to lessen and requires a larger pilot quantity (around 18–30%) [5][15][22]. At high loads, pilot injections are often reported to have little to no influence on combustion [3][6][8][11]; however, some researchers had success with NOx and noise reduction at high loads [13][23][24]. This lessened pilot influence as load progresses has been thought to be a function of the decreasing premixed combustion phase, whereas low load is predominately premixed. This necessitates other means of combustion modulation such as main injection splitting. Furthermore, multiple pilot injections have been reported to be effective [5][14][15][19][25][26], even at high loads [26], with even stronger effects at part load. It is generally agreed upon that double pilot injections are optimal [5][14][15][19][25]. An important takeaway from these results is that the use of a pilot injection allows for delayed main injection timing that benefits NOx emissions, as well as potentially BSFC due to a lengthened combustion event into the expansion stroke [6][7][18][27]. While some researchers found a growth in PM emissions while using pilot injections [13][14][18], many have experienced reductions in PM with little to no penalty in NOx production [7][19][25][26]. Conversely, researchers have seen NOx reductions with little to no penalty in PM levels [28] and even simultaneous NOx and soot reduction [8][11][15][16][24][27][29][30] as compared to a single injection event.
Main injection splits have been successful in reducing NOx levels and combustion noise [9][17][23][31][32][33][34][35] via the same mechanisms discussed via pilot injections. Moreover, main injection splits in conjunction with pilot injections have compounded benefits in NOx and combustion noise [17][32][34]. Main injection splitting has been widely shown to increase fuel air mixing to reduce PM production [17][35]. Here, the reports indicate PM reduction with little to no impact on NOx [17][35], or NOx emission decreases with little to no impact on PM, as well as simultaneous PM and NOx reductions [17][23][31][34]. Furthermore, it has been reported than main injection splits can reduce BSFC by maintaining a high heat release rate for a longer amount of time [32]. Main injection splits also brought forth the witness of an additional soot reducing mechanism. This was explained by the discontinuation of the fuel rich soot producing zone at the tip of the fuel jet that is not replenished since the proceeding injection is sent into a high temperature and pressure setting caused by the preceding injection event. This mechanism was first proposed by Han et al. in 1996 and has since been confirmed by multiple studies [33][36]. Main injection splits have been investigated for up to four injections [32][37] with optimal BSFC and PM reports resulting from two injections [32][35][37][38]. A load dependency has also been observed, albeit with a lessened effect of favorable behavior as compared to pilot injections. Overall, it has been reported that main injection splits can be effective at all loads, with generally dampened effects at high load [11][17][23].
The literature has proven post injections to be effective for PM reduction without increasing NOx emissions while only incurring a potentially small growth of BSFC [11][13][18][19][21][24][34][39][40][41][42][43]. Largely, the mechanisms that make post injections effective operate differently than main injection splits or pilot injections. While post injections do share the same quality of discontinuing the soot producing region at the tip an initial fuel jet, that benefit is lessened due to the lower temperatures and pressures occurring during the expansion stroke. Instead, post injection PM reduction is a function of an improved mixing and continued combustion that influences the soot production and oxidation battle. While temperatures are too low to have any BSFC improvements, temperatures are still high enough to contribute to the late-stage soot oxidation phase seen only minimally during the traditional single injection event strategy. The post injection schemes proven to be the most effective deal with post injection quantities of approximately 10–17% [13][16][19][24][39][40][41][42][43] and SOIs no later than 27° ATDC [16]. Furthermore, post injection effectiveness is highly dependent on load [39][42]. This is similar to pilot injections since the soot producing diffusion phase becomes more predominate as load increases; hence, the soot production and oxidation battle naturally shifts.
While pilot, main, and post injections have been widely studied while employing only one of these respective injection schemes, it is unanimously agreed upon that pilot injection in conjunction with post injection is the most beneficial [13][16][18][19][24][26][34][41], with various methodologies of splitting the pilot, main, and post fuel quantities included [19][24][26]. Furthermore, different injection types benefit from dissimilar fuel pressures. With the reported tests employing fuel pressure ranging from 30–180 MPa [19][43], it has been generally seen that pilot injections are most effective with respectively low fuel pressures [5][19][40][43]. This is to minimize heat release rates from excessive fuel jet penetration, atomization, and mixing; however, this can be combated by splitting the pilot injection. Moreover, high fuel pressures with early pilot injection timing can cause lean misfires due to overmixing [43]. Post injections have been observed to benefit from higher pressures [19][40][43] because of the increased mixing that allows for late-stage soot oxidation, although lower injection pressures still allowed for some post injection benefit. Overall, there has been a wide range of results from the literature with some studies focusing on BSFC improvements, while others concentrate on a single family of emission constituents. Interestingly, favorable literature results with one strategy might incur worse performance for a similar injection scheme in another effort. The overriding agreement is that multiple injection parameter calibration is delicate, and the optimal injection scheme will differ between any two setups. Nevertheless, the literature has illustrated the potential effects and sensitivities of multiple fuel injection events with diesel-fueled CI engines, along with generally agreed upon mechanisms to describe the resultant behaviors. Table 1 provides a more concise visual representation of the generally agreed upon combustion mechanisms that are present with the three main types of multiple fuel injection strategies.
Table 1. Summary of various multiple fuel injection strategy effects using diesel and biodiesel fuels. In this table, use of ↑ and ↓ indicates the respective impact of that injection strategy on the parameter mentioned by either increasing or decreasing the outcome.
Injection Strategy Combustion Emissions Performance
Pilot Injection Mixing ↑, Main Injection Ignition Delay ↓, Peak Heat Release ↓, Sustained Heat Release ↑ NOx ↓, PM ↑,
Noise ↓		 BSFC ↓
Main Injection Split Mixing ↑, Peak Heat Release ↓, Sustained Heat Release ↑ NOx ↓, PM ↑,
Noise ↓		 BSFC ↓
Post Injection Mixing ↑, Sustained Cylinder Temperatures ↑ PM ↓ BSFC ↑
Pilot + Post Injection Mixing ↑, Main Injection Ignition Delay ↓, Peak Heat Release ↓, Sustained Heat Release ↑, Sustained Cylinder Temperatures ↑ NOx ↓, PM ↓,
Noise ↓		 BSFC ↓

2. Summary of Multiple Fuel Injection Strategies Using Biodiesel Fuels

The literature finds similarities with multiple fuel injection operation between conventional diesel and biodiesel fuels with a few differences reported; however, the general behaviors reported in Table 1 are identical between conventional diesel and biodiesel fuels. To begin, the same NOx reduction mechanism has been observed with the use of pilot injections and delayed main injection timing [44][45][46]. Potential benefits in the Indicated Mean Effective Pressure (IMEP) (and effectively BSFC) as a result of pilot injection use was also reported [20] due to the same concept of the combustion process extending further into the expansion stroke. In addition, the same PM reducing tendency of post injections has been reported [47][48]. Conversely, the qualities inherent to biodiesel fuels (i.e., greater cetane number, higher viscosity, and lower energy content compared to petroleum diesel) present deviations in favorable injection parameters compared to conventional diesel. For instance, slightly higher pilot injection quantities (around 15–25%) have been reported with single pilot injections [44][49] when using neat biodiesel to counteract its larger cetane number.
A major difference in biodiesel operation involves the variance in behavior of the soot production and oxidation battle. Due to the higher viscosity of biodiesel fuels, its correspondingly poorer atomization produces an increase in soot concentration in the middle of combustion, but its higher oxygen content and greater adiabatic flame temperature accelerates the soot oxidation process [50]. Furthermore, high post injection quantities (around 20%) have been reported to decrease the activation energy of soot particle oxidation [48]. The beneficial shift in soot production and oxidation inherent to biodiesel use is compounded by the addition of a post injection event, explaining why many researchers have seen lower PM levels with biodiesel compared to diesel when employing multiple injections.
Another difference involves the historical time period when the bulk of efforts have taken place with respect to compression ignition (CI) fuel injection technology. While investigations involving conventional diesel-fueled CI engines employing multiple injections have presented a vast number of experiments involving many injection parameters (e.g., timing and quantity to fuel rail pressure), most biodiesel multiple injection research has taken place when many of these injection parameters are generally understood. Hence, a smaller range for a given parameter has been explored with biodiesel; for example, fuel pressure tested involves a respectively reduced span of about 80–120 MPa [20][44]. Another difference has been a stronger focus on PM emissions. Since Tier 3 PM emissions requirements approach zero, PM emissions must be understood in greater depth; thus, the literature has offered more detailed insight into the PM production and abatement qualities when using multiple injections with biodiesel.
Multiple fuel injection events with biodiesel fuels have been proven to be just as effective, if not more so than with conventional diesel. Most studies have reported the same NOx and PM reduction potentials with biodiesel, and that multiple injections are advantageous as compared to a single injection event. Moreover, a few have attained simultaneous reductions in NOx and PM levels in comparison to conventional diesel with the same injection scheme; however, these results have been limited to neat soybean biodiesel [44][45][51] and coconut biodiesel blends [52][53]. Here, multiple fuel injection events with biodiesel allow for the inherent benefits of biodiesel (e.g., lower PM, HCs, and CO), while also overcoming disadvantages associated with its lower energy content and higher viscosity through flexible control of injection parameters.

This entry is adapted from the peer-reviewed paper 10.3390/en15145214

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