Experimental investigation of ringing intensity and combustion noise level from biodiesel fuel in CRDI diesel engine with various injection timings

1. Introduction

Electronic fuel injectors in diesel engines enable the diversification of fuel-injection methods, such as those pertaining to precise fuel-injection quantities, fuel-injection timings, and fuel-injection-frequency controls. Among these diversification methods, fuel-injection timing is one of the key variables affecting engine performance, including engine noise, vibration, and emission levels. Several studies have indicated that combustion characteristics such as the in-cylinder pressure, peak rate of heat release (ROHR), and peak rate of pressure rise (ROPR) increase as the fuel-injection timing shifts from the top-dead-center (TDC) to advanced [1]-[3]. Advanced injection timing causes a long ignition delay and hence improves the air–fuel mixture. However, fuel injection at low cylinder pressures and temperatures causes significant wall wetting, which results in increased carbon monoxide (CO) emissions [4]-[6]. In addition, combustion as a result of early injection increases the intensity of the premixed combustion and causes the gas temperature and level of nitrogen oxides (NOx) to increase, which results in deteriorated engine performance [7]-[8]. Fuel-injection timing can be improved using a tool that can reduce engine/combustion noise and vibration while utilizing the same amount of fuel. Researchers have introduced ringing intensity (RI) as a quantitative indicator to evaluate engine noise and vibration. It has been reported that high levels of ROHR and ROPR with high premixed combustion during the initial combustion result in high RIs. Meanwhile, the combustion noise level (CNL) is closely associated with the fuel-injection timing and overall combustion duration. It has been reported that as the fuel-injection timing advances, the CA50 (10% MFB to 50% MFB; MFB = mass fraction burned), defined as the rapid combustion period, decreases, and the overall combustion duration (10% MFB to 90% MFB) is reduced. Consequently, the combustion noise level is increased.

Biodiesel is an eco-friendly fuel that contains oxygen and can be mixed with diesel fuel in an appropriate ratio without requiring engine modifications. In fact, biodiesel can reduce ignition delays and reduce the RI. Table 1 summarizes the findings obtained from previous studies pertaining to RIs and CNLs [9]-[15]. The current study was conducted to quantitatively analyze the RI and CNL based on the fuel-injection timing using various combustion parameters by applying biodiesel to an experimental single-cylinder common-rail direct-injection diesel engine. The results of this study revealed that advancing the injection timing adversely affected the RI and CNL.

Table 1:

Review of studies pertaining to RI and CNL

2. Experimental System

2.1 Experimental setup

Figure 1 shows a schematic illustration of the configuration of the test device. A single-cylinder CRDI diesel engine equipped with a sac-type injector was used to perform the test; the specifications of the test engine are summarized in Table 2. Various devices were constructed to perform the test. Table 3 lists the main functions of the devices constructed, as shown in Figure 1. The single-cylinder CRDI diesel engine maintained a constant speed via a 22 kW electric motor. The energy generated by the engine was dissipated by a braking resistor connected to an electric motor.

Figure 1:

Schematic illustration of experimental system

Table 2:

Specifications of test engine

Table 3:

Function of main devices constituting experimental system

2.2 Experimental conditions and test fuel

In this study, a single fuel-injection timing strategy without pilot injection was applied. The test was conducted while adjusting the fuel-injection timing from before the top dead center (BTDC) 30 crank angle (CA) to TDC at 5 CA units. The experiment was performed with the speed of the test engine fixed at 1100 rpm, a fuel injection amount of 10 mg, and a fuel-injection pressure of 40 MPa. The in-cylinder pressure was averaged by measuring 300 cycles to minimize cycle-to-cycle fluctuations. Table 4 lists the experimental conditions for reference. The physicochemical properties of the test fuel, compared with those of the diesel fuel used in naval vessels, are listed in Table 5.

Table 4:

Settings used in experiment

Table 5:

Physiochemical properties of BD100

3. Results and Discussions

3.1 Combustion characteristics

Figure 2 shows the effects of the fuel-injection timing on the in-cylinder pressure, ROHR, and ROPR.

Figure 2:

In-cylinder pressure, ROHR, and ROPR characteristics with various injection timings corresponding to crank angle

As the fuel-injection timing advanced, the peak in-cylinder pressure, ROHR, and ROPR values increased. When the fuel injection was advanced and the in-cylinder pressure and temperature were low, the mixture formation improved significantly owing to a longer ignition delay, thereby resulting in a higher premixed combustion intensity, and hence higher peak ROHR and ROPR values. In addition, because the advanced combustion directly affected the compression stroke, it exerted an adverse effect. A higher mixture-formation intensity and ignition delay reduced the overall combustion period, which consequently decreased the indicated mean effective pressure (IMEP).

The effect of the fuel-injection timing on the engine performance was confirmed. Figure 3 shows a comparison of the results of the IMEP, indicated torque, and thermal efficiency based on the fuel-injection timing. The formulas for calculating the IMEP, indicated torque, and thermal efficiency are as follows:

$\[ IMEP=\frac{∆\theta }{V_d}{\sum }_{i=n_1}^{n_2}P\left(i\right)\frac{dV\left(i\right)}{d\theta } \]$

(6)

$\[ Indicated torque=\frac{imep V_d}{4\pi } \]$

(7)

$\[ Indicated thermal efficiency=\frac{Indicated work}{Q_{in}\left(m_f{Q}_{LEV}\right)} \]$

(8)

Figure 3:

IMEP, Indicated torque, and Normalized indicated thermal efficiency characteristics with various injection timings

In Equations (6), (7), and (8), V_d represents the swept volume, mf the injection fuel mass, and Q_LEV the lower heating value of the biodiesel. As shown in Figure 3, the IMEP, indicated torque, and thermal efficiency increased as the fuel-injection timing approached the TDC. The in-cylinder pressure and temperature were high under retarded conditions. This reduced the ignition delay period, resulting in an improved combustion efficiency. As the fuel-injection timing was delayed, the adverse effects diminished, whereas the positive effects became more prominent; hence, the engine performance improved. In addition, retarding the fuel-injection timing while maintaining a constant heat release (m_fQ_LEV) improved the fuel conversion efficiency and combustion efficiency, which consequently improved the thermal efficiency.

3.2 RI and CNL characteristics

To calculate the RI, combustion parameters, such as the peak ROPR, peak in-cylinder pressure, and peak in-cylinder pressure were considered. According to various studies, an acceptable RI value is 5 MW/m² [19][22]. Figure 4 presents the results of RI, peak ROPR, and peak in-cylinder pressure vs. fuel-injection timing. As mentioned above, the advanced fuel-injection timing increased the intensity of the premixed combustion, which consequently increased the peak in-cylinder pressure and ROPR. In addition, the peak in-cylinder temperature increased, as per Equations (3) and (4). RI is an index used to quantitatively evaluate the strength of knocking by combustion and is associated closely with engine noise and vibration. The highest RI was indicated at BTDC 25 CA; in fact, the RI decreased as the fuel-injection timing was retarded toward the TDC. The RI decreased because the retarded fuel injection suppressed the premixed combustion and improved the combustion stability. In particular, it was evaluated to be below the RI limit of BTDC 15 CA.

Figure 4:

Ringing intensity, peak ROPR, peak in-cylinder pressure, and Peak gas mean temperature vs. injection timing

Figure 5 provides the analysis results of the effects of the fuel-injection timing on the CNL, the peak ROHR, and the overall combustion duration. In this study, the variables for evaluating the CNL based on the fuel-injection timing were the peak ROHR and overall combustion duration that affected the combustion process [16][21]. As the fuel-injection timing advanced, the combustion duration decreased and the peak ROHR increased. Under more advanced conditions, the additional time for mixture formation and ignition delay increased the premixed combustion intensity and reduced the combustion period. A strong premixed combustion and a short combustion period resulted in a high CNL. As the fuel-injection timing was delayed, the CLN decreased gradually as the combustion phase (CA50) was generated in the expansion stroke.

Figure 5:

Combustion noise level, peak ROHR, and combustion duration characteristics vs. injection timing

3.3 Emission characteristics

Figure 6 shows the effect of fuel-injection timing on CO, NOx, and peak gas mean temperature (GMT).

Figure 6:

CO, NOx, and peak gas mean temperature vs. injection timing

Our analysis showed that CO, NOx, and the peak GMT increased as the fuel-injection timing advanced from the TDC. At the early injection point, the spray of the injected fuel developed rapidly because of the lower in-cylinder pressure and temperature. Therefore, local piston wall-wetting was generated, and incomplete combustion occurred owing to the weaker oxidation. This was considered a key contributor to the increase in CO. The NOx was considered thermal NOx, which is associated closely with the combustion gas temperature during the combustion reaction stage. Advancing the injection timing increased the intensity of the premixed combustion via the formation of a superior mixture with a longer ignition delay, as well as via an increase in the peak ROHR and combustion gas temperature. Significant NOx formation was observed as the injection timing became more advanced.

4. Conclusions

An experimental analysis was conducted to quantify the RI and CNL by applying various injection timings of biodiesel fuel in a single-cylinder CRDI diesel engine. The main conclusions of this experimental investigation are summarized as follows:

• 1) Advancing the injection timing resulted in increased peak in-cylinder pressure, ROHR, and ROPR levels. However, the values of engine performance indicators, such as the IMEP, indicated torque, and thermal efficiency, decreased during the compression stroke of combustion.
• 2) RI, as an indicator of engine noise and vibration levels, increased as the fuel-injection timing advanced from the TDC. This was due to the long ignition delays, which increased the intensity of premixed combustion. In particular, our results exceeded the limit of the RI value under premature injection conditions (BTDC 20–30 CA). Advancing the injection timing reduced the overall combustion duration and increased the peak ROHR owing to the more intense premixed combustion. Consequently, the CNL increased in response to each advance in the fuel-injection timing.
• 3) As the fuel-injection timing was advanced, a significant level of CO emissions was observed during incomplete combustion and weaker oxidation reactions. In addition, advancing the injection timing resulted in higher peak ROHR and in-cylinder temperatures, which consequently increased NOx emissions.

Acknowledgments

This study was supported by the 2022 Academic Research Project of the Naval Institute for Ocean Research of the Republic of Korea Naval Academy.

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