2. Test procedure and equipment

The test procedure and equipment used in both approaches are elaborated in this section.

## 2.1. Engine testing and equipment

Many researches have been done in the effort of controlling CAI combustion-based system. Agarwal et al. [7] summarize the various types of combustion method and some method in controlling the combustion process. There are various areas that require improvement in order to reshape the combustion and improve the efficiency while still having a low emission. Development of control on ignition timing [8], method in slowing down the heat release rate at high load [9, 10] and development of intake and exhaust manifold for multicylinder engine are among the few area of improvement that have been identified in the area of CAI combustion system. Focusing on the RCCI combustion control method, Li et al. [11] categorized the control by two main categories, fuel and engine management. The fuel management includes two fuel strategies [12, 13] and single fuel strategy with additives, while the engine management

Combustion mode SI CI HCCI PCCI RCCI

Lambda 1 1.2–2.2 >1 >1 >1

Mixing rate Multipoint or

Flame front Y Y w/o w/o Y

Mixture preparation PFI, GDI DI DI, PFI and DI + PFI DI, PFI DI, PFI and DI + PFI Ignition Spark ignition Autoignition Autoignition Autoignition Autoignition

> dominated by chemical kinetics

spontaneous

Flexible fuels Diesel-like fuels Multifuels

Multipoint or spontaneous

Relatively low Relatively low Relatively high

Premixed Premixed + stratified

a. High-pressure direct injection (12–18 bars) b. Air-assisted low pressure injection (4–6 bars)

Single cylinder (399.25 cc) 88 mm

132 mm 14:1 (Geometric)

4 12 BTDC 132 BTDC 15 BBDC 10 BBDC

Fuel reactivity

Diesel-like fuels

Fuel Gasoline-like

Combustion rate limitation

Combustion temperature

No. of cylinders bore Stroke compression ratio

Exhaust valve open (EVO) Exhaust valve closing (EVC)

No. of valves Valve timing events fuels

56 Improvement Trends for Internal Combustion Engines

Flame propagation

Intake valve open (IVO) intake Valve closure (IVC)

Table 2. Summary of specifications of the engine.

Combustion form Premixed Diffusion Premixed but

High Partially

high

Table 1. Traditional and the controlled autoignition (CAI)-based combustion mode.

Fuel type Gaseous fuel Fuel supply system Direct injection (DI) The engine used for this experimental study houses the fuel system of direct injection of gaseous fuel and has compression ratio of 14. This engine is a single cylinder water-cooled engine coupled to an electric dynamometer that can be used for starting the engine and measuring the brake torque produced by the engine. Figure 4 shows a schematic drawing of the engine.

An electric heater is provided to heat the lubricant oil to help warm up the engine faster. A separate control unit controls the operation of the pumps and the temperature of the oil and water by temperature controllers. The control unit also controls the operation of the dynamometer, which also serves as the starter motor and the engine can be motored at a wide range of speeds. There are standard features of safety included in the control unit such as emergency switch, automatic shut down upon the excessive rise in the oil and/or coolant temperatures, or any abnormal conditions of electrical power supply (Figure 5).

A commercially available gasoline port fuel injector was used and its specifications can be found in Table 3. This injector has low flow rates and was selected to match the requirement of injecting very low quantities of gasoline to operate in HCCI mode with ultralean mixtures. The injector comes calibrated in the factory to inject and precisely meter the volume against the specified injection duration.

Figure 4. The single cylinder research engine.

The change in air flow rate and volumetric efficiency at different engine speeds were taken into account, and the gasoline injection durations were adjusted so as to operate with constant equivalence ratios of gasoline from 0.20 to 0.26. Table 4 shows the matrix of experiments and

Engine speed (N) Equivalence ratio of gasoline (φg) Timing of CNG injection (SOI) CNG quantity (mCNG)

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Fuel compatibility Standard gasoline and ethanol flex fuels

Up to the knocking limit or unstable engine operation

RPM Ratio CAD-BTDC mg/cycle

Fuel pressure 300 kPa Static flow rate at 300 kPa 7.55 g/s Offset 0.67 ms

Minimum linear PW 1.5 ms Linear flow range 8.43 Open/closing time 1.3/0.7 ms Coil resistance 12 Injector inductance 11.6 mH SCOV/DMOV 4.42/4.92 V Spray pattern 26Cone

Gain 0.11 = 9.09 mg/ms

lists the variable parameters of the experiments conducted for this study.

Make/part number Bosch/0280155710 Type High impedance

Fuel injection pressure 3 bar Fuel flow rate 191.8 cc/min Power supply 12 V DC

Table 3. Specifications of the gasoline injector.

0.20 0.22 0.24 0.26

Table 4. Matrix of experiments.

Table 5. Injector technical specification.

Figure 5. Gasoline and CNG fuel supply systems.


Table 3. Specifications of the gasoline injector.


Table 4. Matrix of experiments.


Table 5. Injector technical specification.

Figure 5. Gasoline and CNG fuel supply systems.

Figure 4. The single cylinder research engine.

58 Improvement Trends for Internal Combustion Engines

The change in air flow rate and volumetric efficiency at different engine speeds were taken into account, and the gasoline injection durations were adjusted so as to operate with constant equivalence ratios of gasoline from 0.20 to 0.26. Table 4 shows the matrix of experiments and lists the variable parameters of the experiments conducted for this study.

The CNG quantity was varied and limited by the combustion stability of the mixture by limiting the maximum coefficient variation (CoV) of the combustion to 10%. Thus, the readings were obtained at different speeds, different gasoline flow rates, and different injection timings with various quantities of CNG injected (Table 5 and Figure 6).

Figure 7 depicted the injection timing, and intake and exhaust valve timing of the engine. It shows that injection timing 300, 240, and 180 BTDC take place before the intake valve close,

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Following section describes the detailed experimental setup and equipment used on the constant volume chamber. Constant volume combustion was used as the primary method for characterizing the interaction of the parameters such as fuel compositions, lambda, and mixing ratio. The equipment and control system were designed, manufactured, and calibrated to accommodate the experimental works. The primary data in this experimental setup are combustion images and pressure trace of which acquired by Schlieren and direct measurement

The experiments were carried out in a constant volume combustion chamber with diameter and length 80 and 100 mm, respectively. A 100 W cartridge heater was placed in the middle of the chamber to increase in-cylinder gas temperature with a maximum temperature of 820 C. There were two window access planes to the chamber in order to facilitate the Schlieren image

while 120 and 80 BTDC take place after intake valve close.

method, respectively.

Figure 8. Experimental setup.

visualization method (Figures 8–10).

2.2. Constant volume combustion chamber testing and equipment

Figure 6. Experimental setup.

Figure 7. Injection timing of CNG, and intake and exhaust valve timing.

Figure 7 depicted the injection timing, and intake and exhaust valve timing of the engine. It shows that injection timing 300, 240, and 180 BTDC take place before the intake valve close, while 120 and 80 BTDC take place after intake valve close.

#### 2.2. Constant volume combustion chamber testing and equipment

The CNG quantity was varied and limited by the combustion stability of the mixture by limiting the maximum coefficient variation (CoV) of the combustion to 10%. Thus, the readings were obtained at different speeds, different gasoline flow rates, and different injection timings

with various quantities of CNG injected (Table 5 and Figure 6).

60 Improvement Trends for Internal Combustion Engines

Figure 7. Injection timing of CNG, and intake and exhaust valve timing.

Figure 6. Experimental setup.

Following section describes the detailed experimental setup and equipment used on the constant volume chamber. Constant volume combustion was used as the primary method for characterizing the interaction of the parameters such as fuel compositions, lambda, and mixing ratio. The equipment and control system were designed, manufactured, and calibrated to accommodate the experimental works. The primary data in this experimental setup are combustion images and pressure trace of which acquired by Schlieren and direct measurement method, respectively.

The experiments were carried out in a constant volume combustion chamber with diameter and length 80 and 100 mm, respectively. A 100 W cartridge heater was placed in the middle of the chamber to increase in-cylinder gas temperature with a maximum temperature of 820 C. There were two window access planes to the chamber in order to facilitate the Schlieren image visualization method (Figures 8–10).

Figure 8. Experimental setup.

3. Gasoline-CNG mixtures in RCCI combustion system

tion of low-reactive fuels and method in controlling the combustion process.

3.1. Gasoline-CNG combustion behavior in RCCI combustion-based engine

The gasoline-CNG mixtures performance and combustion in RCCI combustion system from both methods are elaborated in this section. It explained the parameters affecting the combus-

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The degree of stratification of CNG in the total mixture was found to have significant effects on the maximum load in terms of the IMEP attainable and φTotal. The degree of stratification is determined by the injection timing with 300 BTDC representing the homogeneous mixture, while 120 BTDC represents the stratified mixture. The 300 BTDC has a very early injection timing, and the fuel is injected during intake valve is open. It is, therefore, the fuel that has sufficient time to be completely mixed with air and create a homogeneous mixture. While on the other hand, for 120 BTDC, the fuel is injected after intake valve closed and mixing time

between fuel and air is very short and does not allow a complete mixing to take place.

Figure 11. Effect of degree of stratification of CNG on IMEP. (K—limited by knocking; mf—limited by misfire).

reduced IMEP at a given φTotal when compared to the 300 and 240 BTDC cases.

CNG injection rate led to unstable operation or misfire.

Figure 11 shows that at 300 and 240 BTDC injection higher total equivalence ratios could be operated. But with 180 and 120 BTDC, the maximum operable φTotal was limited with

The IMEP results show agreement with investigation from Genchi G and Pipitone E [22] where the increased composition of CNG produces higher IMEP. With the highest degree of stratification, although the maximum load was limited, there was no significant drop in the IMEP and the trend was similar to 300 and 240 BTDC conditions. The corresponding values of indicated thermal efficiencies are shown in Figure 12. The maximum load was observed to be limited by knocking when CNG was injected at 300 BTDC, and for the other cases, increasing

Figure 9. Schematic diagram of the control system and data acquisition.

Figure 10. Injector control system interface.
