1.2) Considering the substrate to be a semi-infinite solid maintained at an initial temperature T i and is
8.2 Experiment of Thermal Sensor in the Combustion Chamber
8.2.1 Fabrication of Coaxial Surface Junction Thermocouple
Fig. 8.1: Schematic of the CSJT fabricated for its application in the combustion chamber of the internal combustion engine
The measurement of instantaneous heat flux inside the combustion chamber becomes very challenging accounting the design feature of the engine. Considering this aspect, a chromel- constantan coaxial surface junction thermocouple (CSJT) of E-type is designed and fabricated in- house; from the available thermocouple material. A thin layer of ceramic paste (a mixture of alumina and sodium silicate) has been used to separate the thermocouple wires. The design methodology is similar to as explained in chapter 3, except that the insulation used for housing the thermocouple material is completely different. The complete assembly is allowed to dry naturally at room temperature such that a bond is developed among the inserted material. The main reason behind using this insulation is such that the paste of this mixture (Alumina and NaSi) can resist very high temperature and it can electrically insulate the thermocouple wires. The mixture has low thermal conductivity and has very high structural stability. Once the assembly gets hardened naturally, the continuity between the two wires is checked using a multimeter, to ascertain whether the used solution had actually created the electrical insulation. Further, after confirming that there is no electrical connectivity between the two wires, the extra layer of the epoxy paste is cleaned using the scalpel blade. The surface junction is created similarly as explained in Chapter 4. The junction formed on the surface has a junction thickness of around 20-24 µm [Irimpan et al., 2015;
Menezes and Bhat, 2010]. Further, the connecting leads were spot welded on to the thermocouple wires using the spot welding machine [Fig. C.8 in Appendix:C]. The dimension of the
thermocouple is chosen based on the thermal penetration depth by using the concept of semi- infinite theory for heat conduction.
Figure 8.1 shows the schematic of the fabricated coaxial surface junction thermocouple. A detailed design methodology of the fabrication process has been explained in Chapter 4. The only difference in the fabrication assembly now lies with the use of ceramic mixture as oppose to the previous choice of epoxy resin (Araldite), as an insulation. The calibration technique of CSJT involves oil-bath experiments in which the relationship between changes in temperature and corresponding change in voltage are obtained. In this case, a linear relationship has been observed for the chosen thermocouple materials (i.e. chromel and constantan). Referring to Fig. 4.1(a), the slope of the curve is the “sensitivity” of the thermocouple i.e. 58.96 µV/°C. The complete details of the calibration methodology are similar to as explained in Chapter 4.
8.2.2 Experimental Facility
(a) (b)
Fig. 8.2. Schematic of the (a) engine head, and (b) CSJT insert for mounting on the engine head
The experimental facility includes a four stroke, air-cooled GK100 Honda engine, having a horizontal shaft, side valve and a single cylinder. The engine has a compression ratio of 4.8. At a rated speed (3000 RPM), the engine has the power of 1.3 kW and a maximum rated torque of 3.92 N.m. For measuring the instantaneous heat flux inside the combustion chamber of the spark ignition engine, an engine head design is suitably modified and fabricated having a dimension of 120 mm 95 mm 10 mm. This head has a provision for holding four number of the thermal
sensor at a time; but for the current set of experiments, only one port out of the four has been chosen (Fig. 8.2-a). The centre of the engine head has a provision for fitting the spark plug.
Fig. 8.3: Layout of the CSJT fitted on the engine head along with its accessories Table 8.1: Specifications of the experimental engine
Type
Side Valve, 4 Stroke, Air Cooled, Horizontal Shaft,
Single Cylinder Displacement (cc) 97
Air-Fuel ratio 14.7 Bore x stroke(mm) 52 x 46 Compression Ratio 4.8:1 Rated Horse
Power/Speed
1.4 HP / 3600 RPM Maximum Horse
Power
1.8 HP / 4200 RPM Maximum
Torque/Speed
0.4 Kg-m / 3000 RPM Ignition System TCI
Ignition Timing 20 BTDC
The detail specifications of the engine are presented in Table 8.1. Furthermore, the barrel for holding the CSJT sensor has been constructed with an insert of 8 mm diameter and an internal hole of 4 mm as shown in Fig. 8.2(b). The sensor is kept flush mounted on the cylinder head, so that the junction is exposed to cyclic heat load as expected in the combustion chamber of the engine [Fig. C.13 in Appendix:C]. The engine head has been fitted in such a way that the combustion chamber was maintained air-tight with the surrounding. Figure 8.3 highlights the complete
Engine Head
Closed
Bore to Insert CSJT
Amplifier
PC Interface CSJT
Spark Plug
Honda GK100
Oscilloscope
factor of 500; frequency 1-40 kHz; Make - Texas, USA), an oscilloscope (sampling frequency of 1 GS/s; Make- Tektronix, USA) and a PC-based LabVIEW interface to visualize the acquired data.
8.2.3 Results and Discussion
(a) (b)
Fig. 8.4: Transient signal (a) change in voltage, (b) rise in surface temperature captured using the CSJT flush mounted on the engine head
Once the calibration and fabrication of the coaxial thermal sensor are over, the sensor is explored to measure instantaneous heat flux inside the combustion chamber. The thermal sensor is flush mounted with the inner surface of the engine head facing the combustion chamber. An isothermal ambience has been maintained throughout the experiment, by wrapping the complete sensor with the layer of alumina and NaSi. Initially, engine is started and as it achieves the steady state condition, the thermal sensor is employed to record the data. The obtained transient variation in voltage signal and the temperature data using the in-house built thermal sensor are plotted in Figs.
8.4 (a-b). Here, the temperature-time history is estimated using the sensitivity value of the E-type thermal sensor as mentioned in Chapter 4 (Fig. 4.7). A sharp peak has been observed from the obtained temporal data depicting the combustion schedule of the engine; which signifies that as soon as the combustion takes place inside the combustion chamber, the thermal sensor captures a sharp shoot. The obtained sharp shoot is kept on repeating after every cycle. Furthermore, the engine has been set to run at a particular fixed RPM (3000 rpm).
In addition, the heat flux data pertaining to cyclic load inside the reciprocating internal combustion engine can be measured using the time-averaged cyclic temperature variation. The principal assumption for calculating heat flux is such that the heat flow through the sensor wall
should be one-dimensional. The experimental temperature data in the present case are discretized using the cubic spline method as discussed in Chapter 3 (Eq. 3.9). The TP values i.e. ck12 plays an important role while predicting the surface heat flux value from Eq. (3.9). The thermal product (TP) value for E-type CSJT in Eq. (3.9) is chosen from the experiment conducted as reported in Chapter 5. Further, using the techniques as mentioned, the surface heat flux histories are predicted from the obtained temperature histories by a numerical algorithm developed in-house [Chapter 3].
Figure 8.5 shows the variation of heat flux with respect to crank angle as well as time in the region of compression and expansion stroke in the combustion chamber. This region is the zone of interest in the heat transfer studies considering maximum heat flux. The obtained results have been compared with the work attempted by previous researchers such as Alkidas and Myers (1982), Alkidas and Cole (1985), and Assanis and Friedmann (1993). The trend of the obtained result matched well with the previous attempted work; thereby validating the use of the developed cost-effective coaxial thermal sensor in the combustion chamber environment of the internal combustion engine. An approximate value of about 2.17 MW/m2 have been obtained from the experiment with an error of ±3 %. The rate of increase in the heat flux and the magnitude of the peak heat flux are the result of gas pressure and temperature of the burnt gases.
Fig. 8.5: Instantaneous heat flux obtained from the temperature history captured at the head of the combustion chamber of the internal combustion engine