The next step in integrating the ICE was to generate torque maps of the engine and program those maps into the hybrid controller to effect IOL control. As discussed in Chapter II, by recording the engine's fuel consumption and torque for a grid of engine speeds and throttle settings, one can generate a map of brake specific fuel consumption as a function of throttle and speed. Plotting the most fuel efficient throttle setting against engine speed forms the operating line necessary to implement an IOL torque split strategy.
The engine was tested on the same dynamometer setup described in Mengistu's thesis [15]. Figure 23 shows the Honda GX25 mounted to the dynamometer inside of the test cell. The dynamometer is a DYNOmite Mini Eddy Dyno 96 V. An extension flange and gear bolted to the flywheel and a timing belt connected the engine to the 96 V direct current braking system, seen as the red coils in the bottom right of Figure 23. Data from the dynamometer is collected by the DYNOmite Pro Data Computer and Controller using a 28 channel wire harness. The controller transfers the data to the DYNO-MAX 2010 Pro Software Suite run on a standalone computer in the laboratory, where it can be recorded, saved, and exported for analysis. The software can also control the load the dynamometer applies to the engine or dynamically adjust
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the load to maintain a target engine speed. As already implied, the dynamometer can measure torque, speed, and power.
To mount the engine to the dynamometer, a plate the size and shape of the bulkhead for the aircraft was manufactured out of aluminum and mounted vertically on a 2.5 cm (1 in) thick aluminum slab. The slab was bolted directly to the dynamometer reaction cradle. The engine then mounted to an intermediate plate, attached to the simulated bulkhead with rubber vibration mounts.
Figure 23: Dynamometer test setup with Honda GX25 engine, no pillow block, electric motor and starter motor not attached
As described and pictured in Figure 23, this set-up leaves the engine cantilevered from behind, as if it were mounted to the aircraft. In the aircraft, the ICE and EM power translates into axial thrust; the dynamometer provides a resistance torque and a corresponding force that pulls downward on the gear and subsequently the engine shaft. The moment arm of this torque, with the engine mounted only from behind, is substantial. Initial runs demonstrated that the moment from loading the dynamometer enhanced engine vibration causing the timing belt between the dynamometer and gear to slip. In a first attempt to overcome this issue, a wooden support block
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was added underneath the engine to reduce the moment arm of the belt on the engine assembly. The block can be seen under the flywheel in Figure 23.
The engine vibrated against the pine wedge, chipping away the wood and requiring a replacement block every 2-3 hours of operation. Therefore, a pillow block was added on the engine flange forward of the dynamometer belt to counter the moment produced by the load. This pillow block is shown schematically in Figure 24 and in a picture of the hybrid system mounted to the dynamometer in Figure 25. The pillow block significantly reduced engine vibration as well as variation in the measured torque. It also reduced the replacement frequency of the pine support block.
Flywheel
Engi
ne Flan
ge
Belt to Dyno
Engine and aircraft/mount Direction of Propeller Pill ow Bloc k Force of Loa d
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Figure 25: Hybrid system on dynamometer showing pillow block and pine support block There are two other quantities necessary for the torque maps outside of those already mentioned: the ICE throttle setting and the fuel flow. Fuel flow was measured using a Max Machinery 213 piston helical flow meter. Designed specifically for low flow rates between 1 and 1800 mL per minute and non aqueous organic fluids such as gasoline, the Max 213 is ideal for this application. The flow meter uses a standard 5 V data signal with published gains, making it possible to hook the flow meter directly into the dynamometer's data acquisition system. The meter would not work for flow measurement on the airframe however; it has a mass of 0.6 kg and is sensitive to inertial orientation. The carburetor of the GX25 stores fuel in a small reservoir before mixing it with incoming air; over a several second period, the fuel flow to the engine is not constant. Thus, the fuel flow should be averaged over at least 30 seconds at a given throttle setting for useful results. One should also purge air from the fuel flow meter before taking data.
The throttle setting measurement was more complicated. The most accurate way to measure the throttle setting is by the position of the carburetor, shown with servo attached in the
Pillow block
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upper right corner of Figure 25. Initially, the throttle setting was measured with throttle position sensors designed to link with the dynamometer's data acquisition system. These throttle position sensors are spring loaded. Their force combined with the spring on the carburetor caused twitching in the servo position. Furthermore, the throttle position sensors had a 20-25% dead zone at either the high end or the low end. Heinley and Koch tried to create an analog gauge for the carburetor position, but the vibration of the engine during operation and the relatively short range of motion of the carburetor made it difficult to read accurately.
The position of the servo controlling the carburetor is easier to measure than the
carburetor itself, but the servo must be calibrated so its range of motion matches the carburetor's range of motion. The servo was controlled via the PIC32 originally intended for use as the hybrid controller. By setting 0% throttle to the carburetor's fully closed position and 100% throttle to the fully open position in the controller's code, the PIC32 then adjusted the servo movement to match the offset and range of the carburetor. Typically, a servo's full range corresponds to a pulse width of 1 ms to 2 ms. However, to match the servo and carburetor, the that range was adjusted to 1 ms to 1.67 ms. The PIC controller calculated the pulse widths for the intermediate throttle settings. Even with the PIC32, changing the linkage between the servo arm and carburetor requires a recalibration of the servo, and thus the throttle setting is not perfectly repeatable. Using the throttle setting to kill the engine as a baseline, tracking the throttle servo is repeatable to within ±5% throttle, an acceptable value when torque map testing is performed in 10% throttle steps.