Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
IC Engine Testing
As has already been noted, we prefer to test each IC engine we are considering in our own test cells rather than simply relying on manufacturers’ data. In the dynamometer cell, seen in Figure 5.1, the basic measurements we take are fuel consumption and torque at varying speeds and throttle settings, often with both gravity-fed and fully pumped fuel lines (a number of small UAV engines will give greater power when used with pressurized fuel lines, while some have built-in fuel pumps). Figure 5.9 shows a typical plot of raw data taken from our dynamometer. We average across the raw data aiming to complete tables such as the one shown in Table 5.1, from which we can then derive the engine power, brake-specific fuel consumption (BSFC), and brake mean effective pressure (BMEP) plots. When combined with the engine mass (including standard exhaust system and ignition system), one can then make like-for-like comparisons between alternative engines. Prior to running tests, we carefully calibrate the torque sensor with a weighted balance bar and also the throttle servo to ensure that 0% and 100% throttle openings are correctly achieved when set by the control system of the dynamometer.
BMEP, which is given from torque T by BMEP= 2nnT/displacement (n = 1 for two-stroke and = 2 for four-stroke engines), allows a comparison of the quality of the combustion system between engines of differing sizes and between different cycle types. Table 5.2 shows typical values of BMEP for a range of engine types, higher values of BMEP being better: the values achieved by the supercharged Rolls-Royce Merlin were clearly outstanding. In the end, however, it is usually BSFC and power-to-weight ratios that drive design calculations and engine selection.
Having completed a dynamometer test, we then run engine and propeller combinations in our dedicated wind tunnel system, also shown in Figure 5.1. This allows static thrust and thrust at varying air speeds to be recorded along with the reaction torque experienced at the engine mountings. The test cell also allows fuel flow rates to be measured. This allows an assessment of the overall efficiency of the propulsive system. Careful matching of propellers to engines is vital in achieving best performance from such systems. Moreover, the propulsive efficiency of small UAV propellers varies quite widely: the extensive test data collected by UIUC demonstrates this most clearly .
Figure 5.9 Raw performance data taken from an engine under test in our dynamometer.
Table 5.1 Typical liquid-fueled IC engine test recording table (maximum rpms are of course engine-dependent).
Note that all engine test results are subject to variability depending on the engine set up and wear, test cell operating conditions, and any losses due to induction, exhaust, or coupling factors. If precise details of the tests carried out are not available, such results should be treated with caution during the design phase.
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