Phase Three Testing

Phase three of engine testing was aimed at testing the liquid fuelling system developed and readying the engine for use on the UAV platform. Various configurations of injectors, fuels and engines were trialled throughout the testing phase.

Test the performance of the Chinese engine on liquid fuel

Initial testing was performed with the injectors placed down the intake in a similar layout as with propane testing. While this setup is not ideal due to both excessive restriction of the intake flow, and poor spray pattern distribution, the layout allowed the modification of the position without altering the engine. This was done to establish a reasonable idea of required injector position. This testing indicated that transition from gas to liquid was not feasible, as the engine would not maintain operation as gas flow was reduced. It was however, possible to start the engine on liquid fuel only, providing that the engine had been brought up to operating temperature on propane.

It was later determined that transitioning could occur with the injectors placed through the side of the intake pipe. It is thought that the inability to do so in initial testing was due to the overheating of the injectors while the engine warms on propane. This would dramatically reduce the flow rate of the injectors to a level that is unable to sustain the engine operation. It was noted from later testing that the fuel flow rates during operation were significantly less than those achieved during flow bench testing, which is most likely also due to injector heating.

The most effective method of startup determined from testing is as follows:

• Turn on ignition circuit and air supply

• Turn on gas supply system and adjust to start engine

• Turn off air supply and ignition circuit

Section 6.1 Engine Testing

• Allow the engine to warm until the combustion chamber begins to glow (~5 seconds)

• Set gas flow to medium throttle, turn on liquid at medium throttle

• Simultaneously reduce gas flow and increase liquid flow, ensuring not to over fuel the engine.

Test the effect of injector position on performance

Testing was performed on the Chinese engine to determine the optimal injector position on liquid fuels. Flow requirements developed from equating energy quantities between fuels suggested that 3 injectors were required for operation. The injectors obtained were of various sizes, denoted ‘6’, ‘8’ and ‘10’, with the number relating to the size of the orifice in tenths of a millimeter.

Initial tests established that having the larger injectors closest to the combustion chamber resulted in the best performance, both in thrust and fuel consumption. This is thought to be due to less wastage of the fuel, as the larger injector is further from the mouth of the intake. It is thought that having the larger injector furthermost from the combustion chamber may allow for easier starting, however transitioning to liquid fuel after operation is established on propane makes this irrelevant. The injector layout is shown in Figure 120

Figure 120 - liquid fuel injectors placed mid way along the intake tube

The effect of injector position was investigated by drilling holes along the intake tube at regular intervals, allowing the injectors to be moved as required. The remaining holes were sealed by fireproof fabric with a metal backing held in place with hose clamps.

Best performance (both thrust and fuel consumption) was achieved with the injectors placed midway along the intake tube. This is unlike the propane tests, where maximum thrust was obtained with the injector at the mouth of the intake. It is possible that excessive losses in fuel are the reason for reduced thrust at the outermost position, as the pump and injector set was unable to over-fuel the engine during testing.

A larger pump was purchased in order to provide a larger fuel flow rate. However, this did not provide significant improvements in engine performance. It is believed that both pumps are able to supply the engine with all the fuel it required, but not enough to make the engine flame out. To determine if fuel supply limitations were affecting thrust output, a fourth injector was added to increase fuel flow. This did not produce any more thrust, but did increase fuel usage, and hence it was determined that extra fuel was not required.

In the interests of improving fuel atomization, the injectors were positioned in an opposing configuration, such that the sprays would interfere with each other. This is setup is shown in Figure 121. This also did not produce positive results, and hence it was determined that the best performance was with the three injectors placed midway along the intake tube, on the same side. Comparison of performance with the various injector setups is shown in Figure 122.

Section 6.1 Engine Testing

Figure 121 - Opposed injector configuration

Figure 122 - Performance of the Chinese engine with different injector placements

Test the fuel mixtures the Chinese engine

In order to achieve the best engine performance, testing was done to determine the best fuel mixture for the engine. Standard unleaded petrol was used as the bas fuel for most testing, with additions of other fuels tested. A single test on kerosene was performed, however, it was noted that the engine produced a lot of fuel vapor during the test. Due to this, and a lack of any performance improvements, kerosene was considered to be unsuitable for the application.

Methanol was trialed as an additive, as on vaporization, a relatively large cooling effect is present. It was thought that this may improve engine performance by increasing the

density and hence mass of air drawn into the engine each cycle. Conversely, the addition of Shellite should increase the heat produced during combustion, and hence potentially improve performance by creating larger combustion pressures.

The best performance was obtained from the addition of 10% methanol to standard unleaded petrol. This addition produced similar thrust, but reduced the fuel consumption of the engine significantly. The relatively consistent thrust levels suggest that the engine is not very sensitive to the degree of variation between the fuels trialed. Engine performance on the trialed fuels is shown in Figure 123.

Figure 123 - engine performance on various fuels

Test the performance of the FWE engine with expanding tail section on liquid fuel

Testing was performed to determine the ability of the FWE engine to operate on liquid fuel. The injector setup used was that which provided the best performance with the Chinese engine, and the engine configuration was that which provided the best performance on propane fuel (350mm extension on the tail section).

During the first and only test performed, sustained operation on straight unleaded petrol was achieved, producing a maximum thrust of 1.5kg. During the test a significant amount of un-burnt fuel vapor was released into the air and was therefore

Section 6.1 Engine Testing

considered a serious safety risk. Hence, no further liquid fuel testing was performed with the FWE engine.

The excessive vapor release from the FWE engine on liquid fuels was a factor that had been expected from the background research stage. It is thought that the FWE style combustion chamber does provide the correct environment to support, which was a primary reason for investigation into the Chinese engine style.

Engine Geometry changes

Due to the relatively poor performance achieved on liquid fuel, the engine was modified with the aim of obtaining more thrust. To do this, the engine was cut at both the intake and exhaust. The pipe was then either extended by wrapping with metal sheet, or shortened by replacement of the cut section with a smaller piece.

It was found that modifications to the engine generally resulted in a reduction in thrust from the engine. It is believed that this results from the interference on the flow due to the irregularities in the engine walls after modification. This is especially evident when the original configuration was retried and produced less thrust than was obtainable previously. Based on this testing, it is thought that the intake length should be increased slightly over the original length, as the extra 5mm produced approximately 0.5kg of extra thrust compared to the original sized engine after cutting.

This is shown in Figure 124, which compares the original performance of the engine with performance after it was cut, as well as the thrust variations with various exhaust lengths. It was also noted that longer exhaust lengths were unable to achieve sustained operation on propane fuel, but would sustain on liquid. Further testing is required in order to determine if a greater extension of the intake will produce better performance, and if the performance degradation post modification can be rectified by re-welding the joins to the original.

Figure 124 - Performance of the Chinese engine for various lengths

The following theories have been developed to explain the reduced performance of the engine on liquid fuels:

• The slower burn rate of the propane fuel means that combustion is still occurring in the expanding section of the exhaust. This combustion creates a positive pressure gradient, which helps to prevent separation. The expansion angle in the engine is relatively large, so on liquid fuel, where the combustion occurs more so in the combustion chamber, the pressure gradient is not present, and separation occurs, causing large losses in the system. Faster combustion may also increase the speed of the gasses at the expansion point, further increasing the likelihood of separation.

• Changes in burn rate have affected the mean temperatures in the exhaust, and hence changed the acoustic length. This therefore means the intake is not tuned correctly to the exhaust, reducing engine performance. This theory is derived from the results of the tests after modification of the engine.

Conclusion

Phase three of engine testing produced positive results, with operation achieved on both the FWE and Chinese engines. Thrust levels obtained while operating on liquid

Section 6.1 Engine Testing

fuel were significantly down on those achieved on propane fuel. The Chinese engine was found to run well on a variety of liquid fuel mixtures, with the best performance achieved with the addition of 10% methanol to unleaded petrol. Liquid fuel operation of the FWE engine was obtained, however excessive fuel misting was produced, posing a safety risk.

The optimal injector setup was found to be with three injectors placed midway along the intake, with the larger injectors closest to the combustion chamber. The maximum thrust achieved was 2.25kg, with a thrust specific fuel consumption of 4.8kg/kg/hr.

Modifications to the Chinese engine suggest that improved performance would be achieved by increasing the length of the intake. Further testing is required to confirm this, as a thrust reduction occurred as a result of cutting the engine. Airframe testing has however, shown that 2.2kg of thrust is enough to successfully power the UAV, and as such greater engine performance is not specifically required in order to be implemented for flight. In current form, the engine is capable of fulfilling the goal of flight, however, it is expected that more thrust would be required in order to achieve the speed goals.

A full report of all tests can be found in Appendix H – Test Log Books.