Final Design

OUTLINE

  1. Safety Modifications
    1. Turbocharger
    2. Oil System
    3. Fuel System
    4. Purged Box
    5. Air Start System
    6. Ignition System
    7. Engine Control
  2. Instrumentation
    1. RPM Measurement
    2. Temperature Measurement
    3. Pressure Measurement
    4. Force Measurement

 

 

SAFETY MODIFICATIONS    

 

Turbocharger

The original turbocharger had a severe structural crack in the turbo housing that made it unsafe to use. A new Garrett T04B25 was chosen as a replacement. This turbochargers larger size makes our system more robust and steady when operating.

 

Oil System


Oil Filter                             Oil Intercooler

The Garrett turbocharger uses journal bearings on the compressor turbine shaft to decrease friction. If the oil system providing the lubrication of the journal bearings fails to work properly or does not work at all, turbo failure will occur. Additional systems were installed to alleviate these problems. Previously a diaphragm pump was used to pump oil from a relatively small reservoir at the base of the jet engine chassis to an inlet point on the turbo, through the turbocharger, then gravity fed out the bottom of the housing. This configuration restricted the flow of oil  exiting the turbo housing, causing insufficient lubrication and a slowing down of the turbo due to higher than normal friction. To counteract this, we enlarged the gravity feed hole at the bottom of the housing. Since the automotive application of this turbo has an oil intercooler being used to properly maximize its service life, it was decided that the system indeed was in need of this device to properly cool the oil. The ideal operating range for engine oil is 180F to 200F (82C to 93C). When operating within this range, the oil works as a lubricant, coolant, and cleansing agent in the engine and turbocharger. The new intercooler is mounted on open side of the cart. A larger reservoir was also added to aid in the cooling of the oil. Attached to the reservoir is an air breather to aerate the oil. To help extend the usage of the oil and to reduce the fatigue on the engine, we also added an oil filter to the system. With these new items being used, we believe that many of the fatigue issues that contributed to the failure of the last turbo will be solved.

 

Fuel System

The original jet engine design used propane fuel. The idea of changing fuels had been considered because the effects of different fuels on engine performance could be used in a laboratory experiment. The usage of other fuels could also improve reliability and safety. Alternate fuels considered were Diesel Fuel, K-1 Kerosene, Jet A, Acetylene, Ethane, and Methane. Some of the considerations we took into account when deciding were cost, availability, safety, reliability, and other fuel specific reasons. Our final decision was to stay with propane, but to implement a few extra safety measures. Switching to another fuel would be expensive and difficult to implant. The first safety measure we decided to use is a flashback arrestor. This device will prevent the flame from going back into the tank if the pressure ever gets too low and ideal conditions are present. A flow meter was also installed to better regulate the amount of propane being added into the system. The addition of the purged box will also help prevent combustion by preventing excess propane from coming in contact with electrical components.

 

Purged Box

Previously there had been no safety considerations when dealing with the propane fuel. With the addition of numerous electrical components, it would be too expensive to purchase all intrinsically safe or hermetically sealed components. We thus came up with the idea of creating a new cart for the engine, half of which would contain a sealed box. Air is to be pumped into this box, creating a greater pressure and thus preventing propane from coming in contact with the electrical components contained inside. The sides of the box are made out of Plexiglas, which has the visual benefit of allowing students to still see every component of the engine. The air being pumped into the box will feed off the air-start system.

 

Air Start System

The starting procedure for the engine was originally accomplished in part by spooling up the turbocharger and providing a continuous volume of air through the combustion chamber. Propane was then introduced and the propane-air mixture ignited. The airflow was provided to the engine by use of a portable household leaf blower. The leaf blower specifications indicated a rated airflow of 170 cfm (4.81 m3/min), however, airflow measurements indicated that the actual airflow through the engine was approximately 100 cfm (2.83 m3/min).

To provide the starting airflow to the engine, the leaf blower outlet was held tightly against the turbocharger compressor inlet. Upon successful ignition, the leaf blower was manually removed from the turbocharger compressor inlet. As a safety precaution, the leaf blower was maintained in the ready position in the event of a failed ignition attempt or other flame out condition. Under these circumstances, the leaf blower was used to provide a purge air stream through the engine to minimize the build-up of a full engine volume of flammable propane-air mixture, which, if ignited, would cause a full volume internal deflagration within the combustion chamber. This event could cause a personnel safety risk, as well as damage the engine assembly.

An alternative to the leaf blower was proposed, prototyped, and installed on the engine. This alternative system utilizes shop compressed air as the air source. This system delivers approximately the same volume of airflow for start up. The air start system is currently activated via a manual valve, however, the system was installed to allow for the addition of a solenoid valve to allow for remote or automated control and integration into any safety systems, which may be added in the future. The advantages to the compressed air source and air start system included integration into the overall engine system and the inherent safety of compressed air vs. the electric motor of the leaf blower which introduced an ignition source into the engine environment where a flammable propane-air mixture could develop under an upset condition. An additional benefit of the compressed air start system includes the ability to tap into the compressed air line at the engine cart to provide low volume/low pressure purge air to the instrument cabinet.

 

Ignition System


Ignition Coils

The original jet engine design used an automotive testing coil as an igniter. It also used a commercially available insulated sparking rod. This system worked, but was not safe or reliable. We deemed the system unsafe because the wires were frayed and the insulation on the wires was cracked and coming off. The sparking rod used is an excellent device, but it could freely rotate out of position thus producing a loss of spark and no ignition. This made the reliability poor. Our intention was to improve the design and make it more reliable and safe.

We looked into alternative methods of high voltage generation. Our first problem was to determine how much voltage would be needed to jump an air gap about one half inch long. From our calculations we found that we need between 10,000 and 15,000 volts. We chose to use a neon sign transformer and it has been working effectively.

Previously there was a single electrode in the combustion chamber, and the spark had been jumping the gap to the side of the flame tube. This was unreliable, because the electrode had frequently been rotating out of position, creating a gap too large to create a spark. It was also unsafe due to the large voltage running through the engine. To solve these problems, we added another electrode in the chamber. This fixed the gap size, and isolated the voltage to make it both more reliable and safe. 

 

Engine Control

For safety reasons, most of the important engine functions are controlled remotely including fuel, starting air and ignition. All functions are routed through a single 20ft (6.1m) cable to a control panel containing toggle switches for these three functions. The air start and fuel systems are controlled with solenoid valves. The sparker unit and both solenoids use AC current due to their large power requirements. However, since the control system has to work with DC, relays are used to provide a link between the AC and DC systems. Solid state panel mount relays have been chosen due to their low power consumption and simplicity. The DC power supply for these systems is shared with the Field Point data acquisition unit. A 24VDC regulated supply is mounted in the cabinet for this purpose. There is also room in the control panel to allow for the addition of new functions.

 

INSTRUMENTATION  

 

 RPM Measurement

RPM measurement is essential for safe operation of the engine. An optical rpm sensor was designed and built by the team for this purpose. The major components of the sensor are a phototransistor, infrared LED, and frequency to voltage converter. The sensor operates by detecting light that is reflected off of a hex nut on the turbocharger shaft at the entrance to the compressor. The LED and phototransistor are mounted close together facing the hex nut. Light emitted by the LED is reflected off of the nut and detected by the phototransistor. The result of this configuration is a voltage waveform output by the phototransistor. The peaks of this waveform correspond to reflections off of the flat sides of the nut. The average voltage of this signal is 23V, a simple high pass filter is used to move the average voltage down to 0V. A frequency to voltage converter is then used to convert this digital signal to an analog 0-5V signal.

 

Temperature Measurement

Thermocouples have been installed in six locations within the system for temperature measurement. Four of the thermocouples are inserted in the airflow path between major engine components: between the compressor and the combustion chamber, between the combustion chamber and the turbine, between the turbine and the nozzle, and at the nozzle exit. These points correspond to the points of interest in the Brayton Cycle. Two additional thermocouples are mounted to the surface of the combustion chamber. They will be used to calculate the amount of heat that leaves the system through the walls of the combustion chamber.

Three of the thermocouples installed on the engine are K type and the other three are T type. T type thermocouples measure temperatures ranging from -330 to 660 F (-201 to 349 C) and K type measure 200 to 2300 F (93 to 1260 C). The three thermocouples located in the flow path after the combustion chamber are K type because the highest air temperatures are present after combustion occurs.

 

Pressure Measurement

A manifold has been constructed that allows multiple pressure measurements to be made with a single sensor. Tubing from multiple pressure taps is connected to a hollow body, which is connected to the pressure transducer. Ball valves on the manifold control which tap the pressure is being measured from.  Three pressure taps have been installed. They are located between the compressor and the combustor, between the combustor and the turbine, and between the turbine and nozzle. Pressure measurement after the compressor is necessary to determine the mass flow rate of air. If engine RPM and pressure rise over the compressor are known the mass flow can be obtained from the turbocharger compressor map. The other pressure taps are located on either side of the turbine because these are significant points in the Brayton cycle.

The Wika S-10 pressure transducer is used for pressure measurement. This transducer has high accuracy (0.25%), low nonrepeatability (0.05%), good long-term stability (0.2%FS/1 yr), low thermal error (0.2%FS/18 F(10C) span and zero shift), and a sufficient compensated temperature range (32 to 176 F (0 to 80C)).

 

Force Measurement


Thrust Stand / Mechanical Stop

The force output of the jet engine is an important measurement in a laboratory experiment. The force sensor will be mounted to the cart, and will come in contact with the thrust stand. The thrust from the jet engine will load the sensor in compression, which will then cause an output voltage that can be directly measured. The base of the triangularly shaped thrust stand rests on two ball transfers, which facilitates the rotating motion. The opposite corner rests on a washer atop a nut attached to a threaded rod. There is a mechanical stop built in to the thrust stand so that it cannot rotate when the cart is being moved. This helps to ensure the force sensor will not be overloaded.

In order to produce measurable thrust, a nozzle was designed for the engine. Since the engine is for laboratory purposes it is desired that the exit area of the nozzle be variable. A variable nozzle also relieves the demand to choose an exact exit diameter with approximated data. In order to design the nozzle the flow through the engine is modeled using the Brayton cycle with corrections [1] made to account for the isentropic efficiency of each component. Additionally the velocity of the fluid is not assumed to be zero as is the case with the standard Brayton cycle. 

The efficiencies of the some of the components were estimated to perform the analysis. In order to prevent compressor stall due to overexpansion in the nozzle, the efficiencies are underestimated in most cases. For the compressor, the exact efficiency for a given operating condition is taken directly from the compressor flow map, which is based on empirical data. 

The efficiency of the combustion chamber is related to the pressure drop across it. A typical isentropic efficiency of 0.9 is assumed for the following two reasons. Since there are no moving parts involved, it is easier to build a combustion chamber which mimics the performance of commercial versions. Secondly the lowest flow velocities in the engine are through the combustion chamber. This minimizes the impact of the inefficiencies in the chamber flow passages. 

No data was available for the turbine efficiency so an efficiency was chosen by comparing turbine data for similar turbochargers. It was observed that the average efficiency increased with the turbine's flow rate. The efficiency used in the calculations is the average efficiency for a turbine of similar flow rate of is 65%. 

For the nozzle, an efficiency of 90% is used. The average value for commercial engines is 95%. This value is has been slightly lowered so as not to overestimate the nozzle efficiency. However, the nozzle efficiency is not expected to be significantly different from other nozzles as long as the expansion angle remains reasonably shallow. Exact relationships between expansion angle and pressure losses indicate that for a half angle less than 30%, the pressure loss will be less than 5%. An additional 5% has been added to account for heat loss through the walls of the nozzle.

The final piece of data sought in the analysis is the exit area of the nozzle. A spreadsheet containing all relevant data is used to output the nozzle exit area. The spreadsheet inputs are the operating point of the compressor, the pressure and temperature of the local atmosphere, fuel heating value and flow rate and the engine component efficiencies. The process for obtaining an exit area is as follows. First the compressor pressure ratio and flow rate are arbitrarily chosen. The fuel flow rate is input as a percent of the fuel flow rate required to achieve a stoichiometric burn at the chosen compressor flow rate. The pressure and temperature drop across the turbine is as required to power the compressor while taking the efficiency of both components into account. The difference between the pressure after the turbine and the atmospheric pressure determines the final nozzle area. Since the exit pressure should be atmospheric, as the pressure after the turbine increases, the nozzle exit area decreases. This corresponds to greater expansion.  In some cases the pressure after the turbine will be below atmospheric pressure. This does not correspond to a valid operating condition of the engine and requires a new fuel flow rate or compressor condition to be chosen so that the gage pressure after the turbine is positive. For maximum engine output, the exit diameter has been calculated to be one inch (25mm).

 

 

 

     
 

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