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Comparison of Wren Micro-turbojet Engine and Wren turbo-drop

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In the following report, one will be analysing and comparing the wren micro-turbojet engine and the Wren turbo-prop, the engine will be tested in the Aviation laboratory in the houldsworth aviation laboratory, here the tests will be carried out, but before the test s are carried out on each of the propeller that needs to be used for the turbo prop must be chosen.

The propeller will be chosen in its efficiency in a certain variable which will be chosen later in the report, there is a choice of a few different propellers, tests shall be ran on each different propeller with the results been recorded by the FADEC.

Once the propeller has been chosen tests shall e ran on both engines, the results will be analysed with numerous variables,

Health and safety

During the course of the lab work, health and safety will play a big part in keeping people safe, Engines pose many possible hazards which are listed below:

  • Noise: The engines that will be used will produce a loud noise that is capable of damaging hearing permanently; therefore the test will be run with the testers wearing ear protection to prevent permanent ear damage.
  • Fuel: fuel poses numerous threats in the lab; the highly volatile liquid can produce vapour that can cause an explosion so therefore all fuel must be kept in a sealed container that must be kept sealed and stored in a cool, dry place away from irresponsible personnel.

Fuel is also irritable when in contact with skin so therefore gloves must be worn when in contact with the substance.

The engines: the engines themselves pose a huge health and safety risk, the engines contain many objects in motion which can cause a hazard if any of the objects become loose, which could result in a projectile being thrown across the room, to eliminate this risk, the tests will be carried out in a separate room from the testers.

The engines also produce fumes which are harmful to humans so an extraction pipe is now attached to the exhausts to remove most of the fumes but testers must be wary that the extraction system may not be 100% efficient so contact time after the runs must be kept minimal.

As engines must be run correctly to be run safely, inexperienced testers must always be supervised by personnel with sufficient experience with the engines.

The fumes that are produced by each engine must be sufficiently vented away from the area as they are a hazard to health.

Extraction system failure

At the beginning of February 2010 the group was told that the lab where the engine runs were to be carried out had been ruled unsafe due to the failure of the ventilation system had failed.

It was possible that the test could not have been carried out, which the whole group was disheartened by, but the group pulled together and all tried to find a solution for the problem, many avenues were explored, even the possible opportunity of the back of a restaurant.

Gareth Atkinson contacted multi flight about the possibility of running the engines at their base as it is located at Leeds-Bradford airport, Multi flight obliged so therefore Gareth Atkinson filled out the risk assessment form shown in appendix A.

By the time Multi flight replied back with their permission the group decided that that time was against us for running the engine tests at Multi flight, results from previous engine runs were used instead.

The Jet Engine

The Engine has proved to be an innovative piece of machinery and has allowed the aviation industry to be what it is today, nearly all of the innovative aircraft of today and conceptual aircraft of tomorrow are all powered by the same type of engine - the jet engine.

The first form of jet propulsion device was first patented by a French engineer René Lori in 1913 but the design remained conceptual as the design needed heat resisting materials that were not available at the time plus the reason that the jet engine was not suitable to fly at the low airspeeds as they did at the time (1996a) The design of which is shown in figure 1.

Not until 1930 did frank whittle receive a patent for a jet engine, Frank whittle went on to build the first jet engine which had its maiden flight 11 years after its patent was received. From this basic foundation more advanced jet engines were produced.

Since this first prototype the jet engine has developed from this early prototype to power aircraft that travel faster than sound. The jet engine works by compressing air adding fuel and combusting the mixture using the pressure increase to accelerate the air rearwards out of a propelling nozzle.

The compression is the first stage of a gas turbine engine, there are many different configurations of compression the first form was the centrifugal compressor, which is the same compressor is fitted to the Wren micro-gas turbines that will be tested in this dissertation.

Centrifugal compressor

The centrifugal compressor essentially "does what it says on the tin" and compress the air using centrifugal forces, the air is compressed by an impeller which is attached and rotated by a shaft that is connected to the rear turbines, which convert the pressure, velocity and heat energy from the exiting gas in to mechanical energy to rotate the shaft which turns the impeller at high speed.

The air introduced continuously throughout the running of the engine through to the eye of the impeller which then enters the rotating guide vanes and due to centrifugal forces causes the air to move towards the tips of the impeller; here the air enters the diffuser which is a system of stationary(Rolls-Royce Limited., 1996) ducts which are designed to convert the kinetic energy of air stream into potential energy, which further increases the pressure of the gas. Below is a diagram showing a cross-section of the impeller and the changes in pressure and velocity throughout the impeller:

From figure 2, the graph at the right shows the pressure and velocity changes at each stage of the centrifugal compressor, as the air enters the eye of the impeller. The air is subjected to a centrifuge which increases the pressure and velocity of the airstream, the air then passes through to the diffuser section where the passage of divergent nozzles convert the kinetic energy into pressure energy. The graph shows that half the pressure rise occurs in the impeller and the other half in the diffuser section. (Rolls-Royce Limited., 1996).

Axial flow compressors

Axial flow compressors have a higher compression ratio than centrifugal compressors provided air which is at a higher pressure therefore used over centrifugal compressors as the higher pressure give better performance.

Axial flow compressor consists of alternating rotor and stator blades, where each stage, which consists of a row of rotating blades followed by a row of stator blades, increases the pressure by a small amount so therefore several stages are required to give the pressure increase required. Where many stages of compression operate on one shaft it becomes necessary to vary the stator vane angle to enable the compressors to operate effectively at speeds below the design conditions. (Rolls-Royce Limited., 1996).

A single spool compressor consists of one rotor assembly and stators with many stages as required achieving the desired pressure ratio, a multi-spool compressor of two or more rotor assemblies, each driven by their own turbine at an optimum speed to achieve higher pressure ratios and to give greater operating flexibility. (Rolls-Royce Limited., 1996).

Principles of operation

During operation the rotor is turned at high speeds by the turbine which makes sure that air is constantly being inducted into the compressor, which then is accelerated by the rotating blades and swept rearwards into the adjacent row of stator vanes. The pressure increase results kinetic energy received by the air by the rotor vane, which is then decelerated (diffused) by the stators vanes turning the kinetic energy into pressure. (Rolls-Royce Limited., 1996).

Combustion Chamber

The combustion chamber has the task of burning large quantities of fuel supplied through the fuel spray nozzles, with large volumes of air supplied by the compressor and releasing heat in a controlled process that the air is expanded and accelerated to give a smooth stream of uniformly heated gas at all conditions required by the turbine. The amount of fuel added will depend on the temperature rise required although the maximum temperature is limited by the materials that make the turbine blades and nozzles. (Rolls-Royce Limited, 1996)

The combustion process

The air arriving from the compressor is travelling at speeds up to 500 feet per second, which if entered the chamber would put out the flame due to the airs high velocity, so therefore the air must be decelerated. The air is diffused so the velocity is reduced also giving the added benefit of increasing the pressure. (Rolls-Royce Limited., 1996).

IIn normal operation the overall air/fuel ration can vary between 45:1 and 130:1, however kerosine will only burn efficiently at, or close to, a ratio of 15:1, so the fuel must be burned with only part of the air entering the chamber, in what is called the primary combustion zone. This is achieved by means of a flame tube that has various devices for metering the airflow distribution along the chamber. (Rolls-Royce Limited., 1996).

As shown in Figure 4, 20% of the air mass flow enters at the snout of the chamber, immediately downstream are a perforated flame and swirl vanes, the swirling air induces a flow upstream of the centre of the flame tube and promotes desired re-circulation. (Rolls-Royce Limited., 1996).

Through the wall of the flame tube body, next to the combustion zone, are a number of secondary holes were a further 20% of the main flow of air passes into the primary zone. The air from the swirl vanes and that from the secondary air holes interacts and creates a region of low velocity recirculation, this take the form of a toroidal vortex, very similar to a smoke ring, this helps stabilise and anchor the flame the flame. (Rolls-Royce Limited., 1996).

The temperature of the gases after combustion is around 1800 to 2000°C, which is far too hot for entry to the nozzle guide vanes of the turbine. As only 40% of the total airflow is used in the combustion process, so the 60% is left to help cool the air, which is introduced progressively into the flame tube. Approximately a third of this is used to lower the gas temperature in the dilution zone before it enters the turbine, whilst the remainder of the air is used for cooling the walls of the flame tube. This is achieved by a film of cooling air flowing along the inside surface of the flame tube, insulating it from the hot combustion gases. (Rolls-Royce Limited., 1996).

Types of combustion chambers

There are three main types of combustion chambers used in gas turbine engines; these are multiple chambers, turbo-annular chambers and annular chamber.

Multiple combustion chambers

This type of Combustion chamber is used on centrifugal compressor engines and primitive axial flow engines. The chambers are disposed around the engine and compressor delivery air is directed by duct to pass into the individual chambers. Each chamber has an internal flame tube around which there is an air casing, the air passes through the flame tube snout and also between the tube and the outer casing as earlier described. All the flame tubes are all interconnected which allows them to operate at the same pressure and also allows combustion to propagate around the flame tubes during engine starting. (Rolls-Royce Limited., 1996).

Tubo-annular combustion chamber

The tubo-annular combustion chamber bridges the evolutionary gap between the multiple and annular types. A number of flame tubes inside a common air casing. (Rolls-Royce Limited., 1996).

Annular combustion chamber

This type of combustion chamber consists of a single flame tube in a completely annular form, which is contained in an inner and outer casing. The main advantage of the annular system is that for the same power output, the length of the chamber is 25% shorter than the tubo-annular system for the same diameter, saving weigh and production costs. . (Rolls-Royce Limited., 1996).

Combustion chamber performance

A combustion chamber must be capable of allowing fuel to burn efficiently over a wide range of operating conditions without incurring large pressure losses. Also if flame extinction occurs, then it must be possible to relight. . (Rolls-Royce Limited., 1996).


The turbine has the job of providing the power to drive the compressor and accessories and sin some engines shaft power for the propeller or rotor. This is done by extracting energy from the hot gases from the combustion system and expanding them to a lower pressure and temperature. The turbines are subjected to high stresses with the turbine tips speeds reaching over 1,500 feet per second with gas temperatures.

Types of turbines

Gas turbine manufacturers have concentrated on the axial flow turbine, although some manufacturers are building engines with a radial-inflow turbine. The radial-inflow turbine has the advantages of being rugged, simples and relatively inexpensive and easy to manufacture compared to the axial flow turbine. On this type of turbine, inlet gas flows through the peripheral nozzles to enter the wheel passages in an inward radial direction. The speeding gas exerts a force on the wheel blades and exhausts the air in a radial direction to the atmosphere. These turbine wheels, used for small engines, are well suited for a lower range of specific speeds and work at relatively high effieciency.

Micro turbines in industry

In this section I will discuss and analyse the role of micro turbines in industry within aviation and out of aviation.

Micro turbines can be used to produce electricity as well as power some ships.


In this section I will be analysing the company of WREN, I will include what they do, their history and the current situation.

I will note that the workers are enthusiast and work at a minimal wage to keep the company in business.

WREN turbo prop

In this section I will analyse the turboprop that will be tested in more detail.

I will go through each component in a fair amount of detail, I will go through the manufacture process in some detail, and analyse the materials that are used.

Wren turbo jet

Here I will do in the same format as the turboprop but with a diary of our engine build: here is some rough notes I have made of the engine build# 26/10/09

Did our Risk assessment with Chris brier, told info on all aspect.

Got on to the engine build, went smoothly until step 11 when inserting the shaft into the middle we dislocated bearings due failing to follow a step properly.

We immediately called Chris brier to help us with our issue; he showed us how to replace the ball bearings into place.

We then progressed onto the next step, at this point we saw etchings on the shaft and the rear turbine which we asked why they we like so

We learnt that they were there for mass balancing, as an imbalance in weight would cause the shaft to distort.

Balancing is done on each individual shaft by the manufacturer by a machine.

Another aspect of the is the inefficiency of the compressor

Compared to lager jet engines the gap between the edge of the compressor blades and outer skin is comparatively large therefore not all of the compressed air is used for the production of thrust, some leaks behind the compressor

Step 16- when fitting turbine shrouds the turbine rests on a rim and not on turbines.

Step 17- aligning case front, case front has been checked and aligned to the best of our abilities.

Step 21- chamber stand-offs successfully crimped

Step 22- comb chamber fixed in place, gas and oil tube is place and 3 stand-offs screwed in

We found that the gas tube didn't fit because of misaligned lubrication pipe. So backtrack in progress was required to re-configure this step.

In the process of correcting the combust chamber bearings in the same place fell apart, so were back to the same stage again


This section I will analyse the results in detail


Evaluate the whole process



Apendix A

Multi-flight risk assessment form

  • Rolls Royce PLC. (1996a). The Jet Engine (5th Edition ed.). Birmingham: Renault Printing Co Ltd.

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