The jet engine is fitted behind the internal combustion engine at the very rear of the car. It is a home built gas turbine jet engine based around a Holset 685 turbocharger from a Volvo FL10 lorry.
Main engine Fuel: Propane delivered at high pressure
After burner fuel: Aviation fuel delivered at 170 psi and 3.75 Lpm
Afterburner ignition – custom built 60,000 volts and enough milliamps to give you guaranteed heart failure if you get it wrong!
With both the main engine and the afterburner at full power the following stats apply:
Boost pressure – 30psi
RPM – 98,500
Compressor vane tip speed – 1450 ft per second (988 MPH)
Decibels – Very Very LOUD !
Turbine outlet temperature – 1200 degrees F
Fuel consumption 6lb per minute (45 gallons per hour)
Once I had decided the concept of the “Flatmobile” was to be a miniature relation of the Batmobile, it wasn’t long until I realised a jet engine would be cool.
I hunted around for some sort of commercial jet engine and soon discovered that they were both very expensive and also rarer than rocking horse shit!
It was whilst I was trawling the net that I came across:
Here I first discovered that it was possible to build a home made jet engine, not only was it possible, but it was relatively simple!
I can’t thank the members of this group enough for all their help and advice.
Amongst others Gary, Nick, Sal, John and particularly Russ are always willing to give all the help and advice needed to help a novice achieve their goal of building a functioning (and safe) jet engine.
All the guidelines and specifications for building an engine can be found on that site.
What follows is the story of how I built my first jet engine.
The heart of every home built gas turbine jet engine is a common or garden car turbocharger, the bigger the better!
I looked on ebay and picked up a brand new Holset 685 turbo from a motor factor.
This turbo was for a Volve 10 litre lorry and fairly large for an automotive one.
Believe it or not, the only other major part needed to build (a very basic) engine is a combustor to burn your fuel in.
The combustor consists of two major parts, the tubular outer casing called the combustion chamber or “CC” and a smaller diameter tube inside the combustion chamber called the flame tube.
Here is my combustion chamber; it is made from an old fire extinguisher!
Any reasonably thick steel tube will do, but stainless steel is best if you can afford it.
These tubes come under a lot of pressure during operation and so it is very important that the tubes construction is sound, especially no dodgy seams !
One end is made from ¼ inch steel plate and has a hole machined in it the same shape and size as the inlet to the turbine side of the turbo, there are 4 studs welded into the plate for bolting the CC onto the turbo.
At the other end (where the fuel is injected) there is a flange welded in and this flange holds a ring of bolts, the end cover plate is bolted on with these bolts.
The afterburner consists of a long tube at the exit of the turbine (The jet pipe).
The end of this tube is coned down to create a smaller exit area, this compresses the exiting gases, which in turn creates more thrust.
There are more fuel nozzles in the jet pipe, and aviation fuel is injected into the jet stream at high volume and pressure to create a massive and instantaneous increase in power.
The flatmobile will have a two-stage afterburner, the second stage being used to create a jet of flame 20ft long !!!
The lubrication system:
Turbochargers may have been designed to run hot, but not to run at the temperatures we subject them to, the combustion chamber pushes a constant flow of high pressure fast moving burning gas into the turbine at around 1200 to 1400 degrees F, so it is essential to have an effective lubrication system to keep oil flowing through the turbo bearings.
The Oil tank :
The oil tank is made from aluminium and holds 5 litres of 0w30 synthetic oil, this is a very thin oil which makes it easier for the turbo to spin and start when the oil is cold.
The 5 litre capacity is plenty to ensure that the oil doesn’t overheat.
The tank has a pick up outlet at one end near the bottom and a return inlet at the other end near the top.
The return pipe must be large enough to ensure that the hot oil can drain away freely from the turbo oil outlet.
Other features of the tank include a large filler cap, a breather, a return inlet from the pressure relief valve and a 12 volt heater to pre warm the oil, again to aid starting.
Other components of the lubrication system :
The oil from the tank is sucked up into the oil pump; the pump was picked up on eBay and will pump up to 100psi.
It is important that a high pressure pump is used because you need to overcome the pressures that are created in the turbocharger, these can be up to 35psi so you want to maintain a pressure of something like 50 psi to keep that oil flowing.
From the pump the oil pipes split off in two directions through one pipe, the oil is pushed through the oil cooler; you can see this in the photo with 3 cooling fans on top.
The other pipe returns to the tank via an adjustable flow valve, the valve has to be fully open when starting the engine, so that oil pressure is reduced to 2.5 psi.
From the cooler the oil goes through the filter and then onto the oil inlet on top of the turbocharger.
The oil filter also houses the oil pressure safety switch, and the oil pressure and temperature sender units.
Monitoring and control:
As I said early these homemade jet engines are potentially very dangerous, we have the combination of copious quantities of fuel, burning at high pressure and a red hot turbine spinning at nearly a thousand miles per hour!
If that turbine overheats, or the temperatures or pressures go too high, or the oil supply fails, then you have a potential explosion on your hands.
Fragments of red hot metal moving faster than a bullet leaves a gun would not be funny, even to my warped sense of humour! So you can see why effective monitoring and control are essential.
Everything is controlled from the control console.
Boost pressure: The pressure in the combustion chamber should not be allowed to rise above 35psi. A copper pipe inserted in the combustion chamber air supply pipe connects directly to the boost gauge to provide this reading.
Oil temperature: The temperature of the oil going into the turbocharger should not exceed 150F. a sender in the filter unit sends the temperature to the oil temp gauge
Oil pressure : The pressure of the oil going into the turbocharger needs to be low when starting and at least 50psi when running the engine at full speed. A sender in the filter unit sends the pressure to the oil pressure gauge.
Exhaust gas temperature (EGT): This is an important indicator of how safely the engine is running. A temperature probe is fitted in the jet pipe, just after the point where the hot gases exit the turbine, this is the most accurate and relevant place to measure the temperature which must be kept below 1200F This is called the TOT (turbine outlet temperature)
R. P. M. :
This is the most important thing to keep an eye on in my opinion. The maximum RPM for my engine is 1450 feet per second or 788 MPH!
If this speed is exceeded then the spinning compressor and turbine blades are in danger of breaking apart with obvious catastrophic consequences!
It is very easy to exceed this speed as well by simply increasing the flow of fuel too much.
The problem is finding a system that will reliably measure up to 100,000 RPM. When you consider that your average car rev counter measures up to only 7000 RPM you begin to appreciate the accuracy and speed required.
Luckily my friend and time served DIY jet engine builder Russ Moore of ‘Bad Bros’ racing came to my rescue with a reliable system that he can supply from the states at a very reasonable price.
Russ can provide all manner of stuff including full engine kits. He can be contacted at: firstname.lastname@example.org or www.badbros.net
Here is the fibre optic sensor unit:
Two glass fibres attach to this unit, one emits a beam of light, the other picks up that beam and returns it to the sensor unit.
The fibres are mounted in a tube as shown:
A hole is then drilled in the compressor housing and the tube mounted in such a way that the fibres point straight at the compressor nut.
The compressor nut is painted half black and half white so that the beam of light will be reflected once every revolution and send a signal back to the sensor.
The sensor then converts this light signal into an electronic signal and sends it to a small computer mounted on the control console.
The computer has a digital readout that displays not only the RPM but also the % of max revs that the engine is running at.
The most popular way to start these engines is to stuff a leaf blower into the compressor inlet and spin up the engine, however this is not ideal for a car based engine.
I opted to build a starting system that uses compressed CO2 blown against the turbine blades to get the engine spinning.
Luckily for me this had already been done by Gary Richards and is well documented on Gary’s site: www.garysjetjournal.com
I basically asked some advice from Gary and then based my system on his.
Firstly I drilled a hole in the turbine housing and inserted a fitting that held a 3mm stainless steel pipe.
The end of this pipe was bent and positioned so that it was almost touching the turbine blades and would blow a stream of gas to make the turbine blades spin in the right direction
This next photo shows the rest of the system:
The CO2 is stored in a 20oz bottle normally used for paintball guns.
This bottle is connected to a manual on / off valve (this will eventually be replaced by an electronic solenoid valve).
This valve is then connected to an expansion bottle where the CO2 can fully evaporate and expand before moving into the turbine via a preset flow valve.
The flow valve is set so that the turbine will spin as fast as possible but without flowing more gas than necessary.
If too much CO2 flows, not only is it used up quickly but the CO2 freezes and pressure quickly drops.
The System provides enough gas for 2 starts and the car will have 3 bottles mounted in the system, so up to 9 engine starts between refills.
The System has to be mounted on the turbine side rather than the compressor side, otherwise the CO2 would put out the fire in the CC!
The flame tube:
The flame tube sits inside the combustion chamber and is around 1.5″ smaller in diameter. The tube has three series of holes drilled in it, small ones near one end where the fuel is injected, medium size holes in the centre section and larger holes nearest to the turbine.
The fuel is injected into the centre flame tube, where it mixes with the air coming in through the holes and ignites, as the flame moves down the tube, more air comes in through the medium size holes and completes the combustion process, the large holes then provide more air to cool the gases before they enter the turbine.
The combustion process causes the gases to rapidly expand and the gas is therefore forced through the turbine, spinning the turbine wheel.
On the other end of the turbine shaft is the compressor wheel which compresses fresh air into the combustion chamber, the more fuel burned then the faster the turbine will spin, the faster the turbine spins, the faster the compressor spins and more air is injected and compressed and so the cycle goes on, with the system “self sustaining”.
In theory if you keep supplying more fuel, then the engine will keep accelerating and eventually explode!
This is why there are many safety measures and features built into the system.
When the exhaust gases exit the turbine only about 30% of the oxygen in the gases has been burned, that means there is still lots of oxygen available to produce more power if we introduce more fuel!
This is why we have an afterburner.