This article was first published in 2007.
John Warnes pauses only momentarily in his
“You’d like to have eye and ear protection?” he
asks. Without waiting for a reply, he adds: “Most people do,” before
disappearing into his workshop to retrieve ear plugs and safety
He hands them out and then equips himself with
both before applying the petrol-powered leaf blower to the compressor intake
bellmouth. A look of concentration turns to a satisfied grin as the machine
starts, an escalating high-pitched whine heralding the arrival of turbine speed
and boost. A few minutes later, after shutting down the machine and swapping the
fuel from petrol to kero, he turns on the second fuel pump, the one that runs
And the need for the ear plugs becomes clear as
a superheated roar fills the backyard, the jet blast sweeping away leaves tens
of metres distant.
We’ve done many stories on different machines,
but this is the only jet-powered scooter we’ve ever covered....
John’s a man after any AutoSpeeder’s heart. We’ve
previously run stories on his turbo RB25-powered Holden Commodore and, more
recently, on the PC installation in his Ford Falcon XR6 turbo. And it was while
chatting during the shoot on the XR6 that we learnt of John’s jet-powered
interests. But at the time, the jet wasn’t powering anything – so while it was
interesting, it wasn’t a must-run story. But then we heard about the jet
scooter. John tends to downplay the work that’s gone into the design and build
of the scooter, but for our money it shows amazing ingenuity and dedication.
Based around a sleeve-bearing TO4E turbo (one
that, incredibly, once did duty in a 10-second rotary drag car!), the scooter is
propelled purely by jet thrust. But how do you turn a TO4 into a jet? Basically,
you feed the compressor outlet into a specially designed combustion chamber. The fuel is squirted in through a high
pressure injector and once ignited by a sparkplug, continues to burn. The expanding gases drive the turbine which in turn spins the
compressor faster, pumping more air into the combustion chamber. The speed of
the turbo is controlled by how much fuel you add; the propulsive power comes
purely from the thrust of the gases blasting out of the turbine exhaust, which
is fitted with a contracting nozzle to increase speed. It’s not dissimilar to
the very first aircraft jets, which also used centrifugal compressors and
external combustion chambers.
But saying it all in one paragraph is easy
compared with actually doing it... not to mention getting the complete assembly
light enough to fit on – and propel – a scooter. The intention was never to make a road-going, practical vehicle - it was much more a case of fun experimentation. And what an experiment!
So let’s have a look at the technology in detail –
like, how do you lubricate the turbo? After trying different oil pumps, John
settled on a Shurflow positive displacement diaphragm pump that supplies oil to
the turbo through a filter. As with all the on-board electrical systems, it’s
powered by a sealed lead acid battery. The pump draws about 2 amps, pumps at 30
psi (oil pressure is monitored with a dedicated oil pressure gauge) and can
handle temps of up to 150 degrees C. But so that limit is not reached, the
scooter runs a large oil cooler with airflow aided by two ex-PC electric fans.
An oil pressure switch shuts down fuel and ignition if a pressure of at least 15
psi isn’t present.
The turbo’s oil drains into a fabricated sump - a
wide, long stainless steel box whose large surface area helps in de-aeration. A
sight glass on the side of the sump shows oil level; about 750mL is used in the
system. The oil is a mix of car and jet oil, thin enough to flow easily but not
so thin that it won’t build pressure.
A vent tube connects the sump to the atmosphere.
This line has an in-line air solenoid which shuts off the vent when the jet is
running but opens the vent when the jet stops. This prevents the pressurised
sump pushing oil into the turbo when the jet is not running, while still
allowing the pressurised oil supply system to aid sealing when the turbo is on
boost. The system was added when it was found that after stopping, oil would otherwise flow into the turbine, creating clouds of white smoke.
The ignition system consists of a single
double-ended coil cut from a VN Commodore coil pack. The low voltage side of the
coil is triggered by a relay that chatters – that is, it cuts itself off when
energised, so opening and closing rapidly. John chose the system because it is
so small, light and simple. One high voltage side of the coil feeds the spark
plug in the combustion chamber and the other is positioned to light the fuel in
the afterburner. However, the latter doesn’t really need the spark plug – the
exhaust gases coming out of the turbine are hot enough to light the afterburner
on their own...
The combustion chamber was fabricated by John in
stainless steel. It consists of vertically orientated inner and outer cylinders,
with the inner cylinder perforated. The top third of the inner cylinder has
small holes drilled in its walls; the next third of the cylinder has larger
holes; and the bottom third of the cylinder has the largest holes – 13mm in
diameter. Fuel is added from the constant flow injector positioned at the top
and air from the compressor flows into the space between the inner and outer
cylinders, entering near the base. The design took many hours to get right,
aided by a sight-glass positioned in the upper wall of the combustion chamber
that allows the burn to be actually seen. Maximum pressure in the combustion
chamber is 1 Bar.
The afterburner uses a fuel ring positioned at the
turbine outlet. Twenty small holes are drilled in the ring for fuel flow. The
exhaust out of the turbo has enough residual oxygen to burn the afterburner fuel
– John comments that you can stand (a fair way back!) from the exhaust and it
smells just like an aircraft jet, without any excessively rich mixtures or
So how does the fuel get to the system? Two fuel
pumps are used – one for the main burner and the other for the afterburner. Both
are high pressure roller cell EFI car fuel pumps. The front-mounted tank feeds
both pumps through a T-piece and fuel filter. From there the fuel from one pump
– a VL Commodore Bosch pump - is pushed to the main combustion chamber nozzle, a
self-designed and manufactured brass item. Pump speed – and so fuel flow – is
controlled by an electronic kit that gives a variable pulse width output. This
effectively runs the pump from 8V to full battery voltage, with rider control by
a lever that turns the control potentiometer.
A safety circuit won’t let the fuel system run
without oil pressure, the fuel switched on and a combustion pressure of at least
0.1 Bar. The latter is so that if the flame goes out, fuel is cut off. An
over-ride switch for the combustion pressure sensor allows easier starting.
Finally, a turbo timer is built in so that the oil supply for the turbo will run
for 10 minutes after shutdown, and a digital voltmeter shows battery
John’s scooter isn’t the sort of machine to take
down the local shops. For starters, the noise is incredible, not to mention that
the machine could easily wreak havoc by burning pedestrians, dogs, car
paint-work... The duration of running is also limited by battery capacity. We
don’t reckon the local constabulary would much appreciate John’s ingenuity
But the jet that John has built has got sufficient
thrust to propel the scooter to 50 km/h - fast enough that John mutters it might have been
better to install the engine on something with more stability than a kid's scooter.....
We love it!