'Gromit' is going freepower! 49cc Puch Magnum/Maxi single speed 'twist and go' scooter engine. My intention is to use the relatively large gear ratio between the crankshaft pinion and drive gear for use in a gearbox for a freepower setup as opposed to using pure thrust for propulsion. With a freepower, the exhaust gases from the jet are passed through a second larger turbine wheel which is connected to the rear wheel via a speed reducing gearbox and chain final drive.  Freepower turbine wheels spin at anything from 30,000-50,000rpm which needs to be brought down to more managable final drive outputs of 5,000-10,000rpm, hence the need for large gear ratios. My reason for deciding to go the freepower route was potential lack of thrust. A freepower setup makes more efficient  use of the exhaust gases giving greater power and acceleration and even with my two relatively small turbos, has the potential to outperform even the largest of the gas guzzling pure thrust types! ;o) 

Engine reduced to essential components. Some fettling work on the crankcases to be done, in particular the shroud around the automatic clutch which needs to be removed.

Top two pictures are  my first design for a freepower based on what others have done. This is going to the biggest challenge to date in particular construction of the wheel itself which has to stand up to high speeds and temperatures.

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Result! Making the turbine wheel would have been a huge task to construct let alone the safety aspect if it ever decided to 'let go'! Luckily a member of the DIYGasturbines group kindly donated a spare first stage turbine wheel from a commercial Allison 250 ( thanks John "racketmotorman" Wallis! ), and has more than likely saved me from potentially spitting turbine blades or worse..! ;o) The wheel in question can be seen most clearly in the lower picture where it has been sectioned for display purposes and is the same as the intact one immediately to the right, only slightly smaller with a diameter of 158mm. If you look closely you can see that there is severe damage to the blades due to over heating, hopefully this won't happen with mine..! ;o)

...and here it is..! ;o) Outer diameter 158mm, hub diameter 128mm, blade height 15mm. Shaft diameter is 20mm, ideal for fitting standard ball bearing. Thread on shaft 20tpi but odd diameter, will have to get a 'nut' machined for this. Heavy for it's size, amazing to think it's going to be spinning at 30-35k rpm!

Top diagram showing preliminary design and section through turbine shaft. Engine cases have been modified by removing the magneto shroud which was redundant. The bottom picture shows the position of the oil burner pump for providing oil to the gears. As the meshing gears are going to be running at high speed, the use of a pump as opposed to an oil bath is necessary to force oil at high pressure into the meshing gears. The pump will be driven via a worm and wheel arrangement of about a 20:1 ratio mounted on the turbine shaft inside the original crankcase housing. Coupling to the pump will be via a double universal joint.

Large gear and pinion with a ratio 5:1. With the turbine spinning at 35,000rpm, gives an output of 7,000rpm, slow enough for a chain and sprocket final drive to the rear wheel.

1:1 scale mock-up of the turbine scroll for the freepower. Adapter tube shown detached. The design is based on what others have done, intuitive guesswork and aesthetics! ;o) The idea is that the exhaust gases from the combined turbos will be fed via a single pipe into the scroll, redirecting the gases correctly onto the turbine wheel and hence converting the kinetic energy of the gases into rotational motion. The real scroll will be made from about 1.5-2.0mm thick mild steel sheet. Stainless would be nice but my welding skills don't stretch that far. ;o)

Adapter tube attached. The bottom two pictures show the turbine as it will be positioned. Not shown is the turbine wheel shroud which will be made from 5mm thick walled mild steel tube. The depth of the tube will be about 20mm, just about enough to cover the blades so that the gases can't bypass the wheel until they have given up as much of their energy as possible. In addition, clearance between the blade tips and shroud will need to be tight, not much more than about 0.5mm, so positional accuracy needs to be ensured in construction.. ;o)