February 2004

More engine frame

The engine frame was a lot more difficult than I first thought, as I wanted the frame to also hold the radiator, water pump, servo unit and ECU. The idea is to follow the army’s approach with the "power pack" replacement in tanks. If I fit all of the engine related components to the engine frame then if I ever need to work on the engine, or swap it for the other engine I can just pull the whole assembly out, and not have to worry about draining it down or disconnecting too many controls. This way, the only connections from the power pack to the rest of the hovercraft are:

The drive coupling from the engine crankshaft to the lower pulley

The petrol pipe from the tank to the pump

The throttle control linkage

One connector carrying:
    rev counter cable
    temperature sensor cable
    engine cut-out cable

It also means I have a safe method of running the engine on its own – as long as I keep the revs down.

I added a few lengths of angle section to support the electric water pump and the original radiator that came with the bike, then plumbed a coolant hose from the outlet on the top of the cylinders to the top of the radiator.

It's coming together...

Having built the engine frame and the fan frame it was time to put it together and see what fitted and what didn't.

The exhaust ports of the TZR250 engine exit downwards, so the pipes were going to need a fair amount of bending to turn them upwards and over the side of the hull, and the radiator needed repositioning closer to the engine, but there's definitely progress there!

The cooling system

The coolant inlet to the TZR engine is via a hole in the casing. To make a pipe union I used a 60mm length of 19mm aluminium tube and filed it down a little at one end to give a tight fit in the inlet hole. The water pump is just a few inches away, but the inlet to the water pump which comes in vertically into the bottom of the pump will need an elbow pipe. Merlin motorsports have a good selection of coolant pipes so I ordered one from them.

The water pump is an Australian made, Davies Craig EBP. Some people recommended a Johnson pump, but I went for the EBP because it was well thought out; it has a magnetic coupling between the motor and the impeller, so no seals and a switching motor. It requires 1.1 Amps at 12Volts.

Fuel system
As the petrol tank will be mounted on the floor of the hovercraft, under the driver’s seat, and the carbs will be about 400mm above the floor of the craft, gravity fed fuel seems a bit unlikely, so I also needed a petrol pump, and a fuel pressure regulator.

Some people recommended using a header tank with a float valve mounted on the side of the duct. I don’t like the idea of a fuel system fitted near the fan, with two or three long lengths of fuel pipe running to and from it. Instead I'm using a Facet fuel pump and a Purolator adjustable fuel regulator, both supplied by Merlin motorsports .

K&M supplied the fuel tank – 12litres, with a very sturdy feel, and also the fuel pipe. The idea is to fit all of the fuel related parts - the primer bulb, filter, and regulator near the fuel tank which will sit in front of the engine. The pump will be the last part in the chain, minimising the length of pipe between it and the carbs. All of the fuel lines and fittings are in 8mm bore tubing, except for the final bit to the carbs, where they are in 6mm bore.

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