Alec Cameron said the following to MIKE ROSS on the subject of
ENGINE DESIGN (15 Oct 97  19:53:06)
 > Hybrid turbine powertrains are currently successfully driving buses
 > cheaply in the San Francisco transit system.
 AC> And I wish them a happy long life. Let's see how that system is
 AC> running ten years from now.
Me too, it's an ongoing evaluation. However, if there was confidence
enough in the state of the art to go from design board, to prototype, to
endurance field tests, it shows a certain amount of maturity in the
technology. Indeed, let's be optimistic.
 AC> When my employer was looking at small gas turbines for electricity
 AC> generation, and attractive aspect was that there was an existing
 AC> service/ repair industry, thanks to the success of aero engines.
 AC> We bought diesel sets instead. Higher first cost, lower maintenance
 AC> cost, long proven reliability. Easily sold [exported] upon retirement.
 AC> The bad news, for the turbines we considered was the absurd low number
 AC> of hours, between the necessary inspections and repairs. The bus
 AC> operator in San Francisco will know already, the downtime arrangements
 AC> for normal diesel engines. Presumably, 200 000 miles or more between
 AC> major checks are done.
You really don't expect me to believe 200,000 miles is realistic. Maybe
you meant the above turbine powered hybrid buses... :-)
Pratt & Whitney makes small turbine driven APU's. Perhaps your boss
should have contacted them 1st before throwing in the towel.
 AC> It is ten years since I retired. Let's know more about those hybrid
 AC> buses, later! Maybe some of the worn bits will work in a HOMEPOWER
 AC> COGENERATION POOL. With a human treadmill on the shaft extension, to be
 AC> walked by failed Fidonet enthusiasts! ;-)   Cheers.......ALEC
The design of a small car turbine would start with consideration given
to the speed/torque ratio required. Since this is not an aircraft or a
ship, this ratio is different for a car.
It can be easily foreseen that a larger diameter to length ratio would
be appropriate for a car turbine. This for a number of reasons, let
alone that it has to fit under the hood.
The larger diameter will result in a lower pressure, lower temperature
system with a lower peak power, but with a gain in torque/speed ratio.
This in direct contradiction to the design philosophy for an aircraft.
In consequence a number of constraints found in conventional turbines
become a bit more relaxed. The lower speed and temperature diminishes
the requirement for very exotic alloys, parts can be cast in one piece,
lubrication is simpler through the use of air bearings, and the exhaust
can be more easily managed to avoid blasting a following car.
All the above also makes for simpler starts but this is not really an
issue since the car can be driven as soon as the electric motors in each
wheel are connected to the battery with an extra boost coming from
energy stored in a flywheel that can rotate on its own for weeks.
 Mike
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