Richard Riley wrote:
On Sat, 9 Jul 2005 21:01:44 +0100, "Keith W"
wrote:
:
:Given enough time and money you can build anything and gas turbine cars
:have been built. The real question is why ?

I agree so far...

:
:Such a vehicle is likely to be unreliable and extremely inefficient in
:using fuel and probably could not be certified for road use.
:A better approach may be a hybrid using a small gas turbine running
:at constant speed to charge a battery for an electrically driven
:vehicle.

But then I have to ask why back? In the size you're talking about, a
piston engine has much better fuel specifics than a turbine, about .4
against .6 or .7 - worse at low power setting. A small turbine just
can't get the compression ratio that a piston can. And since it can't
cool itself between combustion cycles, the metalurgy required means
it's very expensive.


Gas Turbine Vehicles for land traction must be designed for different
characteristics than those for aircraft propulsion.

An aircraft requires takeoff power of about 100% or maximum power
followed by cruising power or around 65%.

A car cruises at about 10% to 20% at the most.

If cars opperated in the same regime as aircraft then they would be
more suitable.

A gas turbine for land traction is NOT designed to have a high
compression ratio. It requires these modifications:
1 a low compression ratio compressor that is more suitable for low load
factors.
2 a centrifugal style compressor that has a broad efficient opperating
range. (possibly variable geometry could be used.)
3 a heat regenerator: a device consisting of a roting mesh or cermaic,
glass or metal that soaks up heat wasted in the exhaust and then
preheats air after compression but before burning with fuel.

Such engines have been made, for instance Rovers engines of the 1950s
and 1960s.

advantages of the gas turbine are
1 about half the zize and weight of IC engines.
2 no torque pulsations therefore lighter drive train.
3 Indifferent to fuel: cetane and octane ratings are irrelevent. Can
run on virtually any gaseous or liquid fuel.
4 Easier to make.

Point 4 might be a suprise but it is in essence true. The problem with
gas turbines has always been not the manufacturing costs (which can be
made tiny by mass production and automation) but the costs of the raw
materials which are exotic alloys of nickel, chromium, rhenium,
zirconium, molydenum and cobalt. There material costs can not be
reduced.

Ceramics have held out much hope and producing Aluminium Nitride and or
Silicon Carbide turbine blades and turbine nozzles has been possible
for decades: they are reliable however they do not outperform advanced
alloys perhaps a little less. When ceramic opperating temperatures are
extended beyond that of metalls they will work but they are no longer
reliable for aviation.




I can see the arguement for piston/storage (battery, flywheel,
compressed gas, whatever) hybrid - a very small piston engine, with a
lot less internal friction than a large piston engine, and running at
or near it's optimal, tuned speed. Even better is a fuel cell hybrid.
But once you're doing a hybrid, why give away half or more of your
fuel specific just to go turbine for the power. Turbines are great
for power to weight and maintence per operating hour - neither of
which are the driving goals in a car.

I question whether maintenance per opperating hour are not a driving
factor. The US Auto industry as the British one before it has failed
to that fundemental lesson in time.

I personally would prefer and engine that never wears out and only
needs new bearings every 30,000 hours and new oil once per year.

They hybrid concept for gas turbines is probably quite practicable.

Another option is the closed cycle gas turbine that transfers heat into
the working fluid via heat exchanger walls and then cools them
likewise. The German divisions of swiss companies attempted such
engines in WW2 but the Swiss Company Bruckner Kanis made some engines
for electricity generation in the 1950s. They have excellent
multifuel capabillity including solids and their partial load abillity
is excellent because the working fluid can have its peressure altered
to suit the load conditions and also because regeneration is relatively
easy to accomplishin the future.



By coincidence I've ridden in three turbine cars. The first was a
1963 Chrystler Turbine, one of 55 built. The second was a 73 'Vette
with a PT-6, the spare engine from the STP Indy turbine car.

The third was a Model T with an Allison 250 in it. It got 4.5 MPG.
But that wasn't the point.

Apart from the infant Chrysler these were all not properly designed for
land traction use and oversized.

The Avco Lycoming AGT-1500 which is of similar configuration to the
T-53 is a case in point. It gave the Abrams tank unbeatable
performance that can only be matched by the latest MAN hyperbaric
diesels. The AGT-1500 has no regenerator but I suspect that tanks
doesn't spend too much time on very low partial load. The t-53 engine
was designed as a personal project intended for Helicopters by Franz
Anselem who was also the Chief Engineer for the Junkers Jumo 004 used
on the Me 262 jet fighter in another time.

As a side note during the second world war the Germans built the AFV GT
101 gas turbine for use in Panther and Tiger tanks. It was loosly a
scaled down BMW 003 turbojet with a similar anular combustion chamber
but with a unique exhaust duct and power takeoff to the rear. It was a
direct drive turbine that required a 3 speed governor opperated gearbox
plus torque converter to keep the main shaft opperating at around 80%
of rate shaft speed before it went to a normal transmission. The
engine had only half the fuel economy of a normal Panther petrol engine
but the Germans were after engines that were
1 Able to run on any fuel (they only had poor grade synthetics)
2 Easy to make (it would proably only have requred 500 man hours)
3 Give their tanks a power to weight ration advantage (critical for
tank combat)

As a side effect that direct drive gave 2600hp of engine braking and
the flywheel effect had the inertia of the entire 45 ton Panther tank
at 26mph which gave a smooth ride.

Further planed engines were the GT102 which had a second independent
turbine for power takeoff and the GT103 which used a quartz glass mesh
heat regenerator for a 30% improvement in fuel consumption.

After the war Rover Cars and Leyland Trucks produced technically
sucessfull engines with heat regenerators. There was a slight smell
of kerosene.

In general the gas turbine if properly designed is competitive with
petrol and diesel engines if a touch less efficient and more expensive.
They continue to be of great interest for trucks, ships etc.

The big advanate of the IC petrol and diesel engine is that they use
cheap materials despite their mechanical complexity. Only the possible
development of higher performance ceramics will change this.