Last week in Alternative Cars Part 2
we looked at the possibility of cars powered by solar energy. This week, we look
at the potential for gas turbine cars.
Aircraft have shown the way with the transition
from piston to turbine engines. During WWII, piston aircraft engines became very
sophisticated, with examples featuring double overhead cams, four valves per
cylinder, variable valve timing and turbo- and supercharging. But within 10
years, piston engines disappeared from nearly all high performance aircraft,
replaced by turbines. Gas turbines are used in helicopters, trains, tanks and
small power stations as well.
Some car manufacturers – especially Chrysler and
Rover - have in the past invested heavily in turbine development, in the case of
Chrysler reaching the stage of having road-going prototypes in public hands. A
turbine-powered race car (pictured) even contested the 1968 Indy 500 – and went
very close to winning it.
So why haven’t turbine-powered cars taken off?
Function
Turbine engines can be divided into two classes –
centrifugal and axial. However, in both cases the fundamental operating
principles are the same.
A turbine uses a compressor and a turbine, with
both mounted on the one shaft. Air enters the engine and is compressed by the
rotating compressor blades. Fuel is then added to the compressed air flow and
combusts, causing the gas to expand through the turbine that in turn extracts
power from the hot gases. Since the turbine and compressor are on the one shaft,
the turbine drives the compressor to further compress the entrance air.
A centrifugal turbine engine (pictured) is rather
like a car turbo in design – the compressor throws air out a side exit and the
turbine is driven through gases reaching its blades from the side. In a
centrifugal turbo, the combustion chambers are in the ‘pipes’ that join the
compressor exit and the turbine inlet. In an axial flow design the air passes
through multiple fan-like blades, staying parallel to the long axis of the
engine and altering pressure at each step. The combustion chamber is usually
internal to the engine. Some gas turbines have both axial and centrifugal
elements in the design.
In a jet aircraft engine, the flow of gas from the
engine produces thrust that in turn pushes the aircraft along. However, in
automotive (and helicopter, train, tank and power station) applications, the
flow of gas from the compressor turbine is used to spin another turbine which is
geared to a shaft that provides the power output. The advantage of using a
separate turbine to extract power is that the main part of the engine can be
revved to full power while the output turbine is stationary. This allows the
development of maximum torque when at rest, with the torque output decreasing as
the vehicle travels faster.
In automotive applications, where small turbines
are used, efficiency is lower than that achieved for large turbines. To improve
efficiency, the hot exhaust gases are used to preheat the intake air, so
reducing the amount of fuel needed to obtain the same internal combustion gas
temperature. This heat exchanger is sometimes known as a recuperator or
regenerator.
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These
rare diagrams show some of the prototype automotive gas turbine engines
developed in the 1950s.
The
1954 120hp turbine engine by Chrysler.
The
Boeing 502 gas turbine had an output of 160hp and used a single stage axial
compressor and turbine. It was installed in a Kenworth truck.
Fiat
designed and built this 200hp turbine car in 1954. The engine used a two stage
centrifugal compressor and a two stage axial turbine.
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Advantages
The advantages of a turbine engine for car use
include a lack of vibration, high power/weight ratio and compact size (although
both aspects depend on whether a recuperator is used) and the ability to burn a
variety of fuels, including those of low octane. A multi-ratio gearbox is not
needed - although step-down gearing is.
Finally, there is a wealth of knowledge available
on the design and construction of turbines.
Disadvantages
However, as is clear by their lack of widespread
use, the disadvantages are large.
The primary negatives are high cost (expensive
high temperature materials needed) and high fuel consumption. The latter is
mostly the case because at part throttle, turbine engines are very thirsty for
the amount of power being produced. Gas flows (both intake and exhaust) are also
very large, so effective filtration is bulky and exhaust silencing needs to be
comprehensive. Emissions performance to car legislated standards is also
problematic.
Solution
It’s unlikely that gas turbine cars will be
commercially produced. However, one possibility is to use a very small turbine
that works continuously at maximum power, driving a generator that charges a
battery and/or makes power available to an electric motor. However, even in this
scenario, battery losses would reduce overall efficiency quite considerably. (In
fact in 1993 General Motors developed a gas turbine hybrid version of its EV1
electric car.)
The major advantage of gas turbines for aircraft
(their ability to work at high altitude being the primary one), and in
helicopter and small power station applications (fuel-efficient when working
continuously at full power) do not apply to cars.
Conclusion
It’s likely that the gas turbine cars produced by
Chrysler and the prototypes developed by Rover (and Boeing, Turbomeca, Laffly,
SOCEMA, Fiat, GM, Renault and Austin) in the 1950s and 1960s will be the only
gas turbine cars to see the light of day.
Next week: human powered vehicles
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