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?
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.
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.
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.
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.
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