INTRODUCTION
In its basic concept, the gas turbine is the simplest of all engines. There are no reciprocating parts. Combustion is continuous, eliminating the need for precise timing of each event in the cy le. The ignition system is needed only when sta mg, a d a cooling system is unnecessary. There IS no slid ing metal-to-metal contact, as in a .piston ng ne; only a few bearings and shafts reqmre lubncahon.
The gas turbine offers several other advantages for motor vehicle use. It is much lighter and smaller than gasoline or diesel engines of com parable power. The lack of reciprocating p arts in the engine makes it inherently smoother. It runs on a wide variety of liquid and gaseous fuels. Because most turbines have built-in “torque con verters,” there is no need for a clutch, and the number of gear ratios in the translnission is usu ally fewer than required for a reciprocating engine.
Although turbines have been installed in a number of experimental passenger cars, trucks and buses, no standard production motor vehicle in the United States is yet available with turbine power.
The term, “gas turbine,” is misleading. The en gine gets its name from the fact that power comes from the force of burning gases striking the blades of a turbine. However, the hot gases may be created by the combustion of either liquid or gaseous fuel-or even finely powdered solid fuel. Turbines have been operated on gasoline, natural gas, diesel fuel, aircraft jet fuel, powdered coal and perfume. Most vehicular turbines burn diesel fuel.
HOW THE TURBINE WORKS
In its simplest form, the turbine consists of a centrifu gal compressor and a turbine wheel, mounted on opposite ends of the same shaf ; a burn er and a means of providing an electncal
spark. ‘Air enters the copressor inlet, hen is compressed to about four times atmosphenc pres sure increasing its temperature by several hun dred degrees. The compressed and heated air enters the burner, or combustor, where fuel is introduced and the mixtur e ignited by an electric spark Once combustion begins, it is continuous; therefore, the ignition system is used only when
starting. The burning gases expand rapidly, strik ing the bla des of the turbine, spinning the shaft and providing power for the compressor.
The earliest turbines developed barely enough power to keep themselves running. Virtually all of the power produced by the turbine was consumed by the compressor. The remaining energy in the burning gases escaped to the atmosphere.
To capture more of the energy of the hot gases, engineers added a second stage, or power, tur bine. The second stage provides power to produee useful work-to drive a vehicle, turn a generator or operate a pump .
The second-stage turbine can be mounted on the same shaft as ‘the first-stage turbine and com pressor or on a separate shaft. Eac des gn as its advantages. The single-shaft vers10n IS srmpler mechanically. It’s good for an application such as a generator, which must maintain a relatively con stant speed, regardless of’fluctuations in load. The high rotating mass develops the inertia to com pensate for sudden changes in electrical load.
Gas turbines designed for vehicular use are of the split-shaft type. The compressor and first-stage turbine are mounted on one shaft; the power turbine is fastened to a second shaft, on the same axis but not connected to the first.
The portion of the turbine containing the com pressor and compressor turbine is called the gasi fier section, while the second-stage turbine, reduc tion gears and output shaft make up the power section.
The split-shaft turbine develops very high start ing torque, making it ideal for vehicular applica tions. When the vehicle is stationary, the power turbine, which is geared to the rear wheels, does not rotate, but the first-stage turbine idles at ap proximately 20,000 rpm. As the vehicle accelerates from a stop, first-stage turbine speed rises, reach ing a maximum of 45,000-50,000 rpm. The burn-ing gases, after passing through the first-stage turbine, strike the blades of the second-stage tur bine, turning it and the driving wheels of the vehicle. At maximum speed, the second-stage tur bine is turning at approximately the same speed as the first stage. The effect is similar to that of the hydraulic torque converter used in automatic transmissions. The engine can’t stall, and maxi mum performance can be achieved with a trans-mission of fewer gear ratios than required with a piston engine.
Because of the high operating speed of the second-stage turbine, reduction gearing is neces sary for turbine-powered vehicles. The shaft on which the second-stage turbine is mounted drives the input gear, which meshes with an output gear mounted on the output shaft. Gear ratio is usually about 10 to 1, reducing output shaft speed to 1/10th of power turbine speed.
Both the single-shaft and split-shaft turbines previously described are known as simple-cycle turbines. Once the burning gases have passed through the two turbine stages, they escape to the atmosphere, still at temperatures of 1,000-1,200 degrees F. Exhausting burned gases at such high temperatures has two big drawbacks. First, much of the energy in the expanding gases is wasted, resulting in high fuel consumption. Second, the emission of hot gases from a motor vehicle could be hazardous.
the exhaust gases to the intake air. While a st a tionary heat exchanger can be used, most automo tive gas turbines employ rotating regenerators, geared to tum very slowly. In one turbine de signed for vehicular use, the two regenerat ors turn at 9 rpm while the gasifier turbine and compressor are spinning at 18,000 rpm.
The regenerator can be made in the form of a disc or a drum. It is of honeycomb construction , similar to an automobile radiator, but ma de of material such as stainless steel, capable of with standing high temperature. As the regenerator rotates, air at relatively low temperature and pres sure passes through one portion, while exhaust gases at high temperature and pres sure flow through another area. The incoming air picks up heat from the hot surfaces of the regenerator , which have been exposed to the exhaust gases. As a result, temperature of the air enterin g the com bustor is raised by several hundred degrees.
REGENERATIVE TURBINES
To extract more energy from the burning gases and reduce exhaust temperature, designers added a regenerator. This device is simply a heat ex changer, which transfers some of the heat from the exhaust gases to the intake air. While a st a tionary heat exchanger can be used, most automo tive gas turbines employ rotating regenerators, geared to tum very slowly. In one turbine de signed for vehicular use, the two regenerat ors turn at 9 rpm while the gasifier turbine and compressor are spinning at 18,000 rpm.
The regenerator can be made in the form of a disc or a drum. It is of honeycomb construction , similar to an automobile radiator, but ma de of material such as stainless steel, capable of with standing high temperature. As the regenerator rotates, air at relatively low temperature and pres sure passes through one portion, while exhaust gases at high temperature and pres sure flow through another area. The incoming air picks up heat from the hot surfaces of the regenerator , which have been exposed to the exhaust gases. As a result, temperature of the air enterin g the com bustor is raised by several hundred degrees.
Without a regenerator, it would be necessary to burn additional fuel to achieve the same increase in temperature. Because heat is transferred from exhaust gases to incoming air, exhaust tempera ture is reduced to a safe level.
Tracing the flow of air, fuel and burning gases in a turbine aids in th e understanding of the en gine’s cycle. The example used is the Chrysler turbine installed in experimental passenger cars loaned to selected drivers as part of Chrysler’s field t esting program, Fig. 3. The turbine was a split-shaft design with dual regenerators, mounted on opposite sides of the engine to ensure even heat distribution and prevent the development of therm al stresses.
Air ent ers the compressor, where it is com pressed to approximately four times atmospheric pressure , increasing its t emperature to about 425 degrees F. It then flows through th e slowly revolv ing regenerators, raising the temperature to 1,100 degrees. The hot air enters th e burner, where fuel is added. When the engine is first started, an electric igniter, similar to an oversized automotive spark plug, produces the spark to ignite the mix ture of fuel and air. Once combustion begins, it is continuous; th e electric spark is no longer needed.
The burning gases reach a temperature of approximately 1,700 degrees, as they leave the burner and flow to the first-stage turbine. A portion of their energy is expended in driving this turbine, which furnishes power for the compressor and all accessories. In the process, the gases give up some of their heat, leaving the first-stage turbine at a temperature of about 1,375 deg. They then strike the blades of the second-stage , or power, turbine, turning the output shaft, which is geared to the driving wheels of the vehicle. After passing through the blades of the second-stage turbine , the gases cool to about 1,200 deg. They then flow through the regenerators, reducing their tempera ture to approximately 525 deg. as they escape from the exhaust stacks.
All of the engine’s useful power comes from a power turbine that is smaller in diameter than an ordinary dinner plate. A 200-hp gas turbin e, with all accessories, weighs less than 500 lb. A diesel truck engine of comparable horsepower weighs 1,600-2,000 lb., not including the cooling system and clutch that are not needed with the turbine.
The split-shaft turbine, even when equipped with dual regenerators, has only four major mov ing parts-the compressor and compressor turbine assembly, the power turbine and the two regen erators. A typical V8 reciprocating engine, either gasoline or diesel, has 83 major moving parts, in cluding a crankshaft, camshaft, flywheel, 8 pis tons, 8 connecting rods, 16 valves, 16 push rods, 16 valv e lifters and 16 rocker arms. The turbine also requires fewer bearings, needs no cooling system and has a much simpler ignition system than the gasoline engine and a fuel system that’s less complicated than that of a diesel.
REDUCED AIR POLLUTION
Because the gas turbine operates on a mixture that contains a high percentage of air in propor tion to fuel, combustion is complete, resulting in extremely low emissions of such pollutants as carbon monoxide and unburned hydrocarbons. As federal and state exhaust emission standards be come increasingly stringent, the turbine’s inher ently clean exhaust characteristics make it more attractive to engineers. Unlike the reciprocating engine, it does not require extensive modi£cations to conform to the new regulations.
COLD-WEATHER OPERATION
The turbine is particularly well-suited to cold weather operation. It spins almost as freely at 0 deg. as it does at 100 deg. because there’s no film of congealed oil between sliding surfaces to put an additional load on the starter motor. The ig niter produces repeated sparks until the fuel mix ture starts to burn . There’s no need for precise timing of the spark.
A turbine engine requires no warm-up. Once combustion starts and the gasifier turbine reaches idle speed, the engine is capable of pickin g up full load, without stalling, regardless of outside tem perature. For the comfort of operator and pas sengers, heat is available a few seconds after the engine starts.
Despite its many ba sic advantages, the turbine has not yet achieved commercial success in vehicular use. Most applications have been in specialized vehicles, such as oil-field servicing trucks, mobile log chippers and other machines in which the turbine’s favorable power-to-weight ratio outweighs its two major disadvantages of high initial cost and heavy fuel consumption.
The gas turbine’s high fuel consumption and expensive selling price are closely related. Effi ciency of the engine goes up sharply with in creases in operating temperature, but the maxi mum permissible operating temperature is limited by the ability of material in the turbine wheels to withstand the tremendous heat. High-priced al loys are capable of operating at elevated tempera tures, but they increase the cost of the engine. At present, temperature at the first-stage turbine inlet is limited to about 1,700-1,750 deg.
FUEL CONSUMPTION
Under the conditions where it works best (heavy load and relatively constant speed), a regenerative turbine can match or exceed the fuel mileage of a gasoline engine and even approach the economy of a diesel. At light load and varying speeds, however, even the most efficient turbine consumes considerably more fuel than does a gasoline engine. Without a regenerator, fuel con sumption is about twice that of a gasoline engine and nearly three times as much as a well-designed diesel.
For vehicular use, the turbine has two other serious handicaps-lag in throttle response and little or no engine braking. The delay in throttle response is due to the necessity of increasing first stage turbine speed from 20,000 to about 45,000 rpm before maximum torque can be applied to the second-stage turbine. There is no engine braking in a split-shaft turbine because there is no me chanical connection between the gasifier section, in which compression takes place, and the power section, which drives the wheels. When a vehicle powered by a piston engine descends a grade with closed throttle, the flow of power is reversed, the driving wheels turning the engine, instead of the engine turning the wheels. Engine compression acts as a brake to help slow the vehicle.
One method of solving both of these problems is a variable nozzle arrangement. In most turbine engines, burning gases are directed against both the first-stage and second-stage turbines through fixed nozzles, each in the form of a ring of airfoil shaped vanes. The vanes are positioned to ensure that the gases strike the turbine blades at the angle that will provide maximum efficiency through most of the engine’s operating range.
To increase efficiency and provide engine braking, one form of variable nozzle system employs a ring gear meshing with sectors attached to each vane. Positioning of the vanes is accomplished by hydraulic pressure, controlled by signals from the accelerator pedal and the transmission. At idle, the vanes are positioned to direct the hot gases straight into the power turbine blades-90 de grees from the direction of rotation. As the ac celerator pedal is depressed, the vanes turn, changing the flow of gas to the direction of the turbine’s rotation.
For engine braking, the variable nozzle re sponds to signals from both accelerator pedal and transmission. A signal from the transmission gov ernor tells the hydraulic control system if the vehicle is coasting at sufficient speed to require engine braking. If it is, releasing the accelerator pedal causes the vanes to shift to reverse position, directing the hot gases against the power turbine opposite to the direction of rotation.
THE FUEL SYSTEM
The gas turbine’s fuel system is basically simple, but it needs several controls to protect it against itself-devices that prevent overheating, over speeding and hung starts. Because the turbine always operates with an excess of air, there is no need to control the proportions of fuel and air, as in a gasoline engine. In most systems, a transfer pump, usually driven electrically, pumps fuel from the supply tank to the high-pressure pump, which is driven by the gasifier section of the turbine. The rate of fuel flow is controlled jointly by accelerator pressure and signals from the gov ernor, also driven by the first-stage, or gasifier, turbine. When the vehicle is coasting, with ac celerator released, fuel flow is shut off.
Should first-stage inlet temperature exceed a safe level, temperature probes signal the pump to reduce the flow of fuel. Some turbines are also protected against hung starts. This condition occurs when the charge of fuel and air ignites but the first-stage turbine does not turn the compres sor fast enough to produce the excess air required to cool the turbine. If the first-stage turbine does not reach the required speed within a specified time, fuel is shut off to prevent overheating of the turbine blades.
THE BURNER
The burner, or combustor, looks like a tin can with a series of holes drilled around its circum ference. Compressed and heated air enters the burner and is mixed with fuel injected from a nozzle mounted in the burner cover. The igniter, which works in the same manner as an ordinary spark plug, is also installed in the burner cover.
THE IGNITION SYSTEM
In an aircraft or industrial turbine, an electric spark is needed to ignite the fuel charge only when the engine is started. Turbines for land based vehicles often incorporate a fuel shut-off, which operates when the vehicle is decelerating, reducing exhaust emissions and conserving fuel. To prevent the engine from stalling, the flame must be re-lit when the engine returns to idle speed or is accelerated. The simplest method of accomplishing this task is to use an ignition sys tem that fires continuously. A typical unit employs a set of cam-driven points, similar to those in a conventional automotive distributor; a coil; a con denser; and a shielded igniter. The igniter fires 80 to 200 times per second, depending upon engine speed. The system is much simpler than that of a gasoline engine because the spark is applied at only one point, instead of six or eight, and it need not be timed to occur at a specific stage in the engine’s cycle. The spark merely starts the fire; if it’s early or late, the engine doesn’t know the difference. The turbine won’t knock, lose power or overheat because of faulty timing.
Review Questions
1. What are the major advantages of the gas turbine for motor vehicle use? …………… .
2. What fuels are suitable for gas turbin es? ………………………………….. .
3. What are the four basic parts of a simple turbine? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. What is the function of the second-stage turbine? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Why is the split-shaft turbine ideal for vehicular use? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6. Why is reduction gearing necessary for turbine-powered vehicles? . . . . . . . . . . . . . . . . . . . .
7. What is the basic difference between the simple-cycle and regenerative turbines? . . . . . .
8. How much does the regenerator increase inlet air temperature? . . . . . . . . . . . . . . . . . . . . . .
9. How many major parts are there in a split-shaft turbine? . . . . . . . . . . . . . . . . . . . . . . . . . .
10. What are the two major disadvantages of the gas turbine? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. How does fuel consumption of the gas turbine compare with that of gasoline and diesel engines? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12. What is the function of a variable nozzle? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13. What is a hung start? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14. Where is the fuel nozzle mounted? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15. Why is it necessary for the ignition system of a vehicular turbine to fire continuously? . . . .