Is Your Turbine Obsolete?
By D.F. Caris
Head of General Motors Research Laboratories Automotive Engines Department.
In the current flood of comparisons between the gas-turbine and free-piston engines with our present-day piston engines, 1 think the fact that pistons are accepted and established and the gas-turbine and free-piston types new and experimental tends to throw the facts out of focus.
So, for a fresh viewpoint, let’s consider the piston engine as having just been invented and trying to break into the passenger car field. Let’s assume the gas turbine had been the power plant introduced with the automobile around the turn of the century. Since its inception, great strides have’ been made to improve it’s overall performance and add an element of economy especially at lower speeds.
Most important of these changes include the gradual improvement of the turbine and compressor, bringing internal efficiencies up around 85% to 88%. A slightly larger but more satisfactory regenerator essentially eliminates the problem of high temperature exhaust by returning a higher percentage of normally wasted exhaust heat to the cycle. This significantly improves cycle efficiency.
And metallurgists have kept pace. New alloys, developed to live in high temperatures allow turbine inlet temperatures to be increased, bringing cycle efficiencies to a yet highest level.
As a result, Mr. Motorist is now enjoying remarkable economies. It is not at all unusual to get 13 miles to a single gallon of fuel at speeds in the order of 40 mph, compared to only eight in 1930. He still, however, suffers with slow throttle response.
Acceleration lag is characteristic of the turbine. Turbine speed is a function of flow and temperature and this cannot be increased by direct means.
Within the past few years, a modification of the gas turbine, called the free-piston engine, has appeared on the market. In this version, the turbine compressor-gasifier portion has been replaced by a cylinder with two opposed pistons. Here’s what happens.
The cylinder in which the pistons reciprocate serves as the combustion chamber. The compression work is done by the pistons as the combustion products drive them ‘toward their bottom-dead-center positions. Expansion of the combustion gases in the, cylinder lowers their temperature considerably before they enter the turbine. This minimizes the use of critical materials (such as nickel and cobalt) in the blades.
This cycle appears to give a fair gain in economy. Noise is still a problem, however, and acceleration lag still exists, although to a lesser extent.
Now here’s the story on the most recent engine development - a new type internal combustion engine based on the’ Otto cycle (constant volume).
The inventors reason this Way:
If the free-piston gasifier serves as a compressor by using part of the energy of the expanding gases, why couldn't a similar principle work using all the energy, part used for the compressor part of the cycle, the remainder going info engine output? This is why the turbine could be eliminated completely.
The XPE-2 is the second completed experimental model of this type engine. It has just finished 30,000miles of test operation - trouble free.
This unit has six vertical cylinders with a single piston in each, operating on a four-stroke cycle. The compression ratio is 7:1. Fuel and air are mixed in proper ratio before entering the cylinder and are admitted by a simple venturi device called a carburetor.
Fuel is ignited after compression by a timed electric spark. The constant volume combustion process tends to give a better cycle efficiency for a particular compression ratio than does the constant-pressure process.
Equally-spaced power impulses to the crankshaft smoothly convert the reciprocating motion of the pistons to rotary motion of the crankshaft It was easy to balance the engine, as balancing technique reached a high level of development with high-speed turbines.
Compared to the turbine, the piston engine compares favorably in both size and ‘weight per’ horsepower output this is also true to a lesser degree in comparison with the free-piston engine. In addition, it has many features of its own that make it especially adaptable for automotive use. All of these add up to one word performance. Here is what it means, one letter at a time.
Proved Reliability was borne out over 30,000 miles of road testing. This included acceleration tests, high-speed operation, and many intermediate constant-speed economy runs. Complete disassembly of the engine after testing showed no, excessive wear.
To be competitive, a new engine design must vie successfully with existing engines. The low-speed and low-temperature operation of the piston engine should result in considerable advantage over the turbine engine in reliability. There is every reason to believe that one engine is capable of operating for 100,000 miles in passenger car service.
Economy is the brightest spot in the entire picture., At the comparatively low compression- ratio of 7:1, the astounding figure of 20 mpg was obtained at 40 mph constant speed on a level road. This, as all of us well know, is considerably better than our present turbine engines deliver at the same
Even more remarkable, 23 mpg was realized at speeds in the order of 30 mph. Generally, economy is good throughout the entire speed range, although maximum economy is at ‘road speeds between’ 20 and 40 mph.
With our present turbine, this is not a particularly good speed range for fuel economy is especially poor at lower car speeds and at idle. Therefore the gain to the city driver is actually considerably greater than the comparative 40 mph constant-speed figures indicate.
For example, a piston engine will idle at 400 rpm for an hour on half a gallon of fuel. The present turbine requires from two to three gallons to idle at several thousand rpm. The significance of this differential will ‘be appreciated by’ those who drive in congested city traffic.
Suppose you had operated your turbine-driven car until you had used 20 gallons of fuel. If you had been operating a piston engine-driven car, you would have used only 12 gallons. As a nation, we would save 24 billion gallons of fuel per year, at least a saving of several billion dollars.
Response is one of the most exciting features of the new piston engine. There is no combustor or gasifier - a varying weight of air and gasoline mixture is admitted to the cylinders by varying the throttle opening.
A car so equipped is agile and quick to respond, with negligible acceleration lag. To appreciate what driving it can he like, it must he experienced. Also, this built-in characteristic provides a greater safety margin, especially for passing. It is an attractive feature that should add a 10 to the salability of the automobile equipped with this type of engine.
Future possibilities of this piston type engine appear to be brilliant. The presently-developed engine shows a high level of desirability; but even at this early date considerable room far improvement is quite evident
The first step suggested in this direction is an increase -in the compression ratio. A theoretical efficiency computation shows that if the 7:1 ratio of the XPE-2 were to be doubled, engine fuel consumption would be reduced by 20.4%. -
Thus, if vehicle performance were to be constant with increased compression ratio (which, of course, it will not) this single step would make a theoretical gain in mpg of 40.8%.
Steps in ‘this direction have already been taken. It is rumored ‘that piston engines have been operated at compression ratios Pin excess of 12:1 without difficulty and that the results substantiate the theoretical predictions. This high-compression operation naturally requires a higher quality gasoline, but there is no reason to believe that the petroleum industry cannot continue to keep pace with engine development
Optimum life can be expected from materials, despite the higher temperatures that occur in the’ Otto cycle, as opposed to the gas turbine cycle. And these already high temperatures must be increased even more if a higher degree of efficiency is to be realized.
However, unlike the steady-flow turbine cycle, these temperatures are a function of time rather than position. They are not higher, throughout the entire cycle, but only part of it. Consequently, no component is subjected to extreme heat continuously, as the, temperature is constantly ranging from the high to the low.
Rotative speed of the new engine, is extremely low. It ranges from about 400 rpm at idle to about 4000 rpm at top road speed. Since these rotative speeds are about one-tenth the speeds of our present engines, exact balance is a lot less critical.
For example, if the piston engine is balanced to .5-oz/in., the present gas turbine would have to be balanced to .005-oz/in. for comparable smoothness.
Materials of a Jess critical nature can be used in the piston engine. The absence of high-sustained operating temperatures allows cast iron and low-alloy steel to be used in presently developed piston engines. However, aluminum and magnesium would play important roles in future – development. They are more readily available, and weigh a lot less.
Airflow is reduced, as only enough air is needed to meet the demands of complete combustion. In fact, an excess of air increases combustion time. This results in deviation from the ideal constant-volume path of the Otto cycle, reducing power and economy.
Thus, another difference from the turbine engine. In fact, airflow with the new piston engine is only about one eighth that of a present turbine engine of equal power. Smaller manifolding and ducting can be used, as well as a smaller air filter.
Noise level of the piston engine can be maintained at a relatively low level. Low rotative speed and low airflow rate contribute toward this end, and an exhaust-line muffler dampens exhaust pulsations. Transmission noise is easier to muffle because of the low step-down that is required.
Control of engine speed is a function of the weight of the charge entering the cylinders; this is regulated very simply by a butterfly-type valve below a carburetor. The air-fuel mixture remains relatively constant throughout the speed range and during acceleration, contributing to a high degree of combustion efficiency.
Compare this control with that of the gas turbine. To begin, combustion must occur with primary air at approximately stoichiometric ratio, the products then diluted with secondary air to produce a very lean overall fuel-air ratio. This is necessary to keep turbine inlet, temperatures below the permissible limit.
Under these conditions, the fuel metering system must continually proportion primary and secondary air, and also be sensitive to both turbine inlet and regenerator outlet temperatures. This is necessary to maintain high combustion efficiency during transient periods of operation.
The regenerator particularly complicates the problem. Its energy-storing capacity results in variable combustion chamber inlet temperature for a given turbine inlet temperature until regenerator equilibrium is established.
Exhaust gas volume from the piston engine is small compared to that from an equal-output turbine, therefore requires less cumbersome exhaust ducting. The engine is also less sensitive to elevated exhaust pressure than the gas turbine.
Thus, piston engines have a few advantages of their own. Combined and Incorporated as automotive power, they spell one thing - performance.
The inventors of this new-type engine are enthusiastic about it’s possible use in this capacity. They realize that the automotive industry is the world’s greatest manufacturer of power, and that anything novel will attract a lot of attention.
They know, too, that to be novel is not enough by itself, just as it is not enough to be just as good as the Incumbent power plant. It has to be better to justify making the change.
The piston engine seems to qualify in every respect. In fact, the Inventors are predicting that it will eventually take over the entire passenger car field.