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Aerospace The advantage the constant-volume combustion type gas turbine engine

  1. Dec 28, 2011 #1
    We all know the constant volume combustion engine acts as a kind of internal combustion engine that is used in the car and truck extremely widely. The cycle is called Otto cycle. I also heard the constant volume combustion gas turbine engine was proposed many years ago and developed a few ones, and soon replaced by constant pressure combustion type. The reason is work frequency and the energy efficiency is low; the engine is rather heavy. But the type of gas turbine engine was made many years ago, so it was restricted by the process and technology at that time.

    Right now I hope to utilize or develop the advantage the constant-volume combustion type: easy to ignite and easy to get the higher gas pressure.furthly inovate its the motion and structure, at last not only on the aspect of energy efficiency, but also the weight to compete the current gas turbine engine.
    Here I start the topic that we discuss to design a new motion and structure for the constant volume combustion gas turbine engine that can avoid the weakness I just mentioned.
    Last edited by a moderator: Dec 28, 2011
  2. jcsd
  3. Dec 30, 2011 #2
    To increase the efficiency of engine(including gas turbine engine ) is a genaral trend, as well as engineers' dream. I hope to apply the constant volume combustion type to gas turbine engine again, totally change the old structure and work courses on this type of combustion gas turbine to get higher efficiency than current gas turbine engine.
  4. Jan 3, 2012 #3
    This section I explain the advantage of this type of combustion and how to realize briefly.I hope readers to comment it.
    -- easy to ignite : The gas under this combustion is static or the flow speed is very low.by this character the gas is easy to ignite. In current jet engine, people do a lot of work to maintain the combustion because the gas flow too quickly in that section.
    -- burning quickly: I did not mention it in the previous post because this advantage need a few condition. If the gas before combustion is already mixed with fuel at proper concentration, the gas can finish combustion extremely quickly, just cost a few milliseconds. The concentration and the pressure of gas before combustion is related to the work performance and safety. The character is very useful to keep high frequency work cycle and smooth work,.
    -- easy to get the high gas pressure: If the gas burns in a closed space, the gas presure will be high definately; If the combustion can finish very quickly, the space is not closed but semi-closed, we still can get high pressure gas after combustion. Normally high presure means high energy efficiency if anything else is same.Though by this way the pressure will be lower a little , the energy efficiency does not change much. We can utilize the solution.
  5. Jan 15, 2012 #4
    I listed the advantages of the constant-volume combustion type for turbine engine, then I will explain how to realize them.
    in order to realize the closed combution or semi-closed space combustion in a certain space,in order to the gas can flow into and out the space in other time. the space need have two doors for input and output for the space at both ends.for high frequency the two doors must adopt rotation to open and close the space.
    One door is at the opening of input of the space(chamber), another is at the opening of output.We can choose the angle between of them to control input and output, including closed or semi-closed combustion .for example the combustion course in completely closed space, the combustion is the closed combustion, if the front door is closed, the back door is opening but the combution has not finished, the combustion is the semi-closed combustion.
    in order to ensure high frequency and reduce the energy loss, I hope each time the old gas after combustion runs out off the chamber by the negative pressure functions. it can suck the fresh gas into the chamber from other end subsequently. The negative pressure is caused by the inertia of the burned gas run out off the chamber .It is possible if we choose the right occasion to open the front door and design proper structure for the burned gas out off the chamber to cause a desired pressure inside the chamber.
  6. Feb 12, 2012 #5
    in order that the engine can work steady and outlet energy can be stable,in order that the two doors for the chamber can rotate continuously and steady,I design several chambers arranged in a circle,at least 6 pairs. thus they cooperate each other, such as the gas in one pair rushs out off the chambers, other pairs are in other courses, such as inlet gas or combustion.The chambers in symmetrical position is in the same course. Though the engine has a energy efficency only by the closed combustion in chambers, in order to increase the efficiency, in front of the chambers, I set the compressor to press the air.(later I will prove the point by the theory formular of the Thermal cycling.)in order to the gas can burn immeidately, before it enters the chambers, I have to make it mixed with fuel uniform. The fuel shall be added into air in the rear section of compressor, or a specific place only for fuel mixed into air before chambers.Two main factors are concerns for the course: the level and uniform of diffuse and the time spent from diffuse to enter the chambers for the mixed gas.
  7. Apr 9, 2012 #6
    I ever said,
    Though the engine has a energy efficency only by the closed combustion in chambers, in order to increase the efficiency, in front of the chambers, I set the compressor to press the air.(later I will prove the point by the theory formular of the Thermal cycling.)

    please read the attached to prove the point. your any comment will be welcome.

    Attached Files:

  8. Apr 23, 2012 #7
    I can also compare the efficiency on two types of turbine engine by formula .
    One is the new one I propose, the other one is the current one.
    Pls read the attached figure

    Estimate the theoretic efficiency; compare the two types of engines: the current engine and I proposed.
    Firstly I state some premises:
    1. Because the efficiency of the current compressor and turbine in current turbo-generator is very high, the below calculation ignore the energy loss in them.
    2. In below analyses I assume that the portion's efficiencies are same if they have the similar function component. Meanwhile I ignore some small loss during courses.

    I set a sample example with data to explain, I assume the pressures are same before combustion in the two type engines.
    for the current jet: efficiency=1-1/{W^[(k-1)/k]} ( ^N means the Nth power)
    W: the pressure rise rate in compressor or blower; I set: W=10, k=1.4, the theoretical efficiency of the current jet is 48.2%.
    As to the new jet: efficiency=1-k*[u^(1/k)-1]/(u-1)/{w^[(k-1)/k]}
    W is increase rate of gas pressure by blower. W=10; here U is the increase rate of pressure by combustion, here U=4.5;I input the data, you can see the theoretical efficiency of this closed combustion jet engine is 60%
    The efficiency of the new type engine is higher than the current one by 25%.
    I try to explain why the new type of engine has more efficiency from another aspect, compared with current jet engine, the new engine uses up the same amount mechanical work and chemical energy but gets higher-pressure gas. The gas can make more work if its pressure is higher. So the new type engine has more efficiency.
    The new engine can get higher-pressure gas because the gas pressure can increase further by the closed combustion course besides by the compressor that also is used in current jet engine.

    b the way, pay attention, my contant-volume type turbine engine is different from the other contant-volume type engines.their thermal cycles are different basically.

    Attached Files:

  9. Jun 3, 2012 #8
    I have introduced the courses of the chambers, you can find the gas is intermittent to flow into one certain chamber.intermitent flow has many disadvantages. such as on energy loss, safety aspect and force load.in order to gas can flow stably and continously in front of chambers as possible as I can,I have to make the flow rotate, Thus I setthe branch pipes in front of the chambers, it is a pairs of the branch pipes(or a space has similar function) that are corresponding to the chambers while the gas enter. The branch pipes rotate with the front door. The branch pipes are short in order to reduce the energy loss.
    the branch pipes do not cost much energy. The mixure gas before chambers always flows in high speed, Thus the gas is hard to burn, even though it has the trend, it has entered the chambers when it burns actually.
    before the inlet brach pipes, there is big pipe after compresser.

    so we encounter similar issue for gas out off the chambers, for a certain chamber, outflow is intetmitent; for whole chambers, outflow is alternately, I hope the final gas current become stable on speed and pressure comparatively. It is beneficial to work for gas by mechanism way, we can get higher efficiency and reliability. So I install one branch pipe corresponding to one chambers, the back door is set between branch pipes and chambers. the door does rotate. the branch pipes are fixed. on their other end the branch pipes converge into one pipe.By the branch pipes and a short period of time the gases expand, pass branch pipes and converge later, as well as a space works as a buffer storage which is after the branch pipes conjunction point , we get the target.
    By this structure,the gas in very high speed in the branch pipes can cause negative pressure inside other branch pipes, so it has suck function to cause gas exchange (fresh gas enter and burnt gas flow out the chambers)in the other chambers.
  10. Jul 6, 2012 #9
    firstly, I correct two sentences in the last post in order to explain more accurate.
    "So I install one branch pipe corresponding to one chambers, the back door is set between branch pipes and chambers. the door does rotate. the branch pipes are fixed" ---corrected to "So I install one branch pipe corresponding to each chambers, the back door is set between branch pipes and chambers. the door does rotate. the branch pipes are static and mounted"

    I have stated the principle of gas flow in side the engine , so i can draw the figue of gas condition in each process ,( including at each position in the engine).
    The figure 3 shows the gas pressure at different courses (in different places), it is rough, and the exact details will is rather complicated. the figure just shows main portion of the flow. Please pay attention, there is a red horizontal line inside the figure, it shows that the pressure at two places have some relation. against the relation much, the engine can't work orderly.

    Attached Files:

    Last edited: Jul 6, 2012
  11. Jul 9, 2012 #10


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    You assume that the benefits from the constant volume combustion offset the extra mechanical complexity required. That may be questionable.
    Just the rotating branch pipes and combustion chambers with front and back doors robust enough to resist deflagration of an air fuel mixture represent major engineering challenges, not to mention the likely resonance issues arising from an intermittent blast of hot gas to spin the turbine.
    There is need for novel designs to push the gas turbine closer to the 100:1 compression engine and this concept may be a possible path towards that goal. However, a conventional axisymmetric compressor, combustor, turbine design such as laid out here may not be the solution.
  12. Jul 10, 2012 #11
    Thank your reply,etudiant
    of course some of what you said are right , any new device has advantage and disadvantage. it usually depends on the position you stand on.
    you said the branch pipes rotating, it is not accurate, the input branch pipe is very short and simple though it rotate; the outlet branch pipes are long and they are mounted.
    I can not undertand your last sentence "push the gas turbine closer to the 100:1 compression engine ", can you explain the sentence more detailed?
  13. Jul 11, 2012 #12
    I have to compensate a little for the upper post.

    the outlet branch pipes are mounted and static.

    because each time the combustion last a very short time, so the combustion type can be the constant volume combustion type though the back door is open during most time of combustion. It is beneficial for the back door to bear the force.
    for the front door , it could be strong because it is assembled with other things, such as input branch pipes. because it does rotate, the main force it bears is comparable stable. That is good.
  14. Jul 28, 2012 #13


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    Current technology turbines have pressure ratios of 20-30 to 1, they compress the air that much before the combustor. A higher pressure ratio such as 100 to 1 would allow a smaller and more efficient unit, but it also becomes more difficult to design an effective turbine to turn a 100x larger compressor.
    A sketch of your concept would be helpful, as the word pictures are not as easily grasped.
  15. Oct 6, 2012 #14
    though i have mentioned key portion for the function when I introduce the main work courses. I still need to state the whole stucture of the engine.
    pls study the figure 1.

    The left one is the section view per axial, it show almost all main parts.
    Another two pictures show the main parts: the front door and back door of the combustion chambers. two doors control and regulate works to complete combustion one by one. Anyone of combustion chambers sometimes sucks mixture gas; sometimes it the combustion happens inside it; sometimes gas exhausts from it. So the chamber sometimes is sealed by the two doors; sometimes it is open on both ends; sometimes it is open on one end. So the door has openings(slots) in it. The two doors rotate at same speed and co-operate each other.
    At any time there is at least a pair of chambers in the same work course that are in the in symmetrical position.
    As to the amount of chambers, I draw 4 couples here, actually it will be more. at least 6 pairs. It depends on the utilization rate of space; their strength; stable work and other factors.
    There is a transmission( gas box) from turbine to the two doors, I set it because the two speeds do not match, I need to reduce speed a lot from turbine to the doors. the transmission unit will not ocuppy much space.
    There is blower or compressor in front of the chambers. There will be also branch pipes in front of the chambers to input gas but they are very short so that they do not appear in the figure.
    The branch pipe, turbine install behind the chambers. after a certain length of branch pipes, these branch pipes merges into one pipe. the turbine should be intalled within the converged pipe.

    Later I will repeat or summarize the work courses of the engine.

    Attached Files:

  16. Dec 20, 2012 #15
    Now I state the work procedure.

    Let me introduce the new type of engine's work procedure: The first step of procedure is that the blower/compressor suck air and increase the pressure to a proper level; spray fuel into air afterwards or at the latter half of the compression; then through a certain length of pipe and a rotary short passage the gas enter the combustion chambers, the rotary short passage is eequivalent to the front door of the chambers. Meanwhile the burnt gas run out off the chambers. When the fresh gas occupies the nearly whole space of the chambers, the front door and back door begin to close. After the doors almost close, the inside mixture gas get ignited, the internal pressure increase rapidly, the combustion last only a few milliseconds and finished, then the back door open; the gas rushes out of chambers and into the branch pipes. thus the chambers begin to suck fresh gas and start the next cycle.
    The chambers are arranged in a circle. There are at least a couple of chambers under a same course. This couple is on the symmetric position. Each chamber works per the sequence I describe upper. Because the front door and back door rotate, the courses change in turn on each chamber corresponding to the rotation.
    The branch pipes are arranged in a circle. The gas flow into the branch pipes which position are symmetric. Later the currents merge and flow inside the general pipe. A turbine is placed afterwards, thus the gas strikes and drives the turbine which gets motion for the front blower/compressor, the rest kinetic energy of the gas is used to push the engine itself forward.
    Inside the pipe/passage in front of the chambers, the flow is continuous.
    The gas spouts out off chambers alternatively, (the spout is intermittent for a certain chamber), but the gases merge in one pipe and flow continuously at last. All the actions repeat with a very high frequency. It provides a consistent push force on the whole engine.

    For each time of exchange gas inside chambers, the gases flow inside chambers are mainly driven by the negative pressure caused by high-speed current in outlet branch pipes.

    Because the combustion in each time can finish within tiny little time, it is not necessary that chambers are closed completely at both sides, then we start ignition as far as the volume of combustion is almost constant.
    If we analyze the principle of each course carefully, we can know how much time each course spends, we can distribute the proper percentage of space on the two doors corresponding to each course in a cycle in order to the engine can get very high work frequency. So the flow in the new engine will not be smaller than it in the current engine.even bigger than it. The flow is a important performance index for jet engine.
  17. Jan 5, 2013 #16
    I set the compressor/blower before the combustion potion for this kind of jet engine, besides its advantage I have said, such as high efficiency, It has other advantages, by the compressor/blower the engine can work under different work condition as current turbojet , so it has the obvious advantage compare with some other new type jet, such as Ramjet engine
  18. Jan 7, 2013 #17


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    Hi gumf,
    I still have trouble understanding how this translates into better performance.
    We have rotating combustion chambers, but you also say the doors don't need to be closed completely at both sides. I envision sort of a six shooter like set of combustion chambers, spinning around their common axis, with an area blanked off by the doors and then as the chamber turns it released the trapped combustion gas into the exhaust stream.
    One challenge I see are that the combustion chamber gets a real pressure surge as the fuel burns in a confined space, so it gets heavy. Another is that the chamber does not get much cooling, it takes in very hot compressed air and that is then further heated by the internal combustion. The cooling will be complicated on a moving chamber.
    The more efficient combustion cycle you propose will have to more than offset the headwinds these and other engineering issues it creates. It is not a slam dunk, imho.
  19. Jan 8, 2013 #18
    That combustion at constant volume is more efficient should be clear, but the whole question is how to achieve this in a turbine, since nobody got convincing results.

    Your design with doors front and aft is the first option people think of. More refined attempts were made without success. In particular, you could have a look at the throughput possible with doors. Because is means stopping the flow, people had arrived to let it oscilate instead, and use air's inertia to achieve the constant volume during a detonation.
  20. Jan 8, 2013 #19
    to etudiant
    thank for your reply.
    I correct you a little. the chambers do not rotate. the doors at the both sides do rotate.
    you can not say " the doors don't need to be closed completely at both sides". It need or not depands on expriment. But i think at the begining of combustion, the space can not completedly closed.
    you also express you worry about something: heavy and heat. You can get result by search the topic to get my articles in other website. the chambers are arranged in a circle is relative to solve the issues. I also will explain my solution here later.
  21. Jan 9, 2013 #20
    to Enthalpy:

    though I will explain the feasibility of this kind of engine later in this web, now you also can search the topic to get my composition on other website.
    Here i say a few words.
    the chambers exhausts gas in turn , and afterwads through a special pipes system to get a steady output at last.

    I know for a certain chamber, the input gas is intermittent. so I set the input pipe rotating (with front door), the input gas flow continuously. thus throughput can be bigger than you thought.
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