Why do turbine engines require a compressor section
Glad you've come over to the dark side, aerobat! 
All the arguments Oggers posted relating to small throttle settings (VE), or flame front speed, or burning fuel faster, or ignition timing blah blah blah - don't relate to the original post about thermodynamic cycle efficiency of low vs. high compression.
In fact, these problems disappear in a constant pressure heat engine like a turbine, where the concept is the same:
Less heat transferred to the fluid = more useable energy available
More useable energy available from same energy input = higher thermodynamic efficiency
I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
All the arguments Oggers posted relating to small throttle settings (VE), or flame front speed, or burning fuel faster, or ignition timing blah blah blah - don't relate to the original post about thermodynamic cycle efficiency of low vs. high compression.
In fact, these problems disappear in a constant pressure heat engine like a turbine, where the concept is the same:
Less heat transferred to the fluid = more useable energy available
More useable energy available from same energy input = higher thermodynamic efficiency
I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
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I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
You were talking about the Otto cycle, not the Brayton cycle
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I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
the original question is well answered but of course a little bit like asking why a piston engine needs pistons . also the ram-jet explanation directs a wrong way since a ram jet is not a turbine engine. the job of a turbine disc is ( beyond driving the propeller via a gearbox at a turboprop) to drive the compressors , and when there are no compressors before the combustion chamber , for what does the turbine disc spin ?
maybe the thread beginner meant a jet engine, since the question for what a compressor in a turbine engine is self answering.
Now that is very clever, change the parameters of your argument.
You were talking about the Otto cycle, not the Brayton cycle
You were talking about the Otto cycle, not the Brayton cycle
I originally chose to talk about the Otto cycle because I thought it was easier to explain why thermodynamic efficiency goes up with higher compression of the fluid. The fact is that in both the Otto and Brayton cycles, more compression before the combustion results in less heat exchange to the fluid - and a thermodynamically more efficient engine.
I obviously made a mistake, because I didn't count on people like yourself and Oggers being unable to understand such a simple concept. Oggers then went off on several tangents, and used a myriad of false arguments about ignition timing, and valve timing, and mixing, and flame fronts, and all sorts of other things to try and squeeze his way out of the fact that nothing of those things have to do with the original question about the thermodynamic efficiency of higher compression.
gents, are you talking turbines or pistons ? valve timing in a turbine ? when it comes to turbine engines we also discuss pressure ratio, not compression. maybe its just a misunderstanding since different things are mixed up.
My switch over to turbine was to show that his arguments don't apply on a turbine, where in fact the same thermodynamic principle does (higher compression before combustion results in less energy wasted heating the fluid = more available work).
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Slippery_Pete,
you stated several time the idea
I may be misunderstanding the point you intend to make, but I think that the statement is either wrong or more or less correct but very misleading.
Your sentence suggests that we add energy to the fluid and that this energy is split between useful work and heat. And so, the less heat the more work.
That's misleading.
Actually, the sequence of events is:
- we take a gas that has an internal energy U1
- optionally, we increase its energy to U2
(this is the compression phase. Mandatory for turbines. For steam machines, just assume U2=U1)
- we pour in some heat Q and get the internal energy U3 = U2 + Q
- we extract a percentage of the internal energy U3 into work through an expansion of the gas (usually adiabatic expansion).
- the resulting energy U4 = U3 - W
The percentage of extracted energy depends on the pressure ratio.
Actually it depends on the temperature ratio T4/T3 and is 1 - T4/T3 .
For adiabatic expansions, it can be shown that 1 - T4/T3 = 1 - (p4/p3)^((γ-1)/γ)
You can see that, for instance if the extraction efficiency is 40%, we extract 40% of U3 which is 40% of U2 plus 40% of Q. The more heat is added to the fluid, the more work is extracted.
This picture is, of course, a simplification. I looked at the efficiency of just one phase of the cycle rather than looking at the efficiency of the whole cycle. However it shows that:
- heat is the object of the energy extraction efficiency and is not an adjustement variable of the efficiency
- theoritical efficiency is just linked to the temperature ratio during expansion phases or to the pressure ratio which can be considered as a surrogate for temperature ratio
Note that the 40% U2 extracted is not magic energy ; it is only if the gas has been compressed initially from U1 to U2 with U1 being 60% of U2 that the final pressure ratio allows a 40% reduction. So the 40% extracted of U2 is just some invested energy that is reclaimed.
If there is no initial compression (steam engine) the increase of pressure obtained by adding Q in a fixed volume results in a pressure increase that allows a very poor extraction efficiency, much less than Q/U1.
Note also that the final state U4 is equivalent to the initial state U1 except that we extracted only a part of Q. So grossly, U4 = U1 + 60%.Q
And the final temperature of the gas is not a variable that can be acted upon ; it is dictated by be heat that could not be extracted.
Luc
you stated several time the idea
Less heat transferred to the fluid = more useable energy available
Your sentence suggests that we add energy to the fluid and that this energy is split between useful work and heat. And so, the less heat the more work.
That's misleading.
Actually, the sequence of events is:
- we take a gas that has an internal energy U1
- optionally, we increase its energy to U2
(this is the compression phase. Mandatory for turbines. For steam machines, just assume U2=U1)
- we pour in some heat Q and get the internal energy U3 = U2 + Q
- we extract a percentage of the internal energy U3 into work through an expansion of the gas (usually adiabatic expansion).
- the resulting energy U4 = U3 - W
The percentage of extracted energy depends on the pressure ratio.
Actually it depends on the temperature ratio T4/T3 and is 1 - T4/T3 .
For adiabatic expansions, it can be shown that 1 - T4/T3 = 1 - (p4/p3)^((γ-1)/γ)
You can see that, for instance if the extraction efficiency is 40%, we extract 40% of U3 which is 40% of U2 plus 40% of Q. The more heat is added to the fluid, the more work is extracted.
This picture is, of course, a simplification. I looked at the efficiency of just one phase of the cycle rather than looking at the efficiency of the whole cycle. However it shows that:
- heat is the object of the energy extraction efficiency and is not an adjustement variable of the efficiency
- theoritical efficiency is just linked to the temperature ratio during expansion phases or to the pressure ratio which can be considered as a surrogate for temperature ratio
Note that the 40% U2 extracted is not magic energy ; it is only if the gas has been compressed initially from U1 to U2 with U1 being 60% of U2 that the final pressure ratio allows a 40% reduction. So the 40% extracted of U2 is just some invested energy that is reclaimed.
If there is no initial compression (steam engine) the increase of pressure obtained by adding Q in a fixed volume results in a pressure increase that allows a very poor extraction efficiency, much less than Q/U1.
Note also that the final state U4 is equivalent to the initial state U1 except that we extracted only a part of Q. So grossly, U4 = U1 + 60%.Q
And the final temperature of the gas is not a variable that can be acted upon ; it is dictated by be heat that could not be extracted.
Luc
Last edited by Luc Lion; 18th Nov 2011 at 13:47.
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I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
aerobat77: thanks for the link to that paper. There won't be any surprises in there for those who've been schooled in piston engine theory and it's a useful case study for those that might be interested.
Specifically on the point of exhaust temperature there is no surprise that it dropped as compression rose: this is not in dispute.
Last edited by oggers; 18th Nov 2011 at 18:38.
Folks when you get all done with this piston engine stuff (no compressors or turbines), could you do a google-idiot style translation into turbine engines for the casual thread reader.
Maybe a distinction could then be made between TIT (Turbine Inlet Temperature), EGT (Turbine Exhaust Gas temperature) out the tailpipe and pressure out of the compressor,
I doubt that timing enters into this for a gas turbine since the flowrate is constant and so is the fuel and ignition.
The casual reader would appreciate simple comparisons with everyday effects e.g. welding torches, bunsen burners, baloons, home oil heaters etc.
Maybe a distinction could then be made between TIT (Turbine Inlet Temperature), EGT (Turbine Exhaust Gas temperature) out the tailpipe and pressure out of the compressor,
I doubt that timing enters into this for a gas turbine since the flowrate is constant and so is the fuel and ignition.
The casual reader would appreciate simple comparisons with everyday effects e.g. welding torches, bunsen burners, baloons, home oil heaters etc.
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Turbine inlet temperature - Not indicated on turbine engines I work on.
Usually ITT, interstage turbine temperature between compressor turbine and power turbine sections is indicated
EGT is idicated on piston engines as an aid to leaning the mixture, not so much on turbine engines.
In any case will be lower than ITT and TIT.
Pressure out of the compressor is a function of the compressor stages in axial flow and compressor size/effenciency in centrifugal compressor; and is expressed as a ration over inlet pressure.
Which leads us to bleed valves and interstage bleeds to decrease stall margins etc.
BH
Usually ITT, interstage turbine temperature between compressor turbine and power turbine sections is indicated
EGT is idicated on piston engines as an aid to leaning the mixture, not so much on turbine engines.
In any case will be lower than ITT and TIT.
Pressure out of the compressor is a function of the compressor stages in axial flow and compressor size/effenciency in centrifugal compressor; and is expressed as a ration over inlet pressure.
Which leads us to bleed valves and interstage bleeds to decrease stall margins etc.
BH
Hi Oggers.
I've already explained why I used the words crank, piston and cylinder in my first post - because I thought it would be easier for people like you to understand the thermodynamic concept ask by the OP if I talked about the Otto Cycle. I used those words to try and explain the concept to most readers on this forum, but I never said anything other than the fact that the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid. All of the false arguments were introduced by you.
I'm listening.
FOR THE FOURTH TIME, I'm still listening. You really are choosing to avoid this.
Luc,
Why don't you read all my posts, rather than glaze over the last one?
The point regarding adding heat to the fluid is the total heat added to the fluid over the entire cycle (ie, once the fluid is returned to atmospheric pressure).
I'll say it again, for either a piston or turbine pressure, then higher the compression ratio or pressure ratio, the less overall energy will have been absorbed by the fluid by the time it has returned to atmospheric pressure.
Higher compression/pressure ratio means for a given fuel flow, less heat exchange has occurred to the exhaust gas by the end of the cycle. It allows more energy to be extracted as useful work.
I've already explained why I used the words crank, piston and cylinder in my first post - because I thought it would be easier for people like you to understand the thermodynamic concept ask by the OP if I talked about the Otto Cycle. I used those words to try and explain the concept to most readers on this forum, but I never said anything other than the fact that the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid. All of the false arguments were introduced by you.
I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
It's about the total fluid heat change from start to finish (once the fluid is returned to atmospheric pressure). It is lower in a high compression engine.
Luc,
Why don't you read all my posts, rather than glaze over the last one?
The point regarding adding heat to the fluid is the total heat added to the fluid over the entire cycle (ie, once the fluid is returned to atmospheric pressure).
I'll say it again, for either a piston or turbine pressure, then higher the compression ratio or pressure ratio, the less overall energy will have been absorbed by the fluid by the time it has returned to atmospheric pressure.
Higher compression/pressure ratio means for a given fuel flow, less heat exchange has occurred to the exhaust gas by the end of the cycle. It allows more energy to be extracted as useful work.
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ok, when we come to turbines i will give here some impressions on the turboprop GA aircraft i currently fly- a cheyenne III with the pt6a-41 engines. the pt6a-41 is a turboprop two shaft engine which incorporates a three stage axial compressor followed by a 1 stage centrifugal compressor driven by a 1 stage compressor turbine. after this we have a 2 stage power turbine which drives the prop via a gearbox. the both shafts are not mechanically coupled and the low pressure turbine drives only the prop - so we call the pt6a is a free turbine.
the engine is like common nowaday design flat rated - so the rated power output is a mechanical limit and the engine is able to keep rated power above ISA or keep rated power in thinner air when you climb until its thermodynamical limit ( ITT or compressor speed) is reached. in other words- the engine could develop on ground more power that the gearbox is approved for.
basicly on this engine you give with the power levers an input to the FCU ( fuel control unit ) to set a target compressor speed. in regard to air density and outside temperature a given amount of fuel is needed to keep this speed. this will result in a given force to the power turbine and a given torque - so power output.
at take off you are mostly torque ( so power output) limited and the turbine is at its mechanical limit . the ITT and compressor speed are below its limit. when you climb out and do not touch the power levers the compressor speed stays the same. the ITT also but torque and fuel flow decreases. this is due the fact the FCU ( fuel control unit) keeps like said a target compressor speed . in a climb out the air gets thinner and the "resistance" on the compressor stages also. so the compressors try to spin faster and the FCU has to decrease fuel flow to keep the same speed. due to less fuel and gas driving the power turbine the torque ans power output also decreases.
when you want to keep the same power output in a thinner air in a climb you will have to push the power levers more and more forward. this will result in a faster and faster spinning compressors and a higher and higher ITT until at a given altitude you match the maximum ITT or compressor speed. here the turbine reaches its thermodynamical limit.
sooo... when the air gets thinner and the compressors deal with a lower pressure ratio ( in pistons compression) the ITT rises .thats a fact. i found and attached a pic at our top of climb in FL 280 with a cheyenne III with pt6a-41 engines so you can have a look what the torque, ITT, compressor speed amd fuel flow is here. at this altitude the engine is at its thermodynamical limit - so the compressor speed is at company limits resulting in a given ITT and torque far below its redline ( mechanical limit)
now we can talk why it is so a a turbine engine.
the engine is like common nowaday design flat rated - so the rated power output is a mechanical limit and the engine is able to keep rated power above ISA or keep rated power in thinner air when you climb until its thermodynamical limit ( ITT or compressor speed) is reached. in other words- the engine could develop on ground more power that the gearbox is approved for.
basicly on this engine you give with the power levers an input to the FCU ( fuel control unit ) to set a target compressor speed. in regard to air density and outside temperature a given amount of fuel is needed to keep this speed. this will result in a given force to the power turbine and a given torque - so power output.
at take off you are mostly torque ( so power output) limited and the turbine is at its mechanical limit . the ITT and compressor speed are below its limit. when you climb out and do not touch the power levers the compressor speed stays the same. the ITT also but torque and fuel flow decreases. this is due the fact the FCU ( fuel control unit) keeps like said a target compressor speed . in a climb out the air gets thinner and the "resistance" on the compressor stages also. so the compressors try to spin faster and the FCU has to decrease fuel flow to keep the same speed. due to less fuel and gas driving the power turbine the torque ans power output also decreases.
when you want to keep the same power output in a thinner air in a climb you will have to push the power levers more and more forward. this will result in a faster and faster spinning compressors and a higher and higher ITT until at a given altitude you match the maximum ITT or compressor speed. here the turbine reaches its thermodynamical limit.
sooo... when the air gets thinner and the compressors deal with a lower pressure ratio ( in pistons compression) the ITT rises .thats a fact. i found and attached a pic at our top of climb in FL 280 with a cheyenne III with pt6a-41 engines so you can have a look what the torque, ITT, compressor speed amd fuel flow is here. at this altitude the engine is at its thermodynamical limit - so the compressor speed is at company limits resulting in a given ITT and torque far below its redline ( mechanical limit)
now we can talk why it is so a a turbine engine.
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Aerobat77:
Turbine engines utilise air for cooling of the combustion chamber and the turbines themselves. ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.
Less mass of air going into combustion chamber, but same mass required for combustion itself = less left over for cooling. I'm not saying this is the only cause (I have a mere pilot's understanding of turbines) but it is the only cause I was taught at the Royal Naval Flying Training School. It is important to helicopter pilots because you can become power limited in the hover due to turbine temp before you reach the service ceiliing - a factor in so called 'hot and high' operations.
now we can talk why it is so a a turbine engine.
Less mass of air going into combustion chamber, but same mass required for combustion itself = less left over for cooling. I'm not saying this is the only cause (I have a mere pilot's understanding of turbines) but it is the only cause I was taught at the Royal Naval Flying Training School. It is important to helicopter pilots because you can become power limited in the hover due to turbine temp before you reach the service ceiliing - a factor in so called 'hot and high' operations.
Last edited by oggers; 19th Nov 2011 at 11:29.
...if you didn't have a compressor it would be a ram jet. Anyone mentioned stoichiometric yet ? As someone said, need to have the pressure gradient to avoid a bonfire - imagine starting a ram jet at zero speed.
Second Law
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Thermodynamics
Gentlemen, as a mere physical chemist and at the risk of causing more confusion, may I suggest we try to stick to what is to me the fundamental process here?
The more we compress the air the more oxygen molecules we supply per unit time to the combustion chambers so we can burn more mostly hydrocarbon fuel per unit time and release more energy to do useful work per unit time.
That's the simple central point here.
Let's dig a little deeper.
The Second Law of Thermodynamics can be expressed as
ΔG = ΔH -TΔS
The combustion products are hotter than the fuel or the air reactants so the ΔS term becomes more positive as their entropy is now larger, the negative sign above then makes the TΔS term negative.
T is large and positive again making the TΔS term negative
The combustion process (yes yes ideally stoichiometrically , we'll have that discussion if you wish but not here for the moment please) is an exothermic reaction that mostly releases energy to the surroundings in the form of heat so ΔH has a negative sign on this side of the pond.
So ΔG (the free energy) is large and negative and we have lots of free energy available to do useful work - such as provide thrust.
Finally, to go back to the original question of "why compress?", we can now say "so as to release more energy per unit time from the fuel"
CW
The more we compress the air the more oxygen molecules we supply per unit time to the combustion chambers so we can burn more mostly hydrocarbon fuel per unit time and release more energy to do useful work per unit time.
That's the simple central point here.
Let's dig a little deeper.
The Second Law of Thermodynamics can be expressed as
ΔG = ΔH -TΔS
The combustion products are hotter than the fuel or the air reactants so the ΔS term becomes more positive as their entropy is now larger, the negative sign above then makes the TΔS term negative.
T is large and positive again making the TΔS term negative
The combustion process (yes yes ideally stoichiometrically , we'll have that discussion if you wish but not here for the moment please) is an exothermic reaction that mostly releases energy to the surroundings in the form of heat so ΔH has a negative sign on this side of the pond.
So ΔG (the free energy) is large and negative and we have lots of free energy available to do useful work - such as provide thrust.
Finally, to go back to the original question of "why compress?", we can now say "so as to release more energy per unit time from the fuel"
CW
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oggers:
Well, this may or may not be true for a given engine type, depending on the characteristics of the control system (whether governing to a constant core speed, constant corrected speed, constant torque, constant CDP, etc etc.)
If an electronic control, it's all done with ones and zeroes; if hydromechanical, then it's bellows and cams and flyweights.
ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.
If an electronic control, it's all done with ones and zeroes; if hydromechanical, then it's bellows and cams and flyweights.
Blimey - PPRuNe at it's best. A simple question asked by someone seeking enlightenment and they get a thesis and a sh!t fight!
I would like to answer the original queries by by answering the second question first. In simple terms, the power output of a reciprocating engine can be expressed as bore x stroke x rpm x compression ratio. Based on this, if you force more of a charge into the engine by using a turbo or superchager, the compression ratio will increase and the power output will go up.
As for the first part, obviously you understand the OTTO cycle an the principle of induction, compression, conmbustion and exhaust, or suck, squeeze bang and blow. The turbojet engine does the same. The suck is the flow into the engine, the squeeze comes from the compressor, the bang occurs in the combustion chamber and the blow is the thurst. If you look at it like this, if you didn't have the compressor, the engine wouldn't work and as someone else said, all you would have is a bonfire!
I would like to answer the original queries by by answering the second question first. In simple terms, the power output of a reciprocating engine can be expressed as bore x stroke x rpm x compression ratio. Based on this, if you force more of a charge into the engine by using a turbo or superchager, the compression ratio will increase and the power output will go up.
As for the first part, obviously you understand the OTTO cycle an the principle of induction, compression, conmbustion and exhaust, or suck, squeeze bang and blow. The turbojet engine does the same. The suck is the flow into the engine, the squeeze comes from the compressor, the bang occurs in the combustion chamber and the blow is the thurst. If you look at it like this, if you didn't have the compressor, the engine wouldn't work and as someone else said, all you would have is a bonfire!
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Barit:
No doubt you're quite right. However, I was responding to aerobat's specific scenario regarding his specific type in which he observes an ITT rise.
If you follow the thread back you will find this comment by aerobat which gives context:
My point is that if you observe such a rise in those circumstances there is a good alternative explanation; namely less cooling air available.
Well, this may or may not be true for a given engine type [that ITT rises in the climb due to reduced mass flow rate of air], depending on the characteristics of the control system (whether governing to a constant core speed, constant corrected speed, constant torque, constant CDP, etc etc.)
If you follow the thread back you will find this comment by aerobat which gives context:
i think slippery pete is pretty right with the higher compression- lower exhaust temperatures at pistons . you can see this effect also in tubine engines where maintaining the same power output / fuel flow in a climb will result in exhaust temperature RISING due to thinner air and worsening compression of the compressor stages
Last edited by oggers; 21st Nov 2011 at 10:58.
Pedant head on.
Internal combustion engine (Otto) cycle - suck, Squeeze, bang, fart.
Gas turbine (I refuse to call it Brayton as Mr Whittle formulated it years before) cycle - Suck, squeeze, burn, blow.
Just sayin'.
Anyway, to answer the original question...
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something.
I suppose in theory you could have a turbo-prop with no compressor and rely on a very efficient ram effect for compression but as no-one has done it yet I suspect it won't work, for all the very clever and frankly complex answers given on the last 3 pages.
Internal combustion engine (Otto) cycle - suck, Squeeze, bang, fart.
Gas turbine (I refuse to call it Brayton as Mr Whittle formulated it years before) cycle - Suck, squeeze, burn, blow.
Just sayin'.
Anyway, to answer the original question...
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something.
I suppose in theory you could have a turbo-prop with no compressor and rely on a very efficient ram effect for compression but as no-one has done it yet I suspect it won't work, for all the very clever and frankly complex answers given on the last 3 pages.
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something
Quote:
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something
actually it won't spin unless there is a pressure drop across it. The turbine needs a forced movement of air through it.
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something
actually it won't spin unless there is a pressure drop across it. The turbine needs a forced movement of air through it.
EG. A ram jet with a turbine stuck up it's arris.
Pointless but you get my drift?
Pointless but you get my drift