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Thread: How much power does a turbo take?

  1. #1
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    How much power does a turbo take?

    Recently when browsing through this technical forum I have noticed the topics of supercharging, turbo charging and the problems posed by “lag” being discussed.

    I recall the question being raised, how much energy does blower actually take from an engine? Whether “blower” was meant to mean supercharger or turbo I am not sure. When asking the question “how much power does a supercharger take?” the answer must of course be “a bit of horse power, after all you can’t get something for nothing!” This is because the compressor is powered mechanically by the engine but the added volumetric efficiency more than makes up for mechanical loss.

    However, has anyone ever thought how much power a turbo takes away? A turbo consists of a compressor which is powered by a turbine which is propelled by exhaust gasses. Although it sounds strange and I am sure not every one will agree with me, a turbo could be looked upon as a second engine altogether. One could say it is a gas turbine engine which is mated to a piston engine. In a piston engine mechanical power is produced by the expansion of gasses in the cylinder. But it is also true that the turbine in the turbo is an expansion engine too. The exhaust gasses are hotter than that of the atmosphere after all. Think of it this way, a gas turbine engine can reach very high power levels, massive when compared with that of a piston engine. A turbo is merely a gas turbine which bleeds off most of its power to a crankshaft, via some pistons in it’s combustion chamber (a number of cylinders).

    Once a turbo is viewed from this perspective, one realizes the question “how much power does a turbo take?” is irrelevant, as it is an entirely different engine altogether. This point is backed up by the fact that rally teams often introduce fuel into the compressor of a turbo to reduce lag. This may be achieved by cutting the ignition briefly or directly spraying fuel in!

    I should point out that Keith Duckworth (founder of Cosworth and very bright chap) used this argument to try and ban turbos from F1, as gas turbines and two engines were both banned at the time. It was only after reading of this in a book that I realised just how much turbos and supercharged engines differ.
    Last edited by M Doe; 07-02-2004 at 09:26 AM.

  2. #2
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    ive never thought of it that way

  3. #3
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    I asked the origianl Q on blowers as I've not seen a number and it was to attempt to point out how large an electric motor it would take to drive it.

    On your comment about turbo's being like turbine.
    They are like the COMPRESSOR part of a turbin.
    But NOT the engine.

    It uses the exhaust gasses and up to a certain point it is 'free' power.
    But it cannot 'create' power in the same way as a turbine.
    It is limited by the CFM of the exhaust pulse and number of cylinders feeding the input chamber.
    Fuel ignition in the impellor is insufficient to produce power and is only capable of keeping it spinning as the ECU has already opened the BOV.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    turbo's will add some backpressure (i dont know the actual term so for the lack of a better word in my vocabulary im using this one) on the exhaust which usually does take a little bit of power away from the engine. To most however this is insignificant because the drop in power is so small that the huge gains you get in the mid and upper rpm band outweigh the disadvantages

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    Quote Originally Posted by KnifeEdge_2K1
    turbo's will add some backpressure (i dont know the actual term so for the lack of a better word in my vocabulary im using this one) on the exhaust which usually does take a little bit of power away from the engine. To most however this is insignificant because the drop in power is so small that the huge gains you get in the mid and upper rpm band outweigh the disadvantages
    Where the designer has planned for a Turbo from the start the exhuast profile and diameter will be omptimised to 'feed' pulses just as in any performance exhaust. So a properly designed system will take close to zero.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    I hope people fully understand what I am saying here. I have tried to keep my explanation as short as I could. If any one has any specific questions on what I have said then please e-mail me.

    May I also point out that a supercharged engine only has one expansion chamber, the cylinder. While a turbo engine has two, the cylinder and the turbo itself. As the turbo feeds the cylinder a virtuous cycle is set up. In formulas where engine capacity is limited but turbos are allowed, the effect I outlined before results in small engine capacity becoming less of a disadvantage. This was evident in the 1980s when some 1.5 litre F1 cars were producing over 1000bhp (666bhp/litre). These days, after 17 years development (not including the DFV era before turbos) we are getting only around 900bhp (300bhp/litre) from normally aspirated engines.

    Does anyone know of a supercharger that can beat 666bhp/litre? Im not being sarcastic I really would be interested to know.

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    Quote Originally Posted by mpackham
    I hope people fully understand what I am saying here. I have tried to keep my explanation as short as I could. If any one has any specific questions on what I have said then please e-mail me.
    Better to explain in UCP forum where lots can better understand things.
    May I also point out that a supercharged engine only has one expansion chamber, the cylinder. While a turbo engine has two, the cylinder and the turbo itself. As the turbo feeds the cylinder a virtuous cycle is set up.
    huh ? Need that one explained.
    By expansion chamber are you considering inlet or exhaust ?
    Turbo and compressor can both feed the same inlet manifold chamber.
    Generally however, it's not does as it's best to move the torbo to the optimum place in the exhaust pipework to prevent excessive restriction and heat.
    For exhaust, the designer only considers the turbo as another restriction in gas flow along with every bend in the pipework and designs it to match for optimum pusle flow through the system.
    In formulas where engine capacity is limited but turbos are allowed, the effect I outlined before results in small engine capacity becoming less of a disadvantage. This was evident in the 1980s when some 1.5 litre F1 cars were producing over 1000bhp (666bhp/litre). These days, after 17 years development (not including the DFV era before turbos) we are getting only around 900bhp (300bhp/litre) from normally aspirated engines.
    That's because of the limits of reciprocating mass in an otto-cycle engine.
    Bigger engines means more pistons, more mass changing direction and lower revs.
    Lower revs means more energy required per bang which means MORE stress on a cylinder and components. To get more power a N/A engine has to go for volume NOT speed, so it is fighting the uphill struggle against mass and can never win.

    Does anyone know of a supercharger that can beat 666bhp/litre? Im not being sarcastic I really would be interested to know.
    Every blower in a top fuel dragster manages that
    But their cylinders are close to hydraulic lock !!!
    A full list of comparative power/cc ( but it's listed the other way round !! ) is at http://www.simetric.co.uk/si_cc2hp.htm
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    What I am suggesting is that the turbine is in itself an expansion engine. The gas entering it is at a higher pressure and density than that leaving it (it is expanding). Because the gasses are trying to escape to the lower pressure atmosphere, some work can be achieved from then. I think this work is additional to that produced from that of the velocity of the gas.

    As for my question: The 1000bhp power figure was achieved using pump fuel (well to the best of my knowledge). And on an engine that had to last for more than a few seconds.

  9. #9
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    Quote Originally Posted by mpackham
    What I am suggesting is that the turbine is in itself an expansion engine. The gas entering it is at a higher pressure and density than that leaving it (it is expanding). Because the gasses are trying to escape to the lower pressure atmosphere, some work can be achieved from then. I think this work is additional to that produced from that of the velocity of the gas.
    But it's in a different chamber, so I'm still confused what you're suggesting.
    The pressure on the exit side isn't significant as a 'pull' force on the gasses, 'cept where as I suggested the designer has chosen an optimal rev range to make the gas pulses work to provide a partial vacuum pulse. This mainly exists in the headers, but isn't fully disappated by the impellors.
    As for my question: The 1000bhp power figure was achieved using pump fuel (well to the best of my knowledge). And on an engine that had to last for more than a few seconds.
    F1 fuels were VERY exotic back then
    Qualifying engines lasted 10-15 minutes tops. Race engines were 1/3 less powerful !!
    We're agreeing on this point, the near 'hydraulic lock' of a top fuel produces foreces that an engine cannot sustain for very long.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    I am in agreement with you on the up hill struggle faced by N/A engines. It was probably not the best comparison I could have used. I was merely attempting to illustrate to people who may not know just how much bhp/litre was being achieved back then in comparison to today.

    A few minutes on the calculator will show you that today’s F1 engines have only half the torque that that of the turbo cars (so I’m not sure what you meant by a bigger bang with N/As). I do agree that the mass of reciprocating parts is going to be a big factor in losses (F=Ma). I have heard the figure of 10,000G is about maximum for modern F1 pistons (I have no idea how accrete this is), that is massive acceleration and must take an extremely large force even with a relatively lite piston.

    Assuming that peak power is made at, or very near maximum revs, then one could estimate a N/A engine would need to rev over 21,000 rpm to match that 1000bhp mark. It would have to rev at over 42,000 rpm to match the 666bhp/litre mark. I’m sure you will agree this is just silly. The frictional losses at this speed would be massive. But who knows maybe 21,000rpm may be achieved in the future?

    As for the argument of a turbo being another engine, I fear that if you do not understand this now, then I have not explained myself well enough. But can I please remind you (with respect), this was argued by Keith Duckworth. He did after all design many successful engines including the twin turbo HB F1 engine.

  11. #11
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    Quote Originally Posted by mpackham
    I was merely attempting to illustrate to people who may not know just how much bhp/litre was being achieved back then in comparison to today.
    n/p
    A few minutes on the calculator will show you that today’s F1 engines have only half the torque that that of the turbo cars (so I’m not sure what you meant by a bigger bang with N/As).
    N/A has to use unforced fuel/air mixture into a cylinder and then compress it so compression ratios become higher. An engine running at half the revs has to ( assuming same cylinders ) produce twice the energy per cylinder. THAT needs a bigger "bang" from the ignition cycle of each cylinder.
    I do agree that the mass of reciprocating parts is going to be a big factor in losses (F=Ma). I have heard the figure of 10,000G is about maximum for modern F1 pistons (I have no idea how accrete this is), that is massive acceleration and must take an extremely large force even with a relatively lite piston.
    Never seen the numbers for modern engines and lighter materials quoted.
    The issue with a piston is the change of direction from the compression through ignition cycle when the forces are their highest and change direction
    Then of course there's the poor little valves struggling to open/close and not bounce/break ( or hit the piston )
    Assuming that peak power is made at, or very near maximum revs, then one could estimate a N/A engine would need to rev over 21,000 rpm to match that 1000bhp mark. It would have to rev at over 42,000 rpm to match the 666bhp/litre mark. I’m sure you will agree this is just silly. The frictional losses at this speed would be massive. But who knows maybe 21,000rpm may be achieved in the future?
    Honda race engines in motorbikes were doing 22-24,000 rpm on the Isle of Man TT road cicruit and tracks around the world.
    I've already posted a link here on UCP to get a chance to hear a recording. I've been lucky to see and hear them live ( though not in full-race throttle )
    But 2-strokes didn't have 'proper' valves so only half as difficult
    As for the argument of a turbo being another engine, I fear that if you do not understand this now, then I have not explained myself well enough.
    The bit where I don't get ( agree? ) with your comment is that a turbine engine the compressor provides ALL the compression of fuel/air mixture for combustion. In a car engine, the pistons provide most of the compression of the mixture prior to ignition. THEN the next ignition cycle in the engine provides the force to exhaust the spent mixture. Fundamentally different mode of operation. So I don't see the point. ( Which it means it's probably so simple I'm overlooking the obvious )
    But can I please remind you (with respect), this was argued by Keith Duckworth. He did after all design many successful engines including the twin turbo HB F1 engine.
    n/p BUT remember that Keith was needing to put forward reasons why he shouldn't be forced to develop a new engine at huge cost and lose out on a VERY lucrative business supplying engines to all but one top F1 team
    So it's not the case that it was necessarily true. Just liek Beryllium and the McLaren/Ferrari debacle. ( Except theat Ferrari won the argument and McLaren lost MAJOR ground in engine developemtn )
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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    Can I just say I am impressed by the 24,000rpm Honda engine! However, I don't think it was a three litre V10 was it? Like you said, it's easier for smaller engines.

    I will have one more attempt at explaining to you about the turbo thing.

    Point 1
    Imagine all the valves were open on an engine at once, yeh ok this would never happen but bare with me. Fuel was still injected in and the sparks were still present too. You would have to agree that then the turbo could be run as a gas turbine engine. Air drawn in by the compressor, combusted with fuel, gasses expand, these gasses escape through exhaust turning turbine in the process. The pressure in the manifolds and combustion chamber would be above that of the atmosphere.

    Point 2
    As a piston descends down the bore of a cylinder the air/fuel mixture is drawn in behind, it may be forced in with a turbo or supercharger. For example, a 250cc cylinder of a N/A engine may contain at BDC, what was 200cc of air/fuel mixture at 1 bar. It would then have a VE of 80%. This mixture is then compressed as the piston rises. It is ignited and it expands pushing the piston down the bore. If the piston could be stopped dead at BDC and the exhaust valve opened, some hot gasses would rush out even though the piston is stationary. If this was done with a blown engine, an even greater volume of gas would escape as slightly more mixture was forced in to begin with (maybe 110% VE). This volume of gas that is expanding as it tries to get back to 1bar could do some useful work on the turbine of the turbo. This work is achieved with no mechanical loss to the engine (remember the piston is stationary).

    Now imagine the piston starts rising in the bore of the cylinder and pushes out the exhaust gasses that do work on the turbine. The movement of this 250cc volume of gas is achieved mechanically by the engine. If this was the only way that gas was forced through the turbine then the engine would be very similar to a supercharged engine. This is because all of the energy put into the compressor would have been derived from mechanical engine work as it is with a supercharged engine.

    Putting Point 1 and Point 2 together.
    Remember that the air fuel mixture expands as a factor when it is combusted. Say for example, 50:1 (don’t start correcting me on this it is only hypothetical). If you force an extra 20cc of mixture through the inlet valve you may find you have an extra 1000cc of gasses coming out of the exhaust valve. This extra gas coming out of the engine can achieve a useful work on the turbo (with no extra mechanical work). As the turbo is forcing more mixture in, the exhaust gasses are increasing by a factor of 50. This of course provides the turbo with more power to push more mixture in! Remember the virtuous cycle I was talking about? Well this is it. We’ve obviously got a waste gate to keep the boost pegged at a pre set level.

    Can you now see how this is similar to a gas turbine engine or at least similar to point 1?

    I took me a couple of hours before I was sure this was right after reading it. I had never thought of it before and I didn’t want to drop my pre conceived ideas of how a turbo functioned either!. That being it is a restriction in the exhaust but it more than pays its way.

    Remember a supercharger only has the mechanical energy of the engine to make the boost. The turbo is benefiting from the mechanical energy (the volume of the cylinder in exhaust gas being pushed out) and that expansion factor as well.

    I hope I have explained my self better now. It has certainly taken some space!

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    ok next time please use english ...

    as far as i can figure out what your saying is that superchargers only get the energy from the rotation of the crank while turbo's get that AND the volume of gasses expelled from each cylinder?

    turbo's use kenetic energy from the exhaust gasses, to expell the gasses from a piston which is undergoing the exhaust phase of the otto cycle another piston is undergoing the combustion phase. although the airfuel mixture inside the piston would be above atmospheric pressure that expansion alone is not strong enough to drive the turbo itself and therefore is irrelevent

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    Gas turbines don't operate with spark igniting the fuel, the heat of the air itself ignites the fuel, but you just made your whole point more confusing.

    You just described how a turbo normally works, more exhaust coming out the faster spins, its just like a normal engine, the more gas you put in, the faster it goes. The virtuous cycle you were talking about isn't that special.

    The supercharger thing, hence if u add more gas, the faster the engine will go. If you accelerate, the engine spins faster which makes the supercharger spin faster to create more boost, yes it does take power to run it, but so does the air conditoning.

    Check out how stuff works and look at the turbocharger, it might help you explain yourself better.
    "We went to Wnedy's. I had chicken nuggest." ~ Quiggs

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    Also, if your idea about all the valves being open ,and the exhaust coming out and spinning the turbo. It can't happen, any extra boost just makes the air/fuel to move out faster, your exhaust would be a mixture of fuel, air and exhaust. It also helps if you take apart an engine to be able to understand all of this.
    "We went to Wnedy's. I had chicken nuggest." ~ Quiggs

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