How much power does a turbo take?

[M Doe]

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.

[Sweeney921]

ive never thought of it that way

[Matra et Alpine]

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.

[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

[Matra et Alpine]

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.

[M Doe]

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.

[Matra et Alpine]

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