Why do turbine engines require a compressor section
 

Hi guys,

Can anyone give me an answer on this? Also why is it that both the piston engine and the turbine engine can have their efficiencies increased by increasing the pressure ratio (compression ratio for piston)? Is there some sort of simple thermodynamic explanation for this?

Cheers,

J

In one rhetorical question or less...

How do you plan on making power/doing work without a compressor of some sort?

Brayton Cycle - go google it.

yah, I have mate.. cheers.. couldn't find anything to simply answer my questions so I came here. :ok:

Fair enough.
Turbo or supercharging is used to increase VE of a piston engine. Simply by increasing the pressure charge will increase volumetric efficiency to above 100%.
The Compressor in turbine engine and is the part that forces the air into the combustion section - a pure jet needs forward airspeed,like the WW2 buzz bomb to work.

Cheers
BH

Blunt answer: a turbine engine does not NEED a compressor in order to work - b ut it does work a great deal better with one.

As mentioned above, it's to do with the efficiency of the combustion process. That applies to combustion engines generally, which is (simplistically) why diesel engines are generally more efficient than petrol engines, why petrol engines using higher grade fuel are more powerful 9because the higher grade allows use of higher compression ratios in the cylinders, hence more power) and why modern gas turbines are more efifcient than the earlier engines, as they run at higher compression ratios.

But there is an example of an engine which doesnt use compression in the combustion process - a steam turbine. You can use atmospheric pressure combustion to create the heat that is used to drive the steam cycle - but it isnt terribly efficient. Yet the turbine will still turn and power is generated.

Why do turbine engines require a compressor section?
l
Turbine engines; Suck, squeeze, bang, blow!

Hi QJB,
Good question. In an internal combustion engine, the direction of rotation is controlled by the ignition timing (you can get a two stroke to run backwards if the timing is too far advanced) and the gas flow direction is controlled by valves.

Since there is no ignition timing or valves as such in a turbine engine, in order to prevent the combustion gases from travelling forwards, it's necessary to "compress" the air flow by increasing its KE towards the rear. It can then enter the combustion chamber through a much smaller area. After combustion the gas finds an easier exit route rearwards, expanding across the turbine.

QJB:

The goal is to get maximum amount (charge) of air/fuel mixture into a given combustion chamber. That is, basically, volumetric efficiency.

Therefore you compress the air to get more in. You also need more fuel to maintain the correct air/fuel ratio (roughly 15:1), so the fuel metering system has to do its bit too.

In a normally aspirated piston engine you merely suck the air in, hence inferior VE. Obviously you still compress the charge once it's in which leads nicely onto...

...the compression ratio in a piston engine: the goal there is to achieve a nice tight charge, evenly mixed at the desired air/fuel ratio where all the fuel molecules are close to the air molecules they've waited so many million years to react with. Hence the use of compression. This helps the so-called thermal efficiency (ie engine efficiency) right up to the point where you go too far with the compression and run into the phenomenon of detonation. But that's another topic.

Hi QJB.

Sorry no-one has answered the question you originally answered. I'm sure you realise the answers given about volumetric efficiency have nothing to do with your question about thermodynamic efficiency of an engine.

The best way to answer you question about the thermodynamic efficiency of an engine increasing with a higher compression ratio, is to consider losses due to heat.

If you consider two cups of water - 1x 50 degrees celsius, 1x 100 degrees celsius... if you put them over a flame of 200 degrees for exactly one second, the cooler cup of water will absorb more heat (because the temperature split between the two is larger).

The same applies in an engine cylinder. When the ignition occurs, a lower compression ratio engine will have a cooler air/fuel charge in the cylinder - and so it will absorb more energy (which is wasted as exhaust gas heat).

A high compression ratio engine will ignite a hotter air/fuel charge which will absorb less heat. Less energy wasted as heat = more energy transferred to the crank.

Of course, it follows then that if you were to have two almost identical piston engines (one low/one high compression) burning exactly the same amount of fuel, the exhaust gases from the higher compression engine would be slightly cooler than the low compression engine.

Make sense?

No not really. It was the crank that turned the piston that compressed the air in the first place, so no gain there only a net loss. :rolleyes:

One might approach this from the ramjet end of the spectrum. What makes a ramjet work, and why won't it create thrust while standing still? Once you internalize this problem, you begin to understand what makes a gas turbine work.

the compressor ensures that the air will stay in the combustor long enough to burn and move rearward out through the turbine.

Take away the compressor and all you have is a bonfire

Best Answer (rated by viewers)

For the turbine to work, the pressure in the combustion chamber needs to be higher than ambient pressure (otherwise there would be no flow across the turbine). But combustion requires air, and that air needs to be forced in against the combustion chamber pressure.

I think that's the simplest answer, though it ignores a lot of stuff.

And let's not forget the other essential function of the compressor which is to finely chop up any meandering birds so that they pass through the combustion chamber without clogging it up. Inferior designs end up falling into water and generating lot's of Pprune traffic ;)
(PS: It's Friday.)

Mate, are you serious? :ugh: The losses incurred by compressing the air have nothing to do with it, because that energy is regained... - ie, if you ignore any fuel going into the engine, if more energy is used to compress the air, at the top of the cycle there is more energy to push the piston down. Compressing an air charge with a piston is like a squashing a spring - you get back what you put in (ignoring friction losses).

I think you are just trying to shoot me down because I said you comments about volumetric efficiency didn't answer the original post. :D You can argue all you like, but I have a physics degree and the principles of thermodynamics have been unchallenged for a few hundred years.

A certain amount of available energy enters the engine in the form of fuel. A heat engine converts this into three types of energy - sound, heat and useable work.

The sound is such a tiny percentage it can be ignored. As a result, the efficiency of an engine is essentially a function of how much waste heat it DOESN'T produce. The difference in heat between the air entering the front of the engine compared to the heat of the exhaust air is proportional to the thermodynamic efficiency of the engine.

QJB, if my previous posts didn't make sense... you might as well start here.

Thermal efficiency - Wikipedia, the free encyclopedia

Fact is that does not need one but (if working) would be considered more of a ramjet with a turbine disk to slow it down inside. The fan is a compression stage...

Slippery...
 

This is my serious face.

I'm not challenging the laws of thermodyamics. What I'm disagreeing with is how you've explained them as they apply to a real engine. You may have a degree in physics but your post will have to stand on its own merit.

His original question was 'why does a turbine engine need a compressor' and that was answered in a number of ways including one direct reference to the Brayton cycle, which is correct.

Volumetric efficiency impacts on thermal efficiency though. If the VE is poor then you are suffering pumping losses and using more energy just to keep the engine running. Idealised heat engine models are all well and good up to a point but you have to move beyond a PV diagram to understand the constraints on the efficiency of a real engine.

I don't think so. You want the working fluid to absorb all available heat from combustion so as to raise the pressure. That's where the umph comes from. Waste heat in the exhaust is simply that which you are unable to turn into work before the power stroke ends.

Again, the method of transferring energy to the crank is to heat the working fluid so it expands against the piston. The fluid needs the heat if it is to do any work. For this reason modern engines are run a little hotter than they used to be so that less of the combustion heat is taken out of the working fluid.

You are barking up the wrong tree. The significance of running the compression ratio high is to get more heat into the charge (without going too far) in order to promote better combustion. It is NOT to get more heat into the charge so the charge then absorbs less heat from combustion because that is not going to work:

As well as friction you have blow-by and heat loss to the coolant (which both increase if CR increases I believe). Therefore, you need to get more out of the power stroke than you took from the crank to achieve the extra compression. That 'more' comes from the improved combustion, not from the heat of compression itself. It is to do with better mixing of the fuel and increased flame speed.

Hi Oggers.

Yes, you are correct that there are other factors at play in a real engine.

The loss of efficiency due to pumping losses (ie the engine having to suck air in and blow it out) are comparatively small. The biggest loss of efficiency in an engine with poor VE is because of a drop in effective compression ratio. For example, if an engine can only suck in half a cylinder full of air because of intake/exhaust restrictions or throttle position - a 10:1 compression ratio engine is actually only effectively producing a 5:1 compression against atmospheric pressure. I still don't think VE is important in the question of the OP.

Exactly. This is exactly what I said. A higher compression ratio adds the heat to a hotter air charge, so once the engine reaches BDC the higher compression engine "fluid" will be cooler. By "absorb less heat", I meant at the end of the cycle the fluid has absorbed less total heat during the cycle (not saying it's cooler at the point of ignition - it is, in fact, hotter as you said).

Sorry, disagree. I do agree higher compression engines have better mixing and better burn, but this is not the main reason they are more efficient.
You need to think again about the scenario I posed earlier. Consider two engines, one high and one low compression ratio, burning 1L of fuel per minute. I guarantee the higher compression ratio will have a lower exhaust gas temperature once back at atmospheric pressure.

Your argument is that higher compression engine will have hotter exhaust gas and that better and more complete combustion is the only reason it will be more efficient. You can't get more "umph", plus hotter exhaust gases too - where is all this extra energy coming from? The difference in percentage of unburnt fuel between a low and high compression engine is quite small - trust me - I've done it on a lab dyno at university.