Without the pressure differential the heated air would still do work, it's just that it wouldn't be much use. Gas flows from a high pressure region to a low pressure region. No compressor and the heated gas, which is now at higher pressure (pv=nkt) sees lower and equal pressure at either end of the tube. Compressor (or ram effect, or reflected shock as in those exhaust pipe trick engines) gives the right gradient to send the gas to the designated exhaust where it expands, cools, and does work. The higher the initial pressure of the incoming gas, the higher its density and so it can support a richer mix, which increases temperature,etc.

First off, I'm just a pilot. My bachelors is in professional aviation. I normally don't do math in public.

I think the answer to the "why" question is simple. It's because they are intended to produce thrust in a specific speed range. (my remarks apply to airplanes only, but could likely be used in other discussions). What is thrust? "Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system."

You can't have an equal and opposite reaction without something to push against. When the exhaust gasses move, they have to push against something in order for there to be an opposite reaction. In a piston type reciprocating engine, the expanding gases push against the piston, which is connected to a crankshaft which converts the linear force to rotating force. In a turbine engine, the expanding gases ultimately have to have something to push against and absorb the energy produced. The compressor blades absorb that push, transfer it to the bearings on their shaft which are connected to the engine cases which are connected to the engine pylon which is connected to the airframe and the entire airframe moves opposite of the exhaust.

I realize that ram/scram jets fly, but they are highly specialized and also work in a specific velocity range. I'm out of my league here, but I think that ramjets also have "something to push against" but that "thing" isn't a structure, it is a pressure wave inside the combustion chamber. Hitler's "buzz bomb" used a pulse jet which was a ramjet with a flapper door at the entrance to the combustion chamber. That flapper provided the expanding exhaust gas with something to push against.

No.
This is a very common misunderstanding of turbine engine.
The gas heating is done at constant pressure not at constant volume !
The temperature increase is exactly compensated by a density decrease.
Without compressor, the heated air would just remain at atmospheric pressure and expand in both forward and backward direction and wouldn't produce any work (no pressure differential).

Luc

Why compress air using a compressor in jet engines ?
 

Mc2

Increase the mass (using compression i.e.take a mass of

air and compress into a small space) to increase power output.

In a jet engine airmass accelleration is caused by heating the air in

combustion chambers.This action (accelleration) causes an equal reaction

(thrust).

To use only ram air,which I think the questioner is thinking of, there'd be

a relatively low mass, hence low thrust. A hypersonic engine, on the other

hand, relies on ram air, but requires very high velocities to compress

before heating.

This is the way I remember and understand how it works.:):)

TTex600:Absolutely NOT true, and the obvious example is a rocket engine in a vacuum. The thrust is the result of the mass flow of gas and its exit velocity, and does not require "something to push against".

So the example of the piston in a cylinder has no relevance to the propulsive utility of a jet or rocket. The piston does not propel the aircraft; its mechanical linkage (connecting rod, crankshaft, perhaps a gearset) is to a propeller, which does.

Does anyone remember the pulsejet engine? The German V-1 used it.

barit1

Add a second nozzle pointing in the opposite direction from the one you started with, on the other side of the reaction chamber.

The Third law is honored, and movement (velocity) is nil.

Hi QJB.

Glad you came back and posted again.

Yup, exactly what I've been saying from the start - this is the thermodynamic reason why higher compression engines are more efficient. Glad it helped you answer the question from your OP.

Obviously how the engine is designed, the operating RPM range, type of fuel etc. etc. are all things which are important for designing an engine for a certain application.

barit1

In your rocket in a vacuum, is the rocket motor open on both ends?

You bust on me, and you didn't even read my post. It doesn't lend you much credibility.

QJB
 

You will not find the answer to your question in the Carnot cycle where, for two given heat reservoirs, you can achieve the same efficiency regardless of compression ratio used. With your statement above you are effectively hoping to use heat from the hot reservoir to increase the temperature difference between the two reservoirs. A hopeless endeavour.

If you want to consider the effect of increased CR in the Otto cycle with a notional perfect constant volume combustion process occuring just after TDC then you are simply left with a longer expansion path, a correspondingly longer compression path and therefore the difference between the two is also maintained over a greater path ie its more; more net work.

But you cannot increase the theoretical efficiency of the Otto cycle by taking energy out the crank and feeding it back to increase the temperature of the fuel/air charge prior to combustion. This is where slippery_pete has given you a bum steer. If you look beyond the cycle itself to the combustion process then the picture changes because you can increase the speed of combustion by increasing the temperature and pressure in the combustion chamber.

Spot on Oggers

At high compression the rate of the combustion reaction increases as the effective concentration of the fuel/air mix increases.

This happens because there is now a greater statistical probability of collisions at above or equal to the Energy of Activation for the reaction

Crucially we're back to time dependence. A more rapid reaction means more bangs per unit time and more power per unit time.

Why do we compress? To make the engine more powerful.

Carnot is predicated on the most efficient cycle for converting a given amount of thermal energy into work

Carnot cycle - Wikipedia, the free encyclopedia

CW

To further Chris's post, compressorless atmospheric jet engines are actually quite common. There's one in most DIY's tool kit, it's his blow torch.

The blow torch makes pretty minimal thrust for the fuel used. The combustion chamber pressure is limited to about atmospheric, and air is induced only by venturi effect from the high speed fuel jet supplying the burner. Without combustion chamber pressure, there is no exhaust jet thrust. I've measured the thrust from a small torch (bored at work one day). Used a torch lighter. It managed to make about 1/10gram of thrust. Nothing.

Compressorless jet engines also exist in the non-air-breathing set. Oxy-fuel torches all the way to liquid fuel rocket engines. These ARE compressorless jet engines. In liquid fuelled rockets, the combustion chamber pressures are limited only by material limitations and the pressure your fuel/oxy pumps can deliver. Thus, the chamber pressure can be high, and so can the thrust.

Without a method to adequately concentrate the oxidizer, and supply it to the burner at a pressure above chamber pressures, jet engines cannot produce meaningful thrust on a continuous basis. It is possible intermittently, as in the Argus pulsejet (or indeed any pulsejet engine) on the V1 (pulsejets aren't truely compressorless either, actually, as they use tuned pipe resonance to achieve compression acoustically).

Food for thought.

J

Exactly, and the oxidizer pump performs exactly the same function as a gas turbine's compressor. Without pressure in the rocket's chamber, there is no thrust. And without a fuel pump supplying a like pressure, fuel would never flow into the pressurized rocket chamber.

TTex600:Sorry, I had missed your point. You are of course correct.

But I was addressing the common misconception of the escaping exhaust of a jet or rocket "reacting" against the atmosphere behind the vehicle. I used to have to shoot down that "logic" in the classes I taught.

You are exactly right, the gases in the rocket chamber force against the forward wall, and that is where the reaction is felt. :)

Hello all, and Season's Greetings to you.

I'd agree with that in principle. It allows useful work to be extracted over a wide range of conditions.

This is misleading. You can't increase the speed of the combustion reaction to extract more energy - we've been over this before. Please see post #70. I'll repost the question I posed for you below so you don't ignore it again, and you can consider them turbine engines (this will prevent you bringing in your multiple false arguments again.

To tell a man something requires belief and acceptance

For a man to learn something you must teach him not tell him

Slippery

It certainly isn't the laws of physics at issue, so let's have another look at your first post:

No. In a cylinder, when you burn a given quantity of fuel 'Q', you get:

ΔU = Q - W

The idealised cycle has a constant volume combustion process and looks like this:

http://upload.wikimedia.org/wikipedi...f/P-V_otto.png

The time may vary by a millisecond or two but ALL the heat of combustion goes into the working fluid because W = 0 during this idealised process [#2 to #3 on the diagram].

Bearing that in mind it is clear that your first post is ill-founded:

To be clear, the energy absorbed during this idealised heat addition process will NOT vary with CR in the way you suggest. No useful work can be done - in theory or in practice - with any of the heat until it HAS been absorbed. The clue is in the phrase 'heat addition process'.

The exhaust gas temperature would vary with the change in PdV work during the power stroke. Not because "A high compression ratio engine will ignite a hotter air/fuel charge which will absorb less heat".

The problem here is a lot of what is being said is true. There is not one answer to the original question, which in turn is rather artificial.

It is true you need a compressor otherwise you haven't got a jet engine; all you've got is an oil fire. Also higher compression tends to lead to more fuel being consumed. However the point of compressors is to raise pressure. The thermodynamic efficiency of internal combustion engines increases with pressure and temperature. The explanations given above may be true; it doesn't really matter. All you need to know is you want to get the pressure as high as you can without suffering from side effects like spontaneous combustion/explosion.

Note that pressure is not the same as compression ratio. That's why superchargers and turbochargers raise pressure - they help stop brake mean effective pressure falling as rotational speed increases or air thins.

Slippery

The edit to your last post is welcome. Crucially you now agree with what CW has been saying in principle, so well done, we are halfway there.

As for the other change:

I'm not going to rehash the whole thread to prove that you are taking my comments out of context. I will just paste your reply from the time:

Which isn't a good concept to be promoting.