CC...Cubic capacity of what?

[Bellow]

I was thinking about efficiency

1 compress
spark
2 push
3 exhaust
4 input

You must lose some (power) efficiency with a turbo because the pressure at the end of 2 should be higher than in a NA engine, yes?

There is greater back pressure that the piston has to work against, but conversely the intake stroke is easier as the air is essentially pumped in creating positive pressure on the pistons (pneumatic transmission) derived from energy that was otherwise going to waste but is expanded through the turbine of the turbo and its energy extracted. Thus the expansion ratio is increased which is what we really are referring to when we talk of compression ratio, the efficiency of an engine being fundamentally linked to that parameter.
For road use, turbos allow for greater efficiency at the same power output as they are made smaller incurring less losses to friction and heat loss. There are limits though - the OEMs are complaining that the reduced compression ratio required to prevent detonation under boost conditions is hurting part load economy..

right ok so what we are sayig is that what ever it is filled with be it air/fuel the more it can hold the more it can combust resulting in more power?

But to make the maximum power, the mixture ratio has to be within a narrow band so knowing the quantity of one, you could deduce the other! Best though to think of the engine as an air pump as it is in the movement of the air that all the work is done. Adding a squirt of fuel is a doddle by comparison.

For anyone who really wants to understand how all the factors play out, have a look at the Ricardo book - The High Speed Internal-Combustion Engine. He lays it all out based on all the research he did in the last century. After the Ladybird book, it's number 2 on the reading list!

 

[Stratman]

Swept volume can be calculated thusly:-

Each cylinder has a diameter, half of this is the radius. The area of the cylinder (effectively the same as the piston crown) is pi x (radius squared).

The stroke is the distance the piston travels from top to bottom in the cylinder.

The volume of one cylinder is its area x piston stroke. Multiply this by the number of cylinders and that's the capacity of the engine.

For example, let's design a four cylinder engine with a cylinder diameter of 10cm and a stroke of 8cm.

The radius is 5cm. 5 squared is 25, multiply by pi (3.142) and we get 78.55 square centimetres. Multiply this by the stroke (8cm) to give 628.4 cubic centimetres. Multiply by the number of cylinders (4) and we have an engine of 2516.3 cc, or just over 2.5 litres.

 

[BillyW124]

Stratman: this is genius!

well explained! thank you very much!

 

[Ted]

What about the combustion area in the head?

And you're assumong flat topped pistons.

 

[renault12ts]

What about the combustion area in the head?

And you're assumong flat topped pistons.

Doesn't count. Engine size is determined by bore and stroke, and pi R squared H.

The other bits determine the compression ratio.

 

[Ted]

I learn something every day - thank you R21 and Stratman.

 

[lxi]

What about the combustion area in the head?

And you're assumong flat topped pistons.

Engine capacity is the swept volume as Stratman explained particularly well.

If you consider a dished (or domed) piston crown, the dish (or dome) is there all the time between BDC & TDC - so doesn't affect the swept volume - nor does any space above the crown, be it in block or head.

Agreed, this is not a measurment of the actual capacity at BDC, but again engine capacity is swept volume.

 

[Bellow]

Swept volume can be calculated thusly:-

Each cylinder has a diameter, half of this is the radius. The area of the cylinder (effectively the same as the piston crown) is pi x (radius squared).

The stroke is the distance the piston travels from top to bottom in the cylinder.

The volume of one cylinder is its area x piston stroke. Multiply this by the number of cylinders and that's the capacity of the engine.

For example, let's design a four cylinder engine with a cylinder diameter of 10cm and a stroke of 8cm.

Taking this a tad further.... The power output can be derived from Power = PxLxAxN.

Back to physics for just a moment, the pressure acting on an area gives rise to a force, which, if then applied over a lever, gives a torque.
The A is the piston area, the L is the lever which is also the stroke.
But as Stratman pointed out, AxL is the swept volume.

P is MEP, mean effective pressure which is the average pressure over the 4 strokes.
So, PxAxL which is also Pxswept volume gives the torque output, and obviously increasing one factor or the other will increase the torque generated. (Turbos and superchargers dramatically raise the MEP)
All that remains to determine the power output is how often in unit time these 4 strokes occur. Hence N, the engine speed in rpm! Simple!

So in summary, the power of an engine is a dependent on its capicity, the pressure it operates at, and the speed it turns at.

Incidentally, in old money, a quirk of the numbers means that at 5250rpm, the numerical values of power in hp, and torque in lbs.ft will be identical.

so doesn't affect the swept volume - nor does any space above the crown, be it in block or head.

But in the case of forced induction engines it can increase the mass trapped in the cylinder as the clearance volume is larger for the cylinder than it would be in a NA engine (to lower the CR) which is than charged with pressurised air leading to greater fill than would have been possible in a NA engine.

For anyone struggling to grasp the above, imagine a CR reduced to something absurd such as 4:1. That would mean the clearance volume would be 33% of the cylinder volume. With unlimited boost available, that represents 33% extra fill over the swept volume!

 

[BillyW124]

THIS IS GREAT STUFF LADS!

very impressed with knowledge on this forum!

although i must admit, i am trying to make sense and breaking it down from what is presented before me!

it is all comming together.

keep it up..

 

[Stratman]

Compression Ratio.

Compression ratio is the ratio between swept and unswept volume. In other words, how hard you squeeze the air/fuel mixture.

When the piston is at the top of its stroke there is still some space above it, maybe a little in the cylinder itself and the combustion chamber in the head (or sometimes in the piston crown). Let's say the cylinder has a total volume from these sources of 500cc when the piston is at the bottom of its stroke. This volume comprises 450cc swept volume and 50cc combustion chamber. This 50cc is the volume of the cylinder with the piston at the top of its stroke. The ratio between the two volumes is 500/50 or 10:1, so the engine has a compression ratio of 10:1

 

[camerafodder]

Here you go Billy, if you click on this link it shows the build and operation of a modern 4 cylinder engine.

YouTube - AWESOME ENGINE ANIMATION....

 

[Bellow]

An engine is an air pump - with a spring.
Springs when compressed (or extended) retain their energy which can then be released for whatever purpose is chosen - wind up watch, catapult, return mechanism on clutch or throttle pedal, door handle, etc, etc, etc. But they can only return the energy that was put into them - never any more than that.
Except for gas springs, in which air can be used for the gas. If you pressurise air and that pressure is maintained, then so is the stored energy. A mechanic uses exactly that principle when he uses air tools. An electrically driven compressor compresses the air into a tank that he can subsequently use to drive his air tools. He could go one step further and employ a unique aspect of a gas spring, namely, that they are the only spring that more energy can be added to and by a different method than more pushing or pulling.
That method is to add heat. If our mechanic took a sizeable blow torch to his primed compressor tank the air on heating would expand - or try to - and as it is contained would instead increase in pressure. He would then have more energy to drive his tools than he had by compression alone. We would call that an external combustion engine.

Now if we were more fly, we could take the compressor part, dispense with the tank, and rather than letting the air out of the compressor after it has been squeezed to a high pressure and is sitting there like a spring, we could once more attempt to heat it. The blow torch is slow and unwieldy, but if we had mixed some fuel in the air before we compressed it (or after in the case of a Diesel) we could ignite it. The compressed air would heat up on burning and expand and the spring would have even more energy than when merely compressed.
And that is all an internal combustion engine is!
Repeat the process hundreds of times each second and soon there is power enough to drive a car. But crucially, some of the fuel energy will be expended in pumping the air into the engine and compressing it and that is the start of the inefficiencies. As is heat lost due to the high flame temperatures, and that all the heat cannot be used and is lost to the exhaust due to lack of an infinite expansion ratio.
But that's probably enough for now...

PS. Expansion ratio and compression ratio are largely the same thing and are aspects of the geometry of the engine as explained by Stratman post#50. Note his (realistic) value of 10:1, far from the infinite expansion ratio we crave for absolute efficiency. A turbocharger however, brings us closer...

 

[oldcro]

What about the combustion area in the head?

And you're assumong flat topped pistons.

As mentioned previously, the volume of the combustion area in the head determines the engines compression ratio.

 

[renault12ts]

As mentioned previously, the volume of the combustion area in the head determines the engines compression ratio.

Post 45.

 

[esox]

What about a ****el,then.....

****el= Rotary enigne - thanks

 

[renault12ts]

Lost me...sorry.

 

[camerafodder]

All this talk of pistons, up and down.
What about a ****el,then.....

Somebody just had to mess things up didn't they?!

 

[camerafodder]

Lost me...sorry.

He means rotary w@nkel but the censor got 'im.

 

[renault12ts]

Somebody just had to mess things up didn't they?!

Oh got it...Wnakel. The rotary has a capacity too...but it can't be compared to the standard piston engine.

 

[renault12ts]

He means rotary w@nkel but the censor got 'im.

Thanks, got there eventually.