Thermodynamics - Adiabatic Compression w/heat loss to cooling - help with equation :)

[Mountain-God]

Hi!

I'm not that clued up on thermodynamics and, especially, this level of mathematics...

I've worked out the changes to temperature and pressure using an adiabatic compression equation, but now I need to account for the impact of heat loss and not sure how to go about it.

The scenario is a simple automotive compression ignition engine - where I now want to work out the impact of a cold engine block on the temperature of air within a cylinder...

I know the general theory, but need to graph the range of effect...

Can someone help me with the equation?

 

[armathas]

I'm certainly no expert..by any stretch of the imagination..but..

Doesn't the Adiabatic Process refer to a thermodynamic process in which there is no heat transfer either way? <--not entirely sure, it has been years since school.

Ooops my bad..I didn't read your post accurately. Doh

 

[Dubai Vol]

The reason heat loss is typically ignored is because there is no time for it to occur. Even at 600 rpm, the entire intake cycle takes 0.1s, and the time the intake charge, heated by compression, spends in the cylinder, is less than that by a considerable factor.

Why exactly are you needing to account for the heat transfer? You're right, it's a bear of an equation, as the temp of the intake charge is not constant, but rising from below ambient at the start of the intake stroke (due to cooling by expansion, so even cold cylinder walls are actually heating the intake charge at first) to the max temp at full compression and also the area available for heat transfer is constantly changing as the gas is compressed. And after the very first cycle the cylinder walls will be warmer than ambient. So lots of variables.

I still have my heat transfer textbook somewhere....

 

[Merkinball]

You're going to have to get more specific. Dubai Vol, I think you're over complicating things here, as well as doing some over simplifying.

First of all, what is your entire thermodynamic system? Are there any other constraints? Is it just the piston and combustion chamber? Or are you looking at the whole block surrounding it? From what you tell me, then Dubai's assertion that the heat loss should be ignored is moot, because it seems to me you're looking for the heat that the engine block is taking on from the "loss", and this should be integratable over time.

I don't think what you're looking for should be all too bearish. If you're dealing with an adiabatic situation, then it shouldn't be too bad. It shouldn't require explicit heat transfer and boundary value problems if you're given a question dealing with an adiabadic system.

Give some more information, perhaps the whole question itself.

 

[plarq]

Assume

Heat transfer: ignored
Working substances: Ideal gas
then
p*V^gamma= const (this constant depends on the system's initial temperature and volume)

gamma is the ratio of heat capacity of constant pressure versus heat capacity of constant volume. For two-atom molecules, it is 7/5. For single atom molecules, it is 5/3.

 

[innonimatu]

Dubai Vol is right, because the system is transient the full equation for heat transfer would have to take into account the outer air temperature, convection, conduction, and surface fouling and fin effects in the engine block. And this would get even worst as the different materials used in the engine had to be considered.

Better to start by dismissing fouling, consider the engine a simple infinite cylinder, and use the conduction and convection equations for steel. Convection inside the cylinder must be accounted for, but the composition of the gas must be known (or a heat transfer coefficient arbitrated), and an idea of it's motion would be useful.

Still a bear of an equation. What you want has been the subject of many papers in technical publications, such as this one.

 

[Mountain-God]

I'm certainly no expert..by any stretch of the imagination..but..

Doesn't the Adiabatic Process refer to a thermodynamic process in which there is no heat transfer either way? <--not entirely sure, it has been years since school.

Ooops my bad..I didn't read your post accurately. Doh

Quite right!

This isn't any kind of exam question, but I'm interested in compression ignition engines - which are often said to have trouble starting in low-temperature conditions.

To cut past the opinion and hearsay, I hoped to work it out myself -yet, with a 24:1 compression ration, it seems the internal temperature is never going to be so low - in fact it remains, from an ambient -40 plus very much higher than diesels ignition point.

So, the next part is to account for heat-loss to the engine block except that I've pretty much exhausted the little skill/knowledge I have with the adiabatic equation

 

[Merkinball]

Mountain-God.

What is EXACTLY that you are trying to find out here. What is your goal?

Sounds to me like you need a lab with a dyno. Not mathematical equations.

 

[Mountain-God]

Mountain-God.

What is EXACTLY that you are trying to find out here. What is your goal?

Sounds to me like you need a lab with a dyno. Not mathematical equations.

Taking one theoretical compression-ignition engine - with a compression ratio of 24:1, breathing air at sea-level @ an ambient temperature of -40 degrees celcius, with the engine-block at the same.

I've worked out that the ambient air temperature will not deter good combustion (as the adiabatic heating raises it so high) - but now I want to determine the effect of the cold engine-block, if any.

I'm told it does - but not to what degree. I don't have an engine or lab to test with - so hoped to rely on some math for an approximate answer. Doesn't need to be exact, but a fudge in the range of 200 degrees.

I'm wondering if the problem is more to do with lower compression ratios...

I'm not necessarily looking for someone to answer the question out-right, but if hope someone might at least point an amatuer in the right direction

 

[Merkinball]

I've worked out that the ambient air temperature will not deter good combustion (as the adiabatic heating raises it so high) - but now I want to determine the effect of the cold engine-block, if any. - Mountain-God

Well, you're gonna have to remember that there are lots of factors that I don't know if you can really take into account to isolate the engine block itself. A few things I can think of is everything from cold oil, with lower viscosity, to the expansion and compression of the block itself, the oil around the crank shaft. Also, as it gets colder, the spacing of the sparkplugs becomes more critical as well. It's easier for water to condense in the air intake...

There's a lot that goes on with an internal combustion as it changes. And by the time you get all your assumptions ironed out, I'm gonna guess you're going to get an erroneous solution, unless you can isolate and control all the other outside factors.

The math is absurd. In order to get the right answers you'd have to do some amazing second order partial differential equations. The integral calculus would also be utterly ridiculous, as you'd have to take into account the reciprication of piston, and you also have to take into account the heat that would be absorbed by the piston and the connecting rod as well.

If I were doing this, I'd be in a lab with a dyno, and be able to work on it for a week with some expensive thermocouples. Either that, or I'd use MatLab, or Solid Works, or Ansys and make simulate it all. There's gotta be stuff on the internet regarding this as well.

 

[Mountain-God]

Thanks for the input.

By my logic, the remaining critical factors are going to be fuel and oil temperature, and that of the engine block, and condensation at intake.

- making a few gross assumptions it may be reasonable to ignore fuel and oil temperatures (they must be moderated anyway, as -40 is already far too low for them), similarly, condensation.

Ok, so two bare questions:

Can anyone suggest some method by which to account for the impact of the engine-block, as a heat-sink, on the temperature?

Or if, according to their understanding, such impact should simply be ignored - considering that any temperature variation less than +/-200degrees is not important

At this stage I suspect the concerns that have been raised to me - about the temperature of the engine block and ignition - are largely based on relatively low 10:1 compression ignition, and may not be valid up around 24:1

 

[Merkinball]

- making a few gross assumptions it may be reasonable to ignore fuel and oil temperatures (they must be moderated anyway, as -40 is already far too low for them), similarly, condensation. - Mountain-God

I wouldn't make such assumptions. The viscosity of motor oil between normal engine operation and -40 degrees C or F is huge, and is going to play a big factor in the initial operation of the engine. And again, I think that spark plug gap has more to do with poor engine operation in cold weather than the block being cold anyway.

I would imagine that the state of the engine block being very cold WILL have some impact. That is certainly the case. But there is no way that I'd do this for a project, make all the assumptions we need to do strict math, and come up with a number. I don't think you could have any sort of certainty in your numbers.

Look at all the intertwined parts on an internal combustion engine. Particularly one for a car. Almost everything on that system is going to be effected by temperature. And almost all those changes will effect it negatively (so far as performance goes.)