Heat transfer inside engine combustion chamber

[RadoTD]

Hi, I'm not sure if I've got this in the right section, but I'm hoping someone can point me in the right direction.

I'm trying to create a model of what's actually happening during the combustion cycle in an engine and where the energy is going.

As the intake valve closes and the piston begins to compress the air in the combustion chamber, as the temperature of the air begins to rise, what type of heat transfer is happening?

The best info I can find is off of here http://en.wikipedia.org/wiki/Thermal_conductivity.
Using the equation where conduction = kA(T/x).. k being the conductivity coefficient, A being surface area, T being temp differential and x being the distance between the two temperatures.

Is this what I'm looking for? Would the variable x be for thickness of the block/head/piston (obviously needing to do a separate calculation for each of them)? And would all the heat transferred be lost from the compressed air? I realize I would need set up a spreadsheet to calculate it every couple of degrees of rotation to keep it accurate.

Just hoping someone could let me know if I'm on the right track or not!

 

[brewnog]

Pretty much. Until addition of heat (spark or fuel injection), the temperature increase in the cylinder is nearly all from compression. This is, as you say, lost to the cylinder head, cylinder walls, and piston (and a small amount of gas escapes through 'blowby').

Determining the heat transfer coefficient for the combustion chamber components after ignition is the tricky bit, because of the rapidly changing in-cylinder conditions (in terms of gas temperature and pressure).

You'll need more than a spreadsheet to do this (a 3D combustion simulation will be required, correlated to heat release data from the engine in question).

 

[Topher925]

You will need more than Fouriers law because the heat transfer from the compressed air is not natural convection, it is forced convection. Most modern engines (if not all) are designed to have turbulent flow inside the combustion chamber upon air intake to promote air/fuel mixing.

I would start by first modeling the heat transfer in an isothermal compressor, then an isentropic compressor and move on from there.

As brewnog pointed out, a simple spreadsheet won't cut it if you want your error less than a couple hundred percent.

 

[RadoTD]

So I got the spreadsheet put together with some interesting results.
I have ignored blowby for now, it would have made it significantly more complicated and I wanted to first see if I could pull any feasible numbers. It's sampling every 6 degrees (30 samples), or 250us at 4000rpm.
This is also just the compression cycle; from BDC to TDC with the valves closed the whole time, gasoline mixed in with the air though... however much of a difference that makes.

First, with parameters set to estimated full throttle, 2L engine, 4000rpm, 8:1 compression, hot engine.
Gave me a max of 251PSI and 827deg Celsius. Seems a little high given that gasoline ignites at 280deg C... obviously including blow-by would bring it in the right direction.
Increasing to 10:1 compression (where engines typically begin to knock (pre ignite the gasoline) brought those numbers up to 487PSI and 1500deg C!
Considering I hadn't added in any cylinder leakage, I was fairly happy with how that ended up.

Now, I set it to simulate running a compression test. 1500rpm from the starter, low volumetric eff (closed throttle plate), cold engine, colder intake air.
Gave me 29PSI at 56deg C...
Strange...
Also, dropping volumetric eff (air sucked in each intake cycle) or RPM too low gave me erratic numbers jumping between positive and negative... (I've got fuel/air mass for the temperature/pressure changes given thermal loss calculated from volumetric efficiency). RPM too low obviously makes the sample rate too long for the thermal transfer.


Something that surprised me is that if I increase intake temp, it increases cylinder temperature, but actually decreases the pressure. I might have to look into that.

And I found http://en.wikipedia.org/wiki/Isentropic_process" on wikipedia about isentropic heat transfer and from a quick read, it looks like that's what I've actually done. I calculated the energy transfer between the combustion chamber and engine bay, multiplied that by specific thermal capacity of the air/fuel mixture to get the change in temperature, then changed the pressure relative to that temp change as well. I'll properly read through it later on and see if I've got it right.

Given that I seem to be in the general ballpark (in the first simulation at least), I'll try to find some way of adding in blow-by and see what that does. Adding in combustion later on might make things interesting!

But... it's midnight and I need to get up early for work tomorrow. The thumbnails below show my input parameters and output data on the first (4000rpm/hot engine/8:1comp) simulation.

http://xs.to/thumb-8B8A_4B63E428.jpg http://xs.to/thumb-FE24_4B63E428.jpg

 

[alphy]

Hi there,

I can definitely help you with understanding the heat transfer inside an engine's combustion chamber. Let's start by discussing the different types of heat transfer that can occur in this situation.

Firstly, there is conduction, which is the transfer of heat through a material or between two materials that are in direct contact. In the case of an engine's combustion chamber, conduction can occur between the hot gases and the walls of the chamber, as well as between the different components of the engine (such as the block, head, and piston).

Secondly, there is convection, which is the transfer of heat through the movement of fluids (such as air or fuel). In an engine, convection can occur as the hot gases move through the combustion chamber and are then expelled through the exhaust.

Lastly, there is radiation, which is the transfer of heat through electromagnetic waves. In an engine, radiation can occur between the hot gases and the surrounding components, as well as between different components within the engine.

Now, to answer your question about what type of heat transfer is happening as the temperature of the air rises during compression, it is a combination of conduction, convection, and radiation. As the air is compressed, its temperature increases and it begins to transfer heat to the surrounding components through conduction. Additionally, the movement of the hot gases through the combustion chamber leads to convection, where the hot gases transfer heat to the walls of the chamber and to other components. Lastly, there is also some radiation occurring between the hot gases and the surrounding components.

In terms of the equation you mentioned, it is a simplified version of the equation for conduction, and it can be useful for calculating the heat transfer between two materials. However, in the case of an engine's combustion chamber, the heat transfer is more complex and cannot be accurately calculated with this simple equation alone. You would need to take into account the different materials and their properties, as well as the geometry and design of the combustion chamber.

To accurately model the heat transfer in an engine's combustion chamber, it would be best to use a computer simulation or modeling software that takes into account all the different factors and variables. This would provide a more accurate and comprehensive understanding of the heat transfer and energy distribution within the engine.

I hope this helps clarify things for you. If you have any further questions, please don't hesitate to ask. Good luck with your model!