Is an Ideal Combustion Cycle Possible? Feedback from Experts.

In summary, an engine could have a very low exhaust gas temperature and does not need any coolant radiator. The aim is to recapture most of the water to avoid bringing it with the engine.
  • #1
Hey guys,

I need to get some feedbacks from all of you here. Do you guys think it is possible for an engine to have a very low exhaust gas temperature and does not need any coolant radiator?

The aim is simple, with lesser heat rejected through exhaust gas and radiator, there will be more heat from either a gas turbine or piston engine to be converted into kinetic energy.

US Army tank command initiated the research back in the 70s and with all the great results that they got, I am surprised why it has never been heard again. The paper can be found here

Someday we should thank these few Army researchers, they have done a lot for mankind.
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  • #2
Welcome to PF.

There are a very large number of engines that don't require a radiator, including many basic internal combustion engines (2 and 4 stroke lawn mowers, for example), many motorcyles and gas turbines (jets).

For the temperature of the exhaust, you can always recover heat from it to lower it and utilize some sort of combined-cycle, but that requires a heat exchanger and costs money, so there is a cost-benefit ratio to consider.

From your link, the idea of water injection has been around for a while and has pros and cons - one of the cons is the need to carry water in addition to fuel.
  • #3

Thanks a lot. the best part about having water injection is that water having far higher specific heat capacity in both liquid and gaseous state absorbs the combustion heat better than nitrogen or carbon dioxide.

Once water is in vapor form, it has higher gas constant R thus it expands better than both nitrogen and carbon dioxide too.

We can also pack in a lot of thermal energy and transport it into the combustion chamber using water as the energy carrier.

As for carrying it, if you read the paper from the US army, by cooling the exhaust gas, we should be able to recover most of the water thus we don't need to bring lots of it.

To me, the use of oxygen and water may enable us to achieve the holy grail of ideal combustion cycle.
  • #4
Depends what you mean by ideal, and what you are getting at. Speaking using the engineering definition of idea:

Ideal in a thermodynamic sense - no.
Ideal in a mechanical sense... sadly no also.

'Ideal' cases are simply impossible to achieve in real life due to physical constraints.

Take the 'ideal' otto cycle, the line representing combustion goes vertically up. It's physically impossible to achieve this as the combustion event takes a finite amount of time. As a practical effect of this is that you never reach the pressures 'ideally' available.

Also take the exhuast gas, yes it's a good idea to cool it after it's done all of its useful work, but you have physical limitations to this. Exhaust gas flows rather quickly, it's a case of just how are you going to cool it in time to recapture energy, and then make use of the energy in a cost efficient manner.

Long term, the internal combustion engine is not a viable option for propelling us round in out little metal boxes.

In the shorter term, technology continues to strive to get more power per unit of fuel. EGR, direct injection, etc etc. Currently the best use for high enthalpy exhuast gas is turbocharging. Simply becuase it allows downsizing.

Bottom line: We can strive for clever ways to work towards the ideal case, but we'll never ge there.
  • #5

I wrote 2 papers which got published by SAE; #2009-01-2808 and 2009-32-0047. If you have time, you can purchase online.

I practically use oxygen to replace air in order to get the best heat release and emissions. I also replace air with water to get the best heat absorption, gas expansion and energy carrier from outside into the system.

In terms of combustion, I feel that nitrogen does not belong there and engine often ends up having to compress more than what it should. Furthermore, it normally ends up as NOx. With the use of stratified oxygen, I can basically use recycled exhaust gas to supply the activation energy for the next fuel auto ignition. The remaining compression work is only required to get the most accurate auto ignition temperature. From the paper that i wrote, the heat release is superb and there is very minimum NOx and soot.

As for the use of water, what I like the most about water, is that I can manipulate it to exist in either liquid or vapor form just by adjusting the temperature and pressure. I keep it in liquid state to transport thermal energy from outside into the system. Once it is inside the combustion chamber, it absorbs the combustion heat far better than nitrogen and CO2. Next, with lots of heat absorb, it expands few times better than both CO2 and nitrogen.

The earlier research by the US army provided a good baseline but I have made improvements in almost every aspect of the system. Thanks to the material and electronics available today.

Well, I agree with you that it's impossible to achieve perfect cycle but I believe that this is the closest that we can climb. If you have a chance, read the papers and I would love to discuss with you on how we can evolve the concept further.
  • #6
Sounds a little bit like a HCCI type engine, with the stratified charge of petrol and EGR + stuff to control cylinder temperatures.

The major problem is acutally getting stuff to do it at engine speeds if I remember. It works in the lab at constant speed, but currently would be undrivable in the same way a current engine is. I've not done anything or read anything on this since university so I'm out of touch on developments to say the least. Also I realize that making lab work into production model is simply an equation involving time and lots of money.

I'm strapped for cash at the moment, but I'll certainly have a look at those papers at some point.

EDIT: I held off asking this becuase I am assuming it is answered in the papers but curiosity is getting the better of me. Where are you getting the pure oxygen from?
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  • #7

I had a choice to run the engine in HCCI but i chose to run it not with homogeneous combustion. I spent the first decade of my career designing SI engines but I realized that homogeneous combustion is not the way to go. The knocking and incomplete combustion are the major issues.

For your information, the engine in those two papers are operated in 2-stroke thus the cylinder temperature near TDC can be accurately controlled by controlling the exhaust valve timing and lift. I can basically operate with HCCI in all engine rpm and load. I can also run it with various liquid or gaseous fuels if I want to.

Still, I chose to stratify both oxygen and fuel inside the piston bowl to ensure full control over the combustion event.

let me know on what you think after you read my papers.
  • #8
If you are after good efficiency then you would need to consider the energy required to produce the Oxygen, in the first place. That would be a significant factor - possibly offset by the fact that (sellable) Hydrogen could also be produced in some processes for producing Oxygen. If you get you Oxygen by fractional condensation then it would just cost ya.
  • #9

You're right, oxygen generation is significant in terms of energy balance. That's why in my paper, i stated that we should use pressure swing adsorption in which the power consumption is quite low for every kg of oxygen produced.

In my paper, I also stated that the benefits of using oxygen far outweigh the cost of producing oxygen.
1) No fancy and high back pressure exhaust aftertreatment is needed. No soot of NOx aftertreatments are needed
2) Broad range of fuels that can be used. This is good because fuel processing can be simplified and minimized
3) Robust combustion with very minimum incomplete combustion. This is crucial to enable high EGR and water to be used
4) Low overall charge means very small compression work is needed to compress the charge above the fuel auto-ignition temperature. Furthermore, the thermal energy from the exhaust gas is the one that raise the cylinder temperature

Here is the link of the presentation slides of the paper that I presented in Engine Expo, you can see how oxygen combustion enables wastewater treatment and direct CO2 sequestration [Broken]
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1. What is an ideal combustion cycle?

An ideal combustion cycle is a theoretical process that describes the most efficient way to convert fuel into usable energy. It involves the complete oxidation of fuel in the presence of oxygen, resulting in the release of heat and the production of carbon dioxide and water.

2. What are the stages of an ideal combustion cycle?

The stages of an ideal combustion cycle include intake, compression, combustion, expansion, and exhaust. During intake, fuel and air are mixed and drawn into the combustion chamber. In compression, the mixture is compressed to increase its temperature and pressure. Combustion occurs when the mixture is ignited and rapidly burns, generating heat and expanding gases. Expansion is when the gases push against the piston, converting the heat energy into mechanical energy. Finally, exhaust involves the removal of the waste products from the combustion chamber.

3. What is the efficiency of an ideal combustion cycle?

The efficiency of an ideal combustion cycle is defined as the ratio of the useful work output to the energy input. In other words, it is a measure of how much of the fuel's energy is converted into usable work. The efficiency of an ideal combustion cycle is typically around 60%, meaning that 60% of the fuel's energy is converted into mechanical work.

4. How does an ideal combustion cycle differ from a real combustion cycle?

An ideal combustion cycle assumes perfect conditions, such as complete combustion and no energy losses. In reality, these conditions are not always met, and there are always some energy losses due to factors like incomplete combustion, heat transfer to the surroundings, and friction. As a result, real combustion cycles are less efficient than ideal combustion cycles.

5. What are some practical applications of the ideal combustion cycle?

The ideal combustion cycle is the basis for many practical applications, including internal combustion engines, gas turbines, and steam power plants. These technologies use the principles of the ideal combustion cycle to convert fuel into usable energy, powering various types of vehicles, machinery, and electrical generators.

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