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Gas Compression & Combustion Efficiency

02/23/2012 9:00 AM

Esteemed CR4'ers,

I like to understand physical phenomena in an intuitive way, to truly get the underlying explanation of why things happen the way they do. Here's one I haven't quite figured out yet:

Why does a higher compression ratio in an engine produce better efficiency? In other terms, if I burn a given amount of fuel in a given amount of air at low pressure, why am I able to extract less energy than if I burn the same amount of fuel in the same amount of air at high pressure? The answer, "it is more efficient at higher pressure" is not a suitable answer for me!

I studied thermo, and dare say that I even understood most of it. I get that the equations of the power cycle dictate that higher pressure combustion yields higher efficiency, and that steam tables indicate this as well. I just don't get why.

It seems counter-intuitive to me that the work I do on a gas to compress it pre-combustion somehow gets magnified many times over once that combustion happens. If anyone could explain the underlying cause of this phenomenon it would be a huge help.

-Tritium

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#1

Re: Gas Compression & Combustion Efficiency

02/23/2012 10:07 AM

Maybe this will help. From WIKI:

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.[citation needed] High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression-causing better mixing and evaporation of the fuel droplets.[citation needed] Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. (Emphasis mine)

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#2

Re: Gas Compression & Combustion Efficiency

02/23/2012 10:23 AM

do you know that work done=pressure x volume. As the compression of air takes place pressure increases, area under p-v diagram represents work done if pressure increase at constant volume , area under p-v diagrams increases. As a result, for a given constant heat input work done increase and thus efficiency increases

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#3

Re: Gas Compression & Combustion Efficiency

02/23/2012 11:45 PM

Tritium, you mentioned "....the same amount of fuel in the same amount of air at high pressure?.." I'm not sure it's the same amount of air if the pressure is higher! Here's why. If the volume of a chamber (cylinder) remains the same, the only way to get higher pressure is to push more air into it. In the case of a cylinder, higher compression means less air is escaping around the piston rings during the compression stroke therefore resulting in higher pressure when the piston reaches the point where it creates the smallest volume ( this volume will always be the same for that particular engine). This would imply that the higher efficiency is the result of more air being burned. The higher pressure is not the cause of the higher efficiency, it's simply a result of more air remaining in the chamber at the end of the compression stroke. Makes sense? Like I said I'm not an expert so someone please correct me if I'm wrong.

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#4

Re: Gas Compression & Combustion Efficiency

02/24/2012 12:35 AM

Tritium, Three factors determine the efficiency of a combustion engine based on the Otto Cycle: combustion efficiency, insulation efficiency, and thermal efficiency. Your question relates to thermal efficiency. The important parameter in thermal efficiency of a heat engine is the expansion ratio of the gas, which is related to but not necessarily the same as compression ratio. Combustion of fuel and air raises the temperature and pressure of the combustion products, ideally carbon dioxide and water for hydrocarbon fuels. The difference in pressure between the combustion products and the gases in the crankcase moves the piston, performing work. As the piston moves in the cylinder away from the cylinder head, the volume in the cylinder increases, and the pressure and temperature of the combustion products decrease, thus trading the potential energy of pressure for the work of turning the crankshaft. The farther the piston can move, the more that the gases expand and the more work that is extracted from the gas, that is, the greater the thermal efficiency. The ideal thermal efficiency is 100% Carnot efficiency, which assumes reversible expansion of the gas (isentropic) with no heat transfer to the environment (adiabatic) to absolute zero temperature and absolute zero pressure, thus extracting every bit of energy available from the fuel and air. In the real world, thermal efficiency is limited to about 72% at an expansion ratio of 472:1 by several physical factors, including the ambient temperature and pressure of the atmosphere and the condensation temperature of steam to water, at which point steam pressure abruptly drops to zero, leaving only the pressure due to carbon dioxide to do work. The expansion ratio of practical engines is further restricted by the auto-ignition temperature of a fuel/air charge. For gasoline engines, the auto-ignition temperature ranges from about 590 F to 720 F, depending on the mixture of hydrocarbons in the gasoline. Under adiabatic compression, a fuel/air charge of 87 octane will auto-ignite at a compression ratio between 8.5:1 and 9:1, depending mostly on ambient air temperature but also on engine speed. Auto-ignition before the piston reaches "top dead center" produces the characteristic pinging sound called "knock". That's why most engines that use 87 octane fuel have a volume ratio in that range. Some engines today, such as the one used in the Toyota Prius, use tricky timing of intake valves to reduce the compression heating of the fuel/air charge while allowing expansion ratios of as much as 12:1, to mimic the effect of a modification of the Otto Cycle known as the Atkinson Cycle. See the Wiki article at http://en.wikipedia.org/wiki/Atkinson_cycle

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#8
In reply to #4

Re: Gas Compression & Combustion Efficiency

02/24/2012 4:19 PM

I think this is quite a good explanation.

Another point worth mentioning is that the energy devoted to compression would be returned (to a large extent) even in the absence of combustion because the air works like a spring. This principal is used in variable displacement engines, to reduce the large pumping losses of a large engine operating at very small throttle openings.

But yes, the expansion ratio is a key to getting the maximum amount of work out of the fuel burn.

leaving only the pressure due to carbon dioxide to do work.

Most of the work is actually done by hot nitrogen (i.e., hot air).

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#5

Re: Gas Compression & Combustion Efficiency

02/24/2012 3:37 AM

I don't know if this is any help but i try when i am attempting to get my head around an engine to visualise it going through its cycles in a cutaway like you see at motor shows or colleges.Taking as read the Higher Compression to power ratio but also trying to approximate valve overlap and profiles/inlet length/valve size effect when the Motor is spinning at high speed.The effect of High Gas/Air/Fuel speed on volumetric efficiency is the key to Power output,obviously valve size,crank throw ect are all important .Try visualising small Generator/Bike engines through to Highly tuned Sports Motors and with practice you can get a "feel" for what you are dealing with.I hope i don't sound insane for this but i have done this for as long as i can remember,I'm no super tuner but i like to think quite intuitive : )

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#6

Re: Gas Compression & Combustion Efficiency

02/24/2012 5:50 AM

My guess is that it's the higher density of fuel times the higher density of oxygen increases the rate of combustion, resulting in more complete combustion at the end of the power cycle.

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#7

Re: Gas Compression & Combustion Efficiency

02/24/2012 9:10 AM

Please be careful not to confuse power and efficiency. As the compression ratio of an IC engine is increased, the power output increases, but the efficiency may, or may not. Also be careful to specify thermal efficiency or volumetric efficiency. In general, the thermal efficiency of an engine isn't nearly as affected by compression ratio as the power output.

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#9

Re: Gas Compression & Combustion Efficiency

02/25/2012 1:49 AM

There are so many good explations are there. I want to add a littile!

At higher compression ratio, the mass flow of air or oxygen is more in the cylinder and the same amount of fuel burns more efficiently in presence of more oxygen. Loss of unburnt fuel is less hence the efficiency improves.

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#10

Re: Gas Compression & Combustion Efficiency

02/26/2012 4:04 AM

Your search for an intuitive interaction with the workings of heat engines is highly commendable. You have received many interesting answers with various intuitive components, but the truth can be looked at several different ways. Here are some basics.

All real heat engines (that is, actually built and working rather than theoretical, imagined constructs in the mind) function with a wide variety of components and actions, all interacting with each other. Some are internal combustion, meaning that the heat is provided by reacting a "fuel", usually a mixture containing hydrocarbons and other components as the reducing agent, with the oxygen present in air as the oxidizing agent in a redox (oxidation-reduction) reaction. There are many different ways that this energy can be used to make mechanical power, as in the Otto and Diesel cycles, Brayton, , Ericsson cycles. The important thing to remember about all these different theoretical cycles is that none of them are accurate descriptions of what happens in a real heat engine from an intuitive standpoint; the internal combustion is not even necessary, it is just a very convenient way to produce power from fuel. A variety of engines can produce power with no combustion at all, just using heat, either heat produced externally, from fuel, or from solar, nuclear, geothermal, or other sources. You need to intuit the thermodynamic transfer of heat energy to mechanical power and understand that transfer intuitively before you can intuitively add in the modifications of all the different engine designs.

Usually there is a working fluid whose physical characteristics change with temperature,even a change of state as in the steam engine, and these changes are used to produce movement, but it is possible to produce mechanical power without a working fluid. Heat can exist in different forms, either electromagnetic radiation, or molecular motion.

A good start for the intuitive approach is an image of a hot object or gas with the molecules moving rapidly, then a second image of the object or gas with the molecules moving slowly. The energy of motion in the first image needs to move away so that you can arrive at the second image. The expansion of gas in a cylinder with the walls of the cylinder moving away, and the fast molecules rebounding from the departing wall, but with the speed reduced, and the distance now greater between molecules, this intuition is valuable. If you play with bouncing balls, or fans or kites in air, or swirl water with a spoon or paddle, you can integrate these feelings into your intuition of how the energy is transferred.

Another good intuition involves understanding the Carnot cycle, which can see the heat as a dammed river, with the water at the top of the dam representing the high temperature gasses, and the water released at the bottom of the dam as the exhaust gasses from the engine. There is still energy in the water released from the dam just as there is in the warm gasses of the exhaust, but there is no way to way to get it out, just as when the water gets to the ocean, it just runs in and can't go any lower.. Using the dam analogy can help to understand why adding the chemical energy of combustion to compressed hot gasses is like adding water to the top of the dam giving it more useable energy, though if you are with me this far, this is Not a direct analogy to the energy of compression and the heat of compression. This is another subject, involving the differences between isothermal and adiabatic compression. Real, actual engine design is limited by chemical and physical facts that have to be taken into account, or gotten around. Till then, W.

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