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Pressure vs Density in diesel

  1. Feb 26, 2013 #1


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    I have been searching for hours and have conflicting data. Basically if engine A has 18:1 CR and 0psi boost and engine B has 13:1 CR and 8.5psi boost, the pressure at TDC-Compression is the same or rather the "effective CR" is the same. However, engine A is 16.7% hotter at TDC but has 43.7% less moles of air. On gasser forums they seem to concur that effective CR is the ticket. On diesel forums they concur that moles of air (density) that gets into the combustion chamber is the ticket. Being that fuel needs air to burn and that there is a 43.7% difference in the amount of air between the two, I would think engine B would have much more power. I realize the higher CR means it needs less boost because the energy density being in such a small area gives it a kick in efficiency that does the same as low compression high boost. But what is the truth? Thanks for any help guys.
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  3. Feb 27, 2013 #2


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    The advantage to supercharging an engine (either SI or CI) is that more air is delivered to the combustion chamber than can be obtained by natural aspiration. Additional air ensures that complete combustion of the fuel can be obtained, making the engine more efficient and increasing power output.

    In a CI engine, the temperature in the cylinder at fuel injection must be higher than the ignition temperature of the fuel. In a supercharged engine, the air will be entering at a higher temperature than ambient, and less compression of the air is required to bring its temperature above that for the fuel to ignite. Lower CR means that the mechanical parts of the engine can be engineered lighted than similar parts in a naturally aspirated engine, which must use higher CR.

    In SI engines which are supercharged, the CR is lowered to prevent predetonation of the fuel.
  4. Feb 27, 2013 #3


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    I did not ask what supercharging does and I did not ask about temperature of ignition. I asked about the truth between effective compression vs. air density, because different forums are saying one is better than the other and contradicting each other. And this is on a CI engine so I don't care about a SI engine, I know it has detonation things but that is not my concern on a CI engine.
  5. Feb 27, 2013 #4
    Well it's best to not even think of the term effective compression. As having more air compressed less, is not the same as having less air compressed more.

    More air = more fuel = more power.
    More compression = more thermal efficiency.

    High pressures in the cylinders mean you need a stonger head (this is where total cylinder pressure matters). As there are limits to practicality you trade one off against the other.

    Though without a specific question, or two specific statements to compare. There's not much more than can be said. Those toughts on the forums are both, in a sense, correct; as they are mirroring what is happening in the industry.

    Petrol engines are moving to direct injection with an increase in compression ratio. <Trying to find a good exmaple apart from the Skyactiv-G>
    Diesels are lowing the compression ratio and increasing boost to improve power and help with emissions, such as Mazda's Skyactiv D.

    Basically both class of engines are converging.
    Last edited: Feb 27, 2013
  6. Feb 27, 2013 #5


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    They specifically said on petrol forums that having one engine with one compression ratio and the same engine with lower CR but more boost which matches effective compression, would have about the same power. So that would mean the thermal efficiency alone on the high compression engine can have the same output as the engine with more fuel/boost and lower CR. That is what I am trying to determine as nobody seems sure.
  7. Feb 27, 2013 #6
    I'm still not sure I know exactly what you are asking....I don't understand why you did not like Steamking's post....what do you think an 8.5 psi 'boost' is?? As I interpret those posted descriptions, he is absolutely correct.

    Depending on the combination of compression and boost, the efficiency and outputs could be comparable..about the same...but that doesn't seem practical....why aim for THAT? [see below]

    If all the fuel is being used in combustion, that is, not a lot of smoke is produced in one engine, [combustion is 'complete'] adding more air doesn't have much effect...but maybe one could argue the burned [consumed] air and exhaust products from the prior combustion stroke is better eliminated with more air flow.
    I think I have seen that regarding Roots blowers...see below.

    I'm not sure I know what 'effective compression' is but I sure know you have to have the right combination of air and fuel for full combustion. In general, colder air is more efficient, produces more power, than hot air. That's true for both gas and diesel engines....you get more expansion of the air/fuel mixture. That's why some high performance engines, drag racers,maybe, use dry ice to chill combustion air and why two cycle turbo charged Detroit diesels [DD] use an inter-cooler to reduce the ambient temperature of combustion in turbo charged engines. [In marine applications, the combustion air is cooled via sea water thru enclosed SS 'radiators'...Two per engine,$1,000 each material cost, one per four cylinder bank, when I had mine!!!! So cooling combustion air is apparently worth that additional cost for improved fuel efficiency!]

    The two cycle DD engine marine engine specifications from the manufacturer say that an 8V71 [ 8 cylinder, 71 cu in per cylinder] natural aspirated [NA] engine [with a roots blower] develops 350 BHP at 2300 RPM and utilizes a compression ratio of 18:1; at the same rated RPM, a turbo charged and inter-cooled model [TI] of that engine, the 8V71TI produces 435 BHP also using a Roots blower and the stated compression ratio is 16:1, if I recall correctly. The fuel consumption of the two engines are different: DD specs show the 8V71TI gets about 10% better fuel economy; the more powerful TI model uses 60mm injectors...I forget the injector specs of the NA engine, but smaller of course.

    The NA engine also comes in a variety of packages including 'work boat' and 'crew boat' versions: In these, smaller injectors and lower RPM's reduce the available output power for longer life and may be rated as 'continuous duty'...meaning they can be run at the rated RPM [say 1600 and 1800 RPM respectively] all day everyday and still achieve long life.

    Hope those examples help.
    Last edited: Feb 27, 2013
  8. Feb 27, 2013 #7


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    I am looking for the principles behind it. Effective compression ratio is like the engines in the original example. Engine A has 18:1 and engine B has lower compression but the boost brings the TDC compression pressure to the same pressure as engine A, effectively making it seem to have an 18:1 CR.

    When you mentioned the cold air expanding more thing, you were starting to talk about what I want. I want to know the technical aspects that make engine A different from engine B, because as far as all the car forums seem to go, they see them as the same engine because the effective pressure is the same. I think there is a lot more to it but I haven't been able to find much. I know how all the basic principles work, I want the finer principles of how the 2 scenarios make power in comparison to each other. I do know the turboed intercooled engine runs cooler because the air is cooled whereas high compression engines have no way to cool air that is in the combustion chamber, other than water injection I guess. The main huge problem I see in their arguments is the 43.7% difference in air density which they seem to ignore. I know that means you can have more fuel but if fueling is kept constant, as in the throttle on a diesel is just clamped in a certain spot, which engine would make more power and why. The engine with 18:1 and no turbo, or the engine with 13:1 and 8.5 psi intercooled boost.

    I'm sorry if I am not explaining it very good.
  9. Feb 27, 2013 #8

    My GUESS is you'll likely have to find somebody who actually designs engines...I suspect such experts have a framework of theory and experience from which they make informed guesses about what might work.....within, say, the confines of materials and fuels economically available. Then my GUESS would be they try different experimental combinations and make trade offs...and pick some combination that they think will work. Some do, some don't.
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