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  1. #21
    Veteran Member Jerry/MT's Avatar
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    Default Re: Compression Ratio

    Compared to a gasser how hot does the combustion chamber get on a diesel. I believe the flash point for diesel in atomized form is around 480 degrees.


    At 17.5 CR and 60 F inlet temperature, ~1000F ideally. At 22.5 CR, ~1135F.


    A gasser at 6CR a gasser gets ideally 550F before combustion and at 8CR it gets about 662F.

    These are ideal, constant mass in the cylinder, valves closed through the compression process, etc temperatures that are not achieved in real engines but they illustrate the realtive effect of compression ratio on "end of compression" temperature.

    On a real engine, these temps would be lower because of valve timing, real gas effects, (and in the case of an SI engine, fuel air ratio), etc.
    Last edited by Jerry/MT; 10-10-2012 at 09:48 PM.

  2. #22
    Platinum Member Raspy's Avatar
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    Default Re: Compression Ratio

    Direct injection diesel engines can have much lower compression ratios than indirect injection engines with pre-combustion chambers.

    It takes about 16 to 1 to reliably run a diesel, but there's more to the story. An indirect injection engine has a much higher surface area to volume ratio in the injection area. So it's harder to get the compression temperature up and reliably start. So much cold surface and so little distance from the injector to the cold wall. Higher compression helps with this and those engines also need glow plugs. An older Mercedes 5 cylinder is a good example.

    Direct injection has a much larger volume where the fuel is injected so they can start well on lower pressure. A longer radius from the center of the volume to the cylinder or head surface. The air gets hotter during compression because it loose less to the cold wall. A Cummins is a good example.

    Next is turbocharging. A low compression direct injection diesel is better to turbocharge than a high compression pre-combustion design. You can add a lot more air and get a lot of power without going to extreme pressures. All you need to do is get the initial compression pressure high enough to reliably start. Then add a huge volume of air with the turbocharger.

    Smaller engines tend to have very high compression. Probably to combat the problem of the heat radius. Very large engines, like ship diesels have lower compression.

    The pre-combustion design comes from the old hot bulb engines. The design evolved to a design intended to better atomize poor fuels and run cleaner by having a lot of turbulence and kind of a two stage burn. But it's a design that has little value in todays world of high performance and consistent light fuels. Plus they are harder to start.

    In the event that a low compression direct injection diesel won't start on some very cold morning, all it takes is a bit of warmed intake air. But a pre-combustion engine takes a glowing object sitting directly in the combustion area to fire the fuel.
    John

    I only exaggerate enough to compensate for being taken with a grain of salt.

  3. #23
    Platinum Member Raspy's Avatar
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    Default Re: Compression Ratio

    Quote Originally Posted by Jerry/MT View Post
    [B]
    On a real engine, these temps would be lower because of valve timing, real gas effects, (and in the case of an SI engine, fuel air ratio), etc.
    And because of the cold cylinder head temperature.
    John

    I only exaggerate enough to compensate for being taken with a grain of salt.

  4. #24
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    Default Re: Compression Ratio

    Quote Originally Posted by Raspy View Post
    Direct injection diesel engines can have much lower compression ratios than indirect injection engines with pre-combustion chambers.

    It takes about 16 to 1 to reliably run a diesel, but there's more to the story. An indirect injection engine has a much higher surface area to volume ratio in the injection area. So it's harder to get the compression temperature up and reliably start. So much cold surface and so little distance from the injector to the cold wall. Higher compression helps with this and those engines also need glow plugs. An older Mercedes 5 cylinder is a good example.

    Direct injection has a much larger volume where the fuel is injected so they can start well on lower pressure. A longer radius from the center of the volume to the cylinder or head surface. The air gets hotter during compression because it loose less to the cold wall. A Cummins is a good example.

    Next is turbocharging. A low compression direct injection diesel is better to turbocharge than a high compression pre-combustion design. You can add a lot more air and get a lot of power without going to extreme pressures. All you need to do is get the initial compression pressure high enough to reliably start. Then add a huge volume of air with the turbocharger.

    Smaller engines tend to have very high compression. Probably to combat the problem of the heat radius. Very large engines, like ship diesels have lower compression.

    The pre-combustion design comes from the old hot bulb engines. The design evolved to a design intended to better atomize poor fuels and run cleaner by having a lot of turbulence and kind of a two stage burn. But it's a design that has little value in todays world of high performance and consistent light fuels. Plus they are harder to start.

    In the event that a low compression direct injection diesel won't start on some very cold morning, all it takes is a bit of warmed intake air. But a pre-combustion engine takes a glowing object sitting directly in the combustion area to fire the fuel.
    Best post I have read so far.
    Any diesel we modded the static compression was dropped just for the purpose of adding more boost. They were a bear to start but once you got some heat in the kitchen and add about 100+lbs of boost, look out. I wonder what kind of cylinder pressure that was developing.
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  5. #25
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    Default Re: Compression Ratio

    I think what confuses most people is making the mistake of confusing static compression ratio #'s, with effective compression. They are 2 different things. Published compression ratios are only a static numeric ratio of the cylinder volume at BDC, compared to TDC.
    Effective compression is the real resulting compression that results mostly from the valve timing, and with diesels, the injection timing, and or turbo addition. All engines have intake valves open during initial piston upward movements, allowing pressures, and vacuums to be fed back through the intake valves. I general, the higher the intended operating speed, the longer the intake and exhaust valves are held open, and the more time that they are open at the same time.
    It's real obvious with gas engines. This is why a static 8.5:1 engine with a low rpm "torque" cam may require preimium fuel, where an 11:1 static engine, such as a motorcyle engine with a cam designed to run up to 11k rpm's can run on 87 octane, even though it has a static 11:1 ratio.

  6. #26
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    Default Re: Compression Ratio

    Quote Originally Posted by DarkBlack View Post
    I think what confuses most people is making the mistake of confusing static compression ratio #'s, with effective compression. They are 2 different things. Published compression ratios are only a static numeric ratio of the cylinder volume at BDC, compared to TDC.
    Effective compression is the real resulting compression that results mostly from the valve timing, and with diesels, the injection timing, and or turbo addition. All engines have intake valves open during initial piston upward movements, allowing pressures, and vacuums to be fed back through the intake valves. I general, the higher the intended operating speed, the longer the intake and exhaust valves are held open, and the more time that they are open at the same time.
    It's real obvious with gas engines. This is why a static 8.5:1 engine with a low rpm "torque" cam may require preimium fuel, where an 11:1 static engine, such as a motorcyle engine with a cam designed to run up to 11k rpm's can run on 87 octane, even though it has a static 11:1 ratio.
    +1 and to add alot of people don't believe that a naturally asperated engine can exceed 100% VE but that is not true also. CJ
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  7. #27
    Platinum Member Raspy's Avatar
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    Default Re: Compression Ratio

    Quote Originally Posted by DarkBlack View Post
    I think what confuses most people is making the mistake of confusing static compression ratio #'s, with effective compression. They are 2 different things. Published compression ratios are only a static numeric ratio of the cylinder volume at BDC, compared to TDC.
    Effective compression is the real resulting compression that results mostly from the valve timing, and with diesels, the injection timing, and or turbo addition. All engines have intake valves open during initial piston upward movements, allowing pressures, and vacuums to be fed back through the intake valves. I general, the higher the intended operating speed, the longer the intake and exhaust valves are held open, and the more time that they are open at the same time.
    It's real obvious with gas engines. This is why a static 8.5:1 engine with a low rpm "torque" cam may require preimium fuel, where an 11:1 static engine, such as a motorcyle engine with a cam designed to run up to 11k rpm's can run on 87 octane, even though it has a static 11:1 ratio.

    That's really well said and sheds a lot of light on modern gas engine compression ratio numbers.

    Here's a diagram that shows it well and an explanation that came with it.



    http://www.cdxetextbook.com/images/v...ingdiagram.jpg


    Engines: Engine Components: Valves & valve trains

















           

    Valve-timing diagram
    Summary
    The time valves in a 4-stroke engine cycle actually open and close can be measured by angles. These angles can be easily read using a valve-timing diagram.

    To see how valve-timing works in a 4-stroke engine cycle, let’s show piston motion as a circle. In this simple cycle, each stroke is shown as a semi-circle.

    This intake valve opens at top dead center, and closes at bottom dead center. The blue line shows that period and it matches the intake stroke.

    The exhaust valve opens at bottom dead center, then closes at top dead center before the new air-fuel mixture enters the cylinder.

    In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.

    The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.

    When the valves actually open and close, can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle.

    This intake valve opens 12 before the piston reaches top dead center.

    And it closes 40 after bottom dead center.

    The exhaust valve opens 47 before bottom dead center - and stays open - until 21 past top dead center. This gives exhaust gases more time to leave.

    By the time the piston is at 47 before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit.

    When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.

    When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.

    On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33.

    Different engines use different timings. Manufacturer specifications contain the exact information.
    John

    I only exaggerate enough to compensate for being taken with a grain of salt.

  8. #28
    Platinum Member Raspy's Avatar
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    Default Re: Compression Ratio

    Quote Originally Posted by MHarryE View Post
    Actually direct injection is quieter than indirect injection. I had a BMW 328 TD indirect injection when I lived in Europe for a few years and the 320 TD direct injection that replaced it ad more power, better economy, and was quieter.

    The modern high pressure common rail engines are quieter because they inject the fuel in multiple events per power stroke.
    Cummins is a good example of this. The same 5.9 engine was very loud back in the 90's, but in 2003 they started with the common rail system and the engines became much quieter.

    There is a small injection to start the burn, then a larger one for power and then another for emissions. I believe there can be four events in the earlier ones and in the latest ones, up to seven events, per power stroke. Not sure how many.

    VWs are the same. Much quieter with electronic injection. It's not because of the chamber design, it's because of the injection strategy.
    John

    I only exaggerate enough to compensate for being taken with a grain of salt.

  9. #29
    Super Member 94BULLITT's Avatar
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    Default Re: Compression Ratio

    Quote Originally Posted by Raspy View Post
    The modern high pressure common rail engines are quieter because they inject the fuel in multiple events per power stroke.
    Cummins is a good example of this. The same 5.9 engine was very loud back in the 90's, but in 2003 they started with the common rail system and the engines became much quieter.

    There is a small injection to start the burn, then a larger one for power and then another for emissions. I believe there can be four events in the earlier ones and in the latest ones, up to seven events, per power stroke. Not sure how many.

    VWs are the same. Much quieter with electronic injection. It's not because of the chamber design, it's because of the injection strategy.
    That makes them more emission friendly too.
    Roger

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  10. #30
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    Default Re: Compression Ratio

    In the days when communications was still tube type receivers and trasmitters. Worked for a company that had micro-wave towers on mountain tops away from any local power. So used Witte flywheel type diesel engines to generate power for the equipment. 24/365 engines were rebuilt every 5 years.
    If engine failed or was shut off the starting procedure was . Release the compression lever. take crank off wall near parts shelf . spin engine about 5 cranks and rehang the crank on the wall. then close the compression release. engine was running. Now if you should keep the crank in the flywheel in your hand the torque of engine starting would spin the crank hitting the person in the jaw. A lesson learned and never forgot.
    I don't know the compresson but could not turn over with out the release opened.
    ken

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