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Thread: To Compress or Not Compress?

  1. #1
    Join Date
    Aug 2019
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    Washington
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    Default To Compress or Not Compress?

    A month ago or however long ago it was I was starting my tractor. I get the pony rolling and snap the pinion clutch in place to get the diesel turning.

    I decide I don't want to stand and wait for the diesel to get oil pressure, so I walk away for a few minutes with the diesel cranking without compression. I figure I would come back and turn on compression and have it crank for the obligatory 3-5 minutes or more necessary to start with compression.

    When I finally go to my tractor, I pop the compression lever and the diesel already sounds ready to pop off. I open the throttle and the diesel fires right off.

    So it leaves me wondering, does the diesel warm up faster without compression?

    I will admit that I believe the weather outside was a bit warmer than the last time I started it.

    I haven't exactly tried to officially experiment to see if the diesel warms faster without compression, but it is something that has me curious and something that does make some sense to me if it is true.

    It seems like the life of a pony motor could be extended if it doesn't have to spend the majority of running time turning the diesel under compression.
    Cat D6 5R

  2. #2
    Join Date
    Nov 2017
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    692

    Default

    i usually don't need more than about 10 seconds of compression to start my d4 in cold weather. after cranking just until i get oil pressure.
    D4 6U,1943 D4 2T,Laplant Choate C41 Carrimor Scraper

  3. #3
    Join Date
    Jan 2007
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    Moriches NY, Hancock Ma
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    compressed gases always are warmer than uncompressed---simple thermodynamics where the work done energy is transformed into heat energy--PV=NRT
    Cat 941B, Cat D2 4U, Cat D3B, Cat D4 7U, Cat D6 9U

  4. #4
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    Aug 2019
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    Washington
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    Quote Originally Posted by dpendzic View Post
    compressed gases always are warmer than uncompressed---simple thermodynamics where the work done energy is transformed into heat energy--PV=NRT
    Yes, but when the compression lever is engaged, all of the warmer gases will head out the exhaust on the exhaust stroke. With the compression released, the warmer gases can travel through the intake and into another cylinder. Thus, you are keeping warmer air in the engine for a longer amount of time rather than constantly adding cooler air.
    Cat D6 5R

  5. #5
    Join Date
    Nov 2006
    Location
    Faunsdale, AL USA
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    Default

    The workload on the pony motor increases when I flip the decompression off/run, so that convinces me that more heat is being produced not only in the diesel cylinders from the heat of compression, but also going out the pony exhaust as waste heat. For those engines that have the heat exchanger in the intake manifold, itís almost certainly heating up both the incoming air and the combustion chambers more than with the decompression on.
    D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time

  6. #6
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    May 2007
    Location
    Watlington, England
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    Default

    There is also all the heat generated in the common cooling system.
    John Gaunt ACMOC Director
    1941 D7 7M
    1940 D4 7J
    1940 D2 5J
    1940 R2 4J

  7. #7
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    Dec 2019
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    Pouce Coupe, BC Canada
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    Default ? ???????????????????????????

    Quote Originally Posted by gauntjoh View Post
    There is also all the heat generated in the common cooling system.
    how would heat created in the common cooling system help?? eventually it would.....but..............

    wouldn't heat in the cylinder/head be better to support actual firing of that cylinder...in short order...

    -how much time do you got....
    Caterpillar D6C 99j

    Tax me, I'm Canadian

  8. #8
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    Nov 2006
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    Faunsdale, AL USA
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    How cold is your engine?
    Below some temp, depending on what type of oil you use, warming the whole engine will make a big difference in actually getting it to run. Not to mention wear on bearings etc until it gets warmed up and oil is flowing well.

    I have wondered if the occasional attempts by Caterpillar to have the pony motor share lube oil with the main engine wasnít to get the oil warmed up faster. Of course, later models with oil to coolant heat exchangers warm the oil without the oil cross contamination problem.
    D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time

  9. #9
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    A cheap no touch thermometer should tell the story of which way the head heats faster.

  10. #10
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    Default

    I am not sure it is reasonable to expect the pony to warm up the coolant enough to make a significant difference when starting. You'd probably have to run the pony for an hour just to get a significant temp increase in the coolant.

    My thoughts are that the air doesn't all escape through the exhaust and instead may cycle back through the intake manifold with each stroke of the pistons. Thus, the warmer air continues to get warmer in the heat exchange tube instead of just completely fresh and cool air being sucked in and then blown out the exhaust.

    A common oil system would be nice, but I think that would involve adding an oil pump to the pony.

    My pony has a water pump, but I actually do question if the water pump is necessary considering that once you engage the starter pinion the diesel water pump starts pumping. So a person might be able to create a new oil line that drains into the pony sump and have an exit line that comes out of the pony oil drain and back into the diesel sump.

    It's all interesting to think about.
    Cat D6 5R

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