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spark plugs vs. no plugs the vacuum difference.

Discussion in 'The Garage' started by colbystephens, Jul 27, 2005.

  1. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    I've been mulling over this question for a while and I can't figure out an answer. Why don't diesel motors create a vacuum like that of a gas motor? To my knowledge, the biggest difference between gas and diesel motors is the combustion process, but I can't see why the spark plug/no spark plug difference changes the vacuum process. Care to enlighten me, anyone? (please don't turn this thread into another worthless diesels suck for this reason, gas motors suck for that reason debate.)

    Colby Stephens.
    for the story of my build on my '83 k5 check out www.web.pdx.edu/colbys
     
  2. 74757685k5

    74757685k5 1/2 ton status

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    diesels are at wide open throtle all the time.as far as air intake go's.so not much vacume then.
     
  3. rumbly truck

    rumbly truck 1/2 ton status

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    no butterflys man, how can you have vacuum with nothing to restrict the air flow?Gasoline engines are speed controlled with air flow through the butterflys diesels are controlled through fuel going trough a injection pump or a unit injector
     
  4. tx_sub

    tx_sub 1/2 ton status

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    diesels have to create enough vac to suck air in the cyl during the downward stroke of the piston.
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Diesel intake...



    [​IMG]


    No throttle whatsoever. For comparison, try to recall how much vacuum a gas engine makes at WOT... :rolleyes:

    Rene
     
  6. rumbly truck

    rumbly truck 1/2 ton status

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    And dont forget they also have atmospheric pressure pushing the air in also.Most diesel that require a source of vacuum require a vacuum pump to provide it. A naturally aspirated diesel does suck but not compared to a gas engine.
     
  7. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    A diesel will suck a cat through a screen door, but it still doesn't create vacuum you can use because it has no throttle.

    Vacuum is measured in inches of mercury and is created when you have suction and some form of restriction to the amount that can be drawn. Take the vacuum cleaner...with the hose end wide open it moves a lot of air, and has a lot of suction but doesn't really create any real vacuum (as measured) until you block the end of the hose.

    A gas engine has a 'blockage' so at idle or low throttle inputs it has measurable vacuum. A diesel which has no restriction on the intake has no measurable vacuum.

    Rene
     
  8. Kiwi John

    Kiwi John 1/2 ton status

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    Sucks

    Hi Colby. Diesels do suck as Rene says. One of the dangerous practices people forget is when they are used to petrol engines they run them without an air cleaner when tuning them up or fiddling with carb settings. No big deal. but run a diesel without an air filter and it'll suck the sleeve off your shirt if you get too close!
    Someone else mentioned it earlier: The vacuum is the depression in atmospheric pressure behind the throttle plate. (Use of the ventuuri effect to suck petrol thru carburettor) The vac is convenient to use for the servo assist of brakes. Most diesels run a vacuum pump off the fan belt to do this work. The Blazer runs hydraulic assist to brakes off power steering pump i think is how you describe it.
    Hows the rebore going?
    I tried to look at your rebuild link at web.pdx.edu/colbys but it wouldn't work for me.
     
  9. Mad-Dog

    Mad-Dog 1/2 ton status

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    no throttle plates homey....normally aspirated diesels depend on the intake stroke to pull air into the cylinder, where a gasoline engine depends on intake stroke and exhaust scavenging to draw air into the cylinder.
    This is a why a diesel is considered "un-finished' until it's turbo-charged. :bow:
     
  10. rumbly truck

    rumbly truck 1/2 ton status

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    Yes but when you put your shirt sleeve over the intake, you have just created a butterfly valve and are restricting air flow. A detroit diesel 2 stroke engine has a gmc roots style blower on it stock. It will run with out it turning the blower, but not very good. Its considered naturally aspirated with the blower. Only when it has a turbo feeding it, is it considered blown. Its the magical restricting orfice that determines whether you you have pressure or vacuum any time you have air flow going through a device for what ever application. A gas engine in compression braking will often blow blue smoke. Thats because the vaccum increases and sucks oil past the valve guides. Gas or diesel engines do the same thing, pump heat. Remeber gas engines controll speed with air flow,diesels do it with fuel . hope this helps.
     
  11. Kiwi John

    Kiwi John 1/2 ton status

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    Sleeves

    :bow:
    Yeah, fair enough. i don't have any experience of the Detroit Diesel 2 stroke engine, but all i was warning against was the inadvertant error of loose clothing near the intake when air filter is off. You guys probably don't have to have long sleeved shirts anyway like we have to wear to keep arms warm.
     
  12. rumbly truck

    rumbly truck 1/2 ton status

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    No bash inended. Diesels can have great suction.its a very good idea to have a backup plan to prevent on a runaway on one. A diesel is a oil burner, be it fuel oil or lube oil, they will burn engine good enough to run past the govenor to the point of destruction. And unless you can shut off the fuel or air you have no control. I have seen small front end loader go on its nose and suck engine oil through the pcv system and only the operater having the sense to load the hydraulics and stall the engine, saved it. Having a thick phone book or a thin steel plate with a rubber backing to put on the intake while you are working on a running is a good idea, like you said if the air cleaner is off, so in case you run to problems you can kill it. A diesels mechanical commpression ratio that is 200%-300% greater than a gasoline engine , will provide the potential to have much more suction on the intake track. I have also seen EMD locomotives suck air very restricted cleaners into their blowers many times in severe service in very dirty conditions.They are basically an overgrown DetroitDiesel. to build pressure or vacuum you have to have a restriction what ever its design.hope the info helps out.
     
  13. jarheadk5

    jarheadk5 1/2 ton status

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    [mild hijack]

    Hey rumbly truck, do you work on those 5000hp EMD locomotives in Utah?

    [/ hijack]
     
  14. rumbly truck

    rumbly truck 1/2 ton status

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    Ive never worked on locomotives that big before. Been on plenty of switchers, gps, a few sds , some baldwins, and even a few alcos. working on stationary power now, some big ugly 10,000 hp units.
     
  15. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    thanks so much for all of your input. sorry i didn't respond sooner - my access to the internet is somewhat limited. it's very much appreciated. kiwi john - i don't know why the site wouldn't work. i appreciate your interest. i know that a couple of the pictures aren't showing up properly, but i'm trying to get that fixed. in any case, the rebuild is slow at the moment as I'm trying to save money for the new pistons and rebuilding the heads. currently i'm just dinking around trying to prep/fix up the chasis without spending much money.

    colby stephens.
    www.web.pdx.edu/colbys
     
  16. rodney chambers

    rodney chambers Registered Member

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    naturally aspirated/turbocharged/blown deisel engines have virtually no inlet air restriction. engine speed is not controlled by throttle plate(butterfly). typical flyweight governor or electronic system is used. since no inlet air is restricted no vacuum is created in the engine.
     
  17. Kiwi John

    Kiwi John 1/2 ton status

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    Hi Colby. Like you, my internet access is limited There's some good stuff for you on the Diesel forum part of the site too. I PM'd one of the moderators the other day to try and get you directd there but not sure if that worked.
    I tried your web address again but still no joy, maybe a problem at my end - perhaps cos i'm not in USA? Lots of guys with lots of experience in this community so i'm enjoying and learning as we go too.
    Best wishes for your rebuild. Keep in touch. John
     

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