Question: Turbo Diesel Engines working

Discussion in 'Basic Guides' started by Codename.47, Aug 2, 2011.

  1. Codename.47

    Codename.47 Guest

    Can any one explain to me about the Diesel engines that are turbocharged.
    What are the differences in them compared to a petrol turbo charged engine.
    And other technical information related to that.
    thanks
     
  2. deep

    deep New Member

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    basic difference between diesel engine and petrol engine is that compression ratio in petrol engine is 8:1 and diesel engine is 16:1 , there is no spark plug needed in diesel engine as fuel is compressed till it ignites on its own.
     
  3. Dhhawal

    Dhhawal New Member

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    The Science

    umm there is a little more difference than that....in the petrol engines of old...the combustible mixture of the air and fuel were compressed which made the carbuerettor quite complicated, with the advent of MPFI only air is inducted and compressed and introduced into the manifold, fuel is injected right at the opening of the intake valve, the mixture is compressed in the compression cycle and is ignited once the piston passes TDC, with a diesel engine the induction of air is the same, only air is compressed in the turbo and is introduced in the manifold here is where the difference starts, only air enters the cylinder and is compressed to a relatively high compression ratio(turbo diesels have a lower compression ratio as compared to naturally aspirated), at the end of the compression stroke, diesel is introduced at high pressure(to aide in better atomisation of the fuel)into the cylinder via an injector and it self ignites due to the high temperature of the compressed air, diesel then continues to be injected till the cylinder reaches BDC, this constitutes the power stroke..! exhaust strokes are much the same..! this is the mechanism...now the other part...because of the higher compression ratio, diesel engines have higher volumetric efficiency(more mileage) but are required to be more robust and strong and so, are heavier..and more expensive to produce, also because of the high compression required the have long strokes and so they have higher torque figures! the reason why diesel engines are easier to turbo charge is that in diesel engines, knocking(self ignition) is essential and compression aides self ignition, whereas in the case of petrol engines, knocking is detrimental to the life of the engine and results in los of power and so it makes it more complicated for a petrol engine to be turbo charged..however in a few years time...gasoline direct injection will make its way through and turbo petrol engines will be all over the place as turbo diesels are..!
     
  4. Dhhawal

    Dhhawal New Member

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    to illustrate it a lil better here are some animations
    petrol engines

    [​IMG]

    and diesel engine

    [​IMG]
     
  5. Codename.47

    Codename.47 Guest

    I wasnt really talking about working of Diesel and petrol engine
    I was asking about the TURBO CHARGING of a diesel engine

    Turbo charging a diesel engine makes it more efficient I read that somewhere
    I am not quiet sure is that similar to petrol turbocharger
    that includes a Intercooler , turbine, compressor and a blow off valve or is it different
     
  6. Dhhawal

    Dhhawal New Member

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    its very important here to stress one thing....self ignition is caused by the increase of pressure and temperature...because fuel is already present when it is compressed in the case of petrol engines self ignition is possible, and ignition before TDC will act against the momentum of the crank(affecting engine life), in the case of a diesel engine there is no fuel present when compression occurs so ignition can't take place, fuel is injected according to the position of the crank which is something that can be controlled, this begs the question...why can't we inject petrol like in the case of a diesel engine?, two things, firstly its to do with the property of the fuel; diesel has good tendency to self ignite, petrol does not(what we refer to as octane number,, higher the octane number the more it will resist self ignition, thats why we use higher octane fuel in turbo petrol engines) and the other thing is that petrol has better combustion when the mixture is homogenous, diesel has better combustion when the mixture is heterogenous...!
     
  7. Dhhawal

    Dhhawal New Member

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    @rahul, yes that is true, because when you compress air...air becomes hot, it needs to be cooled down because you don't want the engine to knock..in the case of a diesel engine..you want it to knock..! thats what the clatteing sound is...! so in the case of a petrol engine you need to cool the incoming charge by a large margin..whereas in the case of a diesel engine you can use a smaller intercooler, as for the blow off valve, you don't want excess boost pressure because more air needs more fuel..if more fuel is not present..it will result in a lean burn...or backfiring..so the pressure in the manifold needs to be monitored correctly, diesel engines can run much leaner and aren't high revving(when the blow off is really needed cuz the turbine is rotating at high rpm at high engine speeds)..most of the power is produced in the lower end of the power range..! meaning to say...the hardware required turbo charging a diesel engine is much lesser than that of a petrol engine..!
     
  8. Codename.47

    Codename.47 Guest

    thats makes sense .
    thanks for that bro

    All the turbo diesel engines have the setup that turbo kicks in at lower RPM if am not mistaken and it cuts off at certain level ?

    Turbo charging a diesel engine is cheaper but Complicated?

    I would imagine if we were to super charge a diesel engine.. its like not possible .
     
  9. Dhhawal

    Dhhawal New Member

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    turbo charging a diesel engine is easier and gives greater benefits..thats why its so widespread...supercharging a diesel engine wouldn't serve the purpose they develop all their power in the lower part of the rev range..!
     
  10. Codename.47

    Codename.47 Guest

    bhai i need visuals to understand the turbo charging principles for diesel engine

    I still dont understand some parts of it

    how the intake manifold will keep up with the excess pressure.
    what will happen to cylinder since its already compressing too much of the air (1/16)
    Boost lag ? does that happen to these engines.
     
  11. Dhillon

    Dhillon Administrator Staff Member

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    What about trubo lag, is it more in a Diesel engine?
     
  12. Dhhawal

    Dhhawal New Member

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    lag is an inherrent weakness of any turbo(not as much as in the case of a VGT), but in the case of a petrol engine...the power and torque are produced in the upper part of the rev range thats when it needs most of the boost, and so you need a larger turbo(more turbo lag) to provide it..and you need a blow off valve to let off excess pressure when you change gear..! in the case of a diesel engine, torque is produced in the lower part of the rev range..and the boost can be provided with a smaller turbo which also has less turbo lag, because of the higher pressures involved in the operation of a diesel engine...the exhaust pressure increase is more immediate, and the torque counteracts any pumping losses at low rpm, so turbo lag is minimal...turbo applications in diesel engines if far more favorable and less demanding...! the newgen gasoline direct injection engines are showing great potential as far as turbo charging is concerned
     
  13. Codename.47

    Codename.47 Guest

    See I learn new things everyday thanks bro
     
  14. Dhhawal

    Dhhawal New Member

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    the intake manifold is under pressure minimal pressure because of a smaller turbo and is not affected by the compression inside the cylinder, turbo charged diesel engines have a slightly lower compression ratio...in the region of 15:1 because of the turbo feeding it more air...!

    what do you think your skyline was doing when it was running at max 8psi boost? at max boost you were squeezing in more air at 8pounds per square inch gauge pressure inside the manifold which in absolute pressure terms(atm pressure=14.7psi) becomes 22.7psi, 22.7/14.7 gives 1.54, 1.54 pressure ratio multiplied by the displacement of your engine(2.5L) gives your actual amount of air going into your engine =3.8L, also if the compression ratio of your engine was originally 8:1, then at max boost your dynamic compression was 12.32:1
     
  15. Codename.47

    Codename.47 Guest

    Good to know abt diesel engines

    Wow that was really simplified techincal information
    Well to be honest skyline had a lot of turbo lag. 1st gear 7 RPM 60kmph then as soon as I change the shift car dropped to 50-55 kmph in 1/10 of seconds and then it would pick up
     
  16. Dhhawal

    Dhhawal New Member

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    hehe...dude give it some credit it was introduced in '93, turbo charging was still considered rocket science..!
     
  17. Codename.47

    Codename.47 Guest

    yeah I would say so too :p
     

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