Air pressure during Combustion

Replies

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  Shashank Moghe

  Well, yes. Such engines are called forced induction (Supercharged or Turbocharged) engines. You must be aware of the typical combustion curve. The four-stroke curve. Imagine the starting Pressure (P1) higher than a naturally aspirated engine pressure - the pressures attained after compression and hence the peak pressure after combustion - both are increased, resulting in better volumetric efficiency, thermal efficiency and more power output. Here is a random simulation PV diagram for a naturally aspirated versus boosted (forced induction) engine cycle, I found on the internet. The boost is 3 bar, here:
  
  
  
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  The limiting factors on the boost are:
  
  1) The increased temperature of the air intake charge: Boost becomes a problem at high pressures, as they might cause knocking and hence, subsequent pre-ignition.
  
  2) Higher peak temperatures during combustion means higher NOx emissions, which are undesirable.
  
  3) The components of the engine have to withstand higher pressure and temperatures, hence need faster replacement/repair than naturally aspirated engines.
  
  I had to go through certain online pages (wiki and random articles) to get updated with these concepts. Again.
  
  Hope this helps, keep this discussion going!
• 

  Sarathkumar Chandrasekaran

  Great thread. We want tp discuss it in detail!!!
  What will happen if we limit the air pressure and increase the input air temperature?
  Will we able to attain similar result?
• 

  Velshankar MJ

  That depends on the volume of the chamber....... Heating will only increase problems as we are already using intercooler for turbocharged engines to decrease some temperature of intake air.
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  Shashank Moghe

  Well, we increase the pressure in a turbocharged/supercharged engine by using compressors. But that automatically increases the temperature of the charge. Hence we use intercoolers to reduce the temperature, but let the density remain, as the whole idea is to have a denser intake charge.
  
  But on the contrary, if we increase the temperature of the intake charge, the pressure sure increases (if the volume is fixed), but the density does not.
  
  Thus, compressor increases the density of the charge (undesirable temperature rise is taken care of by the intercooler), hence serves the purpose. On the other hand, heating the charge does not increase the density (only increases the temperature and pressure), which does no good.
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  Velshankar MJ

  But if we increase the pressure i think the atoms of air molecules will pack together tightly thereby increasing the density..... Because the volume is also fixed...
• 

  Shashank Moghe

  If we increase the pressure by compressing the gas (reducing the volume for the same mass of gas), the density will increase.
  
  But if we just provide heat to a fixed mass of gas within a fixed volume, density (mass/volume) remains the same, although the pressure increases (due to increase in translational and vibrational energy of the gas molecules).
• 

  n1kh1l

  Turbocharging is used to increase the mass of air in combustion chamber. With increased mass of air we can burn more fuel and so get more power.
  The limiting factor for turbocharging pressure is the peak pressure obtained in the cycle.
  A very high peak pressure will stress the cylinder head bolts more also the bearing load will increase.
  But peak pressure does not only depend upon turbocharger pressure. Fuel injection timing or spark timing also affects it.
  In big marine engines a cylinder safety valve is provided to relieve the pressure.
• 

  Shashank Moghe

  Of course. Spark and Injection timing need to be retarded with boost pressure to reduce the probability of knocking, I believe. With boosted pressure, the injection timing needs to be retarded to stop the engine from knocking. As such, the spark timing needs to be retarded as well, to allow for mixing and ideally complete vaporization of the fuel. Since the charge is boosted, the retarded spark timing also reduces the probability of knocking.
  
  The spark timing is around 40 - 20 degrees BTDC (for naturally aspirated automotive gasoline engines, is this the same for marine engines?), #-Link-Snipped-# do you have any stats for the spark timings on any of the boosted/unboosted marine variants? I suppose (most ?) marine engines are diesels, so a comparison is invalid, just asking for information purposes.
  
  
  P.S: Most importantly, I forgot to mention the maximum boost will also very strongly depend on the Octane rating of the fuel used. Lower the Octane rating, lower is the allowable maximum boost.
• 

  n1kh1l

  #-Link-Snipped-# Marine engines dont use spark ignition, they are of compressed ignition type. Generally fuel timing is about 10-12 deg BTDC. But this is not fixed. From about 85-100% load the injection timing is altered to give max peak pressure. This is called as VIT or variable injection timing. Also as you said correctly peak pressure also depends on octane rating or octane number for petrol engines and cetane number for diesel engine. There is a seperate setting called FQS or fuel quality setting to take into account different fuels.
  I you want to have further information about FQS or VIT google MAN B&W VIT
• 

  Shashank Moghe

  That helped a lot. Also, are the marine diesels turbocharged?
• 

  n1kh1l

  Shashank Moghe

  That helped a lot. Also, are the marine diesels turbocharged?

  Yes all marine engines are turbocharged. One advantage of turbocharged engine is that power to weight ratio is higher than normally aspirated engines.
• 

  Shashank Moghe

  Yes, that is a given. Turbocharged engines are better at delivering the power density than NA engines. One good thing about marine diesels is that there is no concern about the turbo lag (it is one of the concerning factors in automobiles - driveability wise), and hence turbocharging makes more sense in marine engines. Also, the exhaust sound is dulled in automotives, which calls for artificial firing noise production (they say its still not the same as NA engines) - again, this is the least of concerns in a marine engine.
• 

  n1kh1l

  Shashank Moghe

  Yes, that is a given. Turbocharged engines are better at delivering the power density than NA engines. One good thing about marine diesels is that there is no concern about the turbo lag (it is one of the concerning factors in automobiles - driveability wise), and hence turbocharging makes more sense in marine engines. Also, the exhaust sound is dulled in automotives, which calls for artificial firing noise production (they say its still not the same as NA engines) - again, this is the least of concerns in a marine engine.

  Turbo lag is present in marine engines. For engines susceptible to turbo lag like generator engines an impulse turbocharging is provided while for other engines a constant pressure turbocharging is provided.
  Also two or more turbochargers are used in series or parallel mode to have a high turbocharger efficiency at reduced loads.
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  Shashank Moghe

  Yes, I understand that Turbo lag is present in marine engines, its just that it must not be as much of a deterrent as it is in automotive engines. Besides, for low RPM operation, does the recent marine engine use the Supercharger to substitute for the Turbocharger as do some of the latest automotive engines?
• 

  n1kh1l

  Shashank Moghe

  Yes, I understand that Turbo lag is present in marine engines, its just that it must not be as much of a deterrent as it is in automotive engines. Besides, for low RPM operation, does the recent marine engine use the Supercharger to substitute for the Turbocharger as do some of the latest automotive engines?

  I think I get this right that turbocharger is driven by exhaust gases and supercharger by engine via a belt and pulley or any other arrangement.
  For low load operation marine engines use auxiliary electric driven blowers or some small engines are provided with a compressed air connection from ships system.
  Efficiency wise only constant pressure turbochargers are susceptible to low load operations while impulse turbocharger are efficient at low load too.
  Now with multiple turbocharger setup in some engines one turbocharger is designed for low load operation and other T/C for high load ops.
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  Shashank Moghe

  Yes a supercharger uses the power at the shaft to run the compressor. Can you mention any links to any of the marine engines for me to read? It should be an excellent read.
• 

  n1kh1l

  Shashank Moghe

  Yes a supercharger uses the power at the shaft to run the compressor. Can you mention any links to any of the marine engines for me to read? It should be an excellent read.

  Google MAN B&W or Sulzer. They are the major makers of marine engines. Also try Mitsubishi. Their websites have good knowledge base.