Can someone explain to me turbo boost....
Discussion
I never understood what 1 bar means or 14 Psi or anything like that. Can someone tell me what the Esprit has and what it means? Thanks
Hi,
A Bar is a unit of pressure measurement which is equal to 100,000 Pascals. It was first 'coined' in Germany in 1910 and comes from the Latin 'Baros' which means pressure. Other equivalents of 1 Bar are: 14.5PSI, 750.1 mm Hg, 33.46 FT water, 100kPa.
The Esprits have many different boost pressures. The early 'G' model non-HC engines used .55 Bar or roughly 8 PSI, the SE uses .655 Bar or roughly 9.5 PSI., the S4 uses 1.0Bar or roughly 14.5 PSI. This is the pressure the turbocharger is allowed to make before opening of the wastegate and releasing any additional pressure created. This pressure is achieved by compressing greater volumes of air of static pressure. So, in other words, you are pushing more air (density) into the engine with each intake stroke. If too much pressure is forced into the cylinder, the air/fuel charge will reach it's detonation threshold and ignite early, before the piston has finished it's compression stroke or the spark has fired. This is known as pre-ignition, pre-detonation, detonation or 'knock'.
This is due to what is termed 'adabiatic heating'. Any gas will increase in temperature when compressed. This heating is equivalent to approx. 11.7°F for every 1.0lb. increase in pressure. So, in other words, boosting a charge to 10 PSI will increase it's temperature 120°F, and this is in addition to it's static temp. So, if you are driving on a 95°F day, you will add 120°F to the air which is already 95°F for a total charge temp of 215°F. This can get close to, or exceed, the detonation threshold of the mixture.
Turbocharging is desireable because it forces much more air into the cylinder. This burns the fuel more completely and releases more power from the fuel being burnt. But, too much pressure and any advantage is defeated by detonation.
To combat this, Turbocharged engines are often lower in compression, timed differently, control the amount of boost via a wastegate and/or an intercooler is often used to keep the fuel/air charge below it's detonation threshold
But, remember, that this pressure is in addition to the static pressure of 1 ATM, 14.7PSI or 1.013 Bar at sea level. This static pressure, of course, drops as one rises above sea level. Hope this helps...Jim '85TE
lotusguy said:
I never understood what 1 bar means or 14 Psi or anything like that. Can someone tell me what the Esprit has and what it means? Thanks
Hi,
A Bar is a unit of pressure measurement which is equal to 100,000 Pascals. It was first 'coined' in Germany in 1910 and comes from the Latin 'Baros' which means pressure. Other equivalents of 1 Bar are: 14.5PSI, 750.1 mm Hg, 33.46 FT water, 100kPa.
The Esprits have many different boost pressures. The early 'G' model non-HC engines used .55 Bar or roughly 8 PSI, the SE uses .655 Bar or roughly 9.5 PSI., the S4 uses 1.0Bar or roughly 14.5 PSI. This is the pressure the turbocharger is allowed to make before opening of the wastegate and releasing any additional pressure created. This pressure is achieved by compressing greater volumes of air of static pressure. So, in other words, you are pushing more air (density) into the engine with each intake stroke. If too much pressure is forced into the cylinder, the air/fuel charge will reach it's detonation threshold and ignite early, before the piston has finished it's compression stroke or the spark has fired. This is known as pre-ignition, pre-detonation, detonation or 'knock'.
This is due to what is termed 'adabiatic heating'. Any gas will increase in temperature when compressed. This heating is equivalent to approx. 11.7°F for every 1.0lb. increase in pressure. So, in other words, boosting a charge to 10 PSI will increase it's temperature some 120°F, and this is in addition to it's static temp. So, if you are driving on a 95°F day, you will add 120°F to the air which is already 95°F for a total charge temp of 215°F. This can get close to, or exceed, the detonation threshold of the mixture very early in the compression stroke which adds further compression and thereby heat.
Turbocharging is desireable because it forces much more air into the cylinder. This burns the fuel more completely and releases more power from the fuel being burnt. But, too much pressure and any advantage is defeated by detonation.
To combat this, Turbocharged engines are often lower in compression, timed differently, control the amount of boost via a wastegate and/or an intercooler is often used to keep the fuel/air charge below it's detonation threshold
But, remember, that this pressure is in addition to the static pressure of 1 ATM, 14.7PSI or 1.013 Bar at sea level and 70°F. This static pressure, of course, drops as one rises above sea level. Hope this helps...Jim '85TE
>> Edited by lotusguy on Friday 3rd January 03:20
LotusV8 said: Thanks! A bit technical and confusing at 1st read but I few times over and I got it. Are there any cars with 2 bars? which would be, let me see, 29psi of boost. Can you or would you even want to tweek the boost on an Esprit to more?
Sorry for the technical nature of my answer, but your question was really technical in it's nature.
As far as your new question, again forced induction (Turbocharging/Supercharging) is a trade off, in fact, many trade offs. Realize that at 2 Bar you are now raising the charge temp some 320°F over the ambient air temp. This may be sufficient in some cases to cause combustion before the charge even gets to the cylinder - VERY BAD! Remember that heating a gas causes it to expand and lowers it's density, but you want a very dense charge so you compress it to get more air in a given volume. Adiabatic heating at such high compression can cause the air density to drop at a rate greater than the density gained by compressing the air. Soon, all you are doing is thrashing the air about.
So, you would have to cool it using an intercooler and in the case of 2 Bar, you'd definitely need a water/air intercooler. Now you have added the complexity and weight of the coolant, plumbing, heat exchanger and pump. Since there is a time component to cooling the charge (heat transfer occurs based upon the time the air is in contact with the heat exchanger and at wide open throttle, the air coming from the turbo is travelling in excess of 400 Ft./sec.), your heat exchanger would need to be enormous to keep up with the load, a definite packaging problem.
Add to that the fact that the engine would need to be practically hermetically sealed to contain all the pressure and what you suggest is simply not practical. Some Top Fuel Dragsters approach these limits, but also use Nitro to cool the charge and the life expectancy of these engines is measured in minutes, not miles.
The best solution rather than increasing boost so dramatically, would be larger displacement or Nitro. Engine buiders have gotten much better at buiding fuel-efficient naturally aspirated engines in somewhat larger displacements. The trend hasn't gone so much from larger Turbos, as it has from Turbocharged 4 cylinders to non-turbo V-6's. Bigger displacement engines now provide power and economy equal to older, smaller, turbo engines. Naturally aspirated low-end torque is improved with tuned-port 'dry' EFI manifolds, in which the fuel is directly introduced by individual port injectors at each intake valve allowing the 'dry' runners of the plenum intake to be optimized for air-flow considerations rather than for concerns for keeping gasoline suspended in the air. Multi-valve technology has matured and high levels of specific horsepower (Power/in.³) are now available from naturally aspirated engines without the complexity of turbocharging. Hope this helps...Jim '85TE
>> Edited by lotusguy on Friday 3rd January 16:29
>> Edited by lotusguy on Friday 3rd January 16:39
lotusguy said:
If too much pressure is forced into the cylinder, the air/fuel charge will reach it's detonation threshold and ignite early, before the piston has finished it's compression stroke or the spark has fired. This is known as pre-ignition, pre-detonation, detonation or 'knock'.
I was led to believe that pre-ignition (sometimes called pinking) was different to detonation. Pre-ignition being caused as described above but also by local hot spots in the cylinder head. Detonation, on the other hand, is caused by shockwaves as the flame front speed becomes supersonic.
I could be wrong though...
ro_butler said:
lotusguy said:
If too much pressure is forced into the cylinder, the air/fuel charge will reach it's detonation threshold and ignite early, before the piston has finished it's compression stroke or the spark has fired. This is known as pre-ignition, pre-detonation, detonation or 'knock'.
I was led to believe that pre-ignition (sometimes called pinking) was different to detonation. Pre-ignition being caused as described above but also by local hot spots in the cylinder head. Detonation, on the other hand, is caused by shockwaves as the flame front speed becomes supersonic.
I could be wrong though...
ro,
You are mostly correct. I just lumped them together for simplicity's sake, but the cause of detonation can be the same as pre-ignition or 'pinging', namely excess pressure (read heat) in the cylinder. But it's true that detonation is most often associated with a charge which has already been ignited by a spark. Here a reference to the topic at hand...Jim '85TE
From the glossary of terms from the American Petroleum Assn. dictionary:
What is pre-ignition?
A:Preignition occurs when the fuel/air mixture in a cylinder ignites before the spark plug fires.
It can be caused by burning contaminates (such as carbon, or a spark plug of the wrong heat range) in the cylinder or by extreme overheating.
What is detonation?
A:Detonation occurs when the flame-front in a cylinder does not proceed smoothly from the point of ignition (the spark plug) to the opposite side of the cylinder. It refers to the spontaneous ignition of the entire charge in the cylinder. This ignition is often caused by the extreme pressure rise in the cylinder that occurs when the charge is first ignited (by the spark plug) or by excess intake pressure such as can occur in turbocharged or supercharged engines.
JonRB said: Also of note is that 1 bar is atmospheric pressure - ie. the average pressure of normal air at sea level and average temperature (ie. pretty much a made-up value as there isn't really such a thing as average temperature and pressure!)
Jon.
You are absolutely correct! 1 Bar is equivalent to 14.7 PSI at 70°F.
This measurement isn't all that arbitrary however. Repeated pressure measurements at sea level all over the world yielded this pressure (14.7 PSI) at a temp of 70°F.
This standardization was established as a Base Line for scientific experimentation so that results could be understood and repeated by anyone, whether they were located in Grenwich or Nepal, once they took this 'constant' into account...Jim '85TE
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