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Old 07-05-2010, 05:18 PM   #61
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Originally Posted by ************* View Post
If the exhaust wasn't "wasted", the exhaust would be at ambient temperature which it obviously is not.
Ah, but the turbo doesn't work on exhaust heat, it works on exhaust velocity. There have been discussions of scavenging exhaust heat too; to do so, you would change the exhaust gas's density, producing more pumping loss (though I don't recall if there was any data showing whether the recovered heat outweighs the increased pumping loss).

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Just because we have headers and other exhaust tuning that takes advantage of exhaust pulses in NA engines which scavenge exhaust in order to improve power, it doesn't mean that a turbo doesn't take this a step further by using this energy to spin a turbine to force more air into the engine.
Forcing more air into the engine requires you to put in more fuel. More air, more fuel, more power. Now you have to back off the throttle, introducing extra pumping loss at the intake (in addition to the turbo's pumping loss at the exhaust).

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Turbo chargers are better than superchargers, especially for our application.
I wasn't talking about superchargers at all. I know someone else was, but I was just addressing turbochargers.

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Originally Posted by pgfpro View Post
But this is your Forum and if you don't feel its necessary to have a discussion on why it works for me and several others(search yourself, outside this forum) then I'll just check out.
It is not my forum. My responsibilities beyond those of a regular user are mainly to keep out troublemakers and spammers; I do NOT direct discussions unless they are grossly misplaced.

It's not that I didn't want to discuss it, just that I didn't want to pollute your thread with a tangent off-topic discussion. I was trying not to step on your toes.

Where does the exhaust gas's movement come from? The pistons pushing it out. A turbocharger is driven by the pistons, using the exhaust as a linkage. It is not using free energy to do its job of shoving more air (and the resulting more fuel) into the engine.

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Originally Posted by DRW View Post
That analogy might apply if turbo cars were driven nearly full throttle all the time.

If an engine has sufficient power without adding a turbo, would adding a turbo increase FE?
This. Many factory turbocharged cars are more efficient because they have a more efficient, less powerful engine plus a turbocharger to provide the missing power only when needed.
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Old 07-05-2010, 05:32 PM   #62
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Quote:
Originally Posted by theholycow View Post
Ah, but the turbo doesn't work on exhaust heat, it works on exhaust velocity. There have been discussions of scavenging exhaust heat too; to do so, you would change the exhaust gas's density, producing more pumping loss (though I don't recall if there was any data showing whether the recovered heat outweighs the increased pumping loss).
You can go ahead and argue with wikipedia but knowing how electric generators at power plants, a jet turbine and a turbo fan works, I'm going to have to say that you're mostly wrong.
http://en.wikipedia.org/wiki/Turboch...ting_principle

They don't work purely off of exhaust gas movement but on the actual heat of the exhaust gas. I know it's hard for you to conceptualize but it's been well established that this is how these things operate. Funny thing actually, it was the development of turbochargers during WWII for Turbo props that led to the development of the Jet engine and then later on the Turbofan engines.. (Turbo fans are much more efficient jet engines...)


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Originally Posted by theholycow View Post
Forcing more air into the engine requires you to put in more fuel. More air, more fuel, more power. Now you have to back off the throttle, introducing extra pumping loss at the intake (in addition to the turbo's pumping loss at the exhaust).
Yes this is true, however in order to quickly win this argument, I'm going to point out that diesel engines experience not only a performance improvement but a fuel economy improvement by going turbo charged. If you buy pretty much anything with a diesel engine today, it's going to be turbo charged due to the increase in efficiency by the utilization of a turbo charger. Gasoline engines are more complicated to talk about but the principle still stands, it's just that it's easier to talk about the benefits of turbos with diesel engines.
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Old 07-06-2010, 06:16 AM   #63
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Originally Posted by ************* View Post
They don't work purely off of exhaust gas movement but on the actual heat of the exhaust gas. I know it's hard for you to conceptualize but it's been well established that this is how these things operate. Funny thing actually, it was the development of turbochargers during WWII for Turbo props that led to the development of the Jet engine and then later on the Turbofan engines.. (Turbo fans are much more efficient jet engines...)
Actually there were no "turboprops" in WWII.
A true turbo prop is a kerosene/jetfuel fueled engine (a jet turbine) with a gear-reducted propellor mounted. Although they were invented around 1940, they did not exist in production aircraft in 1945. Turbocharging was used on reciprocating engines to allow them to make full (or close to full) power at much higher altitudes and fly longer distances.

Unless you are referring that any turbocharged aircraft is a "turboprop" than I need to ammend 700 or so hours in my logbook!
I have way more turbine time than I thought!

Jim
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Old 07-06-2010, 12:55 PM   #64
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Originally Posted by Jim T. View Post
Actually there were no "turboprops" in WWII.
A true turbo prop is a kerosene/jetfuel fueled engine (a jet turbine) with a gear-reducted propellor mounted. Although they were invented around 1940, they did not exist in production aircraft in 1945. Turbocharging was used on reciprocating engines to allow them to make full (or close to full) power at much higher altitudes and fly longer distances.

Unless you are referring that any turbocharged aircraft is a "turboprop" than I need to ammend 700 or so hours in my logbook!
I have way more turbine time than I thought!

Jim
Lol! You're right! I guess I overlooked how a turboprop actually functioned and mistakenly believed it was simply a turbocharged propeller plane running a conventional engine.
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Old 07-06-2010, 09:44 PM   #65
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Originally Posted by theholycow View Post
Ah, but the turbo doesn't work on exhaust heat, it works on exhaust velocity. There have been discussions of scavenging exhaust heat too; to do so, you would change the exhaust gas's density, producing more pumping loss (though I don't recall if there was any data showing whether the recovered heat outweighs the increased pumping loss).



Forcing more air into the engine requires you to put in more fuel. More air, more fuel, more power. Now you have to back off the throttle, introducing extra pumping loss at the intake (in addition to the turbo's pumping loss at the exhaust).



I wasn't talking about superchargers at all. I know someone else was, but I was just addressing turbochargers.



It is not my forum. My responsibilities beyond those of a regular user are mainly to keep out troublemakers and spammers; I do NOT direct discussions unless they are grossly misplaced.

It's not that I didn't want to discuss it, just that I didn't want to pollute your thread with a tangent off-topic discussion. I was trying not to step on your toes.

Where does the exhaust gas's movement come from? The pistons pushing it out. A turbocharger is driven by the pistons, using the exhaust as a linkage. It is not using free energy to do its job of shoving more air (and the resulting more fuel) into the engine.



This. Many factory turbocharged cars are more efficient because they have a more efficient, less powerful engine plus a turbocharger to provide the missing power only when needed.
In a very short description I will try to explain why a turbo engine can be more fuel-efficient.

I believe that the exhaust gases create a pressure and temperature drop across the turbine of the turbo. This is what harnesses the energy to turn the turbine wheel. Today?s turbo?s are very efficient i.e. they can take this energy and utilize it. To the point that at very light engine load there will be very little exhaust pumping losses. Even if there is a slight exhaust pumping loss the advantages on the intake side will out weigh it.

The intake side is where a turbo-charge engine has a few major advantages.
From my data-logs I have noticed that at the same cruise speed with light engine load a turbo charge engine will run at a higher absolute manifold pressure (measured in Kpa). This helps with the intake pumping losses and is a major contributor in fuel atomization. This in turn reduces the BSFC numbers at light load.
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Old 07-07-2010, 12:05 AM   #66
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There was an article (I think on this site) about how new cars were using small turbos for FE. It is a fact. The exhaust gas expands, creating more gas than what was sucked into the engine. That increase is turned into more intake pressure by the turbo for "free" help. If one used a turbo that is the correct size to use only the excess gas's pressure, it is truly "free".
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Old 07-07-2010, 04:02 AM   #67
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pgfpro, that helps begin to explain how a turbo might harness heat energy from exhaust gas. I haven't seen any other explanation of how a turbo would use exhaust heat energy.
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There was an article (I think on this site) about how new cars were using small turbos for FE. It is a fact.
Most of the time, the purpose of that is to put an inherently more efficient, less powerful engine in (and provide the missing power with the turbo, on demand).
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Old 07-07-2010, 07:19 AM   #68
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Originally Posted by theholycow View Post
pgfpro, that helps begin to explain how a turbo might harness heat energy from exhaust gas. I haven't seen any other explanation of how a turbo would use exhaust heat energy.


Most of the time, the purpose of that is to put an inherently more efficient, less powerful engine in (and provide the missing power with the turbo, on demand).
I'm sorry but I'm not understanding your question???

Turbo manufacturers will tell you that there are two things that turn the turbine wheel. Heat and Exhaust flow(temperature and pressure)

I think you agree with the exhaust flow part? On the heat side this is why they use advance materials to make the turbine wheel and when you use header wrap you actually help the turbo spool.

On rear mount systems that I have built and use from a major manufacturer you loose a lot of this energy and usually have to up the size of the turbo hot-side itself because a loss of the turbo's turbine efficiency.
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Old 07-07-2010, 08:46 AM   #69
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I'm sorry but I'm not understanding your question???

Turbo manufacturers will tell you that there are two things that turn the turbine wheel. Heat and Exhaust flow(temperature and pressure)

I think you agree with the exhaust flow part? On the heat side this is why they use advance materials to make the turbine wheel and when you use header wrap you actually help the turbo spool.

On rear mount systems that I have built and use from a major manufacturer you loose a lot of this energy and usually have to up the size of the turbo hot-side itself because a loss of the turbo's turbine efficiency.
Interesting is about heat and gas flow. I once read a dissertation on high performance exhaust systems and the end result was that too big wasn't good because of heat loss. Seems the hotter the exhaust at the tip, the better for scavenging. Something like that.
On the other hand, if keeping the heat in was so important one would think Lockheed would have placed the turbochargers on the engines, not 10 feet away on the booms of the P-38. Or used two stage mechanical superchargers like the other inline engines used..

Is it not true that the reason we need exotic high temperature materials for the impeller because they want the turbo as close to the exhaust valves as possible......to overcome....."turbo-lag?" Is that lag not the reason racers who need instant power do not use turbochargers but default to the good old Roots, in drag racers?
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Old 07-07-2010, 09:03 AM   #70
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Keeping the exhaust temp high (with header wrap) will keep the velocity high- b/c the exhaust gas will contract and slow down as it cools. More heat loss between exhaust valves and turbo will mean less energy to spool the turbo.

I think that there are two perspectives going on here about the whole heat vs. velocity argument about what spins a turbo. The heat of combustion is what provides the energy that causes the gasses to expand. The expanding gasses move to an area of lower pressure and as a result there is a pressure gradient. The movement of gasses toward the atmosphere (their mass and velocity) provide the force that spins the turbo.

So, the heat of combustion initially provided the energy that gave the exhaust gasses the force (a function of their velocity and mass) that that they impart on the turbo impeller.

So both camps are technically correct- they are just looking at it from different viewpoints.

Just like wind energy from a sea breeze is technically a byproduct of solar energy because the sea breeze is a result of differential heating of the earth's surface by the sun.
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