Quote:
Originally Posted by jgeissinger
Put a propane torch on the casing of a turbocharger, heat it until it glows, and then tell me how fast the vanes are turning.
|
They'd turn at about the same speed as if you pointed an air hose at the casing, don't you think?
Quote:
|
It is airflow that spins the turbo! That airflow does come from heated exhaust gasses, but it is not the temperature that spins the air compressor (which is what a turbo is), and forces more air into the combustion chamber. The concept that wasted heat provides the power, because it comes from the exhaust side of the engine, is a common misconception. It is still wrong.
|
First, let's make sure our terminology is aligned. Here's a picture from Garrett:
A turbocharger is a compressor and a turbine on the same shaft.
Is it airflow that powers the turbocharger, or the heat in the exhaust that powers it? Have you ever driven a turbodiesel vehicle and watched the boost gauge? Suppose you're on cruise control in hilly country. Coasting downhill the boost is zero, but going uphill boost is maxed out.
But it's the same engine turning the same RPM. There's just as many exhaust strokes per minute in both cases, so the airflow is just the same. Why doesn't the turbocharger produce boost going downhill, but lots going uphill with the engine running at the same speed?
(Before anyone says the wastegate causes this, hitch a ride on a heavy truck without a wastegate on the turbocharger and see it behaves the same way.)
If it's just airflow driving the turbocharger, why isn't boost constant for a constant engine speed, and why don't you only get high boost at high speed? High RPM gives high airflow. Wouldn't that be necessary for high boost?
Here's some quotes from other sites. From the page Sailboy21 pointed to:
"The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better."
"The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load."
No hot expanding gas, no power for the compressor.
Here's some info from
Scania who make truck engines in Europe:
"Turbocompounding seems to defeat the laws of physics by creating energy out of nothing. It works by recovering energy that would otherwise be lost, or wasted. It is a classic example of recycling. Instead of expelling wasted energy via the exhaust pipe, more heat is extracted from the exhaust gases by a second exhaust turbine downstream from the turbocharger."
They have a
figure and explanation on another page:
" 1. Input of exhaust gases from the manifold, at a temperature approaching 700șC.
2. Exhaust gases are used to drive the conventional turbocharger, where energy is used to boost power and torque in the combustion process. These exhaust gases, instead of being lost to the atmosphere, are then directed to the turbocompound unit.
3. The exhaust gases, on reaching the turbocompound unit, are still at a high temperature (around 600șC); the energy is used to spin the second turbine at up to 55,000 r/min. After passing this point, the gases are down to below 500șC, and are expelled via a conventional exhaust system and silencer.
4. The revolutions of the turbine are stepped down in various stages by mechanical gears and a hydraulic coupling. The hydraulic coupling balances out variations between the rotation of the flywheel and the turbocompound turbine.
5. By the time drive reaches the crankshaft, the rate of rotation is down to around 1,900 r/min.
6. The flywheel's momentum is increased, and its rotation becomes more stable and even."
So, exhaust goes into the turbocharger at 700șC and comes out at 600șC. The 100șC (180șF) difference means a lot of heat energy has been lost from the exhaust.
To get some idea of how much heat is in the exhaust stream, a study at the
University of Alaska measured on a 125 kW Detroit Diesel generator set that 38% of the energy in the
fuel was converted to mechanical energy by the engine, and 30% leaves the engine as heat in the exhaust.
This ratio on the 470hp Scania engine means there's 370 hp of heat in the exhaust. Just to put some numbers on things, suppose the ambient temperature is 0șC (about right today). If the exhaust has 370 hp of heat in it at 700șC, it has 317 hp of heat at 600șC. So going through the turbine of the turbocharger has caused it to lose 53 hp of heat energy.
If you believe in the law of conservation of energy, that 53 hp of heat must be going somewhere. The only other thing coming out of the turbine housing is the shaft to the compressor.
Scania doesn't say how much power the turbocompound unit develops (with another 100șC drop in exhaust temperature), but Detroit Diesel says their similar sized
DD15 engine gets 50 hp off the turbocompound unit.
So 50 hp of heat energy is disappearing from the exhaust stream while 50 hp of mechanical power is showing up on the turbine shaft. And this all occurs inside the turbine housing.
Allow me to humbly suggest that the turbine is converting the heat energy from the hot, expanding exhaust gas into mechanical power.
Here's a part of a letter from
Joshua Berman, MidRange Service, Cummins Engine Company talking about the turbocharger on the B series engines in Dodge pickups (emphasis added):
"That being said, the limit for CPL 1550 ('95 manual, 175HP @ 2500RPM, Chrysler rating) is 900 degrees F, and the limit for CPL 2023/2175 ('96/'97 manual, 215HP @ 2600RPM, Chrysler rating) is 950 degrees F. It's not that we're that concerned about the temperature of the exhaust after the turbo; we're interested instead in what we call TIT (Turbine Inlet Temperature). If this gets too high, then you start to think about damage to manifolds and turbos. Since turbos typically drop 200-250 degrees F across the turbine, and the limit for CPL 2023/2175 is 950 F, the max TIT you should see is right around 1200 F."
The big temperature drop means a lot of heat energy is leaving the exhaust and going somewhere. If it's not being converted to mechanical power by the turbine, exactly where is it going?
Here are some quotes from companies that build turbochargers (emphasis added in each). First, the
Holset Company:
"Unlike the supercharger it does not feed off the power output of the engine. The turbocharger uses the waste energy from the exhaust gas to drive a turbine wheel that is linked to the compressor through a shaft. "
BorgWarner Turbo Systems:
"In exhaust gas turbocharging, some of the exhaust gas energy, which would normally be wasted, is used to drive a turbine. Mounted on the same shaft as the turbine is a compressor which draws in the combustion air, compresses it, and then supplies it to the engine."
Garrett:
"A pressure and temperature drop occurs (expansion) across the turbine, which harnesses the exhaust gas energy to provide the power necessary to drive the compressor."
If the idea that waste heat in the exhaust powers turbochargers is a misconception, it seems pretty commonly held by the companies that actually design and build turbochargers.
The U.S. Department of Energy is sponsoring research on turbocompounding, for instance, they have a project with Catepillar called
"Diesel Engine Waste Heat Recovery Utilizing Electric Turbocompound Technology". The waste heat is in the exhaust, and it's recovered with a turbine.
Maybe one more thing. Here's a schematic of a jet engine:
Notice the similarity to a turbocharger in the figure above -- compressor on the left, turbine on the right and connected by a shaft. The compressor compresses all the air for the engine, and the turbine extracts energy from the exhaust to drive the compressor.
Suppose the jet engine is running along nicely, and the
fuel is shut off. What happens to the airflow (at least initially)? Nothing. It's exactly the same. The only difference is that fuel isn't being burned to heat the air inside the engine before it goes through the turbine. If airflow provides the power to spin the turbine, the same airflow in the turbine should equal the same power, right? If that's the case, the engine would continue to run just as well without fuel. Any bets that would happen?
So, I'll stand by my earlier statement that turbochargers are powered by the heat in the exhaust.
Cheers,
Tim
