How does a turbo use heat from the exhaust?

[ISX]

So I realize that the engine produces pressure but most of the time this pressure is 1:1 with the boost pressure when the turbo is actually doing its job (higher engine speeds/loaded). This raises the question about how you can gain efficiency if the boost you make is used for drive pressure which means the pistons do have a force to shove the exhaust out to drive the turbo. Now a book I have says it also uses the heat from the exhaust which is apparently one of the key factors in reaping the benefits of a turbo. My question is, how? I wouldn't think hot air on it's own would do anything at all. But the pre and post turbo temperatures are 300F apart so apparently it's using it somehow, I just don't get how. Anyone care to enlighten me?

 

[SteamKing]

The pistons do not shove exhaust gases out of a cylinder. When the exhaust valve opens, the hot gas is naturally going to expand, and most of the gas leaves the cylinder due to this expansion.

The turbine portion of the TC will be turned when the hot exhaust gas flows thru the unit during its expansion out of the cylinder. The amount of work extracted by the turbine results in a temperature drop of the gas, which is why the exhaust temp of a naturally aspirated engine is higher than an engine with a turbo.

 

[hondaman520]

Steamking is 100% right. You must focus on the prime phenomenon at work here. Heat particles are expanding as they leave the exhaust valve, which pushes in all directions, if you think about it, this natural phenomenon is somewhat counter-efficient to the structure of an engine. Turbochargers help utilize this phenomenon by means of the turbine, which reacts to this expansion (in the form of a spinning wheel).

 

[Windadct]

ISX - there are few things not 100% in your post -

1st - the Boost is relative to atmosphere - this is a statement of the intake (manifold pressure) relative to naturally aspirated - technically an aspirated engine would be less than 1:1 - because the cylinders are "sucking" in the air. So with a turbo or supercharger - you are pressurizing the intake - more air ( O2) the more fuel you can burn. This side of the turbo/SC is the compressor.

2 - Turbos do not necessarily increase efficiency - they improve the amount of power the engine can produce - well technically torque - another post needed.

3) The pressure to drive the turbo comes from the exhaust of the combustion not really the exhaust stroke of the cylinder - the exhaust is run through the turbo to generate spin - transferred back to the compressor side.
4) At the end of the exhaust stoke - I would say the piston DOES push the gasses out ( with the force coming from another cylinders expansion - but this is lower pressure level than at the initial opening of the E Valve.
5) Heat - typically - my understanding is that heat is the enemy. I guess it is possible that the combustion temp is much higher causing greater expansion of the gasses ( a lean = O2 rich combustion, does run hotter, possibly causing the exhaust gasses to want/need to expand = higher exhaust pressure.) - so I can see how this is important - but I have not heard of this being a significant adder. As far as being an enemy - this in an intake issue - when you compress a gas (air) it is heated, thus the need for intercoolers - since small compact turbos are preferred - hotter Exhaust Gas Temp ( EGT) will raise the temp of the whole turbo assembly... so I am thinking that hotter EGT - is a wash, because the turbo runs hotter and then the incoming air is hotter --- but this last point is just a gut feeling.

 

[ISX]

I guess my question now is if the exhaust has already left the engine, how does any more heat get added? You say the expanding gasses drive the turbo but if it's already out of the engine then where does the heat get added?

 

[Q_Goest]

I guess my question now is if the exhaust has already left the engine, how does any more heat get added? You say the expanding gasses drive the turbo but if it's already out of the engine then where does the heat get added?

Good question. We should be able to apply the first law of thermo to a turbocharger just as we can apply it to any thermodynamic process. If the pressure at the inlet of the exhaust turbine is the same as the pressure on the outlet of the turbine, then the only available energy is from the kinetic energy of the gas. If the kinetic energy isn't being utilized then the thermal energy of the gas isn't going to produce any energy to rotate the turbine.

Have a look at the Garrett web site. They have an explanation of how turbochargers work here. They talk about the exhaust gas coming out of the cylinder and then:

• The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure.
• A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas' energy to provide the power necessary to drive the compressor.

They also include graphs of pressure ratio across the turbine for each model of turbocharger such as this one for the GT35R with 3 different trims:

I read this to say that the absolute pressure ratio of aproximately 2.5 gives the best isentropic efficiency at around 70%, so if the outlet pressure of the turbocharger is 15 psia, the inlet pressure is 37.5 psia (or 2.5 times higher than outlet).

 

[Astronuc]

I guess my question now is if the exhaust has already left the engine, how does any more heat get added? You say the expanding gasses drive the turbo but if it's already out of the engine then where does the heat get added?

One puts the turbine stage as close to the engine as possible and allows the gas expansion (momentum/energy transfer resulting in a cooling and pressure drop) to take place in the turbine. Otherwise, heat transfer is accomplished with a recuperator, which would have a low pressure drop, or conduction through the housing.

The compressor driven by the turbine also heats the cooler inlet air via compression.

In jet engines, the turbine drives the compressor.

 

[ISX]

Can this be thought of as like even just the engine itself? As in you shoot in fuel and the heat expands producing work. In a turbo it is the same idea but you have the pressure driving the turbo but also have the heat driving it. But this leaves in question the fact that the engine is cold then hot with every power stroke but a turbo is seemingly always getting just hot air. Basically I am thinking the air inside the exhaust manifold gets cold and hot very quickly with each exhaust valve opening. This happens so fast that we do not see it on an EGT gauge but see it as a constant temperature. But do those rapid expansions of the air that happens a ton of times each second, actually simulate a pressure rise (driving the turbo) in much the same way the fuel in the combustion chamber does?

 

[Astronuc]

Can this be thought of as like even just the engine itself? As in you shoot in fuel and the heat expands producing work. In a turbo it is the same idea but you have the pressure driving the turbo but also have the heat driving it. But this leaves in question the fact that the engine is cold then hot with every power stroke but a turbo is seemingly always getting just hot air. Basically I am thinking the air inside the exhaust manifold gets cold and hot very quickly with each exhaust valve opening. This happens so fast that we do not see it on an EGT gauge but see it as a constant temperature. But do those rapid expansions of the air that happens a ton of times each second, actually simulate a pressure rise (driving the turbo) in much the same way the fuel in the combustion chamber does?

The hotter the gas, the greater the energy per unit mass or unit volume, assuming a fixed volume, so the greater pressure and potential to do work.

The turbine stage runs of the exhaust stream.

In a piston energy, there are multiple pistons set at different points in the rotational cycle. And the rpms, are such that an individual piston and valve set are moving 20-100 times/sec.

Think 1200 rpm = 20 rps (Hz), and 6000 rpm = 100 rps (Hz), and then multiply this by 4, 5, or 6, or depending on the number of cylinders and how they are phased. We have 4, 5, 6, 8, 12, 16 cylinder engines, and some diesels up to 20 cylinders. Large diesel motors tend to operate at low rpm, < 1000 rpm, and more like 900 rpm, as compared to smaller cylinder automotive engines.