Turbo Efficiency Discussion

[NateD4]

Sizing a turbo should really be based on mass flow, not pressure ratio.

What is critical is to determine your 'design point'. That is where you pick the mass flow and best efficiency point.

For performance turbos you really want to pick your cruise rpm, determine if you need/want boost there and go from there with the 'off design' point checks.

Realistically an accurate computation can be a bit hard to do. But a proper estimate can get you close.

The only thing turbo efficiency buys you is fuel efficiency at boost levels. And to some degree that's arguable (ref Smokey Yunick's hot vapor cycle engine as an anti intercooler design).

What is important is picking the design point and mission of the engine properly, and sizing the turbo accordingly.

[TGP37]

I learned efficiency was the turbos ability to move air mass versus heat generation. Less efficient meaning less air moved and more heat generated. Also requiring more power to drive X amount of air mass.

I learn more everyday, and I'm curious about your understandings and why.

[NateD4]

That is true. But the reason efficiency effects fuel economy (whether mileage or power is under discussion) is because to generate the same mass flow requires more power (extracted from the turbine) to power the compressor.

In the case of intercooling you are trading internal inlet gas energy for air density to allow more addition of fuel. You need to intercool to avoid detonation and increase density. So you are throwing heat away (up to 150-300 degrees F worth of energy).

You CAN make a lot of power with a small-er turbo but you'll need a lot of intercooling and fuel for the same output as a properly sized higher efficiency turbo. For this reason I love Borg Warners S series turbos. They are generally more efficient than Garrett turbos, and give wider boost map with slightly better peak efficiencies, however, they are expensive and harder to find than the standard eBay turbos.

Not sure how much sense that made. I can provide a deeper explanation with more math if you want....

[NateD4]

Forgot to add... The more power you need from the turbine the more mass flow ( or temperature) and thus back pressure is required to drive the turbine. Of course back pressure effects scavenging and VE and requires more boost to overcome with yet more fuel.

[TGP37]

NateD4, I understand now. I never thought about the intercooler wasting heat energy, but heat is energy.

So what does this mean considering a water/meth injection system instead of an intercooler? Knowing a jet just prior to the compressor reduces intake temps, the heat is in the vapor. Drawn from the evap process in the compressor. So when the compressor generates heat, it is being absorbed by the evap process from the h20/meth injection. Being in the vapor state, it should condense when compressed in the cylinder, releasing the heat energy in the combustion chamber instead of ambient air. As condensation releases the heat that was absorbed in evaporation.

Sort of a slick trick happening here? It seems the water/meth injection is just a way to simulate a larger intake, resulting in slightly more efficient VE? It seems that way. So then one would need to inject after the compressor as well to counteract the increase in combustion temps? Just wondering if I got the physics aspect correct.


Sorry 1996Monte_Carlo, no intention of thread jacking. The discussion may help your choice of turbo....lol

[AleroB888]

In the case of intercooling you are trading internal inlet gas energy for air density to allow more addition of fuel. You need to intercool to avoid detonation and increase density. So you are throwing heat away (up to 150-300 degrees F worth of energy).
....

You can view it that way, but I think that's a strange way to put it.

You are not really "throwing away" heat energy, because that particular heat energy is not usable in combustion. It's a by-product of air compression, as you know, and some heat is created even with an "ideal compressor."

The bottom line is the amount of air molecules (density) that gets through the intake system. The loss of energy with an intercooler is due to restriction (which actually adds heat when it is frictional) and pressure (with air density) drop, which means the compressor must work harder to make the same boost. Intercooler restrictions are more troublesome with a turbo, because more heat is retained in the exhaust .

oop, sorry, yet more thread-jacking

[NateD4]

TGP37,

Sorry about the 'thread jacking' but I feel a good technical explanation will really help here....

You are correct the heat is in the vapor. Actually water injection is probably one of the best ways to intercool (however some might say it is less convenient).

In the case of water injection there is a certain amount of heat required to boil (or phase change) the water to steam. Some of the energy is used for cooling of the intake charge, however the biggest advantage is injecting it into the cylinder. Where the water is able to absorb some of the heat energy that would otherwise create detonation. Instead of simply absorbing it however, the heat added to the water injection in the intake manifold helps get the water closer to its boiling point (which could be up to 20 or 30 degrees above normal due to the boos pressure. The combustion 'event' then pushes the water past the boiling point and turns it into steam. The steam adds pressure to the cylinder.

The interesting effect is that if you were to graph the pressure in the cylinder with and without the water injection you would see that with injection the peak cylinder pressure is lower and the duration of the pressure wave (in terms of time / or crank angle) is longer. The cylinder without water injection could potentially have a high pressure and temperature and a spike in pressure and temp when detonation happens and the mixture is instantenously combusted. In this case the pressure graphis very high and extremely short in duration.

In a purely mathematical sense the area under the curve is related to the HP output. So water injection can increase overall efficiency and power up to the practical limit whereby the water molecules block the chain reaction of combustion and you start losing efficiency again.

(aqua mist makes a nice water injection system and at one time had a really nice graph showing the difference I'm talking bout. I'll see if I can't dig the link up...)

inter cooling into the turbo however is a slightly different story. 1) you are preheating the water injection stream which is good because it gets it closer to its boiling point. It also helps to transport the heat energy into the combustion chamber without a large detriment in intake charge air mass. so you don't loose as much heat, but you still pickup inter cooling effects. 2 the turbo is a pretty good blender for air, and thus your water/air mixture is well mixed and in thermal equalibrium. 3) you are however altering the compressor aerodynamics and to some degree the efficiency and effective mass flow of the intake charge. The turbo is (generally speaking) designed around a given set of air conditions (called state properties). When you inject water to it you are effectively increasing the density of the fluid moving through the turbo and thus you change the flow rate. (The real relation of this has to do with compressor vane choking tendencies and the localized Mach number in the vane.... Mach number being the speed of sound which is a function of the composition of the material in discussion: in this case air).

From my point of view there are some trade offs with injecting into the compressor. One of them being blade erosion and wear (though if you don't daily drive with water injection I doubt it'll fail a compressor anytime soon). The other trade off is the reduction of oxygen mass into the engine.

However, I think overall the benefits probably outweigh the disadvantages. As not only can you cool the intake charge but you can absorb additional heat that is being transmitted from the exhaust housing to the compressor housing via the bearing housing. This heat (though perhaps minimal) ends up in the combustion chamber as does the inter cooled heat where it is eventually converted to steam and then to power. (In case I wasn't clear, the energy to boil the water into steam comes from somewhere and is inevitable in the combustion chamber. By using some otherwise waste heat to help get it to boiling point you are using wasted energy and getting something back from it).

[NateD4]

AleroB888,

Sorry another thread jacked I guess....

I think what you neglected is that in order for the compressor to work harder it has to get that work from somewhere. That somewhere is the exhaust system where the turbine requires more flow or greater pressure (or both).

I'm not following your discussion of 'Intercooler restrictions are more troublesome with a turbo, because more heat is retained in the exhaust'. Which exhaust engine or turbo compressor? Additionally I'm not seeing why you are saying that.

Additional explanation:

I'm not really sure why thats a strange way to put it. It's fact. You compress a gas, add energy then expand it to extract the additional energy in a useful way.

I suppose the discussion gets a bit more complicated when we talk about turbo engines. As to me there are two distinct turbocharged engine operation methods. One regular boost in a moderate efficiency range and another extreme boost extreme power where special cams are used to increase exhaust gas flow to feed the turbos. In the later case there is an inordinate amount of combustion gases which aren't being fully expanded. (that gets into a different discussion about expansion ratios).

However, when you intercool any engine you are shrinking the gases (for lack of better terms) which slightly lowers the pressure in the intercooler and changes the equilibrium of the compressor outlet vs inlet.

I may not have been clear enough in explaining that the temperature of the air exiting the turbo outlet directly effects the pressure and density of the gas. So cooling a gas contracts its volume, and lowers its pressure and raising the temperature does the opposite. The difference between any two pressure and temperatures (and thus densities) is related to the internal energy. The compressor adds energy to pressurize the inlet air. A portion of that energy goes directly into potential energy stored as pressure, while some goes to flow losses due to kinetic friction. With the pressure change comes a change in temperature defined by the laws of physics (ideal gas law in this case). The pressure takes energy to generate which comes from the turbine wheel of the turbo.

The turbine wheel of the turbo depending on sizing (and area to radius ratio) and operating point takes the incoming exhaust gas flow and constricts it into the nozzle which raises the pressure in the exhaust system (as compared to a naturally aspirated engine of similar build). The increase in pressure is then exchanged for gas velocity as the exhaust is expanded through the nozzle where it impacts the turbine wheel.

SO in that regard inter cooling removes energy and causes a reduced pressure which allows more mass flow to follow. More mass flow takes more power from the turbine and then takes more energy from the exhaust to power it.

What we are really talking is back pressure to the exhaust costing power, scavenging and in the case of extreme turbo cams fuel that is unburned moving through the exhaust pipes.

The pressure loss adding to the temperature rise in an intercooler is minimal compared to the heat rise due to the mechanical work put into it by the compressor wheel.

Also the more internal energy the gas starts with the more it can be expanded relative to ambient pressure, thus the more power you can extract from that expansion. Which in many ways is the premise behind compression ratios and to some extent turbocharging (where we talk effective pressures and effective compression ratios).

So in the case of an intercooler we have two types of losses. Dynamic flow losses and kinetic flow losses. Dynamic of course being friction and pressure losses, while kinetic would be the internal gas energy.

The balance of losses is in the dynamic part where pressure loses are useful for adding more fuel/air density.

[AleroB888]

......I'm not following your discussion of 'Intercooler restrictions are more troublesome with a turbo, because more heat is retained in the exhaust'. Which exhaust engine or turbo compressor? Additionally I'm not seeing why you are saying that.

Because more exhaust heat is retained in the engine, upstream of the exhaust turbine wheel. A restriction in the intercooler system slows down the compressor wheel, which slows down the exhaust turbine, since they are connected by a common shaft. Therefore more back pressure occurs in the pre-turbine area of the exhaust.

I'm not really sure why thats a strange way to put it. It's fact. You compress a gas, add energy then expand it to extract the additional energy in a useful way.

What you are saying there combines two stages in the induction/combustion process that are not equivalent, for this reason:

First stage (pre-spark) -- charge air is compressed, heated and it expands, but that heat energy is not useful -- if it were, it would offset the resulting loss of air density. That heat expansion is not intentional and is detrimental.

Second stage (post spark) -- the charge air/fuel mixture is compressed by the piston. Then only after the spark ignites, useful heat expansion results, up to the limit of the knock threshold.

But I don't think the explanation for what makes power in the engine needs to be that complex. The amount of oxygen that gets into the cylinder, with the optimum AFR and spark advance.... and that's about it.

The maximum combustion chamber temp before knock is often limited by the intake charge temp. If the engine is on the verge of knock, extra heat expansion introduced through the intake tract is detrimental (that is assuming you already have proper atomization). It will not contribute to useful heat expansion post spark, because it brings with it a charge density loss.

[Joseph Upson]

You guys are spinning your wheels. Without specifications and qualifiers the concern over intercooler restriction is moot to minute at best. The pressure drop across the average size intercooler being used in many of the setups I've seen on forums is so small that it shouldn't be acknowledged because most are not running enough boost to make it significant.

On the Fiero forum there is no shortage of theoretical talk, what irritated me the most was that no one posted real time data to support what they were saying eventhough they were in a position to do so with their own vehicles. That's why I make it a point to post useful data from my experiences when possible, most are on the Fiero forum because for quite a while I could not get pics to post here.

Liquid to air intercoolers are where it's at in my opinion. If you look at the relatively small heat exchangers compared to the cooling they provide as well as the potential to provide greater than 100% cooling (using ice) and the very small pressure drop you'll see what I mean.

A perfect example of the potential of liquid to air intercooling occurs everytime you turn on the heater in your car. My heat blows hot enough to make it difficult to hold my hand directly in front of the vent and my heater core area is about 7"x7"x2", and an inch thinner than the OE unit it replaced about 2 wks ago which was brass and had even better heat transfer.

Brass cores transfer heat so much better than aluminum that it surprises me you can't at least find a brass core intercooler. The switch to aluminum with radiators had to do with weight and the reduced incidence of leaks but on an intercooler that wouldn't be a problem because the liquid system does not have to contain pressure.

Check these guys out: http://www.siliconeintakes.com/index...8c9acab6c5f082

This is the one I plan on using: http://www.siliconeintakes.com/produ...8c9acab6c5f082

[NateD4]

AleroB888,


Your points are well taken. However the key thing most people miss in this type of discussion is discerning between boosting for efficiency and boosting for power. The two have very different thermodynamic goals. When boosting for power you just want to increase oxygen density. When boosting for efficiency you want to get the most out of your turbo system for a specific load point.

The thermodynamics of what I said in any heat cycle is the same. Technically its a 4 stage process: induction, compression, heat addition (combustion), expansion... repeat. The energy in the post compression side matters in efficiency discussions (not as much in power discussions). What makes power is the difference between total post combustion energy (just prior to expansion and or during expansion) and the final expanded energy state. This very thing plus some high tech materials is part of what allows modern jet engines (different discussion of course) to obtain high fuel efficiencies at pressure ratios of 40:1 and greater. Gas turbines don't NEED to have inter cooling, the compressor input energy is transmitted through the system and later becomes part of the turbine drive power as it is extracted again.

The math of this is really where the details are since one design can be detrimental and another can be beneficial depending on how the design is performed (which is mainly what I'm talking about).

Of course this is relevant to turbocharging because the cycles are very similar except there is an expansion engine placed where a combustor normally would be.

I'll have to comment more on your last post later. However, the point that efficiency matters is valid in my opinion since turbo makers spend a lot of time trying to get better efficiency from their turbos, and for many good reasons.

Power discussions usually only deal with cramming as much air as possible, not really so much with how efficient the whole setup is at using fuel to make the power....

[TGP37]

This discussion seems like it deserves it's own thread. I love math and real data as it removes any doubts and levels theories.

I've spent lots of time on Excel working out statistical data from past logs. I once tried to find a way to measure exhaust mass flow. Then it hit me it was as simple as a temp difference of the intake mass. So logging intake and exhaust temps with MAF, I was able to measure exhaust MAF.......now for practical application is yet to be deployed.

Knowing exactly how much fuel is being sprayed, I bet I could write a reliable burn efficiency sensor. Just need to find out the energy for fuel. i.e. Natural Gas is 1,000 BTU per square foot.


Nate, I really appreciate the knowledge you post, thanks.

1996Monte, sorry for starting a thread jacking......any way I can help I will.

[NateD4]

Mass flow is conserved unless you have a leak. That said: exhaust mass = intake mass + fuel mass.

Fuel mass can be accurately approximated by the calibrated fuel flow through the injector at a given pressure * the pulse width. This of course is an approximation but if you correlate fuel flow to fuel pressure you'll have a decent number to mess with.

Density of fuel is commonly available from the oil companies. It's energy content as well. Roughly gasoline is 18,000 BTU/Lbsm

Lbsf= pounds force
Lbsm = pounds mass


The MAF readings you work with are also calculated based on pressure and temperature. They *should* also include humidity content to be accurate, but no one In the industry does that as far as I know.

So at best the data you get is a vet good estimate that works fairly well....

[bszopi]

Thread split up so you guys can now discuss w/o having to discuss thread jacking each time...

[TGP37]

Mass flow is conserved unless you have a leak. That said: exhaust mass = intake mass + fuel mass.

Fuel mass can be accurately approximated by the calibrated fuel flow through the injector at a given pressure * the pulse width. This of course is an approximation but if you correlate fuel flow to fuel pressure you'll have a decent number to mess with.

Density of fuel is commonly available from the oil companies. It's energy content as well. Roughly gasoline is 18,000 BTU/Lbsm

Lbsf= pounds force
Lbsm = pounds mass


The MAF readings you work with are also calculated based on pressure and temperature. They *should* also include humidity content to be accurate, but no one In the industry does that as far as I know.

So at best the data you get is a vet good estimate that works fairly well....

Roughly gasoline is 18,000 BTU/Lbsm - Bingo! That gets logged and stored. Thanks

exhaust maf = intake + fuel mass

I meant volume. Being the volume of exhaust was equal to the intake just a difference of temperature. Using that volume to calc the speed of exhaust considering the cross sectional area of the exhaust pipe.

And that raises another question. I read velocity is important in the exhaust flow. Preventing temp loss helps keep velocity high as the gas contracts as it cools, simple physics. It seems piping too large a diameter would mean the gasses should speed up as they enter the turbine and too small they must slow down.

To make a moving mass change speed introduces wasted energy that could be used to power the turbine. So the manifolds pipe diameter should reflect the turbine used. Considering the temp as it enters the turbine (good reason to use an EGT at the turbine entrance) to figure the exact diameter.

I can see a reflection of inlet diameter for different applications. Being a small diameter = more hp to the turbine and larger diameter = more torque to the turbine.

So a more efficient application would want a smaller inlet manifold diameter and a track turbo needs a larger diameter? .........Knowing the perfect diameter, that's the kind of precision I can appreciate.