N/A vs Boosted Power? 3

[PoorManagement]

I'm having trouble understanding how an engine can produce more power boosted vs naturally aspirated.

I'm assuming that an ideal NA motor - let's say a 2.0L making 220hp is doing so at the limits of the structure of the engine - for arguments sake, with cylinder pressures of 120 bar.

Now you add boost, and the engine can make 400hp.

But the structural limit of the engine is still 120 bar before parts fail.

Is it possible that peak cylinder pressures are roughly the same? (I'm missing a relationship between peak cylinder pressure and mean effective pressure - work done as the volume expands)

How is the additional power achieved while keeping peak cylinder pressures at a level that does not hurt the engine? Spark retard reduces the absolute peak pressure, but the pressure for the entire combustion cycle is higher overall? (Higher MEP?) Is this my answer? I think it is...

I'm assuming that no other physical changes are made - same pistons, rods (so same compression ratio), block, heads, etc.

Assuming the boost device provides additional air at a reasonable temp (intercooled), and the fuel system delivers the appropriate amount of fuel for the available air...


Thanks!

 

[gruntguru]

The combination of in-cylinder and turbine expansion is typically much greater than engines using only one or the other (eg NA reciprocating or Gas turbine). So historically, compound engines (including turbo-charged) are the most efficient ICE's ever produced. (Napier Nomad, Wright Turbo-compound, Marine diesels for example)

je suis charlie

 

[pwildfire]

But, in the given circumstances, the combustion efficiency is bound to fall drastically as expansion is diverted from the power cylinder to the turbine, and completely negate the gain in external VE.

Not so at all. Although it is true that some backpressure is added by the turbine, much of the energy driving the turbine comes from heat and impulse forces that are otherwise wasted as exhaust heat and noise. I would have thought that increased efficiency of turbocharged engines was a well-known enough concept at this point so as to not require a reference, but next time I am bored for a few minutes I will link one.

As a half-ass example, this concept is precisely why many (most?) auto manufacturers are switching from V8s to turbo 6s, and 6s to turbo 4s. A quick review of cars on the market over the last few years which have made this switch, their peak power, and fuel economy shows an obvious correlation.

 

[BrianPetersen]

pwildfire, consider the real-world experiences of Ford Ecoboost owners.

They're OK if you drive gently (off boost) and since the various government test cycles all involve driving gently, small-displacement turbo engines are all the rage.

Hitch up a trailer and put some load on it, now the engine has to run rich to protect itself, and they're thirstier than the V8 gas engine.

They're not the only ones. All of the small-displacement turbo engines have had criticism for not living up to government estimates.

 

[gruntguru]

Rich mixtures on boost. Detonation suppression, thermal stress protection, matt black front on the caravan.

je suis charlie

 

[BrianPetersen]

Yep - and all that starts happening sooner (at less total load on the engine) than it does with a bigger non-turbo engine ...

 

[Lou Scannon]

reduce compression ratio such that peak CP is the same

You are moving expansion from the cylinder to the turbine, which I guarantee will cost net efficiency.
In the passenger car paradigm that this discussion seems to be in, the efficiency gain from turbocharged engines (if any) comes not from the turbocharging, but from downsizing the engine, and in some cases possibly, comcomitant weight reduction of the base vehicle structure & suspension.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Lou Scannon]

The combination of in-cylinder and turbine expansion is typically much greater than engines using only one or the other (eg NA reciprocating or Gas turbine). So historically, compound engines (including turbo-charged) are the most efficient ICE's ever produced. (Napier Nomad, Wright Turbo-compound, Marine diesels for example)

All absolutely true! Did you miss

reduce compression ratio such that peak CP is the same

?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[pwildfire]

You are moving some expansion from the cylinder to the turbo. But you are also capturing expansion that otherwise would take place in the exhaust system (or at its exit).

 

[gruntguru]

Even if CR is reduced to maintain peak CP, the pressure at EO is far higher than at IC so there is a lot of additional expansion available during blowdown - even in the NA engine.

je suis charlie

 

[Lou Scannon]

So then, show me a worked example where keeping the peak cylinder pressure constant and moving expansion from the cylinder to the turbine results in a net power and/or efficiency gain.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Wright Turbo-Compound Link is a good example. Using the same CR and cylinder BMEP (so logically the same peak CP) the output and BSFC were increased substantially simply by recovering some of the blowdown energy.

je suis charlie

 

[Lou Scannon]

The Wright Turbo-compound is a compound engine, which is a different kettle of fish than pwildfire's premise that

For a given engine which operates naturally aspirated at a given peak CP, and with no additional limitations:
If we add a turbocharger which is at peak efficiency at this mass flowrate, and reduce compression ratio such that peak CP is the same, we will find that pumping losses will be reduced, and therefore slightly higher system efficiency will be attained, with slightly higher power output.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Lou Scannon]

If you boost the heck out of the engine at lower RPM where the volumetric efficiency is low, it will indeed break the engine. As the speed increases, the volumetric efficiency drops. The boost compensates that volumetric efficiency and keeps the torque up. A turbo properly matched to the engine does this automatically because the turbine won't produce a lot of power for the compressor at low speed. (For a supercharged engine, something has to regulate the boost or reliability will be affected.)

The big gain from boosting is the torque increase at higher RPM; it shifts the peak horsepower point up. Remember power is torque times rotational velocity. Having the same torque at higher RPM is more power.

Except for the part I put in parenthesis, this whole line of reasoning seems to assume a free floating turbocharger system (i.e. no wastegate or other boost control). The way turbocharging is practiced on modern passenger cars, not to mention all-speed heavy duty engines, is to deliberately emphasize lower rpm torque via the turbocharger system, and use various means (typically wastegate or VG turbine) to limit boost at higher rpms.
There are many heavy duty applications that are thermal loading limited, and have more-or-less flat peak horsepower across a significant rpm range, via control of boost, fuel, and other parameters.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Agree. Furthermore, a turbocharged alternative will usually be designed to make peak power at lower rpm because:

a) It is no longer necessary to turn high rpm to meet the design requirement
b) Durability will improve with lower rpm, higher boost
c) Friction and pumping losses will be lower

je suis charlie

 

[Lou Scannon]

Furthermore, a turbocharged alternative will usually be designed to make peak power at lower rpm because:

a) It is no longer necessary to turn high rpm to meet the design requirement
b) Durability will improve with lower rpm, higher boost
c) Friction and pumping losses will be lower

Quite right.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz