why are turbocharged engines designed with lower compression ratios?

[RobertE]

I've got a friend with a turbo-normalized Lycoming 360 in his F model.  So that engine has, I assume, the same compression ratio as my IO360, paired with a turbo limited to never boosting more than 29.92 inches.  This would seem to me ideal.  Sure, lower compression would allow greater boost before risking detonation but why suffer the extra exhaust backpressure at those higher boost levels?

To put the question succinctly, why is it more efficient to have lower compression but higher boost than higher compression and lower boost?  

Thanks.

[Alain B]

Engine efficiency depend on many factors , but one who is important is compression ratio as it allow a more complete " burn " of the charge within the cylinder . That is one of the reason , including many others , that make car engine efficient nowadays , many of them run 12 to 1 and some 13 to 1 compression ratio compare to the good old days where 9 to 1 was apparently the max . The " boost " Is not directly related to efficiency , but the compression ratio is . For instance , car diesel run low  boost and extremely high compression ratio , in the order of 18 to 1 to 20 to 1 in some recent one , making them extremely efficient . In my old days of flying P&W radial , we use to run up to 45 inches of MP on take off , this would make a lot of power and torque , but those engine where extremely inefficient cuz they where running very low compression ratio , and extremely rich mixture to prevent detonation : in the order of 6 to 1 for compression , and 9 to 1 air-fuel ratio  . Burning in the order of 50 GPH for an R985 . 

 

Edited February 22, 2017 by Alain B

[FoxMike]

Short answer is bigger margin for detonation.  In some cases the higher compression engines have adequate margins but lower compression lets engine manufactures sleep at night.

[carusoam]

Add to the discussion...

Turbo normalizing, uses the same engine and just adds the Turbo/compressor... as an after market situation.

iIRC... The TN'd acclaim has a different, but similar, CR than the NA IO550...

The answer to the OP...

1) adding a compressor improves power as air density decreases at altitudes...

2) decreasing the CR improves engine safety regarding pre-ignition type challenges.

3) measuring the actual CR or effective CR is a bit challenging.  One part geometry and another part MP air pressure....

4) adding a TN to a Mooney is a really cool idea. It adds some responsibility to engine management.  With the additional power you will get additional heat to manage.

5) with more air being ingested, an equal amount of fuel is required.  

6) the cool part...  full power at altitudes where air resistance is lower..!  

PP ideas, not a mechanic....

Best regards,

-a-

[Ned Gravel]

It is certain that a turbocharged engine should probably have lower CR cylinders, but a TN engine only allows up to sea level pressure on the cylinders, which is an environmental condition they already work in.  The beauty of the design of Turbo normalizers is that they do not (are not supposed to) put additional stress on the engine and keep the dialled-in turbo compression at or below 30" MP.

Not certain why someone with a TN engine would need lower CR cylinders.

 

[jlunseth]

9 hours ago, RobertE said:

I've got a friend with a turbo-normalized Lycoming 360 in his F model.  So that engine has, I assume, the same compression ratio as my IO360, paired with a turbo limited to never boosting more than 29.92 inches.  This would seem to me ideal.  Sure, lower compression would allow greater boost before risking detonation but why suffer the extra exhaust backpressure at those higher boost levels?

To put the question succinctly, why is it more efficient to have lower compression but higher boost than higher compression and lower boost?  

Thanks.

Simply stated, it is not more efficient to have lower compression but higher boost.  The reason for the lower compression is the need for a detonation margin, and the need for a detonation margin arises with turbocharged engines, not boosted engines per se.  Without an intercooler, the Induction Air Temp in my TSIO360LB would be in the 250+ range.  Even with an intercooler the temps will be in the 150-200 dF range.  This is where ambient might be -20 dF (in the flight levels).  Compressing this heated air during a compression cycle in a cylinder rapidly heats it further, and that causes the risk of detonation.  The turbocharger is part of the problem because it is heated by gases in the 1600 range, so the air passing through the turbocharger is heated both by compression and by the turbo itself as a heat source.  

A supercharger, on the other hand, is mechanically driven, not driven by 1600 degree exhaust gas.  It will typically run much cooler.  There is still heating due to compression and by the work that is being done in the supercharger, but it is much less than the turbo and the air can still be cooled with an intercooler.  You can put your hand on the supercharger, you would lose your hand if you put it on an operating turbo.  In automotive engines where it is possible to fluid-cool in the intercooler, it is at least theoretically possible where a supercharger is used, to reduce the IAT below ambient.  Higher compression ratio engines are more efficient than low compression, and work is being done in the auto industry to devise a supercharger powered high boost engine.  The problem with superchargers at least until recently, has been that the classic control systems (either a throttle or a wastegate) mean that the supercharger is always doing more work than necessary to produce boost.  Because of this, supercharged engines are inefficient regardless of the boost ratio.  Work is being done on a new control system for superchargers that evens this out, so in the near future, for the sake of fuel economy (efficiency) we may see the combination of a supercharger on a normal (high) compression engine, at least in the auto industry.

There is going to be a paper presented at the April SAE meeting on this new system.  

[jlunseth]

7 hours ago, Ned Gravel said:

 

Not certain why someone with a TN engine would need lower CR cylinders.

 

Two reasons.  One is marketing.  Some TN engines are not truly turbonormalized, they are boosted.  The other is high IAT caused by the turbo, see my prior post.

[carusoam]

JL, one question....

are you going to be presenting at the SAE conference?

That would be cooool!

Whoever thought thermodynamics could actually be interesting and fun.... 

Best regards,

-a-

[N201MKTurbo]

The biggest factor on how much power an engine will make is how many combustible molecules you can get into a cylinder. A lower compression engine has more total volume in the cylinder than a high compression cylinder so you can cram more fuel air mixture into the cylinder. That's why a TSIO360 Continental makes 220 HP and an IO360 Lycoming makes 200HP. The detonation point is controlled by maximum cylinder pressure (and temperature) With higher volume cylinders it takes a higher volume of fuel air mixture to reach the same pressure.

[Yetti]

From my dad who is not an engineer, but used to race power boats. Think of engines as air pumps.  get air in and get air out. think of things that make it a better air pump.

[aviatoreb]

45 minutes ago, carusoam said:

Whoever thought thermodynamics could actually be interesting and fun.... 

Why, Nicolas Léonard Sadi Carnot of course.

[N201MKTurbo]

17 minutes ago, aviatoreb said:

Why, Nicolas Léonard Sadi Carnot of course.

If it wasn't for Carnot our cars would get much better gas mileage!

[aviatoreb]

18 minutes ago, N201MKTurbo said:

If it wasn't for Carnot our cars would get much better gas mileage!

The real problem is the guys who wrote into law the conservation of energy.  I say repeal that law.  Then we could fly a million miles on a thimble full of avgas and still land with ... a thimble full of avgas.

[1964-M20E]

1 minute ago, aviatoreb said:

The real problem is the guys who wrote into law the conservation of energy.  I say repeal that law.  Then we could fly a million miles on a thimble full of avgas and still land with ... a thimble full of avgas.

I can live with the law of conservation of energy.  I'd like to get penalties for violating the law of gravity repealed and I would like to have selective enforcement of gravity then all of your energy is used to get form one place to another and nothing to keep the pane in the air or get to altitude.

 

 

[Shadrach]

Lots of statements in this thread and lots of answers to questions that were not asked.

 

On February 21, 2017 at 9:14 PM, RobertE said:

To put the question succinctly, why is it more efficient to have lower compression but higher boost than higher compression and lower boost?  

It's not more efficient from a BSFC standpoint. compression ratio has a direct effect on thermal efficiency.  However, in terms of  manufacturing or say power to weight ratio, turbo charging (this is to say boosted above a standard atmophere) has many benefits.  There are several 550 c.i. Continental variants that meet or exceed 350hp and they are all TSIO engines. Non of their NA counterparts can match their HP/c.i. High boost increases volumetric efficiency and power (same c.i. is able to pump a larger volume air and fuel).  If Continental were to make a TN engine in 350 or 360hp, it would require more displacement or a much higher compression ratio or both.

[1964-M20E]

One way to look at it is when you turbo charge or super charge an engine you are increasing the compression ratio shoving more fuel and more air into the cylinder to produce more HP.  Like mentioned above lowering the compression rations increases the volume of the cylinder so you have more space to put the fuel air mixture in at a given boost pressure.

I'm only an electrical engineer not mechanical but I'd still like to have a TN on my F (25 to 26MP at 12,000ft would be nice) but it is not worth the cost of adding it.

Of course with any engine there is point where you cannot increase boost any further without destroying the engine.  Look at drag racers the supercharge their engines to the max but they only get about 4secons between overhauls at WOT.  That would get you to the end of the runway but wouldn't make for a long flight.

[RobertE]

Thanks for the input but I may be really dense, so let me put a finer point on it.

I know that turbo charged engines (not turbo normalized ones) have lower compression ratios to provide a detonation margin.  Where I'm struggling is if 1) a higher compression ratio boosts efficiency, and 2) adding boost produces extra power, then 3) why isn't the sweet spot a turbonormalized engine that gets the benefit of the higher compression ratio of a normally aspirated engine yet preserves detonation margin by seeing to it that boost never exceeds the sea level pressure for which the engine was designed?  Alternatively, lowering compression ratio just seems to be unnecessarily taking a step back, the consequences of which require boost in excess of sea level pressure.  And that added boost, I should think, adds burden to the engine via higher exhaust back pressure and higher induction temps to deal with via an intercooler or some other means.  

There must be some thermodynamics that I'm missing.

Thanks again.

[N201MKTurbo]

1 hour ago, RobertE said:

Thanks for the input but I may be really dense, so let me put a finer point on it.

I know that turbo charged engines (not turbo normalized ones) have lower compression ratios to provide a detonation margin.  Where I'm struggling is if 1) a higher compression ratio boosts efficiency, and 2) adding boost produces extra power, then 3) why isn't the sweet spot a turbonormalized engine that gets the benefit of the higher compression ratio of a normally aspirated engine yet preserves detonation margin by seeing to it that boost never exceeds the sea level pressure for which the engine was designed?  Alternatively, lowering compression ratio just seems to be unnecessarily taking a step back, the consequences of which require boost in excess of sea level pressure.  And that added boost, I should think, adds burden to the engine via higher exhaust back pressure and higher induction temps to deal with via an intercooler or some other means.  

There must be some thermodynamics that I'm missing.

Thanks again.

Once again, a lower compression ratio allows more air into the cylinders when it is being boosted then an engine with high compression ratio. This effectively increases the size of the engine without making it physically larger.

[Shadrach]

1 hour ago, RobertE said:

Thanks for the input but I may be really dense, so let me put a finer point on it.

I know that turbo charged engines (not turbo normalized ones) have lower compression ratios to provide a detonation margin.  Where I'm struggling is if 1) a higher compression ratio boosts efficiency, and 2) adding boost produces extra power, then 3) why isn't the sweet spot a turbonormalized engine that gets the benefit of the higher compression ratio of a normally aspirated engine yet preserves detonation margin by seeing to it that boost never exceeds the sea level pressure for which the engine was designed?  Alternatively, lowering compression ratio just seems to be unnecessarily taking a step back, the consequences of which require boost in excess of sea level pressure.  And that added boost, I should think, adds burden to the engine via higher exhaust back pressure and higher induction temps to deal with via an intercooler or some other means.  

There must be some thermodynamics that I'm missing.

Thanks again.

I'll answer with a question that will perhaps bring some understanding. How do you get a TNIO550 to make 360HP?

[RobertE]

7 minutes ago, N201MKTurbo said:

Once again, a lower compression ratio allows more air into the cylinders when it is being boosted then an engine with high compression ratio. This effectively increases the size of the engine without making it physically larger.

I'm not an engineer or know a darn thing about thermodynamics.  But a lower compression ratio provides more volume only due to the smaller "squish" area.  Yet that added volume applies whether boosted or not and didn't someone already observe that higher compression boosts efficiency?  Stated differently, wouldn't my IO360 produce less than 200 HP if compression were reduced from 8.5 (I THINK that's it) to, say, 6?  And if that's true for a normally aspirated engine wouldn't the same be true for a boosted engine?  Or does the simple fact that boosting something 1.33 atmospheres (40 inches vs 29.92) make all the difference?  

I'm sure these engine designers know what they're doing.  I just wonder about the mechanism.  Is it that simply boosting to sea level pressure doesn't make near full use of the excess temps in the exhaust flow so my worry about higher back pressure is wrong headed?  Or is it, indeed, that having a larger squish area is a benefit if and only if there is boost beyond sea level pressure?  As you can see, I'm way out of my depth here.

 

 

[N201MKTurbo]

Just now, RobertE said:

I'm not an engineer or know a darn thing about thermodynamics.  But a lower compression ratio provides more volume only due to the smaller "squish" area.  Yet that added volume applies whether boosted or not and didn't someone already observe that higher compression boosts efficiency?  Stated differently, wouldn't my IO360 produce less than 200 HP if compression were reduced from 8.5 (I THINK that's it) to, say, 6?  And if that's true for a normally aspirated engine wouldn't the same be true for a boosted engine?  Or does the simple fact that boosting something 1.33 atmospheres (40 inches vs 29.92) make all the difference?  

I'm sure these engine designers know what they're doing.  I just wonder about the mechanism.  Is it that simply boosting to sea level pressure doesn't make near full use of the excess temps in the exhaust flow so my worry about higher back pressure is wrong headed?  Or is it, indeed, that having a larger squish area is a benefit if and only if there is boost beyond sea level pressure?  As you can see, I'm way out of my depth here.

 

 

It depends on how many air molecules you cram into the cylinder. A boosted engine will suck more air into the cylinder on each stroke because of the higher MP.

The other side of the coin is maximum cylinder pressure. When you get to about 600 PSI the air fuel mixture will explode instead of burn. This pressure is a combination of the compression from the piston going up and reducing the volume of the combustion chamber and the volumetric expansion of the combustion charge as it burns.

With a high compression engine this pressure will be obtained with a lower volume of air then a low compression engine.

By your own calculations the boosted engine will put 1.33 times the amount of air into the cylinder then the NA engine. The low compression pistons allow that increased volume of air to be compressed to the same pressure as a NA high compression engine.

[RobertE]

Ahhh.  That makes sense.  Thanks, everyone.

[carusoam]

One of the things that makes this all possible is the availability of fine automated machining centers.  Make the first part right, and the next 1,000 parts will be identical...

An improvement will come with fuel injectors that are independently controlled and directly inject into the cylinder.

The materials being used haven't changed much since the 60s, but now they are a more well known and controlled process to produce the proper alloy and machine the finished parts.  Quality control has taken leaps in reliability, so a bad batch of cams will never be produced.  If it were to happen the cams would certainly never reach the customer.

Diesel will be the preferred fuel, with the compression ratios in the 20:1 range are all the rage...

 

Somebody in manufacturing has to put an end to things like...

- Lycoming made a batch of crappy cams...

- Continental can't get their valves centered in the guides...

- Champion spark plugs with their ever increasing electrical resistance...

- Gill batteries with their two years maximum limit...

 

Somebody at Mooney with the help of Continental has it figured out.  An IO550 with a pair of Turbo normalizers with matching intercoolers and pressure controllers turning 2700rpm is a great set-up.  310hp up into the FLs.

Unfortunately there aren't many of them reaching the part of the market many of us live in...

For fun, check the price of factory remanufactured TNIO550 complete with intercoolers... The power plant alone cost twice what my M20C did.  The operating costs are twice as much as well...

all we need is to repeal a few laws of physics and some financial rules...  in that case sign me up for a Turbine Mooney with a few more HP...

It is really cool to be able to understand all the details in producing more power on less fuel.  Go Mooney!

anyone that wants to add a TN to an IO360, there is one for sale listed around here somewhere....

Thinking out loud,

-a-

[jlunseth]

22 hours ago, carusoam said:

JL, one question....

are you going to be presenting at the SAE conference?

That would be cooool!

Whoever thought thermodynamics could actually be interesting and fun.... 

Best regards,

-a-

No, not me, sorry.  Long story, I am a lawyer not an engineer. I just know enough to be dangerous. The authors are people I have worked with for years.  Chief engineer is a Bone pilot.  Good paper, it will be coauthored by an engineer from Ford, and the SC company.  

Every auto company under the sun is boosting their engines now, the movement is towards smaller engines and fewer cylinders, 4 & 3 cylinder engines.  Fewer moving parts, less friction, more efficient.  Then the problem is getting power out of it, and the answer is to boost.  The traditional solution is the turbo, but there is always turbo lag.  All the companies have their "lag" solutions, Ford, GM, Porsche, etc.  None of them truly beat the lag, and the engine has the turbo in the exhaust system.  We have had lots of discussions here about that, and the effects.  It is not free, there is a "price" paid in the form of exhaust back pressure, changes in the engine timing, low compression (inefficient) engine ratio, heat, etc.  Its a good solution in an aircraft, where we want low weight and we don't care about lag.  We don't want the engine starving for O2 in the flight levels.  Auto engine is a different scenario and there seems to be a movement back to superchargers, which have always been thought to be inefficient because of the wasted work.  But there is no lag and no obstructed exhaust, so that may be the future.  Needs a better control mechanism.

It is interesting though, getting down into the details of combustion.

Edited February 23, 2017 by jlunseth

[RobertE]

22 hours ago, jlunseth said:

No, not me, sorry.  Long story, I am a lawyer not an engineer. I just know enough to be dangerous. The authors are people I have worked with for years.  Chief engineer is a Bone pilot.  Good paper, it will be coauthored by an engineer from Ford, and the SC company.  

Every auto company under the sun is boosting their engines now, the movement is towards smaller engines and fewer cylinders, 4 & 3 cylinder engines.  Fewer moving parts, less friction, more efficient.  Then the problem is getting power out of it, and the answer is to boost.  The traditional solution is the turbo, but there is always turbo lag.  All the companies have their "lag" solutions, Ford, GM, Porsche, etc.  None of them truly beat the lag, and the engine has the turbo in the exhaust system.  We have had lots of discussions here about that, and the effects.  It is not free, there is a "price" paid in the form of exhaust back pressure, changes in the engine timing, low compression (inefficient) engine ratio, heat, etc.  Its a good solution in an aircraft, where we want low weight and we don't care about lag.  We don't want the engine starving for O2 in the flight levels.  Auto engine is a different scenario and there seems to be a movement back to superchargers, which have always been thought to be inefficient because of the wasted work.  But there is no lag and no obstructed exhaust, so that may be the future.  Needs a better control mechanism.

It is interesting though, getting down into the details of combustion.

Thanks, Jlunseth.  Keep your thoughts coming.