Turbo Cirrus Blog By Braly

Compressor IAT , CDT vs Altitude - - Mike Busch's Request

2009-02-15T18:53:00Z

Mike Busch, in a response to the blog posting on intercoolers asks this question:

"George, could you post a graph that shows the relationships between OAT, CDT, and IAT as altitude varies from SL to FL250. In other words, how much temperature increase is produced by the turbocompressor and how much temperature decrease is produced by the intercooler as the aircraft climbs up to the Flight Levels? "

So, Mike, your wish is my command !

In this context, CDT is the compressor discharge air temperature (after it has been compressed and experiences the resulting temperature rise from that process.)

The IAT ( Induction Air Temperature) is the air temperature that actually enters the throttle of the engine. In the absence of an intercooler, the compressor discharge air flows directly to the throttle and the CDT will be equal to the IAT, within a very few degrees.

Frankly, doing this for Mike takes a fair bit of work. But, Mike is a good friend! So I decided to try to spend a couple of hours and model it up. The results are consistent with my actual data to a remarkable degree.

As mentioned, when looking at the graph, keep in mind that when one does not have an intercooler, then the CDT = IAT within a very few degrees, by definition.

As a result, I did not plot CDT as an extra curve, but the CDT is evident from the graphs, by reference to the non-intercooled compressor output lines (RED).

Allowing for the various issues described below, the results are consistent with actual data to a remarkable degree.

In the graph below, the assumptions are these:

1) Departure from a 5,000 foot MSL airport on a hot summer day ( OAT = ~ 100 deg F) and then a direct climb to altitude. The further assumption is made that the atmosphere (away from the surface) is an ISA + 40d F day. By making that assumption, anybody can figure the OAT at any altitude without having to plot it out. For easy of reference, at:

Altitude OAT (dF) Pres (in Hg)

10,000 63 20.6

15,000 45.5 16.9

20,000 28 13.75

25,000 10 11.1

2) Further assumptions are that there is an inherent gain in compressor inlet temperature (CIT ) of about 10d F (empirically verified) above the measured ambient (OAT) temperature, just due to the air flow going from the outside to the inside of the engine compartment. I have never seen an installation that did not have more than 10 degrees of rise from OAT to CIT. Further, it assumes some real world pressure losses from the compressor to the throttle of about 0.5" Hg for the non-intercooled configuration and about 1.5" Hg for the intercooled configuration. Those are also based on empirical experience.

3) The mis-matched compressor efficiency number is also from real world experience with older compressor designs or even more recent designs where the compressor is adapted from one design point and used in another application where the speed (RPM) or the mass flow is "off " of the normal compressor map sweet spot.

4) There are multiple well matched and well designed compressors that can operate with efficiencies well into the mid 70's percent range. An Efficiency of 0.74 was chosen as being consistent with real world experience.

5) The intercooler efficiency of 60% is actually "OK" for a single engine aircraft, but low for a twin engine aircraft (higher indicated speeds results in better intercooler efficiencies). It is realistic for a range of aircraft and climb airspeeds and it would be low for cruise for a well designed intercooler.

6) These plots assume the compressor efficiency is constant across the climb profile. That is an over simplification. They are not. One will move out of the sweet spot on the compressor at map at one end or the other of the climb.

7) I did this in a bit of a hurry, so it is possible I made a mistake somewhere!

Some observations:

1) The most significant observation is that, with a well designed turbo & intercooler combination, one can operate a piston aircraft under very hot ambient conditions and the engine will continue to see relatively modest induction air temperatures - - even all the way to 25,000 feet.

2) And, conversely, without the intercooler, one sees some difficult temperatures up into the mid or high 200 degree F range at 25,000 feet. Better compressor efficiencies mitigate this somewhat.

If the compressor is limited in its ability to deliver manifold pressure because of speed or drive shaft torque limitations, then below, is a further example of how that compressor, without an intercooler would perform. Note, the example assumes a compressor with a maximum pressure ratio of about 1.4:1.

So, Mike, is this the data that you wanted to see ?

Regards, George

INTERCOOLERS - - ARE WONDERFUL DEVICES!

2009-02-14T04:31:00Z

INTERCOOLERS - - ARE WONDERFUL DEVICES!

As is indicated in the previous blog on Turbochargers vs Turbonormalizers - - intercoolers are one of the "secret ingredients" in the whole engineering effort that protects the high powered aircraft piston engine from detonation. If you get the CHTs cooled off and you get the induction air temperature (IAT) down close to that experienced by the similar normally aspirated engine, then you really don't threaten the engine with adverse combustion events any more than they already are with a normally aspirated engine.

The value of the intercooler is often misunderstood. Few pilots and mechanics have any notion of the magnitude of protection from detonation that is afforded by a good intercooler installation. The data shown in the graph below helps to quantify the magnitude of protection from detonation that one obtains when good intercoolers are installed on an engine with a supercharger.

This data in this graph is adapted from some rather excellent WWII era data - - all of which was originally obtained with gear driven superchargers. But the same issues apply exactly the same way to any piston engine with a compressor - - whether it is driven from the crankshaft by a gear or belt drive or whether the compressor is driven by a spinning turbine wheel sitting in the exhaust stream.

The left hand vertical axis is in units of horsepower. The scale shows how much LESS horsepower is available to a detonation limited 300 Hp air cooled aircraft engine when one increases the induction air temperature, which is shown across the bottom horizontal axis.

In many text books and papers, it is common to characterize an engine's margin of safety from detonation by establishing how much additional horsepower in excess of its rated horsepower that the engine could make - before it encounters detonation - - assuming all of the other relevant parameters are held constant.

In this case, we have presented the data in a slightly different manner. This graph first establishes a reference point for a maximum detonation free horsepower at an induction air temperature of 120 d F, and then incrementally increases the IAT above the 120d F starting point. This process establishes how much less horsepower the engine is able to make before encountering detonation as the induction air temperature is increased. In order to have a real world example in mind, think of this example engine as a certified engine with something like 470 cubic inches and a normal rated maximum horsepower of 260 Hp at 2700 RPM. For our example, we have tested this engine and found that when boosted up to 300 Hp and with the induction air temperature measured at 120 d F, the engine is just barely free of detonation. This engine would have a "detonation margin" of 40 Hp (300 - 260) at an IAT of 120 d F.

The blue line shows the loss in detonation free horsepower for the engine operating at 2700 RPM. The red line shows the further reduction in detonation margin from the original power that is present if the RPM is reduced to 2500 RPM.

It is clear from the data, that an engine that is capable of making 300 detonation free horsepower at 2700 RPM with the induction air temperature at 120d F - - would only be able to make about 215 to 230 Hp free of detonation if the induction air temperature is allowed to increase into the 165 to 185dF range, and the engine is operated at 2500 RPM . This represents a loss of some 68 to 85 horsepower in the capability of the engine to operate free of detonation. That is a loss in detonation margin of about 22 to 28% of the power of the engine.

That 22% to 28% loss in maximum detonation free horsepower can be recovered with a good intercooler installation.

The industry, the FAA, and the pilot community all consider it an acceptable design configuration for an aircraft piston engine to be able to detonate under certain power and environmental conditions. That is why pilots are trained in engine operation and that is why we have POH limitations that need to be followed. Thus, if those types of engines are inappropriately set up by the pilot with the wrong combination of manifold pressure, RPM & mixture, they can operate outside of the established detonation margins. We operate these engines with these design constraints because it is sometimes essential in order for the aircraft to perform as they are intended. The broad range of pilot selected operating conditions is necessary in order for the pilot to be able to extract the maximum horsepower from the engine during certain critical phases of flight.

This widespread and historically successful aircraft engine operating paradigm has been acceptable because the engines came with manifold pressure, tachometer, temperature & fuel flow gages and the means to manipulate the values, which, if done in accordance with the POH, would allow the pilot to avoid engine operation in areas that would otherwise cause detonation.

Since harmful detonation is most likely to occur at full power during takeoff and climb. Because, under those conditions, the only "tool" at the disposal of the pilot is the mixture control, the FAA defines requirements for a "margin" on the fuel flow so that even if the fuel flow were improperly set up by the mechanic by some "margin" the engine would not detonate on the unsuspecting pilot during a full power takeoff and climb. The certification standards require a demonstration that the engine can be operated free of detonation with the mixture lever positioned so that the fuel flow is as much as 12% below the specified full power set point.

As an example, if one operates a stock 350 Hp Lycoming TIO-540J2BD (Navajo Chieftain) at full power and then foolishly brings the mixture control back to around 13 to 15% below the specified full rich mixture fuel flow - - then that engine is likely to begin to detonate under many environmental conditions.

On the other hand, when operating that engine, if one first reduces the manifold pressure by 8 to 10 inches, then one can lean the mixture to almost any mixture and it will not detonate. The cylinders will get hot if you lean it near peak TIT and leave it there for a while, but it will not detonate.

There are available after-market intercoolers for the Navajo Chieftain. If they are properly installed on those engines, then the detonation margin improves so dramatically, that one can "foolishly" set the mixture in that range from 13 to 15% below full rich and the chances of any detonation are dramatically reduced under almost all environmental conditions, even when the engines are operating at full power.

When we did certification testing on the TN SR 22 - - we did that in flight. We did that during a very hot period in June and July of 2006. The routine daily temperatures were in excess of 100 dF. The purpose of the testing was to insure there were adequate margins to allow operation of the engine at full throttle with the mixture set lean of peak. I was the PIC for the flight tests.

We conducted the tests with half of the intercooler cooling air inlet area blocked off. At various times, we set the manifold pressure to still higher values than the normal 29.6". We exercised the RPM from 2700 down to much lower RPM values while maintaining manifold pressure well in excess of 30". While doing that, and with the intercoolers partially blocked, we were able to force the induction air temperature to values much higher than is possible to obtain with the intercoolers "unblocked" and functioning normally.

The results were fully consistent with the predictions in the old data in the graph above: When the induction air temperature began to rise to values in excess of 160 d F, we were able to measure the onset of detonation at reduced mixture settings.

However, with the intercoolers functioning properly we could not force the induction air temperature above 130dF even on a hot day in a slow climb at 24,000 feet at full power, and detonation was never observed at any mixture setting.

The result of the careful design and thorough testing - - all born out of a lifetime of real world turbocharger engine operating experience and many years of highly instrumented test cell engine operation - - has been a robust Cirrus "Smart Turbo" system that is widely recognized as the most efficient and the easiest turbo system to operate of any general aviation aircraft. Ever.

Turbonormalizing vs Turbocharging - - understanding the important differences

2009-02-10T04:54:00Z

Turbonormalizing Vs Turbocharging.

Understanding the important differences

TN and TC engines share the following characteristics:

Both involve compressing outside ambient air to make it available as denser air to the engine induction system so that the engine can make more power.
The degree to which the air is compressed is just a matter of design choice when one decides to specify the engine and its ability to make horsepower across the normal range of desired altitudes.
Any time you compress air - you make it hotter. (As discussed below - almost everybody in the early days missed the importance of intercoolers.)
Both TN and TC systems use an exhaust driven turbine to drive a centrifugal compressor - at speeds that may range as high as 120,000 RPM on some units.
Both TN and TC systems can use the same turbo components.

The TN system designer typically starts with a normally aspirated engine and asks the following question:

What is the minimal number of changes one can make to this existing NA engine and still have that engine make sea level power at altitude (typically > 18,000 feet)?

The answer includes a laundry list like this:

New exhaust system to get the exhaust to the turbo housing.
Add turbo charger or two of them.
Add some way to control the speed of the turbo charger - ie, a wastegate.
Manual or automatic control over the wastegate?
Intercooler ? Very desirable, but not absolutely mandatory.
Fuel injectors that have the air side referenced to the output of the turbocharger compressor.
Fuel pump and fuel metering changes that will let the engine work gracefully with the higher pressure air over a broader range of operating conditions.
Need to enhance the cooling of the overall system (both cylinder head and possibly oil cooling), since it will be flying in thinner air at altitude that inherently provides less cooling.
Mechanically support the weight of the turbochargers.

The reality is that the TC system designer actually started with the same question. Keep in mind that almost all of the fundamental design decisions for the TCM TSIO series of engines and the Lycoming TIO series of engines were made back in the 1963 to 1968 time frame.

In 1963, at TCM, the question was something like this: "What can we do to modify the existing 260 Hp IO-470, or the new 285 Hp IO-520 normally aspirated engines - to make a turbocharged engine to sell to Cessna?" [History note: The first C-320 aircraft (in 1964) were equipped with TSIO-470 engines. A couple of years later, they switched to TSIO-520 engines with 285Hp, just like the Cessna 310s were also switched to IO-520 normally aspirated engines.]

[Another history note: Remember, Jack Riley was running amok back then putting Lycoming normally aspirated IO-540s on C-310’s with turbochargers and operating them as "turbo normalized" engines and calling them Riley Rockets! TCM just HAD to come up with a turbocharged engine or they ran the risk that Cessna might go to Lycoming for engines for the C-310. The term "turbonormalized" was not used, until later.]

And guess what? TCM ended up developing exactly the same basic list of 9 items (see above) as for the TN engine - but with one additional item.

[Some more history: The then universe of general aviation piston engine engineers mostly had a background in the big supercharged radial engines - all of which had very low compression ratios (often down around 6.5:1). All of those engineers just assumed that they would have to reduce the compression ratio on their new turbo-supercharged flat six cylinder engines in line with their previous big radial engine experience.]

So that is exactly what they did. TCM reduced the CR from 8.5:1 down to 7.5:1. Lycoming typically went from around 8.7:1 down to 7.3:1. There were some variations.

So, now, take the list of nine items above, and add item number 10.

Change the piston geometry to reduce the compression ratio by about 1 point, from ~ 8.5 down to 7.5:1.

When you do that you have identified the only meaningful "difference" in the hardware between our common terminology of turbonormalizing and turbocharging.

Yes, it is true that the typical turbonormalizer on a 285 Hp IO-520 TCM engine only uses around 30" of MP and the typical early Turbocharged engine (TSIO-520D in the 1968 Cessna 320F, for example) uses around 32.5" Hg to make the SAME 285 Hp at sea level.

However, bear in mind, the extra 2.5" of manifold pressure (from 30” up to 32.5” Hg) is there to make up for two things: First, some minor losses in the exhaust system back pressure and second, and more important, to make up for the fact that the air was HOTTER in the non-intercooled induction system in the Cessna 320 and was therefore "LESS DENSE" and it took higher MP to get the same total mass air flow through the engine - in order to make the same 285 Hp - and to make up for the loss in overall efficiency because of the lower compression ratio.

Important note: None of those early turbocharged or turbonormalized engines had intercoolers! Not the Cessna T-210. Not the Cessna 320. Not the Cessna 310T. Not the Bonanza V35TC. Not the Bonanza A36TC nor the Bonanza B36TC. None of them. There were, in the early days, a number of RAY-JAY "turbo normalized" after market STCs that were added to several Mooneys and to some Bonanzas. None of them worked well. Most were really bad. That caused some early heart burn with the OEM’s who condemned those early non-intercooled non-OEM 8.5:1 compression ratio installations - and often for good reason. Had those early aftermarket STC’d installations with the 8.5:1 compression ratios been equipped with good intercoolers and good cylinder and oil cooling, the valid engineering reasons to object to them would have been eliminated, although, undoubtedly, Lycoming and TCM would have still found an excuse to complain, just for marketing reasons.

[Another history note: For a while, the Piper factory did do installations of what we now call turbonormalized engines using the RAY-JAY hardware. There were no intercoolers. This was done on some Comanche and Twin Comanche aircraft and some Apache aircraft with Lycoming IO-540 engines. Then Lycoming changed to low compression pistons and provided an OEM engine to Piper as a TIO-540x series engine. Again, there were no intercoolers. Forty years later, the earlier, higher compression configuration engines are, today, much preferred by knowledgeable pilot-owners, as opposed to the later models with the typical 7.3:1 compression ratio engine configurations.]

If, in the years around the 1963 time frame, TCM (and, later, Lycoming) engineers had readily available cost effective air-to-air heat exchangers (intercoolers) and had the airframe OEM companies then been willing to go to the trouble and expense to integrate the somewhat more complex design required to employ the intercoolers effectively, it is likely that some engineer would have realized that it would be “a good thing” if the early turbocharged engines used in the Cessna T-210 and Cessna 320 and the Piper Comanche would have kept their original 8.5 (8.7):1 compression ratio engines. Thus, the several problems (mostly heat and temperature and fuel consumption) associated with 7.5:1 compression ratio engines would have largely been avoided.

But the OEMs were in a hurry for a quick fix - or a "drop in" turbo engine that would be a “one design” that would fit in a broad cross section of existing Cessna and Piper air frames without extensive airframe modifications. That was a lot easier to do by changing the piston to get a 7.5:1 compression ratio than it was to keep the 8.5:1 compression ratio and figure out a way to stuff a good intercooler into the multiple different cowlings that existed among dozens of different airframes.

A current technology "good" intercooler design can reduce the induction air temperature so that turbocharged or turbonormalized engines see induction air temperatures that are much more comparable at all altitudes and power settings to a hot day normally aspirated engine than they are to a non-intercooled turbocharged or supercharged engine. But nobody back in the 1963-1970 timeframe was properly focused on the importance of the intercooler to optimize and make truly successful the highly desirable change from normally aspirated engines to turbocharged engines.

The routine absence of intercoolers in the design of the early turbocharged (and early turbo-normalized) engines in the 1960s resulted in a series of unsatisfactory engine life operational experiences for the owners. Those unsatisfactory experiences became widely circulated as the prevailing "wisdom" of the time. Those old perceived "truths" are very hard to correct in the often tradition bound aviation community.

Worse, because of the routine absence of the intercoolers in the design of the early turbocharged (and early turbo-normalized ) engines in the 1960s, the owners ended up having a series of unsatisfactory engine life operational experiences. Those less than good experiences became widely circulated as the prevailing "wisdom" of the time. Those old perceived "truths" are very hard to correct in the often tradition bound aviation community.

Intercoolers are Really Important

Example: On a hot day (100d F) day at Denver, the TN SR 22 has a calculated full power induction air temperature of ~ 125 to 130d F.

Many people think the presence of the red hot glowing turbocharger turbine section must cause a large rise in the induction air temperature. But because the mass air flow through the compressor is so large ( think in terms of something short of a TON of air per hour) the temperature rise due to the proximity of the turbocharger is relative small. It gets lost in the calculations it is so small.

As an aside, the actual measurement of the induction air temperature on the TN SR 22 agrees with the calculations to a very close margin ( +- 5 d). (Note, even a normally aspirated aircraft on a 100d F day at Denver will have elevated induction air temperatures around 110dF just due to the heat rise in that simple induction system.

But, if you take away the specially designed (not at all like the standard TCM intercooler design) intercoolers from the calculation you get something like 155 to 160 dF - - even using the extraordinarily efficient compressors that are on the TN SR 22.

If the compressors used were not well matched on the compressor maps to the specific aircraft engine application - - then that number would go much higher.

Something like 170 to 190 dF.

Those kinds of numbers are difficult to deal with. That is why carefully selecting aircraft compressors that have compressor maps that put the operating point near the center of the "island" of maximum efficiency on the compressor maps and designing and using highly efficient intercoolers are all essential ingredients in the "SMART TURBO"(R) which Cirrus decided to bring to market.

Many people only think of detonation in terms of fuel octane or in terms of spark timing. But people knowledgeable in the industry know that detonation is strongly influenced by still another factor - - induction air temperature. There is lots of good data on that subject, going back to WWII.

A couple of more points. Even the TN SR 22 is not "fully turbo normalized". By that is meant, that on hot days, the turbos fully make up for the loss of ambient pressure as one goes to altitude, but without a controller that can compensate for higher than standard outside temperatures, then while the turbo normalized engine does very well in horsepower - - on hot days it will not make full rated Hp unless something more were done to fully compensate for BOTH ambient low pressures and ambient high temperatures. This requires a very expensive hydro-mechanical system to fully compensate for density and the only airplane I know of that even attempts to do that is the Piper Navajo Chieftain.

Last, the pilots and mechanics should really be aware that there is nothing "magic" about 29.9" Hg manifold pressure verses 32.5" Hg manifold pressure or even 36 or 38" Hg manifold pressure. One is not necessarily "harder" on the engine than is the other. The engineering parameters that "make a difference" are evident in other data that the pilot never sees. The important constraints on good engine durability are only evident in the data from the individual combustion events and that is primarily the magnitude of the maximum instantaneous internal cylinder pressure and its occurrence with respect to top dead center of piston travel, along with the magnitude of the operating cylinder head temperature.

If a design engineer properly manages the instantaneous peak internal cylinder pressure (which is a VERY different parameter from the more widely known BMEP) and the cylinder head temperatures, and the induction air temperature, then the magnitude of the manifold pressure becomes very much less important in a discussion of engine durability.

Keep in mind, we have learned over the last 15 years that it really is not how hard you run the engine, but rather, how you run the engine hard.

If you have comments, questions, or criticism about this article, please contact gwbraly@gami.com

New Induction Air Temp probe in the Cirrus Perspective

2008-08-27T01:47:00Z

This is a new "blog" and a new "experience" for me.

Rather than talk about "blogs" and "blogging" - - I prefer to just BEGIN:

The Cirrus Perspective has something new under the cowl. It is a direct reading measurement of the induction air temperature immediately upstream of the throttle plate.

This air temperature, commonly referred to as the "Induction Air Temperature" (or IAT) is often confused with either OAT or less often with compressor discharge temperatures (CDT). The CDT is the temperature of the air coming out of the turbocharger's compressor section - - measured before the air has gone through the intercooler. But the CDT is not measured in the Cirrus. Really no need to do that.

But if you pay even close attention to your Garmin engine display, you will never find any displayed indication of the IAT.

So what is it for ?

The short answer is that the IAT provides the data that works the magic to drive the LOP target fuel flow indicator in the Garmin engine display.

As any of you who may have happened to spend a weekend with the APS engine management class know, manifold pressure and RPM control the mass air flow through the engine.

That is well and good - - but to get it really "right" you need to also know the temperature of the air flowing in the induction system at the point where the MP is measured.

But if you know the rate of mass air flow through the throttle and you know the fuel flow - - then it is just a few short steps away to get the GARMIN to calculate and display a "target" air/fuel mixture ratio.

This is the magic that has enabled the ultra simple "set it and forget it" mixture magic in the Cirrus Perspective.