INTERCOOLERS - - ARE WONDERFUL DEVICES!

 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.