Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Thu, 24 Mar 2005 14:44:59 -0500 Message-ID: X-Original-Return-Path: Received: from imo-m22.mx.aol.com ([64.12.137.3] verified) by logan.com (CommuniGate Pro SMTP 4.3c3) with ESMTP id 818878 for lml@lancaironline.net; Thu, 24 Mar 2005 13:31:30 -0500 Received-SPF: pass receiver=logan.com; client-ip=64.12.137.3; envelope-from=REHBINC@aol.com Received: from REHBINC@aol.com by imo-m22.mx.aol.com (mail_out_v37_r5.33.) id q.15c.4d0532a0 (3924) for ; Thu, 24 Mar 2005 13:30:39 -0500 (EST) From: REHBINC@aol.com X-Original-Message-ID: <15c.4d0532a0.2f74614e@aol.com> X-Original-Date: Thu, 24 Mar 2005 13:30:38 EST Subject: Re: [LML] Superchargers X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1111689038" X-Mailer: 9.0 for Windows sub 5116 -------------------------------1111689038 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en Rob said: All sorts of engines run without intercoolers, without damage. Detonation is= =20 a function of temperature AND pressure in the cylinder at, or near, full=20 compression, not the temperature in the intake. Besides, if you are flying u= p high,=20 where an add on system makes sence, the air starts out colder so it isn't al= l=20 that much warmer than it would have been at ground level.[emphasis mine] =20 The principle reason for adding an intercooler is the same as for adding a=20 supercharger, to make the intake air denser and increase the mass flow throu= gh=20 the engine. =20 =20 In particular, I take issue with your suggestion that intercoolers are not=20 needed at altitude. Let=E2=80=99s take a look at a supercharger (mechanical=20= or exhaust=20 driven) installation on a 7.5:1 compression ratio engine providing 30=E2=80= =9D MAP at 20 =E2=80=99000 pressure altitude and 20C above ISA. While a perfect supercharg= er (100%=20 efficient) might only raise the adiabatic (no heat added or lost in the=20 process) motored (no spark or fuel, just air) compression temperature 9%, a=20= real=20 world supercharger (76% efficient) would raise the motored compression=20 temperature over 31% (using the Rankine temperature scale). I think that is=20= a=20 significant number. Mike, I think you caught me on that one. It's good to know somebody is paying=20 attention out there. I forgot to consider what happens to the energy of the=20 inefficiency and you are correct that in a centrifugal pump/compressor nearl= y all of=20 it goes to heating the fluid. Actually, when I ran the numbers in the=20 calculation, I thought the discharge temperature looked a little low. The ma= th checked=20 out, but I should have looked a bit further.=20 =20 By way of example, this Christmas, I was flying in Texas at 17,000=E2=80=99=20= with a=20 pressure altitude of nearly 20,000=E2=80=99 (13.8=E2=80=9D) and an OAT +22C=20= over ISA (my=20 Density Altitude was 23,500=E2=80=99). My Compressor Discharge Temperature (= =E2=80=9CCDT=E2=80=9D) was 212F=20 at 33=E2=80=9D MAP with a pressure ratio of 2.4 for a compressor efficiency=20= of 76%=20 (TSIO-520R). This is an 180F temperature increase over ambient! I think that= is=20 a significant number. =20 An intercooler with only 60% efficiency would have reduced my inlet and=20 turbine temperatures by over 100F ! That would have moved my TIT from 1622 t= o 1522,=20 well below my max TIT. Similarly, it would decrease the heat rejection=20 requirements of the engine and improve my CHTs a bit. Furthermore, it would=20= result=20 in decreased peak combustion pressures and a slight delay in the pressure pe= ak,=20 resulting in less stress on the engine. All of these factors significantly=20 increase the detonation margin between safe operation and engine damage. Mor= e=20 importantly, the lower temperatures substantially reduce the probability of=20 preignition, and this is the real killer. =20 If you throttled back to maintain the same mass flow rate, then I would=20 completely agree with you. But if you take advantage of the additional mass=20= flow=20 the intercooler permits, then I think you will find that the some of these=20 values go the other direction. The intercooler isn't a polytropic process, l= ike the=20 compressor. If you cool the air 10%, then you increase its density by 10%.=20 Yes the intake air is cooler, but the heat output of 10% additional fuel wil= l=20 result in only 6% reduction in temperature and a 14% INCREASE in pressure in= the=20 cylinder (assuming we avoid detonation/preignition in both cases). Then ther= e=20 are changes in the heat transfer effectiveness at the combustion chamber=20 surface, which further reduce the 6% reduction in temperature. Also, keep in mind that simply cooling the fuel air mixture in the cylinder=20 by 60 degrees does not buy a full 60 degrees of detonation margin. As densit= y=20 increases, both the autoignition temperature and the minimum ignition energy= =20 decrease.=20 In the end, an intercooler still has some favorable benefit on=20 detonation/preignition margin, but this benefit is quite small compared to i= ts effect on=20 power output. Rob -------------------------------1111689038 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en
Rob said:

All sorts of engines run without intercoolers,=20= without damage. Detonation is a function of temperature AND pressure in= the cylinder at, or near, full compression, not the temperature in the inta= ke. Besides, if you are flying up high,= where an add on system makes sence, the air starts out colder so it isn't a= ll that much warmer than it would have been at ground level.[emph= asis mine]

 

The principle reason for adding an intercooler=20= is the same as for adding a supercharger, to make the intake air denser and=20= increase the mass flow through the engine.

 

 

In particular, I take issue with your suggestion th= at intercoolers are not needed at altitude. Let=E2=80=99s take a look at a s= upercharger (mechanical or exhaust driven) installation on a 7.5:1 compressi= on ratio engine providing 30=E2=80=9D MAP at 20=E2=80=99000 pressure altitud= e and 20C above ISA. While a perfect supercharger (100% efficient) might onl= y raise the adiabatic (no heat added or lost in the process) motored (no spa= rk or fuel, just air) compression temperature 9%, a real world supercharger=20= (76% efficient) would raise the motored compression temperature over 31% (us= ing the Rankine temperature scale). I think that is a significant number.

Mike,
 
I think you caught me on that one. It's good to know somebody is p= aying attention out there. I forgot to consider what happens to the ene= rgy of the inefficiency and you are correct that in a centrifugal pump/compr= essor nearly all of it goes to heating the fluid. Actually, when I ran the n= umbers in the calculation, I thought the discharge temperature looked a litt= le low. The math checked out, but I should have looked a bit further.

 

By way of example, this Christmas, I was flying in=20= Texas=20= at 17,000=E2=80=99 with a pressure altitude of nearly 20,000=E2=80=99 (13.8= =E2=80=9D) and an OAT +22C over ISA (my Density Altitude was 23,500=E2=80= =99). My Compressor Discharge Temperature (=E2=80=9CCDT=E2=80=9D) was 212F a= t 33=E2=80=9D MAP with a pressure ratio of 2.4 for a compressor efficiency o= f 76% (TSIO-520R). This is an 180F temperature increase over ambient! I thin= k that is a significant number.

 

An intercooler with only 60% efficiency would have=20= reduced my inlet and turbine temperatures by over 100F ! That would have mov= ed my TIT from 1622 to 1522, well below my max TIT. Similarly, it would decr= ease the heat rejection requirements of the engine and improve my CHTs a bit= . Furthermore, it would result in decreased peak combustion pressures and a=20= slight delay in the pressure peak, resulting in less stress on the engine. A= ll of these factors significantly increase the detonation margin between saf= e operation and engine damage. More importantly, the lower temperatures subs= tantially reduce the probability of preignition, and this is the real killer= .

 

If you throttled back to maintain the same mass flow rate, then I would= completely agree with you. But if you take advantage of the additional mass= flow the intercooler permits, then I think you will find that the some= of these values go the other direction. The intercooler isn't a polytr= opic process, like the compressor. If you cool the air 10%, then you increas= e its density by 10%. Yes the intake air is cooler, but the heat output of 1= 0% additional fuel will result in only 6% reduction in temperature= and a 14% INCREASE in pressure in the cylinder (assuming we avoid= detonation/preignition in both cases). Then there are changes in the heat t= ransfer effectiveness at the combustion chamber surface, which further=20= reduce the 6% reduction in temperature.
 
Also, keep in mind that simply cooling the fuel air mixture in the= cylinder by 60 degrees does not buy a full 60 degrees of detonati= on margin. As density increases, both the autoignition temperature and the m= inimum ignition energy decrease.
 
In the end, an intercooler still has some favorable benefit o= n detonation/preignition margin, but this benefit is quite small compared to= its effect on power output.
 
Rob
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