X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 13 Jun 2011 07:50:04 -0400 Message-ID: X-Original-Return-Path: Received: from mail-fx0-f52.google.com ([209.85.161.52] verified) by logan.com (CommuniGate Pro SMTP 5.4.0) with ESMTPS id 5018164 for lml@lancaironline.net; Sun, 12 Jun 2011 13:51:37 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.161.52; envelope-from=msteitle@gmail.com Received: by fxm6 with SMTP id 6so2339789fxm.25 for ; Sun, 12 Jun 2011 10:51:00 -0700 (PDT) DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; b=MuhvmsLgr7eiG/+Sc67opOy2MWVL0tdPBxM/EkZrG/7DXHh9XkJCpLJOmFmqShgCCL mDuEzIZYCpwtNGtb8TD9o0eevW+zW2+DjuwOaVsetfxdwKmu7SvdCw7UHqaOnXiyyr1F BmjDH6B/ZE0GT+2vTmK4u7fN6KXx9txh3UZ0E= MIME-Version: 1.0 Received: by 10.223.55.200 with SMTP id v8mr4295045fag.82.1307901059977; Sun, 12 Jun 2011 10:50:59 -0700 (PDT) Received: by 10.223.100.12 with HTTP; Sun, 12 Jun 2011 10:50:59 -0700 (PDT) In-Reply-To: References: X-Original-Date: Sun, 12 Jun 2011 12:50:59 -0500 X-Original-Message-ID: Subject: Re: [LML] Re: L-IV Choice of Engine From: Mark Steitle X-Original-To: Lancair Mailing List Content-Type: multipart/alternative; boundary=00151747616e14d34d04a587726f --00151747616e14d34d04a587726f Content-Type: text/plain; charset=ISO-8859-1 Gary, Good analysis of the benefits of piston vs. rotary. The only thing I really disagree on is the compression ratio being limited to 8.5 to 1. The RX-8 is 10.0 to 1. So I would say that point is a draw. From www.Mazdausa.com 2011 RX-8SportGrand TouringR3EngineEngine type1.3-liter 2-rotor rotaryEngine codeRENESISHorsepower- Manual transmission232 hp @ 8500 rpm- Automatic transmission212 hp @ 7500 rpmTorque159 lb-ft @ 5500 rpmRedline- Manual transmission9000 rpm- Automatic transmission7500 rpmDisplacement (cc)1308Compression ratio10:1Fuel systemMulti-port electronic fuel injectionRequired fuelPremium unleaded (91 octane) Mark S. On Sun, Jun 12, 2011 at 11:55 AM, Gary Casey wrote: > Probably no real reason to keep this thread going, but it is interesting. > I'll intersperse my comments with those from Mr. Wilsons, of which I > generally agree. > Gary Casey > From: > "William Wilson" > > Although the Wankel produces twice as many power pulses per revolution as a > piston (leading to a 2-rotor Wankel being equivalent to a 4-cylinder piston, > etc.) the torque ripple is actually better in the rotary because the power > pulses are stretched out over a longer portion of the rotation. The lack of > reciprocating motion also contributes to overall lower vibration. > Yes, the power stroke, in theory at least, lasts 270 degrees instead of 180 > as in a reciprocating engine. But most of the power impulse is created in > the first 20 or so degrees. Still, 1 point for the rotary. And yes, the > inertia forces are a pure sine wave so they can be balanced perfectly with > balance weight, although at a weight penalty. But the non-sinosoidal motion > of the piston has an advantage - the piston spends less time at TDC than at > BDC, helping to reduce the octane requirement and giving more time for > scavenging. Still, another point for the rotary. > > > The fuel consumption problems of the Wankel are greatly overstated. The > Wankel does have this reputation, but there are many ways it is undeserved. > This reputation was earned in the automotive market, and as so many people > are fond of saying, the automotive environment and the aircraft environment > are different. While it's true that the combustion chamber is not ideally > shaped, the Wankel is mechanically much more efficient. There are no valve > springs to push against and no reciprocating parts to force up and down. > Secondly, the Wankel engine is capable of running very lean. With no valves > to burn, and being liquid cooled, it simply has a lot more leeway. The > moving, swirling combustion chamber results in a high turbulence environment > more resistant to knock. The intake and compression cycles take place in a > different part of the engine than the combustion cycle, so they are at lower > temperatures and reduce the risk of preignition. Finally, the Wankel is > inherently a stratified charge engine. The rotating motion produces a > centrifuge effect, and since fuel molecules are heavier than oxygen, they > tend to settle toward the outside of the combustion chamber where the > spark plugs are, resulting in a stratified charge. > Yes, the rotary is more adaptable to a stratified charge concept, but I > don't know how to easily take advantage of that capability within the cost > and time constraints of an experimental aircraft engine application. And > yes, the rotary can more readily tolerate running lean with low octane fuel. > But comparing the BSFC of a piston engine running ROP to a rotary running > LOP isn't realistic. On an even comparison the rotary comes out on the > short end of the stick. I think the BSFC penalty is 5 to 10%. That says > that you have to carry at least 5 gallons (out of a hundred) more with a > penalty of 60 pounds and $25 for a long trip. Does it matter? I don't > know. I think you'll find that the turbulence is actually less, but the > average velocity is higher, since the charge has to be pumped from one lobe > to the next through the slot in the rotor. The high velocity increases the > heat transfer, increasing the BSFC and the heat input to the coolant. Yes, > part of the trochoid is always "cold", but the counter argument is that the > other part is always hot, creating a difficult cooling problem. Preignition > and detonation occur in the hot part. I think the piston engine gets 1 > point. > > > As a practical matter, many or most piston engine aircraft owners do not > run lean of peak to prolong the life of their engine. Rotary owners simply > do not have this concern. Because of this, in practice, rotary engine > aircraft get the same or perhaps even better fuel consumption compared to > piston engines. There are also no concerns about mogas or low octane > (although ethanol can still be a problem for fuel tanks and plumbing, of > course). > Oops, I mentioned this in the previous comment. Since the rotary is > geometrically limited to compression ratios not much more than 8:1 the > rotary will have an octane requirement advantage over a piston engine with a > 10:1 compression ratio. But then you might as well turbocharge the rotary > engine - and reduce the exhaust noise level at the same time. I think the > point goes to the piston engine. > > Anyone who is tempted to disagree because of the bad reputation of > late-model RX-7s for knock, realize that resistance to knock happening is > not the same thing as being able to survive it when it does happen. Mazda's > terrible manifold and fuel injection design on this model car resulted in > plenty of mixture problems, enough to overcome even the favorable attributes > of the engine. The 1974-1991 model engines, which greatly outnumbered the > 3rd generation, had none of these problems. > I don't know about the details of the Mazda design - I can only assume that > an average homebuilder (okay, above average) can do a better job than the > legions of Mazda engineers that have been working the problem for 30 years. > > I agree with Brent's comments below. But the SCORE project was > specifically attempting to create a jet-fuel-powered rotary. Are the > results applicable to a gasoline version? Don't know. Regardless, they put > a ton of money into the project and nothing came of it. It is just hard to > overcome the advantage of the piston engine sealing system of just one > cylindrical sliding seal compared to a complex shape with many more > interfaces and joints. > Gary Casey > > On Thu, Jun 9, 2011 at 12:13 PM, Brent Regan > wrote: > >> >> The Rotary engine does have some advantages for aircraft application. >> Lacking a connecting rod, the primary vibration is a pure sinusoid and the >> one power pulse per revolution per rotor gives a 4 rotor the same torque >> ripple as a 8 cylinder piston engine. The "yes but" of the Wankel design is >> the really bad combustion chamber shape that leads to high fuel consumption. >> Also, having a 10:1 effective compression ratio limit prevents implementing >> a direct injection compression ignition (diesel) cycle. Curtis Wright spent >> years and millions developing their SCORE (Stratified Charge Omnivorous >> Rotary Engine) family of Wankel engines but ultimately sold the design to >> John Deere. >> >> After more than 100 years, the prime mover of choice still has a piston, >> rod and crank. There may be a reason for that. > > > The piston engine was invented first. The rotary is . > > > >> >> >> Regards >> Brent Regan >> > --00151747616e14d34d04a587726f Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Gary,=A0

Good analysis of the benefits of piston vs. rot= ary. =A0The only thing I really disagree on is the compression ratio being = limited to 8.5 to 1. =A0The RX-8 is 10.0 to 1. =A0So I would say that point= is a draw. =A0

Engine
2011 RX-8 Sport Grand Touring R3
1.3-liter 2-rotor rotary
RENESIS
232 hp @ 8500 rpm
212 hp @ 7500 rpm
159 lb-ft @ 5500 rpm
9000 rpm
7500 rpm
1308
10:1
Multi-port electronic fuel injection
Premium unleaded (91 octane)
Mark S.


On Sun, Ju= n 12, 2011 at 11:55 AM, Gary Casey <casey.gary@yahoo.com> wrote:
Probably no real reason = to keep this thread going, but it is interesting. =A0I'll intersperse m= y comments with those from Mr. Wilsons, of which I generally agree.<= /font>
Gary Casey
From:
"William Wilson" <fluffys= heap@gmail.com>

Although the Wankel produces twice as many power pulses per revolution as a= piston (leading to a 2-rotor Wankel being equivalent to a 4-cylinder pisto= n, etc.) the torque ripple is actually better in the rotary because the pow= er pulses are stretched out over a longer portion of the rotation.=A0 The l= ack of reciprocating motion also contributes to overall lower vibration.
Yes, the = power stroke, in theory at least, lasts 270 degrees instead of 180 as in a = reciprocating engine. =A0But most of the power impulse is created in the fi= rst =A020 or so degrees. =A0Still, 1 point for the rotary. =A0And yes, the = inertia forces are a pure sine wave so they can be balanced perfectly with = balance weight, although at a weight penalty. =A0But the non-sinosoidal mot= ion of the piston has an advantage - the piston spends less time at TDC tha= n at BDC, helping to reduce the octane requirement and giving more time for= scavenging. =A0Still, another point for the rotary.


The fuel consumption problems of the Wankel are greatly overstated.= =A0 The Wankel does have this reputation, but there are many ways it is und= eserved.=A0 This reputation was earned in the automotive market, and as so = many people are fond of saying, the automotive environment and the aircraft= environment are different.=A0 While it's true that the combustion chamber is not ideal= ly shaped, the Wankel is mechanically much more efficient.=A0 There are no = valve springs to push against and no reciprocating parts to force up and do= wn.=A0 Secondly, the Wankel engine is capable of running very lean.=A0 With= no valves to burn, and being liquid cooled, it simply has a lot more leewa= y.=A0 The moving, swirling combustion chamber results in a high turbulence = environment more resistant to knock.=A0 The intake and compression cycles t= ake place in a different part of the engine than the combustion cycle, so t= hey are at lower temperatures and reduce the risk of preignition.=A0 Finall= y, the Wankel is inherently a stratified charge engine.=A0 The rotating mot= ion produces a centrifuge effect, and since fuel molecules are heavier than= oxygen, they tend to settle toward the outside of the combustion chamber w= here=A0the spark plugs are, resulti= ng in a stratified charge.
Yes, the = rotary is more adaptable to a stratified charge concept, but I don't kn= ow how to easily take advantage of that capability =A0within the cost and t= ime constraints of an experimental aircraft engine application. =A0And yes,= the rotary can more readily tolerate running lean with low octane fuel. = =A0But comparing the BSFC of a piston engine running ROP to a rotary runnin= g LOP isn't realistic. =A0On an even comparison the rotary comes out on= the short end of the stick. =A0I think the BSFC penalty is 5 to 10%. =A0Th= at says that you have to carry at least 5 gallons (out of a hundred) more w= ith a penalty of 60 pounds and $25 for a long trip. =A0Does it matter? =A0I= don't know. =A0I think you'll find that the turbulence is actually= less, but the average velocity is higher, since the charge has to be pumped from one lob= e to the next through the slot in the rotor. =A0The high velocity increases= the heat transfer, increasing the BSFC and the heat input to the coolant. = =A0Yes, part of the trochoid is always "cold", but the counter ar= gument is that the other part is always hot, creating a difficult cooling p= roblem. =A0Preignition and detonation occur in the hot part. =A0I think the= piston engine gets 1 point.


As a practical matter,= many or most piston engine aircraft owners do not run lean of peak to prol= ong the life of their engine.=A0 Rotary owners simply do not have this conc= ern.=A0 Because of this, in practice, rotary engine aircraft get the same o= r perhaps even better fuel consumption compared to piston engines.=A0 There= are also no concerns about mogas or low octane (although ethanol can still= be a problem for fuel tanks and plumbing, of course).
Oops, I me= ntioned this in the previous comment. =A0Since the rotary is geometrically = limited to compression ratios not much more than 8:1 the rotary will have a= n octane requirement advantage over a piston engine with a 10:1 compression= ratio. =A0But then you might as well turbocharge the rotary engine - and r= educe the exhaust noise level at the same time. =A0I think the point goes t= o the piston engine.

Anyone who is tempted to disagree because of the bad reputati= on of late-model RX-7s for knock, realize that resistance to knock happenin= g is not the same thing as being able to survive it when it does happen.=A0= Mazda's terrible manifold and fuel injection design on this model car = resulted in plenty of mixture problems, enough to overcome even the favorab= le attributes of the engine.=A0 The 1974-1991 model engines, which greatly ou= tnumbered the 3rd generation, had none of these problems.
I don't know about the details of the Mazda de= sign - I can only assume that an average homebuilder (okay, above average) = can do a better job than the legions of Mazda engineers that have been work= ing the problem for 30 years.

I agree with = Brent's comments below. =A0But the SCORE project was specifically attem= pting to create a jet-fuel-powered rotary. =A0Are the results applicable to= a gasoline version? =A0Don't know. =A0Regardless, they put a ton of mo= ney into the project and nothing came of it. =A0It is just hard to overcome= the advantage of the piston engine sealing system of just one cylindrical sliding seal compared to a c= omplex shape with many more interfaces and joints.
Gary Casey

On T= hu, Jun 9, 2011 at 12:13 PM, Brent Regan=A0<brent@regandesigns.com>=A0wrote:

The Rotar= y engine does have some advantages for aircraft application. Lacking a conn= ecting rod, the primary vibration is a pure sinusoid and the one power pulse per revolution per rotor gives = a 4 rotor the same torque ripple as a 8 cylinder piston engine. The "y= es but" of the Wankel design is the really bad combustion chamber shap= e that leads to high fuel consumption. Also, having a 10:1 effective compre= ssion ratio limit prevents implementing a direct injection compression igni= tion (diesel) cycle. Curtis Wright spent years and millions developing thei= r SCORE (Stratified Charge Omnivorous Rotary Engine) family of Wankel engin= es but ultimately sold the design to John Deere.

After more than 100 years, the prime mover of choice still has a piston= , rod and crank. There may be a reason for that.

The p= iston engine was invented first.=A0 The rotary is .

=A0


Regards
Brent Regan
=

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