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Flat Eight Porsche Engines


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#51 MatsNorway

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Posted 24 November 2013 - 06:16

There are old american four bangers with bigger volume but you would be down on power. biggest i know of is NA rallycross spesial moulds of the original volvo B23 block moulds. they go up to 3liters. you can probably get them above 2.5L and go turbo on them. There are lots of 6-700hp+ volvo engines in sweden and norway. Is this a drag racing engine?

 

Many of the volvo engines are straight up replicas or close to it of great historical engines. Their diesel straight five block can be mounted with audi heads. A common thing to see here. Many many cars got this and 800-1000hp. the straight five with audi heads is a better option than the 4 cyls from volvo.

 

Another good 4 cyl is probably the mitsubishi Evo X motor. Or something bought from decendant racing. They run a Scion in time attack with way too much power for FWD configuration. The engine seems to blow up often tho. So yea five syl block..


Edited by MatsNorway, 24 November 2013 - 06:37.


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#52 Greg Locock

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Posted 24 November 2013 - 22:15

A billion years ago, well, in 1980ish, i started work with BL Technology's Advanced vehicle group. At the time they were working on the ECV3, a B/C class car with pretty respectable mpg even in today's environment. (Emissions of course were not a concern)

 

So they needed an engine. As one used to do one got all the SAE papers, all the IMechE proceedings, and all the engine textbooks in the company library and went through them to try and see what other people had done (when I was at Lotus this was SOP in my section, it saved a huge amount of time). Meanwhile the cluey guys would consider first principles, slaughter the fatted calf, gaze in to crystal balls. The end result of all this was that the ideal size for a 4 valve spark ignition petrol engine's cylinder was 400 cc (roughly). If it gets too much bigger then the air can't get in through the valves in time, as the valve area is going up with bore^2, and the volume to be filled is bore^2*stroke.

 

However bore/stroke ratio is also somewhat constrained by efficiency considerations, a high b/s ratio has more loss into the head, and other things I've forgotten.

 

If the general mass flow in the system is at anything much more than 1/3 the speed of sound the chances are that at some point going round a bend or through a pinch point the flow will go locally supersonic, vastly increasing the required pressure. That leads in to mach flow choking.

 

Hopefully Manolis can come up with further reasons to avoid huge cylinders in high speed engines.

 

Incidentally that engine eventually turned into the K series engine. ECV3 actually used a lovingly modified engine out of a Daihatsu Charade, 1200cc 3 cylinder. BL as a whole was a pretty annoying place to work, but Advanced Vehicles was the biz.


Edited by Greg Locock, 24 November 2013 - 22:23.


#53 manolis

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Posted 26 November 2013 - 05:54

Hello Greg Lockok.

The 20m/sec mean piston speed seems to be the limit for good volumetric efficiency in most four-stroke engines.
The 25m/sec mean piston speed seems to be the limit in all four-stroke engines (Formula1 included: with 40mm stroke, at 19,000rm the mean piston speed is slightly above 25m/sec).

At higher revs the air / mixture cannot follow the rhythm of the pistons / valves (from another viewpoint: the energy to do so increases sharply).

It is like moving up and down the upper end of a spring that holds at its lower end a mass. If the spring is hard enough and the up-and-down rhythm slow enough, the mass follows the motion imposed at the upper end. As the rhythm of the upper-end oscillation increases (high rpm), things change and the mass makes its own motion.

The air has mass / inertia and elasticity and friction and runs along pipes and though passages.

Given that the mean piston speed cannot exceed a limit, the small displacement cylinders have the advantage / leverage over the big cylinders.

The “limit” of the 10mKp/lt specific torque of the four strokes (as explained in the http://forums.autosp...-rotary-valves/ thread) combined with the “mean piston speed” limit say that the smaller the cylinders, the more the specific power (power per displacement) because the engine with the small cylinders can keep good volumetric efficiency (and so torque) at higher revs (power = revs * torque).

Things are similar for the turbo-charged engines.

Thanks
Manolis Pattakos

#54 Greg Locock

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Posted 26 November 2013 - 21:37

25 m/s is about 1/10 the speed of sound. The intake valve area is maybe 20% of the piston area at the pinch point, so just for the intake charge to keep up with the piston it is approaching my rule of thumb for the start of compressibility effects and local choking.



#55 manolis

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Posted 27 November 2013 - 04:36

25 m/s is about 1/10 the speed of sound. The intake valve area is maybe 20% of the piston area at the pinch point, so just for the intake charge to keep up with the piston it is approaching my rule of thumb for the start of compressibility effects and local choking.

Greg Locock, you are right.

Your rule of thumb (and, equivalently, the 25m/sec mean piston speed limit) seems as having no exceptions among the four stroke engines.

Has it?

Does anybody know a four-stroke providing a good specific torque above 25m/sec mean piston speed?

By the way, this "rule of thumb" / "mean piston speed limit" is for the best designs (large valves, extreme valve overlap, wild cam lobes providing big lift and long duration, big diameter straight and polished intake pipes etc).
The volumetric efficiency of the typical engines (say car engines, motorcycle engines) drops steeply way sooner.

In the plot below (taken from the http://www.pattakon....takonRoller.htm page wherein the pattakon VVA-roller prototype is presented) the blue curve is the lambda value times the injection duration value, i.e. it is the quantity, per cycle, of the air that is handled by the engine (say volumetric efficiency).

no70Analysis.gif

The engine is a modified 1600cc Honda VTEC (B16A2 cylinder head) having 81mm bore, 77.4mm stroke.

At 9,000rpm (wherein the rev limiter was set) the mean piston speed is (0.0774*2)(9000/60)=23.22m/sec.

Reasonably at 10,000 rpm (25.7m/sec mean piston speed) the volumetric efficiency is expected to significantly drop.


Another example:
The Ducati Panigale (desmodromic valve train) with the 1.86:1 (!) bore to stroke ratio.
At 12,000 rpm (redline) the 60.8mm stroke gives a mean piston speed of 24.3m/sec.
Even if the Ducati Desmo could operate reliably at 13000 rpm ( 26.3m/sec mean piston speed), Ducati knows there is nothing to gain.

Thanks
Manolis Pattakos

#56 r1leysp

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Posted 27 November 2013 - 12:07

If I can throw my spanner in the works -when researching my planned twin engine Riley spesh project -I came up with the firing orders of the straight eight U S A  engines of the 30s-40s -thought it may be of interest that most had firing order of    1-6-2-5-8-3-7-4-  the exception being Cadillac--1-8-7-3-6-5-4-2  and Delage 1-3-7-4-8-6-2-5- don't know what the Mercedes are- but would like to know-I know which one I will be going with to start with . Was told by an old straight 8 engine tuner to get the exhaust timing  right which is what has been said earlier. Corse the flat 8 is different but exhaust timing is the the crucial factor with all engines -interestingly the flathead Ford V8 is --1-5-4-8-6-3-7-2--  All different from the flat 8 ? 



#57 TDIMeister

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Posted 27 November 2013 - 14:57

A couple more data points

 

4.2L V8 Audi engine, understroke design with 84.5mm bore, 92.8mm stroke, max. power developed @7800 RPM (24.1 m/s); 25.5 m/s @8250 RPM redline.

 

And from http://www.epi-eng.c...f_cup_to_f1.htm

F1: 25.5 m/s

Cup: 24.8 m/s

 

From: http://www.f1technic.../features/18858

Cosworth TJ @19k RPM: 33.4 m/s

Cosworth CA @19k RPM: 26.4 m/s

 

I don't have the reference for it now, but an RET article on a Top Fuel engine had MPS of 31.4 m/s with 4.5 inch stroke at the regulated 8250 RPM.

 

Interesting subject, I suggest that we prune it to its own thread.



#58 rory57

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Posted 27 November 2013 - 18:41

When asking the original question I was interested to know if a flat eight had been built with the firing order and crankshaft of a flat four, ie with pairs of cylinders firing simultaneously. It seems not from the above but there has been this

http://en.wikipedia....:Ovalpiston.jpg

the Honda V4 NR 750 which is a 32valve four cylinder, sort of. Oval cylinders are not likely to be a practical solution but they do illustrate where the compromises are re valve area, piston speed and so forth.

#59 manolis

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Posted 28 November 2013 - 04:31

A couple more data points

4.2L V8 Audi engine, understroke design with 84.5mm bore, 92.8mm stroke, max. power developed @7800 RPM (24.1 m/s); 25.5 m/s @8250 RPM redline.

And from http://www.epi-eng.c...f_cup_to_f1.htm
F1: 25.5 m/s
Cup: 24.8 m/s

From: http://www.f1technic.../features/18858
Cosworth TJ @19k RPM: 33.4 m/s
Cosworth CA @19k RPM: 26.4 m/s

I don't have the reference for it now, but an RET article on a Top Fuel engine had MPS of 31.4 m/s with 4.5 inch stroke at the regulated 8250 RPM.

Interesting subject, I suggest that we prune it to its own thread.


Hello TDIMeister.

Interesting your link about the Cosworth TJ and CA engines.

The Cosworth CA has 26.4m/sec mean piston speed at 20.000rpm of the redline. At 19,000rpm wherein it provides its peak power the mean piston speed is 25m/sec. I.e. the Cosworth CA supports the rule of 25m/sec mean piston speed limit.

Strange, but the Cosworth TJ has, at the red line, a 33.6% higher mean piston speed than the "limit" of 25m/sec.
Is this translated into a proportional (or at least significant) increase in engine power, or not?
I guess not; the Cosworth CA returns to a "normal" mean piston speed.

The intake valve area is a little bigger than the 1/3 of the piston area.
The "intake valve headbased mean gas velocity" at PPS (peak power speed?) is a little less than 100m/sec for the TJ and only 70.7m/sec for the CA.

Thanks
Manolis Pattakos

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#60 MatsNorway

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Posted 28 November 2013 - 19:37

Copy pasted from Extreme tuning on Facebook...

1397704_633091183395679_348404802_o.jpg

 

Our new 2014 setup based and developed from 4G63 platform.
Previous setup with the 17:1cr the specs and data was:
All this data are calculated in 1bar -14.7psi - 1020hp per 1 litter – 14.7psi – 12200rpms GTX55 - turbo on methanol, Vol. efficiency 264% - 35.2m/sec piston speed – Mechanical efficiency 70.6% and 29300N of piston force. That was our target. Maximum efficiency from a 4 cylinder engine completed. Stock casting head and stock engine iron block with long rod billet internals. We finish the turbo to 39psi that we had 1840hp.
All dynos, specs 60-130 tests and videos are already covered from speednation.tv. While we built this, to road tests etc.

For the new setup now we have ready the below items.
Billet cylinder head
Billet engine block
tungsten counterweight crankshaft
billet aluminum long rods
17.8:1 cr billet lightweight pistons
Titanium valves
Billet cams

We expect more than 320% efficiency ~ 1250hp per litter at 14.7psi and 13.000rpms using our 98mm T6 11blade unit.
More details and pics from the new setup will follow this month.

 

https://fbcdn-sphoto...244099295_o.jpg

https://fbcdn-sphoto...438762003_o.jpg

 

https://fbcdn-sphoto...294005392_o.jpg

 

Formula 1 v8 titanium rods from our inventory. Unfortunately we can not show the previous year's parts or this year parts.
Naturally aspirated V8's of 2400cc 19.400rpms peak power 21.000 redline, 90º “V” configuration, 97.5mm x 40mm, length rod size 100.2mm c to c, 2.5:1 rod to stroke ratio, Mean piston velocity 25.3m/sec @ 19.000rpms, Instantaneous piston velocity 29.53m/sec, Max. piston acceleration. (@TDC) 95.011m/s2.

 

More here:

https://www.facebook...&type=1


Edited by MatsNorway, 28 November 2013 - 20:03.


#61 Canuck

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Posted 29 November 2013 - 01:49

I pretend to know stuff. I want to say that a 500cc cylinder has somehow become the "perfect" displacement for reasons I don't recall. It was part of the design theory (they claimed) behind the BMW M5 5L V10 of the late 2000s. Perhaps Subaru found that 3L was too much of a deviation from that design theory.

#62 Canuck

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Posted 29 November 2013 - 01:56

Oh for duck sakes. I should refresh my open browser before replying to a post that is days old and well-and-truly answered already by folks much smarter than I. (and thanks BlackBerry Autocorrect for keeping my ducking cursing to a good amend minimum)

#63 manolis

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Posted 30 November 2013 - 05:47

Copy pasted from Extreme tuning on Facebook...
Our new 2014 setup based and developed from 4G63 platform.
Previous setup with the 17:1cr the specs and data was:
All this data are calculated in 1bar -14.7psi - 1020hp per 1 litter – 14.7psi – 12200rpms GTX55 - turbo on methanol, Vol. efficiency 264% - 35.2m/sec piston speed – Mechanical efficiency 70.6% and 29300N of piston force. That was our target. Maximum efficiency from a 4 cylinder engine completed. Stock casting head and stock engine iron block with long rod billet internals. We finish the turbo to 39psi that we had 1840hp.
. . .
Formula 1 v8 titanium rods from our inventory. Unfortunately we can not show the previous year's parts or this year parts.
Naturally aspirated V8's of 2400cc 19.400rpms peak power 21.000 redline, 90º “V” configuration, 97.5mm x 40mm, length rod size 100.2mm c to c, 2.5:1 rod to stroke ratio, Mean piston velocity 25.3m/sec @ 19.000rpms, Instantaneous piston velocity 29.53m/sec, Max. piston acceleration. (@TDC) 95.011m/s2.


Hello MatsNorway.

The naturally aspirated Cosworth TJ V10 has at the red line 33.4 mean piston speed. The return of Cosworth to way lower mean piston speeds with their next generation CA V8 shows that there are problems as the mean piston speed goes too high.

The ethanol / turbocharged engine of the Extreme Tuners provides its peak power at 35.2m/sec.
As Extreme Tuners say "That was our target. Maximum efficiency from a 4 cylinder engine completed."
It is interesting the 70.6% mechanical efficiency: with 1020 hp/lt power output, it provides another 425hp/lt in friction.

It seems that when limitation on the kind/type of fuel, on the fuel consumption (fuel efficiency) , on the emissions, on the reliability of the engine etc get into play, mean piston speeds above 25m/sec are rare.
The Audi 4.2L V-8 mentioned by TDIMeister seems as having one of the highest mean piston speeds among the mass production engines (24m/sec at peak power, 25.5m/sec at red line).

Thanks
Manolis Pattakos

#64 MatsNorway

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Posted 02 December 2013 - 14:58

So it is largely related to the flamefront?

 

There is probably a % number for the values you speak of that follows most fuels and their flamefronts.



#65 TDIMeister

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Posted 03 December 2013 - 03:13

It has almost nothing to do with flamefront, as F1 demonstrates that a flamefront can traverse a >100 mm diameter piston bore at 19k RPM in a mere 400 microseconds, if one assumes a combustion duration of 45°CA.  This would put the radial flame velocity at about 125 m/s, which is about 360X the laminar flame speed of gasoline in air at 1 bar, 298K and lambda near the peak of 0.85.  What gives the increase in flame speed is turbulence and this has been shown to scale with mean piston speed in SI engines.

 

The mean piston speed establishes a physical limit for engines in two ways.  First, on the performance front, the gas dynamics fall off as has been stated in several posts above due to high losses and local choking.  Secondly, tribology becomes a very big problem.  Friction (FMEP) scales with RPM, that is, mean piston speed, so there will be diminishing returns beyond an optimal value of RPM for a given engine geometry and there is also a limit of peak component sliding velocities that lubrication breaks down and drastically curtails engine durability.  That's one reason why drag racing engines can survive only seconds-long stints at mean piston speeds exceeding 30 m/s and so can other race engines can allow briefly higher rev limiters at the consequence of drastically reduced life.