Blown and boosted questions. 2

[540ZCar]

First I want to say there's a wealth of information on sizing a turbo for an engine but I can't find a single think about picking what size twin-screw supercharger for a certain engine, I spent that last week looking for how to do it. How do you know if you need a 1.6L or 2.2L Autorotor supercharger?

Then I've been thinking, for NA builds building compression ratio is a way to get power and for boosted engines they lower the compression ratio so they can put more boost into the system. Would you get the same or less power if you went with a higher compression engine with low boost (think like 10.5:1 on 8 PSI) or low compression with higher boost (9:1 on 20 PSI)? I would think that if your on the limit of your octane, then you would get the same power. But the higher compression one could probably net you more low end torque and power with increases off-boost performance.

My last question about forced induction right now is the comparison of a centrifugal supercharger and a turbo, since they're similar in design do they have the same boost threshold and boost characteristics?

 

[540ZCar]

I'm adding a second part of this because it just popped in my head after do some reading. The Subaru EJ series and Mazda 13B are both acknowledged of being lightweight with a low center of gravity. From what I've found the 13B Renesis is claimed to be 122KG (roughly around 260LBs)engine only and dry. I've found a site that had a photo of a SOHC EJ22 on a scale and it read 214.8LBs wet with no flywheel, exhaust, alternator or damper. The DOHC is probably a little bit heavier. Which do you think would be lighter and/or lower CoG, a blown (yes supercharged) EJ25 or a boosted 13B?

 

[Five9dak]

What type of boost are you looking to run?

As you probably know, the intake manifold pressure comes from differential displacement between the compressor and engine. The autorotors have internal compression (my 2.2L is about 6psig IRC) but ultimately the manifold pressure depends on engine size, blower size, and relative speed.

The 2.2L autorotor for example has a max rpm of 19,000. This can't be exceeded or the rotors can seize in the case. Divide this number by your engines max reasonable rpm, in my case 6500rpm. So my pulley ratio can be at most 2.92.

My engine is 5.9L, but since it's a four stroke you only get that displacement every two revolutions. So take half this, 2.95L.

Take our pulley ratio and multiply it by the compressor size and you'll get 6.42. Now divide this by our engine displacement per rev and you'll get a 2.18 pressure ratio. This is a no loss and no belt slip number, but it yields 17+ psi of boost at max rotor rpm.

These numbers are just for example and what I have in the garage, but you can use this formula to get you in the right ballpark for what you want to do.

(2*BlowerDisplacement*PulleyRatio)/EngineDisplacement=PressureRatio

Boost(psig) =(14.7*PressureRatio)-14.7

The max rotor rpm limitation makes for some interesting situations. For example, if I choose to lower shifts points a bit I can use a larger pulley ratio, which gives me more boost everywhere and more torque across the board, with less piston speed (and rod stresses) up top. The smaller rpm band with more boost may very well get a vehicle down the track quicker.

 

[gt401966]

Hi

I am new to this site and have experience on you question about compression ratio.

This was a back to back test.

This was 10 years ago so I am working off memory.
I have approached it both ways on a turbo engine(3sge Toyota)
1ST tempt
9.5:1 comp 15psi boost
eng made 180rwhp@4500 225rwhp@6.500.
The issue I found was ignition timing playing a major factor in HP & TRQ.
The engine was on 98octane unleaded.
T25 turbo

An easy way to explain it is the fuel has a burn time of say 45deg of crank rotation to propagate then do its job in producing push onto the piston.
When you get max cylinder pressure you want the piston ATDC(say 15deg) so you dont damage the rod or bearings.

Max cylinder pressure at approx 15 deg atdc was not attained due to lack of ignition timing to control knock.
The eng had about 5deg BTDC of ign timing putting max burn at say 35-40deg ATDC,by this time the piston is moving away from the flame front and not utilizing the energy in the fuel to make TRQ.

2ND attempt
7.5:1 comp.
This time I was able to add more timing to utilize the burn time in the fuel with a much better result.
225RWHP@4500 283RWHP@6500. on 12psi ,it was having a small spike of boost in the mid range to 15psi but would settle on 12.
IGN at 283@6500 was around 18-20BTDC.

Most high comp race engines are on restrictor's so they cant get the cylinder fill and try by other means to make power.
If you are going to throw low boost at it then it will more likely be OK as well on higher N/A comp.

A friend built an SR20 Nissan engine on higher comp and experianced the same result as my 1st attemt.

Hope this helps.

 

[540ZCar]

That's actually pretty interesting. So with the higher compression and higher boost (which you would think would make more power) you weren't able to control spark good enough so when you lowered the compression and lowered the boost you were able to optimize the spark enough to make more power. Is that the gist of it?

 

[Noidly1]

Running high comp with boost requires a fuel with a very high motor octane. Some will argue this next part, many high octane fuels do burn slower, however there are some that offer a good burn speed and good octane #'s.

While the theory that you can make more cylinder pressure for more power with high comp with boost, it makes much more sence to run lower comp and have more room for more A/F charge, which inturn makes more power with less engine wear.

 

[Noidly1]

One thing I forgot to mention;

Higher comp pressure doesn't neccesarily = more power,
it does however help pumping efficiency in natural asperated motors but, that is not an issue with blown motors.

 

[gt401966]

540zcar yes that is correct.
If you have to much comp and to much boost you will run out of ignition timing.
Ignition timing is more important than the boost.
Easy to paint yourself into a corner come tune time.

It will relate to the fuel you choose to run with your set up.
My experience with this eng was on pump fuel.

Some say it makes it soft when not on boost but tossing more ignition timing at it will sharpen it up.

 

[gt401966]

When I say soft,due to lower comp

 

[patprimmer]

Do a google search of this site to find detailed discussion of relationship between octane and burn rate or not and why.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[mattsooty]

Gents,

I cant help but feel more than a little surprised at finding this thread on this forum (almost as surprised as I was reading Pats, most restrained, response).


@540zcar
1) the sizing of a compressor is intrinsically linked to the information contained within the compressor map & also the air consumption of the engine. Thats what engineers do.

2) when talking about using a higher compression ratio/low boost & lower compression ratio/high boost you are missing a massive amount of information about this 'comparison'. The stark & salient issue being charge temp......this will obviously change with compressor efficiency and also boost pressure and has a strong influence on the knock resistance (and hence final compression ratio) of an engine.

3) the notion of 'boost threshold' is all but a misnomer for a supercharged application - because the boost pressure in such a case is indepenant of mass air flow through the engine but is a factor of compressor (engine) speed.

@Five9dak
4) 'what boost do you want to run' - what a laugh; for the same reasons as in 2) the real question is what mass airflow does your engine require (do you want) and how can you size a (turbo)compressor to meet this, with the heat rejection constraints of your charge cooling hardware.

@Noidly1
5) Yes, I guess you are stating the obvious in your first post, confusing as it is - but I am not really sure what you are saying here.....:-

"Higher comp pressure doesn't neccesarily = more power,
it does however help pumping efficiency in natural asperated motors but, that is not an issue with blown motors."

That is absolute BS mate, in no way (within reason) does the CR of an engine ever affect its pumping efficiency. Yes in the theorized 4 stroke thermodynamics cycle an infinitely small combustion volume (=0) is assumed, hence there is perfect efficiency for this factor.

@GT401966
6) Another example of the confusion between boost, mass airflow & charge temp. Seriously, its not down to CR, or RON/MON or anything at all in particular - its a system and that means interactions...

For me this thread seems to mark a dangerous watershed, where the Rice banger 'Tuners' start peddaling their piffle instead of good engineering principles..........

MS

 

[Lou Scannon]

Hear, hear...

 

[patprimmer]

Matty

I must have been in a good mood. The kindergarten level and inaccuracies where at least presented in a polite manner and I thought it might just improve a little.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[HOLDAFLAT]

Higher compression ratio will increase the thermal efficiency of an engine, but not by any massive amount.

From memory i believe maximum thermal efficiency occurs at an effective compression ratio of 16:1, and after this there are no real gains to be had on gasoline.

The effective compression ratio is found by:
CR(effective) = ((14.7 + BOOST)/14.7 * CR(static))*VE

eg. the effective compression ratio of a 8:1 engine on 15psi boost with a volumetric efficiency of 85% is;
CRe=((14.7+15)/14.7)*8)*0.85
CRe=13.74

I have found that BP Ultimate will allow an effective CR of about 16:1 to 18:1 before detonation persists.

HP = air+fuel

Figure out how much air and fuel you want to burn, then how much "boost" is required to flow that much air, then figure out what compression ratio you need to run that boost on your fuel.

 

[Five9dak]

@mattsooty:

I labelled the simple approach I provided him as a "ballpark" way to size the compressor. If he wanted to go more indepth I'm sure a little research could lead him to an iterative method with target mass air flow, volumetric efficienceis ratio, PR, temperature gain and density ratio (depending on his charge cooling).

I took for granted that a performance minded individual asking about alternative compressors for a factory boosted and intercooled engine, would use an appropriate intercooler.

I didn't realise all responses on this forum need to come in text book form, if the OP was looking for that I'd think he would be at the library.

 

[Five9dak]

@OP:
ISBN 978-0-8376-0168-7 chapter 7 will get you what you are looking for if the first approach isnt exact enough for you.