Educate me. Turbos and Blowers

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Whitebear
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Educate me. Turbos and Blowers

Postby Whitebear » Thu Jan 07, 2016 11:34 am

Its been nearly 3 decades since I have messed with this stuff so I am guessing both the engineering design and the actual pressure ratios are quite different in today's world. Be it aircraft or automotive engine applications.

Terminology. I had learned the superchargers were gear or belt driven Blowers and that Turbochargers were exhaust gas driven vane type air compressors. Is supercharger a generic term that means either Roots type blower or Turbo compressor? I have some confusion here.

From past memory I remember Blowers being limited to a pressure ratio of about 1.2:1 which would make the output pressure around 17.64 PSI assuming an ambient sea level 14.7 PSI day. Now if that 17.64 is measured with a gage that shows 14.7 PSI as ZERO, isn't this a 120% increase? Shouldn't the gage be calibrated to read 14.7 to actually measure the 1.2:1 pressure ratio? Shouldn't the actual pressure increase be 2.94 PSI if the ratio is 1.2:1 ?? Obviously I am forgetting or totally missing something here.
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Re: Educate me. Turbos and Blowers

Postby Skeeter-Gee » Thu Jan 07, 2016 12:17 pm

The outboards I deal with that are supercharged ,the data stream shows vacuum and not necessarily pressure , i.e. a normal 275 mercury verado is approx -11"hg at idle .That motor is capable of 15 psi of boost under acceleration , which is seen as a 11" hg .Now an engine naturally aspirated at wot should theoretically be equal to atmospheric pressure depending on elevation etc .So if you add a positive boost over natural aspiration then whatever that boost is yes could technically be 120 percent gained at 17 psi nominally.

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Re: Educate me. Turbos and Blowers

Postby Whitebear » Thu Jan 07, 2016 12:30 pm

Ok, I follow what you are saying but that leaves the 1.2:1 pressure ratio out completely. I have some VERY old notes that only talk to pressure ratios, but that isn't the follow up question which is: Does this 120% boost translate to a 120% increase in cylinder pressure at compression?
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Re: Educate me. Turbos and Blowers

Postby digginfool » Thu Jan 07, 2016 2:15 pm

I'm an engineer but not a mechanical so going on memory. A Roots type blower compresses the air within the blower itself at about a 1.2:1 ratio. At sea level, this would mean a 2.94 psi increase. However, as you most likely know, there are supercharged engines that are running way more boost than that. That happens because the blower is cramming more air into the manifold than the engine can digest and that is how the higher pressure is created. It is maintained by the tight seal between the rotor and the case and regulated by the pulley ratio. The manifold pressure is gauge pressure; not absolute. With 18 pounds of boost, your manifold (gauge) pressure would be 18 psi but absolute pressure would be ~32.7 psi (at sea level). By the way, blowers and turbochargers are both means of supercharging. Over time, it became easier to differentiate between the two by calling all mechanical driven means superchargers and exhaust driven means as turbos. Now, the only evidence I can provide regarding one-to-one relationship is this. My DB has the LSA motor on it, which is a supercharged LS3. The pistons are 9.5:1 and the maximum boost is 9 psi with an effective compression ratio of 13.5:1. So, 9 psi would represent a 61% increase over atmospheric pressure, absolute sea level. The compression ratio increase is 4:1, which represents a 42% increase. So, at least with the LSA, there does not appear to be a 1-to-1 relationship. Hopefully, somebody with a fuller understanding will come along but that is my reasoning.
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Re: Educate me. Turbos and Blowers

Postby goldhunter_2 » Thu Jan 07, 2016 2:37 pm

Whitebear wrote:Terminology. I had learned the superchargers were gear or belt driven Blowers and that Turbochargers were exhaust gas driven vane type air compressors. Is supercharger a generic term that means either Roots type blower or Turbo compressor? I have some confusion here.


Blower is generic for supercharger belt or gear drive as you said , if I recall there are three styles Roots type being the most popular.
a supercharger has instant power right off the line but flattens out on type end where a turbo take a few minutes to spool up so slower start but more top end
.


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Re: Educate me. Turbos and Blowers

Postby digginfool » Thu Jan 07, 2016 2:55 pm

I did a little research and found the following equation:

Ptdc=P0*CR^1.3 where: Ptdc is pressure at top dead center
P0 is pressure at bottom dead center (normally atmospheric)
CR is compression ratio
1.3 is specific heat ratio for an air-gasoline mixture

The article where I found this equation pertained to a naturally aspirated engine but does provide an interesting output. If we call P0 as 9 psi (maximum boost for my supercharger) and compare the actual compression ratio of 9.5:1 to the effective compression ratio of 13.5:1, we get the following:

9.5 CR = 169.9 psi
13.5 CR = 268.2 psi

The difference works out to be 58%, which is getting pretty close to that on-to-one relationship. Again, I'm not a mechanical and my education in thermodynamics is quite limited but this seems reasonable to me.
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Re: Educate me. Turbos and Blowers

Postby Whitebear » Thu Jan 07, 2016 3:40 pm

Seems like at BDC the pressure with no carb or supercharger would be 14.7 PSI
at 9.5:1 CR that TDC would be 14.7*9.5=139.65 PSI.

Add the induction system with 9 PSI boost then the BDC pressure would be
14.7+9=23.1 PSI now at TDC that 23.1 would be 23.1*9.5= 219.45 PSI.

Not sure I'm reasoning this correctly either. Anxious to hear more.
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Educate me. Turbos and Blowers

Postby spooledup50 » Wed Dec 07, 2016 4:08 pm

Pressure ratio of 1.2 Pr x bp - bp = boost at sea level 1.2 x14.7-14.7 =2.9 psi of boost
Bp is barometer pressure

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Re: Educate me. Turbos and Blowers

Postby 90chevy396 » Wed Dec 07, 2016 4:55 pm

superchargers are a mechanical drive device to force air volume into the combustion process which is roots and cytrifugal being roots mounted above the intake and cannot be intercooled and cyntrifugal being driven by the engine mechanically also but not mounted directly to the intake thus allowing for an intercooler to densify the air to get more air into the combustion process. turbo chargers are driven from the exhaust so the more air going into the engine the more air coming out the engine which results in more air being forced into the engine at the higher rpm range. turbo chargers run into an intercooler also to densify the air to result more air too.
as for the engineering side of I am lost on raitos and such
pretty sure my rant didn't clear anything up for ya
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Re: Educate me. Turbos and Blowers

Postby br,lc,LA » Wed Dec 07, 2016 10:28 pm

Compression Ratio has to do with the change of volume of the air in the cylinder. The pressure in the cylinder has do do with the volumetric efficiency of the engine or in simple terms how much air it can suck in on the intake stroke. NA engines are totally vacuum dependent basically you have 14.7PSIA air outside the intake, less the pressure drop of the intake train, so the pressure in the cylinder is below 14.7psia.

Both methods supercharging or turbo charging are just designed to increase the volumetric efficiency of the engine by putting the air outside the cylinder above atmospheric and forcing air in along with the natural vaccuum pulling it in. If turbo or super were designed to match the engine air intake rate per cylinder you would see basically a sinusoidal wave of the pressure going up and down as each cylinder intake bc the system in perfectly match... this is never the case... most are oversized and have bypasses set up to control the intake pressure in the manifold or tubing prior to going into the engine and maintain pressure as the cylinders make their intake stroke...

So theoretically for instance say an NA engine may have 13 to 14psia air... maybe as there is a significant pressure drop through the air filter and valvetrain and I run 22psig boost (36.7psia), therefore I have theoretically increased the amount of volume air charge in my cylinder ~2.8x or 280%, but adding in the temperature aspect say outside air for us avg 70-90deg and compressed air can run upwards of 100deg or hotter you actually can negate some of you pressure increase b/c you lost mass of the charge due to lower density hot air so intercooling back down to 70deg becomes a significant power increase after boosting to higher pressures.
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Re: Educate me. Turbos and Blowers

Postby Deano » Wed Dec 07, 2016 10:40 pm

Reading this from the top had me confused, trying to get all the numbers presented to jive. :scratch:

It looks to me like spooledup50 filled in your missing blank Scotty.
I didn't recognize your 2.94 number in the beginning, but that is the result of spooledup's formula.

Not sure what you were trying to figure out, but thought this might help as I had it here anyway.

Image

I would remind you to bear in mind that trying to reconcile formula results with real world numbers such as Diggin posted might give you a headache if you do not allow for the difference between static and dynamic compression ratios. Attaining specific psi numbers would require acknowledging the cam specs. Obviously, the valves do not open and close at TDC and/or BDC respectively. :wink:
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Educate me. Turbos and Blowers

Postby spooledup50 » Thu Dec 08, 2016 12:21 am

1.21 is a pressure ratio for calculating a mass volume of air has very little to do with boost pressure , but more on the volume of air , volume = flow psi = restrictions


Most of and I say. Most aircraft are what we call boost normalized at sea level there is very little boost 2 psi maybe 3 on a cold. Night On the ground atmospheric pressure is 14.7. But altitude increases atmospheric pressure decreases the available pressure at 20,000 feet is almost 50%less about 8psi so the math goes like this 1.2x14.7-14.7=2.9 psi of boost , at 20k 1.2x8-8=1.9 psi of boost .

The supercharger cannot spin faster so with less atmospheric psi it looses efficiency and this is calculated in keeping the engine normalized basically holding the engine at sea level where it will produce the most power , without stressing any of the internals and keeping it safe and reliable ,
Turbochargers have no mechanical connection and can be driven faster this allows them to produce a higher volume of air at a higher altitude allowing aircraft to be certified to fly at higher altitude were a supercharger will run out of volume and the engine will start loosening hp again.

So if you take a 260hp o540 and add 3 psi you get around 315hp
If you were to take a 260hp o540 and ad 17psi you should have close to 560 hp we all no that's not a reality with the stock supercharger impeller ,

If the supercharged 540 is seeing a higher boost pressure at sea level than the above , math than ether you have a restricted intake or the 1.21 pressure ratio given above is incorrect,

Now on to this boost pressure and cranking compression , they are not really related at All

The valve timing has more to do with cranking compression than anything, yes the psi will go up but not as drastic as written above , valve overlap sheds cylinder pressure in both natural aspiration and boosted engines ,

So say you have 10 psi on a gauge the valve opens you don't exactly fill that cylinder with 10 psi you may actually only get 7psi in the cylinder , do to pressure drop of the valve opening and the volume of air that has to move past the valve in that given time , plus air that escapes from valve overlap Now the more efficient the super/ turbo is at producing a large dense volume of air the more power an engine will make at the same boost pressure because it's flowing a higher volume of air






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Re: Educate me. Turbos and Blowers

Postby CactusJack » Thu Dec 08, 2016 2:11 am

Superchargers can also be used on the exhaust side.... I cant remember which plane, might be the spitfire or Mustang had a supercharger pulling the exhaust out of the cylinders as another on forced air in.... and there are different types of superchargers. Some of the modern 'superchargers' are just like the cold half a turbo but driven by a belt, other centrifugal superchargers have just got an impeller in the 'snail shell' type housing.... the rootes type have the intermeshing rollers with nylon tips that actually cram the air/fuel mix into the cylinders with very little air/mixture able to escape nor go anywhere but thru the motor.

The whole idea of turbo/supercharging of engines is fascinating... I love that stuff. It can make such a small capacity motor do so much work (compared to its physical size) .... it is pretty plain to see that motors are going smaller in capacity and bigger on supercharging and turbocharging.

We only need the end of the crankshaft to have X amount of Hp to turn the gearbox/prop.... I am one who loves the raw horsepower of a giant rumbling big block doing the work.... they are absolutely awesome.... but if a 1.5L supercharged and turbocharged motor could provide the same shaft horsepower with the motor weighing well less less than half of that of a big block.... makes for some awesome possibilities.

The more air/fuel mix that can be stuffed into a cylinder and extracted at high speed, the harder it is to get heads to stay on and head gaskets/seals not to blow.... component quality has to be good as well as balance and the fuel quality.... but very exciting times ahead I think .....

Great topic Whitebear :salute:
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