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Old 02-27-2012, 09:30 PM   #21 (permalink)
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Why would I want to bring down dynamic compression?

Its only about 21:1 or 22:1 running up to maybe 20lb of boost.
To control temperature and pressure inside the cylinder I am going to use an intercooler and water injection.

Adiabatic compression calculations put me at about 1100psi and 1400'F just running N/A, Think thats high?
I ran the numbers for hot uncooled boost and I was getting numbers as high as 3000psi and 4000'f
And thats before the fuel get injected.

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Last edited by oil pan 4; 02-27-2012 at 09:41 PM..
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Old 02-27-2012, 10:33 PM   #22 (permalink)
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I think the idea is allowing the turbo to do more of the compression work is more efficient...if you crank up boost, you end up with more excess pressure at the exhaust. If you run more boost but fiddle with the intake timing so that the piston compresses less air, then you can have more of the energy required for compression come from the wasted exhaust impulse energy that the turbine can collect.
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Old 02-27-2012, 11:00 PM   #23 (permalink)
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Any pics of your ram air concept?

I wouldn't mind one on my diesel suburban, I think its the best solution for these rigs, better than even a turbo.
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Old 02-27-2012, 11:42 PM   #24 (permalink)
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Boosting compression inside the engine is more efficient.
The diesel alone is around 30% efficient, the turbocharger is less than 10% efficient.
I have selected the most efficient turbo I can find (also cost efficient, $180).
Increasing compression is proven to increase efficiency in piston engines. The only reason you lower compression in a diesel is to reduce NOx, allow for more boost to be ran to boost power and reduce fuel economy.

Why do all these people want to reduce compression in hopes of increaseing fuel economy?
You think it would take less force to get the piston to TDC on the compression stroke and I can see that but that doesn't mean it will make the engine more efficient.
Notice all the OEMs have gone up in compression since the 1980s not down (except diesels, they all went down for emmissions reasons).

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Here is my old intake setup.
A ram air intake, with 14''x6'' of air filter on a ported J-code intake manifold.
NO EGR.
I sold it on diesel place.

I am going to do a ram air turbo set up but I don't know how it will look, all I know so far is i am going to use a big off the shelf paper air filter and relocate the batteries to make room for it.
Just going to build upon what I already know, diesels love compression and having their cylinders stuffed with air.
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1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
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Last edited by oil pan 4; 02-27-2012 at 11:47 PM..
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Old 02-28-2012, 07:47 AM   #25 (permalink)
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Ideally you'd want as high of a compression ratio as possible, but an even longer expansion ratio to match it. Your compression ratio is very high, but it's a "diminishing returns" sort of thing. It is possible that your raised compression isn't doing as much good for efficiency as reducing the amount of air taken in, and then using more of the pressure that your fuel gets you. If I'm not mistaken your compression ratio is more than high enough to ensure great combustion efficiency, especially with forced induction.

The turbine is not efficient, but it can pick up the leftover pressure in the cylinder that your pistons didn't collect. What reducing dynamic compression will do is reduce the amount of air your cylinders get, and increasing the ratio of piston work to turbine work.

This is especially relevant when you are giving the engine a lot of boost, because the more you boost it, the more energy there is leftover that the piston cannot absorb, and the turbine cannot absorb because it doesn't need all that power to compress the intake air.

Kind of like Miller cycle gasoline engine, but turbo driven.

By the way, it's cool to see all your mods Must be exciting working on that.
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Old 02-28-2012, 02:20 PM   #26 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Increasing compression is proven to increase efficiency in piston engines. The only reason you lower compression in a diesel is to reduce NOx, allow for more boost to be ran to boost power and reduce fuel economy.
Nope, they did it so the motor wouldn't burn valves and fall apart, they could care less about NOx back when they made the change.

Quote:
Originally Posted by oil pan 4 View Post
Why do all these people want to reduce compression in hopes of increaseing fuel economy?
You think it would take less force to get the piston to TDC on the compression stroke and I can see that but that doesn't mean it will make the engine more efficient.
Notice all the OEMs have gone up in compression since the 1980s not down (except diesels, they all went down for emmissions reasons).
The reason is typically reliability but also the fact that your engine is not going to automatically increase in efficiency as compression ratio goes up, in fact it can go down because of the massive dynamic frictional losses, egr and blowby

6.2 based engines like their 21:1 ratio but do not like high boost 5psi is actually the best without the fuel pressure turned up. (for economy.)
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Old 02-28-2012, 02:28 PM   #27 (permalink)
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High compression ratio does improve fuel economy. The advantage of lower compression ratio is that power density improves, that is to say you can get more power out of the same displacement engine. If you look at diesel racing engines they'll have low compression ratio, as will some other applications were fuel economy is less important than power output. Higher compression ratio increases peak cylinder pressures, thus the reduced power density.
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Old 02-28-2012, 10:24 PM   #28 (permalink)
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Quote:
Originally Posted by rmay635703 View Post
The reason is typically reliability but also the fact that your engine is not going to automatically increase in efficiency as compression ratio goes up, in fact it can go down because of the massive dynamic frictional losses, egr and blowby
The highest production compression ratio I could find was 22.5:1, so I find no reason to try and reinvent the wheel. I am sure the OEMs found good reason not to go to say 27:1 or higher.

After every one went NOx crazy that was one of the reasons C/R stayed low.

I have no EGR.
EGR = BAD

The dragster and pulling tractor diesels run C/R as low as 14.5:1 but take over 100lb of boost. Not built for fuel economy.

My water injection pump and pressure control system is all ready to go, just waiting for warmer weather.
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1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.
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Old 02-29-2012, 02:58 AM   #29 (permalink)
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Quote:
Originally Posted by Diesel_Dave View Post
High compression ratio does improve fuel economy. The advantage of lower compression ratio is that power density improves, that is to say you can get more power out of the same displacement engine. If you look at diesel racing engines they'll have low compression ratio, as will some other applications were fuel economy is less important than power output. Higher compression ratio increases peak cylinder pressures, thus the reduced power density.
on a avrege,
lower compression with boost = reliability
higher comp with boost = short engine life.


im very interested in how the water mist injection will help on the temps.


ive also heard of high powered boats running 14/1 compression with large amounts of boost.
there was some N\A diesel VW engines that where 23/1 but there quite common to have head issues with small cooling systems and alumnaum head on iron block.
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Old 02-29-2012, 08:51 AM   #30 (permalink)
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Quote:
High compression ratio does improve fuel economy.
I didn't intend to suggest bringing down total compression, only moving a larger portion of it to before the intercooler, and having the piston preforming a smaller part of the work. All this without negatively impacting the in-cylinder expansion after combustion. (Which is what does all the useful work.)

With more boost from the turbo the intercooler would get more heat from the intake air, and you could end up with more air in the cylinder with the same final pressure.

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