Turbos or superchargers

[patrobertson]

I may have a stupid question but why not run a gear driven or belt driven supercharger instead of a turbo. I know that there is a diesel race truck that has a huge detroit and runs 2 superchargers and turbos but its speed thing. But on our trucks what is the advantage of turbos? I am a young guy so any wiser advice is welcomed. Just curious.



Thanks for enlightening me,



Shane Robertson

 

[FFutch]

It was pobably a v-12,or 16 2-stroke Detroit. They have to have a s-charger,or blower to run. There are no intake valves in them. They get their air through ports in the middle of the liner,just like a chainsaw or 2 stroke motorcycle. All 2 stroke Detroits are considered naturally aspirated with blowers. The reason for the turbo is cause it's much more efficient(runs off of exhaust gasses so it don't rob much power)and takes up less space... . Doug

 

[FFutch]

O. K. not exactly like a chainsaw,you don't have to mix the fuel 50-1 just using as an example... Doug

 

[Deezul1]

Superchargers are driven mechaniclly by the engine through belts or gears and it takes a lot of HP to do it and the drive mechanism is not cheap. Turbochargers being exhaust driven are almost free as far HP consumption goes and no expensive drive and we know how DC doesnt do anything extra that costs money. Super chargers have very little if any lag but are also boosting all the time unless there is some kind of clutch setup like the AC compressor has.

 

[illflem]

Superchargers take a lot of HP to run, sometimes up to 90% of the extra power they provide. Most guys figure that a 50% gain over power used is good. I've heard that on a 5000 HP top fuel dragster it can take 200 HP to run the supercharger.

 

[WadePatton]

Turbos are more efficient also because they're only pumping on demand. That's what allows your fire-breathing 500hp Cummins 5. 9 to idle around like a *****cat. Without the boost it's not a monster. Or at least not a mean monster.



Mounting turbos is a little easier too. Just look at the variations on twin setups.

 

[BushWakr - RIP]

There was a great explanation/comparison of turbo vs. supercharger in one of the more recent TDR mags.

As I understood it the turbo uses what is in essence waste gas/temps to drive the turbo system. For example a given exhaust stream has within it, inherent energy in the form of heat.

Roughly 33% is vented, 33% is converted to drive the turbo and another 33% goes somewere else... (oldtimers disease) and I'm gonna go back and re-read that article.

The point is, NO HP has to be drawn from the engine to produce this, its free since it's actually waste gasses.

The more exhaust you push thru the system the more converted energy (on a percent of total) you have to use.



did that make any sense at all???? If someone else can quote that article better go for it... ... . in the mean time I'm off to look it up for my own sake. .



Bob.

 

[RustyJC]

Not to be nit-picky, but it does take horsepower to drive a turbocharger. This horsepower is associated with pumping losses on the exhaust stroke that result from the higher exhaust back pressure (aka turbo "drive pressure") in the exhaust manifold than would be the case were the turbo not present in the exhaust system. To be fair, however, much of those losses are recovered during the intake stroke due to the higher air manifold pressure present in a turbocharged engine - this tends to reduce intake pumping losses (i. e. , the engine doesn't have to work as much like a vacuum pump to pull air into the cylinder as a naturally aspirated engine must do. )



Having said that, the true engine horsepower losses associated with driving a turbocharger of a given flow/head capacity are less than that required to drive a mechanical supercharger (or "blower" in the case of the 2-stroke Jimmy diesels).



Rusty

 

[NVR FNSH]

I was wondering how long it would be before Rusty chimed in Nice explanation.



Brian

 

[patrobertson]

Thanks guys

You guys rock, I was just wondering and now my question has been answered,



Thanks Rusty, and the others.





Shane

 

[EMD Diesel Power]

Could just be like a EMD 567/645/710 turbo'ed engine... and have the best of both worlds... they use a gear driven turbocharger with an overrunning clutch. Basically the engine doesnt have enough rpms (actually enough air charge to the airbox) to spin the turbo by itself at Runs 1-6. During Run 6 and above... the clutch disengages the geartrain and lets the turbo freewheel. Let er scream baby... .

I used to check this by letting the engine almost stall, then giving it a good heft on the layshaft..... She'd belch black smoke and clear right out if everything works. pretty spectacular... . slug of black smoke being blown straight up by hot gas behind it.

Sometimes the clutch would fail... and basically you're screwed. this is always easy to spot... . the unit will be smoking to beat h*** and sound like its strangling... Wouldnt be able to see 50 feet... . and the bug population drops dead for 500 yards around.

The new 265H engine uses twin turbos (its also a 4-cycle design) that are exhaust driven only. This one is a torquemiester. 6000THP in a SD90MAC (6300 BHP). Big sucker too... barely could fit it into the roundhouse at Point when Conrail had some on WC a year ago. THP is Tractive Horsepower... . BHP is True Horsepower.

 

[Will24]

top fuel blowers use up in excess of 800hp at full boost!

 

[Peter Campbell]

Wow a turbo-supercharger. well I can see the reason. Can't really run a two stroke without forcing air. But is is almost as weird as the opposed piston fairbanks morse diesels. Set the intake and exhaust CRANCKSHAFT timing-weird. They store the head gaskets for those engines right next to the spark plugs for the cummins, and the muffler bearings, and the floogle tubes.

 

[RustyJC]

The oldest mechanically-driven turbocharger I've run across is the Clark TLAB, dating back to the 1950's - it's a 2-cycle port scavenged integral engine-compressor. It used an old Chrysler fluid coupling driven by the crankshaft to chain drive the turbocharger for startup and idling. Once enough load was present for the turbocharger to self-sustain, the fluid was drained from the fluid coupling. Let the chains get loose, and they'd flop around, hit the base and BOOM - a crankcase explosion! Modern 2-cycle pure turbocharged integral engine-compressors drive the turbochargers with starting air (known as "jet assist") during startup and warmup.



The Cooper-Bessemer ET-24 turbocharger (24" diameter impeller) requires 5000 exhaust gas horsepower to drive. 2 are used on the Cooper-Bessemer 20Z-330 (one for each cylinder bank) - a 2-cycle pure turbocharged integral engine-compressor in a V-20 configuration with a 20" bore and 20" stroke - 13,500 BHP @ 330 RPM.



Rusty

 

[drees1]

Why we have turbos

Nice thread. First, and foremost, our engine is an industrial engine, and as such, fuel economy is very important when it is a business expense. With that said, a modern 4 cycle turbo charged diesel engine will produce more boost pressure in the intake than exhaust pressure in the exhaust manifold under moderate operating conditions. Why is this important? Well, the intake boost forces the piston down, and the piston must work to force the exhaust out. If boost is higher than exhaust pressure, then there is net positive work contributing to efficiency. And that in a nutshell is something that cannot be accomplished for free with a blower driven off of the crank. For quick response, the blower wins, but for efficiency, the turbo wins.



Doug Rees

 

[RustyJC]

Re: Why we have turbos

Originally posted by drees1

If boost is higher than exhaust pressure, then there is net positive work contributing to efficiency.

Doug,



Remember, to determine this, one must look at cylinder pressures, not manifold pressures. Pressure drops through valves and ports contribute to pumping losses.



Rusty

 

[drees1]

Right you are, Rusty! The cylinder pressures during the inlet cycle will be higher than the cylinder pressures during the exhaust cycle at and near the match point that the turbo and engine were designed for. You have experience with the big Coopers, it sounds like. I was an Engine design Engineer at Superior for 13 years in Springfield, OH. The 825 engine had tremendous valve overlap to allow raw intake air to pass through the cylinder, thus cooling the exhaust and keeping temps in check. Also needed to pass emissions. If I'm not mistaken, the large 4 cycle natural gas Cooper also has a lot of valve overlap. Of course those axial ET turbos are very efficient and even with all the air passing from inlet to exhaust during overlap, they still maintained a positive pressure differential from inlet to exhaust.



Doug Rees

 

[RustyJC]

Originally posted by drees1

If I'm not mistaken, the large 4 cycle natural gas Cooper also has a lot of valve overlap.

Hi, Doug,



Yep, Honda doesn't have anything new with V-TEC. The 1950's vintage Cooper-Bessemer LSV-SG engines used shifting camshafts (originally designed for direct reversing marine engines) to provide a "naturally aspirated" and a "supercharged (turbocharged, actually)" cam profile. At about 3" Hg boost, the cams (1 per bank) axially shift to the supercharged grind which provides extended duration and overlap. The grinding of the ramps between the low and high load cam lobes was a real work of art.



I agree - At the end of the day, the PV card will tell us if positive or negative work is going on during the exhaust/intake pumping cycles.



Rusty