Stupid Turbocharging Question

[weirdjim]

The difference between supercharging and turbocharging is that a supercharger requires a compressor of its own while turbocharging uses the energy of exhaust gas to do the work via turbine blades running a compressor.

I'm just a poor sparky engineer, not a materials or thermo cat. Matter of fact if it hadn't been for a kindly thermo professor taking pity on me I might not have been an engineer at all.

That's another story. What ever happened to supercharging using, say, an electric compressor motor? Seems you'd have a hell of a lot less problems with something that doesn't run at the temperature of exhaust gas.

Just a question, mindya.

Jim

 

[Skip Miller]

What ever happened to supercharging using, say, an electric compressor motor? Seems you'd have a hell of a lot less problems with something that doesn't run at the temperature of exhaust gas.

Jim, I am no thermodynamic engineer either. I do know that the key word to "why" is efficiency. There is a lot of energy in the exhaust gas, and unless it is harnessed with a turbocharger, that energy is wasted. Superchargers and the electric powered rig you envision will also boost your available power but the efficiency will be less than an equivalent power setting on a turbocharged rig on the same engine.

On ground or marine vehicles, you just bolt on another fuel tank. On airplanes, not so much.

-Skip

 

[Ted]

Definitions are a bit off. Both a turbocharger and supercharger have a compressor of some sort that compresses the intake air. For a centrifugal supercharger, it is similar to a supercharger. Roots and twin-screw superchargers use a different design, but still compress the inlet air. The supercharger gets its power from the engine directly. On a car, typically it's belt-driven. On an aircraft, it's another accessory that is typically gear-driven.

A turbocharger, but comparison, has two turbine wheels. One is a compressor wheel, and then one is the exhaust turbine. The two are connected, and the exhaust gasses spin the turbine wheel, which in turn spin the compressor. This makes it a bit more efficient, since the energy is being extracted from the exhaust (i.e. spent energy already) vs. being an accessory that puts drag on the crank.

As far as the electric supercharger - I think the primary issue is that having an electric supercharger would require a rather substantial electric motor to spin it. The control system probably wouldn't be too bad, but would be a factor. Engine-driven is just easier, and probably ends up being more efficient overall.

 

[Skylane81E]

Most of the heating of the intake aircharge from any supercharger, be it a plane supercharger or a turbo-supercharger (we've dropped the "super" and commonly call it a turbocharger today) is from the compressing of the air, not the proximity to a hot turbine housing.
Example, I have a Saturn Ion Redline with a 2.0L supercharged engine, the air get's PLENTY hot with out passing near the exhaust and requires an air to water aftercooler. So hot that the water in the cooling system can boil if the pump fails to pass it through the radiator.

 

[denverpilot]

On a car, typically it's belt-driven. On an aircraft, it's another accessory that is typically gear-driven.

This add-on supercharger is belt-driven...

http://www.forcedaeromotive.com/product.htm

Been watching it for a while since it's a local development, seems kinda spendy for the O-470 on the 182, and rumors floating around are that it's "fussy" maintenance-wise.

 

[Ted]

Most of the heating of the intake aircharge from any supercharger, be it a plane supercharger or a turbo-supercharger (we've dropped the "super" and commonly call it a turbocharger today) is from the compressing of the air, not the proximity to a hot turbine housing.
Example, I have a Saturn Ion Redline with a 2.0L supercharged engine, the air get's PLENTY hot with out passing near the exhaust and requires an air to water aftercooler. So hot that the water in the cooling system can boil if the pump fails to pass it through the radiator.

In fact, depending on the specifics, a supercharged setup can sometimes heat the intake charge more than a comparable turbocharged setup.

This add-on supercharger is belt-driven...

http://www.forcedaeromotive.com/product.htm

Been watching it for a while since it's a local development, seems kinda spendy for the O-470 on the 182, and rumors floating around are that it's "fussy" maintenance-wise.

Well, the problem I see there is that it looks they used an aftermarket automotive supercharger on the thing, where it's probably spinning much higher continuously than it would in its automotive counterpart. I'd expect it to be fussy.

 

[TripleZ]

To give an example against a purely electrically driven compressor.

18 m^3/min of air (enough for 325hp) compressed to 1.8x atmospheric requires roughly 50 hp with a lysholm style (high efficiency) screw type compressor operating at 78% isentropic and 65% adiabatic efficiencies. That's about as efficient as your going to get it for the air compressing side.

Now, a mechanically driven compressor (supercharger) is typically less desirable because of the mechanical drive losses associated with transmitting 50hp. Since a screw type compressor spins 13K rpm on female and 21.6Krpm on the male rotor......... being direct crankshaft driven is unlikely. So you have step up gears, or belt ratios, which all have their losses.

A turbo charger is more efficient in that respect, but still can't beat the laws of physics for the compressor side. There ARE electrically assisted turbochargers and small electric compressor, but these are all short duration systems. Heat build up and electricity required is the big problem. (Think hybrid car battery pack, 48Volts or more, etc)

Even with the best centrifugal compressors (around 82% isentropic PEAK, in transition), the energy required to drive it is fairly high. Coupled with a turbine side which is maybe 38% efficient...... and you see where Im going. A common myth with turbochargers is that the energy is free (waste exhaust energy). While this is true to a point, there is not 30hp (to keep units consistent) worth of drive energy in the exhaust.

To drive a turbocharger with exhaust, the turbine side uses the Pressure delta across the turbine wheel along with mass flow / temperature / velocity (in a complicated fashion). Basically more mass, more temperature, more pressure...... the more energy you can extract from the turbine. What this means, is that as you demand more work from the compressor, you require more back pressure to drive the turbine. (Waste gate controls back pressure and flow through the turbine)

That pressure comes from the blow down loop in the combustion stroke. Anything greater than atmospheric requires additional energy from the engine to create. So a turbocharger is not really "free" energy.

 

[Henning]

The difference between supercharging and turbocharging is that a supercharger requires a compressor of its own while turbocharging uses the energy of exhaust gas to do the work via turbine blades running a compressor.

I'm just a poor sparky engineer, not a materials or thermo cat. Matter of fact if it hadn't been for a kindly thermo professor taking pity on me I might not have been an engineer at all.

That's another story. What ever happened to supercharging using, say, an electric compressor motor? Seems you'd have a hell of a lot less problems with something that doesn't run at the temperature of exhaust gas.

Just a question, mindya.

Jim

The problem with using an electric drive compressor is the weight of the motor required. They are all "Superchargers" if they increase the manifold pressure above ambient. There are 2 basic types, centrufugal and roots. There is also screw. The other difference is in how you drive them. Direct/gear, belt or exhaust turbine. (electric). The reason for the turbocharger is you already have the raw energy to run it being provided from the BTUs you are just wasting out the pipe. All they cost you is a slight bit of back pressure and the thing is relatively light. Electric drive would mean you need an alternator and motor capable of producing at least 10hp (I don't know the formula to figure it off my head). That is going to be a lot of weight and another 10hp required for the engine to produce and the fuel required to produce in both cost and weight. It's also going to be contained in a hot environment so you have all that extra resistance to deal with.

In fact, I'm been racking my brain trying to think of any installation of an engine that used an electric supercharger and I'm coming up with a blank.

 

[Dave2]

Both are superchargers. They compress (super charge) air in this case. If you compress the same volume of air it will raise the temperature the same amount. PVT

 

[Henning]

That pressure comes from the blow down loop in the combustion stroke. Anything greater than atmospheric requires additional energy from the engine to create. So a turbocharger is not really "free" energy.

Right, but it is the most efficient for the application as well as low weight so it requires little of it's own work "to be flown around".

 

[Dan Thomas]

That pressure comes from the blow down loop in the combustion stroke. Anything greater than atmospheric requires additional energy from the engine to create. So a turbocharger is not really "free" energy.

Right, it's not, really, but some of the energy comes from the temperature drop across the turbine. Same as the turbines in a jet engine. The heat would otherwise be wasted. I've heard it said that at high power settings there is still flame in the exhaust (the old WWII guys used to lean their bomber engines at night by watching the flame color) and if that's so, there's combustion happening in the exhaust system which causes acceleration of gases and plenty of heat. Might as well harness it as dump it overboard.

A turboed engine produces more HP, so there's a net increase even if we create some back pressure and scavenging issues. Even if we only turbonormalize it instead of boosting it beyond standard atmospheric pressure we'll gain performance everywhere except right at sea level.

A curious hybrid: Some old radials (and an Allison V-12) were "turbocompound" engines. They had a turbine wheel, driven by exhaust gases, that was geared to the crankshaft. The turbine added torque to the crank directly instead of compressing intake air.

Dan

 

[TripleZ]

^^^^ Agreed.

 

[Skylane81E]

In fact, I'm been racking my brain trying to think of any installation of an engine that used an electric supercharger and I'm coming up with a blank.

Mention an "electric supercharger" on a car forum, sit back and watch the fireworks

Granted this stems not from it not being possible to electricly supercharge an engine but because of what you can buy that's labeled as such. Basicly it's a very low powered fan (like a bilge fan or over-blown CPU fan) installed into the intake piping

 

[bsdunek]

I remember electric superchargers for cars back in the 50's. They were mostly for drag racing, as the battery couldn't sustain them for long.
On a continous use, like an airplane at altitude, you would have to have a generator to power the supercharger motor. Much easier and lighter to do it with gears or belts.
As some said above, they're all superchargers. Turbocharger is just short for turbine driven supercharger.

 

[airguy]

Right, it's not, really, but some of the energy comes from the temperature drop across the turbine. Same as the turbines in a jet engine. The heat would otherwise be wasted. I've heard it said that at high power settings there is still flame in the exhaust (the old WWII guys used to lean their bomber engines at night by watching the flame color) and if that's so, there's combustion happening in the exhaust system which causes acceleration of gases and plenty of heat. Might as well harness it as dump it overboard.

A turboed engine produces more HP, so there's a net increase even if we create some back pressure and scavenging issues. Even if we only turbonormalize it instead of boosting it beyond standard atmospheric pressure we'll gain performance everywhere except right at sea level.

A curious hybrid: Some old radials (and an Allison V-12) were "turbocompound" engines. They had a turbine wheel, driven by exhaust gases, that was geared to the crankshaft. The turbine added torque to the crank directly instead of compressing intake air.

Dan

The old timers also called these "parts recovery turbines" for their habit of catching debris from disintegrating pistons and valves, and taking out the turbine wheel.

 

[zaitcev]

A curious hybrid: Some old radials (and an Allison V-12) were "turbocompound" engines. They had a turbine wheel, driven by exhaust gases, that was geared to the crankshaft. The turbine added torque to the crank directly instead of compressing intake air.

A modern riff on the same theme is so-called "5-stroke" engine, where the exhaust expands in the additional cylinder.

 

[weirdjim]

(the old WWII guys used to lean their bomber engines at night by watching the flame color)

Which is how the Brit fighter pilots could use onboard radar to get within a quarter mile of the German bombers and then switch to the MK-I eyeball on the exhaust to hit the inboard engine and tear the wing off.

THanks,

Jim

 

[Henning]

A modern riff on the same theme is so-called "5-stroke" engine, where the exhaust expands in the additional cylinder.

Hush now or you'll have Felix starting a thread here too...

 

[TripleZ]

Hush now or you'll have Felix starting a thread here too...

or the 6-stroke with an intermediate steam cycle in the same cylinder.

 

[Skylane81E]

or the 6-stroke with an intermediate steam cycle in the same cylinder.

Love that idea personally

 

[Doggtyred]

That's another story. What ever happened to supercharging using, say, an electric compressor motor? Seems you'd have a hell of a lot less problems with something that doesn't run at the temperature of exhaust gas.


Jim

You'd need a trunkfull of batteries to make it happen. Some of the drag cars use em.

Some of Reno guys use lawnmower engines for boosting during the race.

 

[weilke]

A curious hybrid: Some old radials (and an Allison V-12) were "turbocompound" engines. They had a turbine wheel, driven by exhaust gases, that was geared to the crankshaft. The turbine added torque to the crank directly instead of compressing intake air.

Just yesterday, I found out that the DD15 commercial truck engine from Detroit Diesel uses a turbo-compound design. Downstream from a conventional fixed vane turbocharger, they have a 'axial power turbine' that is geared to the main flywheel. Under full load, it apparently contributes about 50 of the total 560hp the engine produces. Dont know how the systems are holding up in long-term use.

 

[TripleZ]

The turbo-compound or hybrid systems are getting to be popular on commercial diesels. Use an over-ratio supercharger for the low speed, then let the turbo take care of the higher speed once it has exhaust energy to work with. The setup I worked on mainly used the supercharger for emissions reasons down low.

Or use a small turbo, followed by a large turbo. The induction piping gets pretty complicated for one of these.... neat looking though.

 

[Ted]

Or use a small turbo, followed by a large turbo. The induction piping gets pretty complicated for one of these.... neat looking though.

That was what they did on the 2008+ (probably changed it now) F-Series turbo diesels. Get 40 psi of boost pretty much right off idle. Nifty... and helped emissions.

 

[gkainz]

twin turbos setups are what my cummins diesel performance hotrodding buddy has been building ... every time I go into his shop he makes me come to the back and drool over his latest setups ... and slyly remind me that the setup "this one is replacing could EASILY go in" my truck...

 

[Ted]

Greg, something tells me those setups don't help emissions any.

 

[gkainz]

Greg, something tells me those setups don't help emissions any.

Oh they help with emissions, all right ... you ought to see the BBCOD (Big Black Cloud Of Dark!) the Cummins can roll with enough fuel and boost!

 

[Ted]

Oh they help with emissions, all right ... you ought to see the BBCOD (Big Black Cloud Of Dark!) the Cummins can roll with enough fuel and boost!

My fault for using poor wording.

I sometimes get asked "So what're the emissions like on your airplane?" "Oh they're great!" "Really?" "Oh yeah, we make tons of it!"

 

[Graueradler]

At one time, there was a turbocharger for Diesel trucks that used the differential between crankshaft and driveshaft rpm to drive it. Slow moving truck with high engine rpm got a lot of boost.

 

[gkainz]

My fault for using poor wording.

I sometimes get asked "So what're the emissions like on your airplane?" "Oh they're great!" "Really?" "Oh yeah, we make tons of it!"

Ted - I knew what you meant ...

I've added 150 hp to my old 12valve mechanical Cummins and still pass CO emissions checks. With a set of twins, I would still pass emissions, as they (by themselves) wouldn't add fuel, which wouldn't add more black smoke. But who in their right mind would add the boost down low, and the BOOST up high without occasionally using it - if for nothing else, just to make sure it's still there (and the corresponding grin it causes)

 

[Ted]

Ted - I knew what you meant ...

I figured.

I've added 150 hp to my old 12valve mechanical Cummins and still pass CO emissions checks. With a set of twins, I would still pass emissions, as they (by themselves) wouldn't add fuel, which wouldn't add more black smoke. But who in their right mind would add the boost down low, and the BOOST up high without occasionally using it - if for nothing else, just to make sure it's still there (and the corresponding grin it causes)

I had a 2004.5 Cummins with the 325 HP (I think? All I remember for sure was 600 lb-ft torque) 24-valve direct injection diesel. I left it stock, it had plenty of power for what I needed.

I do fully understand the need for more boost. My 3000GT VR-4 is going back together now with the baby 9B turbos upgraded to 15Gs. I'm going for a conservative 15 psi, but that ought to get me about 450 hp.

 

[Skylane81E]

Greg, something tells me those setups don't help emissions any.

Oh they help with emissions, all right ... you ought to see the BBCOD (Big Black Cloud Of Dark!) the Cummins can roll with enough fuel and boost!

Actually from the standpoint of the EPA it does reduce emisions. You get to use a smaller engine to still pull the same payload so durring the govmn't testing at partial load the emisions are much less.

 

[Ted]

Actually from the standpoint of the EPA it does reduce emisions. You get to use a smaller engine to still pull the same payload so durring the govmn't testing at partial load the emisions are much less.

Well, he's talking about putting those on the same engine, same truck.

Putting a then mower powerful, smaller engine in another vehicle is another matter.

 

[Skylane81E]

I'm speaking from a MY to MY view and why they are done "for emisions reasons"

The rest of us know that HP = Fuel burned so from an emissions standpoint....

 

[Henning]

You'd need a trunkfull of batteries to make it happen. Some of the drag cars use em.

Some of Reno guys use lawnmower engines for boosting during the race.

Can you point out a drag car that uses an electric supercharger? I'd love to see that rig because if he made it light enough to be feasible, I need to know what he's using....

 

[Henning]

I wouldn't do 2 series turbos. If I wanted the extra boost I'd turbo on top of a Paxton. I did 3 512BBCs with twin Air Research turbos feeding an 8-92 roots. Even fed it from the bottle to come out of the hole. That was a 47 Fountain that was a hell of a ride, and we'd tow a guy to Catalina and back water skiing...