34 replies to this topic

[Canuck]

By way of conversation genesis, what started out as a discussion about turbocharging engines has turned into debate about the volumetric efficiency of forced induction engines.

Using the assumptions of a .55 BSFC and 12:1 A/F, this translates to an air demand of .11 lbs / hp. The engine in question develops ~775hp @ 4900 rpm (torque peak) and so requires ~85 lbs of air/minute.

Converting pounds to grams I get ~38600 grams / minute;
Converting grams to mols of air I get ~1340 mols / minute (assuming 28.8g/mol)

Using PV=nRT to get Litres/min at 120'F (air temperature at intake under boost), I get either:
i) 35,416 L/min at 4900 RPM in a 3.986 litre engine = 363% Ve
ii) 17,708 l/min using a P pressure of 2 bar (14.5 psi boost) = 181% Ve

Strictly speaking, if Ve is a measure of the engine's actual pumping capacity vs. 100%, wouldn't 363% Ve be accurate?

[McGuire]

With one practical exception, I'm not sure volumetric efficiency is a terribly meaningful or useful value for forced induction engines. Above a given manifold pressure it measures the effectiveness of the charge cooler system as much as anything, the chief attribute of high boost levels being hot air (much like this post).

Even in normally aspirated engines, volumetric efficiency is something of a misnomer. VEs of more than 100 percent are possible in theory and practice, and show me a machine of greater than 100 percent efficiency. All we are measuring is cylinder filling versus atmospheric pressure, stated as a percentage. Meh, BFD. Maybe I could get over it if we called it "volumetric capacity" or something. Our lungs can operate at greater than atmospheric pressure. That's how we exhale.

Now, in the case of engine mapping systems which employ VE-based fuel tables, the VE value must be in reference to the ambeint pressure at the transducer in the plenum, or air mass at the MAF sensor, rather than to atmospheric pressure, or the fuel requirement can never be sorted out properly. In the case of a supercharger that is not a constant-displacement device (turbocharger, centrifugal or axial blower etc) VE is meaningless as it does not pump a fixed air mass or volume relative to an ambient intake pressure. It's just a fan blowing into a tube, see paragraph #1.

[Canuck]

What you say makes perfect sense, so let me share the background on this.

I've been toying with supercharging or turboing my little BMW almost since I bought it (in fact the last Ve thread I started was about supercharger efficiency ratios). Like most of my want-to projects, it's languished on the back burner for various reasons, not least of which is dumping a bunch of money into a body shell with questionable floor pan integrity. I bought a later model a couple of months back that is in beautiful shape, so the FI project rises from the rust and gets some attention.

In examining what I can find in regards to turbocharged BMW M30 engines, the results are quite nice. With well-executed systems, and the addition of only head studs in place of the stock bolts (and a new stock headgasket), users are reporting 15psi all day long with the clutch being the only casualty. The most recent run posted had a datalog trace of 8.5psi on a 3.43L displacement, putting down 300hp at the wheels at ~5500rpm. Assuming a 15% drivetrain loss, that's 350hp at the crank, a shade over 100hp/L.

Using the formulas presented on the Garrett website for turbo-choosing, I can't replicate replicate those results with anything under a 105% engine Ve. According to their formulas:

Required air mass (lbs):
Wa = Target HP x Air/Fuel x (BSFC/60)
=350 x 12 x (.55/60)
=38.5 lbs/min

Manifold Absolute Pressure:
Map = (Wa x R (gas constant of 639.6) x (Temp at intake in Abs)) / (Engine Ve x rpm x .5 x Cu in disp)
= (38.5 x 639.6 x (460+110)) / (.85 x 5500 x .5 x 209)
= 28.73psi

Even assuming no pressure drop at the compressor intake and none through the intercooler, at 14.7psi atmospheric, that's still 14psi of boost. Leaning out A/F works but that data log for the run shows it getting slightly rich at the top end. Dropping the BSFC down to .45 works but that seems...optimistic doesn't it?

[McGuire]

Well, you could look at it this way: we are probably off by no more than ten percent or so. That's not bad for Kentucky windage. Hard to say what all could be slightly askew... first thing I am curious about (as usual) is what dyno correction factor was used. If we had access to more of the dyno data, including actual atmospheric temp and baro, correction factor, BSFC, air temp, and manifold pressure, all would be revealed.

[Canuck]

According to the printed sheet:
Roll on
66.31'F
29.65 in-Hg
25% Humidity
STD. 1.00 (no CF)

The 707whp engine I'd talked to you about earlier - it was run in Denver and has a 20% correction factor for the altitude alone (so I'm told). Actual at the wheels was ~575.

[McGuire]

Sure, but for boosted engines the standard altitude/pressure correction for automotive engines is bogus, especially with turbochargers. With a normally aspirated engine at 100 percent VE and 100 percent throttle opening, manifold pressure is nominally equal to atmospheric pressure. So power is reduced in proportion to elevation above sea level as air density is reduced -- roughly one inch Hg per 1000 ft.

However, with a turbocharged engine: If the boost control valve is calibrated to say 30 PSI absolute, then the manifold pressure will be 30 PSI absolute up to the altitude at which the turbo overspeeds and/or the charge overheats. The greater the altitude the lower the air density and the faster the turbocharger will turn to obtain the same manifold pressure; but meanwhile it takes less work to turn compressor wheel because it is pumping thinner air, and less work to turn the exhaust turbine because it is pumping into thinner air. Most any competently turbocharged engine will produce the same approximate power at Denver altitude (~5250 ft) as at sea level, within a few percent anyway. In aircraft terminology, the altitude at which the turbo system starts falling behind and power falls off is called "Critical Altitude." Some older turbocharged aircraft have throttle-coupled wastegates to control overshoot.

And with a crank-driven supercharger, while the manifold pressure is greater than atmospheric pressure, naturally, it still remains proportional to atmospheric pressure. Or, why some WWII aircraft engines had multi-speed supercharger drives: a low speed for takeoff and near the deck and a high speed for altitude. Takeoff at full boost would fry the engine. But you can see how a supercharged engine that runs only in Denver can be optimized for that altitude with a numerically higher blower drive ratio. At sea level it would overboost and blow up.

In drag racing, the NHRA employs altitude correction factors for tracks above 2100 ft so that et and mph recorded in Denver or Salt Lake City can be compared to those in Florida or New Jersey. However, for boosted engines the correction factor is reduced by 50 percent. That's closer to reality but still not accurate. A more robust correction factor would include boost level and means of supercharging as well as altitude. A system running at 15 PSI boost will be far less influenced by altitude than a system that runs at say 7 PSI, and a turbo far less than a crank-driven blower.

[Canuck]

A-ha! I didn't think 20% was a reasonable CF. I ran into a discussion about SAE 1349 last night about correction factors and it's pretty blatant - turbos and altitude need not apply, anything above 3% air and/or 3% fuel (correction) shall be noted as non-standard.

I'm not at home (which is to say my pile of books and papers isn't in reach) - is there a means to calculate actual BSFC via injector size, pulse width and fuel pressure?

Edit - found something. Based on the datalog information for the 300hp run, I'm calculating anywhere from .58 to .62 BSFC (not knowing what the original injector's test pressure was). Also, intake AT was 117'F.

Using .6, I get 42lbs Wa, piling in the 117'F and 8.5 boost level, that's giving me 116% Ve.

How am I supposed to use Ve to size a compressor when what appears to be real-world stuff doesn't factor back?

[McGuire]

Assuming you did your sums correctly and that the engine in question really does make 350 hp@5500 rpm with 8.5 lbs of boost, all this shows is a limitation of the selection formula -- it's chasing a moving target, due in good part to the fact that a turbocharged engine can operate at greater than 100 percent VE.

We are pumping air through the cylinders at a constant rate with a great big fan. Normal considerations of volumetric cylinder filling with atmospheric depression and all that jazz no longer apply in the same way. With the formula we are sort of supposing that with a pressure ratio of say 1.8 we will pump *exactly* 1.8 times as much air as when normally aspirated when it could be more or less than that, due to any number of factors. An engine is a postitive-displacement pump and here we know its speed, 5500 rpm. A turbo is not a positive-displacement pump, and here its speed is unknown. VE doesn't really mean anything to a turbo as it does not pump a fixed volume. We are back to where I started: VE is not a terribly powerful indicator in a turbocharged engine. I can think of one 3.4 liter six with 2V heads that makes 900+ hp at 60" HG abs: the Buick Indy engine. What VE results if you plug 900 hp into the formula?

Beyond that, it's hard to say anything terribly clever or definitive about this specific engine working from these limited numbers. I can't say 350 hp @ 8.5 psi and 3.4L isn't doable; it is. It is pretty good, but I can't say how it was done without seeing the fuel and air specific numbers. We are trying to wring more info out of the data than is there. And really, I don't know as a matter of reasonable certainty that the engine did that power number at that boost. (This is the Internet, after all.) Maybe it wasn't quite 300 whp, or maybe it was more than 8.5 lbs. If this happened on an inertia dyno I begin to have doubts. There is now a practice in the "hp community" of deducting 20 percent from inertia dyno results. Technically that does not seem to me like a supportable or fair approach, but it just goes to show how these machines have become known for producing higher numbers that can be produced on conventional chassis dynos.

[Canuck]

Too true. I guess that's what I wanted in the end - someone else to conclude that the formula was...well, not quite right somehow. I get so tangled up in the paper engine side, I lose sight of the bigger picture.

In terms of altitude turbo corrections - wouldn't (boost pressure + sea level pressure) / (boost pressure + ambient pressure) be reasonable (if not entirely accurate still).

3.4L, 900hp, 30psia = ~119% Ve if the hp peak is 9000 rpm with a 12 A/F and .55 BSFC. Not that unreasonable for an Indy-level engine I'd guess, but then half the numbers I used are a complete guess.

[McGuire]

Well, the Buick Indy turbo V6 ran on methanol so its BSFC was better than 1.0. In reference to an earlier point... I think one would be hard pressed to beat .60 BSFC with a kit turbo on pump gas (though that view could be slightly dated). Which is another thing to watch for: For whatever reason, many shops advertise their dyno numbers for street engines but running them on race fuel. They will say it doesn't matter because the have validated the combination with pump fuel. True enough if so, but race gas will skew the BSFC somewhat (and the output numbers a little) due to its greater energy content. This practice arose because pump fuel can be a little inconsistent, especially in winter blends. With Rockett 100 or similar you always get the same fuel no matter what.

I recently did an SBC Chevy street engine buildup (normally aspirated, 9.8:1 CR) in which the dyno testing was actually performed with C16 race fuel because the dyno cell was dedicated to Pro Stock development. I'm dead certain the power readings were totally legit but I did not publish the BSFC numbers for the reasons above. I also did a spark advance loop to make sure we were not masking knock sensitivity with octane.

There is an awful lot to dyno testing and a world of data to explore. But ironically, horsepower seems to be the only thing people want to know when by itself it doesn't mean a whole lot. We need more info to provide context. For example, there are two dyno correction standards in common use today: The OE industry standard, SAE J1349: 77F/25C, 29.235" Hg/990 mb, sea level, zero relative humidity; and the performance industry standard: 60F, 29.92" Hg, sea level, zero relative humidity.

These two standards are 4.8% apart. That is, if you take a brand new car off the showroom floor (certified under J1349) and dyno test it at the local speed emporium under the performance industry standard, it can make 4.8% more power. There was originally no attempt to deceive -- this was at one time an SAE standard too (J607) and is also very close to the U.S./International Standard Atmosphere. But you can see how shops can use this wrinkle to blow some smoke, and it would be nice too if everyone could get on the same page. It so happens that the conditions in your first example are roughly halfway between the two standards, so the result is not directly portable to either one. Just sort of in the ball park. As for the Denver example, forget it. The SAE allows a maximum correction factor of 7%.

I know shops that employ no calibration factors at all, as in your first example above. There is nothing wrong with that as long as they are not using the numbers to sell engines or compare with numbers from other shops. In this case the numbers found on the dyno are abstract units -- you can call them "freds" or "krinks" but it is not accurate to call them "horsepower." Inside the shop the numbers are useful for development purposes as long as the ambient conditions are reasonably stable. One of the top race teams in the country uses uncorrected figures only. In relative terms they always know where they are headed, and they don't share their figures with anyone (or believe anyone else's numbers for that matter). But there is a problem with this practice, even if you never do customer work... what if you have a catastrophic dyno failure, or you are forced to move your shop, etc? Now you have lost your baseline and you are lost in the tall grass. And according to Murphy's Law, this is also the week you blow up your best motor.

[Canuck]

Damn...I suppose I should stop futzing with the paper then and just get to it.

Why do turbocharged engines have such a high BSFC? I saw somewhere that even supercharged engines had a lower BSFC, which I thought was odd considering how much crankshaft power is sucked up running the blower itself.

[Greg Locock]

Have you got an example of that?

Turbos on SI engines are notoriously lousy at improving efficiency (for the same engine size, but you can win at part throttle if you downsize), but superchargers are just a joke, we ran a 40 hp s/c to take a 150 hp engine to 250 hp, (on some old GM V6), so the fuel consumption at full power was just awful. Mind you, the customer would never have found out, the engine didn't run very well on a mixture of rich mixture and metal. For those with long memories, that was the attempt to take the Opel Omega into the USA, can't remember what they called it. GM management didn't like our engine, can't blame em.

[McGuire]

Originally posted by Canuck


Why do turbocharged engines have such a high BSFC? I saw somewhere that even supercharged engines had a lower BSFC, which I thought was odd considering how much crankshaft power is sucked up running the blower itself.

No, your instincts are correct. All else being equal, a turbo should beat a crank-driven blower fairly handily.

[Canuck]

Originally posted by Greg Locock
Have you got an example of that?

RC Fuel Injection - Right at the bottom of their page of calculators and what-not is a little footnote stating .45 - .5 for NA, .55 - .60 for supercharged and .60 -.65 for turbocharged. It seemed counter-intuitive when I read it, hence the reason for asking. But, what is it about a turbocharged engine that decreases it's fuel efficiency / hp vs. a NA engine?

[McGuire]

It takes work to turn the turbo even if it is not connected to the crank. You can't make much boost without inducing exhaust backpressure. Also, as a practical matter you generally can't run a boosted engine to the same ragged edge of lean as with NA. With significant boost levels the temps and pressures are too great. Theoretically that is not so but there's theory for you.

I don't know why the source would invert the natural order of things in regard to super vs. turbo. Just as a guess maybe they are presuming a charge cooler for the turbo but not for the supercharger.

[Greg Locock]

Well, I think they are pretty much wrong at or near the optimum efficincy point. Mr Heywood for example shows bsfc maps for an NA and TC engine, the NA tops out at 260 g/kWh, TC at 270. But, at full throttle redline the difference is much greater (the contours are too far apart to tell exactly). I imagine the turbo is using lots of extra fuel to keep the exhaust temp down. And that's what sets the injector size which is what they are interested in.

[Canuck]

Right - both posts make sense. Smaller, shorter pre-turbine exhaust for high velocity, rich mixtures for cooling things at peak power. At part-throttle cruise then, things shouldn't be that radically different than an NA engine?

[Scoots]

Just for a hoot some years ago I strapped my 1994 MX-5 to a Dyno-jet inertial dyno and at 14psi boost on an otherwise stock engine (intake manifold to exhaust ports fully stock, right out of the junkyard) I got 276whp ... that's 276whp out of 1.8L with 91 octane CA pump gas, no correction factor.

Inertial dynos are great for bench racing though.

[J. Edlund]

Originally posted by Canuck
Right - both posts make sense. Smaller, shorter pre-turbine exhaust for high velocity, rich mixtures for cooling things at peak power. At part-throttle cruise then, things shouldn't be that radically different than an NA engine?

Production TC engines have a small pre-turbine volume to keep exhaust pulse amplitudes high, the turbine and turbine housing is undersized for maximum power; exhaust backpressures twice the intake pressure is not uncommon. Essentially the whole turbocharging system is designed for low speed torque, driveability and a fast throttle response. This while the engine itself has a very high compression ratio, which demand late ignition to avoid knocking, which in turn demand rich air/fuel mixtures to keep temperatures under control. Basically, modern automotive turbocharged engines are not designed to be efficient at high loads, and why should they, high loads are used for such a small portion of the total time an engine is used that it really doesn't matter. What they are designed for is to take advantage of the high efficiency offered by a small high compression engine at low power outputs. Replacing say a 300 hp 4 liter engine with a 300 hp turbocharged 2 liter engine offers a significant improvement in the average efficiency of the engine in daily use.

[McGuire]

Ford is taking a run at that with the Eco-Boost engines. Looks promising.

[DOF_power]

>
^ Electric engines are the future, how about F1 focusing on the that ?!

[J. Edlund]

Originally posted by DOF_power
>
^ Electric engines are the future, how about F1 focusing on the that ?!

Let's say that a F1 car require an average of 300 kW during 1.5 hours; that's 450 kWh. If a battery, power electronics and motor have a total efficiency of 80% you have to store 560 kWh on board the car. Current state of the art secondary batteries can provide an energy density of about 0.2 kWh per kg, so storing energy enough for a F1 race would require 2800 kg of batteries, an impossible situation.

As a comparison, even with an efficiency of only 30%, demanding 1500 kWh of energy to be stored, 125 kg of gasoline will do the trick.

Road cars are a completly different situation as a car requires perhaps around 10 kW on average with an average powertrain efficiency of 15%. At least then you have some sort of a realistic chanse to store the amount of energy require for shorter trips.

[DOF_power]

>
^ No one mentioned anything about it being (fully) on batteries.
They could use hydrogen fuel cells.

Or they could use a compressed air engine to generate electricity and power a (couple of) electric engine(s).

If it's 2800 kg of batteries so what, they'll lighter next year, and even lighter the next.

The point I'm trying to make is that racing should return to the days of "improving the breed" and "race on Sunday, sell on Monday" and brake away with the people's idols generating machine non-sense.

[J. Edlund]

Originally posted by DOF_power
>
^ No one mentioned anything about it being (fully) on batteries.
They could use hydrogen fuel cells.

Or they could use a compressed air engine to generate electricity and power a (couple of) electric engine(s).

If it's 2800 kg of batteries so what, they'll lighter next year, and even lighter the next.

The point I'm trying to make is that racing should return to the days of "improving the breed" and "race on Sunday, sell on Monday" and brake away with the people's idols generating machine non-sense.

The first idea was bad, these ideas are worse.

Well, to begin with hydrogen. Hydrogen is really only as 'clean' as the energy it is produced from, which means that hydrigen isn't such a clean fuel. This is made worse by the fact that hydrogen as an energy carrier offers a low efficiency, especially if liquid hydrogen is selected as the fuel of choice which is really the only way we can store the amount of hydrogen in a F1 car within the packaging constraints we have. The efficiency of fuel cells are also not that great, overall system efficiency is typically around 30% or so and decrease with increased load. Since F1 is a high load application, similar to heavy trucks, it's not beneficial to use fuel cells over internal combustion engines.

As for the pneumatic engine, this is potentially even a worse than the fuel cell idea. Pneumatics offers a really low efficiency. Of the energy you use to compress the air you get back perhaps 5-10% of the energy you put in. The use of an electrical drivetrain over a mechanical one consumes an additional 20% of the energy compared to 5% in the case with the mechanical transmission.

[DOF_power]

>
^ With such pessimistic views, how did the auto industry ever got started and went past those early slow, uncomfortable, highly unreliable, inefficient, distrusted "horseless carriages" ?!

Racing that's how.

[Canuck]

Why do you feel you must clog up a rather specifc thread with electric car blather? Go discuss electric motor torque in the V8-V10 torque thread - at least they're arguing about hp and torque over there.

[gruntguru]

Originally posted by McGuire
It takes work to turn the turbo even if it is not connected to the crank. You can't make much boost without inducing exhaust backpressure. Also, as a practical matter you generally can't run a boosted engine to the same ragged edge of lean as with NA. With significant boost levels the temps and pressures are too great. Theoretically that is not so but there's theory for you.

On an efficient turbo setup the compressor work required can actually be free. If the designer acchieves an exhaust pressure lower than inlet pressure the negative crankshaft work during the exhaust stroke is less than the positive crankshaft work obtained during the inlet stroke (neglecting normal losses of course).

The higher BSFC suggested for turbo engines could follow from an assumption that turbo motors run more boost than blown (especially true for Roots) and therefore need much richer mixtures for knock suppression and internal cooling. In general - for a given boost the turbo engine will make more power and have a lower BSFC than the blown engine.

[gruntguru]

Originally posted by Canuck
Why do you feel you must clog up a rather specifc thread with electric car blather? Go discuss electric motor torque in the V8-V10 torque thread - at least they're arguing about hp and torque over there.

Hear Hear Canuck!

[TDIMeister]

The textbook definition of volumetric efficiency is independent of boost pressure, and it is instead a function of engine geometry and design, and has nothing to do with the turbocharger. In other words, if an engine without boost has a VE of 100%, putting 2 bars of boost (absolute) doesn't raise the VE to 200% but rather it stays at 100% (or thereabouts).

Mathematically, VE for forced induction engines is evaluated as the actual mass flow divided by the theoretical mass flow, the latter of which is the swept volume of the cylinder multiplied by the engine speed multiplied by the charge density at the intake ports divided by 2 (for a 4-stroke engine). Therefore even "volumetric" efficiency is a bit of a misnomer, since we're dealing with mass flows. Taking the charge density instead of ambient air density takes into account boost pressure, charge heating, intercooling and port wall heating, so it portrays the true volumetric efficiency of the engine itself, which is a function of manifold/port design, valve diameter, valve lift, cam profle, cam timing, bore-to-stroke ratio, rod length-to-stroke ratio, mean piston speed, etc. This means it's also independent any factors upstream like boost pressure, intercooler effectiveness and charger efficiency.

On this basis I disagree with McGuire, who says that VE is not a terribly meaningful or useful value for forced induction engines. On the contrary, an engine with a high VE will require less boost to make the same HP compared to one with a low VE, which has all sorts of advantages with regards to thermal/mechanical stresses and knock resistance. Alternatively, with a high VE, you will make make more HP with the same boost. At the OEM level, optimizing VE to such an obsessive level with variable length manifolds, etc., is not commonly done because these add even more complexity and cost to the engine, when the same result in target HP can be achieved by increasing the boost pressure -- a less than optimal, but CHEAP solution. Since I believe we're here to wring the best performance out of our engines, VE should also be optimized.

[J. Edlund]

Originally posted by DOF_power
>
^ With such pessimistic views, how did the auto industry ever got started and went past those early slow, uncomfortable, highly unreliable, inefficient, distrusted "horseless carriages" ?!

Simple, they worked on 'the problem'.

Originally posted by TDIMeister
On this basis I disagree with McGuire, who says that VE is not a terribly meaningful or useful value for forced induction engines. On the contrary, an engine with a high VE will require less boost to make the same HP compared to one with a low VE, which has all sorts of advantages with regards to thermal/mechanical stresses and knock resistance. Alternatively, with a high VE, you will make make more HP with the same boost. At the OEM level, optimizing VE to such an obsessive level with variable length manifolds, etc., is not commonly done because these add even more complexity and cost to the engine, when the same result in target HP can be achieved by increasing the boost pressure -- a less than optimal, but CHEAP solution. Since I believe we're here to wring the best performance out of our engines, VE should also be optimized.

Obviously, increasing VE beyond a point with an OEM turbo engine by the use of large valve areas, long valve durations and so on isn't particulary effective since it reduce driveability, probably one of the biggest issues for turbocharged engines, and when exhaust pressure is much higher than the charge pressure it won't do much good.

I have even heard of a case where the inlet manifold length was optimised for a low VE. The plan was that the expansion pulse would cause a temperature drop while massflow was kept up by an increase in boost, by the heat of which could be handled by the intercooler.

Originally posted by gruntguru


On an efficient turbo setup the compressor work required can actually be free. If the designer acchieves an exhaust pressure lower than inlet pressure the negative crankshaft work during the exhaust stroke is less than the positive crankshaft work obtained during the inlet stroke (neglecting normal losses of course).

The higher BSFC suggested for turbo engines could follow from an assumption that turbo motors run more boost than blown (especially true for Roots) and therefore need much richer mixtures for knock suppression and internal cooling. In general - for a given boost the turbo engine will make more power and have a lower BSFC than the blown engine.

Since a turbocharger can increase bmep without significantly increase fmep, it can increase mechanical efficiency.

[TDIMeister]

A good read on the subject by Prof. Blair: http://www.profblair...k_to_basics.pdf

[Canuck]

One of my favourite papers.

[gruntguru]

Originally posted by J. Edlund
Since a turbocharger can increase bmep without significantly increase fmep, it can increase mechanical efficiency.

Even more to the point - the turbine energy required can be provided largely by utilising the blow-down energy when the exhaust valve first opens - energy that is mostly wasted in a naturally aspirated or crank driven blower engine. Blow-down energy is highly utilised in large diesels, and their thermal efficiency exceeds 50% these days.

[McGuire]

Originally posted by gruntguru
Even more to the point - the turbine energy required can be provided largely by utilising the blow-down energy when the exhaust valve first opens - energy that is mostly wasted in a naturally aspirated or crank driven blower engine. Blow-down energy is highly utilised in large diesels, and their thermal efficiency exceeds 50% these days.

If the engine is run at high and constant load to obtain maximum blowdown, and even better, the exhaust turbine can be coupled to the output shaft rather than to a compressor turbine, improvements in efficiency can be realized. First and best example is the Wright R3350 Turbocompound engine used on the DC-7s and Constellations, the first and last piston airliners with transcontinental range. Also some big diesels used in stationary and construction equipment.

On the Wright turbocompound each 18-cyl. (9x2) engine had three Power Recovery Turbines (PRTs) on the exhaust manifolds coupled to the crankshaft via quill shaft and fluid coupling. On startup the turbine did not couple until full system oil pressure was reached, reducing the load on the starter. The exhaust blew in at a very shallow angle from three locations around the turbine to reduce wobble. Optimized BSFC was around .40, although the PRTs proved to be less than reliable.

These big Wright Cyclones -- all were supercharged but only some later models were turbocompounds -- were magnificent machines. Most versions incorporated a hydraulic torquemeter that read in BMEP on the instrument panel, much more sensitive and accurate than manifold pressure in trimming the engine. The Turbocompound versions used a hybrid Bendix fuel injection system which employed a "carburetor" with venturi, but the venturi did not supply fuel but only a pressure signal to a master control, which metered the fuel to the individual port nozzles. If you ever get the chance to look over one of these engines, it's worth it. There is plenty of info on the web as well if you look around.

But on a passenger car the engine does not run at high and constant load but at wildly varying load, so the majority of the time the available blowdown energy is less than significant. Meanwhile, neither the exhaust turbine nor the compressor turbine are anywhere close to 100 percent efficient. So in the true and final test of efficiency, BSFC, turbos are better than crank-driven superchargers but not as good as normally aspirated. The energy required to drive a turbo to significant effect is not "free" but in fact an additional pumping loss. This has been proven enough times to be downright depressing.

As a result, one of the more illusory statements in automotive technology is that turbos operate on "wasted heat energy." Not really. In practical terms, one is better off regarding a turbocharger as simply a pump. You can put a turbo on the bench and heat the turbine wheel to white hot with a torch and it will not budge. To make it rotate with any authority you are going to need some pressure. In terms of "blowdown energy" it is helpful to remember that the typical automotive engine can be stopped in its tracks with a potato. In the old piston aircraft industry when the Wright Turbocompound was developed, "blowdown turbos" and "pressure turbos" were regarded as two very different beasts.

At the moment, the best way to obtain "efficiency" in a SI passenger car turbo application is the technically false but nonetheless practical type of efficiency obtained via the "dual use" concept: a small-displacement engine for economy in normal use, coupled to a turbo for greater performance on occasional demand.

[gruntguru]

Originally posted by McGuire
If the engine is run at high and constant load to obtain maximum blowdown, and even better, the exhaust turbine can be coupled to the output shaft rather than to a compressor turbine, improvements in efficiency can be realized. First and best example is the Wright R3350 Turbocompound engine used on the DC-7s and Constellations, the first and last piston airliners with transcontinental range. Also some big diesels used in stationary and construction equipment.

On the Wright turbocompound each 18-cyl. (9x2) engine had three Power Recovery Turbines (PRTs) on the exhaust manifolds coupled to the crankshaft via quill shaft and fluid coupling. On startup the turbine did not couple until full system oil pressure was reached, reducing the load on the starter. The exhaust blew in at a very shallow angle from three locations around the turbine to reduce wobble. Optimized BSFC was around .40, although the PRTs proved to be less than reliable.

These big Wright Cyclones -- all were supercharged but only some later models were turbocompounds -- were magnificent machines. Most versions incorporated a hydraulic torquemeter that read in BMEP on the instrument panel, much more sensitive and accurate than manifold pressure in trimming the engine. The Turbocompound versions used a hybrid Bendix fuel injection system which employed a "carburetor" with venturi, but the venturi did not supply fuel but only a pressure signal to a master control, which metered the fuel to the individual port nozzles. If you ever get the chance to look over one of these engines, it's worth it. There is plenty of info on the web as well if you look around.

But on a passenger car the engine does not run at high and constant load but at wildly varying load, so the majority of the time the available blowdown energy is less than significant. Meanwhile, neither the exhaust turbine nor the compressor turbine are anywhere close to 100 percent efficient. So in the true and final test of efficiency, BSFC, turbos are better than crank-driven superchargers but not as good as normally aspirated. The energy required to drive a turbo to significant effect is not "free" but in fact an additional pumping loss. This has been proven enough times to be downright depressing.

As a result, one of the more illusory statements in automotive technology is that turbos operate on "wasted heat energy." Not really. In practical terms, one is better off regarding a turbocharger as simply a pump. You can put a turbo on the bench and heat the turbine wheel to white hot with a torch and it will not budge. To make it rotate with any authority you are going to need some pressure. In terms of "blowdown energy" it is helpful to remember that the typical automotive engine can be stopped in its tracks with a potato. In the old piston aircraft industry when the Wright Turbocompound was developed, "blowdown turbos" and "pressure turbos" were regarded as two very different beasts.

At the moment, the best way to obtain "efficiency" in a SI passenger car turbo application is the technically false but nonetheless practical type of efficiency obtained via the "dual use" concept: a small-displacement engine for economy in normal use, coupled to a turbo for greater performance on occasional demand.

Some very interesting info, thanks McGuire.

Large diesels are nowadays achieving a good deal of the turbo-compounding efficiency gains simply by running very high boost and returning the work to the crankshaft via negative pumping losses. In large sizes, turbine and compressor efficiencies exceed 80%.

SI engines in steady state are also able to achieve a negative pumping loss situation (boost pressure higher than exhaust BP) so the compressor work is free. The biggest impediment to thermal efficiency for SI engines is the necessity to operate substantially richer than stoich as boost increases. Every drop of fuel beyond stoich goes straight out the exhaust.

A good deal of "waste heat" is utilised in the exhaust turbine in fact most of the turbine energy comes from internal energy (heat) in the exhaust hence the large temperature difference across the turbine.

Downsizing is the subject of intense work by a number of European researchers.