I'd think that a change of turbo settings or geometry would improve your MPG then.

If your boost is zero at cruise, then you're not using the 'free' exhaust energy to overcome the less-than-100% VE, correct?

In another post, someone mentioned running around 5psi at cruise. That sounds like about the optimum point for best economy.

Since I drive almost all rural 2-lane, in mountain-country; I spend a lot of time at very low throttle...i.e. just maintaining say, 40mph, against wind-resistance...because there's another curve coming up soon where I'll have to brake.

So I've always been interested in setting things up to get boost at very low power-settings. Yet I need a turbo system that works well at near full power as well; because every 10 curves there's another mountain to climb...

In lieu of the $$$ VGT's that I can't afford, I've often wondered about experimenting with a tandem turbo setup. Something tiny from a sub-2L, in tandem with either a stock 617 turbo, or even a slightly larger one (to keep backpressure down at the upper-end of the power/rpm range).

My first guess is to use backpressure to control the gates that'd switch between the two turbos...but I've really not thought it all out in detail yet.

So many experiments to try, so little time....

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

Boost isn't free, it causes backpressure in the exhaust that makes the pistons work harder to push exhaust out of the cylinders.

The 617 is good for 88hp without the turbo and it only takes around 20hp to push the car down the road at a constant 55mph. The lower the boost the less work the engine is doing to make that 20hp.

If you have the $$ for a second turbo, you have the $$ for a $100 VGT off of eBay.

__________________
1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

True, but that's irrelevant....i.e., the power required to push a certain car at speed 'X' isn't relevant to the efficiency of boost vs. no-boost.

No matter -what- the power-level is, the engine operates more economically if the exhaust energy is captured and used to replace pumping-losses which otherwise come out of crankshaft-power.

You're correct of course that back-pressure adds workload to the pistons, but you have to recognize that the -level- of that pressure is -proportional- to the power-level of the engine.

I.e., at the 20hp output level you mentioned, the BP in the exhaust system is a lot less than at 88hp.

It's proportional....at lower output, less HP of boost-pumping is needed AND less HP of backpressure-pumping-work is generated at the pistons.

Also, it's not the case that a turbo has to add a lot of backpressure in order to pump air. A turbo extracts most of its energy from the -heat- of the gases; not from the pressure.

If you take a look at power-output and BSFC curves for both turbo and NA versions of the same engine, you'll see that the turbo version is always more efficient (i.e. bsfc) even in the lower regions of power-output.

I can't claim that every single engine ever made acts this way; but we do a lot of genset, pump, and mine-haulage overhauls and retrofits here, and I've yet to see a single make or model engine whose charts did not show such an across-the-board efficiency advantage for the turbo version.

In any case, I'll stand by what I posted....if he has set up his turbo system so that it's not pumping air at cruise, then he's not getting the best-possible MPG yet, imho.

Of course, I don't know precisely what "zero boost" meant....was the turbo not pumping at all, and there was actually a -vacuum-, which didn't register on a unidirectional gauge?

Or was it pumping just enough to overcome VE losses and truly was literally zero psi at the intake manifold ?

If the latter, then the turbo actually -is- producing 'boost' (i.e. performing pumping work) even though the gauge read 'zero'.

But to overcome port and valve losses, I think one would want to see more than that 'zero'...perhaps 2-3 psi at the manifold?

Even with 2-3psi, it may still be an -overall- benefit to pump to, say, 5psi, and have a bit of excess air. It's possible that more complete combustion would produce more output than the accompanying (if any) increase in backpressure would cost.

There's a balance-point for that, which can only be found by actual measurements....i.e. either dyno testing or long-run fuel and output tracking.....easy with a genset....hard with a car....

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

very true...NOWadays.....

but when I first started thinking about it, VGT's were pretty uncommon at the boneyard...

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

$150 for my new toy.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=130241303955
I would have disassembled and cleaned it anyways so the price is right.

ok, I'm jealous, I admit it...

well, I don't have my -engines- yet....so I guess I can wait a bit on the turbos....

As it happens, I don't currently own any MB cars....got rid of the 300TD in December.....but am preparing to do a pair of conversion-projects for which I'm currently searching for engines.

I will be reading your many excellent posts/threads very carefully when it comes time to select, adapt, and control the turbos for them...

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

Not true. What really does the job is the aerodynamic flow of the expanding gasses across the turbine, not the heat itself. The exhaust gets cooler as it exits the turbine in the same way that refrigerant gets cold as it exits the expansion orifice, rapid expansion. Thats what confuses people into thinking that the exhaust heat is doing the work.

Think of a wind farm, they don't work by heat but by the flow of air across the blades.

Thats how VNT/VGT turbos work, they increase the potential energy of the exhaust by increasing its velocity and expansion ratio across the turbine.

Back pressure is a natural result of gasses slowing as it contacts the working surface (turbine).

No, that's not correct, in two senses....

First, it's not like a wind-genny; which works solely from lift over a wing; and extracts pretty much the same energy >>regardless of the temp of the gases flowing through it<<.

(important point/difference there)

Secondly, I think you're looking at it backwards; because it is the heat that CAUSES the gases to expand. It is the HEAT energy at work here....not the minimal pressure-differential across the expander.

The exact same kind of turbo-expanders are also used as "energy-recovery" machines in all sorts of industrial processes that handle -hot- gases......like refineries, ammonia-manufacture, etc..

Obviously, there does have to be -some- delta-P, to get the gas through the turbine in the first place; but the majority of the rotational power comes from the heat-energy stored in HOT gas....rather than the minimal mechanical-energy in the typically small delta-pressure across the turbine.

As a thought-experiment, imagine putting the same paltry low psi/delta-P of -cold- gas (i.e. ambient temp) into your turbo-turbine...

Even if the in/out pressures and flow-rate were identical to your engine exhaust, you'd hardly get any shaft-power at all.....simply because there just isn't any energy to speak of in 70F gas vs. 1000F gas.

hope I was coherent with the above...it's 1am here...

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

Heat is only what is making the gasses expand. Its the aerodynamic flow of the gasses across the turbine that does the work, not the heat.

A turbine is a turbine, they all work by the push of gasses flowing over the work surfaces, the temperature makes very little difference. Thats why a turbo will make the same boost just as easily with 600*f exhaust as it can with 1600*f exhaust.

No offense intended by the following wording FI....I just want to be very clear:

The above is flat-out incorrect.

I think that by insisting on that 'windmill' view, you're holding yourself back from even greater turbo-mastery than you already have.

The turbine absolutely will NOT "make the same boost just as easily" with 600F gas as with 1,600F gas.

In fact, it will require -considerably- more mass-flow at 600F than at 1600F; to produce the same shaft-power (psi x cfm of boost, or compressor-massflow)

Again, a turbo-turbine is NOT just a windmill....NOT just a simple propeller...it IS a HEAT ENGINE.

....so you gotta remember your Carnot...

I sense that you're (reasonably) resisting changing your view of how turbos work just on my word....so I urge you to look it up in any turbomachinery textbook, and verify for yourself that what I'm saying here is true.

It's clear that you already know a ton of good stuff about turbos from the practical/usage side; so I think that if you give yourself the advantage of correctly viewing them as heat-engines instead of as simple 'fans', you'll find the use and tuning of them more intuitive, and even more rewarding.


PS; the Carnot numbers also imply that not only can more shaft-power be extracted from hotter gas, but also that a higher -percentage- of that higher energy can be extracted. (i.e. higher efficiency too)

That's why the output-power of heat-engines (including turbos) tends to rise NON-linearly with input temp. Double the temp, get four times the power....roughly that sort of relationship....and with much higher efficiency at the same time.

That first-order relationship between temp and both power and efficiency is why the jet-turbine guys are always pushing the limits of materials so they can raise the turbine-inlet temp just another 100 degrees.

It's also a partial factor in why adding an intercooler tends to reduce the boost a few psi at the same engine conditions as before.

It's not just the added flow-resistance of the IC....it's also the lower energy-content of the cooler exhaust, plus the reduced efficiency of the turbine at that new lower temp....it's a double-whammy on the turbine power output.

The same factor is why it's an advantage to wrap the headers....i.e., to maintain the gases as hot as possible going into the turbine.

__________________
WANT to BUY: 3.0L diesel engine.

My other diesel is a....

1962 Cat D9-19A, 2,000 cu-in TD
1961 Cat 966B, D333 TD, powershift
1985 Mack MS300P 8.8L TDI, intercooled, crane-truck
1991 F350 4x4 5spd 7.3 IDI NA
1988 Dodge D50 4x4 5spd 2.4 Mitsu TD
1961 Lister-Petter 14hp/6kw Marine Corp genset weekly charging 5400 lbs of forklift batt for the off-grid homestead.
1965 Perkins 4-108 Fire/water Pump
1960 Deutz 20hp/8kw genset

I'm sorry but the above is flat-out incorrect. If that were true then a turbo would not be able to make any boost if run immediately after a cold start. The simple fact I can make 5+psi with my VNT by revving the engine right after starting disproves your heat engine idea.

I'm sorry but it isn't. They are simple fans.

It reduces boost a few PSI because the air is more dense for the same amount of airflow as well as less boost is needed. Measure before and after an intercooler and you can see the drop.

No, its to maintain the gasses velocity. As the gasses cool they slow down, wrapping headers keeps heat in to keep the exhaust from slowing inside the pipes and causing restriction.

I'm sorry, but you study up a little bit more on the fundamentals of how these things work.

Very interesting thread.
FI, you have loads more experience with turbochargers than I do and I'm chiming in with the utmost respect for said experience but I have to throw in with dozer here.
The expansion valve analogy isn't appropriate because the refrigeration in that case comes primarily from the phase change.
And, to say all turbines work from the flow of gasses across the work surfaces is definitely incorrect. Steam turbines extract a massive amount of heat energy from the steam, otherwise we could just use air pressure.
Turbos are similar. If the exhaust gas is cooler at the turbo outlet than the inlet, where did the energy go? This is a reason exhaust driven superchargers are more efficient than engine driven superchargers. You are using leftover heat energy from the combustion process to compress the inlet charge instead of new mechanical energy (requiring more fuel consumption) from the crank. If it was purely a gas pressure-driven scenario, the pumping losses through the exhaust side of the turbo would not offset the gain from the increased inlet airflow.
Keep the dialogue going. I'm learning from everyone here.

__________________
1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

As said before, if a compressed gas rapidly expands it releases energy and gets cooler. Basic physics. Thats why moisture in compressed air can form ice as it exits a nozzle and steam gets cool as it expands.

There is a huge pumping loss in turbochargers. The only reason they work is because the exhaust contains a much larger gas volume from the combusted fuel, the same way steam engines work by using the expansion properties of water.

If heat was the driving force of turbos then remote turbo systems like what is sold by STS Turbo ( http://www.ststurbo.com/ ) would not work either because in the distance the exhaust has traveled it has cooled significantly. They maintain the exhaust velocity by using small exhaust pipes.

Wow.

Steam is used to drive turbines because heating the water into steam causes the water to expand 1600 times! Heating air comes nowhere close. The water to steam transition carries with it the pressure of expansion, that pressure drives the turbine. It is a very simple and efficient way to turn heat into pressure and then into mechanical power.

You keep the turbo wrapped/hot to keep the air from cooling and thus contracting, as FI said, the contracting air would then have less pressure & thus less flow. I always wrapped the turbo and exhaust pipes to it on my Cats, keep the energy in!

Cooling the air through an aftercooler/intercooler will cause less boost pressure at the intake because you have decreased the volume of the incoming air by cooling it, thus the volume, although the mass-flow is the same. It is also because of the resistance of flowing through the tubing and cooler.Exhaust temps will drop somewhat also simply because of the cooler intake air.

Think of creating steam, the expansion causes a massive outflow from the boiler, if it hits a cooler before the turbine it would contract, if cold enough to re-condense there would be no flow left to get to the turbine (I have a working axial-flow steam turbine here on my desk, hot stuff).

Steam is a gas, air is a gas, just a huge difference in the thermal coefficient of expansion.

Although I respect and strongly agree with all else that Dozer has said, and not just because he's a Cat man, I find myself disagreeing with the turbine being a heat engine. The turbocharger is all about geometry, and coatings/materials that have been experimented with or are in use to control heat in turbines are to avoid absorbing the heat into the turbine and to pass it harmlessly through to the exhaust or into another turbine. Materials are always being experimented with that will withstand the tremendous forces and heat in gas turbine engines so that more heat can be introduced, the more heat the more expansion (pressure) and the more compression the more heat & pressure, heat is necessary only to keep the pressures up. The more heat and thus expansion and pressure you can put into the turbine, the more power per cubic foot of engine or lb of engine weight.

__________________

Gone to the dark side

- Jeff