Fuel economy - PeachParts Mercedes-Benz Forum

dozer · #17 08-02-2008, 03:49 AM

Quote:

Originally Posted by Cervan

Im actually missing the point of this debate.. If i have it right, your saying that the turbocharger is spun, by the heat the engine produces, not the exhaust pressure? Well we can test that pretty easy. Just take the exhaust impeller off leave the shaft in there and see how fast the intake impeller spins. Or are you saying that the turbocharger simply increases the combustion chamber temperatures?

That's OK Cervan......I can't even remember now what the original point was myself....

First, FI has been misquoting me all along....using words like "just", and "only", and "all".....when what I actually said was that the >majority< of the shaft-power produced by the turbine was from the extraction/conversion of the heat, not simply from pressure like a 'fan'. I never said "all" or "only".

I also didn't say that the heat was 'doing' the work. I said that the shaft-power was mainly derived from conversion of the heat-energy into mechanical work.

Second, to answer your question....yes, exactly, the turbo IS mostly powered by the heat-energy contained in the compressed gas.

The turbine-portion of a turbo is technically known as a "turboexpander"; and contrary to FI's assertion, the 150-200 degree drop in gas-temps across the turbo is mostly NOT caused just by heat radiating off the turbo. It IS caused mainly by the heat-energy stored in 1000F gas being usefully -extracted-, i.e. converted into mechanical shaft-power via the process of "expansion" in heat-engine terms.

As I posted earlier, nobody has to take my word for it....you can confirm all this yourself -easily- with just a simple 5-second web-search; or a 5-minute consultation of any turbomachinery handbook ever written.

And no, a brake is not a heat-engine. That's just plain silly. A brake is nothing but a dissipative load. -Exactly- like a resistor in electronics. Nobody would claim that a resistor is a generator.

"Heat Engine" is a specific term defining a machine which converts heat-energy to mechanical work; or converts mechanical-work to cooling.

The extraction of mechanical energy from a gas -requires- expansion. Expansion is how you get work out of any engine.

Thus, any heat-engine -requires- a compression phase -prior- to adding the heat-energy that we want to turn into mechanical work later in the expander.

So, a basic combusting heat-engine requires compression; then the burning of fuel to heat that compressed gas, and finally the expansion of that hot gas (to convert that added heat-energy into useful mechanical work).

Now you know all about heat-engines...it's that simple.

It can be done with pistons and cylinders...or it can be done purely with simple turbomachinery, like that great example that Greasy just posted of a 'jet' engine made from nothing but a car-turbo and a burner (I love that thing!

)

That's nothing but a compressor, a burner, and an expander. You'll note that the expander is a turbo turbine...

(in fact, the very first jet-engine ever made WAS a centrifugal engine, not an axial-flow like today's jet turbines)

In our TD cars, besides the self-contained Diesel-cycle of the piston-engine, there IS an additional "outer loop" heat-engine cycle going on too....which involves the turbo.

This is essentially a Brayton Cycle (which involves compression and expansion being done in seperate devices) heat-engine.

The piston-engine is the compressor/burner half of this "outer loop" Brayton cycle.....and the turbo-turbine is the expander (energy-extraction point) of it.

Again, a 5-second web-search will turn up the definition of heat-engine, description of turboexpanders, and will confirm all of the above; including the fact that the large temp-drop across a turbo is mostly caused by the -work extracted-. Only a small fraction of that temp-drop is from radiation off the housing, etc..

I'm sure this all must've had -something- to do with optimizing fuel-economy on our TD's...but for the life of me, I can't recall what right now...

But in lieu of remembering what started all this, I'll just summarize heat-engine understanding into turbo-system design aspects for best economy...

- Since heat and expansion provides the turbo-power, keep the gases as hot as possible between the head and turbo. Which means, don't expand them (keep the runners narrow) and if the runners have much length, insulate them...or at least shield them from the fast-moving underhood air.

- Conversely, -after- the turbine, provide as much expansion-room (exhaust size) as possible.

- Design the turbo-system so that it's providing at least several psi boost at common cruise-power levels. Even if that's only 1/4 throttle or 30hp, you DO want the turbo to be at least overcoming VE (intake resistance) losses.

Remember, if the turbo (running on otherwise wasted heat-energy) doesn't push that air into the engine, then the crank/pistons (running on fuel you gotta pay for) has to do that vacuum-pumping work.

- Vary the boost proportional to fueling (i.e. to the power-level being demanded). Any excess-air beyond that needed to combust cleanly at your current fueling-level will cause wasted mechanical-energy in the piston-engine.

- Since the turbo is producing mechanical work from normally-wasted heat energy in the exhaust, the best economy comes from replacing as much of the piston-engine's pumping/compressing workload as possible.

In other words, besides replacing the intake-resistance losses with a few psi of boost from the turbo-compressor, also replace as much of the mechanical piston-performed compression-ratio as possible with turbo-compression instead.

Lower the engine's comp-ratio to 17-18 and crank up the boost to 30+ psi. The goal would be to achieve the same total cylinder pressure (BMEP) but with the waste-heat in the exhaust-gas doing as much of the total compression work as practical.

Such a change in engine-CR will have the effect of making starting more difficult; and will also somewhat reduce torque/power prior to the turbo spooling up. However, it will provide the lowest possible steady-state specific fuel-consumption at power.

- Consider adding a 2nd turboexpander to recover more of the substantial remaining heat-energy in the exhaust. This may seem 'exotic', but I've been thinking about it off and on for a few years and I think it's both practical and doable.

Looking at it as HP, and assuming engine eff. of 33%, with another 33% of fuel-value wasted as heat in the radiator and the last 33% of the fuel-value wasted via heat in the exhaust (typical numbers); then when you're running down the freeway at 70, using, say, 30hp, there is >30hp< of heat-energy going right out the tailpipe!

Your primary turbo is recovering a few hp of that; but in a power regime where the turbo-inlet gas is running 900F and the exit-gases at perhaps 700F, there's obviously still a LOT of energy left to capture there.

As one way to capture it, add a 2nd turboexpander sized for the lower-temp gases (it will be larger and lower-rpm) and drive a high-speed alternator from the turboexpander-shaft. The output of the Alt. will be rectified and used to power a DC motor coupled to the engine or drivetrain.

If this secondary turboexpander also recovers several HP while the car is using 30hp to maintain speed, that would be a 10% fuel savings right there.

With good design, it may be possible to recover as much as 5-6hp.....which would be a 20% improvement in MPG. Perhaps not significant to some folks, but us engineers go ga-ga over that kind of efficiency improvement in anything...

A large lag in such a larger turbo wouldn't really be significant in any steady-state driving regime. So, freeway and any steady rural 2-lane driving would see the mpg gains.

To reduce the weight-impact of such a system, an alternator could be modified to serve as both alt. and brushless-DC motor. (remove internal rectifiers and bring out the 3-phase winding leads; and control it with a 4-quadrant 3-phase PWM bridge).

Obviously, there are limits to both belt-capacity (2-5 hp per B belt perhaps?) and HP capacity of the alt/motor itself. It may be difficult to get more than 2hp out of an auto alternator.

Although I've brought out alt-windings and used the alt as a high-power "stepper motor" in the past, I've never tried running one at high speed/power...so I've never happened to measure the output as HP, or gotten an idea of what the thermal issues with a regular car-alt as motor might be.

One could of course source a high power-density brushless-DC motor instead of using a car-alt. That too can operate as an alt. when needed, by using that 4-quadrant PWM controller.

Most of the thinking I've done on this concept has been related to my F350 (7,000 lbs, 7.3L) and I'd thought that the PTO port on the tranny was the ideal place to put a DC-motor. But in using my crane-truck, I've noticed that if I forget to disengage the PTO, then the trans shifts a lot harder (stick trans). Perhaps the PTO-port on the T-case could be used without impacting driveability.

On a 300 series, I can't really think of any easy spot to drive, other than at the engine. Perhaps adding a shaft to the crank-pulley and driving there would be best. It would be a necessity if one was successful in developing 5-10hp from this system.

Anyway, to address the original point of the thread, those are my turbo-related fuel-economy throughts...