underdriven Roots blower for throttling loss reduction
 

Today I was pondering this question instead of doing homework, and I've almost come to the conclusion that it is viable (although perhaps expensive).

We can analyze the (gas cycle) efficiency of a Roots blower run against a negative pressure gradient by looking at the only non-isentropic process that happens in this scenario, when the atmospheric pressure gas "blows down" into the manifold via free adiabatic expansion. This entropy increase is somewhat easy to quantify.

Now looking at it from a pressures point of view, after the blowdown is complete and the pressure against the lobe has equalized with the manifold, the rotor will turn 1/2 of 1/n revolutions (where n is the number of lobes), displacing 1/2 of 1/n of its rated displacement, and work will be done on the rotor. In the real world it's a bit more complicated but just working off this for now...

My problem with this is that if we're actually putting work into the rotor, then the intake temperature and pressure should end up lower than before (for the same air density), except that means the atmospheric pressure air coming in is blowing down to a lower pressure than it would with a throttle plate...which means more losses to the free expansion. Can someone point out the flaw in my reasoning?

Is it perhaps that at the same pressure, the throttle plate is allowing less air mass through?

EDIT: okay thought about it a little more, if the Roots blower is displacing x volume for every 1 unit volume the engine is displacing, then the Roots blower is having slightly less than x * the amount of pumping work the engine is doing since the negative pressure acts on the blower for x times the volume it acts on the engine. So the engine is expending ideally about the same amount of pumping work, but in real life more? The entropy accounting is so much easier...
If the plenum volume is small enough that each atmospheric pressure pulse from the blower increases the pressure appreciably, then the pressure will drop more as each piston descends on the intake stroke and reduce the net work required, and it's easy to see that having the pressure drop less during the blowdown and then having reversible expansion on the intake stroke increases entropy less, but this is pretty iffy. If plenum volume gets really low then it gets interesting and you can probably recover a fairly large amount of energy but that's not practical.

Okay yea I think that's it, with a very tiny intake plenum volume it would help a little bit, but otherwise you're increasing pumping power to generate the power at the blower.

I wonder if people using independent throttle bodies have slightly higher fuel economy. Seems possible, if the throttle bodies are close to the valves, as the amount of volume you're depressurizing is lower.

For sure you have more power if you shorten and divide intake mainfold to the same number as cylinders. But on the other hand if your intake mainfold is long you'll see the gain in torque moved to the lower engine speeds. So you'll loose something and win another. That's why nowadays cars are often equipped with variable intake mainfolds to keep torque as close to the bottom as possible and gain power in high revs.

But interesting idea and I like your way of thinking ;-)

I dropped out of engineering school way too early to understand most of that, but, let me see if I can reduce it into terms we unedumacated types can grasp.

Are you proposing using the blower as a throttle? And harnessing the energy of said blower, back into the engine?

Basically, using the blower as a windmill?

If so, yeah, I think it's feasible. I think it would work well with a constant output engine, less so with an engine requiring variable throttle settings, which is pretty much all of them, unfortunately.

Not as a "windmill", but as a turbine. As I noted in the second post, it doesn't work very well unless the rotor chamber volume is significant compared to the intake manifold volume.

Ideally you have a mini-supercharger placed where independent throttle bodies go, then it would work. Otherwise there is too much blowdown loss. If the supercharger were electrically powered it could have very good transient response too.

Unfortunately you can't really do this if you want boost since an intercooler has to go somewhere in there and such. Additionally, say you take an Eaton M62 with ~1L displacement, with 6 lobes that makes only 0.166L displacement per lobe, so it will depressurize quite a bit in the manifold which probably has a lot more volume than that. If you use a giant supercharger and turn it slowly that would work better, but turning a supercharger too slowly is bad for boost efficiency, although having reduced intake manifold volume is good for boost efficiency.

I was thinking a twin small supercharger setup where you can bypass one of them and disconnect it from the manifold altogether when under light load circumstances, but the remaining supercharger would have so much blowdown loss that the parasitic loss from running the supercharger would probably overcome whatever tiny gains there may be from throttling reduction.

I follow your reasoning on the non-isentropic expansion but your idea is fundamentally circular math and logic.

So you assume isentropic expansion of the air in the manifold from the piston descending creates a pressure differential across the blower from high-atomspheric to low-manifold pressure. And you create an ideal system where all process remain adiabatic and the piston must draw in air and therefore the roots blower must spin because it is both connected by the mechanical belt and is subject to the pressure differential that is acting across it.

OK so this also must be a reversible process in that the if the piston was providing air to the system then the process would remain isentropic.

To remain isentropic and adiabatic the must be no heat transfer so the process must take place instantaneously. There must be no work performed by the working fluid. In order for the work to be transmitted to the rotor you have by definition created a non isentropic process. Lower temperature in the manifold will be subject to heat transfer. It is probably small though.

Fundamentally I don't see how performing work on the working fluid will be able to produce a situation where the work is recovered in a significant amount by the blower to an extent that it is a worthwhile investment from both a fuel and power standpoint. There is so much frictional loss that will be unavoidable in the system. Also the reduced pressure going into the the Otto cycle creates problems. You now have a much bigger pressure differential to overcome between the intake and exhaust sides of the otto cycle which requires more work and therefore entropy creation.

VW created a twin charger system for the 1.4L where they had both a super charger and a turbo to create low end boost quickly but the more efficient turbo for high end. The roots type blower was on a clutch and only engaged for short periods of time at high loads situations.

And what is a turbine?

It is a form of windmill or something which transforms differences in air pressure (aka wind) into rotational mechanical energy.

Some of the 50 cent words in this threat like adiabatic are still confusing my feeble mind, but, I think I get the gist of it.

The bottom like is, rather than throwing away all that pumping loss energy, you want to add it back to the crankshaft.

Am I on the right track here or not?

An adiabatic process is a process by which:

• work is done to a system, or work is performed by a system; and
• where there is no heat energy either dumped into the system, taken out of the system.

That's it.

In an ideal 4-cycle gasoline engine, the compression and power strokes may be thought of as adiabatic processes.

Yes, especially if you replace the throttle plate (a simple metal disk) with a complex system that provides a controlled pressure drop over a wide range of speeds and feeds the resulting energy into the crankshaft.

Now, if throttle loss reduction was the goal, I can think of a couple of ways to do it.

The first would be to slap on a smaller bore throttle body with a nozzle at both ends. As that simple metal disk deliberately causes aerodynamic drag in order to throttle the air, it would make sense to decrease the size of that metal disk. There'd still be a restriction there, but it'd be smaller. As the air is sucked through the smaller bore, it would get sped up, and in the process its static pressure would drop. Once the sped-up air got past the throttle plate, it would be expanded by the nozzle, which would slow it back down to feed the engine.

The other idea would be an extension of the first one above: A variable venturi. Here, no throttle plate is used at all. Instead, the properties of air are used to perform the throttling.

This guy had a pretty good idea: PRV Performance

http://www.prvperformance.com/assets...s/throttle.jpg

Keep in mind that the intake vacuum would still be there, but at least throttling losses would be much reduced.

Nobody's ever done the solenoid controlled valve system yet (that I'm aware of), but with or without boost, it completely negates the need for any type of throttle restriction. That along with it's many other potential benefits, I'd be looking into something like that, long before attempting to use unnecessary addition of parts, drag, expense, as well as extra weight and increased load to justify removing a simple plate or the functionality thereof.

Just me, though. Proceed, sirs.

I really like the concept and have thought of it much a long time ago. I got hung up on having nice, responsive controllability of it and concluded it would really have the most merit on stationary engines or engines that don't see such a wide range of throttling and rpm conditions, maybe tractors, mowers, generators, etc. but then of course there is the complexity and expense. Could be wrong about that though!

The initial benefit I saw was that you could use a CVT or electric motor to drive the supercharger and then be able to control engine output in a very simple and possibly responsive way. However having an electric motor controlling it is going to be complicated anyways, and a CVT probably doesn't change ratios quickly enough to have any sort of acceptable response.

The most efficient way to try to get energy back from the intake would be with an actual turbine, not a compressor run backwards, but at ~70% max efficiency + friction losses + varying performance across rev range it's hard to justify the cost.

http://ecomodder.com/forum/showthrea...tem-19845.html

It even used exhaust heat and an intercooler to have the gasses at an elevated temperature when going through the turbine.

CVT is already a ~20ish % loss in energy to friction losses in most cases.

If I gather what you're saying correctly, you intend to replace the throttle plate with what amounts to a variable restriction that generates torque based on air flow through the intake under vacuum?

If that's the case, abandon hope all ye who enter here. If I remember correctly, the most energy that can be theoretically extracted from any source which requires fluid movement (i.e. wind) is less than 50% of the overall energy. Wind turbines, for example, would not be able to harvest more than 50% of the energy of the wind, because doing so would necessitate the wind to literally stop at the turbine face, returning zero energy for all it's effort.

So now that you're taking about 50% of the energy of a vacuum (oh, no), and putting it through CVT, you're looking at a (conservatively) 15% loss, leaving you with 85% of the original 50%, leaving you with 42.5%.

Let's say your vacuum under full load is sufficient to provide 10 lbft of torque via air movement - you can now only extract a maximum of 5, and after the loss, redeliver 4.25 to the crankshaft, all things theoretically at their best values.

Reconsider, however, that you're technically introducing a new restriction to the intake, increasing the vacuum and, by relation, the subsequent load on the engine, causing more "work" to extract the "Work" that you intended to redeliver to the crank.

If that's not what you were alluding to, please explain further.

With the advent of computer controls and ever faster response times in said controls, I think it's something that could be a reality today.

Using all the same sensors currently in use, possibly a few more, to define the "curves" necessary for each solenoid's map set.

The first thing I'd ever bother trying it out on would be a single cylinder briggs or something, though, because the designs are transcendent and if it works on a single cylinder, it shouldn't be /too/ hard to adapt to more, if we're to treat each cylinder of an engine like it should be, as an individual engine, independent of the other cylinders.

How about no throttle restriction and just randomly pulse injectors to provide fuel for a given power demand. Like the old hit and miss engines of the early 1900's ?

Someone tried that here, using a variable control for the injector pulse instead of a throttle control. The car was a Honda DelSol, I can't remember the username of the character involved in the project, though.

He ended up making a millisecond long mistake and burning a piston crown, IIRC.

However, with controls that would prevent one from over-leaning/overloading at the same time, this is, in fact, a viable option, at least so far as was proven by the experiment.

I apologize to continue to shoot your ideas down but extracting work from the incoming air is different than throttling losses.

Newtonian physics states that work is the product of force and distance. In this case the force is the pressure over the area of the rotor that is being acted upon and the distance is that portion of the rotation, dTheta. In order for work to be extracted from the system you must have a dThetha that is non-zero. You get an easily quantifiable amount of work being performed. Now on a throttle body you have some area but it isn't being moved so there is no work being performed.

Since the piston drawing air into the system is the source of the energy and is creating the potential energy in the form of a pressure differential, it MUST be doing work on the rotor. However it is not doing work on the throttle body because the throttle body does not move. Therefore, for work to be extracted from the blower, it must be extracting work that would otherwise not be extracted from the system.

I already shot my own idea down, the only energy recoverable is miniscule in most cases due to the way Roots blowers are built.

I know no work is performed on the throttle body, the point is that there is an energy loss due to free expansion at the throttle body in the typical case. Cylinders drawing air unevenly reduces this slightly. As I wrote, a Roots blower would suffer nearly the same magnitude of loss to free expansion while having additional inefficiencies, thus it's not a good idea. I was mistaken in thinking it would recover significant energy because that energy would come from the pistons pulling a deeper vacuum.

And again, if the Roots blower exit was coupled more or less directly to the intake valves then it would be a different story but that's very difficult to do.

Actually timed pulses from a roots blower is a good idea and wouldn't be too difficult. You would need a blower running at engine speed with n/2 lobes where n is the number of cylinders. And timing isn't complicated because you could put it on a toothed belt like top fuel dragsters and extremely high performance cars do. And timed pulses could keep a more even pressure inside of the manifold.

I would love to be in engine development. Maybe one day...

Roots blowers generally use a helix type rotor design to help smooth the pulses out. Only very old designs use "impulse" style charging, because it creates a pulse frequency that can make proper fueling very difficult.

Thus, there's really no way to "time" the pressure pulses exactly to a specific event, since the design is meant to provide steady pressure, rather than pulsed pressure.

Even if you designed a blower that provided pulses, it would only serve to enhance the effects of helmholtz tuning (frequency-based freeair supercharging), and wouldn't provide much of an efficiency gain over just charging and maintaining pressure in the intake tract.

Purely hypothetetical. But yes I would specify a roots blower instead of a screw type. But who knows what it would do. You would only have a slight gain over a similar setup at the resonant frequency. Really none of this applies because superchargers don't improve efficiency but power. The work I have done with engines is in the racing sphere and that is very different than efficiency. Max power has been driven into my head.

A blower /can/ improve efficiency. They normally aren't used that way. However, to use a blower as a negative to throttle losses only seems to add complexity and cost without remittance of benefit.