underdriven Roots blower for throttling loss reduction

[serialk11r]

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.

[serialk11r]

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.

[AndrzejM]

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 ;-)

[pete c]

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.

[serialk11r]

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.

[hawk2100n]

Originally Posted by serialk11r

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.

[pete c]

Originally Posted by serialk11r

Not as a "windmill", but as a turbine.

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?

[t vago]

Originally Posted by pete c

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.

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.

[JRMichler]

Originally Posted by pete c

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?

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.

[t vago]

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



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