Vacuum free 4-cycle?

[DKJones96]

Does anybody here know if someone is working on an engine that has no manifold vacuum?

I have a few things i've gome up with that involve an IC engine with no manifold or chamber vacuum at part throttle. The engine basicly purges the charge from the cylinder again into the intake manifold(against a reed valve to create a bit of pressure to force exhaust out on overlap) until it reaches your 'throttle' point. My way of doing it is infinately variable and completely mechanical and to help with this I've developed a completely mechanical way to have variable compression ranging say from 8-12:1 and also thought of a low pressure direct injection system for it. I plan on building this for a Toyota 5M-GE I6 engine starting in a couple weeks.

I was just wondering what you guys thought about efficiency increases and if anyone is working on this kind of stuff. Sadly...I'm only 22 so most people don't take me seriously. Just wondering what you guy thought.

 

[DKJones96]

Most testing i've done to date involved single cylinder OHV/OHC engines. We tested alot at 50% throttle because that's where most poeple spend time accelerating at in most cars before they let off to 5-25% depending on speed. We drove the engine by electric motor to measure power required to maintain a given rpm we ran 3000.

From our testing the pumping losses from the engine taking in and expelling to 50% charge was less than the losses required to pull against a vacuum for an entire intake stroke and 'spring' back to full atmospheric at 90deg BTDC on the compression stroke (like where we closed the valve fully) before it started compressing the charge. In addition to that, when we added the reed valve in the intake for the engine to partially pump against it created a pressure on the back of the intake valve that worked great to expell any extra combustion gasses out the exhaust valve durring overlap which increased our part-throttle torque numbers alot. The reed valve's benefit, however, is only seen at mid/low rpm part-throttle engine use.

Two engines compared at full throttle there was a bit less power output on the engine with the new valvetrain but the losses are on par with the fact it's running an extra cam with extra valvespring.

 

[turbocohen]

Globi5, EGR dilutes the Nitrogen and oxygen from atomsphere with carbon dioxide resulting in more inert gas diluting o2, aka charge dilution. With less O2 per unit volume the manifold pressure is closer to atmospheric while maintaining a more advantageous air/fuel ratio. When EGR and lean burn work together very significant efficiency gains are possible, 10-20% in the lab is not unusual and 5-10% is typical on the street.

EGR, whether via external valve or internal, helps reduce pumping losses, reduces peak cylinder temps for Nox control, Helps increase available charge motion in cylinder and in the case of some higher compression engines helps reduce octane requirements.

 

[globi5]

Thanks turbocohen for the explanation.

Fabrico, no offense, but you can't compare a centrifugal pump (vacuum cleaner) with a piston pump.
A piston driven vacuum cleaner would slow done and not speed up since it has to work against atmospheric pressure and a centrifugal pump simply has to move less air molecules.

 

[wwest]

DK,

What you seem to be doing is trying to apply the Atkinson cycle technique to an engine with less than 4 cylinders or multiples thereof.

With a 4 cylinder engine when you have one piston on a compression stroke and with the intake valve remaining open to "exhaust" some of the charge back into the intake manifold you also have an "opposite" piston on an intake stroke that will, in effect, absorb the charge being "exhausted".

Results being a relatively constant vacuum within the LARGE VOLUME of the intake manifold.

So, with a 4-cylinder Atkinson cycle engine you still have intake throttling, vacuum in the intake manifold, and pumping losses (lowered) associated with same.

The problem you undoubtedly encountered in trying to apply this technique to a single cylinder engine is pulses of reverse airflow, A/F mixture, "spitting" out of the carburator. Made worse on a single cylinder engine by having virtually NO intake manifold VOLUME to act as a pressure vessel, accumulator.

Same as would happen with anything other than a 4 cylinder engine or multiples thereof, no synchronization of airflow into and out of the cylinders resulting in reverse flow "pulses" at the throttle valve.

Which is why Mazda went to the Miller Cycle for their V^ in the Mazda Millenia "S".

You arrived at the obvious solution, use a 2-cycle engine's reed valve at the carburator throat.

As you point out, pressure pulsing is then contained within the intake manifold itself and will actually help "fill" the combustion chamber on the next intake stroke....SuperCharging...??

The one thing you seemed to have missed is raising the mechanical compression ratio to ~13:1 but keeping the delayed closing of the intake valve so the effective compression ratio remains at 10:1.

Then at WOT the two engines would have equal HP/torque output capability.

 

[mishar]

Because it takes less power to pump the air twice than it does to work against the vacuum. Especially the larger the engines displacement is.

Not really. Pumping air twice is all lost work and “working against vacuum” is all conservative because you are getting it back in the very next cycle when that vacuum is “pulling” the piston up. Lost work comes just from vacuum decrease because of heating the gas.

 

[wwest]

An easy example, test, even if only done mentally.

Take an old style bicycle tire air pump and cover up the inlet opening and now try to "prime" the pump by pulling the handle upward.

Pretty hard "travel" after about 50% distance, huh?

That's what it's like, "pumping loses" for a piston on an intake stroke with the throttle butterfly valve virtually closed.

 

[patprimmer]

wwest

When you pull the pump up, try letting the handle go and see what happens on the compression stroke with the butterfly valve closed.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[wwest]

Yes, I agree, the "piston" will spring back.

But just like the upcoming downhill "run" doesn't fully "cover" the energy you used to get to the top in the first place the "spring" doesn't fully return the input energy.

 

[GregLocock]

True, but which causes the bigger loss - pumping the air twice, or whatever you lose in the springback process.

Pumping losses in a typical engine are around 4% of the power output from memory, doubling that would be a bad thing.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CCycle]

Another take on this is to say, well ok, si engines will have pumping loss.
Lets recover the loss.

Look at US Pat 3180079. Freeman in '61 was doing this mechanically with an air motor and roots type blowers.
Kawamura et al of Isuzu have several modern patents to this effect.

The general idea is to put a turbine in the intake. The part load pressure drop is made across the turbine which is coupled to a generator to recover the pumping loss as electrical power.
When extra power is desired from the engine, the generator can be switched to become a motor which turns the turbine wheel to supercharge the engine.

 

[turbocohen]

KKK, Garret or IHI produced a prototype turbo motor-generator a while ago that was shown to aviation buffs years ago. It produced about 0.3bar for brief bursts and a variable vane throttled the nozzle area. The concensus was that it was too risky and expensive and a few of the proto's wound up on a car project. I saw one well over a decade ago on a 1.9 liter benz 4 banger.

 

[DKJones96]

But you see. Pumping losses are about 4% at what throttle?

At full throttle I can see a 4% loss because it doesn't work against much as there is no vacuum or anything. At part-throttle you have a big vacuum, and while there might be a spring back it's not near as much pull as when the valve was open because the cylinder now has air/fuel in it. You lose usable springback at about 45-60 deg crank angle.

 

[GregLocock]

WOT

Figure is from memory, it is in Heywood

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CCycle]

Turbochoen: Yes, there have been several suggestions of a turbocharger with a motor/generator added to the shaft, US Pat 6449950, 4827170 and many more. These are interesting since the device can also turbocompound. Probably the reason they introduced it to the aircraft industry it that turbocompounding is most effective at high engine outputs. Aircraft engines run near full output most the time. Automotive engines however, run at low output most the time and are not good candidates for turbocompounding.

Recovery of pumping loss is most effective at low engine outputs hence automotive applications are good candidates. This same device can also recover pumping loss but there is less pumping loss power than there is exhaust power to be recovered.

DKJones and GregL:
It is hard to determine what they are talking about when pumping loss is expressed as a percent. I assume that the stated percentage is percent of full power. However pumping loss is zero at full power. So the stated 4% would be 4% of full power but this would be available only at some significantly reduced power level.

If we describe pumping loss as the percentage of power output at the condition producing maximum pumping loss, i.e. Pumping Loss/Output Power
This can go over 100% since operating conditions can be chosen where there is more pumping loss than power out of the engine.

 

[wwest]

Don't forget that one of the advantages of super or turbo charging is having an intercooler downstream to reduce the heat gained from pressurization. If the piston does ALL of the compressing no such advantage is available.

 

[turbocohen]

CCycle I beg to differ that aircraft engines run at near full output most of the time. They are hammering out thrust at takeoff and moderate load up to altitude then cruise at moderate loads.. some needs to be there in case of aborted landing.

Autos run at variable loads most of the time.

 

[GregLocock]

CCycle

if you think an engine at WOT has no pumping losses then you are sadly mistaken.

And the concept of a pumping loss as an important parameter in an engine that is not at WOT is a bit of a puzzle.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[wwest]

Aircraft engines...

We always ran our T210 at ~75% power level, hour after hour, in level cruise.

300HP x .75 = 225HP

My 911/996 C4 in "level cruise" at 70MPH uses about 40-50HP. 20-25% of WOT.

Pumping losses...

WOT...As the piston moves downward from TDC on an intake cycle it creates a vacuum at the top of the piston. As we all know, are certainly should know, mother nature HATES a vacuum so air immediately begins to rush into the cylinder bore to fill the area just previously displaced by the piston.

Ideally the cylinder area vacated by the piston would be filled fully, all the way up to local atmospheric pressure.

BUT, and this is where K&N makes their money.

If the outside of the intake valve were truly open, fully open, NO RESTRICTIONS, to the atmosphere then the filling of the cylinder bore would be much easier and quicker.

Just like everything else, air molecules have inertia, so it takes a "moment" to get them moving, and at 5000RPM there are not many "moments" available.

So even at WOT "Otto" engines are not exactly "free-breathing"

Now, fully close that WOT and only rely on the IACV for intake airflow.

The pumping losses quickly become huemongus.

The reason an airplane wing "wants" to move upward, WILL move upward, is to fill the vacuum above. For that same reason the piston "wants" to move upward against the "demands" of the connecting rod/crankshaft.

 

[CCycle]

Wwest Good point if your concern is max power out of a certain displacement. External supercharging with intercooling does have the potential of lowering the work needed to compress the charge.
But look at this from energy efficiency. Any heat rejected by the intercooler is a dead loss. No way in most practical situations of any recovery. However, if you compress it in one step in the cylinder, it does take more work but the work, or most of it at hopefully, is recovered in the subsequent expansion.

Turbocohen: Way back when I was flying, cruising speed was considered 67% of full power. This is over three times the 20% or so that the average auto engine runs at cruising. The point is that aircraft engines work harder hence are better candidates for turbocompounding.

GregL: OK We all realize that there is pumping loss at WOT. Wwest makes a good point (prev. Post), thanks wwest.
Si engines are controlled by making them inefficient.
At WOT any attempt to recover pumping loss with a turbine wheel mounted in the intake will result in reduced power. Yet at low power levels there is significant pumping loss available and it is ok to recover this power since the reduced power level is desired.

 

[turbocohen]

CCycle, In response to your statement "Any heat rejected by the intercooler is a dead loss. No way in most practical situations of any recovery. However, if you compress it in one step in the cylinder, it does take more work but the work, or most of it at hopefully, is recovered in the subsequent expansion."
Got data to back that up? Using an intercooler reduces the net exhaust backpressure for a given mass of compressed air. More dense airat a lower pressure is easier to pump into a manifold than hot lower density air. i.e, If boosting to 10 psi without an intercooler vs boosting to 7 psi with an intercooler (aiming for the same mass flow) then the 7psi system will require less work from the turbine and less backpressure will be working against the piston during the exhaust stroke. Additionally there will be less residual spent gas in the combustion chamber. There are other factors as well.

Just my 0.02 dinars, Turbo