Direct Air Injection Engine Question

[Unknowho]

Why don't automobile engines use direct air injectors just as there are direct fuel injectors to fill the cylinders? I would think it would be easier to move air than fuel. It would require compressed air from a Turbo or Supercharger to create the needed pressure. This would eliminate the need for an intake port and valve. You could even use multiple air injectors for optimum performance. This would also allow the exhaust valve to be relocated to a desired location to take full advantage of this approach. Your thoughts please. Thanks

 

[Tom.G]

Probably because the added mechanics involved would be a duplication of what both 4-cycle and 2-cycle engines already do.

The most common air compressor is a piston moving in a cylinder. That is already a part of most automotive engines - so why not use it, the cost in one extra valve.

Cheers,
Tom

 

[Rive]

The most common air compressor is a piston moving in a cylinder. That is already a part of most automotive engines - so why not use it, the cost in one extra valve.

Actually, those compressors also has valves. And you would need to regulate the airflow anyway, so double work.

Why don't automobile engines use direct air injectors

Automobile engines usually based on reciprocating movement, and that makes these things complicated.

But if you take a look at (gas)turbines, they start with a compressor stage if that counts.

 

[Baluncore]

@Unknowho Welcome to PF.

Why don't automobile engines use direct air injectors just as there are direct fuel injectors to fill the cylinders? I would think it would be easier to move air than fuel.

It is not easier to move air than fuel.

RPM is important as power is proportional to RPM * torque. Twice the RPM gives twice the power from the same lump of engine. The limitation on engine RPM is the injection and time required to complete combustion. The mass of air required is 14.5 times the mass of petroleum fuel, the air is also a bulky gas, compared to a liquid fuel.

A diesel requires heating of air by compression, followed by injection of a fuel, that burns without delay as it is injected. Early diesel engines (pre-1915) used an air blast to inject the fuel, (coal dust, alcohol, or oil), into the cylinder. That was inefficient because the expansion of the injected air cooled the air-charge, and so air blast was replaced by the induction and compression stroke, with jerk pumps, pushing liquid fuel through injectors with atomiser nozzles.

The pre-compressed blast air needed to be distributed to all cylinders. The time delay due to delivery, allowed the air to cool. The extra-high pressure air blast, into a cylinder, cooled the blast air as it entered and expanded. That was not good for maintaining continuous diesel combustion.

With spark ignition, the same cooling would occur as the extra-high pressure air was injected. That would condense the fuel and lead to poor combustion.

 

[Lnewqban]

It would require compressed air from a Turbo or Supercharger to create the needed pressure. This would eliminate the need for an intake port and valve.

Fuel injectors are fighting the pressure of combustion, reason for which the fuel pump needs to keep a constant high pressure to avoid any back-flow.
Yours would have to do the same in order to eliminate the intake valve.

Such air pressure will need a heavy and strong compressor and a way to cool the compressed air prior to injecting the required mass of air into the cylinder, which needs to be about 15 times the mass of injected gasoline for each cycle.

That volume of air is relatively huge for the opening size of an injector, forcing the velocity of the supplied air to be also huge, possibly close to choke conditions.

Turbo compressors achieve pressurized intake strokes in a more economic and light version.

Please, see:
https://en.m.wikipedia.org/wiki/Air–fuel_ratio

https://en.m.wikipedia.org/wiki/Choked_flow

https://en.m.wikipedia.org/wiki/Turbocharger

 

[Baluncore]

Fuel injectors are fighting the pressure of combustion, reason for which the fuel pump needs to keep a constant high pressure to avoid any back-flow.

The energy needed to atomise the liquid fuel, as it is injected, is derived from the differential pressure developed across the injector orifice.

The injection pressure must therefore not only exceed the combustion pressure, but must be very much greater, in order to fully atomise the fuel, to prevent the black soot particulate emission that results from incomplete combustion.

Higher injection pressures lead to lower particulate emissions, which reduces the burden on the Diesel Particulate Filter, allowing the exhaust to flow more freely.

The fuel pressure is rising with every generation of common-rail system. Fuel pressures have now passed 1,000 bar, 14,500 psi.
https://en.wikipedia.org/wiki/Common_rail

 

[Unknowho]

So, essentially air injectors cannot flow at a high enough rate to supply the air needed for the proper air to fuel mixture ratio?

 

[Baluncore]

So, essentially air injectors cannot flow at a high enough rate to supply the air needed for the proper air to fuel mixture ratio?

True.

 

[jack action]

It would require compressed air from a Turbo or Supercharger to create the needed pressure.

There are only two ways of looking at this comment.

1. The actual turbo or supercharger designs are already high-pressure air injected into the cylinder and you want to redesign the valve. Could they be smaller or of different design? If they could be, they would be.

2. You create extremely high-pressure air to be injected with the fuel at TDC. The first problem is that any compressed air that decompresses without actually pushing the piston will be loss energy. It requires a lot of energy to compress air, losing it will require an even greater efficiency to compensate. Second, I think mixing the air and fuel will be more difficult, leading to a very inefficient combustion. In the intake and compression processes, the air swirls, tumbles, and squishes. This is extremely important to induce rapid movement in the airflow.

​

Intuitively, most people think that the spark ignites the mixture and then the first row of burning molecules ignites the next row, and so on until all the mixture is burnt. But that is a process that would be way too slow to burn the air-fuel mixture completely at TDC. Your "constant volume" combustion would become a less efficient "constant pressure" combustion or maybe even worse. To counter this, the airflow is sped up a lot such that the already burning front of the mixture is propelled faster than the burning rate of the mixture. This is the only way we can achieve modern high-speed engines.

The following video for diesel engines might help visualize furthermore the importance of controlling the air motion in the combustion chamber (at the lowest cost possible, obviously):

​

 

[Unknowho]

2. You create extremely high-pressure air to be injected with the fuel at TDC. The first problem is that any compressed air that decompresses without actually pushing the piston will be loss energy. It requires a lot of energy to compress air, losing it will require an even greater efficiency to compensate. Second, I think mixing the air and fuel will be more difficult, leading to a very inefficient combustion. In the intake and compression processes, the air swirls, tumbles, and squishes. This is extremely important to induce rapid movement in the airflow.

NO, NO, NO......................the air would be injected into the cylinder just like it would if using a poppet valve. It would be compressed further as the piston moves to TDC. The fuel would be injected just the same as if there were a valve. Again, just for clarification. The air would be injected on the intake stroke. Then the fuel would be injected after the piston is almost ADTC just as it would with a poppet valve were there.

 

[jack action]

the air would be injected into the cylinder just like it would if using a poppet valve.

As said in point 1, how would that be different from the intake designs already used?

Take a supercharger. Once the cylinder is emptied, the pressure inside the combustion chamber is nearly at atmospheric pressure. The intake valve opens and behind it, you have high-pressure air (5, 10, 15+ psi) ramming into the cylinder.

 

[Unknowho]

As said in point 1, how would that be different from the intake designs already used?

Take a supercharger. Once the cylinder is emptied, the pressure inside the combustion chamber is nearly at atmospheric pressure. The intake valve opens and behind it, you have high-pressure air (5, 10, 15+ psi) ramming into the cylinder.

There would be NO valve, valve spring, retainers, locks nor the giant port to deliver the air. The air injector would serve the role of the intake valve and port. Just like a direct fuel injector delivers fuel to the engine.

 

[jack action]

The air injector would serve the role of the intake valve and port.

How big would it be? Why would it be smaller than the intake port? An injector has some closing mechanism: how would it differ from the poppet valve?

 

[Unknowho]

How big would it be? Why would it be smaller than the intake port? An injector has some closing mechanism: how would it differ from the poppet valve?

Just as a Direct fuel injector sprays fuel into the cylinder a direct AIR injector would do the same. Not hard to understand.

 

[Baluncore]

Just as a Direct fuel injector sprays fuel into the cylinder a direct AIR injector would do the same. Not hard to understand.

Well, it is hard to understand, because you have not reduced the number of possible interpretations.

The fuel is a hydrocarbon liquid, that is atomised, (not sprayed), density ≈ 1000 kg/m³. Air is a gas that must enter the cylinder during the induction stroke, density ≈ 1.2 kg/m³. Then, with stoichiometric air mass being 14.5 times the mass of the fuel, it requires an air-inlet port that is; 14.5 * 1000 / 1.2 = 12,000 times the area of the fuel injector.

What would the air pressure be, just before it was injected?
Are you considering LOX with a density of 1140 kg/m³ ?

How will you dry the air? Or why would the air injector not freeze up, like a carburetor in sub-zero air, due to the pressure drop in the air, as it is injected?

Maybe you are considering replacing the poppet valve in the head, with a sleeve valve, or a port in the wall of the cylinder, uncovered by the piston?

 

[Unknowho]

Well, it is hard to understand, because you have not reduced the number of possible interpretations.

The fuel is a hydrocarbon liquid, that is atomised, (not sprayed), density ≈ 1000 kg/m³. Air is a gas that must enter the cylinder during the induction stroke, density ≈ 1.2 kg/m³. Then, with stoichiometric air mass being 14.5 times the mass of the fuel, it requires an air-inlet port that is; 14.5 * 1000 / 1.2 = 12,000 times the area of the fuel injector.

What would the air pressure be, just before it was injected?
Are you considering LOX with a density of 1140 kg/m³ ?

How will you dry the air? Or why would the air injector not freeze up, like a carburetor in sub-zero air, due to the pressure drop in the air, as it is injected?

Maybe you are considering replacing the poppet valve in the head, with a sleeve valve, or a port in the wall of the cylinder, uncovered by the piston?

It's already been determined the air injector is incapable of supplying the air needed for proper air to fuel ratio. Now, the air could begin being injected from valve overlap, thru the entire intake stroke and just a few degrees before the fuel is injected during the compression stroke. The amount of time the air has to enter the cylinder enables a larger volume of air to enter the cylinder to reach the proper A/R ratio. As for how to treat the air. The same way the ICE with a compressor treats the air. With an intercooler.

 

[Baluncore]

It's already been determined the air injector is incapable of supplying the air needed for proper air to fuel ratio.

Duplication of the airflow path, would seem to add unnecessary complexity. What system makes up the unsatisfied air?

How does an "air injector" differ from a Roots-type supercharger, or a turbocharger, that push air into the intake manifold plenum, then through the inlet valves?

What is an "air injector", how is it physically connected to the engine, and with what flow control valves?

 

[Unknowho]

Duplication of the airflow path, would seem to add unnecessary complexity. What system makes up the unsatisfied air?

How does an "air injector" differ from a Roots-type supercharger, or a turbocharger, that push air into the intake manifold plenum, then through the inlet valves?

What is an "air injector", how is it physically connected to the engine, and with what flow control valves?

Go back and read all of the previous post. They answer your questions.

 

[Baluncore]

Go back and read all of the previous post. They answer your questions.

Do you mean this post?

There would be NO valve, valve spring, retainers, locks nor the giant port to deliver the air. The air injector would serve the role of the intake valve and port. Just like a direct fuel injector delivers fuel to the engine.

Fuel injectors can only inject an incompressible liquid, they cannot inject air which blocks them, so they must be primed. You say what an "air injector" is not, but then what actually is it?

Direct fuel injectors, inject directly into the cylinder, where swirl and mixing with air is often incomplete, that limits engine speed, or increases black soot.
Indirect fuel injectors, inject into a pre-combustion chamber, where an initial partial burn, causes the unburnt fuel to be blasted out through a small port, into the cylinder where there is more air. That makes possible faster and more complete combustion, and so higher engine speeds.

Your "air injector" would appear to not duplicate the mixing function of the pre-combustion chamber, because it does not mix the fuel and air. How can lowering the RPM, or producing black soot, be good?

 

[Unknowho]

Do you mean this post?

Fuel injectors can only inject an incompressible liquid, they cannot inject air which blocks them, so they must be primed. You say what an "air injector" is not, but then what actually is it?

Direct fuel injectors, inject directly into the cylinder, where swirl and mixing with air is often incomplete, that limits engine speed, or increases black soot.
Indirect fuel injectors, inject into a pre-combustion chamber, where an initial partial burn, causes the unburnt fuel to be blasted out through a small port, into the cylinder where there is more air. That makes possible faster and more complete combustion, and so higher engine speeds.

Your "air injector" would appear to not duplicate the mixing function of the pre-combustion chamber, because it does not mix the fuel and air. How can lowering the RPM, or producing black soot, be good?

You need to read the entire thread for an answers. This has all been covered. Take some time to read it all.

 

[Averagesupernova]

This makes no sense. Just what is it that is going to power the pump that compresses the air in the first place? Where will this power come from and how is it different from what superchargers are doing?

 

[renormalize]

You need to read the entire thread for an answers. This has all been covered. Take some time to read it all.

Perhaps so, but if there are still questions being asked, you need to be willing to amplify on your previous postings to address them. Otherwise you are no longer contributing constructively to this discussion.

 

[Baluncore]

I suspect the "Direct Air Injector" is a mythical invention.
Maybe it only exists in the head of @Unknowho.