Overview
Note: This part is a very basic overview of engines, and some tuners will find it boring. Skip to the next section if this includes you.
So most auto tuners and enthusiasts know that an engine is just a glorified air pump. Getting fuel to the cylinders is the easy part thanks to modern electric pumps and injectors. The hard part is pumping the air required for combustion. A typical engine, which breathes due to the vacuum created when the piston moves down the bore, is called naturally aspirated. (There is also the effect of the exhaust gasses rushing out via the exhaust valve, but this is out of scope)
Over the years, inventors have found clever ways to force more air into an internal combustion engine. There are two main schools of thought, and these are forced induction and oxidizers. But superchargers and turbocharges come with some complexities and high cost. Nitrous tends to be the easiest power adder to install, and the cheapest up front, but refill stations can be hard to come by and somewhat expensive. We will use much of the same concepts as N2O.
The Idea
I don't at all think this is a brand new idea. In fact there are probably patents on it, and they are probably long expired. However, I have never seen a system like this before, so let's investigate this a bit. Okay, enough beating around a bush. How about feeding pre-compressed air into your engine? Here are some numbers for you.
A standard scuba tank is holds 80 ft3 air at 3000psi and weighs in at 6.5lbs. In SI units, these are 2.27 m3 of air at 2.07 x 107 N / m2, weighing in at 2.95kg.
Now let's pick a more or less standard engine. Let's say it's a late model 2.0L 4-cylinder four stroke engine. This means the engine breathes in 2.0L worth of air (assuming 100% VE at the given engine speed), every 720° of crankshaft rotation. Lets assume the engine redlines at 7000 RPM. So our engine theoretically inhales a whopping 3500 x 2.0L = 7000L of air a minute at this speed.
Disappointing Results?
That is approximately 247 cubic feet. Remember, our scuba tank only holds a mere 80 ft3. That means that if our engine were to breathe air from our scuba tank, it could sustain just 80/247 = 0.32 min ≈ 19 seconds... which is, if you ask me, not a very impressive duration. It may seem that I have disproved the usefulness of this invention, but I have glossed over a few details, and the keen reader may have already noticed this.
- An engine is not going to be held at or near redline very long, so 19 seconds is several bursts of power, not just one elongated rush.
- This boost would presumably be used at an earlier RPM, which would take linearly less air to satisfy combustion and thus would last longer. This is a corollary of the first point.
- This is assuming 100% VE, which is not true internal combustion engines at all RPM.
- And the biggest, and most important point is... We're not feeding the engine solely with air from the compressed tank! The tank is just for additional air above atmospheric pressure.
There's Still Hope
Let's take a look at that last point. Let's just assume that we want to boost the engines air intake by 50%. That would equate to roughly 124 ft3 of air — which would be roughly 7.5 psi of boost pressure at redline. Please bear with the fuzzy math/physics for the sake of brevity. Then, our burst would last about twice as long (80/124 ≈ 39 seconds). Now we're getting somewhere!
Unexplored Ideas
There are still a few problems I did not research as well as some additional ideas I have related to this topic:
- How do we prevent reversion back through the intake system? with a supercharger or turbo charger this is accomplished by the compressor (the compressor would have to stall to allow the air to flow backwards through it). Could we use some sort of a check valve, like a PCV valve, and plumb it closer to the intake manifold (after the Mass Airflow Sensor, if so equipped)?
- How do we regulate release for a constant flow rate (or alternatively, constant manifold pressure)? I suspect that a device similar to a pressure regulator could be used, but this would have to stand up to the initial tank pressure of 3000psi.
- Is there a convenient way to compress the air "at runtime"? That is, does there exist an electric air compressor with the right characteristics to run in the car and fill the tanks when the boost system is not in use? The right characteristics being size, weight, power draw from the electrical system, max pressure near 3000psi, cost, noise (intake mufflers can do wonders for this).
- Since the tanks are not very heavy (approx: 3 kg), can we run two or more in parallel with couplers to get more use between refills.
- How do we add the additional fuel required? Should we plumb this in before the Mass Airflow Sensor/ Manifold Absolute Pressure sensor and let the ECU take care of fuel enrichment or should we add our own injector like many N2O systems typically do?
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