Compressed air supercharging

[Daox]

I was surfing around on Youtube the other day and came across these guys.

http://casupercharging.com/

They build compressed air supercharging systems. Basically, you fill a high pressure tank with air. You use a scuba compressor to fill them to around 3000 psi. That 3000 psi gets regulated down to something more manageable, say 10 psi so you don't blow your engine to smithereens. Then, the system blocks off your normal intake, and runs solely off the compressed air tank for a short period. The idea sounds pretty cool and does have some very nice advantages over other forms of power adders.

Have you guys ever heard of this?

[timw4mail]

So, would that potentially have enough power to spin the engine without spark?

[Daox]

Nope, it does not spin the engine on the compressed air alone. It uses the compressed air to supercharge the engine. This is just like strapping a turbo or supercharger on, but the air the engine uses is coming only from the tanks, not the ambient air. So, you can basically dial in whatever boost pressure you want.

[Daox]

Here is an overview of the system.

[Loren]

I've heard of the idea. I seem to remember that the CFM requirements of an engine rendered it of limited use.

You can easily get lots of 10 psi boost out of a 3000 psi tank... but, there's only so much VOLUME of air you can put into that tank, which limits the amount of time you can effectively use that boost.

If you take this to its logical conclusion, you get NO2 injection. Not just putting "air" into the intake, but concentrated "oxygen", to which you add more fuel... and off you go.

If my rough calculations are correct, the 1.2 Mirage engine should use about 150 CFM at WOT. If we aim to increase that by 50%, we're adding 75 CFM from our tank.

I'm not a diver, but Google says a typical scuba tank holds 80 CF when pressurized to 3,000 psi. So, 80 CF available... using 75 CFM... it might last about a minute.

Good for limited drag racing, I suppose. If you're making 15-second passes, you could make 4 of them.

No free lunch, though. Once you get beyond about 20% over what the engine was designed for, you're going to have to add more fuel somehow.

[Daox]

Excellent info Loren!

It does look like an 80 cubic foot tank is the 'standard' scuba tank size. However, there are larger ones if you'd like more capacity.

From: https://daniellesdives.wordpress.com...ith-that-tank/





So, limited capacity is definitely a major downside to a system like this. However, there really doesn't seem to be a ton of other downsides to it that I can see.

There are some upsides as well. #1 is very cold intake air. This not only creates more power, but it also allows you to not retard ignition timing as much which makes even more power. It also has zero parasitic losses compared to a turbo or belted supercharger.

[Daox]

I think your calculations are a bit off.

The engine should use ~140cfm at WOT and peak rpm. Most of the time we'll be using less than that due to shifting. However, lets use 140 for worst case calculations .

The tank holds 80 cubic feet.

Lets say we want to hit 100 hp, or ~30% higher than stock horsepower. Sounds like a nice round number to shoot for. This means we need (140 * 1.3 = ) ~180 CFM. This all has to come from the compressed air tank, not just the 40 extra CFM.

Now we need to take 80 / 180 = .44 minutes of run time. This is about 26 seconds.

So, run time is quite limited. Of course, you can always add more or use bigger tanks.

[Loren]

My calcs weren't intended to be accurate. Just pulled some numbers together and used an online calculator to get close.

I still stand by 20% being as far as you'd want to go without adding more fuel, which means either tuning, or larger injectors, or higher fuel pressure, or aftermarket engine management... or some combination thereof.

So, if you want to get accurate... you're looking at 93.6 hp for a simple "bolt-on" compressed air boost system. The benefit of this over NO2 would be that you could recharge your bottle at home, or even in the car with just an air compressor. (though, I don't know what kind of compressor you need to get to 3,000 psi... maybe you could compromise on that, run a bigger tank with lower pressure?)

And then, do you want to run "some" boost all the time, or do you want it to be throttle-triggered to only activate at full throttle?

And then you can get into the fun stuff... do you want to run an on-board compressor that would run as-needed when you're NOT at WOT (sort of like the AC compressor does) to keep the tank pressurized, and take the FE hit for that... and have the boost available for WOT acceleration when you want it?

It's all neat ideas to play with, but at the end of the day, you'd end up with a lot greater efficiency and power by either fitting a larger engine, or going with a traditional supercharger or turbocharger.

[Daox]

I still stand by 20% being as far as you'd want to go without adding more fuel, which means either tuning, or larger injectors, or higher fuel pressure, or aftermarket engine management... or some combination thereof.

That is probably a very good rule of thumb. It would be really nice to get the specs of our fuel pump and injectors. I've looked for quite a while to see what the fuel injector flow rate is, but I could not find any data. I also have no idea what the flow rate is on the fuel pump either. It does not seem to be shared with other vehicles. All the guys running forced injection have also never shared what they've done about the fuel system.

So, if you want to get accurate... you're looking at 93.6 hp for a simple "bolt-on" compressed air boost system. The benefit of this over NO2 would be that you could recharge your bottle at home, or even in the car with just an air compressor. (though, I don't know what kind of compressor you need to get to 3,000 psi... maybe you could compromise on that, run a bigger tank with lower pressure?)

They sell scuba compressors. They aren't horribly expensive, but aren't cheap either. I think the cheapest I found on amazon was ~$280.

And then, do you want to run "some" boost all the time, or do you want it to be throttle-triggered to only activate at full throttle?

IMO it doesn't make much sense to provide boost until you're at WOT. Unless you're at WOT, there is more power already available.

And then you can get into the fun stuff... do you want to run an on-board compressor that would run as-needed when you're NOT at WOT (sort of like the AC compressor does) to keep the tank pressurized, and take the FE hit for that... and have the boost available for WOT acceleration when you want it?

Knowing how big and heavy my 220V compressor is, I'd have to say I wouldn't want to shoe horn it in to my Mirage haha.

It's all neat ideas to play with, but at the end of the day, you'd end up with a lot greater efficiency and power by either fitting a larger engine, or going with a traditional supercharger or turbocharger.

And, that is probably why we have never seen this technology gain any traction. Its just not that practical or appealing. However, it is interesting to see what can be done. I think I may tinker around with a few ideas and post it here. I'd like to explore the idea of a low (shop pressure) setup. Can it be done super cheap with regular off the shelf items and some modding? Its worth exploring IMO, even just for funsies. But, then again I rather enjoy going through the engineering exercises of proving or disproving a modification. I learn and grow through every failure or success and I enjoy that.

[Loren]

IMO it doesn't make much sense to provide boost until you're at WOT. Unless you're at WOT, there is more power already available.

There is potential for greater efficiency. Even 2-3 psi of boost is like adding compression. Plus the cooler charge and all that. You could potentially improve efficiency at low-mid throttle.

And could definitely improve drivability at part-throttle. Let's say right now you can see 70 ft-lbs of torque at WOT from 2500-4500 rpm. With a few pounds of boost, you could either see 90 ft-lbs of torque at WOT... or 60-70 ft-lbs at half to 3/4 throttle. Would be much more satisfying to drive.

But, then again I rather enjoy going through the engineering exercises of proving or disproving a modification. I learn and grow through every failure or success and I enjoy that.

Yeah, I'm with you there. It's fun to think through problems and unconventional solutions, and just DO stupid stuff. This is why I'm autocrossing a Mirage.