Tuesday, July 01, 2008

Air boosting driven by exhaust powered vacuum.

The basic idea goes like this: to make 20% boost on an OM617 engine at full RPM you need about 2.5kW of power in case of adiabatic compression. I used adiabatic process calculator and here are the numbers. 135 L of air compressed to 112.5 Litres from starting pressure of 98.94 kPa and end pressure of 127.71 kPa, start temperature of 24.34°C and end temperature of 46.85°C requires 2526 J of energy. Those are the parameters of air that contains 20% more air molecules than the one second rotation of 3.0 L OM617 engine at 4500 rpm. You see the air has far higher pressure difference than 20% (29%), and it is the result of increased temperature of the compressed gas.

The below assumes air density of 1.3kg/m^3

Now to the crazy point. Previous article explained that properly phased suction vacuum is the power behind the resonant oscillations. Maybe you have seen or heard of devices that create vacuum by using compressed air and ejection nozzle. If we make a similar device utilising the exhaust gases we may too be able to make a reasonably strong vacuum source. How should this help us boost air pressure to the engine? We can use it to suck air out of a long tube to make it move at, say 450 km/h speed (125 m/s) and divert part of this moving air by aerodynamic flap or wing to a longer intake tube where the fast flow will slow down to the ordinary suction speed at the expense of being compressed.

The complexity of the design and calculations is high, and benefits are only small, but it is a device with no moving parts and no maintenance. Plus, when properly designed it can be added to existing supercharger designs and to some extent also with certain turbochargers - those which still provide enough power on the turbine output. There won't be many such systems, and those use three turbocompressors in series already ;-)

What am I aiming to achieve then? 20 to 40 kPa boost pressure for the air filer inlet, which would amount roughly to 15-30% more air molecules. Remember that efficiency of this system is much lower than that of the turbocharger, but it has lower delays and you only need to change muffler bearings once in Platonian year.

The last advantage of this exotic boost system is this: past the divisor the flow that leads to the ejector can be equipped with a small tube to let in it diesel fuel to make flames out of the exhaust, or water (or water + something) to make experiments of other kind (like plumes of stink? or even improving emissions!) or glycerine mix to make white or colored smoke trails behind the bike. For many people, that would outweight the former advantages.

How do Ejectors Work?

Very informative air-combustion gas-water vapour gas ejector desigh considerations from 1956.

First thing that will help us understand how much boost we can getis from the Bernoulli equation. That one only takes STEADY, not pulsed flow into consideration - plus, flow of an non-compressible fluid, like water. But still it provides the basic measure - scale. If you enter the values you will be able to see how much is the kinetic energy compared to the energy already stored in the fluid, because air, as we breathe it is already compressed! Comprerssed to about 1 atmosphjere of pressure. The Bernoulli calculator is here.

Post Scriptum: My math tells me this idea is not that practical because due to practical limitations on the size and flow capacity of the exhaust I can forget about a 20% boost. One meter of 6cm diameter pipe flowing 350 L of fresh air per second carries the kinetic energy of 28 Joules. Very modest compared to the 2.5kW requirement I started with at the top. This can only be useful as a sealant of the resonant pressure peak.

Post Post Scriptum: I was mistaken with the statement above, the kinetic energy of the WHOLE mass of 350 L per second is 3486 Joules and that amounts to 3486 Watts. You just need to take care of losing much less in the pipe in aerodynamic friction. Next you will make the pipe long enough to carry just enough energy to hold any counterpressure pulses from the resonator.


Blogger Mario. said...

error, tube with 6cm diameter (3cm radius) contains 26J of energy in air moving at 124m/s.


of course the amount of air that resides - - MOVES trough the tube will be completely replaced in 8 miliseconds!

Thus, ONLY REASONABLE air flow boosting is trough exhaust powered cross-valve suction during the valve overlap timing.

8:23 PM  
Blogger Mario. said...

I have to explain why the exhaust boosted air flow might be used: if you make enormous air flow by the exhaust ejector, you may tap some 10-20% of the flow, while slowing down, the air will compress.

But still, if you tap a fraction of few kilowatts, it is not much. It would be useful in a race where turbos and superchargers are specifically forbidden. This implies petrol powered engine that is. I may even try that one soon. =]

12:45 PM  

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