DIY bang-bang on turboed singles?

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Blunt Eversmoke
I luv the smell of Diesel...
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Joined: Thu Dec 01, 2011 6:15 pm
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DIY bang-bang on turboed singles?

Post by Blunt Eversmoke » Mon Sep 29, 2014 3:58 pm

Good time o'day everyone! Another stupid idea incoming.
As we all know, our little industrial singles are hard to get a turbo to function on due to two reasons. One is pulsations - a properly sized plenum chamber might mitigate this.
The other one is the insufficient gas flow available: There are few, if any, turbos designed to work with what little gas throughput a single diesel gives before there is boost, most people only having car turbos to work with. Getting these to spin on 800 ccm at 3400 RPM is hard enough, let alone on 400 ccm. Using an electrical, let alone driven supercharger adds weight that is useless most of the time and eats up space in the frame, even though it is technically simple.

Here the bang-bang sets in. While the principle of function and the goal as described in the article are applicable to racing gassers only, the idea itself has got me thinking:
What if one were to release the gasses into the exhaust manifold significantly earlier, say, 90° after TDC, when the combustion process is mostly over but the gasses in the cylinder have not yet worked their complete usable pressure off on the piston? Keep that up until the turbo is up to RPMs and generates enough boost to keep going, then revert to normal operation. That way, you trade about one third of torque for higher exhaust pressure while spinning the turbo up, but that can be dealt with by using a CVT of proper configuration - since CVT is seen as the tranny of choice for a single anyway, it shouldn't be a big problem.

This raises two questions:

First, will or will this not fry your exhaust valve in short order? I see the risk, but since this would function differently from a gasser bang-bang (which works the whole time on less than full trottle because of its very task to keep the turbo spinning the WHOLE time AND has combustion going on partly in the plenum, yikes!), in a diesel under non-racing conditions when we don't care thaaaat much about instant turbo availability, the conditions will be less adverse?

Second, how to get it to work?
Most industrial singles use some sort of decompression mechanism that keeps the intake or exhaust valve open the whole time when handcranking/pullstarting.
For us, the ones with exhaust valve decompressors are the choice for obvious reasons. If one made a provision to actuate the deco not permanently but periodically, only from 90° to 180° after TDC, it would do.
Another way would work on two-stroke diesels only, using an additional exhaust port in the middle of the cylinder wall, closed off during normal operation and opened only when needed, by the means of some waste-gate styled mechanism or somesuch.

Whaddya think?

tappy
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Re: DIY bang-bang on turboed singles?

Post by tappy » Mon Sep 29, 2014 8:47 pm

If you open the exhaust valve very early then you're just reducing engine crank output to drive the turbo instead. This is a much less efficient way to extract energy from the fuel so your fuel consumption will rocket.
You'll need a very heavy exhaust manifold to cope with the pressures and if the decompressor is used at revs for an extended period then some part of it or the valve train is going to let go.

Blunt Eversmoke
I luv the smell of Diesel...
Posts: 119
Joined: Thu Dec 01, 2011 6:15 pm
Location: Somewhere by Bremen

Re: DIY bang-bang on turboed singles?

Post by Blunt Eversmoke » Mon Sep 29, 2014 9:13 pm

tappy wrote:If you open the exhaust valve very early then you're just reducing engine crank output to drive the turbo instead. This is a much less efficient way to extract energy from the fuel so your fuel consumption will rocket.
Not quite. See, once the turbo is up to RPMs so the boost is at a level that turbo will still spin if you stop opening the valve early, there's no reason not to revert to opening it as in stock. Lost crank torque can be compensated by employing a CVT that steps up pretty late but steeply over a narrow RPM band, say, from 2500 on.
You'll need a very heavy exhaust manifold to cope with the pressures
Heavier than normal, probably yes. Heavy as a real bang-bang one, probably no.
and if the decompressor is used at revs for an extended period then some part of it or the valve train is going to let go.
That is the main concern.

Yes, that's why I propose to use it sparingly, i.e. only at spool-up.

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