[Diy_efi] RE: Throttling intake air

[Bill Washington]

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Gents,
    Some of thoughts and questions:
1. An intake air throttle before the compressor will reduce intake air 
pressure to the compressor, thus reducing the load/work it is doing at 
any part throttle position, therefore at any given exhaust airflow it 
will be spinning faster - does this sound logical?
2. Does opening the wastegate cause the turbo to spool down quickly?
3. Has anyone considered a variable flow restrictor in the exhaust 
immediately after the turbine instead of a wastegate?  Here backpressure 
through the system would reduce intake airflow and consequently fuel 
delivery. This would be a little like the jacobs brakes used on trucks, 
only in a different location and progressive. Obviously may be difficult 
to implement due to the environment - temperature etc, although I 
suppose not all that different from a wastegate - in which it would have 
to operate, but it could also have some useful side benefits - emergency 
engine brake - I heard  of one situation where a turbo diesel had the 
oil seal in the compressor let go, the engine was running - over 
speeding - on its own oil and could not be stopped (Diesels have no 
intake air restrictors, speed/power is controlled purely by quantity of 
fuel delivered, and this one had plenty without the injectors - its own 
lubrication)! I believe the vehicle and engine were destroyed, 
fortunately the driver survived.

Regards
Bill W

diy_efi-request at diy-efi.org wrote:

>-----------------------------
>
>Date: Thu, 16 Jan 2003 00:19:17
From: Mike <erazmus at iinet.net.au>
>To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>Subject: Re: [Diy_efi] RE: Throttling intake air -- references
>Message-ID: <3.0.6.16.20030116001917.40bfa9cc at mail.iinet.net.au>
>In-Reply-To: <00cb01c2bcad$8eca9380$92effea9 at a>
>References: <3.0.6.16.20030115003939.5cdf6d18 at mail.iinet.net.au>
> <3.0.6.16.20030115233723.4fe76f86 at mail.iinet.net.au>
>Content-Type: text/plain; charset="us-ascii"
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>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>Message: 4
>
>At 03:48 PM 15/1/2003 -0000, you wrote:
>  
>
>>>Going back to controlling boost via throttle position. I know its possible,
>>>no doubt there, but can I ask:
>>    
>>
> I have seen occasional reference to this method but many years ago 
> when ECU's werent explicity set up to do it compounded by the ready 
> availability of casting which included a wastegate causing a market 
> 'critical mass' and hence momentum for that approach and probably 
> influenced by the feedforward aspect of the control systems complexity...
>
>>>Trying to control boost at 2500 revs using throttle only is more difficult
>>>than at 5000 on the same engine, as the boost rises and falls over a much
>>>smaller throttle movement? What sort of (boost controlling) linkage would
>>>you use that could control boost via the (blow through) throttle plate, but
>>>alter the 'sensitivity' of the throttle so as the boost came and went over
>>>the same amount of pedal travel?
>>    
>>
> Oh yes, I can appreciate that point and its well put. I've not yet 
> considered the linkage issue but should (I expect) be containable via 
> various leverage mechanisms, PID stability plays a major part. These 
> days with extremely powerful 25mips processors for USD$5 and 
> development in sensors and control electronics it might be feasible to 
> revisit the issue armoured with much better implementation tools 
> capable of handling what you well describe - vis a vis the positional 
> nonlinearity or rather angular nonlinearity issue in respect of 
> control dynamic for boost pressure via existing throttle body mechanics.
>
>>>For the same engine, is the exhaust backpressure in the manifold the same
>>>when running wide open throttle and wastegate part open as it is when
>>>running wastegate shut and throttle part open?
>>    
>>
> Well I have recalled, from a magazine sponsored article some years 
> ago, that once the wastegate opens (and stays open) the higher 
> pressure on the pre-turbine side is vented to the post-turbine side, 
> therefore its logical to interpret that the EBP must be higher. These 
> days its easy to measure and when i get time I'll set up a pair of 
> gauges to report what I find. Its common sense to some major degree, 
> if the wastegate needs to be opened and stays open to dump pressure 
> one would expect the pressure post-turbine to be higher - for some 
> other posters to suggest it might be the opposite is ludicrous, sure 
> the turbine will spin down a tad *but* if the need is still there to 
> bypass the turbine then the region post-turbine must be seeing 
> pressures that exist on the pre-turbine side as the wastegate is open 
> and this must be higher as its not all going through the damn turbine 
> <sigh> I also think there is tradition in seeing the wastegate as a 
> control (proportional) element when its really more akin to a digital 
> relief valve, hence is discontinuous in its reponse to dumping exhaust 
> energy and therefore should be logical to control the fuel/air more 
> precisely some time before its combusted. Historically this has been 
> difficult and more expensive, these days it should be much easier if 
> people can intellectualise the control systems algorithms and that was 
> the impetus for this thread in the first place, ;-) 



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