[Diy_efi] RE: Throttling intake air

[Mike]

At 09:55 AM 17/1/2003 +1100, you wrote:
>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?

Not necessarily because the turbine is also doing less work due to
less exhaust flow, classic direct linked negative feedback. Though
your important point is the inlet comp is doing less work, my only
worry is the comp oil seals would have to handle opposite pressure
across them from normal operation, i'd therefore prefer to just
move the normal TB plate which is normally after the comp.

>2. Does opening the wastegate cause the turbo to spool down quickly?

Well we know it spools down and it seems to be fairly quick but its
not clear by how much or how quickly, one day when I get my turbo
speed sensor going then I'll know a little more.

On your last para below, this reminds me of a straight 6 diesel on
a boat in Malaysia someone once told me about. It hadnt been working
for several years but the engine got started after a little difficulty,
seemed ok, then it was shut off, or rather they closed the fuel line
expecting it to shut down but it only slowed a little then sped up
burning its own crankcase oil, the two Malay operators at first
thought it was some ghost - they didnt know the crank oil was
burning. So to stop it they stuffed a Kopra bag into the inlet,
at which point it slowed down, swallowed the bag and promptly
spat it out the exhaust. At this point that was enough to jog
the engine into even more speed and its rpm rose way over its
limit sounding like it was threatening to explode. The senior
operator had 'some' presence of mind to remove his pants and
sit on the air intake at which point it finally slowed to a
stop whilst the operator grimaced in pain. Fortunately there
wasnt enough vacuum to remove his intestines <phew> I wouldnt
care to guess what could have happened if he'd had his army quota of
baked beans the day before...

rgds ~`:o)

mike



>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"
>>MIME-Version: 1.0
>>Precedence: list
>>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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