[Diy_efi] RE: Throttling intake air -- references

[Mike]

At 09:16 PM 14/1/2003 -0600, you wrote:

<snip>

>Therefore my interpretation of the graphs and the text means this:
>If you raise the EBP without a corresponding increase in MAP, you lose
>power.  Therefore it does not make any sense (efficiency-wise) to have a
>turbo working against a partially closed intake throttle body.

Brian,

You are missing the point totally !

We are not talking steady state, we are talking about a control
dynamic - what you tangentially refer to will happen admittedly but,
only in transient for a very shor period of time, then back to
a stready state which is identical to what you do when you drive
normally - which is control the pedal by foot pressure for power
output.

Please Note This:

When I have my engine on a dyno and I select a throttle level
by foot pressure for say 9 pounds boost (some 1.5 pounds before
the wastegate {relief} opens at 10.5 pounds boost), I can easily
throttle the pedal back a little and reduce boost and repeat
this indefinitely without any problem whatsoever, we do this
*all* the time when we drive.

We do that as a matter of course when driving - and it works.

All I suggest is to flick off the wastegate - get better laminar
flow from the improved exhaust geometry post turbo and let an
ECU make the decision to assist me by backing off the throttle
as it gets to the maximum boost selected in such a way that I
cant over-ride it and damage the engine.

That is how turbines from Pratt and Whitney and Rolls Royce do
it, I never knew Cessna made jet turbines - I cant imagine
a wastegate biug enough - but you have to ask - why do cessna
do it when Pratt and Whitney, Rolls Royce and GE dont !

Your response to this idea seem woefully dogmatic and you are
going to some lengths to seem to talk me out of it as if its
a real bad idea (and I will answer your earlier posts when I
have time) but, we do this *normally! when we reduce the throttle
during normal driving. Watch your boost gate when you back off
the throttle - its the same f...... thing.

Please, do as I suggest, as I get the impression you've not
been on a dyno and backed off the throttle when approaching
maximum boost - guess what ! - The system doesnt go into
hysterics, the fuel flow duty cycle rate on the injectors
drops rather dramatically (proportional wise) and there is
no ill effect. *and* you aint losing energy from the turbo
by bypassing it through a wastegate - because at that point
in time the engine is selected to reduce power and the turbo/
engine dynamic operates exactly the same as it does when you
want less power normally by backing off the damn throttle.

Please try it, observe the effect and you'll see it makes
sense, its what we do normally when we back off the throttle.

The '...control systems perspective...' re a reference, well this
is implicit in control systems theory which I studied at the
local uni, I dont have an explicit reference without you entering
into a course regarding control systems and differential equations
as a pre-requisite but, think about
this - it makes far more sense to control the source not the
waste heat from the outcome. Control the nonamplified input
rather than dump power from the amplified result. An engine
which is connected to the foot via a pedal, is after all,
nothing but a power amplifier - translating foot pedal pressure
to engine power - thats it, therefore classic control systems
theory has a great amount to say in terms of application and
the reason its not generally done is predominatly for cost issues,
tradition, complexity and the basic fact that for many years
ICE has not even been looked at seriously as a control system
problem. The people at Pratt and Whitney, Rolls Royce are rather
more sophisticated and do see their turbines in that light :)

After all, we dont *just* throttle an engine via wastegate
set to some arbirarily stupid low level - we do *exactly* as
I suggest but lower down on the control curve by virtue of
the fact we make the decision to throttle back when we dont
want so much boost = we want less power - simple.

I am suggesting looking at the potential discontinuities a bit
further up the control curve when the turbo is working to its
designed capacity but before any pressure relief valve issue
arises, ah lah wastegate needs to be released/opened !

rgds

mike


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