How reliable are turbo engines?

wrote:

> A lot of lower end turbo vehicles do not have knock
> sensors.

Since when does any modern car not have a knock sensor ?

Graham

Eeyore wrote:
>
> wrote:
>
>
>>A lot of lower end turbo vehicles do not have knock
>>sensors.
>
>
> Since when does any modern car not have a knock sensor ?
>
> Graham
>

I'll go you one further- since when does ANY electronically-driven turbo
car not have a knock sensor (and I'm going back to 80s Mopars and
Buicks, here)?

Ed Pirrero wrote:
> Steve wrote:
>
>>Ed Pirrero wrote:
>>
>>
>>>I will stick by a couple of things - boost is related to rpms,
>>
>>And that's EXACTLY where you go off the rails. Boost is NOT related
>>directly to RPM. Sure, there is a threshold RPM below which the engine
>>cannot generate enough mass flow to fully spin up the turbocharger, but
>>that RPM is *very* low compared to engine redline on anything except a
>>heavily modified engine with a grossly oversized turbocharger for street
>>use. Once the engine reaches that threshold RPM, the turbo is generating
>>all the boost that it ever will, because at that point the wastegate
>>will open (or the VNT vanes will move, or whatever mechanism is used) to
>>dump further increases in exhaust flow past the turbine and let the
>>reciprocating engine produce more power by holding the exhaust back
>>pressure constant at the level that fully spins the turbine, rather than
>>continuing to build. Yes, the engine will (up to a point) continue to
>>increase its power output as you increase RPM, but its because of the
>>exact same reason that a normally-aspirated engine does the same thing:
>>power is porportional to force exerted*distance moved per unit time, and
>>higher RPM is more distance moved per unit time. NOT because the turbo
>>is continuing to work "better" at increasing crankshaft RPM. QED.
>
>
> The reason I chose the word "related" instead of "proportional to" is
> very important. As I said, I'm *not* writing a usenet treatise on
> turbocharging. You spent a paragraph essentially agreeing with me and
> giving details.

I'm an engineer, I can't help being rankled by a choice of words that
implied correlations that aren't correlations at all, just enabling
conditions. The MOST you can say is that "a minimum RPM is required to
deliver full boost." Beyond that, there is no correlation,
proportionality, or really ANY other relation between turbo speed and
crankshaft speed. There is, however, a strong correlation with turbo RPM
and throttle position, and between turbo speed and EGT. Now that we've
been around the tree 6 times, I guess I can say that "we agree." I think.

Steve wrote:
> Ed Pirrero wrote:
> > Steve wrote:
> >
> >>Ed Pirrero wrote:
> >>
> >>
> >>>I will stick by a couple of things - boost is related to rpms,
> >>
> >>And that's EXACTLY where you go off the rails. Boost is NOT related
> >>directly to RPM. Sure, there is a threshold RPM below which the engine
> >>cannot generate enough mass flow to fully spin up the turbocharger, but
> >>that RPM is *very* low compared to engine redline on anything except a
> >>heavily modified engine with a grossly oversized turbocharger for street
> >>use. Once the engine reaches that threshold RPM, the turbo is generating
> >>all the boost that it ever will, because at that point the wastegate
> >>will open (or the VNT vanes will move, or whatever mechanism is used) to
> >>dump further increases in exhaust flow past the turbine and let the
> >>reciprocating engine produce more power by holding the exhaust back
> >>pressure constant at the level that fully spins the turbine, rather than
> >>continuing to build. Yes, the engine will (up to a point) continue to
> >>increase its power output as you increase RPM, but its because of the
> >>exact same reason that a normally-aspirated engine does the same thing:
> >>power is porportional to force exerted*distance moved per unit time, and
> >>higher RPM is more distance moved per unit time. NOT because the turbo
> >>is continuing to work "better" at increasing crankshaft RPM. QED.
> >
> >
> > The reason I chose the word "related" instead of "proportional to" is
> > very important. As I said, I'm *not* writing a usenet treatise on
> > turbocharging. You spent a paragraph essentially agreeing with me and
> > giving details.
>
> I'm an engineer, I can't help being rankled by a choice of words that
> implied correlations that aren't correlations at all, just enabling
> conditions. The MOST you can say is that "a minimum RPM is required to
> deliver full boost." Beyond that, there is no correlation,
> proportionality, or really ANY other relation between turbo speed and
> crankshaft speed.

Hmmm. If there's more exhaust, the turbo is going to spool. And to
keep it spooling, and delivering boost, you need exhaust gas flow.
Other than turning higher RPMs (assuming some throttle position other
than closed), I don't see how you keep the turbo delivering boost.

> There is, however, a strong correlation with turbo RPM
> and throttle position, and between turbo speed and EGT.

I see. That's why you get instant boost at 900RPM with full throttle.
Ooops.

Throttle position controls RPM, in any given gear. But again, I'm not
talking about intervals between shifts, of engine braking down an
incline. During those times, the turbo is just along for the ride.

> Now that we've
> been around the tree 6 times, I guess I can say that "we agree." I think.

Well, we agree on all but the semantics of the discussion. I guess
that's OK by me - I don't think we're going to completely agree on the
chicken-egg portion of the discussion.

E.P.