Chris Wilson
22nd February 2004, 08:09 PM
I have received some e-mails regarding turbo primary pipe
design and sizing, integral V external wastegates, and
post turbo pipe sizing, so the below may also be of more
general interest:
N/A cars: As many of you know, the design of turbo exhaust
systems runs contrary to exhaust design for n/a vehicles.
N/A cars utilize exhaust velocity (not backpressure) in
the collector to aid in scavenging other cylinders during
the blowdown process. It just so happens that to get the
appropriate velocity, you have to squeeze down the diameter
of the discharge of the collector (aka the exhaust), which
also induces backpressure. The backpressure is an
undesirable byproduct of the desire to have a certain
degree of exhaust velocity. Go too big, and you lose velocity
and its associated beneficial scavenging effect. Too small
and the backpressure skyrockets, more than offsetting any
gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You
want the exhaust velocity to be high upstream of the turbine
(i.e. in the header). You'll notice that primaries of turbo
headers are smaller diameter than those of an n/a car of
two-thirds the horsepower. The idea is to get the exhaust
velocity up quickly, to get the turbo spooling as early as
possible. Here, getting the boost up early is a much more
effective way to torque than playing with tuned primary
lengths and scavenging. The scavenging effects are small
compared to what you'd get if you just got boost sooner
instead. You have a turbo; you want boost. Just don't go
so small on the header's primary diameter that you choke
off the high end.
Downstream of the turbine (aka the turboback exhaust),
you want the least backpressure possible. No ifs, ands,
or buts. Stick a Hoover on the tailpipe if you can. The
general rule of "larger is better" (to the point of
diminishing returns) of turboback exhausts is valid.
Here, the idea is to minimize the pressure downstream
of the turbine in order to make the most effective use
of the pressure that is being generated upstream of the
turbine. Remember, a turbine operates via a pressure ratio.
For a given turbine inlet pressure, you will get the
highest pressure ratio across the turbine when you have
the lowest possible discharge pressure. This means the
turbine is able to do the most amount of work possible
(i.e. drive the compressor and make boost) with the available
inlet pressure.
Again, less pressure downstream of the turbine is goodness.
This approach minimizes the time-to-boost (maximizes boost
response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0" or even largwr, the "best" turboback
exhaust depends on the amount of flow, or horsepower.
At 350 hp, 2.5" is fine. Going to 3" at this power level won't
get you much, if anything, other than a louder exhaust note.
450 hp and you're definitely suboptimal with 2.5". For over 500
hp, even 3" MAY be on the small side.
As for the geometry of the exhaust at the turbine discharge,
the most optimal configuration would be a gradual increase in
diameter from the turbine's exducer to the desired exhaust
diameter-- via a straight conical diffuser of 7-12? included
angle (to minimize flow separation and skin friction losses)
mounted right at the turbine discharge. Many turbochargers
found in diesels have this diffuser section cast right into
the turbine housing. A hyperbolic increase in diameter (like
a trumpet "snorkel") is theoretically ideal but I've never seen
one in use (and doubt it would be measurably superior to a
straight diffuser). The wastegate flow would be via a completely
divorced (separated from the main turbine discharge flow)
dumptube. Due the realities of packaging, cost, and emissions
compliance this config is rarely possible on street cars. You
will, however, see this type of layout on dedicated race vehicles.
A large "bellmouth" config which combines the turbine discharge
and wastegate flow (without a divider between the two) is certainly
better than the compromised stock routing, but not as effective as
the above.
If an integrated exhaust (non-divorced wastegate flow) is required,
keep the wastegate flow separate from the main turbine discharge
flow for ~12-18" before reintroducing it. This will minimize the
impact on turbine efficiency-- the introduction of the wastegate
flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a
good idea, but if it is necessary, doing it further downstream
is better than doing it close to the turbine discharge since
it will minimize the exhaust's contribution to backpressure.
Better yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is
higher than the inlet temperature, due to the exothermic oxidation
of unburned hydrocarbons in the cat. So the total heat loss (and
density increase) of the gases as it travels down the exhaust is
not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes
place where the flows from separate cylinders merge (i.e. in the
collector). There is no such thing as cylinder scavenging
downstream of the turbine, and hence, no reason to desire high
exhaust velocity here. You will only introduce unwanted backpressure.
Other things you can do (in addition to choosing an appropriate
diameter) to minimize exhaust backpressure in a turboback exhaust
are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius
turns (keep it as straight as possible); avoid step changes in
diameter; avoid "cheated" radii (cuts that are non-perpendicular);
use a high flow cat; use a straight-through perforated core muffler...
etc.
As far as integral wastegate design is concerned there are basically
those that vent out against a flat wall of the turbo casting itself,
and those that vent more progressively into a tapered chamber.
Assuming neither of them have a divider wall/tongue between the
turbine discharge and wg dump, I'd venture that you'd be hard
pressed to measure a difference between the two. The more gradual
taper intuitively appears more desirable, but it's likely that it's
beyond the point of diminishing returns. The latter one sounds like it
will improve the wastegate's discharge coefficient over the first
config, which will constitute the single biggest difference. This
will allow more control over boost creep. Neither is as optimal
as the divorced wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively
large right at the turbine discharge (a large step diameter increase)
, there will be an unrecoverable dump loss that will contribute to
backpressure. This is why a gradual increase in diameter, like the
conical diffuser mentioned earlier, is desirable at the turbine
discharge.
As for primary lengths on turbo headers, it is advantageous to
use equal-length primaries to time the arrival of the pulses at
the turbine equally and to keep cylinder reversion balanced across
all cylinders. This will improve boost response and the engine's VE.
Equal-length is often difficult to achieve due to tight packaging,
fabrication difficulty, and the desire to have runners of the
shortest possible length.
Here's an example (simplified) of how larger exhausts help
turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka
expansion ratio) of 1.8:1. You have a small turboback exhaust that
contributes, say, 10 psig backpressure at the turbine discharge at
redline. The total backpressure seen by the engine (upstream of the
turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
So here, the turbine contributed 19.6 psig of backpressure to the
total.
Now you slap on a proper low backpressure, big turboback exhaust.
Same turbo, same boost, etc. You measure 3 psig backpressure at
the turbine discharge. In this case the engine sees just 17 psig
total backpressure! And the turbine's contribution to the total
backpressure is reduced to 14 psig (note: this is 5.6 psig lower
than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6
psig when you swapped turbobacks in this example. This reduction
in backpressure is where all the engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock
turbo cars-- the turbine compounds the downstream backpressure via
its expansion ratio. This is also why bigger turbos make more power
at a given boost level-- they improve engine VE by operating at
lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small
exhaust (like 2.5" for 500 hp) will vary depending on the match.
At a given power level, a smaller turbo will generally be operating
at a higher turbine pressure ratio and so will actually make the
engine more sensitive to the backpressure downstream of the turbine
than a larger turbine/turbo would. As for output temperatures, I'm
not sure I understand the question. Are you referring to compressor
outlet temperatures?
The advantage to the bellmouth setup from the wg's perspective is
that it allows a less torturous path for the bypassed gases to escape.
This makes it more effective in bypassing gases for a given pressure
differential and wg valve position. Think of it as improving the VE
of the wastegate. If you have a very compromised wg discharge routing,
under some conditions the wg may not be able bypass enough flow to
control boost, even when wide open. So the gases go through the turbine
instead of the wg, and boost creeps up.
The downside to a bellmouth is that the wg flow still dumps right into
the turbine discharge. A divider wall would be beneficial here. And, as
mentioned earlier, if you go too big on the bellmouth and the turbine
discharge flow sees a rapid area change (regardless of whether the wg
flow is being introduced there or not), you will incur a backpressure
penalty right at the site of the step. This is why you want gradual area
changes in your exhaust, without sudden lips or steps, especially close
to the turbine exit.
design and sizing, integral V external wastegates, and
post turbo pipe sizing, so the below may also be of more
general interest:
N/A cars: As many of you know, the design of turbo exhaust
systems runs contrary to exhaust design for n/a vehicles.
N/A cars utilize exhaust velocity (not backpressure) in
the collector to aid in scavenging other cylinders during
the blowdown process. It just so happens that to get the
appropriate velocity, you have to squeeze down the diameter
of the discharge of the collector (aka the exhaust), which
also induces backpressure. The backpressure is an
undesirable byproduct of the desire to have a certain
degree of exhaust velocity. Go too big, and you lose velocity
and its associated beneficial scavenging effect. Too small
and the backpressure skyrockets, more than offsetting any
gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You
want the exhaust velocity to be high upstream of the turbine
(i.e. in the header). You'll notice that primaries of turbo
headers are smaller diameter than those of an n/a car of
two-thirds the horsepower. The idea is to get the exhaust
velocity up quickly, to get the turbo spooling as early as
possible. Here, getting the boost up early is a much more
effective way to torque than playing with tuned primary
lengths and scavenging. The scavenging effects are small
compared to what you'd get if you just got boost sooner
instead. You have a turbo; you want boost. Just don't go
so small on the header's primary diameter that you choke
off the high end.
Downstream of the turbine (aka the turboback exhaust),
you want the least backpressure possible. No ifs, ands,
or buts. Stick a Hoover on the tailpipe if you can. The
general rule of "larger is better" (to the point of
diminishing returns) of turboback exhausts is valid.
Here, the idea is to minimize the pressure downstream
of the turbine in order to make the most effective use
of the pressure that is being generated upstream of the
turbine. Remember, a turbine operates via a pressure ratio.
For a given turbine inlet pressure, you will get the
highest pressure ratio across the turbine when you have
the lowest possible discharge pressure. This means the
turbine is able to do the most amount of work possible
(i.e. drive the compressor and make boost) with the available
inlet pressure.
Again, less pressure downstream of the turbine is goodness.
This approach minimizes the time-to-boost (maximizes boost
response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0" or even largwr, the "best" turboback
exhaust depends on the amount of flow, or horsepower.
At 350 hp, 2.5" is fine. Going to 3" at this power level won't
get you much, if anything, other than a louder exhaust note.
450 hp and you're definitely suboptimal with 2.5". For over 500
hp, even 3" MAY be on the small side.
As for the geometry of the exhaust at the turbine discharge,
the most optimal configuration would be a gradual increase in
diameter from the turbine's exducer to the desired exhaust
diameter-- via a straight conical diffuser of 7-12? included
angle (to minimize flow separation and skin friction losses)
mounted right at the turbine discharge. Many turbochargers
found in diesels have this diffuser section cast right into
the turbine housing. A hyperbolic increase in diameter (like
a trumpet "snorkel") is theoretically ideal but I've never seen
one in use (and doubt it would be measurably superior to a
straight diffuser). The wastegate flow would be via a completely
divorced (separated from the main turbine discharge flow)
dumptube. Due the realities of packaging, cost, and emissions
compliance this config is rarely possible on street cars. You
will, however, see this type of layout on dedicated race vehicles.
A large "bellmouth" config which combines the turbine discharge
and wastegate flow (without a divider between the two) is certainly
better than the compromised stock routing, but not as effective as
the above.
If an integrated exhaust (non-divorced wastegate flow) is required,
keep the wastegate flow separate from the main turbine discharge
flow for ~12-18" before reintroducing it. This will minimize the
impact on turbine efficiency-- the introduction of the wastegate
flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a
good idea, but if it is necessary, doing it further downstream
is better than doing it close to the turbine discharge since
it will minimize the exhaust's contribution to backpressure.
Better yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is
higher than the inlet temperature, due to the exothermic oxidation
of unburned hydrocarbons in the cat. So the total heat loss (and
density increase) of the gases as it travels down the exhaust is
not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes
place where the flows from separate cylinders merge (i.e. in the
collector). There is no such thing as cylinder scavenging
downstream of the turbine, and hence, no reason to desire high
exhaust velocity here. You will only introduce unwanted backpressure.
Other things you can do (in addition to choosing an appropriate
diameter) to minimize exhaust backpressure in a turboback exhaust
are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius
turns (keep it as straight as possible); avoid step changes in
diameter; avoid "cheated" radii (cuts that are non-perpendicular);
use a high flow cat; use a straight-through perforated core muffler...
etc.
As far as integral wastegate design is concerned there are basically
those that vent out against a flat wall of the turbo casting itself,
and those that vent more progressively into a tapered chamber.
Assuming neither of them have a divider wall/tongue between the
turbine discharge and wg dump, I'd venture that you'd be hard
pressed to measure a difference between the two. The more gradual
taper intuitively appears more desirable, but it's likely that it's
beyond the point of diminishing returns. The latter one sounds like it
will improve the wastegate's discharge coefficient over the first
config, which will constitute the single biggest difference. This
will allow more control over boost creep. Neither is as optimal
as the divorced wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively
large right at the turbine discharge (a large step diameter increase)
, there will be an unrecoverable dump loss that will contribute to
backpressure. This is why a gradual increase in diameter, like the
conical diffuser mentioned earlier, is desirable at the turbine
discharge.
As for primary lengths on turbo headers, it is advantageous to
use equal-length primaries to time the arrival of the pulses at
the turbine equally and to keep cylinder reversion balanced across
all cylinders. This will improve boost response and the engine's VE.
Equal-length is often difficult to achieve due to tight packaging,
fabrication difficulty, and the desire to have runners of the
shortest possible length.
Here's an example (simplified) of how larger exhausts help
turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka
expansion ratio) of 1.8:1. You have a small turboback exhaust that
contributes, say, 10 psig backpressure at the turbine discharge at
redline. The total backpressure seen by the engine (upstream of the
turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
So here, the turbine contributed 19.6 psig of backpressure to the
total.
Now you slap on a proper low backpressure, big turboback exhaust.
Same turbo, same boost, etc. You measure 3 psig backpressure at
the turbine discharge. In this case the engine sees just 17 psig
total backpressure! And the turbine's contribution to the total
backpressure is reduced to 14 psig (note: this is 5.6 psig lower
than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6
psig when you swapped turbobacks in this example. This reduction
in backpressure is where all the engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock
turbo cars-- the turbine compounds the downstream backpressure via
its expansion ratio. This is also why bigger turbos make more power
at a given boost level-- they improve engine VE by operating at
lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small
exhaust (like 2.5" for 500 hp) will vary depending on the match.
At a given power level, a smaller turbo will generally be operating
at a higher turbine pressure ratio and so will actually make the
engine more sensitive to the backpressure downstream of the turbine
than a larger turbine/turbo would. As for output temperatures, I'm
not sure I understand the question. Are you referring to compressor
outlet temperatures?
The advantage to the bellmouth setup from the wg's perspective is
that it allows a less torturous path for the bypassed gases to escape.
This makes it more effective in bypassing gases for a given pressure
differential and wg valve position. Think of it as improving the VE
of the wastegate. If you have a very compromised wg discharge routing,
under some conditions the wg may not be able bypass enough flow to
control boost, even when wide open. So the gases go through the turbine
instead of the wg, and boost creeps up.
The downside to a bellmouth is that the wg flow still dumps right into
the turbine discharge. A divider wall would be beneficial here. And, as
mentioned earlier, if you go too big on the bellmouth and the turbine
discharge flow sees a rapid area change (regardless of whether the wg
flow is being introduced there or not), you will incur a backpressure
penalty right at the site of the step. This is why you want gradual area
changes in your exhaust, without sudden lips or steps, especially close
to the turbine exit.