Last week in
Venting Boost, Part 1
we
looked at how a recirculating BOV can be used to control boost, allowing it to
be dropped below wastegate level or be used to give added control if the
wastegate is allowing over-boosting at the top end. This week we install the
system in a car.
The Application
The application of the venting boost control
system was on a Toyota Prius hybrid. The car runs a 1.5-litre four cylinder
engine to which an IHI RHF4 turbo and intercooler have been added. The turbo,
one of the puffers from a twin turbo Subaru Liberty, required that its wastegate
mounting system be modified so that it would fit in the available space. The use
of a spacer ring (arrowed) to allow the wastegate actuator to be placed in a
different orientation put a small preload on the wastegate rod, resulting in a
minimum on-load boost level of 7 psi, even with the wastegate hose connected
directly to the turbo compressor outlet. In other words, boost could not be
dropped below 7 psi.
Seven psi would seem a low enough boost for any
application, but in this complex car, where the petrol engine integrates with a
battery electric system comprising two motor/generators, it increased the power
of the petrol engine sufficiently that at high revs, the control system would
briefly shut the electronic throttle. Dropping boots back to 4 psi at high loads
fixed the problem, without having any affect on power getting to the tyres.
So in this application, it was required that the
boost level smoothly rise to 7 psi in the mid-range and then decrease to 4 psi
(ie below wastegate pressure) at high loads. (As we said last week, a very
similar application is where the wastegate is unable to flow enough exhaust
gases and so at the top end of the engine’s powerband there is
overboosting.)
The Hardware
As also described last week, the approach is to
reduce the boost fed to the BOV, allowing it to open and so leak away manifold
boost pressure. The hardware comprises a T-piece, solenoid and restriction. This
diagram shows how the system is arranged. A T-piece is placed in the
vacuum/boost feed hose and a solenoid plumbed into place on the boost side of
the T. When the solenoid is open, boost pressure reaches the BOV, holding it
shut. When the solenoid closes, boost can no longer reach the BOV and so the BOV
is no longer pushed shut by the boost. The vent allows the air trapped between
the closed solenoid and the BOV to bleed away. The restriction in the vent line
prevents too much boost being bled off when the solenoid is open.
BOV
The venting of boost was achieved with a GFB
‘plumback’ blow-off valve (BOV). This valve is easily disassembled to have its
internal spring swapped or shortened, and external adjustment for spring preload
is provided. The valve is plumbed into place with manifold boost pressure acting
on the base of the piston, ie trying to force it open against the pressure provided by
the internal spring and the boost pressure fed to it down the vacuum/boost hose.
As noted at the bottom of the article, we scored this valve for nothing but
irrespective of that, it is a very good design that we recommend.
Solenoid
The turbo boost control solenoid from a Nissan was
used. These valves are designed to hold boost pressure and come with a good
rubber-mounted bracket. A Japanese importing wrecker should be able to supply
you with one.
Restriction
A small screwdriver-adjustable variable opening
flow control valve was used as the restriction. This neat little valve was
salvaged from a hospital oxygen flowbench but as described last week, this
restriction can be made by soldering-up one arm of the brass T-piece and then
drilling a small (eg smaller than 1mm) hole through the solder plug.
One-Way Valve
The one-way valve salvaged from the brake booster
on a Mazda at the wreckers was used to prevent air from being drawn in through
the vent hose. This allows the BOV to work normally when performing its usual
blow-off valve function.
Simple Voltage Switch
A Simple Voltage Switch (see
The Simple Voltage Switch) working off the airflow
meter output was used to control the action of the solenoid. Note that the very
fine adjustment facilities for trip-point and hysteresis were required in this
application – the trip-point (adjusted by a multi-turn trim-pot) needed to be
set very carefully. We’ll come back to this in a moment.
Testing
The first test was to pull off the vacuum/boost
feed hose to the BOV and assess the boost behaviour. We suggested last week that
this would drop the peak boost level, but the reality is more subtle than that.
The boost venting caused by the opening BOV will usually cause a slower than
normal rate of boost increase and then peak boost will be lower. But as the
turbo winds right up, the boost may rise slightly as revs climb. You really have
to suck it and see – what exactly happens on your car with your turbo and your
BOV. If the boost isn’t low enough with the valve leaking, shorten the internal
spring (or fit a softer one).
In the case of the Prius, the internal spring of
the GFB ‘plumback’ BOV had previously been shortened (when it was being used as
a recirc valve in a supercharger installation on this car – see
Technokill: Building a Blown Hybrid, Part 2). This meant that with the
vacuum/boost hose pulled off, boost rose quite leisurely to a max of only 4 psi.
The low rate of boost increase wasn’t too good,
but the peak boost level was right on the desired money.
The solenoid, T-piece, restrictor and one-way
valve were then installed under the bonnet. The Simple Voltage Switch kit was
built (it’s a very easy kit, even if you’re a beginner) and its input connected
to the voltage signal from the airflow meter. The solenoid was wired using the
Common and Normally Closed terminals on the relay, so that when the ignition was
switched on, the solenoid opened. That is, the solenoid closed only when the
Simple Voltage Switch tripped.
On-road tuning of the switchover point and
hysteresis of the Voltage Switch needed two people – one driving and one
adjusting. A boost gauge was installed and the Voltage Switch set so that it
tripped at about 75 per cent of full load. On the boost gauge the manifold
pressure could be seen to rise smoothly through 1, 2, 3, 4, 6, and then to a
wastegate-regulated 7 psi of boost (it really does rise like this in the Prius
because at full throttle, engine revs are linked to road speed through the ‘CVT’
type of trans), holding at 7 psi for a few moments and then with a click of the
relay from the Simple Voltage Switch, sliding back to 4 psi.
(Incidentally, how rapidly the boost drops back to
the lower pressure can be set by the size of the vent opening. If it’s a very
small vent, the decrease to low boost will be slower as it will take longer for
the pressure trapped between the solenoid and the BOV to bleed out.)
The hysteresis setting is very important.
Hysteresis refers to the difference between the switch-on and switch-off voltage
levels. If the hysteresis is too low, as the boost pressure drops, the voltage
from the airflow meter will decrease to below the switch off level, so opening
the solenoid and causing boost to rise. The result is an ugly merry-go-round of
rising and falling boost. In the case of the Prius, to avoid this, the
hysteresis needed to be set to maximum.
To allow a manual over-ride which selects low
boost, a switch was fitted in series with the relay’s contacts. This lets the
driver select full-time low boost – ideal if only low octane fuel is
available.
Results
The outcome was exactly as desired – at
full-throttle, boost rises to 7 psi boost before the voltage switch trips,
sliding it back to 4 psi. Note that the operation of the system is not
the same as it would be if the was solenoid closed by a boost pressure switch –
at part throttle and/or low revs, it’s now possible to hold 7 psi of boost,
which is important in this car which needs good bottom-end and mid-range
torque.
Conclusion
The Prius is an unusual example, but having the
ability to drop boost below wastegate level is very useful in all turbo cars if
you have an over-small wastegate that allows boost runaway, or want the ability
to select low or no boost with the flick of a switch.
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