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This article was first published in 2005.
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So what’s a supercharged hybrid petrol/electric car like on the road? Pretty
damn good. But first of all, what’s that bloody whining noise!?
Intake Noise
Because in normal use the Prius engine starts and stops frequently, it’s
important that the engine is relatively quiet – otherwise it is too intrusive
when it springs into life.
After a lot of experimentation, the best way of quietening the intake noise
being produced by the supercharger was to ditch the Daihatsu Sirion airbox and
go back to the standard Prius airbox. This box also had to be mounted in the
front guard – but it’s not such a neat fit as the Sirion airbox. In fact, to
change the air filter with the Prius airbox in place, the bumper cover will need
to be removed.
Even with the Prius airbox in place, the intake noise was still quite loud
(though much quieter than with the Sirion airbox). Different length and diameter
snorkels were trialled and it was found that reducing the air inlet diameter to
the box gave the best muffling effect. However, would this restriction drop
airflow? Full-throttle testing was carried out using a Magnehelic gauge to test
the pressure drop through the intake and it was found that this peaked at 20
inches of water – double that of the standard car.
This was a bit high so the intake to the airbox was changed back to 50mm
tube, which caused a peak measured intake pressure drop of 15 inches of water –
and the intake to again be louder, although not as loud as with no snorkel. The
air pick-up point is below the headlight inside the bumper cover and the inside of
the bumper cover is lined with thin foam rubber to help silence the noise at
the inlet.
Another important aspect in keeping the supercharged engine quiet at idle is
to make sure that the bypass valve (from the supercharger’s outlet back to
inlet) is fully open. As described previously, the spring in the GFB blow-off
valve had already been shortened to ensure the valve was open at idle, even with
the often weak vacuum signal available from the engine. However, by measuring
the pressure in this circuit, it could be seen that the valve wasn’t opening
fully – there was a 2 psi pressure build-up between the closed throttle and the
supercharger. The spring was shortened a little more, which resulted in a zero
pressure build-up at idle and a quieter supercharger.
Turbo, Electronic Throttle and BOVs?
The Prius runs an electronic throttle, so the actual throttle blade position
doesn’t always reflect what the driver is doing with the accelerator pedal. For
example, during warm-up, the Prius opens the throttle and then retards ignition
timing to quickly warm up the cat converter. However, this results in a loss of
the vacuum signal to the boost recirculation valve which without the use of a
weak spring would cause it to close. (Supercharger boost pushes open the valve
when there’s no positive pressure signal coming from the manifold.)
The same variation in intake manifold vacuum may happen in electronic
throttle turbo cars. In other words, in some situations the strength and
consistency of the vacuum signal from the intake manifold may be quite different
to what occurs in a manual throttle car. As a result, the opening of a blow-off
valve may be quite variable.
The easiest way to see what’s happening is to plumb a vacuum gauge to the
manifold.
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Supercharger Discharge Noise
In addition to intake noise, there was another source of noise – and it
proved much harder to suppress.
A Roots-type blower has an output flow of air that pulses - and these pulses
cause noise. If the outlet is straight into a large plenum (as is the case with
most Roots blowers which push air into the intake manifold or an intercooler),
the pressure pulsations are damped by the large volume. But in the case of the
Prius, the discharge is into the pipe which leads to the intercooler.
Furthermore, the ex-Subaru Vivio supercharger uses straight two-lobe rotors,
rather than the quieter twisted three-lobe designs used in most Roots blowers.
Four techniques can be used to muffle the outlet sound:
- An absorption muffler (like a straight-through exhaust muffler)
- A dissipative muffler (like a baffled muffler)
- A tee’d-in resonant chamber
- Noise insulation
At low frequencies – like the noise that was occurring on the Prius - an
absorption muffler is ineffective, and while a baffled muffler is very effective
at low frequencies, this type of design will always harm flow. The original
supercharger outlet fitting (which couldn’t be used because of a lack of space)
has a short, closed-end tube attached to the fitting – presumably acting as a
tuned Helmholtz resonator. In this approach, the air vibrating in the resonator
cancels the vibrations occurring in the main pipe. It was therefore decided to
experiment with resonant chambers and in addition, to heavily sound insulate the
outlet pipe.
The outlet pipe between the supercharger and the intercooler was lagged with
3mm lead sheet. Yes, I did say 3mm lead sheet. When all the technobabble is cast
aside, for its thickness, lead has about the best sound suppression performance
of any material you’ll find. The lead sheet was purchased from a scrap metal
yard and cut into small sections with metal shears. A hammer was then used to
form the pieces into overlapping shapes that fitted around the plumbing, then
large diameter heat-shrink was placed over the top to hold the pieces of lead
sheet in place. (Some of the lead was also glued into place with contact
adhesive.)
Warning!
Lead is poisonous. Don’t work the sheet with anything that will result in
small particles (hacksaw, file, grinder, sandpaper) being released. Always wash
your hands thoroughly after working with lead. In the final job, make sure that
the sheet is sealed into place and cannot be easily contacted by humans.
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The lead sheet stopped nearly all noise coming from the outlet pipes.
However, with those now quieter, the noise from the supercharger itself and its
intake became more pronounced. To reduce this, a bonnet sound insulator was then
installed. It comprises 25mm acoustic foam that’s lined with thin metal sheet.
Finally, an intake resonator was added. It is a small 35mm diameter pipe
placed right at the exit to the supercharger. As seen here, the end was
temporarily plugged so that adjustments to its internal length could easily be
made.
After all of this had been done, the supercharger noise was considerably
lessened; however it is still quite obvious over about 50 per cent throttle.
On the Road
So what’s the world’s first supercharged, intercooled, petrol-electric hybrid
like on the road?
The first point is surprising: there is little or no increase in performance
over what was previously able to be achieved in optimal conditions. This point
needs a bit of explaining – after all, does that mean the supercharging project
has been a failure?
The Prius has available from its petrol engine 43kW (at the redline of 4000
rpm), and from its electric motor, 30kW over the range of 940-2000 rpm. However,
maximum electric motor power is available only when the high-voltage battery
pack is ‘full’. The battery pack level is shown on the
dashboard and generally on flat roads, the bar graph sits at about half. This
gives plenty of reserve capacity to store the electric power developed through
regen braking but it also means that when the electric motor is in high demand,
the battery level can fall quickly.
When the battery is empty, a cute tortoise symbol appears on the dash – but
the appearance of the tortoise means just what it implies, terrible
performance!
With a full high voltage battery, on-road performance of the Prius is more
than acceptable. In outright acceleration times - like 0-100 km/h - it is still
woeful, but the amount of available torque gives it an effortless, punchy feel.
But when being driven hard, more likely than not, the tortoise will make an
appearance – and well, then you have just 43kW trying to drag along 1240kg...
But with the supercharger fitted and running just 5 psi boost, the on-road
performance in these difficult conditions has been absolutely
transformed.
In hard driving (eg steep hills, lots of applications of
full power), the battery voltage is typically much higher than it was
previously, so there is more electric power available for a lot longer.
Furthermore, it is now difficult to see a tortoise. (In fact, I haven't seen the
tortoise since the supercharger was added, but I gave the car to its owner – my
partner – and she got a tortoise to appear within 3 minutes. I was amazed. "How
did you do that?" I asked. The answer: she just held full throttle until it came
on...)
Two test drives are indicative of the change.
One is a long,
constantly climbing country road with a huge number of very tight (25 – 40 km/h)
corners. I drove the Prius up the hill as fast as I dared – it was almost one
continuous slide from start to finish – and the car was punchy and consistent. I
was using full throttle out of every corner - and usually in fact full throttle
until I braked for the next corner. By the top of the hill, the battery level
graph was down to its bottom section, but the drop-off in performance was
negligible. Previously, I would have seen the tortoise appear by perhaps
two-thirds of the way up, and performance would have been substantially down
over probably half the climb.
The other test is up an even steeper,
straight hill. Previously, to avoid getting a tortoise by the top required an
excruciatingly careful juggling act by the driver - and a peak speed of about 55
km/h. On one test drive I was stuck behind slow traffic (at 60 – 65 km/h) for
the first half, but once past them I was able to accelerate, in fact reaching 83
km/h at the top of the hill. This is easily – by a margin of at least 18 km/h –
the fastest I have ascended this hill in the Prius. (By comparison, my ‘99 Lexus
LS400 can do 140 km/h at the top of the hill. That's at full throttle all the
way up – it is a very steep hill!) Furthermore, there was no appearance of
Myrtle the Turtle, even by the top of the hill.
So how’s the performance of the supercharged car when it also has a full high
voltage battery? It must be pretty good then, right? Wrong! In fact, in these
conditions the performance over standard is unchanged. It appears that the
hybrid system always optimises itself for the same peak power output – the
amount that is possible when both the petrol engine and the electric motor are
both working at their maxima.
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So what is going on in the hybrid control system? It’s hard to tell but
here’s one perspective:
The High Voltage ECU is programmed with the engine code (and therefore
probably a look-up table of expected engine performance) as well as the battery
capacity. An accelerator input is translated into a power request; the ECU then
calculates a required power and rpm from the engine and sends this information
to the EFI ECU.
Unless the High Voltage ECU has knowledge of the
increased capability of the engine, it won’t know how to take advantage of it,
hence there is no improvement in level road acceleration or peak performance.
But in hard driving, the story is different.
Since one of the motor/generators is used to control the speed of the petrol
engine, a higher than expected current is generated at the rpm requested by the
HV ECU, ie actual engine torque x rpm is greater than the requested power. This
extra power has to go either to the other motor/generator or to the battery.
Under high load, it goes to the other motor/generator (which results in more
power at the wheels), but the total requirement is still fixed by the High
Voltage ECU, and so the battery drain is less. At lower loads the extra power is
used to charge the battery so the battery level under moderate throttle is
higher than usual.
(after bskattebol_aviator on
http://autos.groups.yahoo.com/group/Prius_Technical_Stuff)
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So there’s no awesome peak power gain to talk about – yet performance on the
road is absolutely and dramatically improved. Interesting idea, huh?
Conclusion
So, the big project has a happy ending, right? More on-road performance (and
incidentally, still with brilliant fuel economy) and an all-round good outcome.
Well, no.
It’s that bloody supercharger whine – it’s enough to drive someone mad... To be
honest, I don’t know if I can put up with it – and certainly my lady, Georgina
(and it’s technically her car!) absolutely hates the noise.
So I think, after all that work, the blower is going to come off...
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