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!?
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.
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:
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.)
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
Two test drives are indicative of the change.
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.
So there’s no awesome peak power gain to talk about – yet performance on the road is absolutely and dramatically improved. Interesting idea, huh?
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.
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...