When lower boost doesn’t slow a car…

Posted on December 11th, 2005 in Opinion by Julian Edgar

As regular readers will know, my NHW10 hybrid Toyota Prius has been turbo’d and intercooled. To fit in the available space, the turbo – one of the puffers from a twin turbo Subaru Liberty – required that its wastegate mounting system be modified. A spacer ring was used to allow the wastegate actuator to be placed in a different orientation to standard. This ring put a small preload on the wastegate rod, resulting in a minimum on-load boost level of 7 psi. That’s 7 psi, even with the wastegate hose connected directly to the turbo compressor outlet – ie no bleed or aftermarket boost control fitted.

Now that’s generally well and good, but sometimes at high loads, 7 psi can cause a problem. Intermittently – and for only a very short duration – the hybrid control electronics closes the electronic throttle. I assume that this occurs because I have exceeded a preset internal safety trip-point for the engine or electric motors. That implies that if boost can be dropped a little at the top end, the throttle shut-downs are likely to stop. (And in previous short-term testing with lower boost levels, the problem did in fact disappear.)

Since the minimum boost level the wastegate can be set to is 7 psi, dropping boost below that requires bleeding air from the manifold. One easy way of achieving this is to allow the blow-off valve to leak, something which can be achieved by pulse-width modulating the boost/vacuum feed to the valve. Working with the airflow meter signal, the Simple Voltage Switch kit allows this boost drop to be triggered at a preset load. This boost leak doesn’t cause any fuelling problems, because a recirculating blow-off valve is used and the air is returned to the intake after the airflow meter.

Using this approach, I initially dropped peak boost back to 5 psi at loads over about 80 per cent of max, with little discernible difference in performance. Since dropping from 7 psi to 5 psi apparently made little variation in the available top-end power, I then decided as an experiment to allow the blow-off valve to leak all the time. (The aftermarket GFB valve runs a variable preload on its internal spring, making this easy to achieve with some spring adjustment and pulling off the vacuum/boost feed hose.) This resulted in a slow rise in boost to a max of only 4 psi.

Of course, slow rising boost is an anathema in a turbo car – you always want boost to come up as fast as possible. In a normal car, the difference between this and the previous boost-as-fast-as-possible-to-7-psi-and-then-hold-it-at-that-level would be like chalk and cheese. The configuration with the slow-rise-to-4-psi would feel half-dead and power would be clearly way down.

But in the Prius, the performance difference was minor!

Even up my big open road test hill, the speed at the top was in the same ballpark as in the high boost configuration (I couldn’t do an exact comparative test because of traffic) and in normal point and squirt driving, you’d be hard-pressed to tell the difference. But hold on: how can that be? All else being equal, more boost equals more power – and so less boost equals less power! And just that situation was occurring in the Prius – but the extra power resulting from the higher boost engine wasn’t being fed to the wheels… Nope, what happens is when the 1.5-litre engine makes greater power, more of it is directed into charging the high voltage storage battery – if you like, that’s where the surplus kilowatts go. The same amount of power still reaches the tyres – both directly from the engine and also through powering the generator which runs the electric motor. (Sorry, but it’s a bloody complex car!)

A colour LCD on the dash shows all these power flows and so it can be seen that on low boost, instead of the high voltage battery being charged all the way up the big hill (as it is on 7 psi), on 4 psi boost the power flow is in and out of the battery – the exact direction depending on the throttle angle, speed, etc. In other words, with 4 psi of boost, the factory hybrid control system logic can make use of all the engine power. On 7 psi of boost, the extra power doesn’t get to the wheels…

But it’s very important to note that even on 4 psi boost (and, it should be said, with a free-flow intake and exhaust), the car is completely unlike how it was in standard form. That’s because on this steep ascent, the high voltage battery is not being constantly drained – an outcome which previously resulted in a painfully slow road speed near the top of the hill as the high voltage battery got so low that no electric power was being provided.

You could argue that a slow rise to 4 psi perhaps better matches the hybrid control system logic. On the lower boost level, the measured mass airflow going into the engine is clearly down (measured injector duty cycles are much lower) and so the peak power being produced by the engine is also down. But since when the boost is set high, the extra power is just (uselessly) put into charging the high voltage battery, perhaps it’s better to run the slow-rise-to-4-psi approach and have only sufficient power available to prevent the high voltage battery from being drained at full throttle…

And all still with the Number 1 priority being met – climbing the bloody big country road hill at speed. But it seems such an odd way of doing things – to deliberately pull-back every aspect of turbo boost control that normally makes a car faster!

Footnote: In fact, further testing shows that when the turbo system is set up so that it is slow to boost, low speed fuel economy suffers because the engine revs harder to produce a given power output. The best overall scheme is to have boost rising as fast as possible to a max of 7 psi, then dropping away to 4-5 psi as load reaches max.

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