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The Story of Turbo'ing a Hybrid Prius, Part 4

Finally - finally! - solving the high load fuel problem

by Julian Edgar

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At a glance...

  • Overcoming the auto throttle closing
  • Developing a two-pressure fuel system
  • Fixing the high load mixtures
  • Part 4 of a 5-part series
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This article was first published in 2005.

Last week in The Story of Turbo'ing a Hybrid Prius, Part 3 we ended with two major problems affecting the turbocharged Prius: high load mixtures were completely inconsistent, and at full power, the electronic control system would momentarily shut the throttle. Solving both problems took over two weeks of frustrating work.

Throttle Shut-Down

So what was making the electronic throttle momentarily close itself at full power? If in fact it was because the hybrid system couldn’t cope with the extra power, the effect would be most pronounced when power was at its maximum... say, on a cold night. And that’s just what proved to be the case. When the weather provided me with an 11-degree C evening, I was able to get the throttle-shutdown to repeatedly occur. (But what about those inconsistent full-load mixtures? Well, if the air/fuel ratio meter was watched like a hawk, it was possible to get in peak power runs with the mixtures satisfactorily rich. You just couldn’t do it consistently.)

So the obvious answer to the automatically closing throttle was to lower boost at the very top end, so reducing power. Since the greatest driving improvement over standard occurred due to the mid-range boost, dropping boost a bit at peak power would make very little difference. But how to decrease boost? With the ex-Subaru IHI turbo’s wastegate connected, 7 psi was the minimum boost available – and that’s what I was running. (The boost level had risen from 6 to 7 psi with the redesigned intake plumbing.) Modifying the turbo wastegate actuator to achieve a lower boost was possible, but it would mean taking off the turbo to access the wastegate actuator. Which in turn would mean removing a front driveshaft, taking off the exhaust, draining the ‘gearbox’ of oil, and so on. A lot of work.

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Instead, was it possible to bleed off some of the boost? Already in the system was a GFB blow-off valve running a shortened internal spring – a modification performed on the valve when it was fitted to the supercharged car as a recirc valve. In this design of valve, boost pushes on the base of the piston, trying to open it against both the internal spring pressure and the pressure in the boost-sensing hose. If the boost being fed in via the hose was reduced, it was likely that the internal piston would be pushed open, so allowing some boost to escape. By placing a bleed solenoid in the boost pressure feed hose, the pressure in this hose could be regulated. By using the Independent Electronic Boost Control (IEBC) kit, the action of this solenoid valve could be mapped. (See The Independent Electronic Boost Control, Part 1.)

The system was quickly set up and the solenoid valve opening set to 0 per cent at lower injector duty cycles. (The IEBC kit sets its output duty cycles on the basis of input injector duty cycles. That is, any relationship between injector duty cycle – ie engine load – and output duty cycles can be set via the hand controller.) At higher engine loads, the solenoid was gradually brought on line, until at very high engine loads, it was completely open, so acting as a bleed of the boost pressure in the feed hose to the blow-off valve.

The mapping of the control system was initially done very coarsely but the system soon showed it was possible to drop boost to 5 psi at the top end of the engine power band. This stopped the auto throttle shut-down procedure – the system was sufficiently sensitive that even a 2 psi boost decrease made enough difference.

Hmm, OK then – it was easy enough to stop the auto throttle shut-down with very little loss in performance. But what about these bloody mixtures?

Not Solving the Air/Fuel Ratio Problem

It’s worth recapitulating what had so far been done to try to sort the high load mixtures.

A Simple Voltage Switch (SVS) kit had been fitted that allowed the two oxygen sensors to be disconnected on the basis of measured airflow meter output, so forcing the car into open loop. In the standard car this had resulted in the mixtures automatically going very rich – too rich in fact. The Digital Fuel Adjuster (DFA) kit had then been used to intercept the signal coming from the standard airflow meter, which allowed these ‘oxy sensors disconnected’ mixtures to be leaned out a little. However, once engine power had been increased by the use of the supercharger and then – subsequently - a turbo, these ‘oxy sensors disconnected’ mixtures had become leaner and leaner, and so for forced aspiration, the DFA had been used to richen the mixtures (ie increase the level of the airflow meter signal).

However, mixtures at full load were still too lean.

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To ensure there was sufficient fuel flow and pressure, a new in-tank pump had been installed, together with a new adjustable external regulator (a Malpassi rising rate design) and a new external fuel filter. Installing the external pressure reg had required fitting a new return line to the tank. This system delivered plenty of fuel and allowed the adjustment of fuel pressure. The injectors from a Corolla had also been trial-fitted, but despite coming from a much more powerful engine, at the smaller duty cycles of the Prius system, had proved to flow less fuel than the standard injectors.

Adjustment of the fuel pressure was then carried out which resulted in adequately rich mixtures when the oxy sensors were switched out. In this approach, the DFA did little – mixtures were adjusted by altering fuel pressure. The Malpassi reg was plumbed so that it saw boost but not vacuum. In this way the off-boost fuel pressure remained constant (as it does in the standard system) but rose when on boost. This resulted in full-boost mixtures which were satisfactory – at least at first.

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At this stage the airflow meter sensing element was installed in a custom-made body with a 31 per cent larger cross-sectional area. The DFA was then used to lift the output voltage value of the larger airflow meter to achieve correct closed-lop mixtures. New free-flow intake and intercooler plumbing was also installed at this time.

Next, the mixtures became erratic at full-load. At times they were correct (as set by fuel pressure and the DFA) and at other times, incorrect. Furthermore, in successive full-throttle events they could be seen to be heading back to stoichiometric, even with the oxy sensors disconnected.

An attempt was made to intercept the oxygen sensor outputs, using the DFA to modify their levels at high loads and leaving the system always in closed loop (ie oxy sensors connected). However, the car ignored this and maintained 14.7:1 air/fuel ratios.

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The oxygen sensors were then disconnected at all loads, and an attempt made to tune the mixtures throughout the load range with the DFA. (This was done with great success on a Maxima V6 turbo, where the air/fuel ratio could be maintained very accurately at all loads despite the lack of an oxygen sensor feedback loop.) However, on the Prius it became clear that with the oxygen sensors disconnected, it isn’t just the airflow meter input that is used to set mixtures. This could be clearly seen because even with the engine held at one load site (ie one airflow meter output voltage), the mixtures would gradually slide back from being rich to 14.7:1. (This is what so strongly suggests the presence of a look-up table that compares measured engine power with expected fuel injector pulse width.)

So despite trying each of the following, consistent full-load mixtures could not be obtained:

  • Intercepting the airflow meter signal with the DFA
  • Altering fuel pressure
  • Switching the oxy sensors in and out on the basis of throttle position
  • Switching the oxy sensors out as fuel pressure was progressively increased
  • Switching the oxy sensors out and intercepting the airflow meter signal with the DFA
  • Switching the oxy sensors out, increasing fuel pressure and intercepting the airflow meter signal with the DFA
  • Intercepting the oxy sensor signals with the DFA
  • Disconnecting the oxy sensors at all loads and then setting the mixtures via the DFA working on the airflow meter signal

It’s no understatement to say that by this stage I was pulling my hair out!

I then decided to go back to an approach tried a long time previously – one covered at Electronic Fuel Pressure Increase. This involved switching a restriction into the return line from the regulator and so boosting fuel pressure in one hit. My thoughts were these: if the increase in fuel pressure occurred only when the oxy sensors had been switched out, the ECU couldn’t be aware of the change. Therefore, even at the standard look-up table pulse widths with which it was triggering the injectors after the oxy sensors were disconnected, the mixtures would have to be richer.

(This approach contrasts with using a rising rate pressure reg that increases fuel pressure progressively when on boost. Taking the rising rate fuel pressure route, and switching the oxy sensors out only when boost over say 4 psi, means that often the system is in closed loop with slightly heightened fuel pressure. If the ECU can indirectly learn fuel pressure by looking at the injector pulse widths required to maintain an air/fuel of 14.7:1, then maybe the ECU can pull back all injector pulse widths to compensate for this increased fuel pressure. In other words, despite not having a fuel pressure sensor, the ECU can probably still calculate fuel pressure, and compensate for it. I don’t want it to do that!)

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I then set up the system to allow the fuel return line from the pressure reg to be restricted, increasing fuel pressure. By closing a solenoid, fuel is forced to flow through a ball-valve, which comprises a restriction that can be varied in flow. Shut off the ball valve and when the solenoid closes, the fuel pressure will rise to the maximum the pump can flow (which is normally limited by an internal pressure relief valve). And in fact testing soon showed that to get adequate fuel pressure, the ball valve did have to be completely closed – ie, the pressure reg’s return shut off. This resulted in an immediate increase in fuel pressure of 20 psi – from 50 to 70.

And what were the mixtures like when the oxy sensors were switched out and simultaneously the solenoid was closed? Finally – finally! – they were consistently close to what was desired – around 12.5:1 at full load. I rebuilt the underfloor bracketry to include the solenoid and performed a final on-road test.

But again the mixtures at full load were lean!

WTF was going on this time? Watching the fuel pressure gauge provided the clue: even with the return line shut off, the fuel pressure was now rising to only 60 psi – 10 psi of high load fuel pressure had been lost. Repeated testing showed that with the fuel pressure reg’s outlet closed off, the fuel pressure could jump to anything from 60 to 80 psi...

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Basically, it appeared that the pump couldn’t cope with the increased fuel pressure requirements. I checked the voltage at the pump to make sure that it was getting full battery voltage - it was. This meant another pump was needed – but the in-tank pump had already been replaced and getting a physically larger pump in place would be near impossible. The only alternative was to add a second in-line fuel pump – one that could consistently produce fuel pressure of at least 70 psi. And to regulate this fuel pressure – which is much safer than using the pump’s internal pressure relief valve – a second regulator would be needed.

The New Fuel System

Many turbo and supercharger upgrades use a second fuel pump in series with the first, with the second pump activated only when the extra fuel is needed. In this approach, the first pump flows fuel through the second pump which is usually switched off. However, it is better to use a bypass one-way valve around the second pump so that the fuel can freely flow past it. When the second pump switches on, the extra pressure it generates closes the one-way valve.

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The complete fuel system is shown in this diagram. The in-tank pump feeds the second pump with the one-way valve providing the bypass. The two pressure regs are plumbed in parallel, with a solenoid valve positioned in the feed line of the lower pressure reg. When the fuel pressure needs to be increased, the second pump is switched on and the solenoid closed. This forces the second reg into action and the fuel pressure is then regulated at the higher value. It’s a complex and relatively expensive system but it’s the only approach that gives two fixed (but adjustable) fuel pressures, with the second pressure much higher than conventional fuel systems normally use.

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The system was installed under the car and testing carried out. Initially I just used a manual switch to turn on the new fuel pump and turn off the solenoid (the two steps that result in high fuel pressure). The oxy sensors were being disconnected by a Simple Voltage Switch working on the airflow meter output voltage (ie when airflow reached a certain value, the oxy sensors were disconnected) and when I heard that relay click, I manually toggled the fuel pressure increase switch. And watched the fuel pressure gauge. And watched the boost gauge. And watched the MoTeC air/fuel ratio meter....

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On-road testing showed that the system worked – consistent rich mixtures on high loads with no apparent learning around them occurring. I then added two relays – one the changeover from solenoid-off-to-fuel-pump-on, and the other a power feed relay for both the solenoid and the pump. A second relay was also added to the Simple Voltage Switch to trigger the fuel pressure changeover, so that the oxy sensor switching and fuel pressure switching occurred simultaneously.

I then decided to see what would happen if I swapped the switch-over input signal from being triggered by the airflow meter to being triggered on the basis of accelerator pedal position. Initially, this seemed to work even better – at full throttle, you always got rich mixtures, irrespective of engine airflow – but then I found if the Prius was booted at full throttle from a standstill, the mixtures returned to stoichiometric. (How many times has this return-to-stoich now occurred?!) It appears the car needs to be running in closed loop for at least a little while before the switch-over occurs.

High, High Fuel Pressure

The fuel pressure required to consistently gain a full-load air/fuel ratio of around 12:1 – 12.5:1 is significant – about 85 psi. All the fuel hoses at the rear of the car have been replaced with hose good for 290 psi, and I intend to also replace the underbonnet injector fuel rail feed hose.


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But with the high load air/fuel ratio problem solved, and the throttle shut-down problem consigned to the history books, I could finally see the light at the end of the tunnel: touch wood, that was it for the big problems. (Yes, it takes real skill to get three clichés into one sentence. Was this car really driving me mad or was I always like this?)

Now we could drive the turbo Prius, confident in its mixtures and performance.  Hmmm, performance... so what’s it like? And what’s happened to the fuel economy? Next week, in the final in this series, we’ll find out.

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