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Our Peugeot Diesel - Part 4 - Boost and the Injection System

Winding in the boost and then finding the fuel to go with it

by Julian Edgar

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At a glance...

  • Adjusting boost
  • How distributor-type diesel injection systems work
  • Standard fuel economy figures
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At the end of the last article in this series (see Our Peugeot Diesel – Part 3) we looked at the chassis dyno curve for the later model 406 diesel (that’s the one with the bigger engine and vastly more sophisticated electronic injection system) and concluded that if turbo boost could be brought up much earlier, the 1.9 litre diesel in the 405 wouldn’t be far behind the 406’s engine. In fact not far behind at all... And the obvious answer to achieving better low-rev boost is to use a more sophisticated boost control.

Trouble is, that’s harder to implement than it is to write.

Adjusting Boost

There’s nothing particularly difficult about tweaking turbo boost on a diesel engine. Those that don’t use variable geometry turbos have traditional wastegate controls just like petrol turbo engines. (Variable geometry turbos can use an electric motor control of the variable vane angles.) And, since our Peugeot 405 diesel is 1990s vintage, adjusting the boost should be easy, right? Huh!

Well, as you might have gathered from this slightly embittered discourse, the Peugeot wastegate isn’t anything like I’ve been describing. Well, it might actually be – it’s just bloody impossible to see. The turbo is tucked up behind the engine against the firewall. The transverse engine leans back at such an angle that you can’t look down at the back of the engine and see the turbo; even if you take off the top-mount intercooler, all is still invisible. Instead, to see the turbo, you need to jack up the car, take off the front-right wheel, and then peer in, lying half under the car.

And even then all you’ll see is the compressor cover and a strange-looking wastegate housing. I assume that there is a boost pressure feed, but it is completely impossible to see without taking the engine out of the car (or perhaps taking off the intercooler and then the intake manifold – this engine is a non-crossflow design). Instead, what can be sighted are a lock nut and an adjusting screw. I can’t even show you a pic – it simply can’t be seen sufficiently to take a photo. However, the 306 diesel, which uses the same engine, has a turbo that is much more accessible – you can see some pics of the area here

After I saw how hard everything was to access, I quickly changed my mind away from fitting an external boost controller to instead adjusting the factory boost control. And even then, I initially found it impossible to access the wastegate pressure canister.

In the end I assembled:

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A 2.5mm Allen key...

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...and an 8mm deep cocket (a normal socket was no good)...

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...and a bendy extension...

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...followed by another extension...

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...turned by a T handle.

The long extension and 8mm deep socket allowed the lock-nut to be undone. The Allen key was then able to be manoeuvred into place (using the finger tips of both hands) and then painstakingly turned. I am now an expert - but it’s still a prick of a job.

I went through all this rigmarole, turning the wastegate adjuster clockwise by two turns. I then put the road wheel back on, dropped the car back down and went for a drive.

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There was little of no difference in the peak boost.

I then jacked the car up, took off the wheel, did the rigmarole again.

There was little change in boost.

Really enjoying all this process (not!) I screwed the adjustment clockwise as far as it would go and then went testing.

On this car at least, screwing the adjustment fully in gave a peak boost in normal driving of about 17.5 psi (up from about 12 psi) but under very long periods of load (eg climbing a hill) at full throttle this could rise to about 19.5 psi. (Note: measurements are being taken in front of the intercooler; the pressure drop across the intercooler is currently unknown but you can be sure that there is some.)

So did the car now go really hard? The answer is very interesting: performance, while better, wasn’t hugely improved. Clearly, the fuelling wasn’t keeping up with the boost. Last story we adjusted the rev limiter and the full load fuelling, but to get better results it was by now obvious that we’d have to develop a much better understanding of the fuel system and its adjustments.

Diesels and boost

Now, here’s where is starts getting interesting – and quite diesel-specific.

As I am sure you know, the turbo is powered by the heat energy in the exhaust gases. A diesel’s variation in engine power is governed by how much fuel is added. Therefore, it logically follows that, depending on the maximum wastegate pressure selected, the maximum boost level may well be determined by the amount of fuel being added. And, in the case of the Bosch system used on the Peugeot, the amount of fuel added depends somewhat on the boost pressure level!

So when setting up the boost, you pursue a circular sequence...

Diesel Distributor-Type Fuel Systems

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The fuel system used on the Peugeot consists of just one mechanical assembly that’s connected to the injectors. This single assembly pumps fuel from the tank, pressurises it hugely to the level required to be injected under full cylinder compression, distributes the fuel to the right injector at the right time, and apportions the correct amount of fuel for the conditions.

The injectors can be thought of as simply dumb pressure relief valves – if the pressure in the line to the injector rises sufficiently, the injector opens and sprays a fine mist of fuel into the engine. If the pressure drops, the injector shuts.

But the injection pump is not so dumb - it might be just a mechanical system, but it’s beautifully developed and manufactured.

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The fuel injection pump (which in distributor-type systems is also the metering device) consists of four basic sections. The pump is driven at camshaft speed, normally by the cam drive belt. The first section is a vane pump (red) that draws fuel from the tank. Next there’s a gear (green) that drives a mechanical governor (not shown - more on this in a moment) and then a roller that drives a cam disc (both blue). The cam disc plate turns the rotating motion of the input shaft into an alternating fore-aft and rotary motion. The fore-aft motion operates a plunger (purple) that provides the ultra-high pressure needed for the direct (or indirect) injection of the fuel, while the rotary motion distributes the pressurised fuel to the appropriate injector.

So how is fuel quantity varied? Here’s where it starts to get pretty interesting. The variation in fuel volume is achieved by moving a collar (sometimes called a sleeve – shown above in yellow) along the plunger. The further the collar is moved in one direction, the later the injection pulse is cut off – resulting in more fuel getting to each injector. The further the collar is moved in the other direction, the earlier the fuel is cut off – resulting in less fuel getting to each cylinder.

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OK, that’s all well and good – but the important question becomes: what controls the position of the collar on the plunger? There are two major inputs to the collar position. Firstly, a governor (spun by the gear described above) applies a force to the collar. As the engine (and so the pump) spins faster, the governor force supplies more fuel. If no other forces were acting on the collar, the increase in fuel injected with revs would match the engine’s requirements and maximum power would be developed right through the rev range.

But max power all the time isn’t wanted, so pulling the opposite way on the control collar is a spring connected to the accelerator pedal. This spring pulls most strongly when you foot is off the throttle – that is, it resists the movement of the collar caused by the governor.

So the end result of the collar position – and so the amount of fuel injected – is a tussle between the governor forces (proportional to rpm) and accelerator pedal forces.

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Now if you’ve been keeping up you’ll realise that so far, the system knows nothing about turbo boost pressure. Imagine the fuel injection system working with the accelerator flat to the floor- that is, the increasing amount of fuel with revs is determined by the internal settings of the fuel pump under the influence of the governor. The system works fine – but not if we increase the airflow by (say) 50 per cent by using 9 psi of boost. And the extra fuel can’t be added all the time because the engine might be at 4000 rpm, 9 psi boost – or alternatively, 4000 rpm, 0 psi boost. Or anywhere in between. And remember, to make use of the extra airflow, we have to add the extra fuel or no more power will be produced!

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The way the extra on-boost fuel is added is by means of a boost pressure compensator – a diaphragm that deflects with boost pressure. This diagram works against a spring to operate a control rod (red). The control rod assembly comprises a probe (green) bearing against a specially shaped surface – the shape of the control rod provides appropriate increases in fuel for a given boost diaphragm deflection.

Finally, a few other things to know:

  • Because the time taken for combustion is relatively constant, the timing of the fuel injection is varied with revs - for just the same reasons that ignition timing in a petrol engine is varied with engine speed.

  • When the engine is to be stopped, an electrically-controlled solenoid cuts off the fuel supply. (Yes, nothing more sophisticated than that!)


So now we have the potential to make more boost and have enough knowledge to start to tweak the fuel settings.

Next: making changes to the fuel pump settings. The results were very interesting...

Fuel Economy

The raison d’etre of the Peugeot is fuel economy – as in, our goal is to massively improve performance and driveability while impacting little or not all on the fuel consumption achieved in normal driving. (Of course, cane it and you’d expect fuel consumption to be much poorer than standard – and that’s fair enough.)

So what fuel consumption have we been getting?

Unfortunately, as with any really economical cars without a digital fuel computer, that’s harder than it seems to answer. The old fill-the-tank-and-count-the-kilometres approach if of course still valid, but doing it in this way, the greater the tank fill, the more accurate the final results. And, with a 70 litre tank, in the Peugeot that means travelling more than 1000 kilometres...

However, this is what was gained with the car standard. The first tankful was up and down the steep mountain where I live, plus a little urban and some freeway. Over long experience I have realised that this driving regime penalises small engine cars - they have to work really hard climbing the big hill. So I was stunned to find that the measured economy was 6.9 litres/100km (all these figures take into account a slightly optimistic odometer).

We then did a 700-odd kilometre country drive, two adults, one child and a fair amount of luggage. The drive was done at the speed limits, with the air on for a portion of the trip home. The result was 5.7 litres/100km.

The final tank involved all the performance testing undertaken with the fitting of the exhaust and intake, dyno runs, draining of fuel from the filter to remove water, and up and down the hill and some freeway work. Again the result was 7.0 litres/100km.

So to summarise, on a country drive the standard economy is in the high Fives, while in my normal daily use it’s in the high Sixes / low Sevens. In the real world, they’re excellent fuel economy figures for normal use in (what I at least am happy to call) a small family car.

When the modifications are complete, we’ll bring you the new economy figures.

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