Big Bang Theory: Direct Injection Diesel Tech

The Puegeot 406 HDi was one of the very first cars to use a cutting edge, common rail, electronically injected diesel. Here we look at the technology - this article was first published in 1999.

The Engine

Developed over 3 years at a cost of $A750 million, the DW10ATED HDi engine was launched at the 1998 Geneva Motor Show. It uses an iron block/alloy head, in-line design, 8-valves and four cylinders. A toothed belt drives the overhead cam that uses roller rockers to reduce friction, a feature that improves fuel consumption by 2 per cent. The engine has a compression ratio of 18:1. An oversquare design, it has a bore of 85mm and a stroke of 88mm, giving an actual capacity of 1997cc. Peak power is 82kW (at 4000 rpm) and peak torque of 255Nm occurs at 1750 rpm. Compared with Peugeot's 2 litre naturally aspirated petrol engine also fitted to the 406, power is 18 per cent down but torque is 36 per cent up. In the 406, the HDi engine gives 0-100 km/h performance of 12.5 seconds and a staggering realistic range from the 70 litre tank of 1200km.

When compared with its 2.1 litre Peugeot diesel predecessor, the HDi engine has:

• 20 per cent less CO₂ emissions
• 40 per cent less CO emissions
• 50 per cent less HC emissions
• 60 per cent less diesel particulate (ie smoke) emissions
• 50 per cent lighter valves
• 15 per cent lighter conrods
• 20 per cent better fuel consumption

The Injection System

Used on the engine is a Bosch EDC15C2 system. The Bosch ECU has the following inputs:

• engine speed
• intake airflow
• engine coolant temp
• intake air temp
• fuel temp
• camshaft position
• crankshaft position
• fuel pressure
• atmospheric pressure
• accelerator pedal position

Looking at that list you could have be forgiven for thinking that you were dealing with a petrol injection system - until you start examining HDi in detail. Then you realise it's a whole new ballgame... For example, the airflow meter is used only during acceleration and deceleration, correcting fuel flow and so reducing smoke emissions. The fuel pressure is unbelievably high, and the actual injection process can occur in three separate stages for each induction stroke.

Fuel Supply

The fuel supply system starts with a conventional 140 litre/hour Bosch roller-cell in-tank pump. This pushes the fuel out of the tank to a device that is simultaneously the fuel filter, water decanter, and 2.5 Bar (36 psi) fuel pressure regulator for this low pressure circuit. Fuel is directed from here to the high pressure engine-driven pump, and also (through a separate circuit) to a fuel heater. The thermostatically controlled heating keeps the fuel temperature above 25 degrees C.

Lifting fuel pressure to the heights required by the injectors is a Bosch CP1 3-piston pump. This can take as much as 3.5kW to drive it - it's a helluva pump! Powered by the camshaft drive belt and driven at half engine speed, it increases the fuel pressure to between 200 and 1350 Bar (2900 - 19,575 psi). When starting the engine, even after just 1.5 revs the fuel pressure is already 200 Bar! One of the three pistons within the pump can be switched off by the ECU when it is not required, so decreasing the power being taken to drive the pump and also reducing fuel heating.

From the high pressure pump, the fuel passes to the injector rail. This is made from forged steel - the enormous strength required to withstand such incredible internal pressures. The feeds for the four injectors are incorporated into the rail, as is a fuel temperature sensor and a strain-gauge high pressure fuel sensor. In addition to the fuel supply lines, there is also a fuel return circuit. This takes surplus fuel from the injectors, high pressure pump and low pressure regulator, returning it to the tank through a fuel cooler.

Gas Movement

The air intake uses a conventional filter, filter box and hot-film airflow meter. Air then passes to the turbo compressor where it is boosted to 1Bar (14.5 psi). A small air/air intercooler follows, before the air then passes to the inlet manifold (no throttle body on a diesel, remember).

Exhaust gas is channelled through the turbo, with turbo boost controlled by an ECU-controlled wastegate. Exhaust gas recirculation occurs via another ECU-controlled valve, with the vacuum needed to operate both the EGR and wastegate controls coming from a special vane-type vacuum pump (no throttle = no engine vacuum!). Slight turbo over-boost is permitted to aid acceleration, while at a steady speed between 2500 - 3000 rpm boost is cut to 0.7 bar (10 psi), a move that improves fuel economy by 4 per cent.

The Injectors

The injectors are opened with a peak voltage of 80 volts and a peak current of 20 amps. This initial opening phase is extremely short - only 0.3 milliseconds. A holding voltage of 50 volts is then used to maintain the injectors' open position. These very high voltages are obtained by using capacitors within the ECU - the ECU charges these up by sending electrical pulses to the coils of the injectors that aren't working, with the resulting induced voltage enough to charge the corresponding capacitor. Weird but it works!

The amount of fuel that is actually injected when the injector is open depends on:

• the injector pulse width (ie how long it is open for)
• the opening speed of the injector
• the hydraulic flow characteristics of the injector (size and number of holes)
• the fuel pressure in the injector fuel rail

The very high fuel pressures prevent the ECU from operating the injectors directly - even with the high voltages available. Instead, two chambers are used within the injector - a control chamber and a pressure chamber. The centre shaft of the injector has working surfaces exposed to the pressures in each of these chambers. When the pressures are equal, a spring holds the injector shut. Electronically triggering the injector causes an electrovalve needle to lift, allowing leakage of fuel from the control chamber into the fuel return system. This drop in pressure against the top working surface causes the injector needle to lift, allowing the fuel to spray directly into the combustion chamber. Stopping the 'leakage' equalises the pressures, causing the injector to close.

Unlike a petrol injection engine where fuel pressure is held a constant headroom above intake manifold pressure, in the HDi engine, fuel pressure varies with engine speed. Thus, at idle quite long injector opening times are used, because the lower fuel pressure means that there is less flow for a given pulse width. At higher engine speeds the time available to inject the fuel is less and so the injector opening times must be shorter. To squeeze the greater amount of required fuel through the injectors, the fuel pressure must be high. In fact, the 3D fuel map of the ECU uses these axes: fuel pressure, engine speed and fuel flow. In addition, other 3D ECU maps exist for the:

• accelerator pedal - avoids large variations in fuel flow and gives progressive engine behaviour;
• full load fuel curve - provides correct air/fuel ratio at full load;
• turbo boost pressure - provides the correct turbo boost pressure on the basis of the fuel quantity injected;
• exhaust gas recirculation - operates the EGR valve on the basis of atmospheric pressure, fuel and air inputs;
• smoke limitation - controls fuel flow during transitions eg acceleration;
• fuel pressure - determines the regulated high fuel pressure on the basis of engine speed and fuel amount calculation.

The injection directly into the cylinder can occur in three stages:

• pre-injection
• main injection
• post injection

Pre-injection is designed to reduce combustion noise, and involves the injecting of a very small quantity of fuel (less than 1 milligram) prior to the main injection. This early injection allows the gradual increase in the temperature of the combustion chamber, slowing down spontaneous flame-spread and reducing the amount of fuel burned at the beginning of combustion. This reduces idle noise output by over 3dB. Pre-injection occurs only if the engine speed is less than 3200 rpm.

The start and duration of the main injection depends on the amount of load and whether or not a pre-injection process has already occurred. Main injection is cut if the engine exceeds maximum speed, or if there is insufficient fuel pressure in the injector rail (ie less than 120 Bar). Finally post-injection can occur in engines equipped with a cat converter - presumably to quickly bring the cat up to operating temperature.

If the engine is overly cold, combustion will not occur. In this situation resistor-style glow plugs are used. The pre-heating phase of plugs occurs only at coolant temperatures 10 degrees C and lower, with the maximum expected operating time being 16 seconds at a temp of minus 30 degrees C! During the actual cranking, the plugs operate when the coolant temp is below 20 degrees C or the engine has cranked at more than 70 rpm for 0.2 seconds. After the engine has started, the plugs can stay operating for as long as 3 minutes, reducing post-start emissions.

Conclusion

The Peugeot engine discussed here is only one of a new family of Pug HDi diesel fours - in the pipeline are engines with 4 valve heads and 2.4 litre capacities. A 2.4 litre 16 valve equipped with a bigger intercooler and more boost will probably be pushing 110kW - and perhaps 350Nm! That should give genuine sports car performance while still retaining unbelievably good economy...

www.peugeot.com.au

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