Small but effi ently formed
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Small direct injection diesels have taken a while to catch on, but ever stiffer emissions legislation is beginning to force the pace. Thanks to new types of fuel injection equipment and advances in electronic control we can now look forward to even cleaner and more fuel-efficient light van engines.
13 ig diesels featured direct injection (DI) right from the start when Dr Diesel forced a mixture of coal dust and air via a rubber pipe into the first diesel engine. But refining the DI technique and applying it successfully to smaller, high-speed van engines took a further 90 years. Up until the early 1980s all small diesels used indirect injection (1D1) where the fuel is injected into a pre-chamber before passing to the cylinder. DI engines started well, had relatively clean emissions and were fairly quiet but could not deliver the best in terms of fuel economy
Direct injection promised a fuel saving of at least 15% and that was what motivated the change. Perkins was the first on the market with its 2.0-litre Prima, but it was Iveco and Ford which really made waves in 1984; Iveco with the first turbocharged 1)1 diesel and Ford with its naturally aspirated DI.
What had made these engines possible were developments in fuel pump design. Truck diesels ran at around 2,000rpm, whereas the new small van diesels had to run to 4,000rpm to develop the necessary power. This made tremendous demands on pump technology; the traditional in-line pump could not cope, and it was the compact rotary pump designs of the day that made the higher speeds possible.
The Iveco and Ford engines were successful, they proved robust and above all delivered the fuel saving the designers predicted. But it was to be more than 10 years before other manufacturers followed this lead and Mercedes-Benz, then Volkswagen and most recently Peugeot/Citroen introduced DI engines.
Emissions drive development
However, development work by the major fuel injection equipment manufacturers Lucas and Bosch has continued apace and the future looks rosy for small, high speed DI diesels. The main reason, in a nutshell, is emissions. The operational advantages of the DI diesel are not in doubt. The engines are inherently more efficient, giving better fuel economy and because fewer contaminants reach the lubricating oil, have longer drain intervals—typically 40% greater. But diesels are perceived as polluting, and first the ECE, then the EEC has set increasingly tight emission standards.
To its credit the automotive industry has risen to the challenge and the latest breed of modern DI diesels from Rover, Vauxhall and another from Volkswagen which is yet to be announced, meet these most exacting standards.
Although these are car engines under the latest emissions Directive, 96/69/EEC, cars and some light commercials up to 3.5-tonne GVW have to meet the same tough emissions levels after the vehicle has covered 80,000km.
The allowable levels for this lightest group are: CO 1.0 g/km, HC + NOx 0.9 g/km and particulates 0.1 g/km, representing reductions of between 90% and 60% on 1994 levels.
As in Dr Diesel's day, and again at the dawn of the DI era, the fuel injection system remains the key factor, and in much the same way as the turbocharger and the microchip transformed petrol engines in the 1980s, new highpressure fuel pumps controlled by advanced electronics have rejuvenated Di diesel design. One company insider admits there are so many possible development routes, that for once there is no general industry consensus as to exactly which is the best way to go.
High pressure injection
In general, the higher the injection pressure, the greater the atomisation of the fuel and the better the fuel efficiency. To put this into perspective the first Ford diesel, the DI York, used a pump which delivered a peak line pressure of 350bar (5,145psi). With the advent of the 2.5-litre DI in 1984, thanks to the new Lucas rotary pump, injection pressure increased to 700bar (10,290psi). Today, the Bosch VP44 rotary pump used on Vauxhall's new 2.0litre DI ECOTEC produces pressures of up to 1,500 bar (22,000psi) at the injectors.
But there are even more powerful pump systems waiting in the wings. At these pressures the injector pipes themselves can begin to bulge and the shock waves produced can affect the accuracy of the metering. To get round this, pipe runs have been made shorter or eliminated altogether.
Lucas' Electronic Unit Injector (EU1) is driven directly from an overhead cam, and does away with the need for a pipe between the pump and the injector. As a result, most of the pumping energy that was wasted in compressing and heating the fuel is retained, making for a more efficient system. Importantly this allows peak injection pressures of up to 2,000bar (29,400psi). Bosch started late in the unit injector field but has quickly caught up. Its unit injectors will almost certainly be used on Volkswagen's next DI diesel, due for launch next year.
Unit pump
Bosch has also shown another version of the EUI, which it calls the unit pump. In this design the EUIs—one per cylinder—are mounted low down on the engine where they can be driven perhaps by an existing cam, thus doing away with the conventional pump. This is a novel way of benefiting from EUI technology without the need for a new cylinder head, but the injector pipes are retained, along with the associated prob lems.
Common rail system
Bosch, however, is not putting all its eggs in the EL1 basket and has released details of what it calls a "common rail" system. In essence, this is a reservoir of fuel which is pressurised by a conventional high pressure pump, which in turn supplies the injectors. The system will deliver injection pressures in the order of 1,500bar rather than the 2,000bar of the EUI, but importantly that pressure is available across the engine's speed range, greatly improving the engine management opportunities.
Common rail allows full electronic control including variable-rate injection, and should be quieter in operation than alternative systems.
On the downside, the system incurs high parasitic losses as the fuel has to be continuously pressurised and the injector nozzles have to seal against this considerable pressure at all times rather than just before injection. Nonetheless, Mercedes-Benz has chosen this system for its A-Class car due next year.
The point of all these systems is that they can be controlled electronically, both in terms of the timing of the injection itself and the amount of fuel injected, giving almost unbelievable accuracy. The fuel has to be injected within a given 20° rotation of the crankshaft, and for an engine running at 4,000rpm, that means injecting it within one thousandth of a second.
Calculating the precise timing of the injection and the amount of fuel to be injected from "maps" stored in the engine controller's memory and varying this according to temperature and altitude, say, is relatively easy for today's mini computers, but getting the EUI to respond in time is something else. Yet it does, for every single injection event.
The precise amount of fuel is calculated immediately before the injection takes place, varying if needs be from zero to maximum fuelling between consecutive injection events. It is the complete control of this "shot-to-shot" ability that gives an EUI-equipped engine its smooth control and excellent driveability.
But for the EUI to deliver its best performance, it needs an overhead cam to drive it and ideally, it should be mounted centrally in the cylinder. This is not such a problem on a big, two-litres-per-cylinder diesel, but much more difficult when the cylinder size drops to 0.5 litre.
The first small engine to use EUI, the VW, is still to be announced and it is not known whether it will be a two or four-valve design. But it will be an interesting pointer. Rover and Vauxhall both use conventional rotary pump injection, and both use single overhead cams, driving two valves per cylinder in the case of Rover and four in the Vauxhall.
With all this diversity it is difficult to guess which direction the next generation of van engines might take. Given that the lighter, compact vans have to meet car emissions, they will probably take what is effectively a car engine, whereas the heavier vans will stay with developments of today's larger capacity DI engines, Which ever way it goes the operator is sure to benefit from the breed of quieter, more fuelefficient engines.