An injection of refinement

Diesel motors are usually seen as coming a poor second to their petrol counterparts in terms of performance. A new fuel injection system might change all that.

Ask the typical British motorist about diesels and you’re likely to come up against a perception of ’lack of refinement’ compared with petrol-engined cars.

Now, though, car makers believe a new system of fuel injection will offer a level of refinement that removes that stigma. So excited are they about the potential of so-called ’second-generation common rail’ that its developer, Delphi Diesel Systems, is unable to keep up with demand.

Ford and Renault are the first in the field with the system. Ford’s Focus TDCi (for turbo diesel common rail injection), with a 1.8 litre Duratorq engine, and Renault’s 1.5dCi Clio will be available this summer. A TDCi Mondeo is scheduled later in the year.

The 1.8 Duratorq develops 115bhp at 3,800rpm and 250Nm of torque at 1,850rpm. Renault’s 1.5dCi will be available in two versions, developing 65 or 80bhp and 160-185Nm of torque.

Phil Lake, chief engineer in Ford’s diesel unit, says the new technology offers four key benefits: greater refinement, enhanced performance, better fuel economy and reduced emissions. Manufacturing technology derived from aerospace, with exacting tolerances, was crucial to its development.

The Duratorq was developed – with Delphi as ’technology partner’- at Ford’s Dunton R&D centre and will be built at Dagenham, now designated Ford’s main European centre for diesel production. Ford is currently moving 400 engineers from Dunton to Dagenham, and is building the test cells and clean rooms needed for assembly.

Delphi, basically, had to aim to beat competitor Bosch, which pioneered common rail. ’We knew we were coming out after Bosch and couldn’t just come up with a ’me-too’ product – we had to offer superior performance,’ says Peter Lakin, Delphi’s common rail product line manager.

Traditional diesel systems rely on the fuel pump to deliver bursts of pressurised fuel to the cylinders through individual mechanical injectors. In common rail systems the so-called rail is a reservoir of fuel at high pressure close to the injectors, which are electromechanically actuated. This gives precise control of the injectors and, hence, over how the fuel is delivered to the cylinders. Delphi’s new design builds on this, but differs from the original idea in a number of ways.

More responsive

First, the actuator has been made a lot smaller so that it can fit into the body of the injector; very close to the needle valve it controls instead of on top of the injector with a pushrod connection to the valve. This makes it faster and more responsive, says Lakin. The small size of the injectors – just 17mm in diameter – makes them easy to install in engines with four valves per cylinder.

Injectors are tested during manufacture, and their individual flow characteristics are programmed into the system’s electronic control unit. An accelerometer mounted on the engine ’listens’ to the noise and adjusts the timing to minimise it.

Second-generation common rail also operates at higher pressure than first generation: 1,600bar as opposed to 1,350bar. A high-pressure pump delivers the fuel at a more or less constant pressure, irrespective of engine speed.

The ’big idea’, though, is injectors which can switch in just 0.3milliseconds. The injector valve is balanced by fuel pressure acting on it from both sides. Reducing the pressure on one side achieves that fast operation which is crucial in allowing flexibility in the timing of multiple injections – the key to improved refinement.

’The speed of control means we can tailor injection for a combination of efficiency and smoothness,’ says Lake. This is achieved through ’pilot’ injection; a small delivery of fuel in advance of the main pulse. The effect is perhaps most audible when idling: ’The idea is to get the burn going and avoid a sharp rise in pressure in the cylinder – the cause of the traditional diesel ’knocking’.’

Up to two injections before and two after the main pulse are possible and, though this is not exploited now, it could be used to help meet future emissions regulations.Under the operating pressures being used, close tolerances are essential. Delphi faced the task of implementing the precision techniques needed for high-volume production. ’The challenge required a step-change in our capabilities,’ says Lakin. ’It caused a lot of headaches.’

Each injector has six holes, each less than 50 microns in diameter, manufactured using electric discharge machining. Gun drilling techniques are used for the 150mm long feed holes to the injectors. Electrochemical machining is used for the galleries where fuel collects before going to the nozzle. The seating for the balance valve has to be flat to within a micron to seal properly.

The fuel pipe, extruded from HS alloy steel, has to be polished inside to a finish smooth to within 10 microns.

Extensive deburring and high-pressure washing are needed to clean up components. ’One speck of dust and it won’t work,’ says Lake. And that’s why assembly has to be carried out in Class 0 clean-room conditions.

While production builds up, supplies are having to be rationed. Ford will initially offer the Duratorq only in top-of-the-range Ghia versions of the Focus, and expects to sell 100,000 units across Europe in the first year.

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