| Piezo fuel injectors have been available in diesel engines since 2000. They are starting to find favour in direct injection gasoline applications, including Audi’s own FSi models. Audi announce it is to use the technology in the 2 litre TDi engine for the A3 and A4, with the emphasis on improved power and performance.
With no change to the compression ratio, the new variant produces 170PS @ 4200rpm compared with the 140PS of the existing engine. Maximum torque remains the same, with 320Nm available from the same 1750rpm. The extra power has improved acceleration and top speed:
The A3 hits 62 mph in 8.2s instead of 10.0s and a maximum speed of 138mph instead of 129mph. Consumption on the combined cycle falls from 50mpg to 48.7mpg.
Comparable figures for the A4 are 8.6s (10.0s) 142mph (131mph) although economy is improved from 46mpg to 48.7mpg.
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The boost in performance elevates these models closer to the 2.7 litre V6 TDi A4 (180PS @ 3300, 380Nm @ 1400–3300rpm, 8.4s, 143mph). The larger displacement engine emits more CO2 than the new four cylinder: 185g/km vs. 154g/km, and is not as frugal, but still manages a respectable 40.1mpg combined. It might appear that the new model casts something of a shadow over the 2.7 TDi, but it is intended that the two complement one another rather than compete.
An Audi spokesman said: “It is a question of engine characteristics and delivery of the power. Some customers prefer the six–cylinder with its emphasis on refinement and smooth, linear delivery, and others the more overtly performance and response-oriented four–cylinder unit. Six–cylinder TDi engines, like their petrol counterparts, are still perceived as a ‘step–up’ over four–cylinder alternatives from a status perspective, so demand for them will continue to be strong, but we also need to offer an outstanding alternative for customers wanting the best possible performance, price and economy ratio above all else.”
This is the first time that piezo technology has been used with unit-injectors, called ‘pumpe-düse’ (pump-jet) by Audi. Unit injectors are fitted to the four cylinder engines instead of the common rail system.
Camshaft operated via roller rocker arms, the compact units integrate the function of injector and high pressure pump. The key benefit of unit injectors are higher injection pressures: up to 1600 bar. These deliver particularly good fuel atomisation: essential for more complete combustion, and limiting particulate and hydrocarbon emissions.
Piezo elements allow for much faster injection cycles than solenoids. For this application, the change from solenoid actuation has reduced the moving mass at the injector needle from sixteen to four grams: this allows for greater control over the quantities of fuel injected and the injection timing itself. Up to five injections are possible during each compression stroke, giving engineers greater control over the combustion process, which means increased thermal efficiency, lower emissions and improved refinement.
The crystal stack is smaller than the equivalent solenoid, enabling the injector to be smaller and lighter and more easily packaged into the fuel management system.
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