Piezoelectric Injectors

While many of us are familiar with the use of piezoelectric materials in the automotive industry in the past few years, a new twist has been added and is making its way to our shop floors.

As we are aware, piezoelectric material will produce an electrical charge when compressed or distorted. This property is used in the knock sensors to monitor for detonation. However, this effect can be reversed. If an electrical charge is applied to the material, it will produce a rapid expansion of its physical geometry. This property is now being used to very quickly and very accurately fire fuel injectors. The expansion can be precisely repeated much better than using electro-magnetic solenoid style injectors. This allows for better fuel metering and thus lower fuel consumption and emissions.

Currently, this injection process is seen mostly in diesel applications but, it is also finding its way into the gasoline engine as well–where it is used in place of multiple aperture injectors on direct injection engines. The centre mounted injector can open, unseat the needle and direct a precise amount of fuel towards the spark plug in short easy-to-ignite durations of 0.0002 seconds–which can be further tailored to adjust the density of the charge. This allows the use of flat-top-style pistons rather than relying on wall-guided pistons which deflect the injected fuel back up at the spark plug.

The use of this style of injector in diesel engines has allowed engineers to improve power output, fuel economy, and emission levels while reducing noise levels. Due to the exceptionally quick ON / OFF action, the spray time and amount of fuel injected can be rapidly adjusted for any requirement. Being more than twice as fast as solenoid injectors, the injection process can be divided up into five to seven injection events per combustion cycle. The injectors can generate a considerably finer atomized fuel spray at injection pressures greater than 23,000 psi.

The injectors are made up of four primary sections: the piezoelectric actuator, the hydraulic coupler, the control valve and the injector nozzle.

Located in the top portion of the injector body is the piezoelectric actuator, which includes a piezoelectric stack, a pushing pin and the electrode wires. The piezoelectric stack is where the “magic” happens. There are several hundred layers of piezoelectric material stacked together and, when an electrical charge is applied through the electrode, the material momentarily changes (atoms are mutually displaced) producing a change in the length of the stack by 0.04mm (0.0015.). This change in size extends the pushing pin to apply force to the upper piston of the hydraulic coupler.

The hydraulic coupler increases the stroke of the piezoelectric actuator by using two pistons of different diameter separated by a chamber of fuel. The lower piston has a smaller diameter and is moved hydraulically by the fuel–allowing it to travel further. The fuel helps to compensate for length differences of the individual parts due to thermal expansion rates. A spring bushing is used to apply a preload to the piezoelectric actuator so tensile forces do not build up. Piezo-material can produce a high pushing force but cannot withstand large pulling forces

The control valve directs and controls fuel flow which hydraulically opens and closes the injector nozzle. When injection is required, the control valve blocks a bypass passage, which creates an internal pressure difference within the injector nozzle body allowing the high pressure fuel to raise the needle and fuel to be injected. Injection quantity and duration are determined by the time the ECM keeps the actuator energized.

The injector nozzle generates a fine mist of atomized fuel directly into the combustion chamber. The nozzle tip will have a number of spray holes (varies depending on manufacturer and application) with a diameter of approximately 0.131mm (0.005). These are formed by electrical discharge machining.

The inline design of the injector allows for minimal fuel return flow which decreases the energy and delivery rate required from the high pressure fuel pump. Also, due to the precise repeatable motion of the piezo-material, the needle position can be measured since its position in time correlates directly to fuel flow rate. This then also allows the possibility of eliminating other control sensors used by the managing ECU.

Like many electro-magnetic injectors, these injectors are coded at the time of production; recording the variations which occur in the manufacturing process and in the piezoelectric actuator travel from its target value. This information must be programmed into the controlling module so it can make the appropriate adjustments in activation time to ensure the best performance.

As tighter fuel consumption and emission restrictions are placed upon the industry, expect to see more of this style of injector being used in both gas and diesel engines. While there is a cost penalty for manufacturing them it is almost completely balanced out currently by the improved fuel mileage and performance.

For more information on automotive technology visit CARS OnDemand training at: www.cars-council.ca.