SPE machine helps OEMs test fuel pumps
A novel machine measures the key parameters of automotive diesel fuel pumps and injection systems.
In the common rail fuel injection system of a diesel engine, a high-pressure pump is utilised to pressurise fuel in a rail, or accumulator, that is independent of the engine speed and the quantity of fuel injected. The fuel is then fed through pipes to the injectors, which inject the fuel in a fine spray into the combustion chambers.
The pump and the rail are critical components that must be extensively validated by a manufacturer to determine how their performance might affect the operational characteristics of the engine – before they are manufactured in production quantities.
Today, such accelerated life testing has become more and more important as automotive designers rush to deploy ’greener’ technologies.
One such technology – the stop-start system – automatically shuts down and restarts an engine to reduce the amount of time the engine spends idling, improving fuel economy and reducing emissions.
This, however, places even greater demands on the diesel fuel pump and rail itself and on the equipment needed to test it.
To meet these demands,engineers at Special Purpose Equipment (SPE), based in Rochester, Kent, recently developed a hybrid test machine called the S400 that enables automotive original equipment manufacturers (OEMs) to perform accelerated life testing on a variety of diesel fuel pumps and fuel injection systems.
While developing the machine, the SPE engineering team was well aware that it would need to meet an exacting set of operational requirements.
One of these was that the system would be required to handle pump speeds of up to 7,000rev/min, mimicking the actual speed that it would operate in a vehicle.
The accuracy with which the speed was controlled was considered crucial too – the test system had to deliver a speed holding of +/-1rev/min, even at its top speed.
However, the biggest design challenge was to ensure that the performance of the pump could be measured accurately as it was put through the rigours of the stop-start cycle over hours of testing. ’Previously, the automotive OEM had been unable to perform such testing and the new test requirement meant that we had to develop a means to rapidly ramp the pump speed up and down very quickly and highly accurately,’ said Chris Baker, technical director of SPE.
To do so, Baker and his engineering colleagues deployed an asynchronous 35kW servo motor controlled by a 45kW drive that works in cohort with a fully regenerative braking system to simulate the functionality of the drive shaft on the vehicle that drives the pump.
The use of the regenerative braking system enabled the design team to ensure that the S400 machine could stop a pump rotating at 10,000rev/min in less than a second by capturing its inertial energy as it decelerated and converting that energy back to power that could be fed back into the machine’s power supply.
The solution solved the inherent disadvantages of earlier machines that had used a flywheel to enable diesel pumps to be driven at a constant rate through each revolution.
’While such mechanical systems did overcome the difficulties caused by the inertia of the cams inside the pump, the inertial mass of the flywheel itself meant that they were unable to achieve the acceleration or deceleration times that were now being demanded by the automotive OEM,’ said Baker.
Being unable to decelerate quickly also meant that, should any pump failure such as a seizure occur, the older flywheel-based machines were unable to stop before the pump itself was destroyed. Hence, the OEM had no way to investigate any potential causes of failure after such an event. With the S400 machine, however, there are no such difficulties.
’By comparing the digital output from an encoder on the back of the motor with the output of an encoder on the front of the pump, the new machine is able to quickly detect when a pump might be starting to fail. If it is, then a slip coupling on the front of the machine can disengage the pump from its load and the machine can be brought to a halt so that the pump can be removed and examined in detail,’ said Baker.
As the system exorcises the pump through different cycles, a sophisticated acquisition system collects data from sensors around the machine.
Fluid flow, fluid pressure, the speed of the motor and torque are the key parameters that are captured to enable the machine to then determine the performance characteristics of the pump and the associated rail.
Fluid flow rate data into and out of the pump, and out of the rail, is collected by the system, which also measures the inlet pressure to the pump, the return pressure from the pump and the pressure in the fuel rail, which provides an indication of the pressure generated from the pump.
Furthermore, since the temperature of the pressurised fluid is a critical factor that can influence the dynamics of the system, the system was also required to measure the temperature of the fluid from the inlet return of the pump too, as well as the temperature of the rail itself.
’To accommodate a range of fuels – including newer bio-fuels that might be used with such fuel injection systems – we were also asked to ensure that the temperature of the fluid being pumped could be maintained between 20°C and 120°C +/-1°C accuracy – a tricky task as many of the newer fuels have a flash point at 80°C,’ said Baker.
Once the data has been collected, the system software then correlates the flow, pressure, temperature, speed and torque measurements, allowing test engineers at the automotive OEM to determine how such key system parameters influence one another as the system is put through its paces.
From this, they can determine the pumping efficiency of the system and investigate the cause of any losses that might be occurring over extended periods of time.
Since its launch, the S400 machine has been a resounding success, and SPE has shipped 12 of them to its automotive customer, which is now using them across a number of different sites in Europe to validate a variety of diesel fuel injection systems for cars and trucks.
More recently, Baker and his engineering team have modified the front of the machine so that it can be automatically fed with parts from a conveyor system. While the earlier machines were simply aimed at validating injection systems, the newer version can be used in a full-scale production test environment – a market that the company believes will be equally important for the system.
What is more, Baker said that the system architecture of the machine is flexible enough that it could be used for any applications where any rotary equipment needs to be tested.
According to Baker, one potential future use might be to simulate the dynamic loads experienced by electric motors, which might be used on future electric vehicles.
The key facts to take away from this article
» The machine needed to mimic the speed it would operate in a vehicle
» It tests fluid flow, fluid pressure, motor speed and torque
» Test engineers can then determine the system’s pumping efficiency
» The system architecture of the hybrid machine is said to be very flexible