Locomotive and marine engineers are quite familiar with electric propulsion motors and the diesel–electric powertrain, a configuration now referred to, in automotive engineering, as a series hybrid. Electric and hybrid cars have been designed from the first decade of the twentieth century but now, 100 years later, the environmental pressures applied by government through taxation regimes and the necessary reduction in vehicle use of hydrocarbon fuels are forcing the pace of development of hybrid and electrically propelled vehicles and are requiring facilities capable of testing them. As a reminder, the three major hybrid configurations are:
l Series, in which the vehicle is propelled only by one or more electrical motors and the internal combustion engine acts as a generator to supply power to those motors and to charge the vehicle batteries.
l Parallel, in which both an internal combustion engine and an electric trac- tion motor drive the vehicle in a proportion depending on driver demand and vehicle status (battery charge level, etc.).
Chapter | 4 Powertrain Test Facility Design and Construction 73
l Combined series/parallel, in which either the internal combustion engine or the electric traction motor can be decoupled from propelling the vehicle, again depending on driver demand and vehicle status.
The testing of the non-automotive diesel generator, using a load-bank and propulsion motors using electrical dynamometers, is a mature technology both as individual units and as systems. However, in their automotive form, hybrid vehicles provide a number of new challenges that are largely concerned with testing the sum of the parts, directly or by simulation through HiL.
Batteries and Battery Simulators
The most significant development work that lies at the heart of the future acceptance of hybrid vehicles is that of the battery technology required in most automotive hybrid configurations. This development work is outside the remit of this book and the lives of most automotive test engineers, but for hybrid powertrain testing it is vital to have an electrical power source that simulates the dynamic performance of different battery types and sizes. The voltage range of hybrid vehicle batteries currently varies from 280 to 440 VDC. The devel- opment and increased use of “super-capacitors”, whose ability to charge much faster than batteries makes them particularly suitable for energy recovery through regenerative braking, also require simulation in individual and combined battery–capacitor configurations.
Investment in the latest generation of fully programmable, multiconfiguration, powerful battery–capacitor simulators is increasingly becoming essential for any test facility involved in hybrid powertrain work.
These devices currently take the form of large electrical cabinets having sizes typically of 2200 mm high, 800 mm deep, and between 800 and 3000 mm wide. The typical weight, when fully connected, will be up to 1000 kg;
therefore, adding these devices into an existing cell or service space requires careful planning and an operational risk assessment.
Most hybrid cars use three-phase AC electric motors rated at up to 380 VAC controlled by IBGT control technology similar to that used in AC dynamometer control (Chapter 10).
The similarities may be instructive since both systems can be vulnerable to problems of torsional vibration in the mechanical connection of their engine and motor/generator, and both can be emitters of electromagnetic radiation in the RF bands.
For the automotive test facility there are a vast number of different test tasks and major design changes specific to hybrid configurations that have to be handled. Eight obvious examples are:
1. The development and refinement of model-based methods of testing elec- tronic powertrain control systems.
2. Developing cell test-stand layouts to accommodate UUT that are of a signif- icantly different shape than a single IC engine.
3. Detailed system calibration tasks such as optimizing performance, range, and emissions of the powertrain over the full climatic range.
4. Development and emission optimization of the downsized IC engines running operational cycles required to power and/or charge vehicle batteries over existing and developing urban and highway drive cycles.
5. Drivability is already a significant issue and hybrids provide new problems to solve, such as the “blending” of regenerative braking loads with that of vehicle brakes over the full range of driver force inputs and vehicle speeds.
6. Transmission testing is seeing changes in the type of units tested, which include the operation of the epicyclic units required in the power-splitting units of parallel hybrids.
7. Safety issues concerning the restraint of heavy battery packs in crashes, to the fire safety of high-energy storage systems; all the new designs will have to be examined and tested.
8. The minimization and suppression of electromagnetic emissions from the hybrid power systems provide EMC test facilities with a new generation of testing tasks.
Gearbox and Transmission Test Rigs
The majority of automotive gearbox designs, both manual and automatic, and final drive units have for the last 30 years been developments and refinements of existing units with really novel designs limited to motor-sport or off-road vehicles. Now, in addition to power-splitting hybrid transmissions, the control and actuation systems are undergoing considerable development. The wider use of dual-clutch transmissions (DCT), and the electrical actuation of clutch and gear selection, are requiring new rigs and test routines. Transmission test cells have had to evolve to support this change to highly integrated powertrains and are therefore having to use HiL techniques in order to test the drivability of the transducer and actuator functions.
Meanwhile there continue to be a range of purely gearbox and final drive test rigs that have to fulfill tasks in a number of rig configurations, including:
l Two-wheel drive (three motors) development and endurance rigs
l Four-wheel drive (five motors) development and endurance rigs
l Tilting lubrication rigs
l Gearbox NVH fully and semi-anechoic rig (Chapter 18)
l Durability rigs for both gear-form and gear-shift systems
l Component test rigs, synchronizer, etc.
l Clutch operation loading.
A common factor shared by the test facilities listed above is that they do not have to support IC engine running, so have lower thermal loads and less Chapter | 4 Powertrain Test Facility Design and Construction 75
complex safety interlocking. However, many of the other comments in this book concerning the operators’ work environment, cell layout, rigging, test control, and data acquisition remain the same. Transmission rigs that are designed to run IC engines are classified as full powertrain facilities and are discussed below.
Full Powertrain Test Rigs
Powertrain test rigs can now be built to be capable of using either the engine as the prime mover, or an electric motor simulating the engine. The evolution of such cells has been made possible by the comparatively recent development of permanent magnet motors (PMMs) and their associated controls in the auto- motive power ranges. These units are capable of engine simulation including that of most driveline dynamics and combustion pulses.
The same motor technology has produced dynamometers having low inertia yet capable of absorbing high torque at low speeds, thus providing road wheel load simulation that, with customized controllers, includes tire-stiffness and wheel-slip simulation.
An important logistical justification for such “all-electric” powertrain cells, besides not having to install and maintain all the cell services required by running an IC engine, is that the required engine may not be available at the time of the transmission test.
One cost-effective arrangement that suffers from similar logistical problems of unit availability but which overcomes several rig design problems is shown inFigure 4.12, where a complete (dummy or modified production) vehicle is mounted within either a two-wheel or “four-square” powertrain test rig.
It should be noted that, in spite of advances in motor and drive technology, flywheels still have a valuable part to play in transmission and powertrain testing and are often fitted to the free end of “wheel” dynamometers, a position that allows various flywheel masses to be fitted according to the demands of the test and UUT (see Chapter 11 for a discussion of flywheels).
Powertrain rigs have to be designed to be able to take up different config- urations on a large bedplate, as required by the UUT layout. A large tee-slotted test floor, made up of sections of cast-iron bedplates bolted together and mounted on “air springs” (see Chapter 9), is the usual way to enable the various drive-motor or dynamometer units to be moved and aligned in typical power- train configurations. However, a cheaper alternative for multiconfiguration transmission test rigs, which usually experience lower vibration levels than engine rigs, is shown inFigure 4.13, where steel slideways set into the concrete floor allow relative movement, albeit restricted, of the major dynamometer frames.
To allow fast transition times the various test units should be pallet mounted in a system that presents a common height and alignment to the cell interface points.
Inclined Engine Test Beds
For the simulation of special operating conditions experienced in “off-the- road” vehicles and race cars under high lateral “g” forces, there is a requirement
FIGURE 4.12 A “wheel dynamometer” system installed within a garage workshop space having a simple exhaust gas extract system and being used to carry OBD tests. A large cooling fan would be needed in front of the car radiator when running under power.(Photo courtesy of Rototest Ltd.)
FIGURE 4.13 The five-axis transmission test rig fitted with an electric motor as the engine simulator and a gear-change robot.
Chapter | 4 Powertrain Test Facility Design and Construction 77
for test beds capable of handling engines and gearboxes running with the crankshaft centerline inclined to the horizontal. These tests not only affect inclined oil levels in the sump but have transient effects on oil pump “pickup”
and in turbochargers.
Electric motor-based dynamometers are easily adapted to inclined running, as are eddy-current dynamometers with closed-circuit cooling systems.
However, such arrangements present problems with the high-powered hydraulic dynamometers required in aero-engine rigs having open water outlet connections. Some rigs have been built where the whole engine and dyna- mometer bedplate is mounted upon a system of hydraulic actuators allowing for dynamic movement in three planes while the engine is running. These high- value rigs present high operational complexity and require both a large foot- print and vertical space.
For engines with a vertical crankshaft, e.g. outboard boat engines tested without dummy transmissions, the electrical dynamometer is the obvious choice and may generally be used without modification, although dry gap, eddy current, machines have also been used.
Special arrangements need to be made for torque calibration.
Automotive Engine Production Test Cells (Hot Test)
These cells are highly specialized installations forming part of an automation system lying outside the scope of this book. The objective is to check, in the minimum possible process time, that the engine is complete and runs. Typical
“floor-to-floor” times for small automotive engines range between 5 and 8 minutes.
The whole proceduredengine handling, rigging, clamping, filling, starting, draining, and the actual test sequencedis highly automated, with interventions, if any, by the operator limited to dealing with fault identification. Leak detection may be difficult in the confines of a hot-test stand, so it is often carried out at a special (black-light) station following test while the engine is still warm.
The test cell is designed to read from identity codes on the engine, recognize variants, and to adjust the pass or fail criteria accordingly.
Amongst the measurements made during a production test, two vital build integrity checks are carried out by checking cranking torque and time taken for oil pressure to reach normal level.
Even in the, increasingly rare, cases of gasoline engines being subjected to an end-of-line (EOL) hot test, they are no longer loaded by any form of dynamometer.
During the EOL test of diesel engines, loaded test sequences are still the standard practice, although 100% cold test and some percentage of engines hot tested is becoming more common for small automotive diesels.
Automotive Engine Production Cold-Test Stations
Along with pressure and rotational testing “in process” of subassemblies, cold testing is increasingly being applied to (near) completely built engines. It has considerable cost and operational advantages over hot testing, which requires an engine to be fully built and dressed for running in a test cell with all sup- porting services. Cold-test areas also have the cost advantage of being built and run without any significant enclosure other than safety guarding and can form an integral section of the engine production line.
Cold-test sequences are of short duration and some potential faults are easier to spot than in hot testing when “the bang gets in the way”.
The principal technique of cold testing is that the engine is spun at lower than normal running speed, typically 50–200 rpm, by an electric motor with an in-line torque transducer.
Early in the test sequence, the “torque to turn” and rate of oil pressure rise figures are used to check for gross assembly errors. The engine wiring loom is usually connected through the ECU connector to a slave unit that is pro- grammed to check the presence, connection, and correct operation of the engine’s transducers. Thereafter, vibration and noise patterns are recorded and compared to a developing standard model.
This is a highly automated process that uses advanced computer models and pattern recognition technology. The maximum long-term benefit of cold testing is derived by feeding back field service data to refine the “pass–fail” algorithms that pick out production faults at their incipient stage.
End-of-Line (EOL) Test Station Facility Layout
In the design phase of either type of production EOL test facility, a number of fundamental decisions have to be made, including:
l Layout, e.g. in-line, branch line, conveyor loop with workstations, carousel, etc.
l What remedial work, if any, is to be carried out on the test stand
l Processing of engines requiring minor rectification
l Processing or scrapping and parts recycling of engines requiring major rectification
l Engine handling system, e.g. bench height, conveyor and pallets, “J” hook conveyor, automated guided vehicle
l Engines rigged and de-rigged at test stand or remotely
l Storage and recycling of rigging items
l Test-stand maintenance facilities and system fault detection
l Measurements to be made, handling and storage of data.
Production testing, hot or cold, imposes heavy wear and tear on engine rigging components, which need constant monitoring and spares to be available. The use of vehicle standard plug or socket components on the rig side has proved to Chapter | 4 Powertrain Test Facility Design and Construction 79
be totally unsatisfactory; these components have to be significantly toughened versions to survive in the EOL environment.
Automatic shaft docking systems may represent a particularly difficult design problem where multiple engine types are tested, and where faulty engines are cranked or run for periods leading to unusual torsional vibration and torque reversals.
Shaft docking splines need adequate tooth lubrication to be maintained and, like any automatic docking item, can become a maintenance liability if one damaged component is allowed to travel round the system, causing conse- quential damage to mating parts.
Modular construction and the policy of holding spares of key subassemblies will allow repairs to be carried out quickly by replacement of complete units, thus minimizing production downtime.
Large and Medium-Speed Diesel Engine Test Areas
As the physical size of engines increases, the logistics of handling them becomes more significant; therefore, the test area for medium-speed diesel engines is, more often than not, located within the production plant close to the final build area.
Above a certain size, engines are tested within an open shop in the position in which they have been finally assembled. The dynamometers designed to test engines in ranges about 20 MW and above are small in comparison to the prime mover; therefore, the test equipment is brought to the engine rather than the more usual arrangement where the engine is taken to a cell.
Cells for testing medium-speed diesels require access platforms along the sides of the engine to enable rigging of the engine and inspection of the top-mounted equipment, including turbochargers, during test. There is a design temptation to install services under these platforms but these spaces can be difficult and unpleasant to access; therefore, the maintenance items such as control valves should, where possible, be wall or boom mounted.
Rig items can be heavy and unwieldy; indeed, the rigging of engines of this size is a considerable design exercise in itself. The best technique is to pre-rig engines of differing configurations in such a way that they present a common interface when put in the cell. This allows the cell to be designed with permanently installed semi-automated or power-assisted devices to connect exhausts, intercooler, and engine coolant piping. The storage of rig adaptors will need careful layout in the rig/de-rig area.
Shaft connection is usually manual, with some form of assisted shaft lift and location system.
Special consideration should be given in these types of test areas to the draining, retention, and disposal of liquid spills or wash-down fluids.
Large engine testing is always of a duration exceeding the normal working day, therefore running at night or weekends is common and may lead to complaints of exhaust noise or smoke from residential areas nearby. Each cell or test area will have an exhaust system dedicated to a single engine; traditionally and successfully the silencers have been of massive construction built from the ground at the rear of the cells.
Modern versions may be fitted with smoke dilution cowls and require well-maintained condensate and rain drains to prevent accelerated corrosion.