Engine test benches are easy to buy and hard to install. Choosing the brake, sizing the exhaust extraction, storing fuel legally, keeping the noise out of the lecture theatre next door, and why diagnostic benches answer a different syllabus.
An engine test bench is among the easiest items in an engineering catalogue to specify and the hardest to install. The machine arrives on a pallet, sits on the floor and runs. What it needs is a duct through an external wall, a fuel store that satisfies the fire authority, a room nobody has to teach next to, and make-up air. Departments that budget for the bench and not for those four end up with an expensive object started once a year for visitors.
The EDIBON test bench range runs from TBMC3 for engines up to 2.2 kW, through TBMC8 at 7.5 kW and TBMC12 at 11 kW, to TBMC75 for four cylinder engines up to 75 kW. The temptation is to buy at the top, because a real vehicle engine looks more convincing.
For a thermodynamics or applied thermal engineering course, it is the wrong instinct. The torque and power curves have the same shape at every size, the heat balance is the same calculation, and the specific fuel consumption plot teaches the same lesson. What does scale with engine size is everything the building has to absorb: fuel throughput, exhaust volume, noise, cooling load and heat rejected into the room.
The case for a larger bench is real but narrow. It applies where an automotive programme needs students working on an engine of the type they will meet in a workshop, or where research on fuels or injection strategies is planned. If neither is true, a smaller bench frees enough budget for the exhaust installation, the calorimeter and the gas analyser, which is a far better laboratory than a big engine with none of them.
This specification quietly determines the practical list, and it is easy to miss because both options are described as loading the engine.
An eddy current electromagnetic brake, used on TBMC8, TBMC12 and TBMC75, absorbs power and turns it into heat. It is simple, controllable through its variator, and it does exactly one thing: it takes energy out. It cannot turn the engine.
An asynchronous machine driven from a variable frequency drive, as on TBMC3, can absorb and also motor the engine. That second ability is not a luxury. Motoring an unfired engine is how a class measures friction power directly instead of inferring it, and it is how a cold engine is turned over for demonstration without the fuel system running. If the course teaches mechanical efficiency as a measured quantity rather than a difference between two other measurements, specify a brake that can motor.
Two further checks on any brake. First, how its heat is rejected: an eddy current brake dissipates the whole absorbed power, and if that goes into the room the air conditioning is fighting the experiment. Second, how the torque measurement is calibrated. There should be a documented way of hanging a known mass on a known arm and checking the reading, done at commissioning and then annually. A torque signal never checked against a mass and a length is a number from a supplier, not a measurement.
Engine exhaust contains carbon monoxide, which is colourless, odourless and kills people in enclosed rooms. Everything else here is engineering preference. This is not.
The extraction has to capture at the tailpipe, not at the ceiling: a flexible high temperature hose clamped to the tailpipe, running to a fan and out through an external wall, with a short flexible section to take engine movement and thermal expansion. Size the fan for the exhaust volume at temperature and for the duct's actual resistance, not for the room's air changes. Route the duct so condensate drains away from the engine, and put a drain point at the low part of the run, because a long duct in this climate collects water.
Where the duct discharges matters as much as that it does. Not next to a window, not next to a fresh air intake, not into the light well between two blocks, not at head height on a walkway. On a dense urban campus this often decides which room the laboratory goes in.
Two things complete it. Make-up air: if several hundred litres a second are leaving the room, the same has to enter, or the extraction underperforms, the door becomes hard to open and the room draws air from the corridor. Provide a transfer grille or a supply, sized deliberately. Fixed carbon monoxide detection in the room, at breathing height, with an audible alarm and, ideally, an interlock that stops the fuel supply. Where to mount detectors, and what should happen after the alarm sounds, is covered in designing a laboratory gas detection system. The general principle that capture at source beats dilution is the same one set out in fume hoods and local exhaust ventilation.
An engine laboratory stores flammable liquid on a campus, which puts it into a licensing and inspection conversation with the fire authority. Establish what applies to the quantity you intend to hold before the design is fixed, because the answer changes where the store goes and what it is built from.
Independent of the licensing position, the practice that keeps an engine laboratory out of trouble is small and dull.
An engine at full load in a hard walled concrete room is loud enough that a class cannot be taught over it, and the neighbours will complain within a week. The guard enclosure on a bench such as TBMC75 helps with mechanical noise. It does nothing about the exhaust, which now leaves the building through a duct that is an excellent loudspeaker.
Three measures, in order of value for money. Put a silencer in the extract duct rather than relying on the engine's own. Treat the ceiling or upper walls with absorption, because a reverberant room doubles the problem for free. And site the laboratory where the noise is tolerable: not sharing a wall with a lecture theatre or a research office, and preferably not directly under one, since structure borne noise travels through a concrete frame better than airborne noise travels through a wall.
There is a pleasing circularity available here. The EDIBON TDRC noise control demonstration unit measures the insertion loss of barriers, enclosures and absorbing linings across a frequency range, and TIAC measures the sound absorption coefficient of samples in an impedance tube. Letting a final year group measure and specify the treatment for their own engine laboratory is a better project than most, and it produces a room that works.
The remaining installation items, none of which are difficult and all of which are cheaper before the room is finished.
There are two families here and departments routinely try to buy one item that serves both.
A test bench with instrumentation exists to produce characteristic curves and an energy balance. It gets much more useful with two companions: TBMC-CG, an exhaust gas calorimeter that measures the heat carried away in the exhaust so the energy balance closes properly rather than ending in a large unaccounted term, and TBMC-AGE, a non dispersive infrared exhaust gas analyser that turns combustion quality into a measured concentration. Without those two, the practical stops at torque and power, which is about half of what the syllabus asks for.
A diagnostic bench exists to teach fault finding. TBM/D lays out the intake, exhaust, fuel and cooling systems of a diesel engine for study and diagnosis. TBM/G does the same for a petrol engine with fault simulation and an exhaust system that replicates a real vehicle's, so emissions can be read against introduced faults. TBM/H covers a hybrid drivetrain, with an eddy current system recreating driving loads. These are the right purchase for an automotive or vocational programme and the wrong one for a thermodynamics course.
One more unit deserves a serious look here. TDEGC is a diesel generator set with control, signalling and monitoring panels and a digital synchronoscope for synchronising with the mains, logged through SCADA. Given how much of this country's industry runs on standby generation, a bench that teaches synchronising and load sharing has an unusually direct route to employment, and it links the engine laboratory to the electrical machines and power systems laboratory.