The AC dynamometer uses a 100kW David McLure 155 Amp/415 Volt AC motor capable of very fast transition between absorbing power from the engine and motoring the engine. This allows real-world transients to be followed accurately by the dynamometer. This was previously not possible with the DC dynamometer due to the higher inertia of the DC generator. The old Heenan and Froude water brake dynamometer does not even come close to doing a quick, controlled transient.

picture of AC electric motor of the dyno setup

picture of AC motor attached to Zetec engine. The 3 fast response analyser heads can be seen in the foreground on the right.

picture of control panel of AC dyno

The AC dynamometer is connected to a standalone computer that controls the speed and torque of the AC generator. The software that controls the dynamometer can reproduce any drive cycle that is driven with the suitably instrumented car (a Ford Mondeo for this work). The sensors that are recorded on the in-car data logger while driving are clutch position, throttle position, road speed and engine speed. This data is then reproduced on a nominally similar engine attached to the AC dynamometer, thus subjecting the engine to the same power conditions it was experiencing on the road. The only difference between the two engines would be the temperatures due to the cooling effect of a moving vehicle. Another small discrepancy is the existence of a power steering pump on the car, but not on the dynamometer engine.

picture of the FOrd Zetec 1.8-16V engine attached to the AC dyno

To be able to simulate very cold starting conditions as required for Euro 3 legislation and future US legislation, the engine in the engine bay can be enclosed by thermally insulated panels and cooled to a temperature of up to –10 ° C. This allows research into the effects of cold starts on emissions with much higher consistency than before. Previously, the only way to conduct cold start tests at the University of Leeds was by leaving the car overnight to soak during a cold day in winter. Even then the lowest temperature possible was only –2 or –3 ° C.

To measure fuel flow rate to the engine on the dynamometer, a fuel flow meter is used. This is a gravimetric fuel gauge made by Phoenix Tribology (formerly Plint and partners). The model TE14 has a metering unit that makes use of the principle of buoyancy. A flotation cylinder is suspended from a strain gauge transducer. The cylinder is enclosed in a cylindrical vessel, all surfaces being accurately machined .A change in level of the liquid in the vessel will result in a propionate change in the flotation force exerted on the cylinder. The instrument can operate in two modes; it can either measure the time to consume a selected mass, or it can measure the mass consumed in a selected time. The minimum mass it can measure is 50 grams so it is not possible to obtain real-time readings of fuel flow.

picture of refrigeration unit used to cool the engine enclosure, fuel and coolant to sub zero temperatures

picture of laminar flow element used to measure volume of inlet air to engine

To measure the mass airflow rate of the air consumed by the engine on the test-bed, a second hotwire flow meter is attached upstream of the original Ford airflow meter. The flow restriction caused by this device was found to be acceptable and so it was left in place permanently. The output of the airflow meter is displayed directly on the standalone computer as volumetric airflow, but it also automatically calculates the mass airflow based on input from an inlet temperature thermocouple and an ambient pressure reading.

The AC dyno has two modes of operation: it can either maintain constant load or constant speed. To maintain torque then it needs feedback on the real-time torque being exerted on the engine. To this end a torque transducer is installed inline with the flange coupling the engine to the AC motor. It consists essentially of a flanged torque shaft fitted with strain gauges. The antenna segments are there to recieve the signal transmission which is by short wave radio.

picture of torque transducer

 

The engine is fitted with an additional ‘wide-band' lambda sensor downstream of the catalyst. This sensor measures the actual real-time air-fuel ratio and displays it on the standalone computer. The testbed is also fitted with various thermocouples as listed below.

 

List of variables displayed on dynamometer control screen

Time

Fuel rate

Engine speed

Mass target

Recorded engine speed

Measured time

Volumetric airflow

Live mass

Engine torque

T1 air inlet temperature

Recorded road speed

T4 oil temperature

Mass airflow

T7 exhaust manifold temperature

Power

T10 exhaust temperature d/s of cat

Gear

T2 water in temperature

Throttle position

T5 fuel tank temperature

Torque calculated

T8 Exhaust temperature u/s of cat

Recorded clutch position

T3 water out temperature

Coriolis gauge (fuel flow)

T6 fuel engine inlet temperature

Measured fuel mass

T9 exhaust temperature mid cat