A fully kitted Toyota Land Cruiser 79 was strapped down on the rolling-road dynamometer at Koos Swanepoel Developments (KSD) in Cape Town. Mr Swanepoel was behind the wheel to see if it had the power to back up its macho appearance. Fourth gear, foot flat. The engine roared as the speed increased although the vehicle was going nowhere; the torque was absorbed by the retarder connected to the dyno’s rollers. The power and torque figures eventually flashed up on the screen: 194 kW and 617 N.m. This particular Land Cruiser had clearly spent some time in the gym. Whether you’re developing a new engine, adding some extra oomph to your vehicle, or prepping a racecar, a dynamometer is invaluable.
Why a dynamometer?
To optimise an internal combustion engine over its speed and load range, these need to be fixed to adjust the various engine variables at each point. This may include spark timing, air-fuel ratio and boost pressure in a turbocharged petrol engine, for example. During an engine’s development, countless hours are spent on an engine dynamometer to meet all the necessary performance and emissions targets of the programme.
It requires a large matrix of data points and on a dyno, engine speed can be increased by as little as 100 r/min at a time, from idle to redline, with torque increasing in steps of 5%, for example.
This would be impossible driving on a public road or test track for that matter as there are too many external variables. A dynamometer can provide precise resistance to the powertrain while the vehicle is static in a controlled environment.
Engine dynos are generally employed by OEMs and provide the most accurate measurement for engine performance as torque is measured at the output shaft (crankshaft) of the engine. However, it is impractical for aftermarket tuners to remove an engine from a vehicle before installing it onto an engine dyno; hence, the move to a rolling-road dynamometer that tests the torque output at the driven wheels of a vehicle. But more about that later…
Think of a dyno as a huge brake (also the reason for the term brake horsepower or bhp). When connected to the powertrain, it brakes the rotational motion of either the output shaft of the engine on an engine dyno, or the driven wheels on a rolling-road dyno. This causes a torque reaction that opposes the rotational motion.
On an engine dyno, a load cell is fixed at a known distance to the centre of the output shaft measuring the reaction force from the brake opposing the rotational motion. When this force (N) is multiplied by the distance (m) to the centre of the shaft, the N.m produced is calculated. Power is not measured but calculated, as it is related to the engine speed and torque by the following formula:
P= power (W)
N= rotational speed (r/min)
π= the constant Pi
T= torque (N.m)
An engine dyno is calibrated with weights positioned at a specific distance from the shaft centre on a calibration arm, measuring the force on the load cell. The dyno-brake technology can vary between actual brakes, torque-converter type water brakes, eddy- current electrical brakes (retarders) or even an electrical motor providing resistance torque. With the latter, the engine can be controlled to replicate certain drive cycles such as driving down an incline.
Remember, the dyno brake must absorb the power from the engine to keep the engine speed constant at any particular load. This results in plenty of heat build-up which is dissipated either by water or air cooling. A tricky task when testing engines upwards of 750 kW!
The rolling-road dyno
A company like KSD tests at least five cars per day. It would be impossible to use an engine dynamometer. In the past, a rolling-road dyno made use of two rollers in contact with the driven wheels but today, a single, large-diameter drum is preferred as it is more efficient. The grip force between the tyre and the drum is a limiting factor when testing powerful cars. Employing a higher gear during testing lessens the torque output but increases the speed (power stays constant).
The KSD dyno has four-wheel-drive capability with a roller at each axle. The roller distance can be adjusted to cater for wheelbases of different vehicles. The vehicle is securely strapped down, the exhaust pipe is connected to the extraction unit and a powerful fan is placed in front of the vehicle’s radiator to help with cooling.
According to Swanepoel, adequate ventilation is crucial to remove the spent exhaust gases and heat. Engine calibration can happen only under stable and controlled environmental conditions.
Manufacturers’ claimed power and torque figures are measured at the crankshaft of the engine. In reality, it is the power and torque at the wheels that provide the motive force and are measured by a rolling-road dyno. The problem is that there are many powertrain components sapping energy before the torque reaches the wheels; such as the transmission, shafts, bearings and differential. Therefore, the power at the wheels will always be less than the claimed figure (see Power loss table). By conducting a coast-down test from high speed, it is possible to estimate these losses and add it to the figure measured to get an idea of the true performance figures at the crankshaft.
Swanepoel explains that even though modern dynos automatically compensate for atmospheric conditions such as air temperature and pressure, a rolling-road dyno is best used as a comparative tool rather than a device to measure absolute figures. For example, a change to a free-flow exhaust should be validated on the same dyno (on the same day, if possible) to show any comparative gains. Using a different dyno is bound to create confusion as other correction factors may be employed (also see Can you trust the figures?).
The ability to measure the powertrain performance of a vehicle independent of the power source. We predict that in the future even electric vehicles will eventually put their wheels on a rolling-road dyno albeit without the thundering soundtrack.
The dyno trailer
During vehicle development, a dyno trailer may be employed to control the exact load on the powertrain during real driving conditions. This is needed when it comes to testing the capability of the vehicle’s cooling system in real-world conditions. To provide a constant load, a dyno trailer is used to provide resistance by braking the wheels of the trailer usually via an electrical retarder.
To measure just the maximum power and torque outputs, an inertial dyno can be used. If a large, heavy-diameter drum with known inertia is used, the rotational acceleration under full load can be used to calculate the torque (see equation below) and power. A typical dyno run would last a couple of seconds (power dependent) as the engine speed increases to maximum while the dyno drum speeds up. This type of data is not suited for engine calibration as you cannot keep the engine speed constant but it is great for power shootouts at car or motorcycle meets.
T = torque (N.m)
I = inertia (kgm2)
ω = angular acceleration (rad/s2)
The drivetrain layout influences how much power is lost between the engine and wheels. The table below shows typical values and this explains why a front-wheel-drive vehicle with less power may be faster in-gear (where grip is no issue) than a more powerful all-wheel-drive version.
|Front-wheel drive||Rear-wheel drive||All-wheel drive|
|10-19 kW||20-29 kW||30-48 kW|