A dynamometer (dyno) is a machine that measures power output of a machine or device.  There are many different types of dynos, but this article will focus on ones specifically for the automotive industry.  In the automotive industry, there are chassis (or wheel) dynos and engine dynos.  The two types of dynos fall into one of two specific categories, inertia type and load types.

The chassis or wheel type inertia dynos are ones where an assembled, running vehicle is driven onto the dyno and the drive wheels are stopped atop some type of roller system.  The vehicle is secured in place by mechanical stops and/or chains, straps & cables to prevent any motion of the vehicle.  The vehicle’s engine is connected to the dyno by whatever means the dyno utilizes to sense engine speed measured in revolutions per minute (rpm).  This may be accomplished electrically, optically, mechanically, by vibration measurements or by sensing sound waves.  There may be other methods not mentioned.  When the dyno can sense the engine speed, the vehicle is placed into gear and the roller is turned by the wheel(s) of the vehicle.  The vehicle accelerates the roller up into some gear determined by experience, the size dyno and power output of the vehicle and held at an rpm that is to be the lowest engine speed of the dyno run or “pull”.  The operator hits start, then the vehicle is accelerated at wide open throttle to the maximum engine speed desired.  During this time, as the roller accelerates, the computer is measuring and recording the speed of the engine and of the roller.  The computer then calculates the horsepower and torque by the following methods discussed.

The computer measures all data very precisely.  The roller’s speed is measured continuously and plotted against engine rpm.  The roller’s acceleration is determined by comparing two roller speeds and the time interval between them.  The common engineering terms of expressing the roller speed, Greek letter ω (omega), is in radians per second.  There are 2 PI radians in one revolution (approximately 6.2832 units).  When the computer measures two roller speeds it subtracts the earlier one from the later one and divides by the time elapsed between them.  This gives the difference in speed per unit of time.  This is an acceleration value (positive or negative, depending on whether it is speed up or slowing down).  The math is as follows: later speed (ω₂ in radians per second or rad/s) minus earlier speed (ω₁ in radians per second or rad/s) divided by the time (t in seconds or s) equals acceleration, Greek letter α (alpha),  (in rad/s/s or rad/s²) or (ω₂ – ω₁)/t = α.  Once it calculates the acceleration, the computer then calculates the torque of the roller by the formula T (torque) equals I (inertia) times α (acceleration) or T=I*α.  This provides the torque input into the roller.  However, due to tire size/roller diameter differences & gearing between the engine and the roller, this torque number is not equal to engine torque, so the computer then takes the torque input into the roller and calculates the torque of the engine by ratio.  From engine torque, horsepower can be calculated from the common horsepower formula where horsepower equals torque times rpm divided by 5,252 (HP = T * rpm/5,252). 

The chassis or wheel type load-type dynos are very similar to the inertia types except some type of load is placed onto the roller shaft system.  This load may be in the form of an electrical eddy brake, water brake, clutches, brakes or some other type of load.  This enables a dyno operator to add a load to the dyno to either slow the run down or hold the vehicle at a constant (or near constant) rpm with sufficient load.  This added load is measured and mathematically added to the results obtained by the computer and included into the results.  Some advantages to a load type dyno are the ability to allow turbo cars to “spool up” over a greater period of time, allow combustion chambers to reach higher operating temperatures, more precise tuning of engines by holding them at a constant or near constant rpm to measure air fuel ratios, combustion temperature, etc. There may be other advantages not discussed.

Engine dynos are conceptually similar to chassis dynos; except the engine is coupled directly to the dyno and the drive train losses are eliminated.  In operating principle, they essentially work the same as chassis dynos. 

In conclusion, the main concept of a dyno is to couple an engine to a load, have the engine run wide open against the load and measure the rate at which the engine moves or is prevented from accelerating by the load.  It is a fairly simple concept.  Making it a reality is much, much more complex!