Post by metalbeast on Sept 8, 2005 20:30:16 GMT -5
Dynometers are used to measure the power that an automobiles engine is capable of producing either at the engine flywheel or at the drive wheels of the vehicle. There are two basic types of dynometers: an engine dyno and a chassis dyno. Lets look and see how each of these work.
Engine Dyno
As the name implies, an engine dyno measures the horsepower of the engine at the engine's flywheel. The engine is connected to the dynometer at the engine's crank. The engine is started and is run through its operating engine speeds. While the engine is running, the dynometer will apply a braking force to the engine. The dynometer measures how much force is required to hold that engine at a given engine RPM will yield a torque output. This torque output can then be used to calculate the engine's flywheel horsepower (FWHP).
Chassis Dyno
Chassis dynos measure the power generated at a vehicles driving wheels. A vehicle's drive wheels are placed on drum like rollers and the car is strapped down to prevent it from moving. The car is started and brought up to speed. Once the transmission is in a 1:1 ratio (4th gear for most manual transmissioned cars and 3rd gear for most automatic transmissioned cars) the engine is put under full load and is brought up to its maximum operating speed. Power output and torque to the operating wheels is then measured by the dynometer. This will be the Wheel horsepower (WHP) reading (often referred to as Rear Wheel Horsepower (RWHP) by those driving domestic iron since our vehicles are driven by the rear wheels). Rear wheel horsepower numbers will be significantly lower than the Flywheel horsepower numbers that would be measured for the engine of the vehicle if it were measured on an engine dyno. This is due to parasitic losses of power to the transmission, rear end, wheels, etc. The general rule of thumb is that with a manual transmissioned vehicle, the rear wheel horsepower values are between 88-84% the flywheel horsepower values. For automatic transmissioned cars, the rear wheel horsepower value is between 79-84% of the flywheel horsepower value.
Types of Chassis Dynos
There are two types of chassis dynos that we will cover in this discussion: the water brake and the inertia chassis dynos.
Water Brake Chassis Dyno
With a water brake chassis dyno, a fluid such as water is used to absorb power. The problem with most water brake dynos is that they tend to be ridiculously expensive and less accurate than an inertia dyno.
Inertia Dynos
Inertia dynos differ from water brake dynos in that they do not use a power absorption device to measure the horsepower of a vehicle. This is how the drums on which the car's wheels sit has a know mass equivalent. We know that power is the rate at which work is performed. We can measure the acceleration of the drums as the car accelerates. If we know the circumference of the drums we can calculate power as follows:
P = Md*Ad*Cd*550
Where
P = Horsepower
Md = Mass of the Drum
Cd = Circumference of the drum
Uncorrected and Corrected Power & Torque
Uncorrected horsepower and torque numbers are the actual or raw numbers that the dynometer measures at the ambient temperature, humidity and atmospheric pressure present during testing.
Corrected (also called SAE) horsepower and torque numbers are recalculated horsepower and torque numbers based on the standard values for temperature, humidity and pressure set by the Society of Automotive Engineers (77 degrees F, 0% humidity and 29.2 in/hg).
Dynojet vs Mustang Dynos
The two prevalent dynos that you will encounter are the Dynojet 248 and the Mustang Dyno. Both measure the wheel horsepower produced by a vehicle. If you were to have a Dynojet and a Mustang Dyno and tested the wheel horsepower of a vehicle under identical conditions and compared the numbers from the 2 machines, you would notice that the numbers for the Mustang Dyno are 10-15% lower than the numbers for the Dynojet 248 dyno. The reason for this difference is the way in which the two dynos measure the wheel horsepower output. On a Dynojet, no resistance is applied to the drums therefore allowing the drums to more or less spin freely which will allow the dyno to measure the horsepower at the wheels. A Mustang Dyno applies a resistance or load to the dyno drums that act like the friction between the road surface and the tire. This gives a more accurate measure of how much power a car produces to the ground or the "road wheel horsepower".
Engine Dyno
As the name implies, an engine dyno measures the horsepower of the engine at the engine's flywheel. The engine is connected to the dynometer at the engine's crank. The engine is started and is run through its operating engine speeds. While the engine is running, the dynometer will apply a braking force to the engine. The dynometer measures how much force is required to hold that engine at a given engine RPM will yield a torque output. This torque output can then be used to calculate the engine's flywheel horsepower (FWHP).
Chassis Dyno
Chassis dynos measure the power generated at a vehicles driving wheels. A vehicle's drive wheels are placed on drum like rollers and the car is strapped down to prevent it from moving. The car is started and brought up to speed. Once the transmission is in a 1:1 ratio (4th gear for most manual transmissioned cars and 3rd gear for most automatic transmissioned cars) the engine is put under full load and is brought up to its maximum operating speed. Power output and torque to the operating wheels is then measured by the dynometer. This will be the Wheel horsepower (WHP) reading (often referred to as Rear Wheel Horsepower (RWHP) by those driving domestic iron since our vehicles are driven by the rear wheels). Rear wheel horsepower numbers will be significantly lower than the Flywheel horsepower numbers that would be measured for the engine of the vehicle if it were measured on an engine dyno. This is due to parasitic losses of power to the transmission, rear end, wheels, etc. The general rule of thumb is that with a manual transmissioned vehicle, the rear wheel horsepower values are between 88-84% the flywheel horsepower values. For automatic transmissioned cars, the rear wheel horsepower value is between 79-84% of the flywheel horsepower value.
Types of Chassis Dynos
There are two types of chassis dynos that we will cover in this discussion: the water brake and the inertia chassis dynos.
Water Brake Chassis Dyno
With a water brake chassis dyno, a fluid such as water is used to absorb power. The problem with most water brake dynos is that they tend to be ridiculously expensive and less accurate than an inertia dyno.
Inertia Dynos
Inertia dynos differ from water brake dynos in that they do not use a power absorption device to measure the horsepower of a vehicle. This is how the drums on which the car's wheels sit has a know mass equivalent. We know that power is the rate at which work is performed. We can measure the acceleration of the drums as the car accelerates. If we know the circumference of the drums we can calculate power as follows:
P = Md*Ad*Cd*550
Where
P = Horsepower
Md = Mass of the Drum
Cd = Circumference of the drum
Uncorrected and Corrected Power & Torque
Uncorrected horsepower and torque numbers are the actual or raw numbers that the dynometer measures at the ambient temperature, humidity and atmospheric pressure present during testing.
Corrected (also called SAE) horsepower and torque numbers are recalculated horsepower and torque numbers based on the standard values for temperature, humidity and pressure set by the Society of Automotive Engineers (77 degrees F, 0% humidity and 29.2 in/hg).
Dynojet vs Mustang Dynos
The two prevalent dynos that you will encounter are the Dynojet 248 and the Mustang Dyno. Both measure the wheel horsepower produced by a vehicle. If you were to have a Dynojet and a Mustang Dyno and tested the wheel horsepower of a vehicle under identical conditions and compared the numbers from the 2 machines, you would notice that the numbers for the Mustang Dyno are 10-15% lower than the numbers for the Dynojet 248 dyno. The reason for this difference is the way in which the two dynos measure the wheel horsepower output. On a Dynojet, no resistance is applied to the drums therefore allowing the drums to more or less spin freely which will allow the dyno to measure the horsepower at the wheels. A Mustang Dyno applies a resistance or load to the dyno drums that act like the friction between the road surface and the tire. This gives a more accurate measure of how much power a car produces to the ground or the "road wheel horsepower".
