Differences between Brake Horsepower, Kilowatts along with Pferdestärke or Horsepower can be confusing, we explain the origin and calculation for each along with a handy conversion calculator to help you understand the power of your car.
Powerful cars come in all shapes and sizes. Often confused with speed, the power of a car comes from the engine or the motor in the case of electric vehicles. The transfer of fuel or charge to energy that drives the car forward is measured as power. Brake Horsepower (BHP, Pferdstarke (PS) and Kilowatt (kW) are all different measures of power. Other measures include Horsepower, Cheval Vapeur and Foot-pounds per minute (ft-lb), torque power is also measured in Neuton Metres.
The power produced by any motor or engine will vary depending on where the measurement is taken. An unladen measurement is typically taken from the flywheel. More accurate measurements are taken from the axel with resistance coming from the gearbox. The most accurate figures of the cars true performance can be measured at the wheels, this measures the car as the customer will witness the performance with wheels, tyres, axles and differentials all scrubbing power from the output of the engine.
Power is felt by drivers in two different areas, firstly is torque this is the cars ability to handle stress typically in the form of weight and transfer that into motion or the second area which is speed. The more power a car produces the faster it can travel and as a side product the faster it can accelerate. Although these values are typically travelling hand in hand they aren’t guaranteed. The most extreme examples of this can be seen by comparing a lightweight small engine car like a Lotus Elise, using a 1798 Cubic Centilitre engine that produces.
These stats launch the car weighing no more than 1.2-tonne car from 0 - 62 mph in just 3.9 seconds with a top speed of 155 mph. When compared to a heavier more powerful car such as the Land Rover Range Rover 5.0 V8, a car with significantly different stats weighing more than 3 tonnes and using a 5000 CC engine. The power stats are as follows
Despite the higher power output, the weight of the vehicle significantly reduces the performance of the car in regards to speed. The Range Rover accelerates to 62 mph in 5.5 seconds and stops accelerating at 140 mph. The torque and pulling power of the larger engine is much greater than the smaller engined Lotus.
To calculate the power to weight ratio of any vehicle simply divide the power of the vehicle by the weight. As an example the weight of a Ford Fiesta ST- Line is 1650 Kilograms with 140 BHP. Producing 3.85 horsepower/pounds.
Kilowatts (kW) is a measure of power equal to 1000W of energy. This measurement is less being used more often with the rise in electric and plug-in hybrid vehicles. Although converted into BHP for acceleration and speed figures kW is a more relevant unit for vehicles that don’t measure fuel or MPG instead measuring Kilowatt Hours and Miles per KiloWatt Hours. Manufacturers that typically advertise their engine and motor performance in kW include Tesla, the luxury electric car manufacturer and Toyota producer of the globally renown Prius. The European Union selected KiloWatts as the standard unit of measurement for engines since 1992. However, since the BHP figure is larger than the kW figure and so manufacturers prefer to use the larger figure to sell more cars.
BHP is the most popular measurement of power used by manufacturers today. First developed by James Watt who calculated the power of one horse. One horse is 33,000 ft-lbs of power, it is assumed that 1 horsepower is the energy required to move 550 lbs 1 ft or 249.5 kg travelling 0.3 metres. The difference between BHP and HP is whereabouts in the drive system the power measurement is taken. BHP is the measurement of horsepower taken at the flywheel with no loss of power from drivetrain and gearbox resistance. The laden power figure or HP is taken at the axel or wheels, the lower figure is less popular with consumers and car makers.
PS also known as Pferdestärke, which is German for horse strength or translated as Horsepower. A relatively recent figure that uses Metric horsepower instead of the mechanical horsepower of BHP. The difference between imperial and metric horsepower is considered to be 10.2 watts. German manufacturers including Volkswagen, Audi and BMW have adopted this PS figure to marginally increase the printed power figures of their cars such as the Volkswagen Golf R 300 PS. PS is a popularly used figure for vehicles sold and manufacturers in mainland Europe.
The other side of power is the ability to drive or move mass, torque is the moment of force required for something to rotate one full rotation. Using Archimedes laws of levers the equation for torque is the rotation position vector multiplied by the force required the answer is then multiplied by sin0 equating in the torque figure of an engine. The higher a torque figure for an engine the more powerful it is, typically diesel engines provide more torque than petrol, tractors, and heavy machinery designed for pulling or moving heavy loads will use diesel engines. Such as the power figure of the Mazda MX-5, with a torque value of 205 Nm. Torque is considered to be the strength of an engine, the ability to drive heavy loads, while acceleration and top speed are considered to be fast engines.
To calculate the conversion of Kilowatts of power to BHP the unladen power of an engine is
1 Kilowatt = 1.34 Brake Horse Power or mechanical power.
An example of this is the BMW 140i with 250kW multiplied by 1.34 is 335 BHP. 250kW = 335 BHP
To calculate the conversion from PS or Metric horsepower to BHP is fairly simple. 1 PS or HP is equal to 0.986 BHP.
Using the example of the BMW 140i again which claims to have 340 PS multiplied by 0.986 equals 335 BHP
A similar equation to kW to BHP in reverse HP is Metric horsepower being converted into Kilowatts. 1 HP = 0.735 kW of power.
Taking the BMW 140i as our example the HP is 340 which when converted to Kilowatts is 253.54.
An engine’s power is calculated in Kilowatts as the standard established by EU regulations, however, manufacturers continue to use BHP and more commonly with German manufacturers PS to showcase the engine's power because figures are larger and therefore have a psychological effect on potential customers.