Everyone Talks Horsepower, But how do you actually measure it?
The term “horsepower” is often discussed at a dealer. “How much horsepower does this model have?” “How can I get more horsepower from my bike?” “These two engine are the same size. How come one has more horsepower than the other?” This article will provide a brief history of horsepower as well as the tools used to measure it.
Defining horsepower has always come down to who you ask and what machine (dynamometer) was used to measure it. To complicate things further, there are many definitions of horsepower, including gross, brake, shaft, effective, indicated, relative, SAE Gross, DIN, JIS, ECE, ISO, shaft, watts, kilowatts, advertised and rear wheel. To understand the term – and be able to explain it to your customers – horsepower needs to be put into historical perspective.
How much power an engine produces has been a subject of controversy since the advent of the steam engine in the late 1700s. The dollars and cents of measuring engine power is easy to understand. For example, if an engine made by one company makes 100 horsepower and another manufacturer makes an engine that produces 104 horsepower, and both engines sell for the same price, which is more desirable?
This logic applies not only to engines, but also anything that can be added to an engine to increase horsepower, like exhaust systems. Many aftermarket exhaust manufactureres advertise that their systems will increase the power outlet of an engine. If two similar systems claim different power increases, one will have an advantage in the marketplace. This is also true of motorcycle manufacturers. Honda, Harley-Davidson, Suzuki, Kawasaki, Yamaha, Ducati, BMW, Triump, KTM and other manufacturers are all trying to sell products, and if horsepower is a factor in the equation, more can only be better.
Read the latest review of any motorcycle and more than likely rear wheel or crankshaft horsepower will be listed as a means of comparison between similar bikes in a particular class. Horsepower is measured using a device called a dynamometer, and while these machines don’t produce power like an internal combustion engine, they have something else in common. When it comes to advertising horsepower numbers, more is always better.
Every company that manufacturers dynamometers has a practical reason to steer potential customers away from their competition by pointing out why the other guy’s dyno produces inflated or inaccurate horsepower numbers. Messing around with the numbers that calculate horsepower has been going on for a while. In 1712, Thomas Newcomen designed the first commercially successful steam engine, but it was not very efficient and had limited uses, mostly pumping water out of deep mines.
A scottish mechanical engineer, James Watt, came up with a vastly improved version of the steam engine in 1764 that used 75 percent less coal than the Newcomen engines. Watt’s business plan was to collect royalties from his customers based on the savings in coal, which worked for customers that had existing steam engines and could track their use of coal. But mine operators that still used horses to get their work done need a different way to calculate what they would pay for this cutting-edge technology – the steam engine.
Watt’s plan to entice mine owners to purchase one of his steam engines was based on how many horses the owners could replace. But a question had to be answered: how much work can a single horse accomplish in a given amount of time?
Watt reasoned that if a horse could hoist a bucket of coal weighting 366 lbs. up a mine shaft at the rate of one foot per second, in one minute the horse could raise the bucket 60 feet. With this information, Watt calculated that the horse could raise 21,960 lb.s one foot in one minutes (366 x 60 = 21,960 foot-pounds per minute). Other engineers at the time placed the amount of work a horse could do at 22,916 or 27,500 ft. lbs.
Watt experimented further, and in 1782 found that a brewery horse (a large breed) was able to produce 32,400 ft. lbs. of work per minute. Watt rounded that number up to 33,000 and that became the standard still in use today.
Few horses of even the largest breeds can pull that much weight for any length of time, and there was speculation that Watt had exaggerated the number to his advantage for the purpose of overvaluing his steam engine’s capabilities. Another view is that Watt was just applying good marketing techniques by comparing horses (a familiar form of power and effort at the time) to new technology – the steam engine. With the proliferation of the steam engine, and Watt’s formula for horsepower, a way to measure power output was needed.
(Above: William Froude was born in England in 1810, and in 1877 he invented the hydraulic dynamometer, or water brake. Pictured is a large version of an early water brake circa 1890- a model FA7, Froude Hofmann. The torque arm is easily visible to the right and looks to be almost 15 feet in length. The company was established in 1881 in is still in business, providing design and manufacturing of high technology and specialized test equipment. They produce power measurement products for engines used on ships, automobiles, aircraft even motorcycles.)
The first dynamometer was invented in 1821 by Gaspard de Prony. The de Prony brake, as it was called, was used to measure the performance of engines and other types of machines. Dynamometers have been widely used since the late 1800s to measure the torque of steam engines. The water brake type of dynamometer, sometimes mistakenly called a hydraulic dynamometer, is the oldest type of design and is still used today. These power absorption units can accommodate anything from a Briggs and Stratton lawnmower engine that makes two horsepower to marine diesel engines that can produce hundreds of thousands of horsepower. These early dynamometers basically consist of two half couplings – a rotor and stator.
For measuring horsepower from powersports engines, there are two basic types of dynamometers: engine and chassis. Engine dynamometers are used to measure power directly at the engine’s crankshaft or flywheel. The engine is tested without its transmission or drivetrain connected – in other words, it’s not installed in a motorcycle but rather on a test stand. For the majority of riders, removing their engine for this type of testing is too costly and impractical. The chassis dynamometer measure power at the motorcycle’s rear wheel and the bike simply has to be ridden on to the chassis dyno and strapped down.
( The Dynojet 200i from Dynojet Performance is an inertia type chassis dynamometer that provides a quick way for motorcycle dealers and independent repair shops to verify repairs and diagnose a variety of performance problems. It comes with an atmospheric module that provides a correction factor to ensure consistency between dyno runs made under varying conditions.)
The most common design of dynamometer for powersports use is the inertia type. It doesn’t actually measure torque, but instead calculates it by measuring acceleration. The rear wheel of a motorcycle (or ATV) accelerates a 900-lb. steel drum. Force at the surface of the drum is measured indirectly by measuring its acceleration from one revolution to the next. Force is calculated using Newton’s 2nd law (mass times acceleration). Because the mass, or weight, of the drum is known, force (horsepower) can be calculated.
A typical dyno run begins with the engine running just over idle, in fourth or fifth gear, with the rear tire turning the drum. When the throttle is opened, the engine accelerates the dynamometer’s drum as engine speed increases to redline. Computer software used with inertia dynos can accurately measure acceleration of the drum over small increments of time and calculate a value for torque. Using torque and engine RPM, rear wheel horsepower can be calculated.
(This Dynojet 250i load control (eddy current) dynamometer can hold engine speed steady at any throttle opening. The dyno can measure up to 750 horsepower at speeds of 200 mpg. It can also be configured to run sweep tests like an inertia dynamometer. This type of dynamometer is available in a portable design (pictured above) or for in-ground installations.
(This Dynojet eddy current load absorption unit is ideal for testing motorcycle engines because of its quick response and loading capabilities. The electromagnetic coils can be seen next to the heat absorption rotor. The rotor looks like a disc brake for a car and has large cooling fins and passages to dissipate heat created by a loaded engine.)
Eddy Current Dynamometer
The eddy current brake type dynamometer uses electricity to place a load on an engine by creating a magnetic field. The engine under test is connected to the dyno’s input shaft that spins a metallic rotor creating a magnetic field. When current is increased to the dyno’s internal electromagnetic coils, the rotor shaft becomes harder to rotate and thus loads the engine. Torque load is measured using a strain gauge similar to those used on a water brake dynamometer. The rotor gets hot as the dyno resists the engine’s power and must be cooled. Eddy current dynamometers that are used for testing motorcycle engines are usually air-cooled, employing what looks like an oversized automotive brake rotor with large cooling fins. Eddy current dynoas are accurate and offer the flexibility to perform steady-state load testing or acceleration sweep testing like the inertia dynamometer.
Credit: Tracy Martin via Dealernews
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