Gas Engine Prony Break

A simple and inexpensive method

By Dr. David Cave
Published on October 31, 2023
article image
by David Cave
The first run at the author’s home. This Prony brake was later run at a show to demonstrate how it measured horsepower.

Learn the basics and history of Prony breaks and how you can use them as a simple and inexpensive method to measure horsepower.

If it were convenient to easily apply a load to an old hit-and-miss engine, one could hear its response going from no load to full load, measure horsepower, or test the old expression of “a pint of fuel per horsepower hour.” Whatever the reason, a simple and inexpensive method is desirable.

In the hot-rod culture, a dynamometer (most often called a dyno) is used to load engines. In principle, a dyno is any machine that loads an engine and records its output in horsepower and torque. Now all horses aren’t the same but James Watt, inventor of the steam engine, defined horsepower as 550-foot-pounds-per-second (550 ft-lb/sec) in order to quantify his engines. Lift 550 pounds a foot in one second or lift one pound 550 feet in one second or any combination of weight, distance and time that results in 550 generates 1hp. For example, if a 180lb man runs up a flight of stairs in 3 seconds and those stairs, no matter how long, rose 10 feet, he would generate (10 X 180)/ 3 = 600ft-lb per/sec or about 1.1hp.

The physics folks have converted mechanical horsepower to many other measurement methods. In the electrical world, 1hp is also 745 Watts or 0.212 ton in the refrigeration world. For several years the author ran an electrical dyno at shows (see Figure 1). The generator was a discarded treadmill motor and its load was 300 Watt light bulbs. The hit-and-miss engine could quickly run from full load to no load with stops in between, using toggle switches that controlled the number of bulbs turned on. The load in Watts, and thus horsepower, could easily be computed using an Amp-meter and a Volt-meter. Treadmill motors are generally permanent magnet DC-machines and therefore bidirectional — put Amps and Volts in and they become a motor. But if turned, they become a generator putting out Amps and Volts. Modern electric vehicles use permanent magnet DC-motors and are able to go into generator mode to supplement braking and put that recovered energy back into the battery.

Building an electrical 1 or 2hp dyno (a generator, some light bulbs and switches) is easy and inexpensive if you can find a disbanded treadmill. By 3hp, however, the Amps and Volts have reached a dangerous level and the switches can no longer be cheap Home Depot units. Although the fully loaded 3hp engine with no muffler and bright 300-Watt bulbs was quite an attraction, it was abandoned due to the fact that high Amps and Volts are unsafe around crowds. Prior to this electrical dyno, no engine collector could be found at two shows who had heard a hit-and-miss engine under a 100% load for any measurable time. All had heard engines under a small grinder or sheller load, but never fully loaded.

The oldest dyno, and the traditional method of loading an old stationary engine, is the Prony brake developed by Frenchman Gaspard de Prony in 1826. Prony brakes are not easy to build and operate and are mainly used to measure an engine’s horsepower rather than arbitrarily loading an engine. The Prony brake is a large wheel that’s driven or turned by the engine being tested (see Figure 2). Brake material is wrapped around the bottom of the wheel and attached with bolts to a beam resting on top of the wheel. The outer end of the beam is attached to (or rests on) a scale to measure pounds. As the bolts are tightened, the wheel becomes harder to turn and more pressure is applied to the scale. Calculating horsepower is rather straightforward: If the beam were allowed to spin like a propeller, it would turn a given rpm and the scale point on the beam would travel the circumference of the circle each turn. The variables become the length of the beam (L) measured in feet from the center of the wheel to the point of the beam that sits on the scale, the wheels rpm and the force on the scale (Pd) measured in pounds.

hp = (π X 2 X L X rpm X Pd) / 33000

Reconfigured to calculate the pressure on the scale, Pd becomes:

Pd = (hp X 33000) / (2 X π X L X rpm)

The 33000 is the result of converting revolutions per minute (rpm) to revolutions per second.

Although the measurement of hp is easy once it is built, actual construction of the Prony brake is not. Quick adjustment of the Prony brake to move the engine from full to light loads is rather difficult. How big should the wheel be? What should the brake material be? What is a good rpm thus drive pulley size? Does the brake need to be cooled? These are all questions without solid answers. Many good papers, for example, The Design, Construction, and Use of a Small Prony Brake, have been written to help with the design. Like many others, the cited paper spent 75% of the printed space trying to answer those questions, yet never giving clear answers.

The car in your garage has attached four Prony brakes, one on each corner. Your modern disc, or drum, brake mounted on the wheel assembly is a form of Prony brake. The front brakes of your car are designed to handle a 3,000- to 6,000-pound car, or SUV, on long mountain grades and has resolved the previous questions. They’re fully assembled, inexpensive (if used), quick and easy to operate with a brake pedal, fully balanced, don’t need external cooling and will handle large machines. A vehicle wheel, brake hub and spindle are equivalent to the Prony brake wheel, the caliper and brake pads replace the brake strap on the Prony, and a vehicle frame is the Prony beam. As vehicle brakes are applied, the caliper attached to the frame attempts to push the front of the vehicle down like the Prony beam on the scale.

The following is a description of two Pronys built from front wheel assemblies. The first is a small, engine specific unit built from an ATV brake, capable of 5hp. The second, built from an automobile assembly, could be scaled to handle more than 100hp when mounted on a proper base. Both options are inexpensive to build, quick and easy to operate through the brake pedal, are rpm tolerant, can operate clockwise or counterclockwise, are balanced and self-cooling.

The first Prony brake is small and attached directly to a 3hp John Deere engine. Its purpose is not to measure horsepower but after removing the muffler for effect, to cycle the engine from no load to full load at random. Figure 3 is the front wheel assembly for an ATV found on multiple internet sites for $100 (with free shipping). The kit includes two wheel assemblies, the brake line and master cylinder. This is everything needed for a Prony. It’s only necessary to mount one of the wheel assembly hubs on the hit-and-miss engine flywheel and add the beam to complete the Prony.

There are probably many ways to attach the assembly to the flywheel. One is to use a short aluminum cylinder and drill one set of holes for the brake assembly and another set to fit the engine flywheel (see Figure 4). The mounted wheel assembly must be as centered as possible and planar to the flywheel, or vibrations will be transmitted to the beam.

It is easier to mount the assembly with the lug bolts removed from the ATV hub and replaced with longer, standard 3/8-inch bolts. Also, the second unused brake line is removed at the Y (where its line is mated with the master cylinder line and wheel assembly line being used). The hole caused by that removal is filled with a short 10mm (fine pitch) metric bolt and copper crush washer. The final assembly using a steel rod for the Prony beam is shown in Figure 5.

As mentioned, this Prony was intended to demonstrate the hit-and-miss concept. Rather than a scale, it lifts weights. When the first weight is lifted, the load on the engine is 1hp, two weights is 2hp while the third weight is a full 3hp (see Figure 6).

This small Prony was run at a local show with the engine muffler removed and received many compliments on how well it demonstrated the hit-and-miss concept (see Figure 7).

The ATV Prony easily handled 3hp, was simple to build, and cost less than $125. However, it goes through brake pads rather quickly. A set of pads may last only one show, but are readily available on the internet. Two pairs cost less than $10 and are easy to change. An advantage of modern disc brakes is the ease of changing brake pads. The caliper is held on with only two bolts. Once one bolt is removed, the caliper will swing out using the other bolt as a hinge exposing the brake pads.

A Prony from a small car wheel assembly was built for larger engines. A Hyundai Accent full wheel assembly was purchased online for $100 with free shipping. Full-sized wheel assemblies can easily be found at your local junk yard for $50 with no shipping involved. The Accent is a small car but the assembly still weighs 31 pounds. Full-size U.S. car and SUV assemblies are substantially heavier, some close to 50 pounds. Although a Prony made from a family-sized car could handle a tractor, the target here was not tractors or large engines. The decision was made to use a small car wheel on a cart knowing it would need to be well-staked during operation.

Make sure you get the whole wheel assembly including the knuckle (that attach the assembly to the car), the spindle and hub (with lug bolts that attach to the car wheel), the brake rotor and caliper (as seen in Figure 8). You will not normally get a brake line or a master cylinder with the assembly. Again, as in the case of the ATV brake, Figure 8 is the complete Prony capable of 0 to 1000rpm (forward or reverse), at least 50hp, with no cooling required. The remaining tasks are to mount it on a suitable base with a drive pully, connect the master cylinder and connect the beam with a scale.

Probably the single limitation on this Prony is its cooling capacity. Look for the cooling fins (as seen in Figure 8) when selecting an assembly. Those fins are designed like a fan and force air along the back side of both rotor surfaces, greatly improving the cooling.

As mentioned, although it could handle tractors if on a proper base, the target is 6hp and below engines. A jack shaft is mounted on a cart with the Prony on one end and the drive pully on the other. Two aluminum pucks were drilled to mount the pulley and the Prony on the shaft (see Figure 9).

Figure 10 shows the nearly complete Prony mounted on a cart. The master cylinder and scale still need to be attached. Attaching the master cylinder and mating the beam to a scale would seem to be straight-forward, but they became the tasks that were the most time-consuming.

Brake lines and fittings come in various sizes and flares in both English and metric. Hopefully your local auto parts store can help to connect your caliper to your master cylinder. Less guess work is involved in mating those components if the used master cylinder is from the same car model as the brake assembly. The hydraulic master cylinder is a key part of this Prony. With the master cylinder, the engine or tractor can be instantaneously taken from no load to full load (or anywhere in between) with a mere push of a master cylinder lever.

It was decided that a lever-acting master cylinder mounted on a small table would be easier to work with than a foot-activated master cylinder. Figure 11 shows a simple bracket and lever assembly with six feet of flexible brake line attached.

The car wheel assembly can easily handle 1000rpm (forward or reverse). Therefore, the limitation may fall on the drive pulley’s ability to handle the intended horsepower and rpm. The drive pulley in Figure 10 is a 10-inch cast-iron pulley that would give up well before reaching the limits of the small Hyundai brake assembly. Care should be taken to match the drive pulley to the engine capability.

Even with large flywheels, our engines by nature have a lot of rotational vibration. Unless the drive pulley on the jackshaft is large, and therefor the brake assembly turns rather slowly, a lot of this vibration will be transmitted to the scale beam. Nice professional scales will often have built-in dampers to alleviate the vibration. The simple spring scale used in this build had no damper and resulted in the scale needle appearing as a large blur, covering half the dial. Figure 12 is the water-filled damper built to reduce the vibration. Using less viscus water, rather than the usual oil, required the piston to be only 0.030-inch less than the inside diameter of the cylinder. However water is less messy and easier to tear down and clean up after a show.

After loading the cart with some cement blocks for added stability and marking the scale for 1, 2 and 3hp, an initial driveway run was made (see Page 10). The damper is at the end of the Prony beam but the scale is a shorter distance from the center of the brake assembly and that shorter distance is used for power calculations. A pickup truck cargo bar was used like a jack between the engine and the Prony cart to keep the belt tight. An engine guy who stopped by for the run commented that he had never heard a hit-and-miss engine run under full load for three continuous minutes. It was a hit engine, there were no misses. This first run did reveal a couple of surprises. Even though the engine is only 3hp, the set up doesn’t utilize a power brake and the brake lever arm is short and requires more pressure than expected. For large engines, it would be better to mount the master cylinder on the ground with a foot pedal. Also, it is surprising how cool the caliper remains; after considerable use, the caliper and brake assembly are no more than warm.

A large display board was made describing hit-and-miss and the Prony. With no engine muffler, the Prony on Page 10 was run at a local show and drew large crowds. Admittedly, the main draw was an engine making a lot of noise, then suddenly no noise. Many stopped to read the display board and then watched to understand how it measured horsepower.

Building an automobile disc-brake Prony is easy and inexpensive. The author has a decent scrap pile; aside from the cart and cast-iron drive pulley, the total cost was less than $200. If placed on a substantial base, how much horsepower could a disc-brake Prony handle? Each case and the type of power cycling will vary, but for a modern full-size American SUV, each front disc brake is capable of dissipating over 200hp in a single stop. It would seem a disc brake Prony on a proper base would be capable of meeting our demands. If the builder feels the need for more stopping power, the brake assembly from a front-wheel drive vehicle has another advantage. The drive axle attached to the front wheel creates a pass-through hole in the rotor and spindle. If the jackshaft is extended through that brake assembly hole (see Figure 13), a second brake assembly can be mounted in parallel with the first. The additional work to add a second brake assembly is fairly straightforward. A second mounting adapter is needed and a brake line splitter needs to be added. When connected this way, both knuckles must be connected to the same beam. That may be simplified by mounting the second brake assembly upside-down, or reversed, with no impact on performance.

Dr. David Cave is a regular contributor to Gas Engine Magazine and can be reached at jdengines@cox.net.

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