A Simpler Explanation Of The Measurement Of ONE HORSE POWER

With 400 horsepower and 1550 pounds feet of torque at 1100 PRM coupled to a 10-5-13 transmission this is why Quicksilver went over Donner summit at 65 mph fully loaded, flat towing a 1 ton pickup.

What moves your motorhome is thrust. Thrust is a force. Force is equal to the change in momentum with time. Momentum is the object’s mass times the velocity. On this planet, thrust is sometimes described in pounds. The torque at the drive rear axle, divided by the rolling radius(in feet) of the tire, equals thrust.
Some of this info comes from the Glenn Research Center
Is it correct?

It doesn't matter to me, because I know what performance I want in a diesel pusher. Spartan did an excellent job of matching the torque of the engine to the gear ratios of the transmission and rear axle, plus the weight of my Dutch Star. IMO in order to safely merge with freeway traffic I need to match or exceed the speed of the the traffic, at the correct point.

Also, the "torque" that most are referring to is the engine's crankshaft torque.

That torque can be multiplied as needed for torque to the drive wheels with gearing. The desired spin velocity (vehicle speed) of those drive wheels at that multiplied torque, however, takes more and more horsepower the higher the desired vehicle speed at any given vehicle weight.

I maintain that with the appropriate overall drive system gearing, engine horsepower is what counts in accordance with engine type and design principles versus pure preference - no matter what kind of rotating engine is producing it and no matter what the local driveshaft torque of the engine is.

BTW, one of the most impressive racing engines of all time ran at insane RPMs to tap it's horsepower (the BRM V16) - but boy was it impressive at those spin speeds.

Whenever there is discussion regarding an engine's horsepower engine torque is likely to come up. People often bring up definitions of torque and horsepower and stress that torque is a measure of force where as horsepower is a measure of power or rate at which work is done. The definitions are of some value but the thing that is often overlooked is the fact that power and torque ratings are peak values and are given at a specified rpm. Neither value on its own gives a clear picture of an engine's usable power but wen considered together the two values along with their specified rpm give a clear picture of an engine's power curve. Torque is meaningless without stating an rpm and when the rpm is stated along with the torque value it is an indication of engine power at that rpm.

My high school physics teacher explained it this way... you can exert as much force on an object as you want, generating lots of torque, but if the object doesn’t move, you didn’t accomplish any measurable work, and thus didn’t generate any power, even if you are covered in sweat.

turbojimmy wrote:

larry cad wrote:
Torque is constant over the RPM range, while Horsepower is a straight line function of RPM. Thus, the diesel engine, which operates with a typical limit of 2400 RPM produces more torque than a typical gas engine which operates up to 5500 RPM. Another factor that produces torque is larger displacement (BIG pistons)

The formula for HP is torque * RPM /5252, so HP is a function of RPM and torque. Torque is not constant over the RPM range of any engine. This is why people talk about "torque curves" of engines. Although electric motors have near flat torque curves (which is why train locomotives are diesel/electric). Diesels deliver torque more consistently through the RPM range compared to gasoline engines. But, there's a curve - torque delivery is not a constant. Since diesel engines don't rev as high their horsepower is relatively low (RPM is a variable in the HP calculation) when compared to a gasoline engine because the gasoline engine can produce 2 or 3 times the RPM.

The formula is correct. The problems always come from interpreting the formula. I work with electric motors which operate differently than "engines" in that motors have a zero rpm state while engines have an idle state. Electric motors at zero rpm can produce their maximum torque, while engines have to be rotating, even at idle to produce torque unless you consider the starter motor :). Keep in mind that in physics, torque is not work. No work is done by an electric motor at zero rpm, but it still can produce rated torque. To use this formula for electric motors, you have to use manufacturers nameplate RPM and rated HP. Rated HP is only available at nameplate RPM and tapers off in a straight line to zero RPM.

PaulJ2 wrote:

Dusty R wrote:
I don't know why but I have found that it takes a 2hp gas engine to do the same work that a 1hp electric motor will do.

Dusty

I remember back when growing up many years ago on a farm, we had a power outage. My dad then connected up a 2hp gas engine to the 3/4hp electric jet pump. It would just barely run it at wide open throttle.
The rpm was near the same at 1 to 1 ratio, about 3450-3500 rpm.

Gas engines put out their rated peak horsepower in a relatively narrow range compared to an electric motor. If the gas engine was designed to put out 2hp at 3450-3500 rpm then something was wrong with it, otherwise it just wasn't in its peak power band and needed proper gearing or pulleys to do so.

This is also why people like diesels for towing and tend to get focused on their torque numbers. They typically have a broader power range than gas engines making them easier to gear to get needed power to the wheels for moving heavy loads.

Electric motors are the easiest to gear for because they typically have very broad torque curves including having max torque all the way down to 0 rpm. Many electric cars like a Tesla have just a single speed transmissions as its not necessary to change gear ratios to maintain proper power output. Electric motors will dominate in towing once they become widespread (just look at diesel electric trains) the main issue holding them back is battery cost/weight/capacity.

wnjj wrote:

There are inertia dynos that measure HP by measuring the acceleration of a heavy drum that the drive wheels turn. Math determines the HP that was required to change the drum’s RPM from speed X to Y. Similarly you could measure electrical watts by heating a measured amount of water to a measured temperature change over time. Now a typical watt meter does what you suggest which is voltage and current.

Inertia dynos record RPM and derive power from the rpm change over time. They only measure RPM and time and then you have the known constant of the drum mass.

Measuring watts using heating is a similar issue, you are measuring temperature and time along with the known amount of water and deriving power.

The point is while power is whats important for getting anything done, its always derived from its components which are measured. Those components can be exchange through devices like transmissions while power cannot.

jharrell wrote:

pnichols wrote:


I've always thought that the diesel owners thought that their rigs pulled better because they didn't have to rev the engine very high to tap the engine's horsepower. Kindof a preference thing, not really based on engineering facts.

The V10 in our small motorhome will paste me back into the seatback if I punch it, but it takes 4000-5000 RPM to do it.

You can get any torque number you want at the wheels through gearing, horespower cannot go up (only goes down through losses from engine to wheels). Gears exchange RPM for torque. Your diesel engine puts out 1000 lb-ft of torque, who cares, I can get 10000 out of a lawnmower engine with the right gears.

You cannot measure power directly, dynos measure rpm and torque to derive horsepower. Torque is meaningless on its own, it requires the rpm component to get a meaningful measurement of power.

Many of those here should recognize this relationship as the same one between volts, amps and watts. Watts are power (746 per hp in fact), volts is equivalent to torque and amps equivalent to rpm. Using a transformer/coil you can exchange one for the other but watts stays the same. Just think your spark plug gets 20000v from a 12v system through a coil, you can get 20000 lb-ft of torque from a 12 lb-ft motor through gears.

Again you don't measure watts directly either, you measure voltage and current to derive power.

Good explanation.

There are inertia dynos that measure HP by measuring the acceleration of a heavy drum that the drive wheels turn. Math determines the HP that was required to change the drum’s RPM from speed X to Y. Similarly you could measure electrical watts by heating a measured amount of water to a measured temperature change over time. Now a typical watt meter does what you suggest which is voltage and current.

PaulJ2 wrote:

I remember back when growing up many years ago on a farm, we had a power outage. My dad then connected up a 2hp gas engine to the 3/4hp electric jet pump. It would just barely run it at wide open throttle.
The rpm was near the same at 1 to 1 ratio, about 3450-3500 rpm.

Back in time the mainstream small lawnmower-type engines were rated in "theoretical" horsepower, calculated from the engine design specs. The more industrial engines, like Kohler, gave actual horsepower ratings, so a 15 HP Kohler engine was noticably more powerful than a 15 HP B&S, Tecumseh, etc. engine. One description I read is:
“the theoretical horsepower than an engine could achieve under ideal laboratory conditions with all of the legally required accessories removed from the engine – such as the air filter and exhaust mechanism". Since then lawsuits have changed the HP ratings to be more honest. I'm reading that now B&S doesn't give HP ratings for their small engines, using the torque spec instead.