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Engine Torque and Horsepower


Work The transference of energy that is produced by the motion of the point of
application of a force and is measured by multiplying the force and the
displacement of its point of application in the line of action
Force Strength or energy exerted or brought to bear; cause of motion or change
Foot Pounds The unit of work equal to the work done by a force of one pound acting
through a distance of one foot in the direction of the force.
Torque A measure of twisting force and is measured in foot-pounds.
Horsepower A unit of power equal in the U.S. to 746 watts and nearly equivalent to the
English gravitational unit of the same name that equals 550 foot-pounds of
work per second.


Now that we have a common understanding of terms, let’s talk about engine torque and horsepower. To begin, work is what happens when a force is used to move an object some distance. The energy expended to do the work is not the same as the work itself. For example, you could try to push your house off its foundation with your bare hands but obviously no work would be done even though energy has been expended. You would end up being very tired from the experience, but you would have accomplished nothing. If you selected something a bit smaller than a house, say a piano, you could expend a tremendous amount of energy and move it only a few feet. Some, but not much, work would have been done, and it could be measured in foot-pounds.

Next we need to look at how the idea of horsepower came about. In the 18th century, James Watt made some observations. He concluded that the average horse of the time could lift a 550-pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight-hour shift, more or less. He then published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to one horsepower. Everybody else said, “Okay!” This has been the accepted standard ever since.

When talking about motorized vehicles it is important to also understand how to measure the force from rotating objects such as crankshafts. The term we use for such a “twisting” force is foot-pounds of torque. A foot- pound of torque is the twisting force necessary to support a one-pound weight on a weightless horizontal bar, one foot from the fulcrum.

It is also important to understand that no one on the planet ever actually measures horsepower from a running engine on a standard dynamometer. What we actually measure is torque, expressed in foot-pounds and then we ?calculate? horsepower by converting the twisting force of torque into work units of horsepower. The formula for that process is:


Visualize that one-pound weight we mentioned, one foot from the fulcrum on
its weightless bar. If we rotate that weight for one full revolution against
a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two
foot circle), and, incidentally, we have done 6.2832 foot-pounds of work.

OK. Remember Watt? He said that 33,000 foot-pounds of work per minute was
equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we’ve done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it’s the equivalent of ? horsepower (16,500 foot pounds per minute), and so on.
Therefore, the following formula applies for calculating horsepower from a torque measurement:

This is not a debatable item. It’s the way it’s done. Period.

To get vehicles moving, we use some kind of engine to convert chemical energy (fuel) into force. In an internal combustion engine, the fuel in the combustion chamber is ignited by a spark (gasoline engines) or compression (diesel engines), creating high-pressure gasses that push down on the pistons. As the pistons go down the connecting rods push the crankshaft throws or arms. Bearings control the crankshaft, allowing it to only rotate. Since each crank arm is offset from the centerline of the crankshaft (the rotational axis), we have torque. Voila.

In the MotorHome world this translates to the ability to get a load moving. Diesels produce massive amounts of torque at low RPMs. This of course makes them popular in large trucks, busses, and MotorHomes. Gasoline engines tend to produce maximum torque at higher rpm’s. You will see, feel, and hear the difference between the two types of engines in terms of power production. While climbing a grade of say 6% the “staying” power of a diesel MotorHome can be illustrated by the sound of the engine and the RPMs registering on the tachometer. The engine begins to growl and the RPMs will probably stay in the 1500 to 2000 range with appropriate downshifting, very much the same range as normal highway speeds on level ground. Type of transmission is a key player here as is managing the gears. Diesel engines supply a lot of torque without high RPMs. It is like lifting a heavy object with a long lever and a steady force. A gasoline engine operating in the same environment will require more throttle and more RPMs to achieve the same amount of torque. It is like trying to jack up a car with a small jack. You need to pump on the handle a lot of times to raise the car. A gasoline engine then will not maintain the “staying” power because it cannot produce and maintain a torque range to move the MotorHome uphill at a constant speed. More and more fuel is required. Part of the problem is that gasoline powered MotorHomes typically aren’t geared” appropriately to move the weight. Engine design, fuel systems, etc. all change the shape of the torque curve. In most cases, torque range rises to some peak value and then decreases.

What all this means is that as you make decisions about purchasing a MotorHome, you need to consider exactly what you expect it to do in terms of speed, hill/mountain climbing, and towing. As stated, diesel MotorHomes generally have the appropriate horsepower and torque ratings along with the properly sized transmission to take you down the road safely and in good fashion. The key word here is generally. Just because the coach has a diesel engine doesn’t mean it’s a “cruiser”. Some manufacturers have managed to sell inadequately powered diesel coaches, and the coach owners in a lot of cases don’t attend to the weight ratings of their coach. A good rule of thumb when planning a coach load is you should have a minimum of 10 horsepower for every 1000 pounds of coach weight you want to move. Remember your tow vehicle should be in the total weight equation, too.

Keep in mind. We are not assaulting gasoline powered coaches. For the appropriately sized coach and the total weight you want to move, gas coaches are just fine. In a lot of cases however, we want to move considerably more weight than the gas coach is capable of sustaining.

 

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2 Comments

  • google insights May 29, 2015 at 10:35 pm

    Very nice article, totally what I wanted to find.

    Reply
  • Frank August 6, 2016 at 1:39 am

    Wanted to compare diesel and gas motorhome torque, and this article was very easy to understand.

    Reply

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