Automotive Systems

Formerly Automotive Systems I

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Definitions

DEFINITIONS

WORK is the movement of a body against an opposing force. In the mechanical sense of the term, this is done when resistance is overcome by a force acting through a measured distance. Work is measured in units of foot-pounds. One foot-pound of work is equivalent to lifting a l-pound weight a distance of 1 foot (fig. 2-17). Work is always the force exerted over a distance. When there is no movement of an object, there is no work, regardless of how much force is exerted

ENERGY is the ability to do work. Energy takes many forms, such as heat, light, sound, stored energy (potential), or as an object in motion (kinetic energy).

Energy performs work by changing from one form to another. Take the operation of an automobile for example; it does the following:

  • When a car is sitting still and not running, it has potential energy stored in the gasoline.
  • When a car is set in motion, the gasoline is burned, changing its potential energy into heat energy. The engine then transforms the heat energy into kinetic energy by forcing the car into motion.
  • The action of stopping the car is accomplished by brakes. By the action of friction, the brakes transform kinetic energy back to heat energy. When all the kinetic energy is transformed into heat energy, the car stops.

POWER is the rate at which work is done. It takes more power to work rapidly than to work slowly. Engines are rated by the amount of work they can do per minute. An engine that does more work per minute than another is more powerful.

The work capacity of an engine is measured in horsepower (hp). Through testing, it was determined that an average horse can lift a 200-pound weight to a height of 165 feet in 1 minute. The equivalent of one horsepower can be reached by multiplying 165 feet by 200 pounds (work formula) for a total of 33,000 foot-pounds per minute (fig. 2-18). The formula for horsepower is the following:

 

Hp = (ft-lb. per min)/ 33,000 = (L x W)/(33,000 x t)

 

L = length, in feet, through which W is moved

W = force, in pounds, that is exerted through distance L

t = time, in minutes, required to move W through L

 

A number of devices are used to measure the hp of an engine. The most common device is the dynamometer.

An engine dynamometer (fig. 2-19) may be used to bench test an engine that has been removed from a vehicle. If the engine does not develop the recommended horsepower and torque of the manufacturer, you know further adjustments and/or repairs on the engine are required.

The chassis dynamometer (fig. 2-19) is used for automotive service, since it can provide a quick report on engine conditions by measuring output at various speeds and loads. This type of machine is useful in shop testing and adjusting an automatic transmission.

On a chassis dynamometer, the driving wheels of a vehicle are placed on rollers. By loading the rollers in varying amounts and by running the engine at different speeds, you can simulate many driving conditions.

These tests and checks are made without interference by other noises, such as those that occur when you check the vehicle while driving on the road.

Another device that measures the actual usable horsepower of an engine is the prony brake (fig. 2-20). It is used very little today, but is simple to understand. It is useful for learning the concept of horsepower-measuring tools. It consists of a flywheel surrounded by a large braking device. One end of an arm is attached to the braking device, while the other end exerts pressure on a scale. In operation, the engine is attached to, and drives, the flywheel. The braking device is tightened until the engine is slowed to a predetermined rpm. As the braking device slows the engine, the arm attached to it exerts pressure on a scale. Based on the reading at the scale and engine rpm, a brake horsepower valve is calculated by using the following formula:

 

(6.28 x length of arm x engine rpm x scale reading) /  33,000

 

It must be noted that 6.28 and 33,000 are constants in the formula, meaning they never change. For example, a given engine exerts a force of 300 pounds on a scale through a 2-foot-long arm when the brake device holds the speed of the engine at 3,000 rpm. By using the formula, calculate the brake horsepower as follows:

 

(6.28 x 2 x 3000 x 300) /  33,000 = 342.55 brake horsepower

 

TORQUE is a force that, when applied, tends to result in twisting an object, rather than its physical movement. When the torque is being measured, the force that is applied must be multiplied by the distance from the axis of the object. Torque is measured in pound-feet (not to be confused with work which is measured in foot-pounds). When torque is applied to an object, the force and distance from the axis depends on each other. For example, when 100 foot-pounds of torque is applied to a nut, it is equivalent to a 100-pound force being applied from a wrench that is l-foot long. When a 2-foot-long wrench is used, only a 50-pound force is required. An illustration of a torque wrench in use is shown in figure 2-21.

DO NOT confuse torque with work or with power. Both work and power indicate motion, but torque does not. It is merely a turning effort the engine applies to the wheels through gears and shafts.

ENGINE TORQUE is a rating of the turning force at the engine crankshaft. When combustion pressure pushes the piston down, a strong rotating force is applied to the crankshaft. This turning force is sent to the transmission or transaxle, drive line or drive lines, and drive wheels, moving the vehicle. Engine torque specifications are provided in a shop manual for a particular vehicle. One example, 78 pound-feet @ 3,000 (at 3,000) rpm is given for one particular engine. This engine is capable of producing 78 pound-feet of torque when operating at 3,000 revolutions per minute.

FRICTION is the resistance to motion between two objects in contact with each other. The reason a sled does not slide on bare earth is because of friction It slides on snow because snow offers little resistance, while the bare earth offers a great deal of resistance.

Friction is both desirable and undesirable in an automobile or any other vehicle. Friction in an engine is undesirable because it decreases the power output; in other words, it dissipates some of the energy the engine produces. This is overcome by using oil, so moving components in the engine slide or roll over each other smoothly. Frictional horsepower (fhp) is the power needed to overcome engine friction. It is a measure of resistance to movement between engine parts. Frictional horsepower is power lost to friction. It reduces the amount of power left to propel a vehicle. Friction, however, is desirable in clutches and brakes, since friction is exactly what is needed for them to perform their function properly.

One other term you often encounter is inertia. Inertia is a characteristic of all material objects. It causes them to resist change in speed or direction of travel. A motionless object tends to remain at rest, and a moving object tends to keep moving at the same speed and in the same direction. A good example of inertia is the tendency of your automobile to keep moving even after you have removed your foot from the accelerator.

You apply the brake to overcome the inertia of the automobile or its tendency to keep moving.

The term efficiency means the relationship between the actual and theoretical power output. Volumetric efficiency (fig. 2-22) is the ratio between the amount of air-fuel mixture that actually enters the cylinder and the amount that could enter under ideal conditions. The greater volumetric efficiency, the greater the amount of air-fuel mixture entering the cylinder; and the greater the amount of air-fuel mixture, the greater the power produced by the engine.

Increasing volumetric efficiency increases engine performance. Volumetric efficiency can be increasedin the following ways:

  • Keep the intake mixture cool by ducting intake air from outside the engine compartment. By keeping the fuel cool, you can keep the intake mixture cooler. The cooler the mixture, the higher the volumetric efficiency. This is because a cool mixture is denser or more tightly packed.
  • Modify the intake passages (fig. 2-23). Changes to the intake passages that make it easier for the mixture to flow through will increase the volumetric efficiency. Other changes include reshaping ports to smooth bends, reshaping the back of the valve heads, or polishing the inside of the ports.
  • Altering the time that the valves open or how far they open can increase volumetric efficiency.
  • By supercharging and turbocharging, you can bring the volumetric efficiency figures to over 100 percent.

MECHANICAL EFFICIENCY is the relationship between the actual power produced in the engine (indicated horsepower) and the actual power delivered at the crankshaft (brake horsepower). The actual power is always less than the power produced within the engine. This is due to the following:

  • Friction losses between the many moving parts of the engine.
  • In a four-stroke-cycle engine, a considerable amount of horsepower is used to drive the valve train.

From a mechanical efficiency standpoint, you can tell what percentage of power developed in the cylinder is actually delivered by the engine. The remaining percentage of power is consumed by friction, and it is computed as frictional horsepower (fhp).

THERMAL EFFICIENCY is the relationship between actual heat energy stored within the fuel and power produced in the engine (indicated horsepower). The thermal efficiency figure indicates the amount of potential energy contained in the fuel that is actually used by the engine to produce power and what amount of energy is actually lost through heat. A large amount of energy from the fuel is lost through heat and not used in an internal combustion engine. This unused heat is of no value to the engine and must be removed from it. Heat is dissipated in the following ways:

  • The cooling system removes heat from the engine to control engine operating temperature.
  • A major portion of the heat produced by the engine exits through the exhaust system.
  • The engine radiates a portion of the heat to the atmosphere.
  • A portion of this waste heat may be channeled to the passenger compartment to heat it.
  • The lubricating oil in the engine removes a portion of the waste heat.
  • In addition to energy lost through waste heat, there are the following inherent losses in the piston engine.
  • Much energy is consumed when the piston must compress the mixture on the compression stroke.
  • Energy from the fuel is consumed to pull the intake mixture into the cylinder.
  • Energy from the fuel is consumed to push the exhaust gases out of the cylinder.

The combination of all these factors in a piston engine that uses and wastes energy leaves the average engine approximately 20 to 25 percent thermally efficient.

Figure 2-17.—One foot-pound of work.

Figure 2-18.—Horsepower.

Figure 2-19.—Dynanometers.

Figure 2-20.—Prony brake.

Figure 2-21.—Torque wrench in use, tightening main bearing stud of an engine.

Figure 2-22.—Demonstrating volumetric efficiency.

Figure 2-23.—Port design consideration.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
All rights reserved