1.3 Engine Measurements and Performance

As a automotive technician, you must know the various ways that engines and engine performance are measured. An engine may be measured in terms of cylinder diameter, piston stroke, and number of cylinders. Its performance may be measured by the torque and horsepower it develops, and by efficiency.

Definitions

Work

Work is the movement of a body against an opposing force. In the mechanical sense of the term, this occurs 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 1-pound weight a distance of 1 foot. 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

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

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. The formula for horsepower is the following:

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, which will be discussed later in the chapter.

Torque

Torque, also called moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.

In more basic terms, torque measures how hard something is rotated. For example, imagine a wrench or spanner trying to twist a nut or bolt. The amount of "twist" (torque) depends on how long the wrench is, how hard you push down on it, and how well you are pushing it in the correct direction.

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 1-foot long. When a 2-foot-long wrench is used, only a 50- pound force is required.

Do NOT confuse torque with work or 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.

Friction

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. 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.

Engine Torque

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. For 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.

Chassis Dynamometer

The chassis dynamometer, shown in Figure 1-15, 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.


Figure 1-15 — Chassis dynamometer.

Engine Dynamometer

An engine dynamometer, shown in Figure 1-16, 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.


Figure 1-16 — Engine Dynamometer.

Mechanical Efficiency

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, the considerable amount of horsepower 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

Thermal efficiency is calculated by comparing the horsepower output to the amount of fuel burned. It will be indicated by how well the engine can use the fuel’s heat energy. Thermal efficiency measures the amount of heat energy that is converted into the crankshaft rotation. Generally speaking, engine thermal efficiency is 20-30%. The rest is absorbed by the metal parts of the engine.

Linear Measurements

The size of an engine cylinder is indicated in terms of bore and stroke, as shown in Figure 1-17. Bore is the inside diameter of the cylinder. Stroke is the distance between top dead center (TDC) and bottom dead center (BDC). The bore is always mentioned first. For example, a 3 1/2 by 4 cylinder means that the cylinder bore, or diameter, is 3 1/2 inches and the length of the stroke is 4 inches. These measurements are used to figure displacement.


Figure 1-17 – Bore and stroke of an engine cylinder

Piston Displacement

Piston displacement is the volume of space that the piston displaces as it moves from one end of the stroke to the other. Thus the piston displacement in a 3 1/2-inch by 4-inch cylinder would be the area of a 3 1/2-inch circle multiplied by 4 (the length of the stroke). The area of a circle is R 2 , where R is the radius (one half of the diameter) of the circle. With S being the length of the stroke, the formula for volume (V) is the following:

V = πR2 x S

If the formula is applied to Figure 1-18, the piston displacement is computed as follows:

R = 1/2 the diameter = 1/2 x 3.5 = 1.75 in.
π = 3.14
V = π (1 .75)2 x 4
V = 3.14 x 3.06 x 4
V = 38.43 cu in.

Engine Displacement

The total displacement of an engine is found by multiplying the volume of one cylinder by the total number of cylinders.

38.43 cu in. x 8 cylinders = 307.44 cu in.

The displacement of the engine is expressed as 307 cubic inches in the English system. To express the displacement of the engine in the metric system, convert cubic inches to cubic centimeters. This is done by multiplying cubic inches by 16.39. It must be noted that 16.39 is constant.

307.44 cu in. x 16.39 = 5,038.9416 cc

To convert cubic centimeters into liters, divide the cubic centimeters by 1,000. This is because 1 liter = 1,000 cc.

The displacement of the engine is expressed as 5.0 liters in the metric system.

Engine Performance

Compression Ratio

The compression ratio of an engine is a measurement of how much the air-fuel charge is compressed in the engine cylinder. It is calculated by dividing the volume of one cylinder with the piston at BDC by the volume with the piston TDC, as shown in Figure 1-18. You should note that the volume in the cylinder at TDC is called the clearance volume.


Figure 1-18 — Compression ratio.

For example, suppose that an engine cylinder has a volume of 80 cubic inches with the piston at BDC and a volume of 10 cubic inches with the piston at TDC. The compression ratio in this cylinder is 8 to 1, determined by dividing 80 cubic inches by 10 cubic inches, that is, the air-fuel mixture is compressed from 80 to 10 cubic inches or to one eighth of its original volume.

Two major advantages of increasing compression ratio are that both power and economy of the engine improve without added weight or size. The improvements come about because with higher compression ratio the air fuel mixture is squeezed more. This means a higher initial pressure at the start of the power stroke. As a result, there is more force on the piston for a greater part of the power stroke; therefore, more power is obtained from each power stroke.

Diesel engines have a very high compression ratio. Because the diesel engine is a compression-ignition engine, the typical ratio for diesel engines ranges from 17:1 to 25:1.

Factory supercharged and turbo-charged engines have a lower compression ratio than that of a naturally aspirated engine. Because the supercharger or turbocharger forces the fuel charge into the combustion chamber, it in turn raises the compression ratio. Therefore, the engine needs to start with a lower ratio.

Valve Arrangement

The majority of internal combustion engines are classified according to the position and arrangement of the intake and exhaust valves, whether the valves are located in the cylinder head or cylinder block. The following are types of valve arrangements with which you may come in contact:

L-HEAD —The intake and the exhaust valves are both located on the same side of the piston and cylinder, as shown in Figure 1-19. The valve operating mechanism is located directly below the valves, and one camshaft actuates both the intake and the exhaust valves.


Figure1-19– L-Head engine.

I-HEAD —The intake and the exhaust valves are both mounted in a cylinder head directly above the cylinder, as shown in Figure 1-20. This arrangement requires a tappet, a pushrod, and a rocker arm above the cylinder to reverse the direction of valve movement. Although this configuration is the most popular for current gasoline and diesel engines, it is rapidly being superseded by the overhead camshaft.


Figure 1-20 – I-Head engine.

F-HEAD —The intake valves are normally located in the head, while the exhaust valves are located in the engine block, as shown in Figure 1-21. The intake valves in the head are actuated from the camshaft through tappets, pushrods, and rocker arms. The exhaust valves are actuated directly by tappets on the camshaft.


Figure 1-21 – F-Head engine.

T-HEAD —The intake and the exhaust valves are located on opposite sides of the cylinder in the engine block, each requires their own camshaft, as shown in Figure 1- 22.


Figure 1-22 – T-Head engine.

Cam Arrangement

There are basically only two locations a camshaft can be installed, either in the block or in the cylinder head.

The cam in block engine uses push rods to move the rocker arms that will move the valves.

In an overhead cam engine, the camshaft is installed over the top of the valves. This type of design reduces the number of parts in the valve train, which reduces the weight of the valve train and allows the valves to be installed at an angle, in turn improving the breathing of the engine. There are two types of overhead cam engines: single overhead cam and dual overhead cam.

The Single Overhead Cam (SOHC) engine has one camshaft over each cylinder head. This cam operates both the intake and the exhaust valves, as shown in Figure 1-23.

The Dual Overhead Cam (DOHC) engine has two camshafts over each head. One cam runs the intake valves and the other runs the exhaust as shown in Figure 1-24.


Figure 1-23 – Single Overhead Cam.

 


Figure 1-24 – Dual Overhead Cam.

Induction Type

An air induction system typically consists of an air filter, throttle valves, sensors, and connecting ducts. Airflow enters the inlet duct and flows through the air filter. The air filter traps harmful particles so they do not enter the engine. Plastic ducts route the clean air into the throttle body assembly. The throttle body assembly in multiport injection systems contain the throttle valve and idle air control device. After leaving the throttle body, the air flows into the engine’s intake manifold. The manifold is divided into runners or passages that direct the air to each cylinder head intake port.

Timing

Valve Timing

In an engine, the valves must open and close at the proper times with regard to piston position and stroke. In addition, the ignition system must produce sparks at the proper time, so power strokes can start. Both valve and ignition system action must be timed properly to obtain good engine performance.

Conventional. Conventional valve timing is a system developed for measuring valve operation in relation to crankshaft position (in degrees), particularly the points when the valves open, how long they remain open, and when they close. Valve timing is probably the single most important factor in tailoring an engine for special needs.

Variable. Variable valve timing means that the engine can alter exactly when the valves are open with relation to the engine’s speed. There are various methods of achieving variable timing; some systems have an extra cam lobe that functions only at high speeds. Some others may include hydraulic devices or electro-mechanical devices on the cam sprocket to advance or retard timing.

Ignition Timing

Ignition timing or spark timing refers to how early or late the spark plugs fire in relation to the position of the engine pistons. Ignition timing has to change with changes in engine speed, load, and temperature, as shown in Figure 1-25.


Figure 1-25 – Engine timing.

Timing advance occurs when the spark plug fires sooner on the engine’s compression stroke. The timing is set to several degrees before TDC. More timing is required at higher engine speed to give combustion enough time to develop pressure on the power stroke.

Timing retard is when the spark plug fires later on the compression stroke. It is the opposite or timing advance. It is needed when the engine is operating at lower speed and under a load. Timing retard prevents the fuel from burning too much on the compression stroke that in turn causes spark knock or ping (an abnormal combustion).

Conventional.There are two types of conventional ignition system spark timing: distributor centrifugal advance and distributor vacuum advance.

The centrifugal advance makes the ignition coil and spark plugs fire sooner as the engine speeds up. It uses spring-loaded weights, centrifugal force, and lever action to rotate the distributor cam or trigger wheel on the distributor shaft. By rotating the cam against distributor shaft rotation, spark timing is advanced. Centrifugal advance help maintain correct ignition timing for maximum engine power.

At lower engine speed, small springs hold the advance weights inward to keep timing retarded. As engine speed increases, the weights are thrown outward acting on the cam. This makes the points open sooner causing the coil to fire with the engine pistons farther down in their cylinders.

The distributor vacuum advance system provides additional spark at part throttle positions when the engine load is low. The vacuum advance system is a mechanism that increases fuel economy because it helps maintain ideal spark advance.

The vacuum advance mechanism consists of a vacuum advance diaphragm, a link, a movable distributor plate, and a vacuum supply line. At idle, the vacuum port is covered. Since there is no vacuum, there is no advance in timing. At part throttle, the vacuum port is uncovered and the port is exposed to engine vacuum. This causes the distributor diaphragm to be pulled toward the vacuum. The distributor plate is then rotated against the distributor shaft rotation and spark timing is advanced.

Electronic/Computer

An electronic or computer-controlled spark advance system uses engine sensors, an ignition control module, and/or a computer (engine control module or power train control module) to adjust ignition timing. A distributor may or may not be used in this type of system. If a distributor is used, it will not contain centrifugal or vacuum advance mechanisms.

Engine sensors check various operating conditions and send electrical data representing these conditions to the computer. The computer can then analyze the data and change the timing for maximum engine efficiency.

Sensors that are used in this system include:

• Crankshaft position sensor- Reports engine rpm to the computer.

• Camshaft position sensor-Tells the computer which cylinder is on its power stroke.

• Manifold absolute pressure sensor- Measures engine intake manifold vacuum, an indicator of load.

• Intake air temperature sensor- Checks temperature of air entering the engine.

Engine coolant temperature sensor- Measures the operating temperature of the engine.

• Knock sensor- Allows the computer to retard timing when the engine pings or knocks.

• Throttle position sensor- Notes the position of the throttle.

The computer receives input signals from these many sensors. It is programmed to adjust ignition timing to meet different engine operating conditions.

Summary

In order to be a successful automotive technician, you must know the principles behind the operation of an internal combustion engine. Being able to identify and understand the series of events involved in how an engine performs will enable you to make diagnoses on the job, wherever you may be. During your career, you will apply these and other principles of operation in your daily job routines.