Automotive Engine Designs and Diagnosis

GOALS:

¦ Describe the various ways in which engines can be classified.

¦ Explain what takes place during each stroke of the four-stroke cycle.

¦ Outline the advantages and disadvantages of the inline and V-type engine designs.

¦ Define important engine measurements and performance characteristics, including bore and stroke, displacement, compression ratio, engine efficiency, torque, and horsepower.

¦ Outline the basics of diesel, stratified, and Miller-cycle engine operation.

¦ Explain how to evaluate the condition of an engine.

¦ List and describe nine abnormal engine noises.

+++1 Today's engines are complex, efficient machines.

+++2 A cylinder block for an eight-cylinder engine.

+++3 A cylinder head for a late-model inline four-cylinder engine.

+++4 A typical late-model engine.

INTRODUCTION TO ENGINES

The engine provides the power to drive the vehicle's wheels. All automobile engines, both gasoline and diesel, are classified as internal combustion engines because the combustion or burning that creates energy takes place inside the engine.

The biggest part of the engine is the cylinder block. The cylinder block is a large casting of metal that is drilled with holes to allow for the passage of lubricants and coolant through the block and pro vide spaces for movement of mechanical parts. The block contains the cylinders, which are round passageways fitted with pistons. The block houses or holds the major mechanical parts of the engine.

The cylinder head fits on top of the cylinder block to close off and seal the top of the cylinder. The combustion chamber is an area into which the air-fuel mixture is compressed and burned.

The cylinder head contains all or most of the combustion chamber. The cylinder head also contains ports through which the air- fuel mixture enters and burned gases exit the cylinder and the bore for the spark plug.

The valve train is a series of parts used to open and close the intake and exhaust ports. A valve is a movable part that opens and closes the ports. A camshaft controls the movement of the valves. Springs are used to help close the valves.

The up-and-down motion of the pistons must be converted to rotary motion before it can drive the wheels of a vehicle. This conversion is achieved by linking the piston to a crankshaft with a connecting rod. The upper end of the connecting rod moves with the piston. The lower end of the connecting rod is attached to the crankshaft and moves in a circle. The end of the crankshaft is connected to the flywheel or flexplate.

Engine Construction:

Modern engines are highly engineered power plants.

These engines are designed to meet the performance and fuel efficiency demands of the public. Modern engines are made of lightweight engine castings and stampings; non-iron materials ( For example, aluminum, magnesium, fiber-reinforced plastics); and fewer and smaller fasteners to hold things together.

These fasteners are made possible through computerized joint designs that optimize loading patterns.

Each of these newer engine designs has its own distinct personality, based on construction materials, casting configurations, and design.

These modern engine-building techniques have changed how engine repair technicians make a living. Before these changes can be explained, it’s important to explain the "basics" of engine design and operation.

ENGINE CLASSIFICATIONS

Today's automotive engines can be classified in several ways depending on the following design features:

¦ Operational cycles. Most technicians will generally come in contact with only four-stroke engines.

However, a few older cars have used and some cars in the future will use a two-stroke engine.

¦ Number of cylinders. Current engine designs include 3-, 4-, 5-, 6-, 8-, 10-, and 12-cylinder engines.

¦ Cylinder arrangement. An engine can be flat (opposed), inline, or V-type. Other more complicated designs have also been used.

¦ Valve train type. Engine valve trains can be either the overhead camshaft (OHC) type or the cam shaft in-block overhead valve (OHV) type. Some engines separate camshafts for the intake and exhaust valves. These are based on the OHC design and are called double overhead camshaft (DOHC) engines. V-type DOHC engines have four camshafts-two on each side.

¦ Ignition type. There are two types of ignition systems: spark and compression. Gasoline engines use a spark ignition system. In a spark ignition sys tem, the air-fuel mixture is ignited by an electrical spark. Diesel engines, or compression ignition engines, have no spark plugs. A diesel engine relies on the heat generated as air is compressed to ignite the air-fuel mixture for the power stroke.

¦ Cooling systems. There are both air-cooled and liquid-cooled engines in use. Nearly all of today's engines have liquid-cooling systems.

¦ Fuel type. Several types of fuel currently used in automobile engines include gasoline, natural gas, methanol, diesel, and propane. The most commonly used is gasoline although new fuels are being tested.

+++The linear (reciprocating) motion of the pistons is converted to rotary motion by the crankshaft. Pistons; Connecting rod; Linear motion; Rotary motion; Crankshaft

+++ A cutaway of an engine showing the intake passages (blue) and valve and exhaust passage (red) and valve.

+++ The height and width of a cam lobe determine when and for how long a valve will be open. Closing ramp; Closing flank; Opening flank; Opening ramp; Nose; Heel; Lift Base circle; Duration

Four-Stroke Gasoline Engine:

In a passenger car or truck, the engine provides the rotating power to drive the wheels through the transmission and driving axles. All automotive engines, both gasoline and diesel, are classified as internal combustion because the combustion or burning takes place inside the engine. These systems require an air-fuel mixture that arrives in the combustion chamber at the correct time and an engine constructed to withstand the temperatures and pressures created by the burning of thousands of fuel droplets.

The combustion chamber is the space between the top of the piston and the cylinder head. It’s an enclosed area in which the fuel and air mixture is burned. The piston fits into a hollow metal tube, called a cylinder. The piston moves up and down in the cylinder.

This reciprocating motion must be converted to a rotary motion before it can drive the wheels of a vehicle. This change of motion is accomplished by connecting the piston to a crankshaft with a connecting rod. The upper end of the connecting rod moves with the piston as it moves up and down in the cylinder. The lower end of the connecting rod is attached to the crankshaft and moves in a circle. The end of the crankshaft is connected to the flywheel, which transfers the engine's power through the drive train to the wheels.

In order to have complete combustion in an engine, the right amount of fuel must be mixed with the right amount of air. This mixture must be com pressed in a sealed container, then shocked by the right amount of heat (spark) at the right time. When these conditions exist, all the fuel that enters a cylinder is burned and converted to power, which is used to move the vehicle. Automotive engines have more than one cylinder. Each cylinder should receive the same amount of air, fuel, and heat, if the engine is to run efficiently.

Although the combustion must occur in a sealed cylinder, the cylinder must also have some means of allowing heat, fuel, and air into it. There must also be a means to allow the burnt air-fuel mixture out so a fresh mixture can enter and the engine can continue to run. To accommodate these requirements, engines are fitted with valves.

There are at least two valves at the top of each cylinder. The air-fuel mixture enters the combustion chamber through an intake valve and leaves (after having been burned) through an exhaust valve. The valves are accurately machined plugs that fit into machined openings. A valve is said to be seated or closed when it rests in its opening. When the valve is pushed off its seat, it opens.

A rotating camshaft, driven and timed to the crankshaft, opens and closes the intake and exhaust valves. Cams are raised sections of a shaft that have high spots called lobes. Cam lobes are oval shaped.

The placement of the lobe on the shaft determines when the valve will open. The height and shape of the lobe determines how far the valve will open and how long it will remain open in relation to piston movement.

As the camshaft rotates, the lobes rotate and push the valve open by pushing it away from its seat. Once the cam lobe rotates out of the way, the valve, forced by a spring, closes. The camshaft can be located either in the cylinder block or in the cylinder head.

When the action of the valves and the spark plug is properly timed to the movement of the piston, the combustion cycle takes place in four strokes of the piston: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. The camshaft is driven by the crankshaft through gears, or sprockets, and a cogged belt, or timing chain. The camshaft turns at half the crankshaft speed and rotates one complete turn during each complete four-stroke cycle.

Four-Stroke Cycle-- A stroke is the full travel of the piston either up or down in a cylinder's bore. The reciprocal movement of the piston during the four strokes is converted to a rotary motion by the crankshaft. It takes two full revolutions of the crankshaft to complete the four-stroke cycle. One full revolution of the crankshaft is equal to 360 degrees of rotation; therefore, it takes 720 degrees to complete the four-stroke cycle. During one piston stroke, the crankshaft rotates 180 degrees.

Flywheel--The piston moves by the pressure produced during combustion, but this moves the piston only about half a stroke or one-quarter of a revolution of the crankshaft. This explains why a flywheel is needed. The flywheel stores some of the power produced by the engine. This power is used to keep the pistons in motion during the rest of the four-stroke cycle. A heavy flywheel is only found on engines equipped with a manual transmission. Engines with automatic transmissions have a flexplate and a torque converter. The weight and motion of the fluid inside the torque converter serve as a flywheel.

Intake Stroke-- The first stroke of the cycle is the intake stroke. As the piston moves away from top dead center (TDC), the intake valve opens. The downward movement of the piston increases the volume of the cylinder above it, reducing the pressure in the cylinder. This reduced pressure, commonly referred to as engine vacuum, causes the atmospheric pressure to push a mixture of air and fuel through the open intake valve. (Some engines are equipped with a super- or turbocharger that pushes more air past the valve.) As the piston reaches the bottom of its stroke, the reduction in pressure stops, causing the intake of air-fuel mixture to slow down. It does not stop because of the weight and movement of the air-fuel mixture. It continues to enter the cylinder until the intake valve closes. The intake valve closes after the piston has reached bottom dead center (BDC). This delayed closing of the valve increases the volumetric efficiency of the cylinder by packing as much air and fuel into it as possible.

Compression Stroke-- The compression stroke begins as the piston starts to move from BDC. The intake valve closes, trapping the air-fuel mixture in the cylinder ( +++ - 8B). The upward movement of the piston compresses the air-fuel mixture, thus heating it up. At TDC, the piston and cylinder walls form a combustion chamber in which the fuel will be burned. The volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at TDC determines the compression ratio of the engine.

Power Stroke --The power stroke begins as the com pressed fuel mixture is ignited ( +++8C). With the valves still closed, an electrical spark across the electrodes of a spark plug ignites the air-fuel mixture.

The burning fuel rapidly expands, creating a very high pressure against the top of the piston. This drives the piston down toward BDC. The downward movement of the piston is transmitted through the connecting rod to the crankshaft.

+++8 (A) Intake stroke, (B) compression stroke, (C) power stroke, and (D) exhaust stroke.

Exhaust Stroke The exhaust valve opens just before the piston reaches BDC on the power stroke ( +++8D). Pressure within the cylinder causes the exhaust gas to rush past the open valve and into the exhaust system. Movement of the piston from BDC pushes most of the remaining exhaust gas from the cylinder. As the piston nears TDC, the exhaust valve begins to close as the intake valve starts to open. The exhaust stroke completes the four-stroke cycle. The opening of the intake valve begins the cycle again. This cycle occurs in each cylinder and is repeated over and over, as long as the engine is running.

Firing Order:

An engine's firing order states the sequence in which an engine's pistons are on their power stroke and therefore the order in which the cylinders' spark plugs fire. The firing order also indicates the position of all of the pistons in an engine when a cylinder is firing.

For example, consider a four-cylinder engine with a firing order of 1-3-4-2. The sequence begins with piston #1 on the compression stroke. During that time, piston #3 is moving down on its intake stroke, #4 is moving up on its exhaust stroke, and #2 is moving down on its power stroke. These events are identified by what needs to happen in order for #3 to be ready to fire next, and so on.

The firing order of an engine is determined by its design and manufacturer's preference. An engine's firing order can be found on the engine or on the engine's emissions label and in service manuals.

+++9 shows some of the common cylinder arrangements and their associated firing orders.

+++10 A two-stroke cycle. Intake bypass port Intake port Exhaust port Crankcase

+++9 Examples of cylinder numbering and firing orders. IN-LINE 4-Cylinder

6-Cylinder: COMMON CYLINDER NUMBERING AND FIRING ORDER

Firing Order 1-3-4-2 -- 1-2-4-3 -- Firing Order 1-5-3-6-2-4

+++11 Normal combustion. Federal-Mogul Corporation. 1. Spark occurs 2. Combustion begins 3. Continues rapidly 4. And is completed.

Two-Stroke Gasoline Engine

In the past, several imported vehicles have used two stroke engines. As the name implies, this engine requires only two strokes of the piston to complete all four operations: intake, compression, power, and exhaust ( +++10). This is accomplished as follows:

1. Movement of the piston from BDC to TDC completes both intake and compression.

2. When the piston nears TDC, the compressed air fuel mixture is ignited, causing an expansion of the gases. During this time, the intake and exhaust ports are closed.

3. Expanding gases in the cylinder force the piston down, rotating the crankshaft.

4. With the piston at BDC, the intake and exhaust ports are both open, allowing exhaust gases to leave the cylinder and air-fuel mixture to enter.

Although the two-stroke-cycle engine is simple in design and lightweight because it lacks a valve train, it has not been widely used in automobiles.

It tends to be less fuel efficient and releases more pollutants into the atmosphere than four-stroke engines. Oil is often in the exhaust stream because these engines require constant oil delivery to the cylinders to keep the piston lubricated. Some of these engines require a certain amount of oil to be mixed with the fuel.

Engine Rotation -- To meet the standards set by the SAE, nearly all engines rotate in a counterclockwise direction. This can be confusing because its apparent direction changes with what end of the engine you look at. If one looks at the front of the engine, it rotates in a clockwise direction. The standards are based on the rotation of the flywheel, which is at the rear of the engine, and there the engine rotates counter clockwise.

+++ The cylinder block for an inline engine.

Combustion:

Although many different things and events can affect combustion in the engine's cylinders, the ignition system has the responsibility for beginning and maintaining the combustion process. Obviously when combustion does not occur in all of the cylinders, the engine won’t run. If combustion occurs in all but one or two cylinders, the engine may start and run but will run poorly. The lack of combustion is not always caused by the ignition system. Poor combustion can also be caused by problems in the engine, air-fuel system, or the exhaust system.

When normal combustion occurs, the burning process moves from the gap of the spark plug across the compressed air-fuel mixture. The movement of this flame front should be rapid and steady and should end when all of the air-fuel mixture has been burned. During normal combustion, the rapidly expanding gases push down on the piston with a powerful but constant force.

When all of the air and fuel in the cylinder are involved in the combustion process, complete combustion has occurred. When something pre vents this, the engine will mis? re or experience incomplete combustion. Mis? res cause a variety of driveability problems, such as a lack of power, poor gas mileage, excessive exhaust emissions, and a rough running engine.

+++A V-type engine.

+++A horizontally opposed cylinder engine, commonly called a boxer engine.

+++The basic valve train for an overhead valve engine. Lifter Camshaft Pushrod Valve Rocker arm Spring

Engine Configurations:

Depending on the vehicle, either an inline, V-type, slant, or opposed cylinder design can be used. The most popular designs are inline and V-type engines.

Inline Engine--In the inline engine design, the cylinders are all placed in a single row. There is one crankshaft and one cylinder head for all of the cylinders. The block is cast so that all cylinders are located in an upright position.

Inline engine designs have certain advantages and disadvantages. They are easy to manufacture and ser vice. However, because the cylinders are positioned vertically, the front of the vehicle must be higher. This affects the aerodynamic design of the car. Aerodynamic design refers to the ease with which the car can move through the air. When equipped with an inline engine, the front of a vehicle cannot be made as low as it can with other engine designs.

V-Type Engine--The V-type engine design has two rows of cylinders located 60 to 90 degrees away from each other. A V-type engine uses one crankshaft, which is connected to the pistons on both sides of the V. This type of engine has two cylinder heads, one over each row of cylinders.

One advantage of using a V-configuration is that the engine is not as high or long as one with an inline configuration. The front of a vehicle can now be made lower. This design improves the outside aerodynamics of the vehicle. If eight cylinders are needed for power, a V-configuration makes the engine much shorter, lighter, and more compact.

Many years ago, some vehicles had an inline eight cylinder engine. The engine was very long and its long crankshaft also caused increased torsional vibrations in the engine.

A variation of the V-type engine is the W-type engine. These engines are basically two V-type engines joined together at the crankshaft. This design makes the engine more compact. They are commonly found in late-model Volkswagens.

Slant Cylinder Engine--Another way of arranging the cylinders is in a slant configuration. This arrangement is much like an inline engine, except the entire block has been placed at a slant. The slant engine was designed to reduce the distance from the top to the bottom of the engine. Vehicles using the slant engine can be designed more aerodynamically.

Opposed Cylinder Engine--In this design, two rows of cylinders are located opposite the crankshaft. These engines have a common crankshaft and a cylinder head on each bank of cylinders.

Porsche’s and Subaru’s use this style of engine, commonly called a boxer engine. Boxer engines have a low center of gravity and tend to run smoothly during all operating conditions.

Camshaft and Valve Location:

The valves in all modern engines are placed in the cylinder head above the top of the piston. The valves in many older engine designs were placed to the side of the piston. Camshafts are located inside the engine block or above the cylinder head. The placement of the camshaft further describes an engine.

Overhead Valve (OHV)--As the same implies, the intake and exhaust valves in an OHV engine are mounted in the cylinder head and are operated by a camshaft located in the cylinder block. This arrangement requires the use of valve lifters, pushrods, and rocker arms to transfer camshaft rotation to valve movement. Overhead Cam (OHC) An OHC engine also has the intake and exhaust valves located in the cylinder head.

But as the name implies, the cam is located in the cylinder head. In an OHC engine, the valves are operated directly by the camshaft or through cam followers or tappets. Engines with one camshaft above a cylinder are often referred to as single over head camshaft (SOHC) engines.

+++Basic valve and camshaft placement in an overhead camshaft engine.

Camshaft--Valve spring retainer; Valve stem seal; Hydraulic lash adjuster; Engine Location

The engine is usually placed in one of three locations. In most vehicles, it’s located at the front of the vehicle, in front of the passenger compartment.

Front-mounted engines can be positioned either longitudinally or transversely with respect to the vehicle.

The second engine location is a mid-mount position between the passenger compartment and rear suspension. Mid-mount engines are normally transversely mounted. The third, and least common, engine location is the rear of the vehicle. The engines are typically opposed-type engines.

Each of these engine locations offers advantages and disadvantages.

Front Engine Longitudinal --In this type of vehicle, the engine, transmission, front suspension, and steering equipment are installed in the front of the body, and the differential and rear suspension are installed in the rear of the body. Most front engine longitudinal vehicles are rear-wheel drive. Some front-wheel-drive cars with a transaxle have this configuration, and most four-wheel-drive vehicles are equipped with a transfer case and have the engine mounted longitudinally in the front of the vehicle.

Total vehicle weight can be evenly distributed between the front and rear wheels with this configuration. This lightens the steering force and equalizes the braking load. With this design, it’s possible to independently remove and install the engine, propeller shaft, differential, and suspension. Longitudinally mounted engines require large engine compartments. The need for a rear-drive propeller shaft and differential also cuts down on passenger compartment space.

Front Engine Transverse-- Front engines that are mounted transversely sit sideways in the engine compartment. They are used with transaxles that combine transmission and differential gearing into a single compact housing, fastened directly to the engine.

Transversely mounted engines reduce the size of the engine compartment and overall vehicle weight.

Transversely mounted front engines allow for down-sized, lighter vehicles with increased interior space. However, most of the vehicle weight is toward the front of the vehicle. This provides for increased traction by the drive wheels. The weight also places a greater load on the front suspension and brakes.

Mid-Engine Transverse--In this design, the engine and drivetrain are positioned between the passenger compartment and rear axle. Mid-engine location is used in smaller, rear-wheel-drive, high-performance sports cars for several reasons. The central location of heavy components results in a center of gravity very near the center of the vehicle, which vastly improves steering and handling. Since the engine is not under the hood, the hood can be sloped downward, improving aerodynamics and increasing the driver's field of vision. However, engine access and cooling efficiency are reduced. A barrier is also needed to reduce the transfer of noise, heat, and vibration to the passenger compartment.

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