After studying Chapter 3, the reader should be able to:
Prepare for Engine Repair (A1) ASE certification test content area “A” (General Engine Diagnosis).
Explain how a 4-stroke cycle gasoline engine operates.
List the various characteristics by which vehicle engines are classified.
Describe how engine power is measured and calculated.
Discuss how a compression ratio is calculated.
Explain how engine size is determined.
Describe how turbocharging or supercharging increases engine power.
Prepare for Engine Repair (A1) ASE certification test content area “E” (Fuel, Electrical, Ignition, and Exhaust System Inspection and Service).
Explain how a diesel engine works.
Describe the difference between direct injection (DI) and indirect injection (IDI) diesel engines.
List the parts of the typical diesel engine fuel system.
Explain how glow plugs work.
ENERGY AND POWER
Energy is used to produce power. The chemical energy in fuel is converted to heat by the burning of the fuel at a controlled rate. This process is called combustion. If engine combustion occurs within the power chamber, the engine is called an internal combustion engine.
Engines used in automobiles are internal combustion heat engines. They convert the chemical energy of the gasoline into heat within a power chamber that is called a combustion chamber.
4-STROKE CYCLE OPERATION
Intake stroke. The intake valve is open and the piston inside the cylinder travels downward, drawing a mixture of air and fuel into the cylinder in a gasoline engine.
Compression stroke. As the engine continues to rotate, the intake valve closes and the piston moves upward in the cylinder, compressing the air-fuel mixture.
Power stroke. When the piston gets near the top of the cylinder (called top dead center [TDC]), the spark at the spark plug ignites the air-fuel mixture, which forces the piston downward.
Exhaust Stroke. The engine continues to rotate, and the piston again moves upward in the cylinder. The exhaust valve opens, and the piston forces the residual burned gases out of the exhaust valve and into the exhaust manifold and exhaust system.
A piston that moves up and down, or reciprocates, in a cylinder can be seen in this illustration. The piston is attached to a crankshaft with a connecting rod.
Figure 3-1 The downward movement of the piston draws the air-fuel mixture into the cylinder through the intake valve on the intake stroke. On the compression stroke,the mixture is compressed by the upward movement of the piston with both valves closed. Ignition occurs at the beginning of the power stroke, and combustion drives the piston downward to produce power. On the exhaust stroke,the upward-moving piston forces the burned gases out the open exhaust valve.
Figure 3-2 Cutaway of an engine showing the cylinder, piston, connecting rod, and crankshaft.
THE 720 degrees CYCLE
Each cycle of events requires that the engine crankshaft make two complete revolutions or 720 degrees (360 degrees X 2 = 720 degrees ) . The greater the number of cylinders, the closer together the power strokes occur.
To find the angle between cylinders of an engine, divide the number of cylinders into 720 degrees .
Angle with three cylinders = 720 degrees / 3 = 240 degrees
Angle with four cylinders = 720 degrees / 4 = 180 degrees
Angle with five cylinders – 720 degrees / 5 = 144 degrees
To find the angle between cylinders of an engine, divide the number of cylinders into 720 degrees .
Angle with six cylinders = 720 degrees / 6 = 120 degrees
Angle with ten cylinders = 720 degrees / 10 = 72 degrees
A piston stroke is a one-way piston movement between the top and bottom of the cylinder. During one stroke, the crankshaft revolves 180 degrees (1/2 revolution). A cycle is a complete series of events that continually repeat.
An inline engine places all cylinders in a straight line.
A V-type engine, such as a V-6 or V-8, has the number of cylinders split and built into a V-shape.
Horizontally opposed 4- and 6-cylinder engines have two banks of cylinders that are horizontal, resulting in a low engine. This style of engine is used in Porsche and Subaru engines and are often called the boxer or pancake engine design.
Longitudinal or transverse mounting.
Engines may be mounted either parallel with the length of the vehicle (longitudinally) or crosswise (transversely).
Valve and camshaft number and location.
The valves are opened by a camshaft.
When the camshaft is located in the block, the valves are operated by lifters, pushrods, and rocker arms.
This type of engine is called a pushrod engine, a cam-in-block engine, or an overhead valve (OHV) engine.
When one overhead camshaft is used, the design is called a single overhead camshaft (SOHC) design. When two overhead camshafts are used, the design is called a double overhead camshaft (DOHC) design.
Figure 3-6 Two types of front-engine, front-wheel drive.
Figure 3-7 Cutaway of a V-8 engine showing the lifters, pushrods, roller rocker arms, and valves.
Figure 3-8 SOHC engines usually require additional components such as a rocker arm to operate all of the valves. DOHC engines often operate the valves directly.
Figure 3-9 A dual overhead camshaft (DOHC) V-8 engine with part of the cam cover cut away.
Figure 3-10 Rotary engine operates on the 4-stroke cycle but uses a rotor instead of a piston and crankshaft to achieve intake, compression, power, and exhaust strokes.
Figure 3-11 Inline 4-cylinder engine showing principal and nonprincipal ends. Normal direction of rotation is clockwise (CW) as viewed from the front or accessory belt end (nonprincipal end).
The diameter of a cylinder is called the bore.
Figure 3-12 The bore and stroke of pistons are used to calculate an engine’s displacement.
The distance the piston travels down in the cylinder is called the stroke.
Engine size is described as displacement. Displacement is the cubic inch (cu. in.) or cubic centimeter (cc) volume displaced or swept by all of the pistons.
A liter (L) is equal to 1000 cubic centimeters; therefore, most engines today are identified by their displacement in liters.
1 L = 1000 cc
1 L = 61 cu. in.
1 cu. in. = 16.4 cc
Bore X bore X stroke X 0.7854 X number of cylinders
For example, take a 6 cylinder engine where bore = 4.000 in., stroke = 3.000 in. Applying the formula: 4.000 in. X 4.000 in. X 3.000 in. X 0.7854 X 6 = 226 cu. in.
Because 1 cubic inch equals 16.4 cubic centimeters, this engine displacement equals 3706 cubic centimeters or, rounded to 3700 cubic centimeters, 3.7 liters.
The compression ratio of an engine is an important consideration when rebuilding or repairing an engine. Compression ratio (CR) is the ratio of the volume in the cylinder above the piston when the piston is at the bottom of the stroke to the volume in the cylinder above the piston when the piston is at the top of the stroke.
(Continued) Less ignition timing required to prevent spark knock (detonation) More advanced ignition timing possible without spark knock (detonation) Harder to crank engine, especially when hot Easier engine cranking Better fuel economy. Poorer fuel economy Higher power possible. Lower power If Compression Is Higher If Compression Is Lower
CR = Volume in cylinder with piston at bottom of cylinder (including the combustion chamber)
Volume in cylinder with piston at top center
What is the compression ratio of an engine with 50.3-cu. in. displacement in one cylinder and a combustion chamber volume of 6.7 cu. in.?
CR = 50.3 + 6.7 cu. in. = 57.0 = 8.5
6.7 cu. in. 6.7
Figure 3-13 Compression ratio is the ratio of the total cylinder volume (when the piston is at the bottom of its stroke) to the clearance volume (when the piston is at the top of its stroke).
Figure 3-14 Combustion chamber volume is the volume above the piston with the piston at top dead center.
Figure 3-15 The distance between the centerline of the main bearing journal and the centerline of the connecting rod journal determines the stroke of the engine. This photo is a little unusual because this is from a V-6 with a splayed crankshaft used to even the impulses on a 90 degree, V-6 engine design.
Torque is the term used to describe a rotating force that may or may not result in motion
The metric unit for torque is Newton-meters because Newton is the metric unit for force and the distance is expressed in meters.
1 pound-foot = 1.3558 Newton-meters
1 Newton-meter = 0.7376 pound-foot
Figure 3-16 Torque is a twisting force equal to the distance from the pivot point times the force applied expressed in units called pound-feet (lb.-ft.) or Newton-meters (N-m).
Work is defined as actually accomplishing movement when torque is applied to an object.
Figure 3-17 Work is calculated by multiplying force times distance. If you exert 100 pounds of force for 10 feet, you have done 1000 foot-pounds of work.
The term power means the rate of doing work. Power equals work divided by time. Work is achieved when a certain amount of mass (weight) is moved a certain distance by a force.
The power an engine produces is called horsepower (hp). One horsepower is the power required to move 550 pounds 1 foot in 1 second, or 33,000 pounds 1 foot in 1 minute (550 lb. X 60 sec = 33,000 lb.). This is expressed as 500 foot-pounds (ft. lb.) per second or 33,000 foot-pounds per minute.
The actual horsepower produced by an engine is measured with a dynamometer.
The horsepower is calculated from the torque readings at various engine speeds (in revolutions per minute or RPM). Horsepower is torque times RPM divided by 5252.
Horsepower = Torque X RPM
Figure 3-18 One horsepower is equal to 33,000 foot-pounds (200 lbs. X 165 ft.) of work per minute.
HORSEPOWER AND ALTITUDE
According to SAE conversion factors, a nonsupercharged or nonturbocharged engine loses about 3% of its power for every 1000 feet (300 meters [m]) of altitude.
An engine that develops 150 brake horsepower at sea level will only produce about 85 brake horsepower at the top of Pike's Peak in Colorado at 14,110 feet (4300 meters).
The diesel engine uses heat created by compression to ignite the fuel, so it requires no spark ignition system.
The diesel engine requires compression ratios of 16:1 and higher. Incoming air is compressed until its temperature reaches a high temperature called heat of compression.
A diesel engine uses a precision injection pump and individual fuel injectors.
In a diesel engine, air is not controlled by a throttle as in a gasoline engine. Instead, the amount of fuel injected is varied to control power and speed. The air-fuel mixture of a diesel can vary from as lean as 85:1 at idle, to as rich as 20:1 at full load.
Indirect and Direct Injection
In an indirect injection (abbreviated IDI), diesel fuel is injected into a small prechamber, which is connected to the cylinder by a narrow opening. The initial combustion takes place in this prechamber.
In a direct injection (abbreviated DI) diesel engine, fuel is injected directly into the cylinder. The piston incorporates a depression where initial combustion takes place. Direct injection diesel engines are generally more efficient than indirect injection engines, but have a tendency to produce greater amounts of noise.
Diesel Fuel Ignition
Ignition occurs in a diesel engine by injecting fuel into the air charge which has been heated by compression to a temperature greater than the ignition point of the fuel or about 1000 degrees F (540 degrees C ).
Figure 3-19 Diesel combustion occurs when fuel is injected into the hot, highly compressed air in the cylinder.
Figure 3-20 A typical injector-pump-type automotive diesel fuel-injection system.
Figure 3-21 An indirect injection diesel uses a prechamber and a glow plug.
Figure 3-22 A direct injection diesel engine injects the fuel directly into the combustion chamber. Many designs do not use a glow plug.
THREE PHASES OF COMBUSTION
There are three distinct phases or parts to the combustion in a diesel engine.
Ignition delay. Near the end of the compression stroke, fuel injection begins, but ignition does not begin immediately. This period is called delay.
Rapid combustion. This phase of combustion occurs when the fuel first starts to burn, creating a sudden rise in cylinder pressure. It is this rise in combustion chamber pressure that causes the characteristic diesel engine knock.
Controlled combustion. After the rapid combustion occurs, the rest of the fuel in the combustion chamber begins to burn and injection continues. This is an area near the injector that contains fuel surrounded by air. This fuel burns as it mixes with the air.
Figure 3-23 The common rail on a Cummins diesel engine. A high-pressure pump (up to 30,000 psi) is used to supply diesel fuel to this common rail, which has tubes running to each injector. Note the thick cylinder walls and heavy-duty construction.
Figure 3-24 A rod/piston assembly from a 5.9-liter Cummins diesel engine used in a Dodge pickup truck.
FUEL TANK AND LIFT PUMP
A larger filler neck for diesel fuel. Gasoline filler necks are smaller for the unleaded gasoline nozzle.
No evaporative emission control devices or charcoal (carbon) canister. Diesel fuel is not as volatile as gasoline and, therefore, diesel vehicles do not have evaporative emission control devices.
The diesel fuel is drawn from the fuel tank by a lift pump and delivers the fuel to the injection pump. Between the fuel tank and the lift pump is a water-fuel separator.
Figure 3-25 Using an ice bath to test the fuel temperature sensor.
Injection pumps are usually driven by the camshaft at the front of the engine.
Distributor Injection Pump
A distributor diesel injection pump is a high-pressure pump assembly with lines leading to each individual injector. The high-pressure lines between the distributor and the injectors must be the exact same length to ensure proper injection timing.
Newer diesel engines use a fuel delivery system referred to as a common rail design. Diesel fuel under high pressure, over 20,000 psi (138,000 kPa), is applied to the injectors, which are opened by a solenoid controlled by the computer.
Figure 3-26 A typical distributor-type diesel injection pump showing the pump, lines, and fuel filter.
Figure 3-27 Overview of a computer-controlled common rail V-8 diesel engine.
DIESEL INJECTOR NOZZLES
Diesel injector nozzles are spring-loaded closed valves that spray fuel directly into the combustion chamber or precombustion chamber. Injector nozzles are usually threaded into the cylinder head, one for each cylinder, and are replaceable as an assembly.
Parts of a diesel injector nozzle include:
Diesel injector needle valve.
Injector pressure chamber.
DIESEL INJECTOR NOZZLE OPERATION
The electric solenoid attached to the injector nozzle is computer controlled and opens to allow fuel to flow into the injector pressure chamber.
The fuel flows down through a fuel passage in the injector body and into the pressure chamber.
The high fuel pressure in the pressure chamber forces the needle valve upward, compressing the needle valve return spring and forcing the needle valve open. When the needle valve opens, diesel fuel is discharged into the combustion chamber in a hollow cone spray pattern.
Glow plugs are always used in diesel engines equipped with a precombustion chamber and may be used in direct injection diesel engines to aid starting. A glow plug is a heating element that uses 12 volts from the battery and aids in the starting of a cold engine.
As the temperature of the glow plug increases, the resistance of the heating element inside increases, thereby reducing the current in amperes needed by the glow plugs.
Figure 3-29 A schematic of a typical glow plug circuit. Notice that the relay for the glow plug and intake air heater are both computer controlled.
ENGINE-DRIVEN VACUUM PUMP
Because a diesel engine is unthrottled, it does not create a vacuum in the intake manifold.
Most diesels used in cars and light trucks are equipped with an engine-driven vacuum pump to supply the vacuum for these components.
DIESEL ENGINE ADVANTAGES
More torque output
Greater fuel economy
Longer service life
DIESEL ENGINE DISADVANTAGES
Engine noise, especially when cold and/or at idle speed
Cold weather startability
A vacuum pump is needed to supply the vacuum needs of the heat, ventilation, and air conditioning system
Heavier than a gasoline engine
Figure 3-30 A roller lifter from a GM Duramax 6.6-liter V-8 diesel engine. Notice the size of this lifter compared to a roller lifter used in a gasoline engine.