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.
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.
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.
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.
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).
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
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.
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.
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.
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.
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.
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.
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 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.
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.