Four-Stroke Spark-Ignition Engines
5-04 Explain the Otto four-stroke cycle.
A four-stroke engine design may be either an in-line, V-type arrangement, or opposed cylinder. The SI engine used in today’s vehicles operates on a four-stroke principle. The four-stroke engine receives its name because the piston must travel the distance of the cylinder, known as stroke, four times to complete one engine cycle. During the four strokes, the crankshaft makes two complete revolutions. Only one of the crankshaft’s rotations, however, develops power; the other rotation consumes power. The four strokes that are required to make one cycle are intake, compression, power, and exhaust. All of these events are necessary for the engine to function correctly. In actual operation, these four events can and usually do overlap each other or occur at the same time.
The strokes operate in 180 degrees of crankshaft rotation for simplicity; however, this is not an actual representation of engine operation. When the piston in a cylinder is at the position farthest away from the crankshaft (centerline of the journal), it is at TDC. When the piston in the cylinder is at a position closest to the crankshaft, this is bottom dead center (BDC). When the piston moves from TDC to BDC or from BDC to TDC, one stroke has occurred (FIGURE 5-16). Two or more strokes are called reciprocating motion, meaning an up-and-down motion within the cylinder. Another name for a piston engine is a reciprocating engine.
FIGURE 5-16 Piston movement from TDC to BDC or from BDC to TDC is one stroke.
Basic Four-Stroke Operation
To have a combustion event the intake and exhaust of fuel and fumes, the engine must perform a certain set of tasks. These tasks are categorized into a type of operation—in this case, the basic four-stroke operation of an ICE. These tasks include intake, compression, power, and exhaust events. What happens in each stroke is explained below.
Intake Stroke
The first stroke in a four-stroke cycle is the intake stroke. The air-fuel mixture starts to be introduced into the combustion chamber with the piston at TDC as the intake stroke begins. The intake valve is partially open as it prepares to open during the final stages of the exhaust stroke (valve overlap). Valve overlap is the time when both the intake and exhaust valves are open at the same time and is necessary due to the time in degrees of crankshaft rotation that are required to open the intake valve. The piston is moving down the cylinder from TDC to BDC. As the piston moves down the bore, it creates a larger volume area above the top of the piston. As the volume above the piston increases, it creates a pressure lower than atmospheric (a partial vacuum) in the cylinder. It is commonplace to assume that the vacuum created here is solely responsible for the cylinder filling to reach maximum VE, but that assumption is only partially correct. Piston velocity plays a crucial part in increasing airflow and cylinder filling. The calculation to determine piston speed uses piston stroke, rod length, piston pin offset, and rpm. As a piston reaches its maximum velocity (approximately 70–80 degrees after top dead center (ATDC)), the greatest pressure difference exists between atmospheric pressure and cylinder pressure. This difference in pressure increases airflow substantially. With the intake valve opening, higher outside air pressure (atmospheric) forces an air charge that usually contains fuel (unless in a gasoline direct-injection engine) into the cylinder. As the piston continues traveling down the cylinder, the exhaust valve fully closes while the intake valve, which is fully open, begins to close as the piston reaches BDC. When the piston reaches the bottom of its travel, or BDC, pressure no longer increases, so the pressure differential is now almost equal, slowing airflow inertia. The intake valve is still open slightly after BDC, using the momentum from the incoming intake charge to continue filling the cylinder, even though the piston speed slows at BDC. Holding the intake valve open enhances VE.
Compression Stroke
The compression stroke follows the intake stroke and begins near BDC when the intake valve(s) close and the piston starts moving up the cylinder bore. As the piston reaches BDC, two events occur simultaneously: the intake valve almost closes, and the piston speed slows substantially. The intake valve closes fully after BDC. With both valves fully closed, the cylinder is now completely sealed. The piston then begins the trek back to TDC on the compression stroke, squeezing the air-fuel mixture into a smaller volume that elevates the pressure and temperature in the cylinder. The cylinder pressure continues to increase until the piston reaches TDC, where the air-fuel charge has reached its design limits, or maximum compression ratio. The base compression ratio is determined during engine design and is fixed. Remember that the intake and compression processes started with approximately 15 psi (1 bar) of atmospheric pressure exerted on the air-fuel charge. Compression of the air-fuel mixture facilitates igniting the charge and makes combustion (burning of fuel) more complete and efficient. This event is crucial because the burn needs to be regulated—not be an explosion, which is detonation. A result of detonation is engine damage, usually melted ring lands. Accurately controlled burning returns the piston at maximum velocity to BDC.
TECHNICIAN TIPAn engine with a static compression ratio of 10:1 means the air-fuel mixture is compressed ten times. For example, 10 × 15 = 150 psi. The 10 is the com-pression ratio, and the 15 is the atmo-spheric pressure (in psi) that is in the cylinder when the intake valve opens. In this example, the compression ratio of the engine is 150 psi during the com-pression stroke.
Power Stroke
The power stroke is the third stroke, the only stroke during the four-stroke cycle that generates power, or when internal combustion occurs. Both valves remain closed as the piston reaches TDC of the compression stroke. A spark of more than 20,000 volts, created by the ignition coil jumping the spark plug gap, begins the power stroke by igniting the compressed air-fuel mixture with the piston approaching TDC with both valves closed. The air-fuel mixture burns rapidly, reaching temperatures as high as 4,500°F (2,482°C). The expanding gases quickly force the piston back down the bore by creating a large pressure area across the top of the piston from combustion. The exhaust valve starts to open before the piston has reached BDC. The majority of the gas pressure from combustion diminishes between 45 and 90 degrees of crankshaft rotation ATDC. When the crankpin reaches 90 degrees ATDC, recovery of most of the power is complete and cylinder pressures are low. The exhaust valve begins to open as the piston travels back down the bore before bottom dead center (BBDC), aiding in expelling exhaust gases by increasing the time during which the valve is open. Opening the exhaust valve during the power stroke also assists in reducing cylinder pressure and therefore also pumping losses. Pumping losses occur because of the mechanical movement of the components in the engine requires power to operate, which decreases the available power to use at the flywheel.
Exhaust Stroke
The final stroke of the four-stroke cycle is the exhaust stroke. As the power stroke ends, the piston moves from BDC back to TDC to expel the burned gases from the cylinder, through the open exhaust valve(s). During the exhaust stroke, several other engine operations are taking place simultaneously.
Due to camshaft design, the exhaust stroke actually begins before the piston reaches BDC during the downward movement of the piston from the power stroke, as discussed earlier. Although combustion has ceased, pressure remains in the cylinder from the combustion event. The exhaust valve opens, releasing cylinder pressure through the exhaust port. Then the piston travels back up the bore to TDC, forcing out the remaining gases that are in the cylinder. The exhaust valve is opening rapidly as the piston begins its ascent. As piston velocity increases, the exhaust valve needs to be fully open to reduce resistance (pumping losses), which hampers power and fuel economy. Delaying exhaust valve closure until after the piston has reached TDC and begins its descent takes advantage of the exhaust gas velocity, increasing cylinder scavenging. Delaying exhaust valve closure and starting to open the intake valve on the exhaust stroke (valve overlap) also aids in reducing pumping losses. Valve overlap increases the cylinder fill during the intake stroke, a result of the negative pressure (vacuum) in the cylinder—which is a process known as scavenging. The vacuum created allows more air-fuel mixture to fill the cylinder and ensure that the exhaust gases purge efficiently.
This completes the four-stroke cycle. Note that the crankshaft has completed two full rotations (720 degrees) during the complete four-stroke cycle while the camshaft(s) have completed only one rotation (360 degrees). The piston has traveled from TDC to BDC four times, or strokes, to complete one cycle—hence the name four-stroke cycle.
Engine, Thermal, and Mechanical Efficiency
A combustion engine is a device that transforms the chemical energy stored in a fuel into heat energy and then converts a portion of that heat energy into mechanical work. Most ICEs are incredibly inefficient at turning burned fuel into usable energy, despite current automotive engine designs, including forced induction, lightweight engine materials, variable valve lift and timing, stop/start technology, etc. Most gasoline combustion engines average around 20–30% thermal efficiency. Diesel engines are typically higher, approaching 40%.
There are several ways to find the efficiency of an engine. Thermal efficiency is the percentage of energy that is converted to mechanical work, taken from combustion. The thermal efficiency of a typical low-compression engine is about 26%. Highly modified engines or race engines typically have a thermal efficiency of about 34%.
TECHNICIAN TIPOtto engines suffer a significant loss of efficiency, known as the “pumping loss,” during low-power operation. A throttle plate restricts airflow into the engine. Limiting airflow reduces the amount of fuel required, reducing power output during low- to mid-rpm operation. The throttle plate forces the air to pass through a narrow opening between the plate and throttle body, creating a vacuum. This vacuum acts on top of the piston, slowing its cylinder movement. A slight pressure normally exists in the crankcase on the underside of the piston. This pressure also opposes the piston’s movement in the bore. The pumping loss occurs when the piston must overcome the vacuum and pressure drag created on the crankshaft’s rotation, wasting energy. This effect is present during most engine operating speeds other than wide-open throttle when atmospheric pressure flows into the manifold without a restriction. Therefore, increasing the throttle opening reduces the pumping losses, increasing an engine’s efficiency.
Mechanical efficiency is the percentage of energy that an engine produces after subtracting mechanical losses such as friction and rotational losses, compared to what the engine would deliver without any loss of power. The mechanical efficiency of the majority of engines is about 94%.
Engine Efficiency
The engine efficiency of a thermal engine is the relationship between the total energy contained in the fuel and the amount of energy used (extracted) in performing useful work. Engine efficiency is expressed as a percentage. Calculate an engine’s efficiency by measuring the energy that an engine requires compared to the energy produced.
Thermal Efficiency
Thermal efficiency is a measurement of an engine’s efficiency at turning burned fuel into usable energy, expressed as a percentage. The majority of gasoline engines average close to 20–30%. Diesel engines are slightly more efficient approaching 40%. An approximate rule of thumb is that nearly a third of the fuel energy goes out the exhaust pipe as lost heat. Another third of the fuel energy is lost to the cooling and lubrication systems that carry damaging heat away from the engine (coolant, oil, and surrounding airflow). This leaves roughly only a third of the energy (best case) available for power output, which is reduced further by tire rolling resistance. After totaling all the losses, only about 14%–30% of the energy from the fuel consumed in a modern conventional automotive engine is left to propel the vehicle, depending on the drive cycle (city, highway, or combined). Engine and driveline inefficiencies consume the remaining energy or are used to power the vehicle’s accessories.
Automotive manufacturers continue to strive for improved fuel efficiency by using and developing advanced technologies that will find their ways into a technician’s service bay. Forced induction, GDI, water injection, and increased use of the Atkinson engine design are current examples. Greater use of friction and thermal-reducing metallic, aluminum, and ceramic engine components, lower-friction piston rings, engine oil, and other vehicle lubricants are normal. Manufacturers continue to develop new technologies that may or may not make it into production vehicles in the future. An example is a new free piston engine: The linear generator (FPEG) from Toyota Central, in Maine, with a claimed thermal efficiency for the device approaching 42%, is under development and could power hybrids in the future.
Mechanical Efficiency
Mechanical efficiency is a percentage rating that shows the amount of power developed by the expansion of the gases in the cylinder to the total power capable of being delivered without any power loss. Friction in an ICE has the greatest effect on mechanical efficiency. Internal friction between an engine’s moving parts remains constant, for the most part, regardless of the engine’s rpm range. Operating an engine under low- to part-load reduces its mechanical efficiency. Consequently, an engine’s mechanical efficiency will be at its maximum when the engine is running at an rpm that delivers it maximum base horsepower.