10212 Air Standard Otto Cycle

The Otto cycle is the idealized air-standard version of the practical cycle used in SI engines. The airstandard Otto cycle assumes that heat addition occurs instantaneously under constant volume when the piston is at the TDC. The cycle is illustrated on ap-v(pressure-volume) diagram in Figure 10.6. The intake stroke starts with the intake valve opening at the TDC to draw a fresh charge into the cylinder. The intake valve is open between 1 to 5 to take the fresh charge, which is a mixture of fuel and air. The volume of the cylinder increases as the piston moves down to allow more charge into the cylinder. The stroke ends with the piston reaching the BDC

FIGURE 10.6 p-v diagram of an air-standard Otto cycle.

FIGURE 10.6 p-v diagram of an air-standard Otto cycle.

when the intake valve closes at that position. This state point at the BDC is labeled as 1. In the next process between 1 and 2, work is done on the charge by the piston to compress the charge, thereby increasing its temperature and pressure. This is the compression cycle, when the piston moves up with both valves closed. Process 1 to 2 is an isentropic (constant entropy) compression, as the piston moves from the BDC to the TDC. The combustion starts near the end of the compression stroke in SI engines, when the high-pressure, high-temperature fluid is ignited by the spark plug. The pressure thus rises at constant volume to state point 3. Process 2 to 3 is the rapid combustion process when heat is transferred at constant volume to the air from the external source. The next stroke is the expansion or power stroke, when the gas mixture expands, and work is done by the charge on the piston, forcing it to return to the BDC. Process 3 to 4 represents the isentropic expansion when work is done on the piston. The final stroke is the exhaust stroke, which starts with the opening of the exhaust valve near 4. During Process 4 to 1, the heat is rejected, while the piston is at BDC. Process 1 to 5 represents the exhaust of the burnt fuel at essentially constant pressure. At 5, the exhaust valve closes, and the intake valve opens; the cylinder is now ready to draw in fresh charge for a repeat of the cycle.

SI engines can be four-stroke or two-stroke engines. Two-stroke engines run on the two-stroke Otto cycle, where the intake, compression, expansion, and exhaust operations are accomplished in one revolution of the crankshaft. The two-stroke cycles are used in smaller engines, such as those used in motorbikes.

Most SI engines, or gasoline engines as more commonly known, run on a modified Otto cycle. The air-fuel ratio used in these engines is between 10/1 to 13/1. The compression ratios are in the range of 9 to 12 for most production vehicles. The compression ratio of the engine is limited by the octane rating of the fuel. If the octane number of the fuel is too low, a high compression ratio may lead to auto-ignition of the air-fuel mixture during compression, which is completely undesirable in a SI engine. SI engines were originally developed by limiting the amount of air allowed into the engine using a carburetor. The carburetor is the throttling valve placed on the air intake. However, fuel injection, which is used for diesel engines, is now common for gasoline engines with SI. The control issue for fuel injection systems is to compute the mass flow rate of air into the engine at any instant of time and to mix the correct amount of gasoline with it, such that the air and fuel mixture is right for the engine running condition. In recent years, requirements to meet the strict exhaust gas emission regulations have increased the demand for fuel injection systems.


FIGURE 10.7 Torque-speed characteristics of a gasoline engine.

The torque-speed characteristics of a SI or gasoline engine are shown in Figure 10.7. The engine has a narrow high torque range, which also requires high enough rpm of the engine. The narrow high-torque region burdens the transmission gear requirements of SI engines.

SI engines are widely used in automobiles, and continuous development has resulted in engines that easily meet current emission and fuel economy standards. Currently, SI engines are of the lowest cost engines, but the question remains as to whether it will be possible to meet future emission and fuel economy standards at a reasonable cost. The SI engine also has a few other drawbacks, which include the throttling plate used to restrict the air intake. The partial throttle operation is poor in SI engines due to throttle irreversibility, a problem that is nonexistent in diesel engines. In general, the throttling process leads to a reduction in efficiency of the SI engine. The losses through bearing friction and sliding friction further reduce the efficiency of the engine.

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