Internal Combustion - Spark ignition engine

Internal combustion engines are devices that generate work using the products of combustion as the working fluid rather than as a heat transfer medium. To produce work, the combustion is carried out in a manner that produces high-pressure combustion products  that can be expanded through a turbine or piston. The engineering of these highpressure  systems introduces a number of features that profoundly influence the formation of  pollutants.

Cross section showing one cylinder of a four-stroke internal-combustion engine. In the first stroke (credit: © Merriam-Webster Inc.)

There are three major types of internal combustion engines in use today:

 (1) The spark ignition engine, which is used primarily in automobiles;

 (2) The diesel engine, which is used in large vehicles and industrial systems where the improvements in cycle efficiency make it advantageous over the more compact and lighter-weight spark ignition engine;

 (3) The gas turbine, which is used in aircraft due to its high power/weight

ratio and also is used for stationary power generation.

Each of these engines is an important source of atmospheric pollutants. Automobiles are major sources of carbon monoxide, unburned hydrocarbons, and nitrogen oxides.

Probably more than any other combustion system, the design of automobile engines has been guided by the requirements to reduce emissions of these pollutants. While substantial progress has been made in emission reduction, automobiles remain important sources of air pollutants. Diesel engines are notorious for the black smoke they emit.

Gas turbines emit soot as well. These systems also release unburned hydrocarbons, carbon monoxide, and nitrogen oxides in large quantities.

SPARK IGNITION ENGINES

The operating cycle of a conventional spark ignition engine is illustrated in Figure 1.

The basic principle of operation is that a piston moves up and down in a cylinder,

transmitting its motion through a connecting rod to the crankshaft which drives the vehicle.

The most common engine cycle involves four strokes:

1. Intake. The descending piston draws a mixture of fuel and air through the open

intake valve.

Figure 1 Four-stroke spark ignition engine: stroke 1. intake; stroke 2. compression; stroke 3. power; stroke 4, exhaust.

2. Compression. The intake valve is closed and the rising piston compresses the fuelair

mixture. Near the top of the stroke, the spark plug is fired, igniting the mixture.

3. Expansion. The burning mixture expands, driving the piston down and delivering

power.

4. Exhaust. The exhaust valve opens and the piston rises, expelling the burned gas

from the cylinder.

The fuel and air mixture is commonly premixed in a carburetor. Figure 2 shows how engine power and fuel consumption depend on equivalence ratio over the range commonly used in internal combustion engines. Ratios below 0.7 and above 1.4 generally are not combustible on the time scales available in reciprocating engines. The maximum power is obtained at a higher ratio than is minimum fuel consumption. As a vehicle accelerates, high power is  needed and a richer mixture is required than when cruising at constant speed. We shall  return to the question of the equivalence ratio when we consider pollutant formation, since this ratio is one of the key factors governing the type and quantity of pollutants formed in the cylinder.

The ignition system is designed to ignite the air-fuel mixture at the optimum instant.

Prior to the implementation of emission controls, engine power was the primary concern in ignition timing. As engine speed increases, optimal power output is achieved by advancing the time of ignition to a point on the compression stroke before the piston reaches the top of its motion where the cylinder volume is smallest.

 

Figure 2 Variation of actual and indicated specific fuel consumption with equivalence ratio and load. BSFC denotes "brake specific fuel consumption. "

 

This is because the combustion of the mixture takes a certain amount of time, and optimum power is developed if the completion of the combustion coincides with the piston arriving at socalled top dead center. The spark is automatically advanced as engine speed increascs. Also, a pressure diaphragm senses airflow through the carburetor and advances the spark as airflow  increases.

Factors other than power output must be taken into account, however, in optimizing the  engine operation. If the fuel-air mixture is compressed to an excessive pressure, the mixture temperature can become high enough that the preflame reactions can ignite the charge ahead of the propagating flame front. This is followed by very rapid combustion of the remaining charge and a correspondingly fast pressure increase in the cylinder. The resultant pressure wave reverberates in the cylinder, producing the noise referred to as knock (By et al., 1981). One characteristic of the fuel composition is its tendency to autoignite, expressed in terms of an octane rating.

High compression ratios and ignition spark timing that optimize engine power and efficiency lead to high octane requirements. The octane requirement can be reduced by using lower compression ratios and by delaying the spark until after the point for optimum engine performance. Emission controls require additional compromises in engine design and operation, sacrificing some of the potential engine performance to reduce emissions.