ENGINE COMBUSTION AND EMISSIONS

B URNING V ELOCITY

4.3. ENGINE COMBUSTION AND EMISSIONS

combustion duration results in higher power output, and therefore increased thermal efficiency.

Since burning rates are generally highest close to the stoichiometric air–fuel ratio, operating an spark ignition (SI) engine lean, with an equivalence ratio of less than one, results in increased combustion duration. As can be seen from Figure 4.3, this then reduces power output and thermal efficiency, thereby tending to counteract the in- creased efficiency of lean operation due to an increased ratio of specific heats, as seen in Figure 4.2. It is important, therefore, when choosing to operate an SI engine under lean-burn conditions to design the combustion system to provide a high burning rate.

diesel engine, for example. The homogeneous fuel–air mixture always present in the cylinder results in another characteristic of the spark-ignition engine– knock. Knock occurs when unburned mixture self-ignites due to the increasing cylinder pressure as a result of combustion of the bulk of the mixture. Persistent knock causes very rough engine operation, and can cause engine failure if it is not controlled. This problem is exacerbated by high compression ratios and fuels which readily self-ignite at the temperature achieved following compression (low octane fuels). Knock is the principal reason why spark-ignition engines are usually limited to a compression ratio of less than approximately 10:1 with currently available fuels. This relatively low compression ratio results in lower thermal efficiency compared to diesel engines operating at approximately twice the compression ratio, as we have seen in the previous section.

4.3.2. S

^TRATIFIED

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^HARGE

E

^NGINES

The stratified-charge engine is something of a hybrid between the homogeneous charge spark-ignition engine and the diesel engine. The concept is aimed at incorpor- ating some of the design features of each engine in order to achieve some advantages of both. The result has been an engine more nearly like the spark-ignition engine, but one in which much leaner operation can be achieved and which is able to burn a wide variety of fuels. One example of a stratified-charge engine combustion chamber is shown schematically in plan view in Figure 4.4, taken from Benson and Whitehouse (1979). Air is introduced into the cylinder without fuel as in the diesel engine, although a throttle is still used to regulate the quantity of air, as in the spark-ignition engine. The inlet port is shaped so that the air has a strong swirling motion as it enters the combustion chamber, as shown in the figure. Fuel is introduced directly into the chamber through the fuel injection nozzle, mixed with the swirling air and then swept past the spark plug. The injection of fuel and firing of the spark plug are timed so that the local air–fuel ratio near the spark plug is nearly stoichiometric at the instant the spark is discharged. The mixture then burns near the spark plug and downstream of the spark plug as a premixed flame, while in the rest of the cylinder the mixture is very lean.

The fact that fuel is introduced at one point in the chamber and burnt immediately means that the overall air–fuel ratio can be very lean, while the local ratio is near stoichiometric, and is a direct result of the stratified nature of the charge. The stratified- charge engine is thus able to operate over a much wider range of air–fuel ratios than a

1 2

3

4 Spark plug

Nozzle Direction of air swirl

Figure 4.4 Stratified-charge engine combustion chamber (Benson and Whitehouse, 1979).

conventional spark-ignition engine. This results in higher efficiency due to leaner overall air–fuel ratios and reduced pumping losses, since less throttling is required.

Mention should be made of some pre-chamber engines, such as the Honda CVCC which might be called PSC engines. These engines usually employ a second intake valve and fuel system to introduce a rich mixture into a small pre-chamber. This mixture is ignited with a spark plug, and the hot combustion products pass as a jet into the main chamber which has been filled with a very weak mixture through the main inlet valve and fuel system. The expanding flame front from the pre-chamber is able to ignite a much leaner mixture in the main chamber than can a conventional spark plug. These engines are then able to operate at leaner overall air–fuel ratios and higher efficiency than conventional spark-ignition engines. More recently, the PSC approach has been developed without the use of a separate pre-chamber, and these will be discussed in more detail in a later section.

4.3.3. S

^PARK

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^GNITION

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^NGINE

E

^MISSIONS

The emission levels of a spark-ignition engine are particularly sensitive to air–fuel ratio. This can be seen in Figure 4.5, taken from Heywood (1988), which shows schematically the level of emissions from a spark-ignition or Otto cycle engine as a function of relative air–fuel ratio. At rich air–fuel ratios, with f greater than 1.0, unburned HC levels are high since there is not enough air to completely burn all the fuel. Similarly, CO levels are high, because there is not enough oxygen present to oxidize the CO to CO2. For lean mixtures, withfless than 1.0, there is always excess air available, so that CO almost completely disappears, while HC emissions reach a minimum near f¼0:9. Forf less than about 0.9, some increased misfiring occurs because of proximity to the lean misfire limit, and HC emissions begin to rise again. The main factor in production of NO is combustion temperature: the higher the temperature,

Lean

NO

HC

CO

Fuel/air equivalence ratio 0.7

NO, CO, and HC concentrations (not to scale)

0.8 0.9 1.0 1.1 1.2 1.3

20 17 15 14 13 12

Stoichiometric Air/fuel ratio

Rich

Figure 4.5 Emissions as a function of fuel air equivalence ratiof(Heywood, 1988).

the greater the tendency to oxidize nitrogen compounds into NO. Since the combustion temperature is at a maximum near stoichiometric conditions wheref¼1:0, and falls off for both rich and lean mixtures, the NO curve takes the bell shape shown in Figure 4.5.

In addition to the higher efficiency discussed previously, an examination of Figure 4.5 clearly shows another benefit of lean operation. At values offless than about 0.9, CO production is negligible, HC emissions are near the minimum level, and NO emissions are greatly reduced. Asfdecreases even further, there is a trade-off between further reduction in NO emissions and an increase in HC emissions. Lean operation is, in fact, an excellent control strategy for reducing emissions, and is the reason why some engines operating with very lean overall mixtures could meet early emission standards without the need for exhaust gas clean-up devices.