Environmental Engineering Reference
In-Depth Information
the name indicates, combustion is initiated by a high-energy electrical discharge, a spark, which
causes the fuel-air mixture locally to ignite. Once the fuel-air mixture starts to burn, the flame
propagates through the combustion chamber, consuming the premixed fuel and air in a controlled
manner. To avoid uncontrolled combustion, the autoignition of the fuel-air mixture at any stage
of this must be prevented. Gasoline and other SI fuels must have a high resistance to autoignition,
which is measured by the octane rating of the fuel. The higher the octane, the more resistant the fuel
is to autoignition (see values in Table 10.2, column H). If the octane rating is not high enough, com-
bustion knock (unwanted autoignition) will occur, reducing efficiency and potentially damaging the
engine. The SI biofuels in Table 10.2 all exhibit octane numbers higher than gasoline, indicating
that they are better than the standard petroleum gasoline regarding resisting autoignition. However,
additional fuel properties need to be considered as they influence performance, startability, and
emissions.
10.2.2.1 Influence of Fuel on operating characteristics of sI engines
Two very important pathways for improving the efficiency of an engine are (1) to increase the com-
pression ratio (CR), which is a measure of how much the fuel and air mixture is compressed before
ignition, and (2) to reduce the amount of work that is wasted pumping air and fuel into the cylinder
and out the exhaust port.
In SI engines, the maximum compression ratio is limited by the fuels octane number. A higher
compression ratio leads to higher temperatures during the combustion process, and this can cause
a portion of the fuel to autoignite and burn rapidly at the wrong time in the engine cycle, leading
to combustion knock. Fuels with a higher octane rating, such as alcohol-based fuels like ethanol,
methanol, or some isomers of butanol, will resist autoignition, which allows engine developers to
increase an engine's compression ratio and thus improve efficiency. However, because these alterna-
tive fuels, even ethanol, remain a low portion of the overall fuel supply, vehicle OEMs must produce
engines that are capable of running on gasoline in addition to alternative fuels. Unfortunately, the
requirement that these vehicles, called FFVs, operate on gasoline limits the engines compression
ratio, and thus limits the full benefit of the higher octane of the alternative fuel. Engines with vari-
able compression ratio (VCR) systems are a developing technology that shows promise in alleviat-
ing this limitation because these systems would allow the compression ratio to be automatically
optimized as the fuel in the tank changes (Drangle 2002; Moteki 2003; Rosso 2006).
The load in SI engines is controlled by throttling the incoming air and controlling the prior
cycle residuals in the cylinder by continuously adjusting the phase of the intake and exhaust valves.
By restricting the air, less air enters the cylinder during the intake stroke. To maintain the proper
air-to-fuel ratio, less fuel is injected, resulting in less heat being released during combustion and
resultant power being reduced. Similarly, by increasing the amount of prior cycle residuals, which
displace air, less fuel is injected. The disadvantage of the throttling process is that although the
engine ingests less air, the added restriction in the intake air path causes the engine to do negative
work during the intake stroke to bring the air into the cylinder, thus reducing the efficiency. The
alternative method to control residuals via valve phasing reduces this pumping work within engines.
The burned exhaust gases (primarily CO
2
, H
2
O, and N
2
) do not participate in the combustion reac-
tion, but instead fill space in the cylinder that would have otherwise been occupied by the air and
fuel mixture. To compensate and maintain engine load, the throttling effect is reduced, which in
turn reduces the pumping work. A limiting factor in this is the stability of the combustion process.
As more residuals are added to the cylinder, the combustion process becomes prolonged and less
stable, eventually leading to misfire. The type of fuel and its properties affect this in that some fuels
can tolerate greater amounts of dilution than others, and some fuels such as gaseous fuels require
less throttling because of the low density of the fuel.
Another approach to decreasing pumping losses is to reduce the size (displacement) of the
engine. A smaller engine requires less throttling to achieve the same load as a large engine and
thus runs more efficiently. However, to get the same maximum power out of the engine, intake air
