Advanced Automotive Technology: Visions of a Super-Efficient Family Car

Increasing Thermodynamic Efficiency

Increasing the thermodynamic efficiency of SI engines can be attained by optimum control of

spark timing, by reducing the time it takes for the fuel-air mixture to be fully combusted (burn

time), and by increasing the compression ratio.

Spark timing

For a particular combustion chamber, compression ratio and air fuel mixture, there is an

optimum level of spark advance for maximizing combustion chamber pressure and, hence, fuel

efficiency. This level of spark advance is called MBT for “maximum brake torque. ” Owing to

production variability and inherent timing errors in a mechanical ignition timing system, the

average value of timing in mechanically controlled engines had to be retarded significantly from

the MBT timing so that the fraction of engines with higher than average advance owing to

production variability would be protected from knock. The use of electronic controls coupled

with magnetic or optical sensors of crankshaft position has reduced the variability of timing

between production engines, and also allowed better control during transient engine operation.

More recently, engines have been equipped with knock sensors, which are essentially vibration

sensors tuned to the frequency of knock. These sensors allow for advancing ignition timing to the

point where trace knock occurs, so that timing is optimal for each engine produced regardless of

production variability. Manufacturers expect that advanced controls of this sort can provide small

benefits to future peak efficiency.

Faster combustion

High-swirl, fast-bum combustion chambers were developed during the 1980s to reduce the

time taken for the air fuel mixture to be fully combusted. The shorter the burn time, the more

closely the cycle approximates the theoretical Otto cycle with constant volume combustion, and

the greater the thermodynamic efficiency. Recent improvements in flow visualization and

computational fluid dynamics have allowed the optimization of intake valve, inlet port, and

combustion chamber geometry to achieve desired flow characteristics. Typically, these designs

have resulted in a 2 to 3 percent improvement in thermodynamic efficiency and fuel economy.^42

The high swirl chambers also allow higher compression ratios and reduced “spark advance” at the

same fuel octane number. More important, manufacturers stated that advances in this area are

particularly useful in perfecting lean-bum engines.

Increased compression ratios

Compression ratio is limited by fuel octane, and increases in compression ratio depend on how

the characteristics of the combustion chamber and the timing of the spark can be tailored to

prevent knock, or early detonation of the fuel-air mixture, while maximizing efficiency. Improved

electronic control of spark timing and improvements in combustion chamber design are likely to

increase compression ratios in the future. In newer engines of the 4-valve dual overhead cam

42 J. W. Walker et al., "The GM 4.3 Valve V-6 Gasoline Engine,” SAE paper 841225,1984..