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.
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.