ENGINE DESIGN PARAMETERS

The following engine parameters have large influence on emissions and hence have undergone substantial modifications since the pre-emission control era.

• Engine compression ratio,
• Combustion chamber design – low crevice volume, high turbulence
• Spark timing
• Air-fuel ratio
• Fuel system design: carburetor giving way to fuel injection
• Multivalves and variable valve actuation
• Engine temperature control

Engine Compression Ratio

Engine compression ratio affects

• Surface to volume ratio  of the combustion chamber
• Engine combustion temperature
• Thermodynamic efficiency
• Fuel octane number for knock free  engine operation

The premium high performance car engines during 1960s employed CR of 10 to 11:1. The engine CR was lowered to 8.5 to 9.0:1 when stringent emission standards were legislated for the first time in 1975.
The combustion chamber with a lower CR has lower surface/volume ratio resulting in a reduction in volume of quench layer on the combustion chamber surface. Typical effect of surface/volume ratio of combustion chamber is shown on Fig, 5.1. Moreover, for a low CR engine the crevice volume would also be proportionately lower. These factors in turn would reduce HC emissions.  Higher exhaust temperatures resulting with low CR would promote oxidation of HC and CO in exhaust system.
At low CR the peak combustion temperatures are lower and hence low NOx formation.
Introduction of catalytic converters made the use of unleaded gas necessary. As the petroleum refinery economics dictated the use of unleaded gasoline of a low octane number, engine CR had to be lowered.
The principal disadvantage of low CR engine is that it has a poorer fuel efficiency that also results in higher CO2 emissions.
 The premium high performance car engines during 1960s employed CR of 10 to 11:1. The engine CR was lowered to 8.5 to 9.0:1 when stringent emission standards were legislated for the first time in 1975.

However, due to development of high turbulence engine combustion chambers and use of port fuel injection system it has been possible to use a somewhat higher engine CR. The modern gasoline engines have CR mostly in the range of 9.0 to 9.5:1.