lotus



previous page: 7.1 What is the Octane Number Requirement of a Vehicle?
  
page up: Gasoline FAQ
  
next page: 7.3 What is the effect of changing the air-fuel ratio?

7.2 What is the effect of Compression ratio?




Description

This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.

7.2 What is the effect of Compression ratio?

Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation

Efficiency = 1 - (1/compression ratio)^gamma-1

where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 for
gasoline fuels in an SI engine [23].

The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:-

   Compression       Octane Number    Brake Thermal Efficiency       
     Ratio            Requirement         ( Full Throttle )
      5:1                 72                      -
      6:1                 81                     25 %
      7:1                 87                     28 %
      8:1                 92                     30 %
      9:1                 96                     32 %
     10:1                100                     33 %
     11:1                104                     34 %
     12:1                108                     35 %

Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.

 

Continue to:













TOP
previous page: 7.1 What is the Octane Number Requirement of a Vehicle?
  
page up: Gasoline FAQ
  
next page: 7.3 What is the effect of changing the air-fuel ratio?