Engine Parameter |
Volumetric Efficiency |
Flame Front Velocity |
Combustion Time |
Ignition Advance Requirement |
Engine RPM |
VE peaks near torque peak |
Increased at VE peak |
Reduced at VE peak |
Less relative advance at VE
peak. However, predominant effect is that more advance is required
as RPM increases due to less time for crank to sweep through a given
angle - thus requiring spark initiation at a greater angle BTDC. |
Increased compression ratio |
Minimal effect |
Increased |
Reduced |
Less advance |
More radical camshaft (increased
duration and overlap) |
Less at low RPM; greater at
high RPM |
Less at low RPM; greater at
high RPM |
Less at low RPM; greater at
high RPM |
More advance at low RPM; less
advance at high RPM |
Improved exhaust scavenging
or less back pressure |
Varies throughout RPM range |
Lower levels of exhaust
gas residuals in cylinder increases velocity |
Reduced |
Less advance within the RPM
range where exhaust is most efficient |
Improved intake system efficiency
(bigger throttle body or low restriction air cleaner) |
Generally greater at high RPM
for H-D� engines |
Increased |
Reduced |
Less advance |
Increased fuel octane |
No direct effect |
Reduced; less likely to reach
knock limit |
Reduced |
More advance; increased knock
limit |
Air/fuel ratio |
No direct effect |
Optimum near stochiometric 14.7
A/F ratio |
Optimum near stochiometric 14.7
A/F ratio |
More advance required for rich
mixtures |
Improved fuel atomization |
Minimal effect |
Small fuel droplets burn faster |
Reduced |
Less advance |
Increased intake air temperature |
Lower |
Increased; may reach knock limit
where end gases ignite |
Reduced |
Less advance; lower knock limit
as temperature increases |
Increased humidity |
Slight reduction as water displaces
air |
Reduced |
Increased |
More advance. Extreme example
is water injection used to increase knock limit. |
Increased cylinder head temperature |
Minimal effect |
Increased; may reach knock limit
where end gases ignite |
Reduced. |
Less advance; lower knock limit
as temperature increases |
Spark plug position in head;
number of spark plugs |
No direct effect |
Minimal effect |
Affected by distance from plug
to farthest cylinder wall. Ideal location for single plug is center
of squish area |
Less advance for centered spark
plug or dual spark plug designs |
Greater bore/stroke ratio |
Minimal effect unless valve
shrouding occurs in large bore designs |
Short stroke increases rate
of compression and results in higher velocity |
Large bore requires more time
to burn from spark plug to cylinder walls. |
Very long stroke or large bore
(over-square) engines may require up to 10� more advance than
an equivalent CID engine with optimum bore/stroke ratio |
Combustion chamber design with
high squish and swirl |
Minimal effect |
High swirl increases velocity |
High squish designs take
less time to burn to farthest reaches |
Less advance for efficient combustion
chamber designs |