Re: Why Can't A Fuel injected Petrol Engine be as Efficient as a Diesel?
- From: bernxard@xxxxxxxxxxxx
- Date: 20 May 2005 07:45:09 -0700
A gasoline engine always burns its fuel 'stoichiometrically' that is 14
parts by weight air to 1 part by weight gasoline so that there is a
complete burn of both oxygen and fuel.
When running at partial power this leads to suction losses as the
airflow is throttled. If you halve the fuel flow you also have to
halve the airflow. (strictly speaking it is airflow that determines
fuel flow)
The diesel opperares with an over supply of air, it never throttles the
airflow and only controls power by controlling fuel flow. It's as if
the piston bounces of air at the top of the stroke as a spring would
and all of the energy is returned.
when running at near 100% of power there is little differences in fuel
efficiency: the gains are mainly in the area of partial load.
There is a type of petrol engine called a 'lean burn engine' which
tries to opperate on partially lean mixtures. Getting good ignition
tends to require fancy injection systems to try and get a 'stratefied
charge' ie a rich mixture near the sparking plug. Can be done with
direct injection AFTER the compression stroke is complete rather than
multipoint injection during the induction stroke.
Iridium based catalysts that destroy nirtrous oxides also appear part
of the solution to stratefied charge engines and this requires low
sulfur fuels.
High compression does lead to improvements in efficiency but only up to
a point (about 13:1) because more of the energy can be expanded. Part
of the reason is for diesels high compression is to get good ignition.
.
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