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Dual
fuel
Dual fuel engines use pilot diesel for ignition with
most of the power coming from burning gas. The combustion
process changes when the engine transitions from using
diesel to using the combination of diesel and gas. The
gas is injected into the inlet air and mixed with air,
the air fuel ratio is controlled by a throttle plate
so that in a sense on dual fuel the engine becomes an
Otto cycle engine with pilot diesel ignition.
This change in combustion dynamics presents some challenges
when developing a dual fuel engine to match the diesel
engine torque and power performance and modern emissions
standards. DieselGas engineers use the benefits of each
combustion type to help meet project targets for the
engines.
The gas percentage varies from 0% at idle to 90% at
full load. The dynamic transition from diesel to gas
is handled with special engine control algorithms and
various settings and adjustments to ensure the dual
fuel engine driveability is similar to diesel.
Because some of the dual fuel operating cycle is on
diesel the vehicle driving cycle will effect the total
substitution of dual fuel vehicles. For mid city operations
where idle may account for more than 50% of the running
time, substitution may be reduced as low as 60%. For
less congested inner city driving and urban driving
substitution should increase to between 65% and 75%
and for highway driving substitution can be as high
as 75% and 85%.
Dedicated gas
engine
Dedicated gas engines use spark ignition Otto cycle
combustion processes. This means one fuel and 100% gas
in all operating conditions. This would at first appear
to have large economic benefits if CNG is cheaper than
diesel.
At idle and at light loads dedicated gas engines are
much less fuel efficient than diesel or dual fuel engines.
In typical city driving cycles dedicated gas buses have
been shown to use between 25% and 40% more fuel energy
than diesel engine buses. Lean burn combustion engines
are at the lower end and stoichimetric combustion engines
are at the higher end of this difference. By comparison
dual fuel engines use only about 5% more fuel energy
than a diesel engine bus on the same driving cycle.
Economics
Fuel cost comparisons between diesel, dual fuel and
dedicated gas engines are not easy to establish. To
have accurate fuel consumption records kept of bus operations
over say 1,000km to 10,000km is a difficult task. Short
tests say of one days operations has the difficulty
of accurately measuring the fuels used for the day.
Expansion and contraction of the gas and diesel because
of temperature change can distraught the results by
10% or up to 50%.
DGTech substitution logging facility allows consistent
data to be accumulated and is useful for comparative
performance, say comparing different driving cycles
with the same setting. Results for comparison with other
fuels on the same driving cycle need to be confirmed
under controlled conditions. [ See Benefits economics
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