Piston displacement is the volume (cm³, l) displaced by a piston in a cylinder in a single stroke, i.e.
between the bottom and . top dead-canter positions (BDC and TDC, respectively). The total cylinder
capacity (Vtot) comprises the swept volume (Vs) and the compression volume (Vc), i.e. Vtot =
Vs+Vc.
The compression ratio (E) is the ratio of the maximum to the minimum volume of the space
enclosed by the piston, i.e. prior to compression (Vtot) as compared to the end of the compression
stroke (Vc). The compression ratio can be used to calculate the pressure and temperature of the
compressed fuel mixture (E = Vtot/Vc).
The efficiency (rl = Pc/Pf) is the ratio between the power applied to the crankshaft (Pc) and the
amount of energy introduced with the fuel (Pf = V x n.c.v.).
Ignition and combustion: The firing point (diesel: flash point; spark-ignition engine: ignition point) is
timed to ensure that the peak pressure is reached just after the piston passes top dead center
(approx. 10° - 15° crankshaft angle). Any deviation from the optimal fiash/ignition point leads to a
loss of power and efficiency; in extreme cases, the engine may even suffer damage. The
flash/ignition point is chosen on the basis of the time history of combustion, i.e. the rate of
combustion, and depends on the compression pressure, type of fuel, combustion-air/ fuel ratio and
the engine speed. The ignition timing (combustion) must be such that the air/fuel mixture is fully
combusted at the end of the combustion cycle, i.e. when the exhaust valve opens, since part of the
fuel's energy content would otherwise be wasted.
Air/Fuel-ratio and control: Proper combustion requires a fuel-dependent stoichiometric air/fuel-ratio
(af-ratio). As a rule, the quality of combustion is maximized by increasing the air fraction, as
expressed by the air-ratio coefficient (d = actual air volume/stoichiometric air volume).
For gasoline and gas-fueled engines, the optimal air/fuel ratio is situated somewhere within the
range d = 0.8 - 1.3, with maximum power output at 0.9 and maximum efficiency (and clean exhaust)
at 1.1. The power output is controlled by varying the mixture intake and, hence, the cylinder's
volumetric efficient and final pressure, via the throttle. Diesel engines require an air-ratio of d = 1.3
at full load and 4 - 6 at low load, i.e. fuel intake is reduced, while the air intake remains constant.
Converting diesel engines
Diesel engines are designed for continuous operation (10 000 or more operating hours). Basically,
they are well-suited for conversion to biogas according to either of two methods:
The dual-fuel approach
Except for the addition of a gas/air mixing chamber on the intake manifold (if need be, the air filter
can be used as a mixing chamber), the diesel engine remains extensively unmodified. The injected
diesel fuel still ignites itself, while the amount injected is automatically reduced by the speed
governor, depending on how much biogas is introduced into the mixing chamber. The biogas supply
is controlled by hand. The maximum biogas intake must be kept below the point at which the engine
would begin to stutter. If that happens, the governor is getting too much biogas and has therefore
turned down the diesel intake so far that ignition is no longer steady. Normally, 15 - 20% diesel is
sufficiency, meaning that as much as 80% of the diesel fuel can be replaced by biogas. Any lower
share of biogas can also be used, of course, since the governer automatically compensates with
more diesel.
As a rule, dual-fuel diesels perform just as well as a comparable engine operating on pure diesel.
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