COMPONENTS AND ITS FUNCTIONS
different autogas systems generally use the same type of filler,
tanks, lines and fittings but use different components in the engine
bay. Liquid injection systems use special tanks with circulation
pumps and return lines similar to petrol fuel injection systems.
are three basic types of autogas system.
oldest of these is the conventional converter-and-mixer system,
which has existed since the 1940s and is still widely used today.
The other two types are known as injection systems, but there are
significant differences between the two.
regulator-mixer system uses a regulator to change liquid fuel from
the tank into vapour, then feeds that vapour to the mixer where it
is mixed with the intake air. This is also known as a venturi
system or "single
Vapour phase sequential injection systems also
use a regulator, but unlike the mixer system, the gas exits the
converter at a regulated pressure. The gas is then injected into the
air intake manifold via a series of electrically controlled
injectors. The injector opening times are controlled by the autogas
control unit. This unit works in much the same way as a petrol fuel
injection control unit. This allows much more accurate metering of
fuel to the engine than is possible with mixers, improving economy
and/or power while reducing emissions. Liquid
phase injection systems do
not use a converter, but instead deliver the liquid fuel into a fuel
rail in much the same manner as a petrol injection system. These
systems are still very much in their infancy. Because the fuel
vaporises in the intake, the air around it is cooled significantly.
This increases the density of the intake air and can potentially
lead to substantial increases in engine power output, to the extent
that such systems are usually de-tuned to avoid damaging other parts
of the engine. Liquid phase injection has the potential to achieve
much better economy and power plus lower emission levels than are
possible using mixers or vapour phase injectors.
fuel is transferred into a vehicle tank as liquid by connecting the
bowser at the filling station to the filler fitting on the vehicle.
type of filler used varies from country to country and in some cases
different types are used within the same country.
three types are:
type has a threaded fitting onto which the bowser nozzle is screwed
before the trigger is pulled to establish a seal before fuel
transfer. This type is used in Australia, USA, Germany, Austria,
Belgium, Republic of Ireland.
type establishes a gas proof seal by a push and twist action. This
type is used in the United Kingdom, Netherlands and Switzerland.
type is used in Italy, France, Poland, Scandinavia and Portugal.
Adaptors that allow a vehicle fitted with a
particular system to refuel at a station equipped with another
system are available.
fill valve contains a check valve so that the liquid in the line
between the filler and the tank(s) does not escape when the bowser
nozzle is disconnected.
installations where more than one tank is fitted, T-fittings may be
used to connect the tanks to one filler so that the tanks are filled
simultaneously. In some applications, more than one filler may be
fitted, such as on opposite sides of the vehicle. These may be
connected to separate tanks, or may be connected to the same tanks
using T-fittings in the same manner as for connecting multiple tanks
to one filler.
Fillers are typically made of brass to avoid the
possibility of sparks when attaching or removing the bowser that
might occur if steel fittings were used.
HOSES, PIPES AND FITTINGS
lines and fittings. Containment hose, liquid hose, copper pipe with
flared end plus brass elbows and T fitting.
hose between the filler and tank(s) is called the fill hose or fill
line. The hose or pipe between the tank(s) and the converter is
called the service line. These both carry liquid under pressure.
flexible hose between the converter and mixer is called the vapour
hose or vapour line. This line carries vapour at low pressure and
has a much larger diameter to suit.
the tank valves are located inside an enclosed space such as the
boot of a sedan, a plastic containment hose is used to provide a gas-tight
seal between the gas components and the inside of the vehicle.
Liquid hoses for LPG are specifically designed and rated for the
pressures that exist in LPG systems, and are made from materials
designed to be compatible with the fuel. Some hoses are made with
crimped fittings, while others are made using re-usable fittings
that are pressed or screwed onto the end of the hose.
sections of liquid line are usually made using copper tubing,
although in some applications, steel pipes are used instead. The
ends of the pipes are always double-flared and fitted with flare
nuts to secure them to the fittings.
Liquid line fittings are mostly made from brass. The
fittings typically adapt from a thread in a component, such as a BSP
or NPT threaded hole on a tank, to an SAE flare fitting to suit the
ends of pipes or hoses.
Vehicles are often fitted with only one tank, but multiple tanks are
used in a some applications. In passenger car applications, the tank
is typically either a cylindrical tank mounted in the boot of the
vehicle or a toroidal tank or set of permanently interconnected
cylinders placed in the spare wheel well. In commercial vehicle
applications, the tanks are generally cylindrical tanks mounted
either in the cargo space or on the chassis underneath the body.
tanks have fittings for filling, liquid outlet, emergency relief of
excess pressure, fuel level gauge and sometimes a vapour outlet.
These may be separate valves mounted into a series of 3 to 5 holes
in a plate welded into the tank shell, or may be assembled onto a
multi-valve unit which is bolted into one large hole on a boss
welded into the tank shell.
FILL VALVE AND AFL (MULTIVALVE)
Modern fill valves are usually fitted with an automatic fill limiter
(AFL) to prevent overfilling. The AFL has a float arm which
restricts the flow significantly but does not shut it off entirely.
This is intended to cause the pressure in the line to rise enough to
tell the bowser to stop pumping but not cause dangerously high
pressures. Before AFLs were introduced, it was common for the filler
(with integral check valve) to be screwed directly into the tank, as
the operator had to open an ullage valve at the tank while filling,
allowing vapour out of the top of the tank and stopping filling when
liquid started coming out of the ullage valve to indicate that the
tank was full. Modern tanks are not fitted with ullage valves.
liquid outlet is usually used to supply fuel to the engine, and is
usually referred to as the service valve. Modern service valves
incorporate an electric shut-off solenoid. In applications using
very small engines such as small generators, vapour may be withdrawn
from the top of the tank instead of liquid from the bottom of the
emergency pressure relief valve in the tank is called a hydrostatic
pressure relief valve. It is designed to open if the pressure in the
tank is dangerously high, thus releasing some vapour to the
atmosphere to reduce the pressure in the tank. The release of a
small quantity of vapour reduces the pressure in the tank, which
causes some of the liquid in the tank to vaporise to re-establish
equilibrium between liquid and vapour. The latent heat of
vaporisation causes the tank to cool, which reduces pressure even
gauge sender is usually a magnetically coupled arrangement, with a
float arm inside the tank rotating a magnet, which rotates an
external gauge. The external gauge is usually readable directly, and
most also incorporate an electronic sender to operate a fuel gauge
on the dashboard.
CONVERTER - REGULATOR
converter (also known as regulator or vaporiser) is a device
designed to change the fuel from a pressurised liquid to a vapour at
around atmospheric pressure for delivery to the mixer or vapour
phase injectors. Because of the refrigerant characteristic of the
fuel, heat must be put into the fuel by the converter. This is
usually achieved by having engine coolant circulated through a heat
exchanger that transfers heat from that coolant to the LPG. There
are two distinctly different basic types of converter for use with
mixer type systems. The European style of converter is a more
complex device that incorporates an idle circuit and is designed to
be used with a simple fixed venturi mixer. The American style of
converter is a simpler design which is intended to be used with a
variable venturi mixer that incorporates an idle circuit.
Engines with a low power output such as; scooters,
quad bikes and generators can use a simpler type of converter (also
known as governor or regulator). These converters are fed with fuel
in vapour form. Evaporation takes place in the tank where
refrigeration occurs as the liquid fuel boils. The tanks large
surface area exposed to the ambient air temperature combined with
the low power output (fuel requirement) of the engine make this type
of system viable. The refrigeration of the fuel tank is proportional
to fuel demand hence this arrangement is only used on smaller
engines. This type of converter can either be fed with vapour at
tank pressure (called a 2 stage regulator) or be fed via a tank
mounted regulator at a fixed reduced pressure (called a single stage
mixer is the device that mixes the fuel into the air flowing to the
engine. The mixer incorporates a venturi designed to draw the fuel
into the airflow due to the movement of the air.
Mixer type systems have existed since the 1940s and
some designs have changed little over that time. Mixers are now
being increasingly superseded by injectors.
VAPOUR PHASE SEQUENTIAL TYPE INJECTORS
vapour phase injection systems mount the solenoids in a manifold
block or injector rail, then run hoses to the nozzles, which are
screwed into holes drilled and tapped into the runners of the intake
manifold. There is usually one nozzle for each cylinder. Some vapour
injection systems resemble petrol injection, having separate
injectors that fit into the manifold or head in the same manner as
petrol injectors, and are fed fuel through a fuel rail.
ELECTRICAL AND ELECTRONIC CONTROLS
There are four distinct electrical systems that may be used in
autogas systems - fuel gauge sender, fuel shut-off, closed loop
feedback mixture control and injection control.
some installations, the fuel gauge sender fitted to the autogas tank
is matched to the original fuel gauge in the vehicle. In others, an
additional gauge is added to display the level of fuel in the
autogas tank separately from the existing petrol gauge.
most modern installations, an electronic device called a tachometric
relay or safety switch is used to operate electrical shut-off
solenoids. These work by sensing that the engine is running by
detecting ignition pulses. Some systems use an engine oil pressure
sensor instead. In all installations, there is a filterlock (consisting
of a filter assembly and a vacuum or electric solenoid operated shut-off
valve) located at the input to the converter. In European converters,
there is also a solenoid in the converter to shut off the idle
circuit. These valves are usually both connected to the output of
the tachometric relay or oil pressure switch. Where solenoids are
fitted to the outputs of fuel tanks, these are also connected to the
output of the tachometric relay or oil pressure switch. In
installations with multiple tanks, a switch or changeover relay may
be fitted to allow the driver to select which tank to use fuel from.
On bi-fuel, the switch used to change between fuels is used to turn
off the tachometric relay.
Closed loop feedback systems use an electronic controller that
operates in much the same way as in a petrol fuel injection systems,
using an oxygen sensor to effectively measure the air/fuel mixture
by measuring the oxygen content of the exhaust and control valve on
the converter or in the vapour line to adjust the mixture. Mixer
type systems that do not have a closed loop feedback fitted are
sometimes referred to as open loop systems.
Injection systems use a computerised control system which is very
similar to that used in petrol injection systems. In virtually all
systems, the injection control system integrates the tachometric
relay and closed loop feedback functions.
- CONVERTER (REGULATOR) AND MIXER SYSTEM OPERATION
designs of converters and mixers are matched to each other by
matching sizes and shapes of components within the two.
European style systems, the size and shape of the venturi is
designed to match the converter. In American style systems, the air
valve and metering pins in the mixer are sized to match the
diaphragm size and spring stiffness in the converter. In both cases,
the components are matched by the manufacturers and only basic
adjustments are needed during installation and tuning.
autogas carburettor may simply consist of a throttlebody and a mixer,
sometimes fitted together using an adapter.
start enrichment is achieved by the fact that the engine coolant is
cold when the engine is cold. This causes denser vapour to be
delivered to the mixer. As the engine warms up, the coolant
temperature rises until the engine is at operating temperature and
the mixture has leaned off to the normal running mixture. Depending
on the system, the throttle may need to be held open further when
the engine is cold in the same manner as with a petrol carburettor.
On others, the normal mixture is intended to be somewhat lean and no
cold-start throttle increase is needed. Because of the way
enrichment is achieved, no additional choke butterfly is required
for cold starting with LPG.
temperature of the engine is critical to the tuning of an autogas
system. The engine thermostat effectively controls the temperature
of the converter, thus directly affecting the mixture. A faulty
thermostat, or a thermostat of the wrong temperature range for the
design of the system may not operate correctly.
power output capacity of a system is limited by the ability of the
converter to deliver a stable flow of vapour. A coolant temperature
lower than intended will reduce the maximum power output possible,
as will an air bubble trapped in the cooling circuit or complete
loss of coolant. All converters have a limit, beyond which mixtures
become unstable. Unstable mixtures typically contain tiny droplets
of liquid fuel that were not heated enough in the converter and will
vaporise in the mixer or intake to form an excessively rich mixture.
When this occurs, the mixture will become so rich that the engine
will flood and stall. Because the outside of the converter will be
at or below 0°C when this happens, water vapour from the air will
freeze onto the outside of the converter, forming an icy white layer.
Some converters are very susceptible to cracking when this happens.