Biofuel Engine Technology

Fuel System Design

Diesel engines require a constant supply of fuel to maintain operation.  In part, in order to guarantee adequate supply, most engine designs actually pump far more fuel to the engine than what is being consumed with the excess being returned to the fuel tank.

Diesel engine fuel injection equipment rely on very fine tolerances.  Injector and pump parts have to be precisely machined to provide the exacting fuel regulation necessary for acceptable engine operation.

These finely machined parts are susceptible to damage from contaminates.  Both water and solids can seriously damage injection equipment, degrading performance or stopping the engine.  For this reason diesel engines are fitted with fuel filters and some have sedimenters or water separaters.

Fuel filters have to be sized as not to give excessive flow restriction over the desired filter service life. It is necessary to consider flow reduction due to build up of contaminates on the filter media over its lifetime.

Where diesel engines are operated at ambient temperatures below the cloud point of the fuel it is necessary to heat the fuel to stop it waxing.  Wax will otherwise gather at restrictions within the fuel system, main concern being any filters or gauzes, and gradually reduce fuel flow.

Component location

Components are best mounted to provide the shortest reasonable overall fuel run.  Positioning should also consider ease of service, wind chill and the possibility of damage.

Mounting components or running fuel pipe close to hot engine parts and the exhaust system will provide some degree of fuel heating.  Care should be taken to ensure that components or pipe are not overheated - plastic and rubber pipes will suffer from excessive temperatures.

Installation above electrical components should be avoided where possible.

Fuel tank

The fuel tank should be vented with the vent positioned to avoid water or dirt entry or clogging.

A drain plug fitted at the lowest point of the tank allows for draining of settled contaminates.  Ideally this should be positioned below the fuel pick up.

Fuel return pipes should be positioned to return fuel close to the pick up pipe to help prevent fuel waxing.

Fuel pipes and hoses

Fuel pipes and hoses should be

• sized as large as possible to provide minimal fuel restriction.
• kept as short as possible as flow restriction is cumulative.  Where long runs are unavoidable a non return valve should be fitted to avoid fuel draining back to the tank.
  
• non metal due to corrosion resistance and less chance of fracturing.
• kept as straight as possible - bends increase restriction and U bends can collect water
• continuous - unions generally increase restriction and increase the possibility of leaks
• clear or opaque to allow fuel flow or air ingress to be observed
• adequately supported
• positioned to avoid rubbing or protected

Pipe and hose connections

Connections should be kept to a minimum.   Hoses are best pushed onto suitable barbs and fitted with clips.  Nylon pipe is best forced onto a suitable barb.  Heating a metal barb can aid attachment of nylon pipe.

Hose Clips

A number of different designs of hose clip are available.  Most common are screw or jubilee type clips.  Tabbed sprung clips provide good performance due to maintaining tension on the hose.  These are often used on heated fluids as they self adjust to take up any thermal expansion / contraction.

Water trap / sedimenter

Water traps/ sedimenters should be installed before any fuel pumps so that they can catch water droplets before they are broken up by the pump.  Fitting as close to the tank as possible allows catchment of any free water before it could freeze blocking fuel flow.  Installation below the fuel level in the tank allows for easy draining.  Traps and sedimenters should be included when high levels of particle or water contamination are likely.

sedimentor unit complete and dissembeled

Lift pump

Many engines are fitted with lift pumps to provide a constant flow of fuel to the engine.  Mechanical pumps driven by the engine are often used.

Electric pumps are best fitted close to the tank to provide positive pressure in the rest of the fuel system.  This will reduce the chances of air being drawn into the system.  Electric pumps can be used to prime the fuel system.

Use of a lift pump allows a permanent air bleed from the fuel filter with associated advantages when priming the fuel system or under operation.

Lift pumps should be selected to provide under 1 bar of pressure to standard fuel filters.  A releif valve can be fitted to reduce the pressure at the filter to an acceptable level.

A mesh pre filter should be fitted before the lift pump to protect it from damage from large particle contaminates.

Hand pump

Hand pumps can be fitted to assist fuel priming.  Some types of mechanical lift pumps have integral hand pumps.

Diaphragm or plunger type pumps are sometimes built into the fuel filter and are also available in a design that can be attached to threaded filter ports.

Bulb or plunger pumps are available that should be fitted prior to the fuel filter.

Returned fuel

Fuel returned from the injectors should always be sent to the fuel tank to minimise back pressure on the injector.

Fuel returned from the injection pump can be looped to the dirty side of the filter, helping to avoid filter waxing and reducing the amount of fuel being drawn from the tank.  On systems with lift pumps a small air bleed allows air to be returned to the tank.  With suction systems oil de-aerators can be fitted to help avoid problems from air ingress and aid system priming.  Any fuel heaters should be fitted within the loop to ensure maximum possible fuel heating.

Looped fuel return kits

Fuel filter

Primary fuel filters are fitted to every diesel engine and are the main defence against contaminates.  FIlters should be selected to suit the required flow restriction, service life and expected level of contamination.  Filter performance also requires consideration particularly for high pressure commonrail and unit injection systems which require finer filtration to maintain injection equipment longevity.

Ideally filters should be mounted  pressure side of any lift pump and above the level of the fuel injection pump to prevent siphoning and assist venting.

Although there are lots of different types of fuel filter, most common are canister type filters where the filter media is contained within a disposable metal canister.  There are two common type of canister filter the Bosch spin-on type and the Lucas / CAV / Delphi type sandwich type filter.

Bosch spin-on type

Bosh spin on filters are widely fitted to European vehicles, particularly those from German manufacturers.  The fuel filter screws onto the head with by an M16 thread.  Filter heads are available to take single or twin filters.  Single heads are available in numerous different configurations with inbuilt hand pumps and suitable to take a thermal T which returns the warm fuel from the injection pump to the dirty side of the filter at low temperatures. Head ports are either threaded for M14x1.5 fittings or have 8mm hose barbs.

Coolant and electric heaters are available for these filters that screw under the head.

A number of different sizes of filter can fit onto this type of head.  Fuel flows in the top of the cartridge, down through the filter media then up through a tube in the centre of the filter - see http://aa.bosch.de/aa/en/static/produkte/filter/kraftstoff/dieselfilter/ds_funktion.htm

Most filters are designed to encourage water to drop out of the fuel and gather in the bottom of the filter.  Generally a water drain tap is fitted to the bottom of the filter.

Lucas / CAV / Delphi type

Fuel filters are sandwiched in between a filter head and filter base.  The filter range is modular with numerous options for head, filter element and base.

Heads include single and twin with single heads with integral hand pumps being available in a number of different configurations.  Ports are threaded for either M14x1.5 or 1/2 UNF.

Long and short filter elements are available in both filter flow and agglomerator flow types.

Agglomerator flow filters flow in a similar fashion to Bosch spin-on type.  The filter media encourages water that is entrained in the fuel to agglomerate into large droplets that fall to the base of the filter.

Filter flow filters are designed to have fuel flow through the other direction.  This gives a slightly longer filter life and marginally lower protection against water compared to agglomerator flow filters.

A special water diffuser / sedimenter unit is available which fits in place of the filter cartridge.  This unit is designed to encourage water and other contaminates to fall out of the fuel.

A number of different bases are available in aluminium and glass and of different shapes and sizes.  Options include a coolant heated base designed for use with "filter flow" filter elements and a number of different bases have a water sensor option.  There is also a glass section available that can be fitted between the filter and base.

Coolant and electric heaters are available that fit between the filter head and the filter designed for use when running agglomerator flow.