Engine Suitability

Version 2.0 (20/4/2004)

Engine Suitability-Version 1.0

1 The Diesel Engine
2 Theory of Vegetable Oil Use as a Fuel
3 Engine suitability
4 Heating the Oil
5 Biodiesel
6 Micro Emulsions and Blends
7 Vegetable Oil Engine Design
8 Vegetable Oil Furnaces and Heaters
9 Oil Types and Filtering
10 Taxation
11 Implications of Vegetable Oil Fuel Use
12 Sources

Engine Suitability

There are many variations on diesel engine design. Various design elements effect suitability for vegetable oil fuel use.

Combustion Chamber

Direct Injection (DI)

Problems can occur with deposits building within DI engines when running on vegetable oil. The design of the combustion chamber and the injector along with their positioning will affect suitability

The Elsbett DI multifuel engine [34] [35] produced from the 1970s could be fuelled with vegetable oil or diesel. This engine has a deep, almost spherical, combustion chamber within the piston. The injector nozzle protrudes into the cylinder and delivers fuel directly to the centre of the combustion chamber. The unique design ensures that fuel is burnt without contacting the combustion chamber walls.
Fuel oil with a substantially greater viscosity will cause degraded atomisation from injectors that are deigned for a thinner fuel. Combustion will be adversely affected causing an increase in emissions and giving a greater potential for harmful deposits on the injectors and within the combustion chamber.

Hemmerlein et al.[28] tested three unmodified DI engines, one 2.6 litre air cooled and two larger, 6.6 litre and 12 litre, turbo charged and intercooled with liquid cooling, running rapeseed oil. All three engines failed durability tests due to problems caused by carbon build ups.

Karaosmanoglu et al.[30] tested a Panccar Motor 1 litre single cylinder DI engine running sunflower oil. The engine successfully completed long-term engine testing when it was started and shut down for 5 minutes with diesel fuel. The engine was run at a constant low speed under partial load.
Both Pflanzenoeltechnik-Nord GmbH [49] and Albrecht Transporte [50] report up to 250,000 km travelled using vegetable oil in a number of different modern DI trucks.
DI engines with a re-entrant bowl type combustion chamber - where the bowl has a lip around the top to encourage fuel to stay within the combustion chamber, away from the cylinder walls and piston rings – should have lower frequencies of ring land fouling associated problems. The lip of a re-entrant bowl also causes micro turbulence within the chamber, which aids fuel/air mixing and thus gives superior combustion.

Deeper combustion chambers, which are generally of a smaller diameter give rise to increased swirl which aids atomisation and injection pressure can be set higher as the fuel does not have to penetrate as far to reach the combustion chamber walls (which allows wall wetting and proper combustion). The increased squish area of this design gives a larger overall surface area and a greater output of hydro carbons due to heat loss.

Indirect Injection (IDI)

The fuel is injected as a jet and atomised in a separate combustion chamber before it enters the cylinder and completes combustion. The atomisation processes, caused by high air speeds, within the annexed combustion chamber after injection along with the remoteness of injection from the piston rings make these units less prone to problems from using thicker fuel oil.

Hemmerlin et al. [28] tested three unmodified IDI engines running on rapeseed oil. A small 1.6 liter swirlchamber engine failed durability testing due to carbon build up within the engine. Two larger IDI engines, a 6.2 litre prechamber engine and a 5.7 litre swirlchamber engine completed the durability testing.

Fuls et al.[31] found that an unmodified Caterpillar IDI engine in a tractor successfully completed extended service tests using sunflower oil as a fuel. This test led to Caterpillar(South Africa) providing a warranty on this engine running this fuel.
Togashi et al. [29] found that a small Yanmar IDI engine could be reliably operated on refined or deacified rapeseed oil.

Niegsch [10] found a Mercedes prechamber engine can be operated on refined, food grade rape oil for over 200,000 km with only minor problems.

Injector Pump (IP)

Mechanical Injection

In-Line Pump

These pumps have proved to be very reliable when fuelled with vegetable oil fuels. Mercedes IDI engines with in-line pumps have been run, without modification, fuelled with rapeseed oil for extended periods.

Rotary Pumps

The single pumping mechanism which pumps fuel to all cylinders has to work harder than its in-line equivalent. On a four cylinder engine the pump mechanism is pumping four times to every one pump of an in-line. The increased stress on the pumping mechanism, of pumping more viscous fuel, is multiplied.

Rotary pumps generally contain a sliding vane type transfer pump to supply a constant excess of fuel at pressure to the injector pump.

There are also two common designs of rotary pump mechanisms

Bosch Type: The entire rotating valve system moves backwards and forwards pumping the fuel.

Lucas/CAV Type: Uses two plungers that are flung outwards by the rotor, they are pushed inwards by a cam to expel the fuel.

The Lucas/CAV units has been found to be susceptible to malfunctions when running on vegetable oil, thought to be caused by their less rugged construction and possible accumulation of dirt, held by centrifugal force, within the plunger cylinders which would be cleared by the backwards and forwards pumping motion of the Bosch unit.

The Lucas/CAV units come with either metal or fibre vanes in the transfer pump which pulls the fuel into the IP. Transfer pumps have been damaged, examination of damaged pumps has lead to the conclusion that the damaged was caused by the increased heat created pulling thicker fuel or a swelling of fibre vanes, metal vanes can be fitted to alleviate this possible problem. Another possible cause of problems is expansion or contraction caused by fuel of a radically different temperature suddenly being introduced. Great care should be taken when using this type of pump. The fuel should be filtered very thoroughly and vegetable oil heated carefully.

If an engine is to be started with cold (thick) oils it is advisable to keep the engine speed low until the oil has been heated to reduce stresses on the pumps, this is especially the case with rotary pumps.

Injector pumps have been modified to allow them to function more reliably with vegetable oil. Lucas/CAV rotary pumps can have fibre vanes up rated. The main pumping mechanism(s) within an IP can be modified by the use of lapping paste on the pump plunger(s) so that it requires a similar force to move when pumping SVO as when using diesel or channels can be cut into the rotor in a rotary pump to increase lubrication and reduce heat. This will reduce stresses on the pump but in many cases may be unnecessary. Injector pumps are very precise systems and modifications should only be undertaken by a suitably experienced engineer.

Electronic Injection

The more precise injection controls and higher injection pressures of new electronically controlled systems are capable of providing superior combustion. However equipment may need tuning to run vegetable oil fuels. Injection events are governed by computers, which monitor a number of different factors to give the best combustion performance. The properties of vegetable oil based fuels have the potential to confuse the computer as the fuel may have properties that may be outside the parameters expected by the computer or under a given injection program the fuel may behave differently to how the computer expects and not combust effectively. Damage to sensing equipment is also possible.

Owen [36] reported a damaged incremental angle sensor within a Bosch VP44 injector pump fitted to an Vauxhall Astra Diesel Tdi while running 80% recycled used vegetable oil, 10% industrial methylated spirits (denatured ethanol), 5% butanol and 5% petrol mix.

Cummins [37] reported, “There are confirmed accounts of ISB engine fuel pump failures due to the effects of alcohol induced de-lamination of an internal timing sensor component. Robert Bosch, the fuel pump manufacturer prohibits alcohol blended fuel in the VP44 fuel pump on the ISB/QSB Cummins engine."

Frei [38] described modifications to a Mercedes Sprinter CDI (common rail). The engine would not start on vegetable oil when the engine was warm. The engine control computer monitors engine temperature when starting. Injection quantity is reduced when starting with a warm engine. The computer had to be tricked to think that the engine was cold starting so that it increased the fuelling and enabled initial combustion.

Burton[51] reported that Elsbett engineer Alexander Noack from Elsbett used a laptop to reprogram the onboard computer so that it would think that the engine is cooler than it is and heat the glow plugs for a longer duration – allowing for smooth starts with vegetable oil.


Pilot Injection

Injection systems are increasingly being designed with a pilot injection. A small quantity of fuel is introduced before the main fuel load is injected. Ignition of this fuel commences and ‘conditions’ the combustion chamber, providing heat that reduces the ignition delay of the main fuel load.

Some electronically controlled injection systems provide the pilot fuel as a separate injection. High-speed fuel delivery control solenoids allow multiple injections of fuel during the fuel delivery phase. The ferocity of the main combustion event is reduced as less fuel is delivered during the now reduced ignition lag period of the main fuel load.

Fuel introduced during the ignition delay or lag period, which is the time between fuel being introduced to the combustion chamber and conditions becoming correct for ignition to commence, combusts quickly, the larger the amount of fuel present at the onset of ignition the greater the noise.Ignition lag is increased with cold engines and at low engine loads, as less heat is available within the engine and combustion begins later. The fuel properties and quality of atomisation also effect ignition lag. Ignition lag is the cause of ‘diesel knock’ or ‘clatter’.

Injectors with a pilot injection function were introduced to reduce engine noise by reducing the amount of fuel injected during the ignition lag period.

Experiments using a shock tube at high temperature and increased pressure found sprays of vegetable oil fuels burnt in a different manner to diesel, where the flame was reasonably uniform. Vegetable oil fuels produced pockets of burning, some of which continued for longer than would be available in an engine. It was thought that these were larger fuel droplets. These part burnt droplets would cause engine deposits. A second injection of fuel into high temperature and pressure conditions was found to rapidly combust in a uniform manner over the entire reaction volume. [2]

These experiments would suggest that pre injection systems should help to enable vegetable oil fuels to combust more completely.

DI engines sometimes utilize two spring nozzle holders to give a pre injection. The valve is held shut by two springs. The increasing fuel pressure overcomes the first spring, which allows a small quantity of fuel to be injected. As the fuel pressure increases it overcomes the pressure of the second spring and the main injection event begins.

IDI engines use a number of different pintle nozzles designs to achieve a pilot injection effect.

Delay or Throttle Pintle

As the needle lifts from its seat the longer throttle pintle still largely blocks the injection orifice allowing a small amount of fuel to be injected. As injection continues the throttling pintle clears the injection orifice allowing the main fuel charge to be delivered. Sometimes the throttling pintle is slightly tapered to provide a gradually increasing injection rate until the main injection takes place towards the end of the needle lift. [39,41]

Standard Pintle Nozzle

Throttle/Delay Pintle Nozzle

Hole Type Pintle

With this throttle pintle design, as the needle begins to lift, the fuel is able to flow into small bores that provide a pilot injection from a spray hole in the centre of the pintle.

Pintaux Pintle Nozzle

The Pintaux nozzles are similar to hole type except the pilot injection bore is in the nozzle body rather than the needle. The fuel jet is aimed at the throat from the engine cylinder to the prechamber. This aids starting as at low needle lift conditions (such as when the engine is being started) the main fuel charge is directed at the hot air rushing into the prechamber where combustion is most likely to occur.

Flat-Cut Pintle

This is a throttling pintle with an additional flat cut to allow a greater volume of fuel to flow at low levels of needle lift. As with the hole type pintle the gap between the throttling pintle and injection orifice is very small and during the pilot injection phase most fuel flows through the flat cut area
Industrial and marine engines are available equipped to burn heavy fuel oils. Injectors fitted to these engines are designed to provide good atomisation and avoid coking with these thicker fuels.

Anti-Coking Nozzle Designs

Flat-cut pintle nozzles have been found to be far less prone to coking due to the increased flow volume within the flat-cut area that stops deposits forming. [39]

Seat hole or valve covered orifice (VCO) injectors are multi hole injectors in which the start of the spray hole is located within the valve seat. With the traditional multi hole injector design the fuel left downstream of the valve, in the sac and nozzle holes, is heated by combustion and some fuel enters the combustion chamber late within the combustion event. This fuel only partially combusts and causes associated hydrocarbon emission problems and deposits on and within the nozzles. VCO injectors help to negate these problems by significantly reducing the amount of fuel that can leave the injector after the injection event and by blocking the flow of combustion products into the injector sac [39,40,41]

Multi hole nozzle

VCO Nozzle

Non VCO injectors are available with a reduced sac capacity that perform somewhere between standard multi hole and VCO injectors. Conical sac hole injectors are available which have a reduced sac volume compared to standard cylindrical sac injectors. Also injector needles are used with an extension from the tip to decreases sac volume with the valve closed.

Needle motion sensor, optical ignition sensors, injection lag, ignition lag and vegetable oil fuels.

NOx emissions have been found to increase with vegetable oil fuels, it has been suggested that this is due to the larger molecules found in these fuels which are less compressible and would lead to a reduced injection lag, that is the time between fuel being pumped by the IP and being delivered by the injector nozzle. Advancing the time of combustion is known to increase combustion pressures and temperatures, which leads to higher NOx levels. Injectors fitted with needle motion sensors detect the exact time of fuel delivery and the pump timing is modified to compensate [39][52].

Toyota introduced an optical ignition sensor that enables the ignition delay to be observed. The control unit can optimise the timing to compensate for differing fuels and engine wear [42].

Glow Plugs
Some vehicles glow plugs have an after glow function where the glow plugs stay on after starting for a few minutes to smooth cold running. This function improves poor combustion within a cold engine.

Upgrade kits are available for some vehicles from both Bosch and Buru. The replacement plugs are of a heavy-duty construction to withstand the extended heating times. Buru after-glow plugs are marked GN for Nachglueh (post-glow) compared to GV for Vorglueh (pre-glow). Bosch produce Duraterm glow plugs which are designed for use with afterglow systems. The later Duraterm Chromium are of a more rugged design.

1 The Diesel Engine
2 Theory of Vegetable Oil Use as a Fuel
3 Engine suitability
4 Heating the Oil
5 Biodiesel
6 Micro Emulsions and Blends
7 Vegetable Oil Engine Design
8 Vegetable Oil Furnaces and Heaters
9 Oil Types and Filtering
10 Taxation
11 Implications of Vegetable Oil Fuel Use
12 Sources

© All original material on this website is copyright Darren Hill, unless otherwise stated, and may be copied and distributed for non-commercial purposes only as long as the source of the material is stated and a reference to the vegburner website URL is included (http://vegburner.co.uk/). All material is provided "as is" without guarantees or warranty of any kind, either expressed or implied.