Vegburner

Version 2.0 (17/4/2004)

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

The Diesel Engine

Dr. Diesel

The term diesel derived from a German engineer, Dr Rudolph Diesel, who is widely credited with developing the compression ignition (CI) engine. In 1892 he took out a patent (no.7241) on a CI engine which ran on coal dust this was built in 1893 but exploded when it was run.

In 1894 he filed a patent for a CI oil engine which he successfully ran in 1897. In 1900, at the world fair in Paris, a small diesel engine was demonstrated running on peanut oil. The Otto Company provided this demonstration at the request of the French Government which was keen to use peanut oil as an energy source for its African colonies.

There is however some controversy as to weather he was the inventor of this type of engine as a British engineer, Herbert Ackroyd-Stuart took a patent (No.7146) in 1890 for an oil fired engine which was the result of development he had been doing on low-compression oil engines. This patent describes "first of all compressing the necessary quantity of air for the charge, and then introducing into this quantity of compressed air the necessary supply of combustible liquid, vapour or gas, to produce the explosive mixture." The engine was in production in 1892. Also Ruston & Hornsby Ltd claimed, in a 1937 advertisement “Before Dr. Diesel took out his first oil engine patent, we were making successful oil engines at our Granthan Works.”

As the engine could be fuelled on a variety of oil fuels, Dr. Diesel saw his engine as a way of freeing small producers from the grip the steam industry had on the energy markets of the time. He worked on the development of the engine until his untimely death. Much controversy surrounds his death on board a boat to England some say he committed suicide, others suggest an accident, others still that something more sinister was afoot.

Due to economics fossil oil became the energy sources of choice for the developed nations and a distillate of crude that was suitable for CI engines came to be known as diesel fuel. CI engine development since this time has largely been with diesel fuel in mind. Most CI engines are engineered to optimise the burning of this fuel. It is however possible to tune these engines so that they can be fuelled with vegetable oil.

C.I. Engine Basic Principles

Diesel engines come in all shapes and sizes with refinements made to the basic principles developed in the late 19th century.

The Direct Injection Engine (DI)

The engine uses the heat caused by compressing air in a cylinder, with a piston, to ignite fuel oil which is injected into the cylinder. This oil burns and creates an increase in pressure which forces the piston back down the cylinder, providing the power. The burnt gasses are exhausted from the cylinder and replaced by fresh charge through valves, commonly positioned above the piston in the cylinder head.

The fuel oil is pushed into the engine under high pressure at the correct time by an injector pump. The oil is sprayed into the engine cylinder as a very fine mist from the nozzle of an injector. The fine fuel oil mist readily mixes with hot air in the cylinder, ignites and provides an efficient burn. The injector pump also meters the amount of fuel delivered. The more fuel the more power/faster the engine.

Operating cycle of the common four stroke engine – the engine provides one combustion event during the four piston strokes illustrated.

Large DI Engines

Large slow running DI engines (under 1500 rpm) commonly used to power machinery and large boats have 'open chambers' where the piston has a wide shallow dish in the centre of its upper surface. Air movement within the combustion chamber is minimal and the multi-hole injectors (often 8-12 holes) are set to inject a fine mist into the dish. Due to the size of the combustion chamber and the distribution of fuel sufficient air is present to supply the fuel with sufficient oxygen and the fuel combusts before it contacts surfaces of the combustion chamber.

Small DI Engines

Small high speed direct injection engines have a deep hollow in the top of the piston which contains most of the air when the piston is at the top of the engine. As air is drawn into the engine a swirling motion is initiated. The air is forced into a horizontal rotary motion by the design of the air inlet port or valve. Some engines have a helical or partial vortex form of inlet port to encourage the swirl in others the inlet valve is partially masked to initiate the desired motion. As compression of the air begins it is forced or squished into the specially shaped piston hollow which initiates a vertical swirl achieving ever increasing speeds and heat as the air is compressed. These combined movements create an air vortex within the piston into which the fuel is injected. The fuel spray, finely atomised by the high-pressure injector, passes through this air vortex where it is provided with sufficient oxygen and heat to combust. DI engines generally utilise a multi-hole injector to provide a good distribution of fuel.

Squish air motion in a DI engine

Air spiralling through a helical intake into engine cylinder

Photo: Bosch
Cut away view of a
small DI cylinder

The Indirect Injection Engine (IDI)

A later development was the IDI engine that utilises a separate combustion chamber, connected to the engine cylinder, into which the fuel is injected and combustion is initiated.

A heat resistant insert with low heat conductivity is located within the combustion chamber so it quickly heats up and retains heat from combustion, providing extra heat to enable quicker ignition. The fuel is injected into the hot combustion chamber as a jet at a low pressure compared to the fine high-pressure spray of a DI engine. The fuel jet hits the hot insert where ignition is initiated; the fuel is distributed around the combustion chamber as combustion continues. The expanding burning fuel, along with partially burnt and unburnt fuel, is carried into the hot engine cylinder where further oxygen is available and combustion continues.

The most common prechamber format utilised is the Ricardo Comet design developed by Ricardo and Company of Shoreham, Sussex, UK. With this design air is pushed from the cylinder into a circular ‘swirl chamber’ through a tangentially aligned throat. The bottom half of the chamber along with the throat is constructed from a nimonic alloy designed to maintain high temperatures during engine operation. The temperature of the compressed air is raised further while passing through the throat. A vigorous swirl motion is initiated as the air is forced into the circular swirl chamber. The fuel is injected into the swirl chamber and rapidly atomised within the mass of hot turbulent air.

Photo:Bosch
IDI engine – Picture shows fuel being injected into a swirl chamber with a heated glow plug – not shown is the throat from the engine cylinder to the swirl chamber - Note the lower half of the chamber constructed from nimonic alloy.

Mercedes IDI engines utilised a different design in which a jet of fuel is fired at a ball like baffle surface. The jet is broken by the baffle surface and distributed around the prechamber being finely dispersed by the turbulent air charge. Upon combustion the fuel/air mixture is carried through several bores into the main engine cylinder. Mercedes later improved this design, reshaping the prechamber, creating a swirling air motion to improve combustion.

The advantage of IDI engines is that they can operate at higher engine speeds as the more efficient fuel and air mixing provides faster combustion. Cars and small commercial vehicles require a small, light engine which must be able to operate at higher speeds to provide the necessary power, with the advent of the IDI engine the use of diesel engines in such vehicles became widespread.

The heat lost due to the increased surface areas of the combustion chamber and the pressure drop between cylinder and combustion chamber make it necessary for the engines to operate at higher compression ratios to provide enough heat for ignition. The lost heat and force required to push the air into the combustion chamber is wasted energy making IDI engines around 10-15% less efficient than DI units.

IDI engines became the engine of choice in small vehicle applications as a small engine could produce more power at higher speed providing a suitable power/weight ratio for such applications. Recent advances in fuel injection technology, which provide more precise fuel delivery, allow faster combustion within a DI engine. The improved efficiency of the DI cycle has spurred the fitment of such engines to become more common in small vehicles.

Fuel Injection Equipment

Timing, pressure and quantity of fuel delivery are crucial for efficient combustion. The important task of fuel delivery is performed by the fuel injection equipment.

Mechanically Controlled Fuel Injection

Fuel is supplied to the injector by an injector pump. The injector pumps are mechanically driven from the crankshaft. The drive from the crankshaft is set so that the injector pump delivers fuel at the correct time in the engine operation cycle.

Single Element Injection Pump (Jerk Pump)

The simplest type of injection pump found on single cylinder engines. The single pumping element delivers fuel to the injector via a high pressure pipe. Multi-cylinder engines sometimes utilise multiple single element pumps. This is most often seen in stationary or marine applications.

In-Line Injector Pump

In-line pumps use a similar design to the single element pump, for a multi-cylinder engine. A number of jerk pumps are combined into a single component. The pumps are driven by a camshaft held within the pump body.

Rotary Injector Pump

Rotary pumps are similar in appearance to a petrol engine distributor. A single pumping mechanism rotates and supplies fuel to each cylinder in turn.

Electronically Controlled Fuel Injection

Recently developed electronically controlled injectors can provide exacting amounts of fuel at very high pressure, very precise timing and even multiple injections within each cycle to give greatly improved combustion and in turn increased fuel economy and lower engine noise and toxic emissions. Both in-line and rotary injection pumps have been developed with electronic components to help achieve more accurate injection timing and metering. By monitoring the engine using a number of sensors the electronic controller can modify the fuel injection characteristics to improve combustion.

Common Rail Injection Systems (CDI)

With this system a pump constantly supplies fuel at a very high pressure to the common rail - a tube with thick walls. From the common rail fuel is supplied to electronically controlled injectors.
The higher pressure injection gives a finer spray and improved combustion.

Unit Injector System and Unit Pump Systems

Unit injector systems combine the pump and injector into one unit. The pump is driven from the engines camshaft. Fuel delivery is timed and metered by electronically controlled valves. Unit pump systems are similar with the pump and injector separated and connected by a short high pressure fuel line.

Diagram: Bosch

Bosch Electronic Injection Systems:
Illustration shows a VP44 electronically controlled rotary pump and 3 other systems

Turbocharger

Diesel engines are particularly suited to turbocharging. Turbochargers use a specially designed turbine and centrifugal compressor linked by a shaft to utilise the energy of the exhaust gas being forced from the engine to compress extra air into the engine. The compressed air contains more oxygen. The amount of fuel that can be effectively combusted is directly proportionate to the amount of oxygen within the engine. Thus the maximum fuel delivery can be adjusted and engine power increased by as much as 60%. Due to more efficient combustion fuel efficiency is improved by about 10%. Turbocharged engines are generally of a more rugged construction to withstand the greater forces and temperatures encountered.

Intercooler

Intercoolers are often added to turbo engines to cool the air charge between the turbocharger and the engine. Compression of the air and proximity to the hot exhaust gases increase the temperature of the air charge and cause it to expand. The intercooler, most often an air to air heat exchanger, uses air flow in the same way as an automobiles radiator to bring the temperature of the air charge down. The cooled air contracts and more of it – and thus more oxygen – can be forced into the engine cylinder.

Turbocharged and intercooled engine.

Combustion in a Diesel Engine with Mechanical Fuel Injection System

The combustion process is initiated when the fuel is injected into hot air within the combustion chamber. The temperature reached in the combustion chamber will be effected by;

compression ratio : the more the air is compressed the higher the pressure and the hotter it becomes

the speed of the engine : the faster the air is compressed the less time there is for heat to be lost from the combustion chamber and air to seep by the piston rings reducing the pressure.

the temperature of the incoming air

The fuel is delivered by the injector pump pushing fuel through the injector. The pumping action produces a rise in pressure in the fuel line as the pressure rises it overcomes a spring which holds the injector needle valve closed. The injection lag is the time between the pump pumping and the fuel being delivered and is affected by the length of the high pressure injection pipe, the load on the injector spring and the density and thus compressibility of the fuel.

The fuel is injected into the hot air mass within the combustion chamber by the injector which delivers a fine spray of fuel. This spray breaks into small droplets which begin to evaporate in the intense heat. Each droplet becomes surrounded by a layer of vapour which begins to react with the surrounding oxygen. For any delivered amount of fuel, the finer the spray, the larger the surface area of fuel available to vaporise and then react with oxygen. However a fine spray will not penetrate far into the combustion chamber where further oxygen would be available and would be easily dragged around with any air flow which would otherwise be introducing further oxygen. A spray of large droplets would travel further through the combustion chamber less affected by air motion but would take longer to ignite and longer to combust entirely.

Multi Hole Injector

The oxidation process creates heat which causes more vaporisation and increased oxidation until the self-ignition temperature of the fuel is reached and, at a limited number of points within the chamber, ignition of the vapour begins. The time between the fuel being introduced and start of ignition is called the ignition lag or delay period.

Combustion then spreads quickly through the fuel which has been injected during ignition lag causing a rapid pressure rise. A long ignition lag allows a greater amount of fuel to be delivered which will cause a more rapid and greater pressure. The increased engine noise and rough running ,known as diesel knock, is a direct result of these greater pressures. The rate of burn will depend on the speed at which the vaporising fuel droplet can come into contact with unreacted oxygen. The speed of the fuel and motion of the air, if any, affect the burn rate. The rapid pressure rise continues, slowing as the temperature increases to a level where fuel droplets are burnt soon after being injected.

At each fuel droplet the liquid oxidation reaction of the vapour will continue rapidly as long as sufficient oxygen is available. As the combustion continues less oxygen is available close to the injector and the fuel droplets take longer to find enough oxygen to burn. Combustion continues at a steady level until the fuel delivery is completed. The remaining fuel and unburnt products of the oxidation reaction continue to burn with decreasing vigour as the temperature in the chamber falls. Some of these products remain unburnt and are largely expelled with the exhaust.