Carbureted compression ignition model engines
Encyclopedia
Simple compression ignition engines are made for model propulsion, usually model aircraft
but also model boats. These are quite similar to the typical glow-plug engine that runs on a mixture of methanol
-based fuels with a hot wire filament to provide ignition. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engine
s.
Full-size diesel engine
s, such as those found in a truck
, are fuel injected
and either two-stroke or four-stroke. They use compression ignition to ignite the mixture: the compression within the cylinder heats the inlet charge sufficiently to cause ignition, without requiring any external ignition source. A fundamental feature of such engines, unlike petrol (gasoline) engines, is that they draw in air alone and the fuel is only mixed by being injected into the combustion chamber separately. Model diesel engines are instead a carbureted two-stroke using the crankcase for compression. Their carburettor supplies a mixture of fuel and air into the engine, with the proportions kept fairly constant and their total volume throttled to control the engine power. Apart from sharing the diesel's use of compression ignition, their construction has more in common with a small two-stroke motorcycle or lawnmower engine. In addition to this, model diesels have variable compression ratio
s. This variable compression is achieved by a "contra-piston," at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.
Model diesels are found to produce more torque
than glow engines of the same displacement
, and are thought to get better fuel efficiency
, because the same power is produced at a lower RPM, and in a smaller displacement engine. However, the specific power
may not be significantly superior to a glow engine, due to overbuilding, to assure that the engine can withstand the much higher compression, sometimes reaching ratio
s of as high as 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.
, ether
, castor oil
or vegetable oil, and Amsoil
cetane
or amyl nitrite
booster. The fuel used contains Diethyl ether
, which is highly volatile and has an extremely low flash point
, combined with kerosene
and a lubricant plus a very small proportion (typically 2%) of ignition improver such as Amyl nitrite
or preferably Isopropyl nitrate
nowadays.
, gradually increasing the compression while cranking the engine. The compression is increased until the engine starts running. The mixture can then be leaned out and the compression increased. Compared to glow plug engines, model diesel engines exhibit much higher fuel economy, thus increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propeller at slower speed. Since the combustion occurs well before the exhaust port is uncovered, these engines are also considerably quieter (when unmuffled) than glow-plug engines of similar displacement
. Compared to glow plug engines, model diesels are more difficult to throttle over a wide range of powers, making them less suitable for radio control models than either two or four stroke glow-plug engines although this difference is claimed to be less noticeable with the use of modern schnuerle-ported engines.
Model aircraft
Model aircraft are flying or non-flying models of existing or imaginary aircraft using a variety of materials including plastic, diecast metal, polystyrene, balsa wood, foam and fibreglass...
but also model boats. These are quite similar to the typical glow-plug engine that runs on a mixture of methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
-based fuels with a hot wire filament to provide ignition. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s.
Full-size diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s, such as those found in a truck
Truck
A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...
, are fuel injected
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
and either two-stroke or four-stroke. They use compression ignition to ignite the mixture: the compression within the cylinder heats the inlet charge sufficiently to cause ignition, without requiring any external ignition source. A fundamental feature of such engines, unlike petrol (gasoline) engines, is that they draw in air alone and the fuel is only mixed by being injected into the combustion chamber separately. Model diesel engines are instead a carbureted two-stroke using the crankcase for compression. Their carburettor supplies a mixture of fuel and air into the engine, with the proportions kept fairly constant and their total volume throttled to control the engine power. Apart from sharing the diesel's use of compression ignition, their construction has more in common with a small two-stroke motorcycle or lawnmower engine. In addition to this, model diesels have variable compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...
s. This variable compression is achieved by a "contra-piston," at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.
Model diesels are found to produce more torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
than glow engines of the same displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
, and are thought to get better fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
, because the same power is produced at a lower RPM, and in a smaller displacement engine. However, the specific power
Specific power
In physics and engineering, surface power density or sometimes simply specific power is power per unit area.-Applications:* The intensity of electromagnetic radiation can be expressed in W/m2...
may not be significantly superior to a glow engine, due to overbuilding, to assure that the engine can withstand the much higher compression, sometimes reaching ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
s of as high as 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.
Fuel
These "diesels" run on a mixture of keroseneKerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
, ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
, castor oil
Castor oil
Castor oil is a vegetable oil obtained from the castor bean . Castor oil is a colorless to very pale yellow liquid with mild or no odor or taste. Its boiling point is and its density is 961 kg/m3...
or vegetable oil, and Amsoil
Amsoil
AMSOIL Inc. is a corporation based in Superior, Wisconsin that primarily formulates and packages synthetic lubricants and filters. The company's advertising slogan is The First in Synthetics.-History:...
cetane
Cetane number
Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality.- Definition :...
or amyl nitrite
Amyl nitrite
Amyl nitrite is the chemical compound with the formula C5H11ONO. A variety of isomers are known, but they all feature an amyl group attached to the nitrito functional group. The alkyl group is unreactive and the chemical and biological properties are mainly due to the nitrite group...
booster. The fuel used contains Diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, which is highly volatile and has an extremely low flash point
Flash point
The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source...
, combined with kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
and a lubricant plus a very small proportion (typically 2%) of ignition improver such as Amyl nitrite
Amyl nitrite
Amyl nitrite is the chemical compound with the formula C5H11ONO. A variety of isomers are known, but they all feature an amyl group attached to the nitrito functional group. The alkyl group is unreactive and the chemical and biological properties are mainly due to the nitrite group...
or preferably Isopropyl nitrate
Isopropyl nitrate
Isopropyl nitrate is a colorless liquid monopropellant. It is used as a diesel cetane improver. IPN is a low-sensitivity explosive, with a detonation velocity of approximately 5400 m/s....
nowadays.
Starting
The engine is started by reducing the compression and setting the spray bar mixture rich with the adjustable needle valveNeedle valve
A needle valve is a type of valve having a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates.- Construction and operation :...
, gradually increasing the compression while cranking the engine. The compression is increased until the engine starts running. The mixture can then be leaned out and the compression increased. Compared to glow plug engines, model diesel engines exhibit much higher fuel economy, thus increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propeller at slower speed. Since the combustion occurs well before the exhaust port is uncovered, these engines are also considerably quieter (when unmuffled) than glow-plug engines of similar displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
. Compared to glow plug engines, model diesels are more difficult to throttle over a wide range of powers, making them less suitable for radio control models than either two or four stroke glow-plug engines although this difference is claimed to be less noticeable with the use of modern schnuerle-ported engines.