Glow igniter engine
Glow igniter engines ( also simply called glow igniters ) are internal combustion engines in reciprocating piston design with external mixture formation by carburettors , which do not have a controlled ignition device (high-voltage ignition modules with ignition coil , distributor and spark plug ). They can be built in two-stroke and four-stroke versions. Also Wankel -Glühzünder be established.
They are used almost exclusively in model making. Airplane , car , boat and tractor models in larger scales (about 1:10 and 1: 8 for cars) are equipped with glow igniters. But there are also two- and four-stroke engines with electronic spark ignition in model making.
2-stroke glow igniters are manufactured in a stroke volume of up to about 30 cm³. In the RC car sector, 2 cm³ to 6.5 cm³ motors are mainly used. Engines with a displacement of 3 cm³ now work as standard up to around 35,000 rpm (OS Engines 2012), and sometimes even higher. Airplane models have engines with displacements between 0.163 cm³ and 30 cm³, multi-cylinder engines also up to about 60 cm³. Helicopter engines have between 5 and 15 cm³, ship model engines between 3 cm³ and 15 cm³.
functionality
The functional principle is similar to the hot-head motor . A glow igniter engine is a two-stroke or four-stroke engine with external mixture formation that does not require controlled ignition. The fuel ignites on a permanently glowing wire coil that is coated with platinum - iridium . The filament is electrically preheated for the starting process.
The two-stroke version (most common design) is usually a slot-controlled engine with crankcase scavenging , the inlet timing is determined by a milling (hollow shaft) on the crankshaft and the exhaust is controlled by the piston. To enhance performance, one is often Schnürle - Reverse Circulation used.
Methanol is used as fuel in model making . Up to about 35% nitromethane can be added to the methanol to increase performance . Nitromethane also has a positive effect on ignitability and causes quieter idling .
Engine lubrication
Glow igniter engines usually do not have their own lubricant circuit , which is why the lubricating oil required for all moving components of the engine is added to the fuel .
Today, the lubricant consists of a synthetic oil especially for glow igniter engines. In the past, castor oil was often used, but it has the disadvantage of residue formation compared to modern oils. With some models, castor oil is prescribed during the running-in process because it creates a low-wear surface layer on steel. The usual total amount of oil in the fuel is 18%.
With four-stroke engines, oil can be filled into the crankcase before operation. During operation, the parts of the engine that are located in the crankcase are otherwise only lubricated by the oil that runs down the combustion chamber walls into the crankcase, which is sufficient under normal conditions.
cooling
Car and aircraft engines are air-cooled , but there are also exceptions where car models are water- cooled . However, these water-cooled motors often have thermal problems when temperatures exceed 130 ° C. Ribs on the engine surface and (in the case of car and helicopter engines ) large heat sinks that are mounted on the cylinder head create the conditions for efficient cooling. In aircraft models, the cooling is reinforced by the propeller ensured (propeller), car models by the wind and in some cases, liquid cooling , boat motors with liquid cooling are performed. For this purpose, the cylinder head is cooled by a liquid jacket in the head instead of a heat sink. The surrounding water is used as the cooling medium. Occasionally one also finds evaporative cooling with a water bath.
Other factors involved in cooling glow igniters are the fuel mixture and the amount of lubricant in the fuel. Glow igniters must not be operated at the lean limit (lambda = 1), otherwise they will overheat. A slightly bold setting (lambda <1) of the mixture prevents this. A higher proportion of oil also reduces the operating temperature.
Fuel processing
Glow igniter engines predominantly have simple throttle slide - carburetors , which are designed as rotary slide or axial slide . Setting options are only available for the mixture preparation ( main jet ), for the idle speed and the partial load mixture ( idle mixture ).
Exhaust system
The exhaust gas routing is crucial for smooth running and the development of the glow igniter's torque , especially in the two-stroke version . In most cases, resonance tubes designed for the displacement and the speed are used, the length, shape and diameter of which are adapted to the vibration behavior of the engines. Optimal performance can only be achieved through good coordination of the exhaust tract .
Specific performance
The specific displacement of the glow igniter is very high. An engine with a displacement of 2.5 cm³ develops an output of 0.9 kW at 29,000 rpm, which corresponds to a liter output of 360 kW.
The 3.5 cm³ motors, which have been developed to the limit of what is currently feasible, already have models with over 2.25 kW in regular operation. With tuning measures , a significantly higher performance can be achieved with a greatly reduced service life. The class of 3.5 cm³ engines is especially predestined for these performance increases because the displacement of model cars in professional races is usually limited to this value.
Engine start
In order to save weight, glow igniters are used in professional applications (competitions, races) without their own starting device. Starting is then done via a so-called start box (an electric motor with a friction roller) that attaches to the flywheel of the motor and starts it. In car models, this electric motor is usually located in a metal box on which the vehicle can be precisely placed and only needs to be pushed down slightly to start.
There are three starting methods for car models that are not designed for racing. The engine can be started either with a cable pull , EZ starter or an electric starter, in which - in contrast to the starter box - the power is not transmitted via a friction roller, but with a hexagon or cardan shaft to one at the rear of the crankcase Plate is transferred to the crankshaft. With the EZ System from Traxxas, the engine is started by an electric motor permanently mounted on the RC model. Starting takes place via the EZ-Start, which supplies the glow plug and the electric motor with power.
An electric starter can also be used for model aircraft , which is placed axially on the spinner with a rubber insert and starts the engine; manual throwing on the propeller is also usually possible. Reversing starters or permanently attached electric starters can be attached. To start, the glow plug is connected to a voltage source of around 1.2 V to 2 V, whereby the flow of current heats the glow plug strongly and thus enables the engine to start. The voltage can be removed a few seconds after the start. The glow plug filament is catalytically coated, and the fuel mist that is brought up to the plug when the engine is running keeps the plug glowing. This is why restarting a hot engine is only possible by re-energizing the candle. Automatic glow devices are conceivable here, for example to remotely start an engine or, more often, to enable a lower and more stable idling. This is used particularly frequently in four-stroke engines, as these are particularly susceptible to the incandescent filament going out due to the double time interval between the individual ignitions due to the operating principle.
Control of ignition ( ignition timing )
The operation of glow plug motors requires precise setting of the influencing variables.
As for all internal combustion engines with spark ignition, it is also true for the glow igniter that a certain compression ratio depending on the fuel mixture, as well as an ignition point in a narrow angular range before top dead center, must be observed, otherwise the engine will not work.
The ignition takes place via a permanently glowing glow plug , so that no ignition point can be set on the engine. It depends on influencing variables that are outside the motor. They are essentially
- Fuel mixture
This applies to the effect of the fuel on the ignition point. The more nitromethane, the earlier the ignition point, the more oil, the later the ignition point.
- Heat value of the glow plug
The glow plugs are manufactured in different heat values. A hot glow plug causes an earlier ignition, a cold one a correspondingly later ignition. The heat values range from 1 (hot) to 9 (very cold). The ignition point shifts significantly to late ignition due to colder candles.
- compression
The compression of the engine is fixed and changes to this value inside the engine are only possible by machining the piston or cylinder head. However, there are ways to change the compression ratio by adding an additional or thicker head gasket , which can be fitted to the two-stroke glow igniter in a few minutes. Higher compression leads to earlier ignition, lower compression leads to later ignition.
External igniter or compression ignition?
The type of ignition of the glow igniter motor cannot be clearly categorized; however, the tendency is to classify it as a compression-ignition engine similar to the hot-head engine .
history
Gottlieb Daimler and Wilhelm Maybach made the gas engine developed by Nikolaus August Otto usable for the use of liquid fuels, especially gasoline . In 1883 Daimler and Maybach registered a so-called "gas engine with glow tube ignition" under the patent number DRP No. 28022. This was the world's first high-speed 4-stroke petrol engine. High-voltage ignition with an ignition spark did not come about until over two decades later. The glow tube ignition was an externally heated glowing tube in the cylinder head that ensured that the mixture ignited. So the first gasoline engines were actually glow-igniter engines.
Other factors
Methanol is very hygroscopic , so it attracts water from the ambient air. This reduces the calorific value (the fuel ages). When using such a fuel it may be necessary to change the carburetor setting. The aging effect can also be compensated by a slightly increased nitromethane content. The oil content also has an influence on the performance of the engine. The lower it is, the better the combustion.
safety rules
Glow igniter motors are used in model making. However, models with internal combustion engines are not toys , as the fuel methanol is poisonous and nitromethane is harmful and both substances must be used properly. Swallowing very small amounts can have serious consequences and even death. Both substances are also easily flammable. The engine itself is also dangerous if used improperly. In Germany, children and young people are only allowed to operate combustion engines if they are accompanied by a parent or legal guardian.
literature
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden 2001, ISBN 3-528-13114-4 .
- Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
Web links
- Model engine archive by Torsten Steiche
- Accessories for the operation of internal combustion engines
- Running-in and adjustment of internal combustion engines