A nitro engine generally refers to an engine powered with a fuel that contains some portion (usually between 10% and 40%) of nitromethane mixed with methanol. Nitromethane is a highly combustible substance that is generally only used in very specifically designed engines found in Top Fuel drag racing and miniature internal combustion engines in radio control, control line andfree flight model aircraft.
The term "nitro" has only come into use in the last few years to describe these engines, and has its origins in marketing hype in the model car market. For the fifty or so years prior to this term since the engines were first developed, they were simply referred to as "glow engines", but the term "nitro" has more impact in ad copy. These engines are actually fueled by methanol, but the fuel is often doped with nitromethane as a performance additive. The ignition system consists of a glow plug - hence the older term "glow" engine - which has a coil of platinum containing wire alloy, usually platinum-iridium. The glow plug is heated with electric current for starting, after which power is disconnected and the combination of residual heat and catalytic action of the platinum alloy with methanol ignites the fuel mixture.
When starting, the glow plug is electrically preheated by electric current. The glow plug is not to be confused with a spark plug - there is no spark in the glow plug. Catalysis from methanol vapor on the heated platinum element keeps it red-hot even after voltage has been removed, which ignites the fuel and keeps the engine running. Whereas spark plugs are constantly used to ignite the fuel/air mix every time the piston comes up, as seen in the petrol engine where the spark plug is used, the fuel cannot be ignited with compression alone. It is the plug's temperature, still red-hotfrom previous ignition and from catalysis with the new compressed mixture, that ignites the fuel.
Nitros engines typically use a carburetor to mix the fuel and air together, although for some applications where throttling is not required they have a simple venturi with a spraybar and needle valve. The carburetor can either be sliding or rotary. On a rotary carburetor, the slide is opened as the arm is turned by the servo. On a slide carburetor the slide is opened by sliding the arm out by the servo. Both are held open slightly by an idle screw which allows the engine to receive a very small amount of fuel to keep the engine running when the vehicle is at a stop. The carburetors usually feature 2 needles used to tune the mixture. A high speed needle tunes how much fuel is allowed into the carburetor at mid to high RPM, and a low speed needle determines how much fuel is allowed into the carburetor at low to mid range RPM. Turning either needle in a clockwise motion will thin the fuel mixture. Lean describes the amount of fuel in the fuel / air mixture. To a point this will make the engine run faster with better performance, but once too lean the engine will overheat, and wear out prematurely due to not receiving enough lubrication. Turning either needle counterclockwise will enrich the fuel mixture (unless the low speed needle is an air bleed in which case the opposite is true). Rich is the opposite of lean, it means more oil (fuel mixture) is entering the engine. If the engine is too rich, it will run poorly, and fuel that has not yet been burnt may start to spit out of the exhaust. The engine will run very slowly and seem to have no power and possibly cut out from being flooded with fuel. Although, being too rich is better than being too lean, because being too rich just means the engine is getting too much oil which is perfectly fine, although performance may not be as good as if the engine were lean. An excessively lean mixture can damage an engine in a short time, as it will run above its design temperature. A properly tuned engine will last a long time with good performance throughout its life. -wiki