A nitro engine generally refers to an engine powered with a fuel that contains some portion (usually between 10% and 30%) of nitromethane mixed with methanol. Nitromethane is highly combustible substance that is generally only used in very specifically designed or modified engines.
Nitro engines used for R/C models are highly advanced engines that can turn in excess of 20,000 RPMs. With this much movement, a lot of friction is generated and the fuel used for these engines usually contains between 12-25% oil content depending on the nitromethane and methanol percentage. Most engines in R/C cars today are 2 stroke engines, which means that it takes 2 strokes of the piston to complete the engine cycle. On the first stroke as the piston travels downward, a mixture of nitro and oxygen is sucked into the crankcase, from the carburetor and travels into the induction port and finally into the combustion chamber. As the piston travels upward the mixture is compressed and ignited by the glow plug which causes the fuel to explode forcing the piston down. As the piston travels downward the spent exhaust gases escape out of the combustion chamber through the exhaust port, and the cycle starts over by the fuel mixture being again sucked into the induction port.
Nitro engines use a carburetor to mix the nitro and air together. 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 RPMs, and a low speed needle determines how much fuel is allowed into the carburetor at low to mid range RPMs. Turning either needle in a clockwise motion will lean the engine out. 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 richen the engine. Rich is the opposite of lean, it means more oil 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 slow 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. A properly tuned engine will last a long time with good performance throughout its life.
There are several different types of R/C engines. There are onroad, offroad, aircraft, marine, and monster truck engines. Onroad engines are designed to come into their powerband from mid to high RPM. These engines are can be used in offroad vehicles but are normally used in onroad sedans where very high rpms and high speed is required. Offroad engines have a less abrupt power curve compared to onroad engines. Offroad engines have a powerband that extends through most of the rpm range. Offroad engines do not rev as high as onroad engines, but they do have more torque that can easily propel the vehicle it is in to impressive speeds. Offroad engines are usually used in 1/8 scale buggies where high speeds and good acceleration are required. Aircraft engines are manufactured to be able to sustain high RPMs. The biggest difference between all other nitro engines and aircraft engines is the ability to sustain RPMs. Other nitro engines tend to break if run at full throttle for a full tank of fuel. Marine engines are cooled with water rather than air like other nitro engines. Monster truck engines are generally very large compared to onroad and offroad engines. Where an offroad engine may be .21ci size, a monster truck engine may be as much as .50ci. Monster truck engines generate much of their torque and horsepower at low to mid range RPMs. They are usually used in large and heavy trucks where all that power is needed to get good performance out of the vehicle.
Members of the drag racing industry mix the fuel themselves to create the appropriate fuel for their car and the day's atmospheric conditions.