RC Nitro Engines
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An Overview of RC Nitro Engines
Now on to the heart of any RC nitro truck, the fire breathing nitro RC engines that feeds off a diet of potent nitro fuel. An RC nitro engine is an engineering marvel. These tiny engines can scream to speeds in excess of 40,000 RPM! This is an extremely HIGH RPM for a reciprocating style engine (where a piston goes up and down constantly "stopping" and changing directions). Just for comparison, a typical V8 engine may run out of steam somewhere around 6,000 RPM or so. Even a typical "high revving" 4 cylinder car engine will usually be limited to around 7,500 RPM. There are exceptions with some racing type automotive engines able to rev up a few thousand RPM higher, but even so it becomes very clear how amazing these little screaming nitro RC engines can be! The sound of one of these engines screaming by at 30,000+ RPM in a RC truck is truly impressive. Let's just say it's not something you want to do at 6 AM on a Saturday morning while the neighbors are still sleeping! Once you better understand what these little engines are really doing, then you can better appreciate the engineering marvel that these tiny internal combustion engines are. The power-to-displacement ratio is mind boggling. In other words, if you scaled up one of these tiny engines into a full size engine, the amount of power it could produce would be shocking.
To give you a point of comparison for how much power these nitro engine really produce, let's say you have a powerful full size hot rod car with a 350 cubic inch displacement engine that cranks out around 375 HP. That's a powerful engine that will make a full size car really move! Here's where it really gets interesting. If you take the displacement of a typical "big block" nitro RC engine (.21 cubic inches or above), you will find that they will usually produce somewhere between 2 - 3 HP. If you figure the power to displacement ratio of one of these nitro RC engines, you quickly realize that the power output of these little engines is staggering. For comparison purposes, that hot-rodded 350 V8 would not just be pumping out in excess of 300 HP, but it would need to thump out in excess of 3,000 HP to match the power to displacement of typical RC nitro engines! Did you get that?! 3,000 HP! That's even a conservative number based on the more conservative power output of a typical stock nitro engine found in many RC trucks. There are high performance aftermarket engines that have even higher power/displacement outputs.
S-25 Nitro RC Engine from Savage 25 Truck
The picture above shows what typical nitro rc engines found in RC trucks might look like. This particular engine is found in the popular Savage 25 monster trucks. A typical nitro engine found in RC trucks operates on the 2 cycle (aka 2 stroke ) engine operating principle. Unlike most normal 2 cycle engines (such as are found in chain saws, weed eaters, etc...), nitro engines do not have a spark plug. Instead, a glow plug does the work of igniting the air/fuel mixture in the combustion chamber. In this respect, nitro engines are somewhat similar to a diesel engine. In both a nitro and diesel engine, a glow plug is energized during starting which causes the element to heat up and glow (hence the name "glow" plugs). This hot, glowing plug ignites the air/fuel mixture. Once the engine is started, the power source to the glow plug is turned off and the element continues to glow from the heat of combustion. So, the glow plug is energized only during initial start up, and after the nitro engine starts to run the combustion process is self sustaining and the engine will usually continue to run until the fuel runs out in the tank.
Another unique aspect of a typical nitro RC engine is that the piston does not have rings like most conventional piston engines. In most conventional internal combustion piston engines, the rings help to seal and keep the combustion gases inside the combustion chamber instead of leaking past the piston. However, a nitro engine is different. Instead of piston rings, nitro rc engines will typically just have a precision fit with the cylinder sleeve. To ensure a good seal during the compression stroke, the cylinder liner is actually slightly tapered so that the piston fits tighter at the top of the cylinder stroke. Many nitro engines have pistons with at least 1 small groove at the top of the piston which act as an oil retainer and is supposed to help keep the piston to cylinder contact well lubricated. Click link to see more about the operation of 2 stroke engines.
Most nitro rc engines are of the "ABC" type design. The ABC stands for ALUMINUM BRASS CHROME. This means that the piston is aluminum, the cylinder liner is brass, and the internal wear surface of the cylinder is plated with chrome. This hard chrome plating offers a much better wear surface than the soft brass underneath, and helps to prolong engine life. Sometimes, you will see an ABN engine. Usually the N stands for NICKEL plating in the cylinder liner which is cheaper and less durable. However, occasionally expensive, high end rc nitro engines will have an "ABN" design in which case the N stands for a NICASIL plated cylinder sleeve. This is a premium plating found on only a very few very expensive engines, because it offers even better wear resistance than chrome plating. This tougher Nicasil plating comes at additional expense. By far, the majority of good rc nitro engines utilize ABC technology with excellent results and long life potential.
The picture below shows some of the internal parts of a typical reciprocating 2 cycle nitro engine including the crankshaft, connecting rod, piston, & cylinder sleeve.
Crank, rod, piston, and cylinder of S-25 engine
In order to survive at nearly 40,000 RPM, internal components in RC nitro engines must be precision machined from high quality materials. These particular nitro engine parts that are shown in the picture above are the high quality components found in the HPI S-25 engine.