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The RC nitro car engine

The rc nitro car engine has evolved over the years into a very robust and reliable power unit, and modern nitro engines are much more reliable and powerful than they ever were.

They vary greatly in size although the most popular and widely used engines are the small-block ones which generally range between the .10cu.in and .17cu.in capacity (cu.in. stands for cubic inch).

They can have different features, such as rear or side exhaust; barrel or sliding carb; pullstart or non-pullstart etc. and can be modified too by replacing certain parts to increase performance, but an average rc car or truck owner will almost certainly just keep his or her nitro engine exactly as it was when it came out the factory, as the performance of the engine will be more than adequate for the model.

Two typical rc model car nitro engines are shown below; a pullstart one (left) and non-pullstart (right):


Different rc nitro engines



The basic parts of a nitro engine

A model nitro engine consists of just the fundamental parts of any internal combustion engine.
The cylinder head sits on top of the crankcase and has 'fins' which greatly increase the surface area to help cool the engine as it runs.

Inside the cylinder head is the combustion chamber which is home to the piston. This is the part that moves up and down very quickly as the fuel and air mixture within the combustion chamber ignites.
The piston is connected to the crankshaft, which is a horizontal shaft running through the crankcase and is connected to the clutch of the car.

Sitting at the front of the engine on top of the crankshaft is the carburetor, which is the part of the engine that introduces the fuel into the crankcase. The fuel / air mixture is ignited by a glow plug which is screwed into the top of the cylinder head.

The basic parts visible from the outside are shown in the diagram below:


external parts of a typical nitro engine



How a nitro engine operates

RC car nitro engines are two-cycle (or two-stroke), whereas most larger engines are almost always four-cycle - with the exception of things like small lawn mowers or scooter engines.
Being 2-cycle means that it really is a very straightforward thing, as there are no valves, cams or lifters that are found in a 4-cycle engine.

Two-cycle means that the piston just needs to complete one down-stroke and one up-stroke within the combustion chamber to complete the process of drawing the fuel and air in and then converting the ignited mixture into raw power and also expelling the exhaust gases.

The piston will keep on moving up and down (hopefully!) so long as the fuel/air mixture is being ignited, but how does the whole process start? Well, the fuel/air mixture needs to be introduced into the crankcase at the bottom of the combustion chamber before anything can happen.
This is done by 'priming' the engine, which usually means holding a finger over the carb venturi or exhaust opening while slowly turning over the engine by hand, or just squirting fuel directly into the carb from a plastic bottle with a fine nozzle.
Either action makes sure that fuel is put into the crankcase.

Now that fuel is in the crankcase and has mixed with air, the glow plug needs to heat up and the piston needs to move up and down. So to start, the nitro engine needs to be turned over manually, which is either done with the pull cord, if it is a pullstart engine, or by an electrical engine starter.
At the same time, the plug is made to glow red hot by connecting a special battery to it for a few seconds; the core of the plug heats up immediately.

With the fuel/air mix sitting in the crankcase, it needs to be moved up to the top of the chamber and ignited. It's moved to the top as the piston moves down; as it does so it pressurizes the area of the crankcase where the fuel/air mix is and forces it up small ports, or channels, that run up to the top of the chamber. The mixture is then ignited by the glow plug on the top of the engine.
Unless the engine is in poor condition or badly tuned, it should fire up after just a few seconds of being turned over.

When the piston is at the point just before the mixture ignites, it is covering the exhaust ports which run from the sides of the combustion chamber through to the silencer. When the piston has almost reached the top, the fuel/air mix ignites because of the glow plug, and the resulting explosion forces the piston back down again. On the way down, the exhaust ports are uncovered by the piston and the burnt gas mixture leaves the chamber via the ports.
The momentum of the spinning crankshaft makes the piston move upwards again and the whole process starts over.

Although the glow plug was made to glow red hot initially by a battery, once the engine has begun to run then the high temperature inside the combustion chamber keeps the plug glowing. If the plug fails, then the fuel/air mix will not get ignited and the engine will stop.

 

Related pages

Nitro RC cars.


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