Lesson 3: Helicopter engines

In our lesson 2, we explained what autorotation is and how we perform it. One scenario in which we have to perform an autorotation is when, for example, the engine fails. Modern engines, however, are reliable, so once the engine is running, the chance of it failing is very small, unless you forgot to refuel.

Another cause of an engine misfire can be ice build-up in the carburettor. It therefore seems useful to go into the causes and dangers of ice build-up in the carburettor in more detail. To better understand the operation and purpose of the carburettor, we should first take a look at how a piston engine works.

We often call this a combustion engine, but combustion also occurs in a turbine engine, hence our correct term of the piston engine. Among piston engines there are also various operating principles, yet the most common piston engine in light aviation is the four-stroke engine.

A four-stroke engine consists of a cylinder containing a piston that can move up and down, a bit like a bicycle pump. The aim is to convert the up-and-down motion of the piston into a rotary motion, and for this we have the connecting rod and the crankshaft.

The top of the cylinder is closed and contains two or more openings that can be closed by valves; these are the inlet and exhaust valves. There are also one or two ignition plugs, commonly known as spark plugs, at the top in the cylinder head.

A complete cycle of a four-stroke engine consists of four strokes, namely the intake stroke, the compression stroke, the power stroke and the exhaust stroke.

At the start of the intake stroke, the piston is at the top of the cylinder. We also call this point top dead centre (TDC) because the piston momentarily comes to a standstill here at the top of the cylinder during operation. Both valves are closed, and as soon as the piston moves downwards, the inlet valve opens.

Due to this downward movement of the piston, air is drawn in past the inlet valve, as it were. In some four-stroke engines this is not just air but a mixture of air and petrol vapour coming from the carburettor (more on which later). This all happens during the intake stroke.

Once the piston is at its lowest point, also known as bottom dead centre (BDC), the inlet valve closes. When the piston now moves upwards again, the mixture of air and fuel is compressed by the ever-decreasing volume in the cylinder. This increases the pressure and temperature, but the intention is not for our mixture to ignite yet. We call this the compression stroke. This combustion of the fuel/air mixture occurs through the ignition of this mixture by the “spark plug” or ignition plug at the moment the piston is once again at the top of the cylinder.

Due to this combustion, the temperature and of course also the pressure in the cylinder increase even further. This increase in pressure pushes our piston downwards. This is actually the only moment in the entire cycle of the four-stroke engine when our engine performs work, and this stroke is therefore called the power stroke.

Once the piston has moved all the way down (BDC), the other valve opens, the exhaust valve. During the following upward movement of the piston, the combustion gases can be pushed out through the exhaust valve to continue on to the engine's exhaust system. When the piston is at the top of the cylinder (TDC), the exhaust valve closes again, the inlet valve can open again and the entire four-stroke cycle begins anew.

So this is a rather simple explanation, but it is sufficient to later understand what the carburettor will be used for. To improve the engine's efficiency, manufacturers will also ensure that the inlet and exhaust valves open earlier and stay open longer, and the moment at which the “spark plug” delivers the spark to ignite the mixture is also brought forward.

Multiple inlet and exhaust valves can also be provided, as well as multiple “spark plugs” per cylinder. Just think of the gleaming “16V” badge that is often seen on the back of a car. This actually just means that this is a car with 4 cylinders, where each cylinder has 2 inlet and 2 exhaust valves.

Imagine: a Robinson R44 Raven II 16V. So I explained the operation of the four-stroke using one cylinder with its associated piston. Just like in cars, piston engines in aviation are equipped with several cylinders. A very common design consists of an arrangement of 4 or 6 cylinders placed in opposing pairs, also known as the boxer engine (in English, flat-opposed). This type of engine is easy to install and is usually provided with air cooling.

We now know that our engine needs a mixture of fuel and air. One way to supply the engine with the necessary fuel is to equip it with a fuel injection system, as in the Robinson Raven II. In the Robinson R44 Raven I and the Robinson R22, this is done by means of a carburettor. In our next lesson we will see how a carburettor works and what the advantages and disadvantages of it are.

Credits:

• Photos, drawings and text © C. Comyn – Toran Helicopter Acdemy

Article source: https://www.helispot.be/hs/page/detail.asp?oid=e7e5c2D3