Two-Stroke / Two-Cycle Engines - How they work

The basic internal combustion engine is a very simple machine, in principle. A piston moving up and down within a cylinder is connected (by a connecting rod (con-rod)) to a rotating crankshaft. The piston is forced downwards by the expandng gases of a fuel-air mixture burning in the cylinder space above it. This causes the crankshaft to rotate. The momentum of the rotating crankshaft then forces the piston back up again, allowing the next cycle to occur. Often, a heavy flywheel is attached to the crankshaft, to maintain the momentum of rotation.

Four-Stroke Engines

The complete cycle of an internal combustion engine is easiest to understand by referring first to a simple four-stroke petrol (gasoline) engine with a conventional carburettor and ignition system.

Air is drawn into the engine through the carburettor, where fuel is added. The resultant fuel-air mixture is then ducted to the top of the engine, ready to be drawn into the cylinder. The four strokes are:-

  1. Induction - the piston moves downwards, drawing the fuel-air mixture through an open inlet valve into the top of the cylinder, an area known as the combustion chamber.
  2. Compression - the inlet valve closes and the piston moves upwards, compressing the fuel-air mixture, which causes it to become quite hot. As the piston approaches the top of the stroke, known as Top Dead Centre (TDC), an electrical discharge from the spark plug ignites the fuel-air mixture. As it burns, the mixture produces very hot gases, which expand because of the heat.
  3. Ignition / Power - with all the valves still closed, the expanding gases force the piston downwards again.
  4. Exhaust - an exhaust valve opens and the piston moves upwards again, pushing the spent gases out of the cylinder. At the end of the exhaust stroke, the exhaust valve closes and the inlet valve opens, ready for the next induction stroke.
Four-cylinder Four-stroke Engine - SchematicLess formally, the four strokes are sometimes called "Suck", "Squeeze", "Bang", "Puff".

The schematic diagram shows a four-cylinder engine, for simplicity, but you can have any number of cylinders from one upwards. For engines with only one cylinder, the momentum of a heavy flywheel attached to the crankshaft keeps the crankshaft turning during the "non-power" strokes. Bigger engines also have flywheels, which help to keep the rotation smooth.

The inlet and exhaust valves are opened and closed by a series of rods connected to a camshaft, which makes the mechanics of the four-stroke engine slightly more complicated. And all these moving parts must be lubricated, necessitating a system for pumping oil to them and adding even more to the complexity. So, although the four-stroke engine is a simple concept, it is mechanically complex.

Two-Stroke Engines

The two-stroke engine is mechanically very simple. There are no valves, camshafts, etc., just the piston connected by the con-rod to the crankshaft. Lubrication is achieved by mixing oil with the fuel, the resulting mixture then bathing all the moving parts. However, the complete cycle takes only one upstroke and one downstroke of the piston, so some elements of the four phases of operation must occur simultaneously.

The fuel and air are mixed in the carburettor in the usual way, but instead of going directly to the top of the cylinder, the mixture enters the sealed crankcase, i.e. the space underneath the piston. The crankcase is connected to the combustion chamber in the cylinder by an inlet port, sometimes known as a transfer port. Opposite the inlet port there is another port, the exhaust port. Both the inlet and exhaust port are uncovered by the piston in the bottom part of its stroke, thus replicating the function of the valves in the four-stroke engine and allowing gases to enter and leave the cylinder. In the top part of the stroke both ports are covered, sealing the cylinder. This allows compression to occur at the top of the upstroke and allows the power of the expanding gases to be harnessed at the top of the downstroke.

Bottom of Downstroke

Bottom of Downstroke - Induction/Exhaust

The piston has moved down, uncovering both the inlet (transfer) and exhaust ports. The descending piston has increased the pressure in the crankcase, so the fuel-air mixture is being pumped from the crankcase via the transfer port into the combustion chamber. In some engines (not in a Vire 7) there is a valve between the carburettor and the crankcase which stops any tendency for some of the fuel-air mixture to blow back through the carburettor.

Top of Upstroke - Compression

Top of Upstroke - Compression

The piston has passed Bottom Dead Centre (BDC) and is now rising. The inlet and exhaust ports are both covered, so the fuel-air mixture in the combustion chamber is being pressurised and is heating up. At the same time, a vacuum is developing in the crankcase, so a fresh charge of fuel-air is being drawn into the crankcase from the carburettor. As the piston rounds TDC, a high voltage discharge from the spark plug ignites the mixture in the combustion chamber.

Top of Downstroke - Ignition/Power

Top of Downstroke - Ignition/Power

With both ports still closed, the pressure of the expanding gases forces the piston down again. The pressure in the crankcase is already rising. Later in the downstroke the exhaust port will be uncovered, allowing the spent gases to escape. Very shortly after that the inlet/transfer port will also be uncovered, which takes the engine back to where it was at the start of the cycle.

The moving image is flawed. The skirt of the piston is drawn too short, so that the ports are shown uncovered underneath the piston as it rises. That doesn't happen.

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