Engine Cycles 101

Story by Costa Mouzouris//
July 1 2013

Two men can be credited with inventing the internal combustion engine in the nineteenth century: Frenchman Alphonse Beau de Rochas and German Nikolaus August Otto. The former came up with the principles of the four-stroke cycle engine; the latter made it work. Today the concept might seem rudimentary and simple, but to these men, the idea that a fuel could be burned inside an engine, converting reciprocating motion into spinning motion, was entirely unproven. Fuel, in the form of coal, had already been used to perform that function in steam engines, but it was burned outside the engine and used to heat water, whose hot vapour was then introduced into the engine; effective, but not efficient. The four strokes of Beau de Rochas’ and Otto’s engine, in order, are: intake (downward stroke), compression (upward stroke), combustion (downward stroke) and exhaust (upward stroke). Here’s how they work.

First, an engine must convert reciprocating motion to rotating motion, for which a crankshaft is needed. Rotation is critical, because it is the type of motion that transfers best through gearboxes, belts, chains or shafts to the wheels of a vehicle.

The crankshaft has a crankpin offset from its central axis, connected to one end of a rod; the other end is connected to a piston. The piston slides up and down in a cylinder, and as it does, it pushes on the connecting rod, which in turn spins the crankshaft. To understand this motion, just think of riding a bicycle; you’re essentially doing the same thing. Your knees (where the pistons would be in an engine) move up and down, but as you push on the pedals, you’re converting that up-and-down motion to rotating motion, which is then transferred to the rear wheel by a chain. Your leg muscles provide the power to do this, and you are burning fuel in the form of calories.

In an engine, gasoline is the fuel, and the power used to push on the piston comes from the gasses that expand inside the cylinder when that fuel is burnt. For gasoline to burn efficiently enough to push on the piston with force, it must first be mixed with air and compressed; otherwise, it would burn like a flame at the end of a match. The air and fuel mixture is blended at the right ratio either in a carburetor, or by a fuel injection system.

That mixture is introduced into the engine through ports that lead into the combustion chamber, located at the top of the cylinder. However, this must be timed properly, as the piston makes its way down the cylinder. This is the intake stroke, and it is the downward movement of the piston that draws the fuel mixture into the cylinder. Because the cylinder must be sealed for the combustion to be effective, there are valves at the end of the ports that open and close at the right time, allowing the intake mixture to enter the cylinder and combust, and allowing the spent exhaust gasses to escape. These valves are operated by the camshaft, which is geared to the crankshaft at a ratio of 1:2, and is what times their opening and closing.

The intake valve will open as the piston begins to descend, and will close as the piston reaches the bottom of its stroke (called bottom dead centre or BDC). Inertia keeps the crankshaft spinning, forcing the piston back up the cylinder, and because the intake and exhaust valves are closed at this time, it compresses the fuel mixture into the combustion chamber. At a critical time, just before the piston reaches the top of its stroke (top dead centre or TDC), a spark, delivered by the ignition system through the sparkplug, ignites the compressed mixture, which burns progressively, yet very quickly. This forces the piston back down the cylinder, providing the power stroke. If you lug a single-cylinder engine at a low speed in a high gear, the chugging you will feel is exactly at this moment.

As the piston reaches BDC again, the exhaust valve opens, and crankshaft inertia again swings the piston back up the cylinder, pushing the exhaust gasses out the exhaust port, through the exhaust system and out the tailpipe. As the piston swings past TDC again, the intake valve opens and the entire process is repeated. The engine will not begin running without some help, so the crankshaft must be spun manually to get the process started, either by a starter, a kick start or even a cord. Once spinning, the engine will run as long as it has fuel or until it is physically shut off.
The two-stroke cycle engine, invented by Belgian engineer Jean Lenoir, preceded the four-stroke engine; however, the latter proved more efficient, more reliable and more practical, eventually enabling the invention of the motorcycle, the automobile and the airplane. It’s the design that has stood the test of time, and today it is a highly advanced and relatively inexpensive powerplant. It’s the reason we spend summers exploring undiscovered roads on our favourite machines.

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.


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