The four-stroke internal combustion engine running on petroleum spirit or petrol¹ forms the basis for the power plants of the majority of modern automobiles and has changed little since its invention in the late nineteenth century.

A Little History

The original four-stroke engine was patented in 1862 by a french scientist called Alphonse Beau de Rochas. Unfortunately, having perfected the theory, the Frenchman failed to build an operational engine and it was left to german engineer Nikolaus A. Otto who built a working prototype around 1867.

Engines based on Otto's principles were designed and built by another, more famous² german engineer Gottlieb Daimler, who was the first to use the now widespread vertical cylinder in 1885 and then a two cylinder engine in the original 'V' arrangement in 1889.

The Otto Cycle

Otto's eventual reward for his effort was the naming of the four-stroke cycle which gives the engine its name. The Otto cycle forms the basic principle of operation of the engine but to understand how it functions it is necessary to know a little bit of engine anatomy.

Basic Anatomy of the Four-Stroke Engine

The basic operating unit of a four stroke engine is the cylinder³. It normally takes the form of a cylindrical space in a large metal block⁴, within which sits a flush fitting piston, also cylindrical and attached via a piston rod⁵ through the open end of the cylinder to the crankshaft. The latter is a long metal rod which usually runs the length of the engine. There are U-shaped bends offset from the crankshaft sometimes called crankpins (similar to an old fashioned hand drill) which are normally at 90 but can also be at 180 degrees to each other. Looking along the axis of the shaft, you would see one at each of the four points of the compass in a 4 cylinder engine. Each piston rod is attached to one of these crankpins and as the piston moves up and down in the cylinder, its 'reciprocating' motion is translated into rotation as the crankpins move around the crankshaft. The rotational motion of the crankshaft is later used to move the wheels of the vehicle via the clutch, gearbox and final drive⁶.

Four Strokes, One Cycle

Each up or down movement of a piston is termed a stroke, hence the piston moves up and down twice and the crankshaft rotates twice in once cycle. Each stroke is associated with a different phase of the cycle. The four are termed Induction (or intake), compression, power (or combustion) and exhaust. An alternative terminology, much more memorable but disliked by purists is Suck, Squeeze, Bang, Blow.

Induction

In this nominally first phase, the piston moves down, away from the top of the cylinder, increasing the space available between the two. This creates a negative pressure which, when an inlet valve in the top of the cylinder opens, draws a fuel and air mixture into the cylinder.

Compression

The inlet valve closes and the piston moves back toward the top of the cylinder, compressing the fuel and air mixture. The difference between the volume of fuel and air mixture when the piston is maximally down and maximally up is termed the compression ratio. It is normally between 8:1 and 10:1 but can be 12:1 or more if high octane⁷ fuels are used. Compression increases the potential of the fuel/air mixture to burn.

Power

An electrical spark generated by the spark plug in the roof of the cylinder ignites the fuel/air mixture, generating large amounts of heat and causing the mixture to expand rapidly⁸, driving the piston downward and actively turning the crankshaft.

Exhaust

The last stroke involves the opening of the second valve (the exhaust valve) in the cylinder and the return of the piston to the upper position. The returning piston forces the remaining gasses in the cylinder out through the exhaust valve and into the engine's exhaust system.

Other Important Engine Parts

The four-stroke cycle applies to a single cylinder operating a crankshaft but many practical engines have several cylinders. There is also the problem of controlling the opening and closing of the valves. These are explained below.

Multiple Cylinders

The vast majority of internal combustion engines use more than one cylinder. This is entirely a question of efficiency. The limitation of the Otto Cycle is that it only provides power to turn the crankshaft a quarter of the time. The logical solution is to have four cylinders with pistons turning the crankshaft so at any time there is always one cylinder in the power stroke and the crankshaft is turned at a fairly even rate. An even more powerful method is to use extra cylinders at intermediate points in the cycle so that one power stroke starts before the previous one has finished. Engines with 6, 8, 10, 12 or more cylinders have been created, either in straight rows or in a V structure with the cylinders split into two banks in a V shape converging on the crankshaft. This allows for efficient transfer of power to the crankshaft without making the engine too large. Horizontally opposed engines with the banks of cylinders directly opposite each other have also been designed but these are generally limited to aircraft⁹

Camshafts and Valves

The inlet and exhaust valves of each cylinder need to open and close at specific times during the cycle. The most appropriate method of achieving this was found to be the mechanical linking of the valve timing to the crankshaft. Thus, when the engine turns more quickly, the valve openings speed up in proportion. This is achieved by driving another shaft, the camshaft via a flexible belt¹⁰ from the crankshaft. The camshaft is so named because distributed along its length are cams. These teardrop shaped¹¹ devices, rotate at half the speed of the crankshaft, and thus the pushrods, which rest on the cams are pushed upward by the tail of the teardrop once every four strokes¹². The pushrods raise one end of the see-saw like rockers, the other end of which presses down on the valve. The valves are shaped like mushrooms with their flattened heads inside the cylinder, held in the upward position by springs. The downward motion of the rockers pushes the head of the valve further into the cylinder and creates a space around the 'stem' of the valve, allowing the flow of gas into or out of the cylinder.

What Exactly is a 24 Valve Twin Overhead Cam?

A common phrase used to make modern engines sound hi-tech, overhead cams are actually a simplification of the design which places the camshaft directly over the valves so they can be depressed directly by the cams, eliminating the need for pushrods and rockers and making the whole process more efficient. The number of valves an engine has refers to the total over all the cylinders so a 4 cylinder engine with 24 valves has 6 valves per cylinder (3 intake and 3 exhaust) allowing more efficient transfer of gas into and out of the cylinder.

Fuel Systems

The fuel and air mixture which is supplied through the inlet valve is traditionally supplied by a device called a carburettor. The most important bit of the carburettor is a narrow tube known as a venturi, through which air flows on its way to the inlet valve. Because a vacuum is created in the far end of the venturi by the intake stroke of the Otto Cycle, fuel is drawn from a jet into the venturi, vaporising as it passes through the tiny nozzle of the jet. The flow through the venturi is controlled by the engine's throttle, the higher the setting, the more air, and therefore the more fuel is drawn through the carburettor and the faster the engine can work.

Fuel Injection

Most modern cars have fuel injection systems which involve the direct injection of vapourised fuel into the inlet pipe, removing the necessity for a carburettor. The injection units are normally electronically controlled, allowing for much more accurate regulation of the amount of fuel which enters the cylinder, producing more power, more economy and fewer emissions. See the article on Fuel Injection for more details.

Electrical Systems

As has already been alluded to, combustion in a four-stroke petrol engine has to be initiated by an electrical spark when the piston is at the top of the cylinder after the compression stroke¹³. This spark is provided by a spark plug which is the end point of an engine's electrical system. The important bit of a spark plug is the two closely placed contacts inside the cylinder between which an electrical pulse arcs¹⁴ creating a spark. The electricity originates from the car's battery, which is being constantly charged by an alternator¹⁵. The electrical charge, which normally has a relatively low voltage (12V) is transmitted to the coil, which is essentially a transformer which converts it to high voltage at the expense of current. Low tension leads then carry this charge to the distributor.

Distributor

The distributor is possibly the most technical part of the electrical system. Its task is to connect the electrical energy from the coil to the spark plug supplying each cylinder in turn. This is achieved by a rotating arm at the centre of the distributor¹⁶ which touches each of four equally spaced contacts in turn. The contacts being connected to one of the high tension leads attached to the spark plugs. As the engine speeds up, the cylinders need to fire more quickly after one another so the speed of the rotating arm is controlled by a small tube from the carburettor which transmits the increasing vacuum in the venturi to the distributor causing the rotating arm to spin more quickly.

Starter Motor

The electrical starter motor is essentially quite a simple gadget which turns the cranshaft and allows fuel and air to be sucked into the cylinders to start the Otto cycle¹⁷. It is powered from the battery and can be activated once the ignition has been switched on, the latter providing power to the coil and thereby the sparkplugs.

Summary

As can hopefully be seen from this article, the petrol powered four-stroke internal combustion engine is something approaching a technological miracle. A complicated piece of equipment in which each component is intimately related to several others and dependant on them for its proper functioning. It is a testament to the original design that the essential principles have changed little in 130 years despite the improvements in materials science and electronics which have lead to significant improvements in the power, efficiency and reliability of these engines. There are few other inventions which can claim such lasting success.